THIOETHER COMPOUNDS FOR USE IN PREPARING BIFUNCTIONAL CHELATING AGENTS FOR THERAPEUTIC RADIOPHARMACEUTICALS

Abstract: A compound consisting essentially of a multidentade ligand including at least two thioether groups for being complexed to Rhodium-105 in specific activities that are sufficiently high for use in formulating therapeutic <105>Rh-radiopharmaceuticals.

Description: THIOETHER COMPOUNDS FOR USE IN PREPARING BIFUNCTIONAL CHELATING AGENTS FOR THERAPEUTIC RADIOPHARMACEUTICALS This invention was made with Government support under Contract No. DE-FG02-89ER60875 awarded by the Department of Energy. The Government has certain rights in the invention. TECHNICAL FIELD The present invention relates to compounds which chelate radioactive atoms and have chemical properties which can be used in designing radiotherapeutic agents. More specifically, the present invention relates to a chelate which can complex preferably with rhodium-105, a radioactive form of rhodium for use as a radiotherapeutic. BACKGROUND OF THE INVENTION Radiotherapy using "non-sealed sources" by way of radiolabeled pharmaceuticals has been employed for several decades for cancer treatment [1,2]. Unfortunately, only a very limited number of therapeutic radiopharmaceuticals are currently in routine use, as being approved by the FDA. There is a great deal of interest in developing new agents due to the emergence of more sophisticated biomolecular carriers that have high affinity and high specificity for in vivo targeting of tumors. Several types of agents are being developed and investigated at a rapid pace, including monoclonal antibodies (MAbs) , antibody fragments or Single Chain Antibodies (SCAs) and peptide-based and non- peptide receptor-avid molecules [3-7]. Radiolabeling of these types of molecules with gamma- or positron-emitting radionuclides have produced effective agents for scintigraphic and PET imaging for diagnostic utility in cancer patients [8-10] . Development of radiotherapeutic agents is also occurring, however, at a much slower rate and is more problematic. For example, the choice of the particle emitting radionuclides for labeling of biomolecules for specific applications is not trivial [11-13]. Many factors must be considered when selecting the appropriate therapeutic radionuclide, including particle energies, matching of half-life with pharmaco inetics, availability, specific activity, suitability of an appropriate chelation system for coupling the radionuclide to the vector, _Ln vivo stability, etc. [13,14]. Most specific molecular probes target tumor cell populations that have a limited number of binding sites or receptors which in turn limits the quantity (i.e., usually greater than 100 nmoles and often less than 20 nmoles) of the pharmaceutical that can be administered [15,16]. As a result, the specific activity of these radiolabeled drugs must usually be high (i.e., > 100 m (i/nmole) [15,16]. Thus, the most attractive and perhaps the only functional radionuclides that can be used with these types of pharmaceuticals are those readily available in high specific activities. Relatively few beta-particle emitting radionuclides can be produced in sufficient quantities for treatment of very large numbers of patients [11-13]. One of these radionuclides is rhodium-105 ( 105 Rh) [13,17]. The moderate energy beta particles [E^- (max)=560 keV (70%) and 250 keV (30%)] emitted by 105 Rh make it attractive for therapy while the 306 and 319 keV gamma rays emitted in relatively low abundance (5% and 19% respectively) could be used for imaging in conjunction with therapy applications, if desired. The half-life of 105 Rh is 1.44 d which could be a good match for the pharmacokinetics of receptor binding agents or MAb fragments. It can be readily produced in large quantities "indirectly" in a no-carrier-added (NCA) form by bombardment of w Ru (>99% enriched) to produce 105 Ru which decays (t^ - 4.4 hr) to 105 Rh. The 105 Rh can be separated from the Ru to obtain the high specific activity 105 Rh [18]. It is also possible to obtain samples containing high activities (i.e., 10 3 curies) of 105 Rh as a fission product, if required [17]. Therapeutic agents have been primarily labeled with beta-particle emitting radionuclides. Most of the promising radionuclides are produced in nuclear reactors, however, some are accelerator produced [11-13]. Several different chelating structures have been employed to maintain the association of these beta emitters with the drug [19-23]. Many of these structures are not sufficiently stable and most, if not all, do not provide appropriate routes or rates of clearance of radioactivity from non-target tissues [23,24]. Accordingly, there is a delivery of high radiation doses to normal tissues and a reduction of the therapeutic ratio. This lowers the amount of radiation doses that can be safely delivered to a target tissue. Development of new radionuclide chelates that link the radioactive metal to a radiopharmaceutical is necessary. Further, new approaches must be taken in order to identify radiolabeling techniques that produce chelates that are highly stable in vivo but have improved clearance characteristics from normal tissues. Bifunctional chelating agents have been used to form stable metal complexes that were designed to minimize in vivo release of the metallic radionuclide from the radiopharmaceutical. For example, diethylenetriaminepentaacetic acid (DTPA) forms rather stable chelates with a variety of metals. However, coupling of this ligand to monoclonal antibodies by one of its five carboxyl groups resulted in unacceptable in vivo stability with a variety of radionuclides [14]. Linking of this compound by a side group attached to one of the carbon atoms on an ethylene bridging group provides improved stability in vitro and in vivo. Unfortunately, the stability characteristics of this chelate and its analogues with all radioactive metals are not ideal resulting in poor clearance of activity from certain non-target organs. The fact that Rh (III) forms a variety of complexes that are chemically inert under physiologic conditions [25] makes 105 Rh (III) particularly attractive for formulating new 105 Rh- labeled bifunctional chelating agents to form new therapeutic radiopharmaceuticals. Unfortunately, the formation of desirable Rh (III) chelates in aqueous media usually requires rather harsh conditions (e.g. refluxing for greater than two hours) [26-29]. In addition, polymerized forms of Rh are often produced, even with a large excess of the ligand [30,31]. Complexation of Rh with a variety of thioether compounds has been reported recently [28- 32]. Rh (III), considered a moderately soft acid, will usually form stable complexes with ligands containing "soft" donor atoms (e.g., thioethers) [32,33]. Blake et al., (1989) [32] showed that 1,5,9,13-tetrathiacyclohexadecane (16-ane-S 4 ) forms trans Rh(III)Cl- complexes with Rh(III) chloride. The ability of thioether compounds to complex Rh(III) in high yields at low ligand concentrations has not been investigated. Unfortunately, almost all metal chelates with high thermodynamic stabilities are not sufficiently stable in vivo and will not form in high yields using low quantities of ligand. The applicant investigated the possibility of 105 Rh to form well-defined 105 Rh chelates with tetrathiomacrocycles or their analogues using quantities of the ligand in the nanomole range under relatively mild conditions. Unlike other prior art chelates, these chelating agents show the ability to form highly stable 105 Rh chelates in high yields and high specific activities using very low quantities of the ligand (i.e., < 20 nmoles). SUMMARY OF THE INVENTION In accordance with the present invention, there is provided a compound consisting essentially of a multidentate ligand including at least two thioether groups for being complexed to rhodium- 105. The present invention further provides stable complexes of Rhodium-105 with 16-ane-S 4 -diol, 14- ane-NS- and 14-ane-N-S- ligand. BRIEF DESCRIPTION OF THE FIGURES Other advantages of the present invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein: Figure 1 is a radiochromatogram of electrophoresis analysis of (a) 105 Rh-chloride as received from MURR in acidic solution, and (b) 105 Rh(III) after complexation to 16- ane-S 4 -diol; Figure 2 shows the complexation yield of 105 Rh-l6-ane-S 4 -diol at different temperatures as a function of heating time, studies being performed using 10 μg 16-ane-S 4 -diol in 0.5 ml solutions at pH 4 containing 15% ethanol; Figure 3 shows the stability of 105 Rh-l6- ane-S 4 -diol at pH 7.4, 8.5 and in human serum at pH 7.4-7.8 for greater than 4 days, samples being maintained at (a) pH 7.4 in .09% saline (N. saline) at room temperature (RT) using 0.05M sodium phosphate; (b) pH 8.5 in N. saline at RT using 0.1M sodium bicarbonate and (c) pH 7.4-7.8 in human serum at 37 * C; Figure 4 is a radiochromatogram of electrophoreses of: SUBmUT-SHEET β UE» A. 05 Rh-chloride and B. 105 Rh-l4-ane-NS 3 ; Figure 5 shows complexation yield of 105 Rh(III) with 14-ane-NS 3 as a function of quantity of 14-ane-NS 3 used, lowest quantity of ligand = 1 μg (about 4 nmoles) , prepared by heating 105 Rh-chloride and 14-ane-NS 3 at pH 4 in 20% ethanol for 1 hour and analyzed by TLC and electrophoresis; and Figure 6 shows complexation yield of 105 Rh-l4-ane-NS 3 as a function of temperature, 12 μg 14-ane-NS 3 being incubated in aqueous solutions at pH 4 containing 20% ethanol for one hour at 55, 60, 70 or 80'C. DETAILED DESCRIPTION OF THE INVENTION Generally, the present invention provides a compound consisting essentially of a multidentate ligand including at least two thioether groups for being complexed to rhodium-105. That is, the compound can contain an 105 Rh core and a multidentate ligand containing two or more thioether (TE) groups for bonding to the metal. The resulting 105 Rh-TE complexes have high in vitro and in vivo stability and are formed using low quantities of the ligand (i.e., < 50 nmoles). The TE containing ligand is complexed to 105 Rh to form a chelate where the metal to ligand ratio is 1:1. The method of complexation permits the formation of the 105 Rh-chelate in a one step, high yield reaction (exemplified in the Experimental Section) , especially with 105 Rh- synthons that are currently available or can be made commercially available. The 105 Rh-chelates are produced in high yields (i.e. greater than 85%) under relatively mild conditions (i.e., heating at 60-80*C for one hour at moderate pHs) following mixing of the 105 Rh (III) synthon and the thioether ligand in aqueous solutions containing very small quantities of the ligand (i.e., less than 50 nmoles) . This is critical to solve the problems of prior art. 105 Rh chelates required more severe conditions (e.g. , refluxing for greater than two hours) and the use of larger quantities of the complexing ligands. Synthesis of 105 Rh chelates under these conditions will normally destroy the specificity or binding affinity of sensitive biomolecules and produce 105 Rh-compounds with specific activities that are usually too low for use as radiopharmaceuticals. The 105 Rh chelates made in accordance with the present invention have been found to be stable in aqueous solutions and in human serum at 37"C. These chelates are also stable over a wide pH range (i.e., pH 1-10). This high stability is critical with regard to permitting localization of the compound in areas of the body having different pH's as well as being stable through different administration routes. More specifically, the thioether (TE) containing multidentate ligands used for complexing 105 Rh can be characterized by the following formulas: A. Macrocyclic ligands containing two or more thioether groups: Essentially X's are or contain "donor" atoms that will complex Rh-105 (i.e., S, 0, or N) . A wide variety of thioether macrocycles have the above structure and can be made using well described methods [34,35]. R 1 , R 2 , R 3 , and R 4 are all the same or different and are selected from the group consisting of -(CH-)--, -(CH 2 ) 3 -, -CH-CH (C 3 CH--, -(CH-) 4 -, -CH 2 CH(R 5 )-, -CH 2 CH(R 5 )CH 2 )-, -CH(R 5 )CH 2 CH 2 - and -CH-CH(CH-R 5 )CH 2 ~. R 5 is -H, or any side chain containing groups commonly used for linking (e.g.,-OH, NH 2 , COOH -NCS, activated ester) . R 5 also can be selected from the group consisting of -OCH-, -OC 2 H 5 and groups for attaching a linking group used to modify lipophilicity