Patent Publication Number: US-2022233726-A1

Title: Novel radiolabelled compounds for diagnosis or treatment of prostate-specific membrane antigen-expressing cancer

Description:
FIELD OF INVENTION 
     The present invention relates to radiolabelled compounds for selective imaging or treatment of cancer, particularly compounds that target prostate-specific membrane antigen. 
     BACKGROUND OF THE INVENTION 
     Prostate-specific membrane antigen (PSMA) is a transmembrane protein that catalyzes the hydrolysis of N-acetyl-aspartylglutamate to glutamate and N-acetylaspartate. PSMA is not expressed in most normal tissues, but is overexpressed (up to 1,000-fold) in prostate tumors and metastases (see, e.g.: Silver et al.,  Clin. Cancer Res.  1997, 3:81-85; and Sokoloff et al.  Prostate  2000, 43:150-157). Due to its pathological expression pattern, various radiolabeled PSMA-targeting constructs have been designed and evaluated for endoradiotherapy of prostate cancer. 
     The common radiolabeled PSMA-targeting endoradiotherapeutic agents are derivatives of lysine-urea-glutamate (Lys-urea-Glu), including  131 I-MIP-1095,  177 Lu-PSMA-617 and  177 Lu-PSMA I&amp;T (see, e.g.: Afshar-Oromieh et al.,  Eur. J. Nucl. Med. Mol. Imaging  2017, 44:950-959; Heck et al.,  J. Urol.  2016, 196: 382-391; and Kratochwil et al.,  J. Nucl. Med.  2016, 57:1170-1176). Among them,  177 Lu-PSMA-617 is the most studied agent, and is currently being evaluated in multi-center trials. Preliminary data demonstrated that  177 Lu-PSMA-617 was effective in treating metastatic prostate cancer with 32-60% of patients having &gt;50% reduction in PSA levels, and without severe side effects (Kratochwil, et al., supra; Rahbar et al.,  Clin. Nucl. Med.  2016, 41:522-528; Fendler et al., Oncotarget2017, 8:3581-3590; Rahbar et al.,  J. Nucl. Med.  2017, 58:85-90; Ahmadzadehfar et al.,  Eur. J. Nucl. Med. Mol. Imaging  2017, 44:1448-1454; Brauer et al.,  Eur. J. Nucl. Med. Mol. Imaging  2017, 44:1663-1670; and Yadav et al.,  Eur. J. Nucl. Med. Mol. Imaging  2017, 44:81-91). In a phase 2 Australian study, an objective response was observed in 82% of patients with measurable nodal or visceral disease (Hofman et al.,  Lancet Oncol.  2018, 19:825-833). However, the complete response rate was low (&lt;7%), and up to 33% of the patients still had progressive disease after 177 Lu-PSMA-617 treatment (see: Kratochwil et al., supra; Fendler et al., supra, Rahbar et al., supra; Abmadzadehfar et al., supra; Brauer et al., supra; and Yadav et al., supra). One approach to increase the radiotherapeutic efficacy is to increase the radiation dose deposited in tumors per unit administered radioactivity of the  177 Lu-labeled agents. Improving the delivery of  177 Lu to tumors can also reduce the cost of therapeutic radiopharmaceuticals by decreasing radioisotope costs. 
     No admission is necessarily intended, nor should it be construed, that any of the preceding information constitutes prior art against the present invention. 
     SUMMARY OF THE INVENTION 
     Disclosed herein are novel compounds targeting the PSMA. 
     This disclosure provides a compound, wherein the compound has Formula I (shown below) or is a salt or a solvate of Formula I: 
     
       
         
         
             
             
         
       
     
     wherein: each of R 1a , R 1b  and R 1c  is independently —CO 2 H, —SO 2 H, —SO 3 H, —SO 4 H, —PO 2 H, —PO 3 H or —PO 4 H; R 2  is a linear or branched, cyclic or acyclic, and/or aromatic or non-aromatic C 1 -C 20  alkylenyl or alkenylenyl, or a linear or branched, cyclic or acyclic, and/or aromatic or non-aromatic X 2 -X 20  heteroalkylenyl or heteroalkenylenyl; R 3  is —O—, —S—, —S(O)—, S(O) 2 —, —NHC(O)—, —O(O)N H— 
     
       
         
         
             
             
         
       
     
     R 4  is —(CH 2 ) 0-3 CH(R 7 )(CH 2 ) 0-3 —, wherein R 7  is —(CH 2 ) 5 CH 3 , 
     
       
         
         
             
             
         
       
     
     each of R 5  and R 6  is independently hydrogen or methyl; Xaa 1  is an amino acid of formula —N(R 8 )R 9 C(O)—, wherein each R 8  is independently hydrogen or methyl, and wherein each R 9  is independently: a linear or branched, cyclic or acyclic, and/or aromatic or non-aromatic C 1 -C 20  alkylenyl, alkenylenyl or alkynylenyl; or a linear or branched, cyclic or acyclic, and/or aromatic or non-aromatic X 2 -X 20  heteroalkylenyl, heteroalkenylenyl or heteroalkynylenyl; and R X  is a radiolabeling group independently selected from: a radiometal chelator optionally bound by a radiometal; an aryl substituted with a radioisotope; a prosthetic group containing a trifluoroborate; and a prosthetic group containing a silicon-fluorine-acceptor moiety. 
     This summary of the invention does not necessarily describe all features of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and other features of the invention will become more apparent from the following description in which reference is made to the appended drawings wherein: 
         FIG. 1  depicts the chemical structures of HTK03026, HTK03027, HTK03029 and HTK03041. 
         FIG. 2  shows representative displacement curves of  18 F-DCFPyL by Ga-PSMA-617, Ga-HTK03026, Ga-HTK03027, Ga-HTK03029, and Ga-HTK03041. 
         FIG. 3  shows PET/CT images of (A)  68 Ga-HTK03026, (B)  68 Ga-HTK03027, (C)  68 Ga-HTK03029 and (D)  68 Ga-HTK03041 in mice bearing LNCaP tumors (t: tumor, k: kidney, b:bladder). 
         FIG. 4  shows maximum intensity projection SPECT images of 177 Lu-HTK03041 acquired at 1 h, 4 h, 24 h, 72 h, and 120 h post-injection (t: tumor, k: kidney, b:bladder). 
     
    
    
