CONTRAST AGENTS

Abstract: The invention provides a composition of matter of the formula (I): V - L - R where V is a non-peptidic organic group having binding affinity for an endothelin receptor site, L is a linker moiety or a bond, and R is a moiety detectable in in vivo imaging of a human or animal body. This composition of matter may be used to image diseases and disorders, particularly of the cardiovascular system.

Description: CONTRAST AGENTS This invention relates to diagnostic imaging techniques in which a disease state may be imaged using a targeted contrast agent and to targeted contrast agents suitable for use in such techniques. More particularly the invention relates to the use of such contrast agents in which the targeting vector binds to endothelin receptors . The lining of the blood vessels, the endothelium, secretes various agents which can cause relaxation and contraction of the vessel, thereby controlling blood flow and pressure. Many cardiovascular diseases are associated with imbalances in the secretion of such agents, leading for example to long term structural damage . The endothelins (ETs) , are a family of related oligopeptides which are generated as a result of stimuli such as hypoxia, shear stress, circulating hormones, growth factors and cytokines . The endothelins are vasoconstrictors and indeed the dominant isoform, ET-1, is one of the most highly potent known mammalian vasoconstrictors and is associated with various pathological effects in several diseases and disorders. ET-1 is a 21-amino acid peptide which has vasoconstrictor, bronchoconstrictor and mitogenic activities and is found in many cell types, eg. neurons, glia, vascular smooth muscle, endothelium and gastric mucosal cells. The ability of ET-1 to elevate blood pressure is related to its induction of vascular contraction through the activation of various hormones. This effect however is preceded by a transient vasodilatory effect. ET-1 can also act as a mitogen to induce smooth muscle cell proliferation and thus contributes to vascular proliferative disorders such as restenosis. Two types of ET receptors have been identified, ET A and ET B . The ET A receptors found on smooth muscle cells have high affinity for ET-1 and ET-2 while the ET B receptors found in endothelial cells have affinity for all ET isoforms. The levels of ET B receptors are higher than those of ET A receptors in organs such as kidney and brain, while ET A receptors predominate in the heart and vasculature . ET A and ET B receptors both contribute to vasoconstriction while vasodilation appears to be soley dependent on ET B receptors in endothelial cells. In view of the range of potentially harmful effects of the endothelins, various ET receptor antagonists have been developed, eg. for use in improving haemodynamics (eg. restoring blood flow) , reducing inflammation, preventing mitogenesis, etc. Thus such antagonists have been proposed for use in a wide variety of treatments, eg. of unstable and variant angina, asthma, atherosclerosis, cerebral ischemia, myocardial isochemia, renal isochemia, stroke, cerebral vasospasm, congestive heart failure, vascular complications of diabetics, gastric cancer, gastric mucosal injury, hypertension (eg. pulmonary hypertension), idiopathic pulmonary fibrosis, migraine, nephropathy, Reynaud's disease, subarachnoid hemorrhage, and restenosis and other hyperproliferative vascular diseases. It has now been found that it is possible to image endothelin receptor sites in vivo using targeted contrast agents in which the targeting vector has affinity for endothelin-receptor sites. The endothelin receptors are generally located within the cardiovascular system and the gastrointestinal tract and are accessible to such contrast agents when they are administered into the blood stream or the gi tract . Accordingly, using such targeted contrast agents it is possible to detect diseases and disorders such as heart failure, atherosclerosis, restricted blood flow, gi tract surface damage as well as other vascular and gastric diseases and disorders, and also to monitor the progression of treatment for such diseases and disorders . Viewed from one aspect therefore the invention provides a composition of matter of formula I V - L - R (I) where V is a non-peptidic organic group having binding affinity for an endothelin receptor site, L is a linker moiety or a bond, and R is a moiety detectable in in vivo imaging of a human or animal body. Preferably where R is a radionuclide it is an iodine bonded directly or indirectly to V or it is a metal ion chelated by a chelant group in L, and/or it is separable from V by biodegradation of an organic linker group L. (It should be noted that hydrogen radionuclides, while detectable in vitro, may not satisfy the requirement that R be detectable in in vivo imaging) . In many instances, the composition of matter of formula I will be a characterisable compound. In others it may be a combination of compounds bonded or otherwise associated, eg. conjugated, with each other. For convenience sake, the composition of matter will be referred to hereinafter as an agent . Viewed from a further aspect the invention provides a pharmaceutical composition comprising an effective amount (eg. an amount effective to enhance image contrast in in vivo imaging) of an agent of formula I together with at least one pharmaceutically effective carrier or excipient. Viewed from a still further aspect the invention provides the use of an agent of formula I for the manufacture of a contrast medium for use in a method of diagnosis involving administration of said contrast medium to an animate subject and generation of an image of at least part of said subject. Viewed from a still further aspect the invention provides a method of generating an image of an animate human or non-human (preferably mammalian or avian) animal subject involving administering a contrast agent to said subject, eg. into the vascular system or the gi tract, and generating an image of at least a part of said subject to which said contrast agent has distributed, eg. by X-ray, MR, ultrasound, scintigraphic, PET, SPECT, electrical impedance, light or magnetometric imaging modalities, characterised in that as said contrast agent is used an agent of formula I. Viewed from a further aspect the invention provides a method of monitoring the effect of treatment of a human or non-human animal subject with a drug to combat or provoke effects associated with endothelin, in particular ET-1, said method involving administering to said subject an agent of formula I and detecting the uptake of said agent by endothelin receptors, eg. ET A and/or ET B receptors, said administration and detection optionally but preferably being effected repeatedly, eg. before, during and after treatment with said drug. Viewed from a yet further aspect the invention provides a process for the preparation of an agent of formula I, said process comprising conjugating (i) an organic non- peptidic compound having binding affinity for an endothelin receptor to (ii) a compound detectable in a diagnostic imaging procedure or a chelant compound and if necessary metallating chelant groups in the resultant conjugate with a metal ion detectable in a diagnostic imaging procedure . The agents of formula I have three characteristic components: a vector (V); a linker (L) ; and a reporter (R) . The vector must have the ability to target the compound to endothelin receptors, the reporter must be detectable in an in vivo diagnostic imaging procedure; and the linker must couple vector to reporter, at least until the imaging procedure has been completed. Vectors As the vector, one may use any non-peptidic compound having affinity for endothelin receptors. Non-peptidic compounds are used as peptidic vectors will generally have poor biological stability and may provoke undesired ET-1ike responses by the body. By the term non-peptidic, compounds are excluded which possess a -CHR-CO-NH-CHR-CO-NH- entity. Moreover, the vector may be a compound (such as BMS-182874) which has more pronounced affinity for one type of endothelin receptor than for the other type, or a compound (such as bosentan) which has affinity for both types of endothelin receptors. Compounds having more pronounced affinity for a particular type of endothelin receptors will generally be preferred. Preferably the agent is a compound which does not elicit any unacceptable biological response, particularly compounds which act as endothelin receptor antagonists and do not elicit the responses associated with endothelin itself, especially the blood pressure modifying and mitogenic responses. However, biological responses may if desired be modified by administration of a therapeutic agent, eg. before, at the same time or after administration of the agent of formula I. Among non-peptidic vectors, bosentan, Shionogi ' s 97-139, Shionogi's 50-235, BMS-182874, Merck's L-744453, PD-155719, PD-155080, PD-156707, Fujisawa's FR 901366, FR 901367, PD-160672, PD-160874, SB 209670, SB 217242, L-749329, -751281, IRL 2500, IRL 2659, IRL 2796, RO 468443, RO 46-2005, RO 470203, BMS 193884, TBC 11251, CGS-26303, CGS-26393, CGS-27830, L-751281, L-747844 and L-754142 are preferred. Examples of suitable endothelin receptor antagonists are given in O-95/03295 , US-A-5420123 , O-94/27979, O- 95/03044, and EP-A-405421 and by Sakurawi et al . in Chem. Pharm. Bull. 44: 343-351 (1996). The development of appropriate non-peptidic endothelin receptor antagonists is discussed for example by Ferro et al . in Drugs 5_1: 12-27 (1996), Battistini et al . in TiPS 16: 217-222 (1995) , and Doherty, Chapter 4 in "Chemical and Structural Approaches to Rational Drug Design", Ed. einer et al . , CRC Press, 1995. Suitable non-peptidic vectors will generally be polycyclic compounds containing a phenyl group linked to a 5- or 6-membered carbocyclic or heterocyclic ring by one or two bridging groups, one of which contains or has pendant therefrom a heteroatom interrupted, oxo- substituted moiety (eg. a -S0 2 -NH- or -CO-S- or -CO-CH 2 -0- group) . Thus the vector compound may for example be a compound of formula II 0-Bd-Rg (II) where 0 is an optionally substituted phenyl group, optionally part of a fused polycyclic structure containing up to 4 fused rings; Bd is a 1 to 5 atom long optionally unsaturated organic bridge containing backbone atoms selected from C, N, S and 0 optionally carrying side chain substituents and optionally forming part of a fused ring, and containing a 0 II -X-(CH 2 ) n -Y- moiety where X is C or SO, n is 0 or 1 and Y is 0, S or an amine nitrogen; and Rg is a 5- or 6-membered saturated or unsaturated optionally substituted carbocyclic or heterocyclic ring, optionally part of a fused polycyclic structure containing up to 5 fused rings; wherein the optional substituents are selected from halogen and oxygen atoms, carbon and sulphur oxyacid groups, hydroxy, alkyl, aryl, aralkyl, acyl , alkoxy, alkylenedioxy, alkylthio, alkylamino, acyloxy, aralkylamino, aralkyloxy, acylthio, amino, and acylamino groups and combinations thereof (eg. haloalkyl) , and wherein ring heteroatoms are selected from 0, N and S, aryl groups are mono or bicyclic carbocyclic or heterocyclic rings containing up to 10 ring atoms, and no more than 7 rings and no more than 6 fused rings are present . In the compounds of formula II, alkyl or alkylene groups