for conjugation of the uncomplexed ligand or the ligand chelated to 105 Rh to a biomolecular targeting agent and R 1 " 4 can also contain another coordinating atom, e.g., R 6 -X 4 -R 6 , wherein R 6 is -(CH 2 ) 2 -, -(CH-)--, -CH-CH(CH)-CH--, -(CH 2 CH(R 5 )CH 2 )-, -CH(R 5 )CH-CH--, -CH-CH(CH-R 5 )CH 2 ~, wherein R 5 is -OH, NH 2 , COOH. X 1'4 is an atom or group containing S, 0 or N donor atoms that can also coordinate 105 Rh(III) , wherein X 1 , X 2 , X 3 and X 4 are all the same or different in which one of the X's is an -S- and the others are -S, -0-, NH, NR 7 , wherein R 7 is H, -CH 3 or a group attached to the N- atom to alter lipophilicity or to link the uncomplexed ligand or the "preformed" 105 Rh-chelate to the biomolecular targeting agent. As stated above, the ligand may be uncomplexed; that is, not complexed to the Rhodiu - SU8STITUTE SHEET(RULE 28) 105. Alternatively, the ligand may be complexed to the Rhodium and referred to as "precomplexed". Hence, either an uncomplexed or precomplexed ligand can be linked to a receptor avid molecule. Several chemical methods for conjugating ligand or metal chelates to biomolecules have been well described in the literature [36,37] and one or more of these methods will be used to link the uncomplexed TE ligands or 105 Rh-TE complexes to the receptor avid biomolecular targeting molecules. These include the use of acid anhydrides, aldehydes, arylisothiocyantes, activated esters or N-hydroxysuccinimides [36, 37]. The ligands can also be linear, open chain-ligands, containing at least one thioether group. The compounds are exemplified by the following formula: wherein R 1 , R 2 and R 3 are all the same or different and are selected from the groups consisting of -(CH 2 ) 2 -, -(CH 2 ) 3 -, -CH 2 CH(CH) 3 CH 2 -, -(CH 2 ) 4 -, -CH 2 CH(R 5 )-, -CH 2 CH(R 5 )CH 2 )-, -CH(R 5 )CH 2 CH 2 - and -CH 2 CH(CH 2 R 5 )CH 2 -. R 4 and R 5 can be the same or different and can be H or an alkyl group or a linking group containing functional groups such as -OH, NH 2 , COOH. -OCH 3 , -OC 2 H 2 and other functional groups for attaching a linking group used to modify lipophilicity for conjugation of the uncomplexed ligand or the ligand (chelated or not to 105 Rh) to the bimolecular targeting agent. X 1*4 is an atom or group that can also coordinate 105 Rh(III) . X 1 , X 2 , X 3 , and X 4 are all the same or different when at least one X is a S-atom and the remaining are selected from the group consists of -S-, -0-, -SH, NH and NR 7 . R 7 is selected from the group consisting of H, -CH 3 and a group attached to the N- atom to alter lipophilicity or to link the uncomplexed ligand or the ligand complexed to 105 RH to the biomolecular targeting agent. The above formulas characterize the present invention as providing capabilities for ligand modifications in order to tailor the ligands, that when chelated to 105 Rh, can be designed to optimize in vivo binding and pharmacokinetic properties for specific localization on target tissues (i.e., cancerous cells or tumors) . For example, the uncomplexed ligand or corresponding 105 Rh-chelate can be conjugated to peptides and other receptor avid molecules (targeting molecules) such as antibodies and antibody fragments by using side chains previously used for conjugation to bioactive molecules [36,37]. Conjugation reactions can involve reactive groups such as benzyl isothiocyanate, bromoacetamide, N-hydroxy-succinimides, activated esters and aldehydes (15) . Other side chains can be added to functional groups to make the chelate more polar or more hydrophilic. Charged groups, such as carboxyl or hydroxyl groups can be added to either the C- backbone or other sites (i.e., N-atoms) to increase the hydrophilic character of the resulting chelate. Alternatively, non-polar groups (e.g. , alkyl, alkoxy, etc.) can be added to the ligand backbone in a similar manner to increase the hydrophobic character of the resultant 105 Rh- chelate. Also, if one of the terminal groups on the linear ligand is a thiol group, a neutral- lipophilic 05 Rh chelate should be formed. These modifications can be systematically tuned to provide a 105 Rh pharmaceutical that