     DETAILED DESCRIPTION 
     As used herein, the terms “comprising,” “having”, “including” and “containing,” and grammatical variations thereof, are inclusive or open-ended and do not exclude additional, unrecited elements and/or method steps. The term “consisting essentially of” if used herein in connection with a composition, use or method, denotes that additional elements and/or method steps may be present, but that these additions do not materially affect the manner in which the recited composition, method or use functions. The term “consisting of” if used herein in connection with a composition, use or method, excludes the presence of additional elements and/or method steps. A composition, use or method described herein as comprising certain elements and/or steps may also, in certain embodiments consist essentially of those elements and/or steps, and in other embodiments consist of those elements and/or steps, whether or not these embodiments are specifically referred to. A use or method described herein as comprising certain elements and/or steps may also, in certain embodiments consist essentially of those elements and/or steps, and in other embodiments consist of those elements and/or steps, whether or not these embodiments are specifically referred to. 
     A reference to an element by the indefinite article “a” does not exclude the possibility that more than one of the elements is present, unless the context clearly requires that there be one and only one of the elements. The singular forms “a”, “an”, and “the” include plural referents unless the content clearly dictates otherwise. The use of the word “a” or “an” when used herein in conjunction with the term “comprising” may mean “one,” but it is also consistent with the meaning of “one or more,” “at least one” and “one or more than one.” 
     Unless otherwise specified, “certain embodiments”, “various embodiments”, “an embodiment” and similar terms includes the particular feature(s) described for that embodiment either alone or in combination with any other embodiment or embodiments described herein, whether or not the other embodiments are directly or indirectly referenced and regardless of whether the feature or embodiment is described in the context of a method, product, use, composition, compound, et cetera. 
     As used herein, the terms “treat”, “treatment”, “therapeutic” and the like includes ameliorating symptoms, reducing disease progression, improving prognosis and reducing recurrence (e.g. reducing cancer recurrence). 
     As used herein, the term “diagnostic agent” includes an “imaging agent”. As such, a “diagnostic radiometal” includes radiometals that are suitable for use in imaging agents and “diagnostic radioisotope” includes radioisotopes that are suitable for use in imaging agents. 
     The term “subject” refers to an animal (e.g. a mammal or a non-mammal animal). The subject may be a human or a non-human primate. The subject may be a laboratory mammal (e.g., mouse, rat, rabbit, hamster and the like). The subject may be an agricultural animal (e.g., equine, ovine, bovine, porcine, camelid and the like) or a domestic animal (e.g., canine, feline and the like). In some embodiments, the subject is a human. 
     The compounds disclosed herein may also include base-free forms, salts or pharmaceutically acceptable salts thereof. Unless otherwise specified, the compounds claimed and described herein are meant to include all racemic mixtures and all individual enantiomers or combinations thereof, whether or not they are explicitly represented herein. 
     The compounds disclosed herein may be shown as having one or more charged groups, may be shown with ionizable groups in an uncharged (e.g. protonated) state or may be shown without specifying formal charges. As will be appreciated by the person of skill in the art, the ionization state of certain groups within a compound (e.g. without limitation, CO 2 H, PO 3 H 2,  SO 2 H, SO 3 H, SO 4 H, OPO 3 H 2  and the like) is dependent, inter alia, on the pKa of that group and the pH at that location. For example, but without limitation, a carboxylic acid group (i.e. COOH) would be understood to usually be deprotonated (and negatively charged) at neutral pH and at most physiological pH values, unless the protonated state is stabilized. Likewise, OSO 3 H (i.e. SO 4 H) groups, SO 2 H groups, SO 3 H groups, OPO 3 H 2  (i.e. PO 4 H 2 ) groups and PO 3 H groups would generally be deprotonated (and negatively charged) at neutral and physiological pH values. 
     As used herein, the terms “salt” and “solvate” have their usual meaning in chemistry. As such, when the compound is a salt or solvate, it is associated with a suitable counter-ion. It is well known in the art how to prepare salts or to exchange counter-ions. Generally, such salts can be prepared by reacting free acid forms of these compounds with a stoichiometric amount of a suitable base (e.g. without limitation, Na, Ca, Mg, or K hydroxide, carbonate, bicarbonate, or the like), or by reacting free base forms of these compounds with a stoichiometric amount of a suitable acid. Such reactions are generally carried out in water or in an organic solvent, or in a mixture of the two. Counter-ions may be changed, for example, by ion-exchange techniques such as ion-exchange chromatography. All zwitterions, salts, solvates and counter-ions are intended, unless a particular form is specifically indicated. 
     In certain embodiments, the salt or counter-ion may be pharmaceutically acceptable, for administration to a subject. More generally, with respect to any pharmaceutical composition disclosed herein, non-limiting examples of suitable excipients include any suitable buffers, stabilizing agents, salts, antioxidants, complexing agents, tonicity agents, cryoprotectants, lyoprotectants, suspending agents, emulsifying agents, antimicrobial agents, preservatives, chelating agents, binding agents, surfactants, wetting agents, non-aqueous vehicles such as fixed oils, or polymers for sustained or controlled release. See, for example, Berge et al. 1977. ( J. Pharm Sci.  66:1-19), or Remington—The Science and Practice of Pharmacy, 21st edition (Gennaro et al editors. Lippincott Williams &amp; Wilkins Philadelphia), each of which is incorporated by reference in its entirety. 
     As used herein, the expression “Xy-Xz”, where y and z are integers (e.g. X 1 -X 15 , X 1 -X 30 , X 1 -X 100,  and the like), refers to the number of carbons (for alkyls, whether saturated or unsaturated, or aryls) in a compound, R-group or substituent, or refers to the number of carbons plus heteroatoms (for heteroalkyls, whether saturated or unsaturated, or heteroaryls) in a compound, R-group or substituent. Heteroatoms may include any, some or all possible heteroatoms. For example, in some embodiments, the heteroatoms are selected from N, O, S, P and Se. In some embodiments, the heteroatoms are selected from N, O, S and P. Such embodiments are non-limiting. Alkyls and aryls may alternatively be referred to using the expression “Cy-Cz”, where y and z are integers (e.g. C 3 -C 15  and the like). 
     Unless explicitly stated otherwise, the terms “alkyl” and “heteroalkyl” each includes any reasonable combination of the following: (1) saturated alkyls as well as unsaturated (including partially unsaturated) alkyls (e.g. alkenyls and alkynyls); (2) linear or branched; (3) acyclic or cyclic (aromatic or nonaromatic), the latter of which may include multi-cyclic (fused rings, multiple non-fused rings or a combination thereof); and (4) unsubstituted or substituted. For example, an alkyl or heteroalkyl (i.e. “alkyl/heteroalkyl”) may be saturated, branched and cyclic, or unsaturated, branched and cyclic, or linear and unsaturated, or any other reasonable combination according to the skill of the person of skill in the art. If unspecified, the size of the alkyl/heteroalkyl is what would be considered reasonable to the person of skill in the art. For example, but without limitation, if unspecified, the size of an alkyl may be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100 or more than 100 carbons in length, subject to the common general knowledge of the person of skill in the art. Further, but without limitation, if unspecified, the size of a heteroalkyl may be 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22, 23, 24, 25, 26, 27, 28, 29, 30, 31, 32, 33, 34, 35, 36, 37, 38, 39, 40, 41, 42, 43, 44, 45, 46, 47, 48, 49, 50, 51, 52, 53, 54, 55, 56, 57, 58, 59, 60, 61, 62, 63, 64, 65, 66, 67, 68, 69, 70, 71, 72, 73, 74, 75, 76, 77, 78, 79, 80, 81, 82, 83, 84, 85, 86, 87, 89, 90, 91, 92, 93, 94, 95, 96, 97, 98, 99, 100 or more than 100 carbons and heteroatoms in length, subject to the common general knowledge of the person of skill in the art. In the context of the expression “alkyl, alkenyl or alkynyl” and similar expressions, the “alkyl” would be understood to be a saturated alkyl. Likewise, in the context of the expression “heteroalkyl, heteroalkenyl or heteroalkynyl” and similar expressions, the “heteroalkyl” would be understood to be a saturated heteroalkyl. 
     As used herein, in the context of an alkyl/heteroalkyl group of a compound, the term “linear” may be used as it is normally understood to a person of skill in the art and generally refers to a chemical entity that comprises a skeleton or main chain that does not split off into more than one contiguous chain. Non-limiting examples of linear alkyls include methyl, ethyl, n-propyl, and n-butyl. 
     As used herein, the term “branched” may be used as it is normally understood to a person of skill in the art and generally refers to a chemical entity that comprises a skeleton or main chain that splits off into more than one contiguous chain. The portions of the skeleton or main chain that split off in more than one direction may be linear, cyclic or any combination thereof. Non-limiting examples of a branched alkyl group include tert-butyl and isopropyl. 
     The term “alkylenyl” refers to a divalent analog of an alkyl group. In the context of the expression “alkylenyl, alkenylenyl or alkynylenyl”, “alkylenyl or alkenylenyl” and similar expressions, the “alkylenyl” would be understood to be a saturated alkylenyl. The term “heteroalkylenyl” refers to a divalent analog of a heteroalkyl group. In the context of the expression “heteroalkylenyl, heteroalkenylenyl or heteroalkynylenyl”, “heteroalkylenyl or heteroalkenylenyl” and similar expressions, the “heteroalkylenyl” would be understood to be a saturated heteroalkylenyl. 
     As used herein, the term “saturated” when referring to a chemical entity may be used as it is normally understood to a person of skill in the art and generally refers to a chemical entity that comprises only single bonds, and may include linear, branched, and/or cyclic groups. Non-limiting examples of a saturated C 1 -C 20  alkyl group may include methyl, ethyl, n-propyl, i-propyl, sec-propyl, n-butyl, i-butyl, sec-butyl, t-butyl, n-pentyl, i-pentyl, sec-pentyl, t-pentyl, n-hexyl, i-hexyl, 1,2-dimethylpropyl, 2-ethylpropyl, 1-methyl-2-ethylpropyl, 1-ethyl-2-methylpropyl, 1,1,2-trimethylpropyl, 1,1,2-triethylpropyl, 1,1-dimethylbutyl, 2,2-dimethylbutyl, 2-ethylbutyl, 1,3-dimethylbutyl, 2-methyl pentyl, 3-methylpentyl, sec-hexyl, t-hexyl, n-heptyl, i-heptyl, sec-heptyl, t-heptyl, n-octyl, i-octyl, sec-octyl, t-octyl, n-nonyl, i-nonyl, sec-nonyl, t-nonyl, n-decyl, i-decyl, sec-decyl, t-decyl, cyclopropanyl, cyclobutanyl, cyclopentanyl, cyclohexanyl, cycloheptanyl, cyclooctanyl, cyclononanyl, cyclodecanyl, and the like. Unless otherwise specified, a C 1 -C 20  alkylenyl therefore encompasses, without limitation, all divalent analogs of the above-listed saturated alkyl groups. 
     As used herein, the term “unsaturated” when referring to a chemical entity may be used as it is normally understood to a person of skill in the art and generally refers to a chemical entity that comprises at least one double or triple bond, and may include linear, branched, and/or cyclic groups. Non-limiting examples of a C 2 -C 20  alkenyl group may include vinyl, allyl, isopropenyl, I-propene-2-yl, 1-butene-I-yl, I-butene-2-yl, I-butene-3-yl, 2-butene-I-yl, 2-butene-2-yl, octenyl, decenyl, cyclopropenyl, cyclobutenyl, cyclopentenyl, cyclohexenyl, cycloheptenyl, cyclooctenyl, cyclononanenyl, cyclodecanenyl, and the like. Unless otherwise specified, a C 1 -C 20  alkenylenyl therefore encompasses, without limitation, all divalent analogs of the above-listed alkenyl groups. Non-limiting examples of a C 2 -C 20  alkynyl group may include ethynyl, propynyl, butynyl, pentynyl, hexynyl, heptynyl, octynyl, nonynyl, decynyl, and the like. Unless otherwise specified, a C 1 -C 20  alkynylenyl therefore encompasses, without limitation, all divalent analogs of the above-listed alkynyl groups. Without limitation, the above-defined saturated C 1 -C 20  alkyl groups, C 2 -C 20  alkenyl groups and C 2 -C 20  alkynyl groups are all encompassed within the term “X 1 -X 20  alkyl”, unless otherwise indicated. Without limitation, the above-defined saturated C 1 -C 20  alkylenyl groups, C 2 -C 20  alkenylenyl groups and C 2 -C 20  alkynylenyl groups are all encompassed within the term “X 1 -X 20  alkylenyl”, unless otherwise indicated. 
     Without limitation, the term “X 1 -X 20  heteroalkyl” would encompass each of the above-defined saturated C 1 -C 20  alkyl groups, C 2 -C 20  alkenyl groups and C 2 -C 20  alkynyl gruops, where one or more of the carbon atoms is independently replaced with a heteroatom. Likewise, without limitation, the term “X 1 -X 20  heteroalkylenyl” would encompass each of the above-defined saturated C 1 -C 20  alkylenyl groups, C 2 -C 20  alkenylenyl groups and C 2 -C 20  alkynylenyl groups, where one or more of the carbon atoms is independently replaced with a heteroatom. The person of skill in the art would understand that various combinations of different heteroatoms may be used. Non-limiting examples of non-aromatic heterocyclic groups include aziridinyl, azetidinyl, diazetidinyl, pyrrolidinyl, pyrrolinyl, piperidinyl, piperazinyl, imidazolinyl, pyrazolidinyl, imidazolydinyl, phthalimidyl, succinimidyl, oxiranyl, tetrahydropyranyl, oxetanyl, dioxanyl, thietanyl, thiepinyl, morpholinyl, oxathiolanyl, and the like. 
     Unless further specified, an “aryl” group includes both single aromatic rings as well as fused rings containing at least one aromatic ring. non-limiting examples of C 3 -C 20  aryl groups include phenyl (Ph), pentalenyl, indenyl, naphthyl and azulenyl. Non-limiting examples of X 3 -X 20  aromatic heterocyclic groups include pyrrolyl, imidazolyl, pyrazolyl, pyridinyl, pyridazinyl, pyrimidinyl, pirazinyl, quinolinyl, isoquinolinyl, acridinyl, indolyl, isoindolyl, indolizinyl, purinyl, carbazolyl, indazolyl, phthalazinyl, naphthyridinyl, quinoxalinyl, quinazolinyl, cinnolinyl, pteridinyl, phenanthridinyl, phenazinyl, phenanthrolinyl, perimidinyl, furyl, dibenzofuryl, xanthenyl, benzofuryl, thiophenyl, thianthrenyl, benzothiophenyl, phosphorinyl, phosphinolinyl, phosphindolyl, thiazolyl, oxazolyl, isoxazolyl, and the like. 
     As used herein, the term “substituted” is used as it would normally be understood to a person of skill in the art and generally refers to a compound or chemical entity that has one chemical group replaced with a different chemical group. Unless otherwise specified, a substituted alkyl is an alkyl in which one or more hydrogen atom(s) are independently each replaced with an atom that is not hydrogen. For example, chloromethyl is a non-limiting example of a substituted alkyl, more particularly an example of a substituted methyl. Aminoethyl is another non-limiting example of a substituted alkyl, more particularly an example of a substituted ethyl. Unless otherwise specified, a substituted compound or group (e.g. alkyl, heteroalkyl, aryl, heteroaryl and the like) may be substituted with any chemical group reasonable to the person of skill in the art. For example, but without limitation, a hydrogen bonded to a carbon or heteroatom (e.g. N) may be substituted with halide (e.g. F, I, Br, Cl), amine, amide, oxo, hydroxyl, thiol, phosphate, phosphonate, sulfate, SO 2 H, SO 3 H, alkyls, heteroalkyls, aryl, heteroaryl, ketones, carboxaldehyde, carboxylates, carboxamides, nitriles, monohalomethyl, dihalomethyl or trihalomethyl. 
     As used herein, the term “unsubstituted” is used as it would normally be understood to a person of skill in the art. Non-limiting examples of unsubstituted alkyls include methyl, ethyl, tert-butyl, pentyl and the like. The expression “optionally substituted” is used interchangeably with the expression “unsubstituted or substituted”. 
     In the structures provided herein, hydrogen may or may not be shown. In some embodiments, hydrogens (whether shown or implicit) may be protium (i.e.  1  H) , deuterium (i.e.  2 H) or combinations of  1 H and  2 H. Methods for exchanging  1 H with  2 H are well known in the art. For solvent-exchangeable hydrogens, the exchange of  1 H with  2 H occurs readily in the presence of a suitable deuterium source, without any catalyst. The use of acid, base or metal catalysts, coupled with conditions of increased temperature and pressure, can facilitate the exchange of non-exchangeable hydrogen atoms, generally resulting in the exchange of all  1 H to  2 H in a molecule. 
     The term “Xaa” refers to an amino acid residue in a peptide chain (linear or branched) or an amino acid that is otherwise part of a compound. Amino acids have both an amino group and a carboxylic acid group, either or both of which can be used for covalent attachment. In attaching to the remainder of the compound, the amino group and/or the carboxylic acid group may be converted to an amide or other structure; e.g. a carboxylic acid group of a first amino acid is converted to an amide (i.e. a peptide bond) when bonded to the amino group of a second amino acid. As such, Xaa may have the formula —N(R a )R b C(O)—, where R a  and R b  are R-groups. R a  will typically be hydrogen or methyl. The amino acid residues of a peptide may comprise typical peptide (amide) bonds and may further comprise bonds between side chain functional groups and the side chain or main chain functional group of another amino acid. For example, the side chain carboxylate of one amino acid residue in the peptide (e.g. Asp, Glu, etc.) may be bonded to and the amine of another amino acid residue in the peptide (e.g. Dap, Dab, Orn, Lys). Further details are provided below. Unless otherwise indicated, “Xaa” may be any amino acid, including proteinogenic and nonproteinogenic amino acids. Non-limiting examples of nonproteinogenic amino acids are shown in Table 1 and include: D-amino acids (including without limitation any D-form of the following amino acids), ornithine (Orn), 3-(1-naphtyl)alanine (Nal), 3-(2-naphtyl)alanine (2-Nal), α-aminobutyric acid, norvaline, norleucine (Nle), homonorleucine, beta-(1,2,3-triazol-4-yl)-L-alanine, 1,2,4-triazole-3-alanine, Phe(4-F), Phe(4-Cl), Phe(4-Br), Phe(4-I), Phe(4-NH 2 ), Phe(4-NO 2 ), homoarginine (hArg), 2-amino-4-guanidinobutyric acid (Agb), 2-amino-3-guanidinopropionic acid (Agp), B-alanine, 4-aminobutyric acid, 5-aminovaleric acid, 6-aminohexanoic acid, 7-aminoheptanoic acid, 8-aminooctanoic acid, 9-aminononanoic acid, 10-aminodecanoic acid, 2-aminooctanoic acid, 2-amino-3-(anthracen-2-yl)propanoic acid, 2-amino-3-(anthracen-9-yl) propanoic acid, 2-amino-3-(pyren-1-yl)propanoic acid, Trp(5-Br), Trp(5-OCH 3 ), Trp(6-F), Trp(5-OH) or Trp(CHO), 2-aminoadipic acid (2-Aad), 3-aminoadipic acid (3-Aad), propargylglycine (Pra), homopropargylglycine (Hpg), beta-homopropargylglycine (Bpg), 2,3-diaminopropionic acid (Dap), 2,4-diaminobutyric acid (Dab), azidolysine (Lys(N 3 )), azido-ornithine (Orn(N 3 )), 2-amino-4-azidobutanoic acid Dab(N 3 ), Dap(N 3 ), 2-(5′-azidopentyl)alanine, 2-(6′-azidohexyl)alanine, 4-amino-1-carboxymethyl-piperidine (Pip), 4-(2-aminoethyl)-1-carboxymethyl-piperazine (Acp), and tranexamic acid. If not specified as an L- or D-amino acid, an amino acid shall be understood to encompass both L- and D-amino acids. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 List of non-limiting examples of non-proteinogenic amino acids. 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
            