preferably contain 1 to 6 carbon atoms, aryl moieties preferably are phenyl or naphthyl groups or 5-membered heterocycles optionally carrying a fused benzene or 6 membered aromatic heterocyclic ring, and halo atoms are preferably F, Cl , Br or I . In such compounds, the amino, hydroxy, carbonyl or oxyacid functions may readily be used to conjugate the vector to the reporter. Thus examples of such polycyclic vectors include triterpenoids of formula III wherein R 1 and R 2 , which may be the same or different, are hydroxy, sulphur oxyacid, acylamino, or carboxyacylamino groups (wherein the acyl moiety conveniently contains 1 to 6 carbons) . Examples of such triterpenoids include compounds in which the pendant phenyl group is of formula In these compounds the hydroxyl and/or OS0 3 H groups on the pendant phenyl groups may conveniently be used for vector attachment . Further examples of polycyclic vectors of formula II include anthraquinone compounds of formula IV (wherein each R 3 which may be the same or different is hydrogen or an acyl group; R 4 is a carboxy or acyl group; and R 5 is hydrogen, hydroxy or an acyloxy group, the acyl groups being for example C^g groups such as formyl, acetyl or propionyl) . In these compounds, the bridging group Bg is part of the fused tetracyclic structure. Examples of vectors of formula IV include compounds wherein R 3 on the tetracyclic ring structure are all hydrogen, the pendant alkylthio group is 2-carboxy-2- acetylamino-ethylthio and R 5 is hydrogen (compound FR 901366) or hydroxy (FR 901367) . In these compounds the amino and/or carbonyl functions on the pendant alkylthio groups may conveniently be used for vector attachment . Still further examples of the polycyclic vectors include dibenzodiazepines of formula V (where R 6 is hydrogen or alkyl; R 7 is aryl, aralkyl, arylcarbonyloxy or aralkylcarbonyl (eg. where the aryl moiety is a phenyl, naphthyl or indolyl, eg. indol-3-yl group and any alkyl moiety may have 1 to 6 carbons) ; Z is an optionally oxa substituted C x _ 6 alkylene group optionally with a CH 2 moiety replaced by an amine nitrogen or an oxygen atom (eg. a -CH 2 -, -OCH 2 -, 0 0 0 0 II II II II -0-C-, -NH-C-, -0-C-CH 2 - or -NH-C-CH 2 group); Z 10 is hydrogen or a group X x as defined in US-A-5420123 ; and R 8 is hydrogen or optionally substituted C x _ 6 alkyl, eg. carboxyalkyl , aminoalkyl , sulphonyloxyalkyl , etc. group) . Examples of R 7 groups thus include phenyl , naphthyl , indol-3-yl, benzyl, naphthylmethyl , indole-2-carboxyl , indole-3-carboxyl and 3 -indolyl-acetyl . Examples of these dibenzodiazepines of formula V include compounds wherein R 6 is hydrogen or propyl, R 8 is carboxymethyl and ZR 7 is indol-3yl -methyl, indol (2 or 3) yl-carbonyloxy, indol (2 or 3) ylcarbonylamino, indol-3-yl- methylcarbonyloxy, indol-3-ylmethylcarbonylamino, 1-naphthyl-methylamino, 1-naphthylmethoxy, benzyloxy, and benzylamino . With such compounds one may conveniently use the R 8 group for vector attachment . Still further examples of polycyclic vectors of formula I include N-isoxazolyl-sulphonamides, in particular sulphonamides of formula VI (where the isoxazoyl group is attached at the 5 or 3 positions; R 9 is an optionally substituted, optionally fused benzene ring (eg. amine or oxyacid substituted), fused pyridine ring, a phenyl group, or a carboxyl or amino group where the ring to which it is attached is unsaturated or a fused 2 , 2-propylidene group or an oxygen atom where the ring to which it is attached is saturated; and R 10 is a straight chain or branched C j __ 6 alkyl group or a halogen atom) . Examples of vectors of formula VI include compounds of formula (BMS 182874) With such compounds, vector attachment may be affected conveniently using active substituents in R 10 or R 9 , eg. amine or oxyacid groups. Further examples of polycyclic vectors of formula II include phenoxyphenylacetic acid substituted benzimidazolinones of formula VII (where R 11 is hydrogen or C λ . 6 alkyl optionally substituted by oxyacid or other reactive substituents for vector linkage (eg. carboxyl) ; R 12 is C^g alkyl optionally substituted by oxyacid or other reactive substituents for vector linkage; R 13 is hydrogen, halogen or C λ . 6 alkyl or a group R 10 as defined in WO95/03044; and R 14 is hydrogen, C^g alkoxy or methylenedioxy or a group R 1 , R 2 , R 3a , R 3b or R 10 as defined in WO95/03044) . Thus examples of vectors of formula VII include Vector attachment of such compounds to linker-reporter moieties may be achieved using the carboxyl or other reactive substituents. Yet still further examples of polycyclic vector compounds of formula II include 1 , 2-diphenylethylene compounds of the formula I described in WO 95/03295, eg. compounds of formula VIII (where Z" is selected from alkylsulphonylaminocarbonyl, arylsulphonylaminocarbonyl, aryl, arylaminocarbonyl, and carboxy; R 15 is hydrogen, alkyl, oxyacid or another reactive group for vector attachment; R 16 is hydrogen, halogen, carboxyl , λ . 6 alkoxy- carbonyl , or C 1 _ 6 alkyl, alkoxyl, hydroxy-alkyl or haloalkyl; and R 17 is hydrogen, halogen, nitro, carboxyl, aminocarbonyl, acetylamino, alkoxycarbonylamino, alkoxy or alkylenedioxy) . Where Z" contains an aryl function this may be carbocyclic (eg. phenyl) or heterocyclic (eg. tetrazolyl) and may be optionally substituted, eg. by halogen atoms, carboxy groups or C x _ 6 alkyl or haloalkyl groups . Examples of vector compounds of formula VIII include (where each R 151 independently is hydrogen, halgen or carboxy or C^g alkyl, alkoxy, hydroxy-alkyl or haloalkyl, or alkoxy-carbonyl , or C 3 _ 7 cycloalkyl ; R 16 is hydrogen or C^g alkyl; and each R 171 independently is hydrogen, halogen, nitro, alkyl, alkoxy, aminocarbonyl, alkoxycarbonyl, alkoxycarbonylamino or acetylamino) Thus for example such compounds include compounds of formulae (where each R is independently halogen, n-propyl or i-butyl; Z" is 4-i-propyl-phenyl-S0 2 NHCO; R is hydrogen or methyl ; and each R 171 is independently hydrogen , chlorine or methoxy) , 51 (where R is hydrogen, carboxyl or hydroxymethyl ; each R 151 is independently hydrogen or n-propyl; Z" is carboxy, 4-i-propyl-phenyl-S0 2 NHCO, 4 -t-butyl -phenyl - S0 2 NHCO, tetrazol-5-yl or tetrazol-5-yl-NHCO; R 16 is hydrogen or methyl; and R 171 is hydrogen, chlorine or methoxy) , (where R 152 is carboxy or hydroxy ethyl ; each R 151 is independently hydrogen or n-propyl; Z" is carboxy, 4-i-propyl-phenyl-S0 2 NHCO, 4 -i-butyl -phenyl -S0 2 NHCO, 4- bromo-phenyl -S0 2 NHCO, tetrazol-5-yl , tetrazol- 5 -yl-NHCO or i -propyl -S0 2 -NHCO; and R 16 is hydrogen or methyl) , and (where R 152 is carboxyl or hydroxymethyl; R 151 is hydrogen or n-propyl; R 16 is hydrogen or methyl; and Z" is carboxy, 4 -i-propyl -phenyl -S0 2 NHCO, 4-i-butyl-phenyl- S0 2 NHCO, 4 -bromo-phenyl -S0 2 NHCO, tetrazol-5-yl , tetrazol- 5 -yl-NHCO or i -propyl -S0 2 -NHCO; and R 16 is hydrogen or methyl) . Further examples of polycyclic vectors of formula II include 3-benzoyl-2-phenyl -3- (benzyl or cyclohexylmethyl) propeneoic acids and the furan analogs of formulae IX and X (where each R 18 which may be the same or different is an alkoxy or alkylenedioxy group) , eg. the compounds Yet further examples of polycyclic vector compounds of formula II include compounds of formula XI (where each R 18 which may be the same or different is an alkoxy or alkylenedioxy group), eg. the compounds Still further examples of vector compounds of formula II include phenylsulphonamides of formulae XII and XIII (where R 19 is C λ . 6 alkyl; R 18 is as defined above; Z" ' is 0 or CH 2 ; and R 20 is alkoxyaryloxy, hydroxyalkoxy, or alkoxyaryl) , eg. compounds of formulae (L7493 2 9) (L751281) Still further examples of the polycyclic vectors of formula II include the compounds of formula XIV (where R is as defined above; R ι21 is a 5 or 6 ring membered aryl group, eg. a phenyl, thiophenyl or isoxazolyl group; and each R 22 , which may be the same or different is a hydrogen or C^g alkyl group) , eg. compounds of formulae (1RL2500) (IRL2659) (IRL2796) Still further examples of polycyclic vector compounds of formula II include the phenoxy-phenyl acetic acids of formula XV (where R is as defined above; R is an aralkyl group, eg. a 3 , 6-diazaindol-1-yl-methyl group; and R 24 is an aralkyl) , eg. a compound of formula (L744453) Still further examples of polycyclic vectors of formula I include compounds of formula XVI (where R 25 is an oxyacid or an ester thereof, eg. an alkoxy carbonyl group; R 18 is as defined above; and R 26 is a hydrogen atom or an optionally substituted Cτ__ 6 alkyl group, eg. carrying a reactive functional group capable of use for vector attachment) , eg. a compound of formula Other compounds suitable for use as vectors include compounds such as those described in WO 95/03295, WO 95/03044, WO 94/27979, US-A-5420123 , EP-A-405421, and WO 95/08550, and by Schin-ichi et al . in Eur. J. Pharmacol. -Mol . Pharmacol. 247: 219-221 (1993) and by Sakurawi et al . in Chem. Pharm. Bull. 4_4: 343-351 (1996) . The vectors of formulae II to XVI may be prepared by the techniques described in the patent and other publications referred to herein, the disclosures of which are incorporated herein by reference. CAM-D and other candidate identification and evaluation techniques as mentioned above can also be used to find or assess further candidate non-peptide vectors. Thus it is also possible to obtain molecules that bind specifically to endothelin receptors by direct screening of molecular libraries. Screening of peptidic libraries may also be used to identify generally effective peptidic structures of which non-peptidic analogs may be generated by conventional or combinatorial chemistry. Binding moieties identified in this way may be coupled to a linker molecule, constituting a general tool for attaching any vector molecule (or molecules) to the reporter. Vector molecules may be generated from combinatorial libraries without necessarily knowing the exact molecular target, by functionally selecting ( in vi tro, ex vivo or in vivo) for molecules binding to the region/structure to be imaged. As mentioned above, the agents of formula I comprise vector, linker and reporter moieties. A linker moiety may serve to link one vector to one reporter; alternatively it may link together more than vector and/or more than one reporter. Likewise a reporter or a vector may be linked to more than one linker. Use in this way of a plurality of reporters (eg. several linker-reporter moieties attached to one vector or several reporters attached to one linker itself attached to one vector) may enable the detectability of the contrast agent to be increased (eg. by increasing its radioopacity, echogenicity or relaxivity) or may enable it to be detected in more than one imaging modality. Use in this way of a plurality of vectors may increase the targeting efficiency of the contrast agent or may make the contrast agent able to target more than one site, eg. different receptors for an agent which has receptor heterogeneity. Thus for example the agent of formula I may include vector moieties with affinity sites other than endothelin receptors, eg. with affinities for cell surfaces on body duct wall surfaces. Accordingly, the agent may include vectors such as antibody fragments and oligopeptides, eg. containing RGD or analogous cell surface binding peptide motifs (for