has properties that maximize selective and maximal binding affinity to tumor cells while minimizing non¬ specific binding and localization in normal, radiosensitive tissues. All of the aforementioned modifications demonstrate the flexibility of compounds made in accordance with the present invention and further the ability to modify these compounds to alter binding, elimination and absorption of the compounds in order to tailor the resulting radiopharmaceutical for specific in vivo targeting, dosing and metabolism. The thioether ligands used in accordance with the present inventions were purchased commercially or could be made by methods similar to those outlined in the literature [34,35]. Attachment of side chains to functionalizable atoms on the ligand backbone (e.g., N-atoms) or attached to the ligand backbone (e.g., -OH, amines or carboxyl groups) are performed by standard methods. For example, the available 14-ane-NS 3 macrocyclic ligand shown below is derivatized by reaction of an alkyl halide with the lone ring N-atom to produce a variety of thioether derivatives. Similarly, the commercially available 16- ane-S 4 diol (1,5,9,13 tetrathiacyclo-hexane-3,11- diol) can be modified as shown below for attaching a single side chain (R) to the ligand. The R group can then be used for conjugation to bioactive molecules [36,37]. By radiopharmaceutical, it is meant that the chelate linked to a targeting molecule can be used to localize sufficient levels of 105 Rh at a site to provide radiotherapeutic properties. Accordingly, the chelate including the 105 Rh is bound to a targeting molecule, such as a peptide, antibody or other receptor avid molecule directed towards a specific antigen or other receptor on a target cell. Such compounds formed in high specific activities (i.e., greater than 100 curies/nmole) with sufficient stability in accordance with the present invention can be injected, circulate through the patient's system, and bind at target tissue to then provide radiotherapy at that site. The preferred compounds of the present invention, as designated above, contain two or more thioether groups that form high specific activity complexes with the rhodium-105 at high yields, as demonstrated below. The tetrathiamacrocycle (16-ane-S 4 diol) which is an example of the present invention as indicated above, chelates rhodium-105 to form a single species at low ligand concentrations permitting production of high specific activity chelates. As demonstrated below, the formed rhodium-105-16-ane-S 4 -diol chelate is stable for up to and greater than four days at physiological pH. This model S 4 ligand used in the experimental studies below includes two hydroxyl groups which can be used for linking either the macrocycle alone or the rhodium-105 chelate to biomolecules. Hence, there is significant potential for S 4 ligands and analogs thereof for use in formulating new rhodium- 105 therapeutic radiopharmaceuticals. The advantages of the present invention are numerous. The compound made in accordance with the present invention forms a stable, well defined, single species. The rhodium-105(III)-l6-ane-S 4 -diol chelate is formed in high yield under relatively mild conditions (i.e., 65*C for 60-90 minutes). Since these mild conditions will not result in significant 105 Rh(III) complexation with other groups on proteins, such as amines, carboxyls, or hydroxyls etc. or most other molecules, the 105 Rh(III) is able to selectively chelate to S 4 moieties already linked to biomolecular targeting molecules. The resulting 105 Rh-pharmaceutical can be used to selectively localize the 105 Rh on target cells. In addition to the S 4 ligand system, other examples follow demonstrating that substitution of N-atoms for the thioether groups also results in high 105 Rh-complexation yields. The following are examples of the use of three different specific thioether macrocyclic ligands to form 105 Rh chelates in high yields using exceptionally low quantities (less than 50 nmoles and often less than 20 nmoles of the thioether ligands) in accordance with the present invention. Several chemical methods for conjugating ligand or metal chelates to