               
                 Any D-amino acid of a proteinogenic amino acid 
               
               
                 ornithine (Orn) 
               
               
                 3-(1-naphtyl)alanine (Nal) 
               
               
                 3-(2-naphtyl)alanine (2-Nal) 
               
               
                 α-aminobutyric acid 
               
               
                 norvaline 
               
               
                 norleucine (Nle) 
               
               
                 homonorleucine 
               
               
                 beta-(1,2,3-triazol-4-yl)-L-alanine 
               
               
                 1,2,4-triazole-3-alanine 
               
               
                 Phe(4-F), 
               
               
                 Phe(4-Cl), 
               
               
                 Phe(4-Br), 
               
               
                 Phe(4-I), 
               
               
                 Phe(4-NH 2 ), 
               
               
                 Phe(4-NO 2 ), 
               
               
                 homoarginine (hArg) 
               
               
                 4-(2-aminoethyl)-1-carboxymethyl-piperazine (Acp) 
               
               
                 2-(5′-azidopentyl)alanine, 2-(6′-azidohexyl)alanine 
               
               
                 2-amino-4-guanidinobutyric acid (Agb) 
               
               
                 2-amino-3-guanidinopropionic acid (Agp) 
               
               
                 β-alanine 
               
               
                 4-aminobutyric acid 
               
               
                 5-aminovaleric acid 
               
               
                 6-aminohexanoic acid 
               
               
                 7-aminoheptanoic acid 
               
               
                 8-aminooctanoic acid 
               
               
                 9-aminononanoic acid 
               
               
                 10-aminodecanoic acid 
               
               
                 2-aminooctanoic acid 
               
               
                 2-amino-3-(anthracen-2-yl)propanoic acid 
               
               
                 2-amino-3-(anthracen-9-yl)propanoic acid 
               
               
                 2-amino-3-(pyren-1-yl)propanoic acid 
               
               
                 Trp(5-Br), 
               
               
                 Trp(5-OCH 3 ), 
               
               
                 Trp(6-F), 
               
               
                 Trp(5-OH) 
               
               
                 Trp(CHO), 
               
               
                 2-aminoadipic acid (2-Aad) 
               
               
                 3-aminoadipic acid (3-Aad) 
               
               
                 propargylglycine (Pra) 
               
               
                 homopropargylglycine (Hpg) 
               
               
                 beta-homopropargylglycine (Bpg) 
               
               
                 2,3-diaminopropionic acid (Dap) 
               
               
                 2,4-diaminobutyric acid (Dab) 
               
               
                 azidolysine (Lys(N 3 )) 
               
               
                 azido-ornithine (Orn(N 3 )) 
               
               
                 amino-4-azidobutanoic acid Dab(N 3 ) 
               
               
                 tranexamic acid 
               
               
                 4-amino-1-carboxymethyl-piperidine (Pip) 
               
               
                 NH 2 (CH 2 ) 2 O(CH 2 ) 2 C(O)OH 
               
               
                 NH 2 (CH 2 ) 2 [O(CH 2 ) 2 ] 2 C(O)OH 
               
               
                 NH 2 (CH 2 ) 2 [O(CH 2 ) 2 ] 3 C(O)OH 
               
               
                 NH 2 (CH 2 ) 2 [O(CH 2 ) 2 ] 4 C(O)OH 
               
               
                 NH 2 (CH 2 ) 2 [O(CH 2 ) 2 ] 5 C(O)OH 
               
               
                 NH 2 (CH 2 ) 2 [O(CH 2 ) 2 ] 6 C(O)OH 
               
               
                   
               
            
           
         
       
     
     The wavy line “  ” symbol shown through or at the end of a bond in a chemical formula (e.g. in the definitions R 3 , R 7 , etc.) is intended to define the R group on one side of the wavy line, without modifying the definition of the structure on the opposite side of the wavy line. Where an R group is bonded on two or more sides (e.g. R 3 ), any atoms shown outside the wavy lines are intended to clarify orientation of the R group. As such, only the atoms between the two wavy lines constitute the definition of the R group. When atoms are not shown outside the wavy lines, or for a chemical group shown without wavy lines but does have bonds on multiple sides (e.g. —C(O)NH—, and the like.), the chemical group should be read from left to right matching the orientation in the formula that the group relates to (e.g. for formula —R a —R b —R 3 —, the definition of R b  as —C(O)NH— would be incorporated into the formula as —R a —C(O)NH—R 3 — not as —R a —NHC(O)—R 3 —). 
     In various aspects, there is disclosed a compound, wherein the compound has Formula I or is a salt or a solvate of Formula I: 
     
       
         
         
             
             
         
       
     
     wherein:
     each of R 1a , R 1b  and R 1c  is independently —CO 2 H, —SO 2 H, —SO 3 H, —SO 4 H, —PO 2 H, —PO 3 H or —PO 4 H;   R 2  is a linear or branched, cyclic or acyclic, and/or aromatic or non-aromatic C 1 -C 20  alkylenyl or alkenylenyl, or a linear or branched, cyclic or acyclic, and/or aromatic or non-aromatic X 2 -X 20  heteroalkylenyl or heteroalkenylenyl;   R 3  is —O—, —S—, —S(O)—, S(O) 2 —, —NHC(O)—, —C(O)NH—,   

     
       
         
         
             
             
         
       
         
         R 4  is —(CH 2 ) 0-3 CH(R 7 )(CH 2 ) 0-3 —, wherein R 7  is —(CH 2 ) 5 CH 3 , 
       
    
     
       
         
         
             
             
         
       
         
         each of R 5  and R 6  is independently hydrogen or methyl; 
         Xaa 1  is an amino acid of formula —N(R 8 )R 9 C(O)—, wherein each R 8  is independently hydrogen or methyl, and wherein each R 9  is independently: a linear or branched, cyclic or acyclic, and/or aromatic or non-aromatic C 1 -C 20  alkylenyl, alkenylenyl or alkynylenyl; or a linear or branched, cyclic or acyclic, and/or aromatic or non-aromatic X 2 -X 20  heteroalkylenyl, heteroalkenylenyl or heteroalkynylenyl; 
       
    
     R X  is a radiolabeling group independently selected from: a radiometal chelator optionally bound by a radiometal; an aryl substituted with a radioisotope; a prosthetic group containing a trifluoroborate; and a prosthetic group containing a silicon-fluorine-acceptor moiety. 
     In some embodiments, the compound has Formula I-a or is a salt or solvate of Formula I-a: 
     
       
         
         
             
             
         
       
     
     wherein R 1a , R 1b , R 1c , R 2 , R 3 , R 4 , R 5 , R 6 , Xaa 1  and R X  are as defined for Formula I. 
     In some In some embodiments, the compound has Formula I-b or is a salt or solvate of Formula I-b: 
     
       
         
         
             
             
         
       
     
     wherein R 1a , R 1b , R 1c , R 2 , R 3 , R 4 , R 5 , R 6 , Xaa 1  and R X  are as defined in Formula I. 
     In some embodiments, R 1a  is —CO 2 H. In some embodiments, R 1b  is —CO 2 H. In some embodiments, R 1c  is —CO 2 H. In some embodiments, R 1a  and R 1b  are each —CO 2 H. In some embodiments, R 1a  and R 1c  are each —CO 2 H. In some embodiments, R 1b  and R 1c  are each —CO 2 H. In some embodiments, R 1a , R 1b  and R 1c  are each —CO 2 H. 
     In some embodiments, R 2  is a linear or branched C 1 -C 20  alkylenyl in which 0-5 carbons are replaced with N, S and/or O heteroatoms. In some embodiments, R 2  is a linear acyclic C 3 -C 15  alkylenyl. In some embodiments, R 2  is a linear acyclic C 3 -C 15  heteroalkylenyl having 1-5 N, S and/or O heteroatoms. In some embodiments, R 2  is a linear acyclic saturated C 3 -C 10  alkylenyl, optionally substituted with 1-5 amine, amide, oxo, hydroxyl, thiol, methyl or ethyl groups. In some embodiments, R 2  is —(CH 2 ) 3-15 —. In some embodiments, R 2  is —(CH 2 ) 1-8 —. In some embodiments, R 2  is —CH 2 —. In some embodiments, R 2  is —(CH 2 ) 2 —. In some embodiments, R 2  is —(CH 2 ) 3 —. In some embodiments, R 2  is —(CH 2 ) 4 —. In some embodiments, R 2  is —(CH 2 ) 5 —. In some embodiments, R 2  is —CH 2 —O—CH 2 —. In some embodiments, R 2  is —CH 2 —S—CH 2 —. 
     In some embodiments, R 3  is —O—. In some embodiments, R 3  is —S—. In some embodiments, R 3  is —NHC(O)—. In some embodiments, R 3  is —C(O)NH—.In some embodiments, R 3  is 
     
       
         
         
             
             
         
       
     
     In some embodiments, R 3  is 
     
       
         
         
             
             
         
       
     
     In some embodiments, R 3  is —S(O)—. In some embodiments, R 3  is —S(O) 2 —. 
     In some embodiments, R 2  is —(CH 2 ) 3-15 — and R 3  is—C(O)NH—. In some embodiments, R 2  is —(CH 2 ) 3-5 — and R 3  is —C(O)NH—. In some embodiments, R 2  is —(CH 2 ) 1-8 — and R 3  is —C(O)NH—. In some embodiments, R 2  is —(CH 2 ) 4 — and R 3  is —C(O)NH—. 
     In some embodiments, R 4  is —(CH 2 ) 3 CH(R 7 )(CH 2 ) 3 —. In some embodiments, R 4  is —(CH 2 ) 2 CH(R 7 )(CH 2 ) 2 —. In some embodiments, R 4  is —CH 2 CH(R 7 )CH 2 —. In some embodiments, R 4  is —CH 2 CH(R 7 )(CH 2 ) 0-3 —. In some embodiments, R 4  is —(CH 2 ) 0-3 CH(R 7 )CH 2 —. In some embodiments, R 4  is —CH 2 CH(R 7 )—. In some embodiments, R 4  is —CH(R 7 )CH 2 —. In some embodiments, R 4  is —CH(R 7 )—. 
     In some embodiments, R 7  forms the side chain of an L-amino acid residue. In other embodiments, R 7  forms the side chain of a D-amino acid residue. 
     In some embodiments, R 7  is —(CH 2 ) 5 CH 3 . In some embodiments, R 7  is 
     
       
         
         
             
             
         
       
     
     In some embodiments, R 7  is 
     
       
         
         
             
             
         
       
     
     In certain embodiments, R 7  is 
     
       
         
         
             
             
         
       
     
     In some embodiments, R 7  is 
     
       
         
         
             
             
         
       
     
     In certain embodiments, R 7  is 
     
       
         
         
             
             
         
       
     
     In some embodiments, R 7  is 
     
       
         
         
             
             
         
       
     
     In certain embodiments, R 7  is 
     
       
         
         
             
             
         
       
     
     In some embodiments, R 4  is —CH(R 7 )— and R 7  is 
     
       
         
         
             
             
         
       
     
     In some embodiments, R 5  is hydrogen. In some embodiments, R 5  is methyl. In some embodiments, R 6  is hydrogen. In some embodiments, R 6  is methyl. In some embodiments, R 5  and R 6  are both hydrogen. In some embodiments, R 5  and R 6  are both methyl. In some embodiments, R 5  is methyl and R 6  is hydrogen. In some embodiments, R 5  is hydrogen and R 6  is methyl. 
     In some embodiments, the compound has Formula II or is a salt or solvate of Formula II: 
     
       
         
         
             
             
         