example as described in EP-A-422937 and EP-A-422938 (Merck)) or other vectors as described in GB 9700699.3. Such extra vectors may also be selected from any of the molecules that naturally concentrate in a selected target organ, tissue, cell or group of cells, or other location in a mammalian body, in vivo . These can include amino acids, oligopeptides (e.g. hexapeptides) , molecular recognition units (MRU's), single chain antibodies (SCA's), proteins, non-peptide organic molecules, Fab fragments, and antibodies. Examples of site-directed molecules include polysaccharides (e.g. CCK and hexapeptides) , proteins (such as lectins, asialofetuin, polyclonal IgG, blood clotting proteins (e.g. hirudin) , lipoproteins and glycoproteins) , hormones, growth factors, clotting factors (such as PF4) , polymerized fibrin fragments (e.g., E x ) , serum amyloid precursor (SAP) proteins, low density lipoprotein (LDL) precursors, serum albumin, surface proteins of intact red blood cells, receptor binding molecules such as estrogens, liver-specific proteins/polymers such as galactosyl-neoglycoalbumin (NGA) (see Vera et al . in Radiology 151: 191 (1984)) N- (2-hydroxy-propyl) methacrylamide (HMPA) copolymers with varying numbers of bound galactosamines (see Duncan et al., Biochim. Biophys. Acta 880 : 62 (1986)), and allyl and 6-aminohexyl glycosides (see Wong et al . , Carbo. Res. 170.:27 (1987)), and fibrinogen. The site-directed protein can also be an antibody. The choice of antibody, particularly the antigen specificity of the antibody, will depend on the particular intended target site for the agent. Monoclonal antibodies are preferred over polyclonal antibodies. Preparation of antibodies that react with a desired antigen is well known. Antibody preparations are available commercially from a variety of sources . Fibrin fragment E λ can be prepared as described by Olexa et al . in J. Biol. Chem. 254 :4925 (1979) . Preparation of LDL precursors and SAP proteins is described by de Beer et al . in J. Immunol. Methods 5.0:17 (1982) . The above described articles are incorporated herein by reference in their entirety. It is especially preferred that such extra vectors should bind so as to slow but not prevent the motion of the agent in the blood stream and to anchor it in place when it is bound to an endothelin receptor site. Linker The linker component of the contrast agent is at its simplest a bond between the vector and reporter moieties. More generally however the linker will provide a mono- or multi-molecular skeleton covalently or non-covalently linking one or more vectors to one or more reporters, eg. a linear, cyclic, branched or reticulate molecular skeleton, or a molecular aggregate, with in-built or pendant groups which bind covalently or non-covalently, eg. coordinatively, with the vector and reporter moieties or which encapsulate, entrap or anchor such moieties. Thus linking of a reporter unit to a desired vector may be achieved by covalent or non-covalent means, usually involving interaction with one or more functional groups located on the reporter and/or vector. Examples of chemically reactive functional groups which may be employed for this purpose include amino, hydroxyl, sulfhydryl, carboxyl, and carbonyl groups, as well as carbohydrate groups, vicinal diols, thioethers, 2- aminoalcohols, 2-aminothiols, guanidinyl, imidazolyl and phenolic groups. Covalent coupling of reporter and vector may therefore be effected using linking agents containing reactive moities capable of reaction with such functional groups. Examples of reactive moieties capable of reaction with sulfhydryl groups include α-haloacetyl compounds of the type X-CH 2 CO- (where X=Br, Cl or I) , which show particular reactivity for sulfhydryl groups but which can also be used to modify imidazolyl, thioether, phenol and amino groups as described by Gurd, F.R.N. in Methods Enzymol . (1967) 11, 532. N-Maleimide derivatives are also considered selective towards sulfhydryl groups, but may additionaly be useful in coupling to amino groups under certain conditions. Reagents such as 2- iminothiolane, e.g. as described by Traut , R. et al . in Biochemistry (1973) 12, 3266, which introduce a thiol group through conversion of an amino group, may be considered as sulfhydryl reagents if linking occurs through the formation of disulphide bridges. Thus reagents which introduce reactive disulphide bonds into either the reporter or the vector may be useful, since linking may be brought about by disulphide exchange between the vector and reporter; examples of such reagents include Ellman's reagent (DTNB) , 4,4'- dithiodipyridine, methyl-3-nitro-2-pyridyl disulphide and methyl-2 -pyridyl disulphide (described by Kimura, T. et al . in Analyt. Biochem. (1982) 122, 271) . Examples of reactive moieties capable of reaction with amino groups include alkylating and acylating agents. Representative alkylating agents include: i) α-haloacetyl compounds, which show specificity towards amino groups in the absence of reactive thiol groups and are of the type X-CH 2 CO- (where X=C1 , Br or I), e.g. as described by Wong, Y-H.H. in Biochemistry (1979) 24, 5337; ii) N-maleimide derivatives, which may react with amino groups either through a Michael type reaction or through acylation by addition to the ring carbonyl group as described by Smyth, D.G. et al . in J. Am. Chem. Soc . (1960) 82, 4600 and Biochem . J. (1964) 91, 589; iii) aryl halides such as reactive nitrohaloaromatic compounds ; iv) alkyl halides as described by McKenzie, J.A. et al. in J. Protein Chem. (1988) 7, 581; v) aldehydes and ketones capable of Schiff 's base formation with amino groups, the adducts formed usually being stabilised through reduction to give a stable amine ; vi) epoxide derivatives such as epichlorohydrin and bisoxiranes, which may react with amino, sulfhydryl or phenolic hydroxyl groups; vii) chlorine-containing derivatives of s-triazines, which are very reactive towards nucleophiles such as amino, sufhydryl and hydroxy groups ; viii) aziridines based on s-triazine compounds dET A iled above, e.g. as described by Ross, W.C.J. in Adv. Cancer Res . (1954) 2, 1, which react with nucleophiles such as amino groups by ring opening; ix) squaric acid diethyl esters as described by Tietze, L.F. in Chem . Ber. (1991) 124, 1215; and x) α-haloalkyl ethers, which are more reactive alkylating agents than normal alkyl halides because of the activation caused by the ether oxygen atom, e.g. as described by Benneche, T. et al. in Eur. J. Med. Chem. ( 1993 ) 28 , 463 . Representative amino-reactive acylating agents include: i) isocyanates and isothiocyanates, particularly aromatic derivatives, which form stable urea and thiourea derivatives respectively and have been used for protein crosslinking as described by Schick, A.F. et al . in J " . Biol . Chem . (1961) 236, 2477; ii) sulfonyl chlorides, which have been described by Herzig, D.J. et al . in Biopolymerε (1964) 2, 349 and which may be useful for the introduction of a fluorescent reporter group into the linker; iii) Acid halides; iv) Active esters such as nitrophenylesters or N- hydroxysuccinimidyl esters; v) acid anhydrides such as mixed, symmetrical or N- carboxyanhydrides ; vi) other useful reagents for amide bond formation as described by Bodansky, M. et al . in ^ Principles of Peptide Synthesis ' (1984) Springer-Verlag; vii) acylazides, e.g. wherein the azide group is generated from a preformed hydrazide derivative using sodium nitrite, e.g. as described by Wetz, K. et al . in Anal . Biochem . (1974) 58, 347; viii) azlactones attached to polymers such as bis- acrylamide, e.g. as described by Rasmussen, J.K. in .Reactive Polymers (1991) 16, 199; and ix) Imidoesters , which form stable amidines on reaction with amino groups, e.g. as described by Hunter, M.J. and Ludwig, M.L. in J " . Am. Chem . Soc . (1962) 84, 3491. Carbonyl groups such as aldehyde functions may be reacted with weak protein bases at a pH such that nucleophilic protein side-chain functions are protonated. Weak bases include 1, 2-aminothiols such as those found in N-terminal cysteine residues, which selectively form stable 5 -membered thiazolidine rings with aldehyde groups, e.g. as described by Ratner, S. et al . in J " . Am. Chem . Soc . (1937) 59, 200. Other weak bases such as phenyl hydrazones may be used, e.g. as described by Heitzman, H. et al. in Proc . Natl . Acad . Sci . USA (1974) 71, 3537. Aldehydes and ketones may also be reacted with amines to form Schiff 's bases, which may advantageously be stabilised through reductive amination. Alkoxylamino moieties readily react with ketones and aldehydes to produce stable alkoxamines, e.g. as described by Webb, R. et al . in Bioconjugate Chem . (1990) 1, 96. Examples of reactive moieties capable of reaction with carboxyl groups include diazo compounds such as diazoacetate esters and diazoacetamides, which react with high specificity to generate ester groups, e.g. as described by Herriot R.M. in Adv. Protein Chem . (1947) 3, 169. Carboxylic acid modifying reagents such as carbodiimides, which react through O-acylurea formation followed by amide bond formation, may also usefully be employed; linking may be facilitated through addition of an amine or may result in direct vector-receptor coupling. Useful water soluble carbodiimides include 1- cyclohexyl-3- (2-morpholinyl-4-ethyl) carbodiimide (CMC) and l-ethyl-3- (3-dimethylaminopropyl) carbodiimide (EDC) , e.g. as described by Zot, H.G. and Puett, D. in J " . Biol . Chem . (1989) 264, 15552. Other useful carboxylic acid modifying reagents include isoxazolium derivatives such as Woodwards reagent K; chloroformates such as p- nitrophenylchloroformate; carbonyldiimidazoles such as 1, 1 ' -carbonyldiimidazole; and N- carbalkoxydihydroquinolines such as N- (ethoxycarbonyl) - 2-ethoxy-l, 2-dihydroquinoline . Other potentially useful reactive moieties include vicinal diones such as p-phenylenediglyoxal , which may be used to react with guanidinyl groups, e.g. as described by Wagner et al . in Nucleic acid Res . (1978) 5, 4065; and diazonium salts, which may undergo electrophilic substitution reactions, e.g. as described by Ishizaka, K. and Ishizaka T. in J " . Immunol . (1960) 85, 163. Bis-diazonium compounds are readily prepared by treatment of aryl diamines with sodium nitrite in acidic solutions. It will be appreciated that functional groups in the reporter and/or vector may if desired be converted to other functional groups prior to reaction, e.g. to confer additional reactivity or selectivity. Examples of methods useful for this purpose include conversion of amines to carboxylic acids using reagents such as dicarboxylic anhydrides; conversion of amines to thiols using reagents such as N-acetylhomocysteine thiolactone, S-acetylmercaptosuccinic anhydride, 2- iminothiolane or thiol-containing succinimidyl derivatives; conversion of thiols to carboxylic acids using reagents such as -haloacetates ; conversion of thiols to amines using reagents such as ethylenimine or 2-bromoethylamine; conversion of carboxylic acids to amines using reagents such as carbodiimides followed by diamines; and conversion of alcohols to thiols using reagents such as tosyl chloride followed by transesterification with thioacetate and hydrolysis to the thiol with sodium acetate. Vector-reporter coupling may also be effected using enzymes as zero-length crosslinking