biomolecules have been well described in the literature [36,37] and one or more of these methods is used to link the uncomplexed TE ligands or 105 Rh-TE complexes to the SUBSTITUTE SHEET(RULE 26 biomolecular targeting molecules. These include the use of acid anhydrides, aldehydes, arylisothyiocyantes, activated esters or N-hydroxysuccinimides [14]. EXAMPLE 1 Formation of 105 Rh-l6-ane-S 4 -diol The 16 -ane-S 4 -diol ( 1 , 5 , 9 , 13 - tetrathiacyclohexadecane-3,11-diol)ligand, obtained from Aldrich Chemical Co., was used to react with the " 105 Rh-chloride reagent" that was obtained from the Missouri University Research Reactor (MURR) . The 105 Rh-chloride reagent contains a mixture of 105 Rh (III) species, presumed to include 105 RhCl 3 (H 2 O) 3 , 105 RhCl 4 (H 2 O) 2 ' , 105 RhCl 5 (H 2 O) '2 and 105 RhCl 6 "3 [38,40]. Electrophoresis of this reagent typically demonstrates a mixture primarily composed of three different anionic 105 Rh-species, presumably tetra, penta- any hexachloro- 105 Rh(III) anions. These samples contained only no-carrier-added (NCA) or trace quantities of 105 Rh(III) in HCl acidified solutions (- 0.1-1 M HCl). After adjusting the pH of the 105 Rh-chloride reagent, containing 1-10 mCi (37-370 MBq) of 105 Rh, to a desired pH between 1.5- 8.5, 0.5 ml aliquots were added to 0.1-0.2 ml solutions (containing some ethanol) containing between 0.2 to 10 μg (1-50 nmoles) of 16-ane-S 4 - diol. After heating the samples at temperatures ranging from 55' to 80" for up to 3 hours, the % labeling efficiency was determined. It was shown that the 105 Rh-complex with 16-ane-S 4 -diol is cationic and is a single species as determined by electrophoresis performed at 300 V for 1 hour on paper strips saturated with 0.075M sodium phosphate buffer at pH 4.5 (Figure 1). Silica-TLC also was used to routinely measure complex yield. The silica-TLC plates were developed with 0.9% aqueous NaCl (i.e., N saline), on which the uncomplexed 05 Rh-chloride reagent has a R f of 0.9-1.0 while the R f of the 105 Rh-16-ane-S 4 -diol is 0.05-0.10. Development of the C-18-reverse-phase TLC plates with 0.02 M hexane sulfonic acid in 10% CH 3 CN in water showed 105 Rh-l6-ane-S 4 -diol remained at the origin while the 105 Rh-chloride reagent exhibits an SUBSTITUTE SHEET (RULE 261 in 50% CH 3 CN, the R f , of 105 Rh-16-ane-S 4 -diol was 0.70. The results of a series of experiments in which systematic, independent variation of temperature, quantity of ligand, percent of ethanol and pH are summarized in Tables 1-3 and Figure 2 . These results show that excellent yields of 105 Rh- 16-ane-S 4 -diol are achieved using as low as 0.5 μg (2.5 nmoles) of ligand and heating for 1 to 1.5 hours at temperatures greater than 60°C. Maximal yields, under these conditions are achieved at pH ranging between 3-6, which can be used to incubate most bioactive molecules. Also, only small quantities of ethanol (i.e. 10%) are required to produce optimal 105 Rh-complex yields. Based on chromatographic comparisons with a non-radioactive chelate of Rh(III) complexed with 16-ane-S 4 -diol made under similar conditions, the 105 Rh-16-ane-S 4 -diol is assigned to be [ 105 Rh(III) (16-ane-S 4 -diol)Cl 2 ] + as shown below: SUBSTITUTE SHEET (RULE 2® [Rh(III)16-ane-S 4 -diol)Cl 2 ] + was prepared by a method similar to that described by Blake, et al. [32] for a similar S 4 -macrocycles. The PF 6 " salt of this chelate was crystallized and fully characterized by NMR, UV-spectroscopy, elemental analysis and X-ray crystallography. Since the migration distance of this yellow [Rh(III)16-ane- S 4 -diol)Cl 2 ] + on electrophoresis and the silica and reverse-phase TLC R f was identical to 105 Rh-16-ane- S 4 -diol, the assignment of the 105 Rh chelate as [ 105 Rh- (l6-ane-S 4 -diol)Cl 2 ] + could be made with a high degree of confidence. This chelate was shown to be stable for > 4 days in aqueous solutions near physiological pH and at room temperature and in human serum at 37°C (Figure 3) . EXAMPLE 2: 105 Rh-complexation with 14-ane-NS 3 Between 1-60 μg of 14-ane-NS 3 (4 , 8 , ll-trithia-8- amino-cyclotetradecane) , obtained as a gift, was reacted with the 105 Rh- chloride