       
     
     wherein R X  and Xaa 1  are as defined in Formula I. 
     In some embodiments, (Xaa 1 ) 0-4  is (Xaa 1 ) 2-4 . In some embodiments, (Xaa 1 ) 0-4  is Xaa 1 . In some embodiments, (Xaa 1 ) 0-4  is (Xaa 1 ) 2 . In some embodiments, (Xaa 1 ) 0-4  is (Xaa 1 ) 3 . In some embodiments, (Xaa 1 ) 0-4  is (Xaa 1 ) 4 . In some embodiments, (Xaa 1 ) 0-4  is absent. 
     In some embodiments, (Xaa 1 ) 0-4  consists of a single amino acid residue. In some embodiments, (Xaa 1 ) 0-4  is a dipeptide, wherein each Xaa 1  may be the same or different. In some embodiments, (Xaa 1 ) 0-4  is a tripeptide, wherein each Xaa 1  may be the same, different or a combination thereof. In some embodiments, (Xaa 1 ) 0-4  consists of 4 amino acid residues connected by peptide bonds, wherein each Xaa 1  may be the same, different or a combination thereof. In some embodiments, each Xaa 1  is independently selected from proteinogenic amino acids and the non-proteinogenic amino acids listed in Table 1, wherein each peptide backbone amino group is optionally methylated. R X  may form an amide bond with Xaa 1 . The linkages between Xaa 1  groups and between R X  and Xaa 1  may be amide bonds. 
     In some embodiments, each R 9  is independently: a linear or branched C 1 -C 18  alkylenyl in which 0-5 carbons are replaced with N, S and/or O heteroatoms; or —CH (CH 2 R 19 )— wherein each R 19  is independently a C 5 -C 16  cyclic or multicylic system in which 0-5 carbons are replaced with N and optionally substituted with 0-5 hydroxy groups, and which is non-aromatic, partially aromatic or fully aromatic. Each R 8  is independently hydrogen or methyl. In some embodiments, each R 8  is hydrogen. In some embodiments, each R 8  is methyl. In some embodiments, the R 8  groups are a combination of hydrogen and methyl. 
     In some embodiments, R X  comprises a radiometal chelator optionally bound by a radiometal. The radiometal chelator may be any radiometal chelator suitable for binding to the radiometal and which is functionalized for attachment to an amino group. Many suitable radiometal chelators are known, e.g. as summarized in Price and Orvig,  Chem. Soc. Rev.,  2014, 43, 260-290, which is incorporated by reference in its entirety. Non-limiting examples of radioisotope chelators include chelators selected from the group consisting of: DOTA and derivatives; DOTAGA; NOTA; NODAGA; NODASA; CB-DO2A; 3p-C-DEPA; TCMC; DO3A; DTPA and DTPA analogues optionally selected from CHX-A″-DTPA and 1B4M-DTPA; TETA; NOPO; Me-3,2-HOPO; CB-TE1A1P; CB-TE2P; MM-TE2A; DM-TE2A; sarcophagine and sarcophagine derivatives optionally selected from SarAr, SarAr-NCS, diamSar, AmBaSar, and BaBaSar; TRAP; AAZTA; DATA and DATA derivatives; H2-macropa or a derivative thereof; H 2 dedpa, Haoctapa, H 4 py4pa, H 4 Pypa, H 2 azapa, H 5 decapa, and other picolinic acid derivatives; CP256; PCTA; C-NETA; C-NE3TA; HBED; SHBED; BCPA; CP256; YM103; desferrioxamine (DFO) and DFO derivatives; and H 6 phospa. Exemplary non-limiting examples of radioisotope chelators and example radioisotopes chelated by these chelators are shown in Table 2. In alternative embodiments, R X  comprises a radioisotope chelator selected from those listed above or in Table 2, or is any other radioisotope chelator. One skilled in the art could replace any of the chelators listed herein with another chelator. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Exemplary chelators and exemplary isotopes which bind said chelators. 
               
            
           
           
               
               
            
               
                 Chelator 
                 Isotopes 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 Cu-64/67 Ga-67/68 In-111 Lu-177 Y-86/90 Bi-203/212/213 Pb-212 Ac-225 Gd-159 Yb-175 Ho-166 As-211 
               
               
                   
                 Sc-44/47 
               
               
                   
                 Pm-149 
               
               
                   
                 Pr-142 
               
               
                   
                 Sn-117m 
               
               
                   
                 Sm-153 
               
               
                   
                 Tb-149/161 
               
               
                   
                 Er-165 
               
               
                   
                 Ra-223/224 
               
               
                   
                 Th-227 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 Cu-64/67 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 Pb-212 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 Bi-212/213 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 Cu-64/67 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 Cu-64/67 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 Cu-64/67 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 Cu-64/67 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 Cu-64/67 Ga-68 In-111 Sc-44/47 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 Cu-64/67 Ga-68 Lu-177 Y-86/90 Bi-213 Pb-212 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 Au-198/199 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 Rh-105 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 In-111 Sc-44/47 Lu-177 Y-86/90 Sn-117m Pd-109 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 In-111 Lu-177 Y-86/90 Bi-212/213 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 Cu-64/67 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 Cu-64/67 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 In-111 Lu-177 Y-86/90 Ac-225 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 Ac-225 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 In-111 Ac-225 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 In-111 Lu-177 Ac-225 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 In-111 Lu-177 Ac-225 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 In-111 Ga-68 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 In-111 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
                 Cu-64/67 
               
               
                   
               
               
                 H2-MACROPA (N,N′-bis[(6-carboxy-2-pyridil)methyl]- 
                 Ac-225 
               
               
                 4,13-diaza-18-crown-6) 
               
               
                   
               
            
           
         
       
     
     In some embodiments, R X  is polyaminocarboxylate chelator attached through an amide bond. In some embodiments, R X  is: DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) or a derivative thereof; TETA (1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid) or a derivative thereof; SarAr (1-N-(4-Aminobenzyl)-3,6,10,13,16,19-hexaazabicyclo[6.6.6]-eicosane-1,8-diamine or a derivative thereof; NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid) or a derivative thereof; TRAP (1,4,7-triazacyclononane-1,4,7-tris[methyl(2-carboxyethyl)phosphinic acid) or a derivative thereof; HBED (N,N0-bis(2-hydroxybenzyl)-ethylenediamine-N,N0-diacetic acid) or a derivative thereof; 2,3-HOPO (3-hydroxypyridin-2-one) or a derivative thereof; PCTA (3,6,9,15-tetraazabicyclo[9.3.1]-pentadeca-1(15),11 ,13-triene-3,6,9,-triacetic acid) or a derivative thereof; DFO (desferrioxamine) or a derivative thereof; DTPA (diethylenetriaminepentaacetic acid) or a derivative thereof; OCTAPA (N,N0-bis(6-carboxy-2-pyridylmethyl)-ethylenediamine-N,N0-diacetic acid) or a derivative thereof; or H2-MACROPA (N,N′-bis[(6-carboxy-2-pyridil)methyl]-4,13-diaza-18-crown-6) or a derivative thereof. In some embodiments, R X  is DOTA. In some embodiments, R X  is a chelator moiety in complex with radioisotope X wherein X is  64 Cu,  67 Cu,  90 Y,  111 In,  114m In,  117m Sn,  153 Sm,  149 Tb,  161 Tb,  177 Lu,  225 Ac,  213 Bi,  224 Ra,  212   Bi,    212 Pb,  227 Th,  223 Ra,  47 Sc,  186 Re or  188 Re. In some embodiments, X is  177 Lu. In some embodiments, R X  is a chelator moiety in complex with radioisotope X wherein X is  64 Cu,  68 Ga,  86 Y,  111 In,  94m Tc,  44 Sc,  89 Zr, or  99m Tc. In some embodiments, X is  68 Ga. 
     In some embodiments, the radioisotope chelator is conjugated with a radioisotope. The conjugated radioisotope may be, without limitation,  68 Ga,  61 Cu,  64 Cu,  67 Ga,  99m Tc, 111In,  44 Sc,  86 Y,  89 Zr,  99 Nb,  177 Lu,  117m Sn,  165 Er,  99 Y,  227 Th,  225 Ac,  213 Bi,  212 Bi,  211 As,  203 Pb,  212 Pb,  47 Sc,  166 Ho,  188 Re,  186 Re,  149 Pm,  159 Gd,  105 Rh,  109 Pd,  198 Au,  199 Au,  175 Yb,  142 Pr,  114m In,  111 In, and the like. In some embodiments, the chelator is a chelator from Table 2 and the conjugated radioisotope is a radioisotope indicated in Table 2 as a binder of the chelator. 
     In some embodiments, the radioisotope chelator is not conjugated to a radioisotope. 
     In some embodiments, the chelator is: DOTA or a derivative thereof, conjugated with  177 Lu,  111 In  213 Bi,  68 Ga,  67 Ga,  203 Pb,  212 Pb,  44 Sc,  47 Sc,  90 Y,  86 Y,  225 Ac,  117m Sn,  153 Sm,  149 Tb,  161 Tb,  165 Er,  213 Bi,  224 Ra,  212 Bi,  212 Pb,  227 Th,  223 Ra,  64 Cu or  67 Cu, H2-MACROPA conjugated with  225 Ac; Me-3,2-HOPO conjugated with  227 Th; H 4 py4pa conjugated with  225 Ac,  227 Th or  177 Lu; H 4 pypa conjugated with  177 Lu; NODAGA conjugated with  68 Ga; DTPA conjugated with  111 In; or DFO conjugated with  89 Zr. 
     In some embodiments, the chelator is TETA (1,4,8,11-tetraazacyclotetradecane-1,4,8,11-tetraacetic acid), SarAr (1-N-(4-Aminobenzyl)-3,6,10,13,16,19-hexaazabicyclo[6.6.6]-eicosane-1,8-diamine), NOTA (1,4,7-triazacyclononane-1,4,7-triacetic acid), TRAP (1,4,7-triazacyclononane-1,4,7-tris[methyl(2-carboxyethyl)phosphinic acid), HBED (N,N0-bis(2-hydroxybenzyl)-ethylenediamine-N,N0-diacetic acid), 2,3-HOPO (3-hydroxypyridin-2-one), PCTA (3,6,9,15-tetraazabicyclo[9.3.1]-pentadeca-1 (15), 11,13-triene-3,6,9,-triacetic acid), DFO (desferrioxamine), DTPA (diethylenetriaminepentaacetic acid), OCTAPA (N,N0-bis(6-carboxy-2-pyridylmethyl)-ethylenediamine-N,N0-diacetic acid) or another picolinic acid derivative. 
     In some embodiments, R X  is a chelator for radiolabelling with  99m Tc,  94m Tc,  186 Re, or  188 Re, such as mercaptoacetyl, hydrazinonicotinamide, dimercaptosuccinic acid, 1,2-ethylenediylbis-L-cysteine diethyl ester, methylenediphosphonate, hexamethylpropyleneamineoxime and hexakis(methoxy isobutyl isonitrile, and the like. In some embodiments, R X  is a chelator, wherein the chelator is mercaptoacetyl, hydrazinonicotinamide, dimercaptosuccinic acid, 1,2-ethylenediylbis-L-cysteine diethyl ester, methylenediphosphonate, hexamethylpropyleneamineoxime or hexakis(methoxy isobutyl isonitrile). In some of these embodiments, the chelator is bound by a radioisotope. In some such embodiments, the radioisotope is  99m Tc,  94m Tc,  186 Re, or  188 Re. 
     In some embodiments, R X  is a chelator that can bind  18 F-aluminum fluoride ([ 18 F]AIF), such as 1,4,7-triazacyclononane-1,4-diacetate (NODA) and the like. In some embodiments, the chelator is NODA. In some embodiments, the chelator is bound by [ 18 F]AIF. 
     In some embodiments, R X  is a chelator that can bind  72 As or  77 As, such as a trithiol chelate and the like. In some embodiments, the chelator is a trithiol chelate. In some embodiments, the chelator is conjugated to  72 As. In some embodiments, the chelator is conjugated to  77 As. 
     In some embodiments, R X  comprises an aryl substituted with a radioisotope. In some embodiments, R X  is 
     
       
         
         
             
             
         
       
     
     wherein A, B, C, D and E are independently C or N, and R 15  is a radiohalogen. In some embodiments, R X  is 
     
       
         
         
             
             
         
       
     
     In some embodiments, R X  is 
     
       
         
         
             
             
         
       
     
     In some embodiments, R X  is 
     
       
         
         
             
             
         
       
     
     In some embodiments, R X  is 
     
       
         
         
             
             
         
       
     
     In some embodiments, R X  is 
     
       
         
         
             
             
         
       
     
     In some embodiments, R X  is 
     
       
         
         
             
             
         
       
     
     In some embodiments, R X  is 
     
       
         
         
             
             
         
       
     
     In some embodiments, R X  is 
     
       
         
         
             
             
         
       
     
     In some of these embodiments, R 15  is independently  211 At,  131 I,  124 I,  123 I,  77   13 r or  18 F. In some of these embodiments, R 15  is  18 F. In some embodiments, the radioisotope (e.g. R15) is  131 I. 
     In some embodiments, R X  is a prosthetic group containing a trifluoroborate (BF 3 ), capable of  18 F/ 19 F exchange radiolabeling. The prosthetic group may be R 16 R 17 BF 3 , wherein R 16  is 
     independently 
     
       
         
         
             
             
         
       
     
     and R 18  is absent, 
     
       
         
         
             
             
         
       
     
     The group —R 17 BF 3  may independently be selected from one ora combination of those listed in Table 3 (below), Table 4 (below), or 
     
       
         
         
             
             
         
       
     
     wherein R 19  and R 20  are independently C 1 -C 5  linear or branched alkyl groups. For Tables 3 and 4, the R in the pyridine substituted with —OR, —SR, —NR—, —NHR or —NR 2  groups is C 1 -C 5  branched or linear alkyl. In some embodiments,—R 17 BF 3  is selected from those listed in Table 3. In some embodiments, —R 17 BF 3  is independently selected from one or a combination of those listed in Table 4. In some embodiments, one fluorine is  18 F. In some embodiments, all three fluorines are  19 F. 
     