agents; thus, for example, transglutaminase, peroxidase and xanthine oxidase have been used to produce crosslinked products. Reverse proteolysis may also be used for crosslinking through amide bond formation. Non-covalent vector-reporter coupling may, for example, be effected by electrostatic charge interactions, through chelation in the form of stable metal complexes or through high affinity binding interaction. A vector which is coupled to a peptide, lipo- oligosaccharide or lipopeptide linker which contains a element capable of mediating membrane insertion may also be useful. One example is described by Leenhouts, J.M. et al . in Febs Letters (1995) 370(3), 189-192. Coupling may also be effected using avidin or streptavidin, which have four high affinity binding sites for biotin. Avidin may therefore be used to conjugate vector to reporter if both vector and reporter are biotinylated. Examples are described by Bayer, E.A. and Wilchek, M. in Methods Biochem . Anal . (1980) 26, 1. This method may also be extended to include linking of reporter to reporter, a process which may encourage association of the agent and consequent potentially increased efficacy. Alternatively, avidin or streptavidin may be attached directly to the surface of reporter particles. Non-covalent coupling may also utilise the bifunctional nature of bispecific immunoglobulins. These molecules can specifically bind two antigens, thus linking them. For example, either bispecific IgG or chemically engineered bispecific F(ab) '2 fragments may be used as linking agents. Heterobifunctional bispecific antibodies have also been reported for linking two different antigens, e.g. as described by Bode, C. et al. in J " . Biol . Chem . (1989) 264, 944 and by Staerz, U.D. et al . in Proc . Natl . Acad. Sci . USA (1986) 83, 1453. Similarly, any reporter and/or vector containing two or more antigenic determinants (e.g. as described by Chen, Aa et al . in Am. J. Pathol . (1988) 130, 216) may be crosslinked by antibody molecules and lead to formation of cross-linked assemblies of agents of formula I of potentially increased efficacy. So-called zero-length linking agents, which induce direct covalent joining of two reactive chemical groups without introducing additional linking material (e.g. as in amide bond formation induced using carbodiimides or enzymatically) may, if desired, be used in accordance with the invention, as may agents such as biotin/avidin systems which induce non-covalent reporter-vector linking and agents which induce electrostatic interactions . Most commonly, however, the linking agent will comprise two or more reactive moieties, e.g. as described above, connected by a spacer element . The presence of such a spacer permits bifunctional linkers to react with specific functional groups within a molecule or between two different molecules, resulting in a bond between these two components and introducing extrinsic linker- derived material into the reporter-vector conjugate. The reactive moieties in a linking agent may be the same (homobifunctional agents) or different (heterobifunctional agents or, where several dissimilar reactive moieties are present, heteromultifunctional agents) , providing a diversity of potential reagents that may bring about covalent bonding between any chemical species, either intramolecularly or intermolecularly. The nature of extrinsic material introduced by the linking agent may have a critical bearing on the targeting ability and general stability of the ultimate product. Thus it may be desirable to introduce labile linkages, e.g. containing spacer arms which are biodegradable or chemically sensitive or which incorporate enzymatic cleavage sites. Alternatively the spacer may include polymeric components, e.g. to act as surfactants and enhance the stability of the agent. The spacer may also contain reactive moieties, e.g. as described above to enhance surface crosslinking. Spacer elements may typically consist of aliphatic chains which effectively separate the reactive moieties of the linker by distances of between 5 and 30 A. They may also comprise macromolecular structures such as poly (ethylene glycols) . Such polymeric structures, hereinafter referred to as PEGs, are simple, neutral polyethers which have been given much attention in biotechnical and biomedical applications (see e.g. Milton Harris, J. (ed) "Poly (ethylene glycol ) chemistry, biotechnical and biomedical applications" Plenum Press, New York, 1992) . PEGs are soluble in most solvents, including water, and are highly hydrated in aqueous environments, with two or three water molecules bound to each ethylene glycol segment; this has the effect of preventing adsorption either of other polymers or of proteins onto PEG-modified surfaces. PEGs are known to be nontoxic and not to harm active proteins or cells, whilst covalently linked PEGs are known to be non- immunogenic and non-antigenic. Furthermore, PEGs may readily be modified and bound to other molecules with only little effect on their chemistry. Their advantageous solubility and biological properties are apparent from the many possible uses of PEGs and copolymers thereof, including block copolymers such as PEG-polyurethanes and PEG-polypropylenes . Appropriate molecular weights for PEG spacers used in accordance with the invention may, for example, be between 120 Daltons and 20 kDaltons. The major mechanism for uptake of particles by the cells of the reticuloendothelial system (RES) is opsonisation by plasma proteins in blood; these mark foreign particles which are then taken up by the RES. The biological properties of PEG spacer elements used in accordance with the invention may serve to increase the circulation time of the agent in a similar manner to that observed for PEGylated liposomes (see e.g. Klibanov, A.L. et al . in FEBS Letters (1990) 268, 235- 237 and Blume, G. and Cevc, G. in Biochim. Biophys . Acta (1990) 1029, 91-97) . Increased coupling efficiency to areas of interest may also be achieved using antibodies bound to the terminii of PEG spacers (see e.g. Maruyama, K. et al . in Biochim . Biophys . Acta (1995) 1234, 74-80 and Hansen, C.B. et al . in Biochim . Biophys . Acta (1995) 1239, 133-144) . Other representative spacer elements include structural - type polysaccharides such as polygalacturonic acid, glycosaminoglycans, heparinoids, cellulose and marine polysaccharides such as alginates, chitosans and carrageenans ; storage-type polysaccharides such as starch, glycogen, dextran and aminodextrans; polyamino acids and methyl and ethyl esters thereof, as in homo- and co-polymers of lysine, glutamic acid and aspartic acid; and polypeptides, oligosaccharides and oligonucleotides, which may or may not contain enzyme cleavage sites. In general, spacer elements may contain cleavable groups such as vicinal glycol, azo, sulfone, ester, thioester or disulphide groups. Spacers containing biodegradable methylene diester or diamide groups of formula -(Z) ra .Y.X.C(R 1 R 2 ) .X.Y. (Z) n - [where X and Z are selected from -0-, -S-, and -NR- (where R is hydrogen or an organic group) ; each Y is a carbonyl, thiocarbonyl, sulphonyl, phosphoryl or similar acid-forming group: m and n are each zero or 1; and R 1 and R 2 are each hydrogen, an organic group or a group -X.Y. (Z) m -, or together form a divalent organic group] may also be useful; as discussed in, for example, WO-A- 92/17436 such groups are readily biodegraded in the presence of esterases, e.g. in vivo, but are stable in the absence of such enzymes. They may therefore advantageously be linked to therapeutic agents to permit slow release thereof. Poly [N- (2-hydroxyethyl)methacrylamides] are potentially useful spacer materials by virtue of their low degree of interaction with cells and tissues (see e.g. Volfova, I., Rihova, B. and V.R. and Vetvicka, P. in J " . Bioact . Comp . Polymers (1992) 7, 175-190) . Work on a similar polymer consisting mainly of the closely related 2- hydroxypropyl derivative showed that it was endocytosed by the mononuclear phagocyte system only to a rather low extent (see Goddard, P., Williamson, I., Bron, J. , Hutchkinson, L.E., Nicholls, J. and Petrak, K. in J. Bioct . Compat . Polym . (1991) 6, 4-24.). Other potentially useful poymeric spacer materials include : i) copolymers of methyl methacrylate with methacrylic acid; these may be erodible (see Lee, P.I. in Pharm. Res . (1993) 10, 980) and the carboxylate substituents may cause a higher degree of swelling than with neutral polymers ; ii) block copolymers of polymethacrylates with biodegradable polyesters (see e.g. San Roman, J. and Guillen-Garcia, P. in Biomaterials (1991) 12, 236-241); iii) cyanoacrylates, i.e. polymers of esters of 2- cyanoacrylic acid - these are biodegradable and have been used in the form of nanoparticles for selective drug delivery (see Forestier, F., Gerrier, P., Chaumard, C, Quero, A.M., Couvreur, P. and Labarre, C. in J " . Aπtimicro . Chemoter. (1992) 30, 173-179) ; iv) polyvinyl alcohols, which are water-soluble and generally regarded as biocompatible (see e.g. Langer, R. in J " . Control . Release (1991) 16, 53-60); v) copolymers of vinyl methyl ether with maleic anhydride, which have been stated to be bioerodible (see Finne, U. , Hannus, M. and Urtti, A. in Int . J. Pharm . (1992) 78. 237-241) ; vi) polyvinylpyrrolidones, e.g. with molecular weight less than about 25,000, which are rapidly filtered by the kidneys (see Hespe, W. , Meier, A. M. and Blankwater, Y. M. in Arzeim . -Forsch . /Drug Res . (1977) 27, 1158-1162) ; vii) polymers and copolymers of short-chain aliphatic hydroxyacids such as glycolic, lactic, butyric, valeric and caproic acids (see e.g. Carli, F. in Chim. Ind. (Milan) (1993) 75, 494-9) , including copolymers which incorporate aromatic hydroxyacids in order to increase their degradation rate (see Imasaki, K. , Yoshida, M. , Fukuzaki, H., Asano, M. , Kumakura, M. , Mashimo, T., Yamanaka, H. and Nagai . T. in Int . J. Pharm. (1992) 81, 31-38) ; viii) polyesters consisting of alternating units of ethylene glycol and terephthalic acid, e.g. Dacron R , which are non-degradable but highly biocompatible; ix) block copolymers comprising biodegradable segments of aliphatic hydroxyacid polymers (see e.g. Younes, H. , Nataf, P.R., Cohn, D. , Appelbaum, Y.J., Pizov, G. and Uretzky, G. in Biomater. Artif . Cells Artif . Organs (1988) 16, 705-719) , for instance in conjunction with polyurethanes (see Kobayashi , H. , Hyon, S.H. and Ikada, Y. in "Water-curable and biodegradable prepolymers" - J " . Biomed . Mater. Res . (1991) 25, 1481-1494); x) polyurethanes, which are known to be well-tolerated in implants, and which may be combined with flexible "soft" segments, e.g. comprising poly(tetra methylene glycol), poly (propylene glycol) or poly (ethylene glycol)) and aromatic "hard" segments, e.g. comprising 4 , 4 ' -methylenebis (phenylene isocyanate) (see e.g. Ratner, B.D., Johnston, A.B. and Lenk, T.J. in J " . Biomed . Mater. Res : Applied Biomaterials (1987) 21, 59- 90; Sa Da Costa, V. et al . in J. Coll . Interface Sci . (1981) 80, 445-452 and Affrossman, S. et al . in Clinical Materials (1991) 8, 25-31); xi) poly (1, 4-dioxan-2-ones) , which may be regarded as biodegradable esters in view of their hydrolysable ester linkages (see e.g. Song, C. X., Cui, X. M. and Schindler, A. in Med. Biol . Eng. Comput . (1993) 31, S147-150) , and which may include glycolide