reagent obtained from MURR by the method previously described for reacted with the 105 Rh-chloride reagent obtained from MURR by the method previously described for complexation of 105 Rh with 16-ane-S 4 -diol (See Example 1) . The results of 105 Rh(III) complexation with 14-ane-NS 3 are summarized in Figure 4-6. These data show that the 14-ane-SN 3 ligand forms cationic complexes with 105 Rh in yields > 90% at pH 4-5 using only 1 μg of the NS 3 ligand (Figure 5) . High yields are obtained at 60 ' C incubation temperature. The temperature dependence (Figure 6) is observed with 105 Rh-l6-ane-S 4 -diol (Figure 2) . Results with this ligand demonstrate that the presence of one N-atom in the chelate in combination with three thioether groups will form a well defined 105 Rh-chelate in yields similar to the tetrathia ligand. The 105 Rh-14-ane-NS 3 chelate has the same electrophoretic migration distance as 105 Rh-l6-ane-S 4 -diol, indicating the two chelates have the same overall charge (i.e., +1). 105 Rh-l4-ane-NS 3 was also shown to be stable in aqueous solutions for greater than 4 days. EXAMPLE 3: 105 Rh-complexation with 14-ane-N 2 S 2< Between 10-50 μg of 14-ane-N 2 S 2 (l,4,8,11 dithia-8, 11-diamino-cyclotetradecane) was mixed with the 105 Rh-chloride reagent from MURR by the method previously described for complexation with 105 Rh-l6-ane-S 4 -diol (see Example 1) . After heating the solution at pH 4 at 80*C for 1 hour, a cationic 105 Rh-complex (with a similar migration distance in electrophoresis as 105 Rh-l6-ane-S 4 -diol) was produced in >80% yield. The results of these studies demonstrate that high 05 Rh-complexation yields can be achieved using a ligand with two thioether group and two amine functionalities. The above experimental results demonstrate that the ligands made in accordance with the present invention that contain at least two thioether groups that form complexes with rhodium-105 can be made in high yields. The specific ligands examined form a single species on complexation to Rhodium-105, in low concentrations, SU&TITUTESHEET(RULE26) thereby permitting production of high specific activity 05 Rh-complexes. The ligands are stable for greater than four days at physiological pH. The _ligand used having at two hydroxyl groups attached to the ligand backbone can be easily used for linking the macrocycle or rhodium chelate acrocycle to biomolecules as known in the art. Accordingly, there is great potential for the present invention and analogs thereof in formulating therapeutic pharmaceuticals. The invention has been described in an illustrative manner, and it is to be understood that the terminology which has been used is intended to be in the nature of words of description rather than of limitation. Obviously, many modifications and variations of the present invention are possible in light of the above teachings. It is, therefore, to be understood that within the scope of the appended claims wherein reference numerals are merely for convenience and are not to be in any way limiting, the invention may be practiced otherwise than as specifically described. Table 1. Effect of pH on 10S Rh chloride complexation yields with 16S*-diol PH % Complexation' (±SD) 1.5 93.0 ± 3.6 3.0 99.3 ± 6 4.0 99.7 ± 0.6 5 97.0 ± 1.5 6 97.9 ± 2.9 7 90.0 ± 5.0 8.5 79.3 ± 10.0 ' The compiexation experiments were performed using 10 μg 16S 4 -diol in 0.5 ml solutions containing 15% ethanol and heated at 80 °C for 1 hr. analysis was performed using silica-TLC; N = between 3-8. Table 2. Effect of 16S 4 -diol concentration on 105 Rh-4S16-diol complex yield at pH 4. μg 4S16-diof % Complexation Yield ( ±SD) f 10 μg 97.7 ± 2.6 i //g 96.3 ± 4.3 0.5 μg 94.7 ± 5.9 0.2 μg 64.5 ± 14.8 ' This is the # μg of ligand present in between 1.2 to 0.5 ml of solution. Samples containing 15% ethanol were heated at 80°C for 1 hr (N=between 3-13). Table 3. Effect of ethanol concentration on 105 Rh-16S Λ -diol yields at pH 4 % Ethanol (VV) % Compiexation Yields " 0 61.0 ± 19.5 10 95.9 ± 2.2 15 97.6 ± 3.8 20 97.0 ± 3.1 30 97.5 ± 3.7 50 98.0 ± 2.2 'Reactions were carried out using 10 μg 16S 4 -diol in 0.5 ml solutions and heated at 80°C for 1 hr (N=4).