       
         
           
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Exemplary R 17 BF 3  groups. 
               
               
                   
               
             
            
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 4 
               
               
                   
               
               
                 Exemplary R 17 BF 3  groups. 
               
               
                   
               
             
            
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 
                   
                     
                     
                         
                         
                     
                   
                 
               
               
                   
               
            
           
         
       
     
     In some embodiments, R 17 BF 3  may form 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     in which the R (when present) in the pyridine substituted —OR, —SR, —NR—, —NHR or —NR 2  is a branched or linear C 1 -C 5  alkyl. In some embodiments, R is a branched or linear C 1 -C 5  saturated alkyl. In some embodiments, R is methyl. In some embodiments, R is ethyl. In some embodiments, R is propyl. In some embodiments, R is isopropyl, In some embodiments, R is n-butyl. In some embodiments, one fluorine is  18 F. In some embodiments, all three fluorines are  19 F. 
     In some embodiments, R 17 BF 3  may form 
     
       
         
         
             
             
         
       
       
         
         
             
             
         
       
       
         
         
             
             
         
       
     
     in which the R (when present) in the pyridine substituted —OR, —SR, —NR— or —NR 2  is branched or linear C 1 -C 5  alkyl. In some embodiments, R is a branched or linear C 1 -C 5  saturated alkyl. In some embodiments, R is methyl. In some embodiments, R is ethyl. In some embodiments, R is propyl. In some embodiments, R is isopropyl, In some embodiments, R is n-butyl. In some embodiments, —R 17 BF 3  is 
     
       
         
         
             
             
         
       
     
     In some embodiments, one fluorine is  18 F. In some embodiments, all three fluorines are  19 F. 
     In some embodiments, —R 17 BF 3  is 
     
       
         
         
             
             
         
       
     
     In some embodiments, R 19  is methyl. In some embodiments, R 19  is ethyl. In some embodiments, R 19  is propyl. In some embodiments, R 19  is isopropyl. In some embodiments, R 19  is butyl. In some embodiments, R 19  is n-butyl. In some embodiments, R 19  is pentyl. In some embodiments, R 20  is methyl. In some embodiments, R 20  is ethyl. In some embodiments, R 20  is propyl. In some embodiments, R 20  is isopropyl. In some embodiments, R 20  is butyl. In some embodiments, R 20  is n-butyl. In some embodiments, R 20  is pentyl. In some embodiments, R 19  and R 20  are both methyl. In some embodiments, one fluorine is  18 F. In some embodiments, all three fluorines are  19 F. 
     In some embodiments, R X  is a prosthetic group containing a silicon-fluorine-acceptor moiety. In some embodiments, the fluorine of the silicon-fluorine acceptor moiety is  18 F. The prosthetic groups containing a silicon-fluorine-acceptor moiety may be independently selected from one or a combination of the following: 
     
       
         
         
             
             
         
       
     
     wherein R 21  and R 22  are independently a linear or branched, cyclic or acyclic, and/or aromatic or non-aromatic C 1 -C 10  alkyl, alkenyl or alkynyl group. In some embodiments, R 21  and R 22  are independently selected from the group consisting of phenyl, tert-butyl, sec-propyl or methyl. In some embodiments, the prosthetic group is 
     
       
         
         
             
             
         
       
     
     In some embodiments, the prosthetic group is 
     
       
         
         
             
             
         
       
     
     In some embodiments, the prosthetic group is 
     
       
         
         
             
             
         
       
     
     In some embodiments, the prosthetic group is 
     
       
         
         
             
             
         
       
     