units to improve their absorbability (see Bezwada, R.S., Shalaby, S.W. and Newman, H.D.J. in Agricul tural and synthetic polymers : Biodegradabili ty and utilization (1990) (ed Glass, J.E. and Swift, G.), 167-174 - ACS symposium Series, #433, Washington D.C., U.S.A. - American Chemical Society) ; xii) polyanhydrides such as copolymers of sebacic acid (octanedioic acid) with bis (4-carboxy-phenoxy) propane, which have been shown in rabbit studies (see Brem, H., Kader, A., Epstein, J.I., Tamargo, R.J., Domb, A., Langer, R. and Leong, K.W. in Sel . Cancer Ther. (1989) 5, 55-65) and rat studies (see Tamargo, R.J., Epstein, J.I., Reinhard, C.S., Chasin, M. and Brem, H. in J " . Biomed. Mater. Res . (1989) 23, 253-266) to be useful for controlled release of drugs in the brain without evident toxic effects; xiii) biodegradable polymers containing ortho-ester groups, which have been employed for controlled release in vivo (see Maa, Y.F. and Heller, J. in J " . Control . Release (1990) 14, 21-28); and xiv) polyphosphazenes, which are inorganic polymers consisting of alternate phosphorus and nitrogen atoms (see Crommen, J.H. , Vandorpe, J. and Schacht, E.H. in J". Control . Release (1993) 24, 167-180). The following tables list linking agents which may be useful in targetable agents in accordance with the invention. Heterobifunctional linking aσents Notes: (1) =iodinable; (2) =fluorescent Homobifunctional linking agents Biotinylation agents Notes : DPPE=dipalmitoylphosphatidylethanolamine ; LC=long chain Agents for protein modification In addition to the already contemplated straight chain and branched PEG-like linkers (e.g polyethylene glycols and other containing 2 to 100 recurring units of ethylene oxide) , linkers in the VLR system can be independently a chemical bond or the residue of a linking group. The phrase "residue of a linking group" as used herein refers to a moiety that remains, results, or is derived from the reaction of a vector reactive group with a reactive site on a vector. The phrase "vector reactive group" as used herein refers to any group which can react with functional groups typically found on vectors, the derivatization of which only minimally effects the ability of the vector to bind to its receptor. However, it is specifically contemplated that such vector reactive groups can also react with functional groups typically found on relevant protein molecules. Thus, in one aspect the linkers useful in the practice of this invention derive from those groups which can react with any relevant molecule which comprises a vector as described above containing a reactive group, whether or not such relevant molecule is a protein, to form a linking group. Preferred linking groups are derived from vector reactive groups selected from but not limited to:- a group that will react directly with carboxy, aldehyde, amine (NHR) , alcohols, sulfhydryl groups, activated methylenes and the like, on the vector, for example, active halogen containing groups including, for example, chloromethylphenyl groups and chloroacetyl [ClCH 2 C (=0) -] groups, activated 2- (leaving group substituted) - ethylsulfonyl and ethylcarbonyl groups such as 2- chloroethylsulfonyl and 2-chloroethylcarbonyl ; vinylsulfonyl ; vinylcarbonyl ; epoxy; isocyanato; isothiocyanato; aldehyde; aziridine; succinimidoxycarbonyl; activated acyl groups such as carboxylic acid halides; mixed anhydrides and the like. A group that can react readily with modified vector molecules containing a vector reactive group, i.e., vectors containing a reactive group modified to contain reactive groups such as those mentioned in (1) above, for example, by oxidation of the vector to an aldehyde or a carboxylic acid, in which case the "linking group" can be derived from reactive groups selected from amino, alkylamino, arylamino, hydrazino, alkylhydrazino, arylhydrazino, carbazido, semicarbazido, thiocarbazido, thiosemicarbazido, sulfhydryl, sulfhydrylalkyl, sulfhydrylaryl, hydroxy, carboxy, carboxyalkyl and carboxyaryl . The alkyl portions of said linking groups can contain from 1 to about 20 carbon atoms. The aryl portions of said linking groups can contain from about 6 to about 20 carbon atoms; and a group that can be linked to the vector containing a reactive group, or to the modified vector as noted above by use of a crosslinking agent. The residues of certain useful crosslinking agents, such as, for example, homobifunctional and heterobifunctional gelatin hardeners, bisepoxides, and bisisocyanates can become a part of a linking group during the crosslinking reaction. Other useful crosslinking agents, however, can facilitate the crosslinking, for example, as consumable catalysts, and are not present in the final conjugate. Examples of such crosslinking agents are carbodiimide and carbamoylonium crosslinking agents as disclosed in U.S. Patent No. 4,421,847 and the ethers of U.S. Patent No. 4,877,724. With these crosslinking agents, one of the reactants such as the vector must have a carboxyl group and the other such as a long chain spacer must have a reactive amine, alcohol, or sulfhydryl group. In amide bond formation, the crosslinking agent first reacts selectively with the carboxyl group, then is split out during reaction of the thus "activated" carboxyl group with an amine to form an amide linkage between thus covalently bonding the two moieties. An advantage of this approach is that crosslinking of like molecules, e.g., vector to vector is avoided, whereas the reaction of, for example, homo- bifunctional crosslinking agents is nonselective and unwanted crosslinked molecules are obtained. Preferred useful linking groups are derived from various heterobifunctional cross-linking reagents such as those listed in the Pierce Chemical Company Immunotechnology Catalog - Protein Modification Section, (1995 and 1996) . Useful non-limiting examples of such reagents include: Sulfo-SMCC Sulfosuccinimidyl 4- (N- maleimidomethyl) cyclohexane-1-carboxylate . Sulfo-SIAB Sulfosuccinimidyl (4- iodoacetyl) aminobenzoate . Sulfo-SMPB Sulfosuccinimidyl 4- (p- maleimidophenyl) butyrate . 2-IT 2-Iminothiolane. SATA N-Succinimidyl S-acetylthioacetate . In addition to the foregoing description, the linking groups, in whole or in part, can also be comprised of and derived from complementary sequences of nucleotides and residues of nucleotides, both naturally occurring and modified, preferably non-self-associating oligonucleotide sequences. Particularly useful, non- limiting reagents for incorporation of modified nucleotide moieties containing reactive functional groups, such as amine and sulfhydryl groups, into an oligonucleotide sequence are commercially available from, for example, Clontech Laboratories Inc. (Palo Alto California) and include Uni-Link AminoModifier (Catalog # 5190) , Biotin-ON phosphoramidite (Catalog # 5191) , N- MNT-C6-AminoModifier (Catalog # 5202) , AminoModifier II (Catalog # 5203), DMT-C6-3 ' Amine-ON (Catalog # 5222), C6-ThiolModifier (Catalog # 5211), and the like. In one aspect, linking groups of this invention are derived from the reaction of a reactive functional group such as an amine or sulfhydryl group as are available in the above Clontech reagents, one or more of which has been incorporated into an oligonucleotide sequence, with, for example, one or more of the previously described vector reactive groups such as a heterobifunctional group on the vector. By attaching two complementary oligonucleotide sequences one to the vector and the other to the reporter the resulting double-stranded hybridized oligonucleotide then comprises the linking group between the vector and reporter. Other polymer systems that serve as linkers include :- Poly(L or D or DL- amino acids) = proteins and peptides; naturally occuring or synthetic Pseudo Poly (amino acids) = (amino acids linked by non- amide bonds) Poly (L or D or DL-lactide) and the co-polymers e.g Poly (L -lactide/DL -lactide) Poly (glycolide) L-lactide/glycolide co-polymers Poly-caprolactone and its co-polymers Polyanhydrides Poly (ortho esters) Polyphosphazenes Long-chain straight or branched lipids (& phospholipids) Sugars and carbohydrates Oligonucleotides (see above) as well as mixtures of the above. Linking agents used in accordance with the invention will in general bring about linking of vector to reporter or reporter to reporter with some degree of specificity, and may also be used to attach one or more therapeutically active agents. The present invention accordingly provides a tool for therapeutic drug delivery in combination with vector- mediated direction of the product to the desired site. By "therapeutic" or "drug" is meant an agent having a beneficial effect on a specific disease in a living human or non-human animal. Therapeutic compounds used in accordance with the present invention may be encapsulated in the interior of a molecular aggregate or particulate linker or attached to or incorporated in the encapsulating walls of a vesicular linker. Thus, the therapeutic compound may be linked to a part of the surface, for example through covalent or ionic bonds, or may be physically mixed into an encapsulating material, particularly if the drug has similar polarity or solubility to the material, so as to prevent it from leaking out of the product before it is intended to act in the body. The release of the drug may be initiated merely by wetting contact with blood following administration or as a consequence of other internal or external influences, e.g. dissolution processes catalyzed by enzymes or the use of where the linker-reporter is a gas containing vesicle. The therapeutic substance may be covalently linked to the encapsulating membrane surface of a vesicular linker using a suitable linking agent, e.g. as described herein. Thus, for example, one may initially prepare a phospholipid derivative to which the drug is bonded through a biodegradable bond or linker, and then incorporate this derivative into the material used to prepare the vesicle membrane, as described above. Alternatively, the agent may initially be prepared without the therapeutic, which may then be coupled to or coated onto particulate (eg. vesicular) agents prior to use. Thus, for example, a therapeutic could be added to a suspension of microbubbles in aqueous media and shaken in order to attach or adhere the therapeutic to the microbubbles . The therapeutic may for example be a drug or prodrug known for use in combatting restenosis, hypertension, stroke, congestive heart failure or other cadiovascular therapy, or any other drug or prodrug having ET- combatting effects. By targeting an agent according to the invention containing a prodrug-activating enzyme to areas of pathology one may image targeting of the enzyme, making it possible to visualise when the agent is targeted properly and when the agent has disappeared from non- target areas. In this way one can determine the optimal time for injection of prodrug into individual patients. Another alternative is to incorporate a prodrug, a prodrug-activating enzyme and a vector in the same particulate linker reporter in such a way that the prodrug will only be activated after some external stimulus. Such a stimulus may, for example, be a bursting of vesicles by external ultrasound, light stimulation of a chromophoric reporter, or magnetic heating of a superparamagnetic reporter after the desired targeting has been achieved. So-called