Claims: What is claimed is: 1. A compound consisting essentially of a multidentate ligand including at least two thioether groups and two or more other donor atoms (including S, N or 0) for being complexed to Rhodium-105 in high specific activities to use in formulating therapeutic radiopharmaceuticals. 2. A compound according to claim 1 including three thioether groups. 3. A compound according to claim 1 including four thioether groups. 4. A compound according to claim l including three thioether groups and one group containing a nitrogen donor atom. 5. A compound according to claim l wherein said ligand is a macrocycle containing at least two thioether groups of the formula: wherein R 1 , R 2 , R 3 , and R 4 are all the same or different and are selected from the group consisting of -Ch 2 -, -(CH 2 ) 2 -, -(CH 2 ) 3 ~, -CH 2 CH(C 3 CH 2 -, -(CH 2 ) 4 -, -CH 2 CH(R 5 )-, -CH 2 CH(R 5 )CH 2 )-, -CH(R 5 )CH 2 CH 2 -, -CH 2 CH(CH 2 R 5 )CH 2 - and R 5 is -H or a group containing -OH, NH 2 , COOH, -OCH 3 -, -OC 2 H 5 and groups for attaching a linking group used to modify lipophilicity for conjugation of the uncomplexed ligand or the ligand chelated to Rh-105 to a biomolecular targeting agent and R 1'4 can also contain another coordinating atom (R 6 -X 4 -R 6 ) wherein R 6 is -Ch 2 (CH-) 2 -, -(CH 2 ) 3 -, -CH 2 CH(CH) 3 CH 2 -, -(CH-CH(R 5 )CH-)-, -CH(R 5 )CH 2 CH 2 -, -CH 2 CH(CH 2 R 5 )CH 2 - and R 5 is a group containing -OH, NH 2 , COOH, X being an atom or group containing S, O or N donor atoms that also coordinates 105 Rh(III), wherein X 1 , X 2 , X 3 and X 4 are, all the same or different in which at least one or more X-atoms or groups is -S-, and the others are -S, -0-, NH, NR 7 , R 7 being H, -CH- or a group attached to the N-atom to alter lipophilicity or to link the uncomplexed ligand or the "preformed" 105 Rh-chelate to the biomolecular targeting agent. 6. A compound according to claim l wherein said ligand is an open-chain multidentate ligand containing at least two thioether groups of the formula: wherein R 1 , R 2 , and R 3 are all the same or different and are selected from the groups consisting of - (CH 2 ) 2 -, -(CH 2 ) 3 -, -CH 2 CH(CH) 3 CH 2 -, -(CH 2 ) 4 -, -CH 2 CH(R 5 )-, -CH 2 CH(R 5 )CH 2 )-, -CH(R 5 )CH 2 CH 2 -, -CH 2 CH(CH 2 R 5 )CH 2 CH 2 CH(CH 2 R 5 . R 4 and R 5 can be the same or different and can be H or an alkyl group or a linking group containing functional groups such as OH, NH 2 , COOH -OCH 3 , -0C-H 2 and a functional group for attaching a linking group used to modify lipophilicity for conjugation of the uncomplexed ligand or the ligand chelated to said Rhodium-105 to the biomolecular targeting agent; X being an atom or group containing S, N. Or 0 donor atoms that can also coordinate 105 Rh(III), X 1 , X 2 , X 3 and X 4 being all the same or different and at least two are -S- and the others being selected from the group consisting of -S-, - 0-, -SH, NH, NR 7 , R 7 being selected from the group consisting of H, -CH 3 and a group attached to the N- atom to alter lipophilicity or to link said uncomplexed ligand or said ligand complexed to said Rhodium-105 to the biomolecular targeting agent. R 4 and R 5 can also contain a group containing one or more atoms that will complex 105 Rh selected from the groups containing 0, S or N donor items. 7. A compound according to claim 6 or 7 wherein R 5 is selected from the group consisting of benzyl isothiocyanate, bromacetamide, an activated ester, N-hydroxy succinimides, a cleavable ester and an aldehyde. 8. A compound as set forth in claims 6 or 7 wherein any one or all of R 1"7 are carboxylated, hydroxylated, alkylated or alkoxylated to render said ligand more or less polar. 9. 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Inventors: JURISSON SILVIA, KETRING ALAN R, SCHLEMPER ELMER DI, VENKATESH MEERA, VOLKERT WYNN A, JURISSON, SILVIA, KETRING, ALAN, R., SCHLEMPER, ELMER +DI, VENKATESH, MEERA, VOLKERT, WYNN, A.
Applicants: UNIV MISSOURI, THE CURATORS OF THE UNIVERSITY OF MISSOURI
Application year: 1995
Main classification: C07F13
IPC codes: A61K38, A61K39, A61K51, C07B59, C07C321, C07C323, C07C381, C07D285, C07D327, C07D331, C07D339, C07D341, C07F15, C09K3