     In certain embodiments, the compound is conjugated with a radioisotope for positron emission tomography (PET) or single photon emission computed tomography (SPECT) imaging of PSMA expressing tumors, wherein the compound is conjugated with a radioisotope that is a positron emitter or a gamma emitter. Without limitation, the positron or gamma emitting radioisotope may be  68 Ge,  67 Ga,  61 Cu,  64 Cu,  67 Ga,  99m Tc,  110m In,  111 In,  44 Sc,  86 Y,  89 Zr,  90 Nb,  18 F,  131 I,  123 I,  124 I or  72 As. 
     When the radioisotope (e.g. X) is a diagnostic radioisotope, there is disclosed use of certain embodiments of the compound for preparation of a radiolabelled tracer for imaging PSMA-expressing tissues in a subject. There is also disclosed a method of imaging PSMA-expressing tissues in a subject, in which the method comprises: administering to the subject a composition comprising certain embodiments of the compound and a pharmaceutically acceptable excipient; and imaging tissue of the subject, e.g. using positron emission tomography (PET). When the tissue is a diseased tissue (e.g. a PSMA-expressing cancer), PSMA-targeted treatment may then be selected for treating the subject. There is therefore disclosed the use of certain compounds of the invention in imaging a PSMA-expressing cancer in a subject, wherein R X  comprises or is complexed with a diagnostic or imaging radioisotope. In some embodiments, the subject is human. 
     In certain embodiments the compound is conjugated with a radioisotope that is used for therapy of PSMA-expressing cancer. This includes radioisotopes such as  165 Er,  212 Bi,  211 At,  166 Ho,  149 Pm,  159 Gd,  105 Rh,  109 Pd,  198 Au,  199 Au,  175 Yb,  142 Pr, 177Lu (β-emitter, t 2/1 =6.65 d),  111 ln,  213 Bi,  203 Pb,  212 Pb,  47   Cc,    90 Y (β-emitter, t 2/1 =2.66 d),  225 Ac,  117m Sn,  153 Sm,  149 Tb,  161 Tb,  165 Er,  213 Bi,  224 Ra,  212 Bi,  212 Pl,  225 Ac (α-emitter, t 2/1 =9.95 d),  227 Th,  223 Ra,  77 As,  131 I,  64 Cu or  67 Cu. 
     When the radioisotope (e.g. X) is a therapeutic radioisotope, there is disclosed use of certain embodiments of the compound (or a pharmaceutical composition thereof) for the treatment of PSMA-expressing diseases (e.g. cancer) in a subject. Accordingly, there is provided use of the compound in preparation of a medicament for treating PSMA-expressing disease in a subject. There is also provided a method of treating PSMA-expressing disease in a subject, in which the method comprises: administering to the subject a composition comprising the compound and a pharmaceutically acceptable excipient. For example, but without limitation, the disease may be a PSMA-expressing cancer. There is therefore disclosed the use of certain compounds of the invention for treating a PSMA-expressing cancer in a subject, wherein R X  comprises or is complexed with a therapeutic radioisotope. In some embodiments, the subject is human. 
     PSMA expression has been detected in various cancers (e.g. Rowe et al., 2015,  Annals of Nuclear Medicine  29:877-882; Sathekge et al., 2015,  Eur J Nucl Med Mol Imaging  42:1482-1483; Verburg et al., 2015,  Eur J Nucl Med Mol Imaging  42:1622-1623; and Pyka et al., J Nucl Med November 19, 2015 jnumed.115.164442). Accordingly, without limitation, the PSMA-expressing cancer may be prostate cancer, renal cancer, breast cancer, thyroid cancer, gastric cancer, colorectal cancer, bladder cancer, pancreatic cancer, lung cancer, liver cancer, brain tumor, melanoma, neuroendocrine tumor, ovarian cancer or sarcoma. In some embodiments, the cancer is prostate cancer. 
     The compound may be HTK03026, HTK03027, HTK03029, HTK03041, (each shown in  FIG. 1 ) or a salt or solvate thereof, optionally conjugated with a radiometal. In some embodiments, the DOTA chelator in these compounds is replaced by an alternative polyaminocarboxylate chelator. In some embodiments, the alternative chelator is selected from derivatives of DOTA, TETA, SarAr, NOTA, TRAP, HBED, 2,3-HOPO, PCTA, DFO, DTPA, OCTAPA or H2-MACROPA. In some embodiments, a radiometal is not conjugated in the chelator. In some embodiments, a radiometal is conjugated to the chelator. In some embodiments, the radiometal is  64 Cu,  68 Ga,  86 Y,  111 In,  94m Tc,  44 Sc,  89 Zr,  99m Tc. In some embodiments, the radiometal is  64 Cu,  67 Cu,  90 Y,  111 In,  114m In,  117m Sn,  153 Sm,  149 Tb,  161 Tb,  177 Lu,  225 Ac,  213 Bi,  224 Ra,  212 Bi,  212 Pb,  225 Ac,  227 Th, 223Ra,  47 Sc,  186 Re, or  188 Re. In some embodiments, the radiometal is  177 Lu. In some embodiments, the compound is  177 Lu-HTK03041 or a salt or solvate thereof. In some embodiments, the chelator in these compounds is replaced with a group that incorporates a radio-halogen (e.g.  131 I). Such halogens can be incorporated via an electrophilic substitution reaction rather than complexation via the chelator portion of the compound. Compounds comprising  131 I,  90 Y,  177 Lu and  225 Ac (e.g.  177 Lu-HTK03041) may be useful for radiotherapy of cancer (e.g. prostate cancer and other diseases/disorders characterized by PSMA expression). The compounds presented herein incorporate peptides, which may be synthesized by any of a variety of methods established in the art. This includes but is not limited to liquid-phase as well as solid-phase peptide synthesis using methods employing 9-fluorenylmethoxycarbonyl (Fmoc) and/or t-butyloxycarbonyl (Boc) chemistries, and/or other synthetic approaches. 
     Solid-phase peptide synthesis methods and technology are well-established in the art. For example, peptides may be synthesized by sequential incorporation of the amino acid residues of interest one at a time. In such methods, peptide synthesis is typically initiated by attaching the C-terminal amino acid of the peptide of interest to a suitable resin. Prior to this, reactive side chain and alpha amino groups of the amino acids are protected from reaction by suitable protecting groups, allowing only the alpha carboxyl group to react with a functional group such as an amine group, a hydroxyl group, or an alkyl halide group on the solid support. Following coupling of the C-terminal amino acid to the support, the protecting group on the side chain and/or the alpha amino group of the amino acid is selectively removed, allowing the coupling of the next amino acid of interest. This process is repeated until the desired peptide is fully synthesized, at which point the peptide can be cleaved from the support and purified. A non-limiting example of an instrument for solid-phase peptide synthesis is the Aapptec Endeavor 90 peptide synthesizer. 
     To allow coupling of additional amino acids, Fmoc protecting groups may be removed from the amino acid on the solid support, e.g. under mild basic conditions, such as piperidine (20-50% v/v) in DMF. The amino acid to be added must also have been activated for coupling (e.g. at the alpha carboxylate). Non-limiting examples of activating reagents include without limitation 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HBTU), 2-(1H-benzotriazol-1-yl)-1,1,3,3-tetramethyluronium tetrafluoroborate (TBTU), 2-(7-Aza-1H-benzotriazole-1-yl)-1,1,3,3-tetramethyluronium hexafluorophosphate (HATU), benzotriazole-1-yl-oxy-tris(dimethylamino)phosphoniumhexafluorophosphate (BOP), benzotriazole-1-yl-oxy-tris(pyrrolidino)phosphoniumhexafluorophosphate (PyBOP). Racemization is minimized by using triazoles, such as 1-hydroxy-benzotriazole (HOBt) and 1-hydroxy-7-aza-benzotriazole (HOAt). Coupling may be performed in the presence of a suitable base, such as N,N-diisopropylethylamine (DIPEA/DIEA) and the like. For long peptides or if desired, peptide synthesis and ligation may be used. 
     Apart from forming typical peptide bonds to elongate a peptide, peptides may be elongated in a branched fashion by attaching to side chain functional groups (e.g. carboxylic acid groups or amino groups), either: side chain to side chain; or side chain to backbone amino or carboxylate. Coupling to amino acid side chains may be performed by any known method, and may be performed on-resin or off-resin. Non-limiting examples include: forming an amide between an amino acid side chain containing a carboxyl group (e.g. Asp, D-Asp, Glu, D-Glu, Aad, and the like) and an amino acid side chain containing an amino group (e.g. Lys, D-Lys, Orn, D-Orn, Dab, D-Dab, Dap, D-Dap, and the like) or the peptide N-terminus; forming an amide between an amino acid side chain containing an amino group (e.g. Lys, D-Lys, Orn, D-Orn, Dab, D-Dab, Dap, D-Dap, and the like) and either an amino acid side chain containing a carboxyl group (e.g. Asp, D-Asp, Glu, D-Glu, and the like) or the peptide C-terminus; and forming a 1, 2, 3-triazole via click chemistry between an amino acid side chain containing an azide group (e.g. Lys(N 3 ), D-Lys(N 3 ), and the like) and an alkyne group (e.g. Pra, D-Pra, and the like). The protecting groups on the appropriate functional groups must be selectively removed before amide bond formation, whereas the reaction between an alkyne and an azido groups via the click reaction to form an 1,2,3-triazole does not require selective deprotection. Non-limiting examples of selectively removable protecting groups include 2-phenylisopropyl esters (O-2-PhiPr) (e.g. on Asp/Glu) as well as 4-methyltrityl (Mtt), allyloxycarbonyl (alloc), 1-(4,4-dimethyl-2,6-dioxocyclohex-1-ylidene))ethyl (Dde), and 1-(4,4-dimethyl-2,6-dioxocyclohex-1-ylidene)-3-methylbutyl (ivDde) (e.g. on Lys/Orn/Dab/Dap). O-2-PhiPr and Mtt protecting groups can be selectively deprotected under mild acidic conditions, such as 2.5% trifluoroacetic acid (TFA) in DCM. Alloc protecting groups can be selectively deprotected using tetrakis(triphenylphosphine)palladium(0) and phenyl silane in DCM. Dde and ivDde protecting groups can be selectively deprotected using 2-5% of hydrazine in DMF. Deprotected side chains of Asp/Glu (L- or D-forms) and Lys/Orn/Dab/Dap (L- or D-forms) can then be coupled, e.g. by using the coupling reaction conditions described above. 
     Peptide backbone amides may be N-methylated (i.e. alpha amino methylated). This may be achieved by directly using Fmoc-N-methylated amino acids during peptide synthesis. Alternatively, N-methylation under Mitsunobu conditions may be performed. First, a free primary amine group is protected using a solution of 4-nitrobenzenesulfonyl chloride (Ns-Cl) and 2,4,6-trimethylpyridine (collidine) in NMP. N-methylation may then be achieved in the presence of triphenylphosphine, diisopropyl azodicarboxylate (DIAD) and methanol. Subsequently, N-deprotection may be performed using mercaptoethanol and 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) in NMP. For coupling protected amino acids to N-methylated alpha amino groups, HATU, HOAt and DIEA may be used. 
     The PSMA-binding moiety (e.g. Lys-ureido-Glu, and the like) may be constructed on solid phase via the formation of a ureido linkage between the amino groups of two amino acids. This can be done by attaching an Fmoc-protecting amino acid (for example Fmoc-Lys(ivDde)-OH) to Wang resin using standard activation/coupling strategy (for example, Fmoc-protected amino acid (4 eq.), HATU (4 eq.) and N,N-diisopropylethylamine (7 eq.) in N,N-dimethylformamide). The Fmoc-protecting group is then removed by 20% piperidine in N,N-dimethylformamide. To form the ureido linkage, the freed amino group of the solid-phase-attached amino acid is reacted with the 2 nd  amino acid which has its carboxylate group protected with a t-butyl group and its amino group activated and converted to an isocyanate group (—N═C═O). The activation and conversion of an amino group to an isocyanate group can be achieved by reacting the amino group with phosgene or triphosgene. After the formation of the ureido linkage, the side chain functional group of the amino acid (for example ivDde on Lys) can be removed, and then the linker, albumin-binding motif, and/or radiolabeling group (e.g. radiometal chelator and the like) can be subsequently coupled to the PSMA-binding moiety. 
     The formation of the thioether (—S—) and ether (—O—) linkages (e.g. for R 3 ) can be achieved either on solid phase or in solution phase. For example, the formation of thioether (—S—) linkage can be achieved by coupling between a thiol-containing compound (such as the thiol group on cysteine side chain) and an alkyl halide (such as 3-(Fmoc-amino)propyl bromide and the like) in an appropriate solvent (such as N,N-dimethylformamide and the like) in the presence of base (such as N,N-diisopropylethylamine and the like). The formation of an ether (—O—) linkage can be achieved via the Mitsunobu reaction between an alcohol (such as the hydroxyl group on the side chain of serine or threonine, for example) and a phenol group (such as the side chain of tyrosine, for example) in the presence of triphenylphosphine and diisopropyl azidicarboxylate (DIAD) in an aprotic solvent (such as 1,4-dioxane and the like). If the reactions are carried out in solution phase, the reactants used are preferably in equivalent molar ratio (1 to 1), and the desired products can be purified by flash column chromatography or high performance liquid chromatography (HPLC). If the reactions are carried out on solid phase, meaning one reactant has been attached to a solid phase, then the other reactant is normally used in excess amount (≥3 equivalents of the reactant attached to the solid phase). After the reactions, the excess unreacted reactant and reagents can be removed by sequentially washing the solid phase (resin) using a combination of solvents, such as N,N-dimethylformamide, methanol and dichloromethane, for example. 
     Non-peptide moieties (e.g. radiolabeling groups, albumin-binding groups and/or linkers) may be coupled to the peptide N-terminus while the peptide is attached to the solid support. This is facile when the non-peptide moiety comprises an activated carboxylate (and protected groups if necessary) so that coupling can be performed on resin. For example, but without limitation, a bifunctional chelator, such as 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) tris(tert-butyl ester) may be activated in the presence of N-hydroxysuccinimide (NHS) and N,N′-dicyclohexylcarbodiimide (DCC) for coupling to a peptide. Alternatively, a non-peptide moiety may be incorporated into the compound via a copper-catalyzed click reaction under either liquid or solid phase conditions. Copper-catalyzed click reactions are well established in the art. For example, 2-azidoacetic acid is first activated by NHS and DCC and coupled to a peptide. Then, an alkyne-containing non-peptide moeity may be clicked to the azide-containing peptide in the presence of Cu 2+  and sodium ascorbate in water and organic solvent, such as acetonitrile (ACN) and DMF and the like. 
     The synthesis of radiometal chelators is well-known and many chelators are commercially available (e.g. from Sigma-Aldrich™/Milipore Sigma™ and others). Protocols for conjugation of radiometals to the chelators are also well known (e.g. see Example 1, below). The synthesis of the silicon-fluorine-acceptor moieties can be achieved following previously reported procedures (e.g. Bernard-Gauthier et al.  Biomed Res Int.  2014 2014:454503; Kostikov et al.  Nature Protocols  2012 7:1956-1963; Kostikov et al.  Bioconjug Chem.  2012 18:23:106-114; each of which is incorporated by reference in its entirety). The synthesis or acquisition of radioisotope-substituted aryl groups is likewise facile. 
     The synthesis of the R 16 R 17 BF 3  component on the PSMA-targeting compounds can be achieved following previously reported procedures (e.g.: Liu et al.  Angew Chem Int Ed  2014 53:11876-11880; Liu et al.  J Nucl Med  2015 55:1499-1505; Liu et al.  Nat Protoc  2015 10:1423-1432; Kuo et al.,  J Nucl Med  2019 60:1160-1166; each of which is incorporated by reference in its entirety). Generally, the BF 3 -containing motif can be coupled to the linker via click chemistry by forming a 1,2,3-triazole ring between a BF 3 -containg azido (or alkynyl) group and an alkynyl (or azido) group on the linker, or by forming an amide linkage between a BF 3 -containg carboxylate and an amino group on the linker. To make the BF 3 -containing azide, alkyne or carboxylate, a boronic acid ester-containing azide, alkyne or carboxylate is first prepared following by the conversion of the boronic acid ester to BF 3  in a mixture of HCl, DMF and KH F 2 . For alkyl BF 3 , the boronic acid ester-containing azide, alkyne or carboxylate can be prepared by coupling boronic acid ester-containing alkyl halide (such as iodomethylboronic acid pinacol ester) with an amine-containing azide, alkyne or carboxylate (such as N, N-dimethylpropargylamine). For aryl BF 3 , the boronic acid ester can be prepared via Suzuki coupling using aryl halide (iodine or bromide) and bis(pinacolato)diboron. 