prodrugs may also be used in agents according to the invention. Thus drugs may be derivatised to alter their physicochemical properties and to adapt agent of the invention; such derivatised drugs may be regarded as prodrugs and are usually inactive until cleavage of the derivatising group regenerates the active form of the drug. Therapeutics may easily be delivered in accordance with the invention to the heart and vasculature in general, and to the liver, spleen and kidneys and other regions such as the lymph system, body cavities or gastrointestinal system. By way of example, where the reporter is a chelated metal species (eg. a paramagnetic metal ion or a metal radionuclide) , the linker may comprise a chain attached to a metal chelating group, a polymeric chain with a plurality of metal chelating groups pendant from the molecular backbone or incorporated in the molecular backbone, a branched polymer with metal chelating groups at branch termini (eg. a dendrimeric polychelant) , etc. What is required of the linker is simply that it bind the vector and reporter moieties together for an adequate period. By adequate period is meant a period sufficient for the contrast agent to exert its desired effects, eg. to enhance contrast in vivo during a diagnostic imaging procedure. Thus, in certain circumstances, it may be desirable that the linker biodegrade after administration. By selecting an appropriately biodegradable linker it is possible to modify the biodistribution and bioelimination patterns for the vector and/or reporter. Where vector and/or reporter are biologically active or are capable of exerting undesired effects if retained after the imaging procedure is over, it may be desirable to design in linker biodegradability which ensures appropriate bioelimination or metabolic breakdown of the vector and/or reporter moieties. Thus a linker may contain a biodegradable function which on breakdown yields breakdown products with modified biodistribution patterns which result from the release of the reporter from the vector or from fragmentation of a macromolecular structure. By way of example for linkers which carry chelated metal ion reporters it is possible to have the linker incorporate a biodegradable function which on breakdown releases an excretable chelate compound containing the reporter. Accordingly, biodegradable functions may if desired be incorporated within the linker structure, preferably at sites which are (a) branching sites, (b) at or near attachment sites for vectors or reporters, or (c) such that biodegradation yields physiologically tolerable or rapidly excretable fragments. Examples of suitable biodegradable functions include ester, amide, double ester, phosphoester, ether, thioether, guanidyl , acetal and ketal functions. As discussed above, the linker group may if desired have built into its molecular backbone groups which affect the biodistribution of the contrast agent or which ensure appropriate spatial conformation for the contrast agent, eg. to allow water access to chelated paramagnetic metal ion reporters. By way of example the linker backbone may consist in part or essentially totally of one or more polyalkylene oxide chains. Thus the linker may be viewed as being a composite of optionally biodegradable vector binding (V b ) and reporter binding (R b ) groups joined via linker backbone (L b ) groups, which linker backbone groups may carry linker side chain (L sc ) groups to modify biodistribution etc. and may themselves incorporate biodegradable functions. The R b and V b binding groups may be pendant from the linker backbone or may be at linker backbone termini, for example with one R b or V b group at one L b terminus, with R b or V b groups linking together two L b termini or with one L b terminus carrying two or more R b or V b groups. The L b and L sc groups will conveniently be oligomeric or polymeric structures (eg. polyesters, polyamides, polyethers, polyamines, oligopeptides, polypeptides, oligo and polysaccharides, oligonucleotides, etc.), preferably structures having at least in part a hydrophilic or lipophilic nature, eg. hydrophilic, amphiphilic or lipophilic structures. The linker may be low, medium or high molecular weight, eg. up to 2MD. Generally higher molecular weight linkers will be preferred if they are to be loaded with a multiplicity of vectors or reporters or if it is necessary to space vector and reporter apart, or if the linker is itself to serve a role in the modification of biodistribution. In general however linkers will be from 100 to 100 000 D, especially 120 D to 20 kD in molecular weight. Conjugation of linker to vector and linker to reporter may be by any appropriate chemical conjugation technique, eg. covalent bonding (for example ester or amide formation) , metal chelation or other metal coordinative or ionic bonding, again as described above. Examples of suitable linker systems include the magnifier polychelant structures of US-A-5364613 and WO90/12050, polyaminoacids (eg. polylysine), functionalised PEG, polysaccharides, glycosaminoglycans, dendritic polymers such as described in WO93/06868 and by Tomalia et al . in Angew. Chem. Int. Ed. Engl . 29 : 138- 175 (1990) , PEG-chelant polymers such as described in W94/08629, WO94/09056 and W096/26754, etc. Where the reporter is a chelated metal ion, the linker group will generally incorporate the chelant moiety. Alternatively, the chelated metal may be carried on or in a particulate reporter. In either case, conventional metal chelating groups such as are well known in the fields of radiopharmaceuticals and MRI contrast media may be used, eg. linear, cyclic and branched polyamino- polycarboxylic acids and phosphorus oxyacid equivalents, and other sulphur and/or nitrogen ligands known in the art, eg. DTPA, DTPA-BMA, EDTA, D03A, TMT (see for example US-A-5367080) , BAT and analogs (see for example Ohmono et al . , J. Med. Chem. 15: 157-162 (1992) and Kung et al. J. Nucl. Med. 25: 326-332 (1984)), the N 2 S 2 chelant ECD of Neurolite, MAG (see Jurisson et al . Chem. Rev. 13: 1137-1156 (1993)), HIDA, DOXA (l-oxa-4 , 7 , 10- triazacyclododecanetriacetic acid), NOTA (1,4,7- triazacyclononanetriacetic acid), TETA (1,4,8,11- tetraazacyclotetradecanetetraacetic acid), THT 4'-(3- amino-4-methoxy-phenyl) -6, 6 "-bis (N' ,N' -dicarboxymethyl- N-methylhydrazino) -2,2 ■ :6 ' , 2" -terpyridine) , etc. In this regard, the reader is referred to the patent literature of Sterling Winthrop, Nycomed (including Nycomed Imaging and Nycomed Salutar) , Schering, Mallinckrodt, Bracco and Squibb relating to chelating agents for diagnostic metals, eg. in MR, X-ray and radiodiagnostic agents. See for example US-A-4647447 , EP-A-71564, US-A-4687659, WO89/00557, US-A-4885363, and EP-A-232751. Reporters The reporter moieties in the contrast agents of the invention may be any moiety capable of detection either directly or indirectly in an in vivo diagnostic imaging procedure, eg. moieties which emit or may be caused to emit detectable radiation (eg. by radioactive decay, fluorescence excitation, spin resonance excitation, etc.), moieties which affect local electromagnetic fields (eg. paramagnetic, superparamagnetic, ferrimagnetic or ferromagnetic species) , moieties which absorb or scatter radiation energy (eg. chromophores, particles (including gas or liquid containing vesicles) , heavy elements and compounds thereof, etc.), and moieties which generate a detectable substance (eg. gas microbubble generators), etc. A very wide range of materials detectable by diagnostic imaging modalities is known from the art and the reporter will be selected according to the imaging modality to be used. Thus for example for ultrasound imaging an echogenic material, or a material capable of generating an echogenic material will normally be selected, for X-ray imaging the reporter will generally be or contain a heavy atom (eg. of atomic weight 38 or above) , for MR imaging the reporter will either be a non zero nuclear spin isotope (such as 19 F) or a material having unpaired electron spins and hence paramagnetic, superparamagnetic, ferrimagnetic or ferromagnetic properties, for light imaging the reporter will be a light scatterer (eg. a coloured or uncoloured particle) , a light absorber or a light emitter, for magnetometric imaging the reporter will have detectable magnetic properties, for electrical impedance imaging the reporter will affect electrical impedance and for scintigraphy, SPECT, PET etc. the reporter will be a radionuclide . Examples of suitable reporters are widely known from the diagnostic imaging literature, eg. magnetic iron oxide particles, gas-containing vesicles, chelated paramagnetic metals (such as Gd, Dy, Mn, Fe etc.) . See for example US-A-4647447, WO97/25073, US-A-4863715 , US- A-4770183, WO96/09840, WO85/02772, W092/17212, W097/29783, EP-A-554213, US-A-5228446 , W091/15243, WO93/05818, W096/23524, W096/17628, US-A-5387080 , WO95/26205, GB9624918.0, etc. Particularly preferred as reporters are: chelated paramagnetic metal ions such as Gd, Dy, Fe, and Mn, especially when chelated by macrocyclic chelant groups (eg. tetraazacyclododecane chelants such as DOTA, D03A, HP-D03A and analogues thereof) or by linker chelant groups such as DTPA, DTPA-BMA, EDTA, DPDP, etc; metal radionuclide such as 90 Y, 99m Tc, m In, 47 Sc, 67 /Ga, 51 Cr, 177m Sn, 67 Cu, 167 Tm, 97 Ru, 188 Re, 177 Lu, 199 Au, 203 Pb and 141 Ce; superparamagnetic iron oxide crystals; chromophores and fluorophores having absorption and/or emission maxima in the range 300-1400 nm, especially 600 nm to 1200 nm, in particular 650 to 1000 nm; vesicles containing fluorinated gases (ie. containing materials in the gas phase at 37°C which are fluorine containing, eg. SF 6 or perfluorinated C^g hydrocarbons or other gases and gas precursors listed in W097/29783) ; chelated heavy metal cluster ions (eg. W or Mo polyoxoanions or the sulphur or mixed oxygen/sulphur analogs) ; covalently bonded non- metal atoms which are either high atomic number (eg. iodine) or are radioactive, eg 123 I, 131 I, etc. atoms; iodinated compound containing vesicles; etc. Stated generally, the reporter may be (1) a chelatable metal or polyatomic metal-containing ion (ie. TcO, etc), where the metal is a high atomic number metal (eg. atomic number greater than 37) , a paramagentic species (eg. a transition metal or lanthanide) , or a radioactive isotope, (2) a covalently bound non-metal species which is an unpaired electron site (eg. an oxygen or carbon in a persistant free radical) , a high atomic number non- metal, or a radioisotope, (3) a polyatomic cluster or crystal containing high atomic number atoms, displaying cooperative magnetic behaviour (eg. superparamagnetism, ferrimagnetism or ferromagnetism) or containing radionuclides, (4) a gas or a gas precursor (ie. a material or mixture of materials which is gaseous at 37°C) , (5) a chromophore (by which term species which are fluorescent or phosphorescent are included), eg. an inorganic or organic structure, particularly a complexed metal ion or an organic group having an extensive delocalized electron system, or (6) a structure or group having electrical impedance varying characteristics, eg. by virtue of an extensive delocalized electron system. Examples of particular preferred reporter groups are described in more detail below. Chelated metal reporters : metal radionuclides. paramagnetic