       18 F-Fluorination of the BF 3 -containing PSMA-targeting compounds via  18 F- 19 F isotope exchange reaction can be achieved following previously published procedures (Liu et al.  Nat Protoc  2015 10:1423-1432, incorporated by reference in its entirety). Generally, ˜100 nmol of the BF 3 -containing compound is dissolved in a mixture of 15 μl of pyridazine-HCl buffer (pH=2.0-2.5, 1 M), 15 μl of DMF and 1 μl of a 7.5 mM KH F 2  aqueous solution.  18 F-Fluoride solution (in saline, 60 μl) is added to the reaction mixture, and the resulting solution is heated at 80° C. for 20 min. At the end of the reaction, the desired product can be purified by solid phase extraction or by reversed high performance liquid chromatography (HPLC) using a mixture of water and acetonitrile as the mobile phase. 
     When the peptide has been fully synthesized on the solid support, the desired peptide may be cleaved from the solid support using suitable reagents, such as TFA, tri-isopropylsilane (TIS) and water. Side chain protecting groups, such as Boc, pentamethyldihydrobenzofuran-5-sulfonyl (Pbf), trityl (Trt) and tert-butyl (tBu) are simultaneously removed (i.e. deprotection). The crude peptide may be precipitated and collected from the solution by adding cold ether followed by centrifugation. Purification and characterization of the peptides may be performed by standard separation techniques, such as high performance liquid chromatography (HPLC) based on the size, charge and polarity of the peptides. The identity of the purified peptides may be confirmed by mass spectrometry or other similar approaches. 
     The present invention will be further illustrated in the following examples for the synthesis and evaluation of radiolabeled HTK03026, HTK03027, HTK03029, and HTK03041. 
     EXAMPLES 
     Materials and Methods 
     General Methods 
     All chemicals and solvents were obtained from commercial sources, and used without further purification. PSMA-targeted peptides were synthesized using solid phase approach on an AAPPTec (Louisville, Ky.) Endeavor 90 peptide synthesizer. Purification and quality control of cold and radiolabeled peptides were performed on Agilent HPLC systems equipped with a model 1200 quaternary pump, a model 1200 UV absorbance detector (set at 220 nm), and a Bioscan (Washington, D.C.) Nal scintillation detector. The operation of Agilent HPLC systems was controlled using the Agilent ChemStation software. The HPLC columns used were a semi-preparative column (Luna C18, 5μ, 250×10 mm) and an analytical column (Luna C18, 5μ, 250×4.6 mm) purchased from Phenomenex (Torrance, Calif.). The HPLC solvents were A: H 2 O containing 0.1% TFA, and B: CH 3 CN containing 0.1% TFA. The collected HPLC eluates containing the desired peptide were lyophilized using a Labconco (Kansas City, Mo.) FreeZone 4.5 Plus freeze-drier. Mass analyses were performed using an AB SCI EX (Framingham, Mass.) 4000 QTRAP mass spectrometer system with an ESI ion source. C18 Sep-Pak cartridges (1 cm 3 , 50 mg) were obtained from Waters (Milford, Mass.).  68 Ga was eluted from an iThemba Labs (Somerset West, South Africa) generator, and was purified according to the previously published procedures using a DGA resin column from Eichrom Technologies LLC (Lisle, Ill.) (e.g. Lin et al.,  Cancer Res.  2015, 75:387-393). Radioactivity of  68 Ga-labeled peptides was measured using a Capintec (Ramsey, N.J.) CRC®-25R/W dose calibrator, and the radioactivity of mouse tissues collected from biodistribution studies were counted using a Perkin Elmer (Waltham, Mass.) Wizard2 2480 automatic gamma counter. 
     Synthesis of HTK03026, HTK03027, HTK03029, and HTK03041 
     Solid-phase synthesis of HTK03026, HTK03027, HTK03029 and HTK03041 (see  FIG. 1  for structures) was modified from literature procedures (e.g., see: Benesova et al.,  J. Med. Chem.  2016, 59:1761-1775). Fmoc-Lys(ivDde)-Wang resin (0.3 mmol, 0.61 mmol/g loading) was suspended in DM F for 30 min. Fmoc was then removed by treating the resin with 20% piperidine in DMF (3×8 min). The isocyanate derivative of di-t-butyl ester of glutamate (3 eq.) was prepared according to literature procedures (e.g. Benesova et al.,  J. Med. Chem.  2016, 59:1761-1775), and added to the lysine-immobilized resin and reacted for 16 h. After washing the resin with DMF, the ivDde-protecting group was removed with 2% hydrazine in DMF (5×5 min). Fmoc-2-Aoc-OH (for HTK03026), Fmoc-Ala(2-Anth)-OH (for HTK03027), Fmoc-Ala(1-Pyrenyl)-OH (for HTK03029) or Fmoc-Ala(9-Anth)-OH (for HTK03041) was then coupled to the side chain of Lys using Fmoc-protected amino acid (3 eq.), HBTU (3 eq.), HOBT (3 eq.) and N,N-diisopropylethylamine (8 eq.). Afterwards, elongation was continued with the addition of Fmoc-tranexamic acid, and finally DOTA-tris(t-bu)ester (2-(4,7,10-tris(2-(t-butoxy)-2-oxoehtyl)-1,4,7,10)-tetraazacyclododecan-1-yl)acetic acid). 
     The peptide was then deprotected and simultaneously cleaved from the resin by treating with 95/5 trifluoroacetic acid (TFA)/triisopropylsilane (TIS) for 2 h at room temperature. After filtration, the peptide was precipitated by the addition of cold diethyl ether to the TFA solution. The crude peptide was purified by HPLC using the semi-preparative column. The eluates containing the desired peptide were collected, pooled, and lyophilized. For HTK03026, the HPLC conditions were 27% acetonitrile in water with 0.1% TFA at a flow rate of 4.5 mL/min. The retention time was 10.7 min. ESI-MS: calculated [M+H] +  for HTK03026 C 45 H 75 N 9 O 16  986.5; found [M+H] +  986.6. For HTK03027, the HPLC conditions were 32% acetonitrile in water with 0.1% TFA at a flow rate of 4.5 mL/min. The retention time was 7.1 min. ESI-MS: calculated [M+H] +  for HTK03027 C 53 H 74 N 9 O 16  1092.5; found [M+H] +  1094.6. For HTK03029, the HPLC conditions were 33% acetonitrile in water with 0.1% TFA at a flow rate of 4.5 mL/min. The retention time was 7.3 min. ESI-MS: calculated [M+H] +  for HTK03029 C 55 H 74 N 9 O 16  1116.5; found [M+H] +  1116.6. For HTK03041, the HPLC conditions were 31% acetonitrile in water with 0.1% TFA at a flow rate of 4.5 mL/min. The retention time was 7.2 min. ESI-MS: calculated [M+H] +  for HTK03041 C 53 H 74 N 9 O 16  1092.5; found [M+H] +  1092.6. 
     Synthesis of Ga-Labeled Standards 
     To prepare Ga-labeled standards, a solution of each precursor was incubated with GaCl 3 (5 eq.) in NaOAc buffer (0.1 M, 500 μL, pH 4.2) at 80° C. for 15 min. The reaction mixture was then purified by HPLC using the semi-preparative column, and the HPLC eluates containing the desired peptide were collected, pooled, and lyophilized. For Ga-HTK03026, the HPLC conditions were 27% acetonitrile in water with 0.1% TFA at a flow rate of 4.5 mL/min. The retention time was 9.4 min. ESI-MS: calculated [M+H] +  for Ga-HTK03026 C 44 H 73 N 9 O 16 Ga 1052.4; found [M+H] +  1052.5. For Ga-HTK03027, the HPLC conditions were 32% acetonitrile in water with 0.1% TFA at a flow rate of 4.5 mL/min. The retention time was 9.5 min. ESI-MS: calculated [M+H] +  for Ga-HTK03027 C 53 H 72 N 9 O 16 Ga 1159.4; found [M+H] +  1161.4. For HTK03029, the HPLC conditions were 33% acetonitrile in water with 0.1% TFA at a flow rate of 4.5 mL/min. The retention time was 10.3 min. ESI-MS: calculated [M+H] +  for Ga-HTK03029 C 55 H 72 N 9 O 16 Ga 1183.4; found [M+H] +  1183.4. For Ga-HTK03041, the HPLC conditions were 31% acetonitrile in water with 0.1% TFA at a flow rate of 4.5 mL/min. The retention time was 9.3 min. ESI-MS: calculated [M+H] +  for Ga-HTK03041 C 53 H 72 N 9 O 16 Ga 1159.4; found [M+H] +  1159.4. 
     Synthesis of Lu-Labeled Standard 
     To prepare Lu-labeled standard HTK03041, a solution of precursor was incubated with LuCl 3  (5 eq.) in NaOAc buffer (0.1 M, 500 λL, pH 4.2) at 80° C. for 15 min. The reaction mixture was then purified by HPLC using the semi-preparative column, and the HPLC eluates containing the desired peptide were collected, pooled, and lyophilized. The HPLC conditions were 29% acetonitrile in water with 0.1% TFA at a flow rate of 4.5 mL/min. The retention time was 8.6 min. ESI-MS: calculated [M+H] +  for Lu-HTK03041 C 53 H 71  N 9 O 16 Lu 1264.4; found [M+H] +  1264.6. 
     Synthesis of  68 Ga-Labeled Compounds 
     The radiolabeling experiments were performed following previously published procedures (e.g. Lin et al.,  Cancer Res.  2015, 75:387-393). Purified  68 Ga in 0.5 mL water was added into a 4-mL glass vial preloaded with 0.7 mL of HEPES buffer (2 M, pH 5.0) and 50 μg of the DOTA-conjugated precursor (HTK03026, HTK03027, HTK03029 or HTK03041). The radiolabeling reaction was carried out under microwave heating for 1 min. The reaction mixture was purified by HPLC using the semi-preparative column. The eluate fraction containing the radiolabeled product was collected, diluted with water (50 mL), and passed through a C18 Sep-Pak cartridge that was pre-washed with ethanol (10 mL) and water (10 mL). After washing the C18 Sep-Pak cartridge with water (10 mL), the  68 Ga-labeled product was eluted off the cartridge with ethanol (0.4 mL), and diluted with saline for imaging and biodistribution. The decay-corrected radiolabeling yields for the  68 Ga-labeled HTK03026, HTK03027, HTK03029 are HTK03041 were &gt;50%, and their radiochemical purities were &gt;95%. 
     Synthesis of  177 Lu-Labeled HTK03041 
       177 LuCl 3  (329.3-769.9 MBq in 10-20 μL) was added to a solution of HTK03041 (25 μg) in NaOAc buffer (0.5 mL, 0.1 M, pH 4.5). The mixture was incubated at 90° C. for 15 min, and then purified by HPLC using the semi-preparative column. The eluate fraction containing the radiolabeled product was collected, diluted with water (50 mL), and passed through a C18 Sep-Pak cartridge that was pre-washed with ethanol (10 mL) and water (10 mL). After washing the C18 Sep-Pak cartridge with water (10 mL), the  177 Lu-labeled HTK03041 was eluted off the cartridge with ethanol (0.4 mL), and diluted with saline for imaging and biodistribution. The decay-corrected radiolabeling yield for  177 Lu-labeled HTK03041 was &gt;50%, and its radiochemical purity was &gt;95%. 
     Cell Culture 
     LNCap cell line was obtained from ATCC (LNCaP clone FGC, CRL-1740). It was established from a metastatic site of left supraclavicular lymph node of human prostatic adenocarcinoma. Cells were cultured in PRMI 1640 medium supplemented with 10% FBS, penicillin (100 U/mL) and streptomycin (100 μg/mL) at 37° C. in a humidified incubator containing 5% CO 2 . Cells grown to 80-90% confluence were then washed with sterile phosphate-buffered saline (1× PBS pH 7.4) and trypsinization. The collected cells number was counted with a Hausser Scientific (Horsham, Pa.) Hemacytometer. 
     In Vitro Competition Binding Assays 
     The LNCaP Cells (400,000/well) were plated onto a 24-well poly-D-lysine coated plate for 48 h. Growth media was removed and replaced with HEPES buffered saline (50 mM HEPES, pH 7.5, 0.9% sodium chloride) and allowed to incubate with the cells for 1 h at 37° C.  18 F-DCFPyL (0.1 nM) was added to each well (in triplicate) containing various concentrations of tested compounds. Non-specific binding was determined in the presence of 10 μM non-radiolabeled DCFPyL. The assay mixtures were further incubated for 1 h at 37° C. with gentle agitation. Then, the buffer and hot ligand were removed, and cells were washed twice with cold HEPES buffered saline. Trypsin solution (0.25%, 400 μl) was then added to each well to harvest the cells. Radioactivity was measured on the gamma counter. Data analyses of K i  were performed using the nonlinear regression algorithm of GraphPad Prism 7 software. 
     PET/CT, SPECT/CT Imaging and Biodistribution 
     Imaging and biodistribution experiments were performed using NODSCID 1L2RγKO male mice. Mice were anesthetized by inhalation with 2% isoflurane in oxygen, and implanted subcutaneously with 1×10 7  LNCaP cells behind left shoulder. Mice were imaged or used in biodistribution studies when the tumor grew up to reach 5-8 mm in diameter during 5-6 weeks. 
     PET imaging experiments were conducted using Siemens Inveon micro PET/CT scanner. Each tumor bearing mouse was injected 6-8 MBq of Ga-68 labeled tracer through the tail vein under anesthesia (2% isoflurane in oxygen). The mice were allowed to recover and roam freely in their cage. After 50 min, the mice were sedated again with 2% isoflurane in oxygen inhalation and positioned in the scanner. A 10-min CT scan was conducted first for localization and attenuation correction after segmentation for reconstructing the PET images. Then, a 10-min static PET imaging was performed to determined uptake in tumor and other organs. The mice were kept warm by a heating pad during acquisition. For imaging studies acquired at 3 h post-injection (p.i.), the mice were placed in the micro PET/CT scanner at 170 min p.i. Then, the CT acquisitions were conducted as described above, a 15-min static PET imaging was performed to determined uptake in tumor and other organs. 
     SPECT/CT imaging experiments were conducted using the MlLabs (Utrecht, the Netherlands) U-SPECT-II/CT scanner. Each tumor bearing mouse was injected with ˜37 MBq of  177 Lu-labeled HTK03041 through the tail vein under anesthesia (2% isoflurane in oxygen). The mice were allowed to recover and roam freely in their cage and imaged at 1, 4, 24, 72 and 120 hours after injection. At each time point, the mice were sedated again and positioned in the scanner. A 5-min CT scan was conducted first for anatomical reference with a voltage setting at 60 kV and current at 615 pA followed by a 60-min static emission scan acquired in list mode using an ultra-high resolution multi-pinhole rat-mouse (1 mm pinhole size) collimator. Data were reconstructed using the U-SPECT II software with a 20% window width on three energy windows. The photopeak window was centered at 208 keV, with lower scatter and upper scatter windows centered at 170 and 255 keV, respectively. Reconstruction parameters used maximum-likelihood expectation maximization (3 iterations), pixel-based ordered subset expectation maximization (16 subsets), and a post-processing filter (Gaussian blurring) of 0.5 mm. Images were decay corrected to injection time in PMOD (PMOD Technologies, Switzerland) then converted to DICOM for qualitative visualization in Inveon Research Workplace (Siemens Medical Solutions USA, Inc.). 
     For biodistribution studies, the mice were injected with the radiotracer as described above. At predetermined time points (1 or 3 h for  68 Ga studies; 1, 4, 24, 72, or 120 h for  177 Lu studies), the mice was anesthetized with 2% isoflurane inhalation, and euthanized by CO 2  inhalation. Blood was withdrawn immediately from the heart, and the organs/tissues of interest were collected. The collected organs/tissues were weighed and counted using a Perkin Elmer (Waltham, Mass.) Wizard2 2480 gamma counter. The uptake in each organ/tissue was normalized to the injected dose using a standard curve, and expressed as the percentage of the injected dose per gram of tissue (% ID/g). 
     Results 
     Binding affinity 
     The binding affinities of Ga-HTK03026, Ga-HTK03027, Ga-HTK03029, and Ga-HTK03041 were measured by in vitro competition binding assays using PSMA-expressing LNCaP cells and  18 F-DCFPyL as the radioligand. As shown in  FIG. 2 , all tested compounds inhibited the binding of  18 F-DCFPyL to LNCaP cells in a dose-dependent manner. The calculated K values for Ga-HTK03026, Ga-HTK03027, Ga-HTK03029, and Ga-HTK03041 were 48.2±16.8 (n=3), 22.0±8.9 (n=2), 16.6±3.6 (n=3) and 0.63±0.06 nM (n=3), respectively. 
     Biodistribution of  68 Ga-Labeled PSMA-Targeting Tracers 
     Detailed uptake values of  68 Ga-labeled HTK03026, HTK03027, HTK03029 and HTK03041 in various tissues/organs of LNCaP tumor-bearing mice are provided in Table 5.  68 Ga-labeled HTK03026, HTK03027 and HTK03029 showed comparable tumor uptake (˜13% ID/g) at 1 h post-injection.  68 Ga-HTK03041 had the highest uptake (23.1±6.11% ID/g) in LNCap tumor xenografts at 1 h post-injection, and the uptake value increased to 28.2±9.17% I D/g at 3 h post-injection. 
     PET/CT Imaging of  68 Ga-Labeled PSMA-Targeting Tracers 
     Representative PET/CT images of  68 Ga-labeled HTK03026, HTK03027, HTK03029 and HTK03041 are provided in  FIG. 3 . Consistent with the biodistribution data provided in Table 5, higher uptake was observed in LNCap tumor xenografts, kidneys and urinary bladders. 
     Biodistribution of  177 Lu-Labeled HTK03041 
     Detailed uptake values of  177 Lu-HTK03041 in various tissues/organs of LNCap tumor-bearing mice are provided in Table 6. The tumor uptake of  177 Lu-HTK03041 peaked at 4 h post-injection (18.7±2.96% ID/g), and remained at the similar level throughout the investigated time points (up to 120 h post-injection). 
     SPECT/CT Imaging of  177 Lu-Labeled HTK03041 
     Representative SPECT images of  177 Lu-HTK03041 at different time points are provided in  FIG. 4 . Consistent with the biodistribution data, the high uptake of  177 Lu-HTK03041 in tumor sustained over the course the study (up to 120 h post-injection). In contrast, the uptake of  177 Lu-HTK03041 in background tissues including kidneys cleared quickly over time. 
     
       
         
           
               
             
               
                 TABLE 5 
               
             
            
               
                   
               
               
                 Biodistribution data and tumor-to-background contrast ratios of  68 Ga-labelled HTK03026, HTK03027, HTK03029,  
               
               
                 and HTK03041 in mice bearing PSMA-expressing LNCAP cancer xenografts. 
               
            
           
           
               
               
               
               
               
            
               
                   
                   68 Ga-HTK03026 
                   68 Ga-HTK03027 
                   68 Ga-HTK03029 
                   68 Ga-HTK03041 
               
            
           
           
               
               
               
               
               
               
            
               
                 Tissue 
                 1 h 
                 1 h 
                 1 h 
                 1 h 
                 3 h 
               
               
                 (% ID/g) 
                 (n = 7) 
                 (n = 5) 
                 (n = 5) 
                 (n = 6) 
                 (n = 6) 
               
               
                   
               
               
                 Blood 
                 0.70 ± 0.19 
                 1.56 ± 0.57 
                 3.93 ± 0.91 
                 1.43 ± 0.30 
                 0.83 ± 0.27 
               
               
                 Urine 
                 575 ± 245 
                 844 ± 645 
                 175 ± 178 
                   173 ± 93.01 
                  171 ± 93.3 
               
               
                 Fat 
                 0.45 ± 0.11 
                 1.03 ± 0.67 
                 1.57 ± 0.42 
                 2.06 ± 0.59 
                 1.06 ± 0.20 
               
               
                 Seminal 
                 0.43 ± 0.31 
                 10.6 ± 10.9 
                 6.74 ± 10.7 
                 1.03 ± 0.27 
                 4.56 ± 9.00 
               
               
                 Testes 
                 0.35 ± 0.22 
                 0.56 ± 0.18 
                 1.02 ± 0.13 
                 1.34 ± 0.22 
                 0.89 ± 0.12 
               
               
                 Intestine 
                 0.29 ± 0.06 
                 0.44 ± 0.12 
                 0.77 ± 0.11 
                 1.14 ± 0.18 
                 0.67 ± 0.09 
               
               
                 Stomach 
                 0.10 ± 0.03 
                 0.17 ± 0.10 
                 0.31 ± 0.08 
                 0.41 ± 0.11 
                 0.26 ± 0.06 
               
               
                 Spleen 
                 1.34 ± 0.54 
                 3.36 ± 2.54 
                 7.08 ± 5.08 
                 8.95 ± 3.22 
                 4.19 ± 1.58 
               
               
                 Liver 
                 0.24 ± 0.06 
                 0.46 ± 0.10 
                 1.32 ± 0.63 
                 1.38 ± 0.25 
                 0.82 ± 0.15 
               
               
                 Pancreas 
                 0.71 ± 1.04 
                 0.57 ± 0.20 
                 0.94 ± 0.18 
                 1.47 ± 0.16 
                 0.71 ± 0.12 
               
               
                 Adrenal glands 
                 1.84 ± 0.95 
                 3.55 ± 2.33 
                 4.91 ± 1.57 
                 8.90 ± 2.56 
                 4.04 ± 1.20 
               
               
                 Kidneys 
                 68.5 ± 28.2 
                 85.6 ± 73.5 
                  198 ± 49.3 
                 170 ± 26.4 
                  121 ± 37.6 
               
               
                 Lungs 
                 0.82 ± 0.26 
                 1.66 ± 0.67 
                 3.59 ± 0.69 
                 4.32 ± 0.62 
                 2.14 ± 0.39 
               
               
                 Heart 
                 0.30 ± 0.06 
                 0.56 ± 0.24 
                 1.25 ± 0.27 
                 1.82 ± 0.21 
                 0.93 ± 0.17 
               
               
                 Tumor 
                 12.5 ± 2.90 
                 13.3 ± 5.44 
                 13.9 ± 6.58 
                 23.1 ± 6.11 
                 28.2 ± 9.17 
               
               
                 Muscle 
                 0.28 ± 0.13 
                 0.48 ± 0.37 
                 0.63 ± 0.10 
                 0.75 ± 0.09 
                 0.40 ± 0.06 
               
               
                 Bone 
                 0.54 ± 0.20 
                 0.70 ± 0.33 
                 0.92 ± 0.14 
                 1.29 ± 0.45 
                 0.98 ± 0.40 
               
               
                 Brain 
                 0.06 ± 0.02 
                 0.06 ± 0.03 
                 0.09 ± 0.02 
                 0.10 ± 0.05 
                 0.06 ± 0.02 
               
               
                 Tail 
                 0.90 ± 0.56 
                 0.98 ± 0.38 
                 2.18 ± 0.65 
                 1.36 ± 0.45 
                 0.86 ± 0.73 
               
               
                 Thyroid 
                 0.35 ± 0.08 
                 0.76 ± 0.33 
                 1.57 ± 0.32 
                 2.48 ± 0.44 
                 1.19 ± 0.23 
               
               
                 Salivary 
                 0.83 ± 0.22 
                 1.86 ± 0.91 
                 3.27 ± 1.02 
                 4.99 ± 0.88 
                 2.11 ± 0.57 
               
               
                 Lacrimal 
                 1.33 ± 0.90 
                 1.00 ± 0.56 
                 1.06 ± 0.49 
                 2.44 ± 1.79 
                 2.82 ± 3.84 
               
               
                 Tumor: Blood 
                 18.6 ± 5.54 
                 8.57 ± 2.83 
                 3.39 ± 1.01 
                 17.3 ± 7.24 
                 36.6 ± 15.4 
               
               
                 Tumor: Muscle 
                 52.8 ± 24.9 
                 35.0 ± 18.7 
                 21.3 ± 7.50 
                 31.6 ± 12.1 
                 73.8 ± 30.9 
               
               
                 Tumor: kidney 
                 0.22 ± 0.15 
                 0.27 ± 0.20 
                 0.07 ± 0.02 
                 0.14 ± 0.04 
                 0.26 ± 0.13 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 6 
               
             
            
               
                   
               
               
                 Biodistribution data and tumor-to-background contrast ratios of  177 Lu-labelled HTK03041 in mice bearing 
               
               
                 PSMA-expressing cancer xenografts. 
               
            
           
           
               
               
            
               
                   
                   177 Lu-HTK03041 
               
            
           
           
               
               
               
               
               
               
            
               
                 Tissue 
                 1 h 
                 4 h 
                 24 h 
                 72 h 
                 120 h 
               
               
                 (% ID/g) 
                 (n = 5) 
                 (n = 5) 
                 (n = 5) 
                 (n = 5) 
                 (n = 5) 
               
               
                   
               
               
                 Blood 
                 0.84 ± 0.15 
                 0.31 ± 0.04 
                 0.07 ± 0.00 
                 0.02 ± 0.00 
                 0.01 ± 0.01 
               
               
                 Urine 
                  160 ± 49.6 
                 84.6 ± 39.3 
                 23.8 ± 8.50 
                 0.67 ± 0.22 
                 0.29 ± 0.04 
               
               
                 Fat 
                 1.09 ± 0.20 
                 0.56 ± 0.16 
                 0.16 ± 0.02 
                 0.03 ± 0.01 
                 0.05 ± 0.09 
               
               
                 Seminal 
                 1.22 ± 0.85 
                 0.57 ± 0.12 
                 0.23 ± 0.13 
                 0.03 ± 0.01 
                 0.02 ± 0.01 
               
               
                 Testes 
                 1.01 ± 0.17 
                 0.92 ± 0.10 
                 0.40 ± 0.08 
                 0.08 ± 0.02 
                 0.06 ± 0.01 
               
               
                 Intestine 
                 1.25 ± 0.15 
                 0.95 ± 0.08 
                 0.30 ± 0.06 
                 0.13 ± 0.04 
                 0.06 ± 0.03 
               
               
                 Stomach 
                 0.47 ± 0.18 
                 0.37 ± 0.06 
                 0.18 ± 0.04 
                 0.19 ± 0.09 
                 0.05 ± 0.04 
               
               
                 Spleen 
                 8.80 ± 2.53 
                 3.22 ± 1.01 
                 0.59 ± 0.19 
                 0.12 ± 0.04 
                 0.22 ± 0.11 
               
               
                 Liver 
                 1.91 ± 0.38 
                 1.46 ± 0.17 
                 0.42 ± 0.08 
                 0.13 ± 0.05 
                 0.13 ± 0.04 
               
               
                 Pancreas 
                 1.61 ± 0.24 
                 1.15 ± 0.13 
                 0.26 ± 0.04 
                 0.05 ± 0.02 
                 0.04 ± 0.01 
               
               
                 Adrenal glands 
                 5.53 ± 1.90 
                 3.41 ± 0.54 
                 0.76 ± 0.10 
                 0.12 ± 0.05 
                 0.11 ± 0.11 
               
               
                 Kidneys 
                 98.6 ± 9.27 
                  101 ± 13.3 
                 20.3 ± 6.51 
                 1.27 ± 0.58 
                 1.06 ± 0.74 
               
               
                 Lungs 
                 4.41 ± 0.49 
                 3.20 ± 0.41 
                 0.83 ± 0.12 
                 0.14 ± 0.04 
                 0.10 ± 0.04 
               
               
                 Heart 
                 2.44 ± 0.43 
                 1.69 ± 0.13 
                 0.41 ± 0.07 
                 0.07 ± 0.02 
                 0.06 ± 0.02 
               
               
                 Tumor 
                 10.8 ± 1.57 
                 18.7 ± 2.96 
                 19.8 ± 6.13 
                 15.2 ± 4.72 
                 22.3 ± 6.90 
               
               
                 Muscle 
                 0.94 ± 0.20 
                 0.56 ± 0.15 
                 0.15 ± 0.03 
                 0.02 ± 0.01 
                 0.02 ± 0.02 
               
               
                 Bone 
                 0.29 ± 0.07 
                 0.45 ± 0.53 
                 0.07 ± 0.02 
                 0.02 ± 0.01 
                 0.02 ± 0.00 
               
               
                 Brain 
                 0.04 ± 0.01 
                 0.05 ± 0.05 
                 0.01 ± 0.00 
                 0.01 ± 0.00 
                 0.01 ± 0.00 
               
               
                 Tail 
                 3.41 ± 3.03 
                 1.06 ± 0.60 
                 0.34 ± 0.17 
                 0.13 ± 0.08 
                 0.11 ± 0.07 
               
               
                 Thyroid 
                 3.38 ± 0.44 
                 2.10 ± 0.19 
                 0.47 ± 0.09 
                 0.09 ± 0.02 
                 0.08 ± 0.03 
               
               
                 Salivary 
                 5.88 ± 0.82 
                 2.18 ± 0.56 
                 0.40 ± 0.06 
                 0.08 ± 0.03 
                 0.06 ± 0.04 
               
               
                 Lacrimal 
                 0.03 ± 0.04 
                 0.02 ± 0.01 
                 0.00 ± 0.00 
                 0.01 ± 0.01 
                 0.01 ± 0.01 
               
               
                 Tumor: Blood 
                 13.4 ± 4.08 
                 60.3 ± 4.81 
                  282 ± 82.6 
                 805 ± 164 
                 1670 ± 272  
               
               
                 Tumor: Muscle 
                 12.1 ± 4.21 
                 34.4 ± 8.39 
                  142 ± 30.1 
                 647 ± 119 
                 1215 ± 512  
               
               
                 Tumor: kidney 
                 0.11 ± 0.02 
                 0.19 ± 0.02 
                 0.97 ± 0.15 
                 13.1 ± 3.75 
                 25.4 ± 9.59 
               
               
                   
               
            
           
         
       
     
     Discussion 
     Here we disclose the design and synthesis of four  68 Ga-labeled PSMA-targeting tracers ( 68 Ga-labeled HTK03026, HTK03027, HTK03029 and HTK03041), and their evaluation in LNCaP prostate tumor-bearing mice by in vivo PET/CT imaging and ex vivo biodistribution studies. The top candidate HTK03041 was selected, radiolabeled with  177 Lu, and further evaluated in LNCaP prostate tumor-bearing mice by in vivo SPECT/CT imaging and ex vivo biodistribution studies for up to 120 h post-injection. 
     The radioligand with the highest commercialization potential is  177 Lu-HTK03041 for radiotherapy of metastatic prostate cancer. Compared with the gold standard  177 Lu-PSMA-617 which is currently under multicenter clinical trials,  177 Lu-HTK03041 has higher peaked tumor uptake (˜20% ID/g for  177 Lu-HTK03041 vs ˜15% ID/g for  177 Lu-PSMA-617 reported previously: see Kuo HT et al. Molecular Pharmaceutics 2018; 15: 5183-5191). Most importantly, the tumor uptake of  177 Lu-HTK03041 sustained over the course of the study (up to 120h post-injection), whereas the tumor uptake of  177 Lu-PSMA-617 was reported to gradually drop to ˜8% ID/g at 120 h post-injection (see Kuo HT et al. Molecular Pharmaceutics 2018; 15: 5183-5191). Therefore, the radiation dose delivered to tumors by  177 Lu-HTK03041 is expected to be much higher than  177 Lu-PSMA-617, potentially leading to superior treatment efficacy of  177 Lu-HTK03041.