metal ions, fluorescent metal ions, heavy metal ions and cluster ions Preferred metal radionuclides include 90 Y, 99 Tc, lu In, 4 4 7 7S Scc,, 6 6 ° 7 7'GGaa,, 5 5 3 1 i C/ r , 177m Sn , 6 °7'/Cu , 167 Tm, 97 Ru , "Re , 177',Lu , 199 Au , 203 Pb and 141 Ce. Preferred paramagnetic metal ions include ions of transition and lanthanide metals (eg. metals having atomic numbers of 6 to 9, 21-29, 42, 43, 44, or 57-71), in particular ions of Cr, V, Mn, Fe, Co, Ni, Cu, La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu, especially of Mn, Cr, Fe, Gd and Dy, more especially Gd. Preferred fluorescent metal ions include lanthanides, in particular La, Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, and Lu. Eu is especially preferred. Preferred heavy metal -containing reporters may include atoms of Mo, Bi, Si, and W, and in particular may be polyatomic cluster ions (eg. Bi compounds and W and Mo oxides) as described in WO91/14460, W092/17215, WO96/40287, and W096/22914. The metal ions are desirably chelated by chelant groups on the linker moiety or in or on a particle, (eg. a vesicle or a porous or non-porous inorganic or organic solid) , in particular linear, macrocyclic, terpyridine and N 2 S 2 chelants, such as for example DTPA, DTPA-BMA, EDTA, D03A, TMT . Further examples of suitable chelant groups are disclosed in US-A-4647447 , WO89/00557, US-A- 5367080, US-A-5364613, etc. The linker moiety or the particle may contain one or more such chelant groups, if desired metallated by more than one metal species (eg. so as to provide reporters detectable in different imaging modalities) . Particularly where the metal is non-radioactive, it is preferred that a polychelant linker or particulate reporter be used. A chelant or chelating group as referred to herein may comprise the residue of one or more of a wide variety of chelating agents that can complex a metal ion or a polyatomic ion (eg. TcO) . As is well known, a chelating agent is a compound containing donor atoms that can combine by coordinate bonding with a metal atom to form a cyclic structure called a chelation complex or chelate. This class of compounds is described in the Kirk-Othmer Encyclopedia of Chemical Technology, Vol. 5, 339-368. The residue of a suitable chelating agent can be selected from polyphosphates, such as sodium tripolyphosphate and hexametaphosphoric acid; aminocarboxylic acids, such as ethylenediaminetetraacetic acid, N- (2 -hydroxy) ethylene- diaminetriacetic acid, nitrilotriacetic acid, N,N-di(2- hydroxyethyl) glycine, ethylenebis (hydroxyphenylglycine) and diethylenetriamine pentacetic acid; 1 , 3-diketones, such as acetylacetone, trifluoroacetylacetone, and thenoyltrifluoroacetone; hydroxycarboxylic acids, such as tartaric acid, citric acid, gluconic acid, and 5- sulfosalicyclic acid; polyamines, such as ethylenediamine , diethylenetriamine , triethylenetetraamine, and triaminotriethylamine; aminoalcohols, such as triethanolamine and N- (2- hydroxyethyl) ethylenediamine; aromatic heterocyclic bases, such as 2 , 2 ' -diimidazole, picoline amine, dipicoline amine and 1, 10-phenanthroline; phenols, such as salicylaldehyde, disulfopyrocatechol , and chromotropic acid; aminophenols, such as 8- hydroxyquinoline and oximesulfonic acid; oximes, such as dimethylglyoxime and salicylaldoxime; peptides containing proximal chelating functionality such as polycysteine, polyhistidine, polyaspartic acid, polyglutamic acid, or combinations of such amino acids; Schiff bases, such as disalicylaldehyde 1,2- propylenediimine; tetrapyrroles, such as tetraphenylporphin and phthalocyanine; sulfur compounds, such as toluenedithiol , meso-2 , 3-dimercaptosuccinic acid, dimercaptopropanol , thioglycolic acid, potassium ethyl xanthate, sodium diethyldithiocarbamate, dithizone, diethyl dithiophosphoric acid, and thiourea; synthetic macrocyclic compounds, such as dibenzo [18] crown-6, (CH 3 ) 6 - [14] -4 , 11] -diene-N 4 , and (2.2.2-cryptate) ; phosphonic acids, such as nitrilotrimethylene-phosphonic acid, ethylenediaminetetra (methylenephosphonic acid) , and hydroxyethylidenediphosphonic acid, or combinations of two or more of the above agents. The residue of a suitable chelating agent preferably comprises a polycarboxylic acid group and preferred examples include: ethylenediamine-N,N,N' ,N' -tetraacetic acid (EDTA); N,N,N' ,N" , N" -diethylene-triaminepentaacetic acid (DTPA) ; 1,4,7, 10-tetraazacyclododecane-N, N ' ,N" ,N" ■ - tetraacetic acid (DOTA); 1 , 4 , 7 , 10-tetraazacyclododecane- N,N' ,N"-triacetic acid (D03A) ; l-oxa-4 , 7 , 10- triazacyclododecane-N,N' ,N" -triacetic acid (OTTA) ; trans (1,2) -cyclohexanodiethylene-triamine-pentaacetic acid (CDTPA) . Other suitable residues of chelating agents comprise proteins modified for the chelation of metals such as technetium and rhenium as described in US Patent No. 5078985, the disclosure of which is hereby incorporated by reference. Suitable residues of chelating agents may also derive from N3S and N2S2 containing compounds, as for example, those disclosed in US Patent Nos. 4444690; 4670545; 4673562; 4897255; 4965392; 4980147; 4988496; 5021556 and 5075099. Other suitable residues of chelating are described in PCT/US91/08253, the disclosure of which is hereby incorporated by reference. Preferred chelating groups are selected from the group consisting of 2-amiomethylpyridine, iminoacetic acid, iminodiacetic acid, ethylenediaminetetraacetic acid (EDTA) , diethylenetriaminepentaacetic acid (DTPA) , 1,4,7, 10-tetraazacyclododecane-l, 4,7, 10-tetraacetic acid (DOTA) , carbonyliminodiacetic acid, methyleneiminoacetic acid, methyleneiminodiacetic acid, ethylenethioethylene- iminoacetic acid, ethylenethioethyleneiminodiacetic acid, TMT, a terpyridinyl group, a chelating agent comprising a terpyridyl group and a carboxymethylamino group, or a salt of any of the foregoing acids. Especially preferred chelating groups are DTPA, DTPA- BMA, DPDP, TMT, DOTA and HPD03A. Representative chelating groups are also described in US 5559214 A, WO 95/26754, WO 94/08624, WO 94/09056, WO 94/29333, WO 94/08624, WO 94/08629 Al , WO 94/13327 Al and WO 94/12216 Al . Methods for metallating any chelating agents present are within the level of skill in the art. Metals can be incorporated into a chelant moiety by any one of three general methods: direct incorporation, template synthesis and/or transmetallation. Direct incorporation is preferred. Thus it is desirable that the metal ion be easily complexed to the chelating agent, for example, by merely exposing or mixing an aqueous solution of the chelating agent-containing moiety with a metal salt in an aqueous solution preferably having a pH in the range of about 4 to about 11. The salt can be any salt, but preferably the salt is a water soluble salt of the metal such as a halogen salt, and more preferably such salts are selected so as not to interfere with the binding of the metal ion with the chelating agent. The chelating agent-containing moiety is preferrably in aqueous solution at a pH of between about 5 and about 9, more preferably between pH about 6 to about 8. The chelating agent-containing moiety can be mixed with buffer salts such as citrate, acetate, phosphate and borate to produce the optimum pH. Preferably, the buffer salts are selected so as not to interfere with the subsequent binding of the metal ion to the chelating agent. In diagnostic imaging, the vector-linker-reporter (VLR) construct preferably contains a ratio of metal radionuclide ion to chelating agent that is effective in such diagnostic imaging applications. In preferred embodiments, the mole ratio of metal ion per chelating agent is from about 1:1,000 to about 1:1. In radiotherapeutic applications, the VLR preferably contains a ratio of metal radionuclide ion to chelating agent that is effective in such therapeutic applications. In preferred embodiments, the mole ratio of metal ion per chelating agent is from about 1:100 to about 1:1. The radionuclide can be selected, for example, from radioisotopes of Sc, Fe, Pb, Ga, Y, Bi, Mn, Cu, Cr, Zn, Ge, Mo, Ru, Sn, Sr, Sm, Lu, Sb, W, Re, Po, Ta and Tl . Preferred radionuclides include ""Sc, 64 Cu, 67 Cu, 212 Pb, 68 Ga, 90 Y, 153 Sm, 212 Bi, 186 Re and 188 Re . Of these, especially preferred is 90 Y. These radioisotopes can be atomic or preferably ionic. The following isotopes or isotope pairs can be used for both imaging and therapy without having to change the radiolabeling methodology or chelator: Sc 21 ; 141 Ce 58 ; 188 R . 177 τ . 199 , , . 47 ς . 131 -y . 67 p . 131 -,- rf 123 -r . Ke 75 J'- / ΛU79 , OC 21 , 1 53 , V-U 29 , 1 53 dUU 53 188 Re 75 and 99ra Tc 43 ; 90 Y 39 and 87 Y 39 ; 47 Sc 21 and 44 Sc 21 ; 90 Y 39 and 123 I 53 ; 146 Sm 62 and 153 Sm 62 ; and 90 Y 39 and ιn In 49 . Where the linker moiety contains a single chelant, that chelant may be attached directly to the vector moiety, eg. via one of the metal coordinating groups of the chelant which may form an ester, amide, thioester or thioamide bond with an amine, thiol or hydroxyl group on the vector. Alternatively the vector and chelant may be directly linked via a functionality attached to the chelant backbone, eg. a CH 2 -phenyl-NCS group attached to a ring carbon of DOTA as proposed by Meares et al . in JACS 110:6266-6267(1988) , or indirectly via a homo or hetero-bifunctional linker, eg. a bis amine, bis epoxide, diol , diacid, difunctionalised PEG, etc. In that event, the bifunctional linker will conveniently provide a chain of 1 to 200, preferably 3 to 30 atoms between vector and chelant residue. Where the linker moiety contains a plurality of chelant groups, the linker preferably is or contains portions of formula Li Li-C h or Li (Ch) Ch where Ch is a chelant moiety and Li is a linker backbone component, ie. the linker preferably has pendant chelants, in-backbone chelants or terminal chelants or a combination thereof. The pendant and in-backbone polymeric structures may be branched but more preferably are linear and the repeat units (LiCh) or other repeat units in the polymer may have in-backbone or pendant biodistribution modifying groups, eg. polyalkylene groups as in WO94/08629, WO94/09056, and WO96/20754. The terminal chelant structures Li(Ch) n , which may be dendritic polymers as in WO93/06868, may have biodistribution modifying groups attached to termini not occupied by chelants and may have biodegradation enhancing sites within the linker structure as in W095/28966. The chelant moieties within the polychelant linker may be attached via backbone functionalization of the chelant or by utilization of one or more of the metal co-ordinating groups of the chelant or by amide or ether bond formation between acid chelant and an amine or hydroxyl carrying linker backbone, eg. as in polylysine- polyDTPA, polylysine-polyDOTA and in the so-called magnifier polychelants , of WO90/12050. Such polychelant linkers may be conjugated to one or more vector groups either directly (eg. utilizing amine, acid or hydroxyl groups in the polychelant linker) or via a bifunctional linker compound as discussed above for monochelant linkers . Where the chelated species is carried by a particulate (or molecular aggregate, eg. vesicular) linker, the chelate may for example be an unattached mono or polychelate (such as Gd DTPA-BMA or Gd HP-D03A) enclosed within the particle or it may be a mono or polychelate conjugated to the particle either by covalent bonding or by interaction of an anchor group (eg. a lipophilic group) on the mono/polychelate with the membrane of a vesicle (see for example WO96/11023) . Non-metal atomic reporters Preferred non-metal atomic reporters include radioisotopes such as 123 I and 131 I as well as non zero nuclear spin atoms such as 18 F, and heavy atoms such as I. Such reporters, preferably a plurality thereof, eg. 2 to 200, may be covalently bonded to a linker backbone, either directly using conventional chemical synthesis techniques or via a supporting group, eg. a triiodophenyl group. In an embodiment of this invention, the use of radioisotopes of iodine is specifically contemplated. For example, if the vector or linker is comprised of substituents that can be chemically substituted by iodine in a covalent bond forming reaction, such as, for example, substituents containing hydroxyphenyl functionality, such substituents can be labeled by methods well known in the art with a radioisotope of iodine. The iodine species can be used in therapeutic and diagnostic imaging applications. While, at the same time, a metal in a chelating agent on the same vector- linker can also be used in either therapeutic or diagnostic imaging applications. As with the metal chelants discussed above, such metal atomic reporters may be linked to the linker or carried in or on a particulate linker, eg. in a vesicle (see WO95/26205 and GB9624918.0) . Linkers of the type described above in connection with the metal reporters may be used for non-metal atomic reporters with the non-metal atomic reporter or groups carrying such reporters taking the place of some or all of the chelant groups. Organic Chromophoric or Fluorophoric Reporters Preferred organic chromophoric and fluorophoric reporters include groups having an extensive delocalized electron system, eg. cyanines, merocyanines , phthalocyanines, naphthalocyanines, triphenylmethines , porphyrins, pyrilium dyes, thiapyrilium dyes, squarylium dyes, croconium dyes, azulenium dyes, indoanilines, benzophenoxazinium dyes, benzothiaphenothiazinium dyes, anthraquinones, napthoquinones , indathrenes, phthaloylacridones , trisphenoquinones , azo dyes, intramolecular and intermolecular charge-transfer dyes and dye complexes, tropones, tetrazines, bis (dithiolene) complexes, bis (benzene-dithiolate) complexes, iodoaniline dyes, bis (S , O-dithiolene) complexes, etc. Examples of suitable organic or metallated organic chromophores may be found in "Topics in Applied Chemistry: Infrared absorbing dyes" Ed. M. Matsuoka, Plenum, NY 1990, "Topics in Applied Chemistry: The Chemistry and Application of Dyes", Waring et al . , Plenum, NY, 1990, "Handbook of Fluorescent Probes and Research Chemicals" Haugland, Molecular Probes Inc, 1996, DE-A-4445065, DE-A-4326466 , JP-A-3/228046 , Narayanan et al . J. Org . Chem. 60: 2391-2395 (1995), Lipowska et al . Heterocyclic Comm. 1 : 427-430 (1995), Fabian et al . Chem. Rev. 92.: 1197 (1992), W096/23525, Strekowska et al . J. Org. Chem. 57: 4578-4580 (1992), and W096/17628. Particular examples of chromophores which may be used include xylene cyanole, fluorescein, dansyl, NBD, indocyanine green, DODCI, DTDCI, DOTCI and DDTCI . Particularly preferred are groups which have absorption maxima between 600 and 1000 nm to avoid interference with haemoglobin absorption (eg. xylene cyanole) . Further such examples include: cyanine dyes: such as heptamethinecyanine dyes, e.g. compounds 4a to 4g Table II on page 26 of Matsuoka (supra) 4a where Y= s, X=I, R=Et 4b where Y= s, X=C10 4 , R=Et 4c where Y= Cme 2 , X=I, R=Me 4d where Y= CMe 2 , X=C10 4 , R=Me 4e where Y= CH=CH, x=ι, R=Et 4f where Y= CH=CH, X=Br, R=Et 4g where Y= CH=C H, X=C10 4 , R=Et and in Table III on page 28 of Matsuoka (supra] i . e . where Y= O , X= I , R=Me where Y= CMe 2 , X= I , R=Me where Y= S , X=Br R=Et ; chalcogenopyrylomethine dyes, e.g., compounds 12 on page 31 of Matsuoka (supra), i.e. where Y= Te, Se, 0 or NR; monochalcogenopyrylomethine dyes, e.g. compounds 13 on page 31, of Matsuoka (supra) i.e. where n = 1 or 2 ; pyrilium dyes, e.g., compounds 14 (X= 0) on page 32 of Matsuoka (supra), i.e. where X = 0, S, or Se; thiapyrilium dyes, e.g. compounds 15 on page 32, and compound I on page 167 of Matsuoka (supra) , i.e. where n = 1 or 2 ; squarylium dyes, e.g. compound 10 and Table IV on page 30 of Matsuoka (supra), i.e. where X = CH=CH, Y = H, and R=Et, X = S, Y = H, and R=Et, and X = CMe 2 , Y = H, and R=Me, and compound 6, page 26, of Matsuoka (supra), i.e where X = CH=CH, Y = H, and R=Et; croconium dyes, e.g. compound 9 and Table IV on page 30 of Matusoka (supra), i.e. where X = CH=CH, Y = H, and R=Et, X = S, Y = H, and R=Et, X = CMe 2 , Y = H, and R=Me, and compound 7, page 26, of Matsuoka (supra), i.e where X = CH=CH, Y = H, and R=Et ; azulenium dyes, e.g. compound 8 on page 27 of Matsuoka (supra) , i.e. merocyanine dyes, e.g. compound 16, R= Me, on page 32 of Matsuoka (supra), i.e. indoaniline dyes such as copper and nickel complexes of indoaniline dyes, e.g. compound 6 on page 63 of Matsuoka (supra) , i.e. where R = Et, R' = Me, M = Cu, R = Et, R' = Me, M = Ni, R = Me, R' = H, M = Cu, or R = Me, R' = H, M = Ni, benzo [a] phenoxazinium dyes and benzo [a] henothiazinium dyes, e.g. as shown on page 201 of Matusoka (supra) , i.e. where X = O or S ; 1 , 4-diaminoanthraquinone (N-alkyl) -3 ' -thioxo-2 , 3 - dicarboximides, e.g. compound 20, on page 41 of Matusoka (supra) indanthrene pigments, e.g. see compound 21 on page 41 of Matsuoka (supra) ; 2 -arylamino-3 , 4-phthaloylacridone dyes, e.g. compound 22 on page 41 of Matsuoka (supra) trisphenoquinone dyes, e.g. compound 23 on page 41 of Matsuoka (supra) azo dyes, e.g. the monoazo dye, compound 2 on page 90 of Matsuoka (supra), i.e. where X = CH=C(CN) 2 , R x = R 2 = Et, R 3 = R„ = H, X = C(CN) =C(CN) 2 , R x = R 2 = Et, R 3 = R 4 = H, or X = and Y = C=0, R x = R 2 = Et , R 3 = R 4 = H, or Y = S0 , Rn = H, R 2 = CH(Me)nBu, R 3 = OMe, and R 4 = NHAc; azo dyes, e.g. the polyazo dye, compound 5 on page 91 of Matsuoka (supra) , i.e. intramolecular charge transfer donor-acceptor infrared dyes, e.g. compounds 6 and 7 on page 91 of Matsuoka (supra) , i.e. and nonbenzenoid aromatic dyes, e.g. compound 8, a tropone, on page 92, of Matsuoka (supra), i.e. tetrazine radical dyes, e.g. compound 9 on page 92 of Matsuoka (supra), i.e. in which, X = p-phenylene or X = p-terphenylene as well as compound 10 on page 92 of Matsuoka (supra), i.e. in which X = p-biphenyl ; cationic salts of tetrazine radical dyes , e . g . compound 11 on page 92 of Matsuoka ( supra) in which X = p-phenylene; donor-acceptor intermolecular charge transfer dyes, e.g. CT complexes of compounds 13b and 14a to 14c on page 93 of Matsuoka (supra), i.e. where X = CH=N-N(Ph) 2 in the donor and a) Y = CN, Z = N0 2 b) Y = CN, Z = H or a) Y = Cl , Z = N0 2 in the acceptor; anthraquinone dyes, e.g. compounds 12 (X = S or Se) on page 38 of Matsuoka (supra), i.e. wherein X = S or Se and Y = tetrachloro, tetrabromo, 2 , 3-dicarboxylic acid, 2 , 3 -dicarboxylic anhydride, or 2 , 3-dicarboxylic acid N-phenyl imide; naphthoquinone dyes, e.g. compounds 2, 3, and 4 on page 37, of Matsuoka (supra), i.e. and metallated azo dyes such as azo dyes containing nickel, cobalt, copper, iron, and manganese; phthalocyanine dyes, e.g. compound 1 in Table II on page 51 of Matsuoka (supra), e.g. naphthalocyanine dyes, e.g. compound 3 in Table II on page 51 of Matsuoka (supra) , e.g. metal phthalocyanines such as phthalocyanines containing aluminum, silicon, nickel, zinc, lead, cadmium, magnesium, vanadium, cobalt, copper, and iron, e.g. compound 1 in Table III on page 52 of Matsuoka (supra) , e.g. in which, for example, M = Mg; metal naphthalocyanines such as naphthalocyanines containing aluminum, zinc, cobalt, magnesium, cadmium, silicon, nickel, vanadium, lead, copper, and iron, see compound 3 in Table III on page 52 of Matsuoka (supra) , e.g. in which, for example, M = Mg; bis (dithiolene) metal complexes comprising a metal ion such as nickel, cobalt, copper, and iron coordinated to four sulfur atoms in a bis (S, S ' -bidentate) ligand complex, e.g. see Table I on page 59 of Matsuoka (supra) where R L = R 2 = CF 3 , M= Ni, R j = R 2 = phenyl, M= Pd, R λ = R 2 = phenyl, M= Pt , Rj = C4 to CIO alkyl, R 2 = H, M= Ni , R x = C4 to CIO alkyl, R 2 = H, M= Pd, Rj = C4 to CIO alkyl, R 2 = H, M= Pt , R x = R 2 = phenyl , M= Ni , R j = R 2 = p-CH 3 -phenyl, M= Ni , R j = R 2 = p-CH 3 0-phenyl, M= Ni , R j = R 2 = p-Cl -phenyl, M= Ni , R λ = R 2 = p-CF 3 -phenyl, M= Ni , R j = R 2 = 3 , 4, -diCl -phenyl, M= Ni , R 1 = R 2 = o-Cl -phenyl, M= Ni , R x = R 2 = o-Br-phenyl, M= Ni , R x = R 2 = 3,4, -diCl -phenyl, M= Ni , R j = R 2 = p-CH 3 , M= Ni, R x = R 2 = 2-thienyl, M= Ni , Ri = p- (CH 3 ) 2 N-phenyl, R 2 = phenyl, M= Ni , and R : = p- (CH 3 ) 2 N-phenyl, R 2 =p-H 2 N-phenyl , M= Ni ; bis (benzenedithiolate) metal complexes comprising a metal ion such as nickel, cobalt, copper, and iron coordinated to four sulfur atoms in a ligand complex, e.g. see Table III on page 62 of Matsuoka (supra), i.e. NBn where X = tetramethyl, M = Ni , X = 4, 5 -dimethyl, M = Ni , X = 4 -methyl, M = Ni , X = tetrachloro, M = Ni , X = H, M = Ni, X = 4 -methyl, M = Co, X = 4 -methyl, M = Cu, and X = 4 -methyl, M = Fe; N, O-bidentate indoaniline dyes comprising a metal ion such as nickel, cobalt, copper, and iron coordinated to two nitrogen and two oxygen atoms of two N, O-bidentate indoaniline ligands, e.g. compound 6 in Table IV on page 63 of Matsuoka (supra), e.g. where R = Et , R' = Me, M = Cu, R = Et, R 1 = Me, M = Ni, R = Me, R 1 = H, M = Cu, and R = Me, R' = H, M = Ni , bis (S , O-dithiolene) metal complexes comprising a metal ion such as nickel, cobalt, copper, and iron coordinated to two sulfur atoms and two oxygen atoms in a bis(S,0- bidentate) ligand complex, e.g. see US Patent 3,806,462, e.g. a-diimine-dithiolene complexes comprising a metal ion such as nickel, cobalt, copper, and iron coordinated to two sulfur atoms and two imino-nitrogen atoms in a mixed S,S- and N, -bidentate diligand complex, e.g. see Table II on page 180, second from bottom, of Matsuoka (supra) (also see Japanese patents: 62/39,682, 63/126,889 and 63/ 139,303) , e.g. and tris (a-diimine) complexes comprising a metal ion coordinated to six nitrogen atoms in a triligand complex, e.g. see Table II on page 180 of Matsuoka (supra) , last compound, (also see Japanese Patents 61/20,002 and 61/73,902), e.g. Representative examples of visible dyes include fluorescein derivatives, rhodamine derivatives, coumarins, azo dyes, metalizable dyes, anthraquinone dyes, benzodifuranone dyes, polycyclic aromatic carbonyl dyes, indigoid dyes, polymethine dyes, azacarbocyanine dyes, hemicyanine dyes, barbituates, diazahemicyanine dyes, stryrl dyes, diaryl carbonium dyes, triaryl carbonium dyes, phthalocyanine dyes, quinophthalone dyes, triphenodioxazine dyes, formazan dyes, phenothiazine dyes such as methylene blue, azure A, azure B, and azure C, oxazine dyes, thiazine dyes, naphtholactam dyes, diazahemicyanine dyes, azopyridone dyes, azobenzene dyes, mordant dyes, acid dyes, basic dyes, metallized and premetallized dyes, xanthene dyes, direct dyes, leuco dyes which can be oxidized to produce dyes with hues bathochromically shifted from those of the precursor leuco dyes, and other dyes such as those listed by Waring, D. R. and Hallas, G., in "The Chemistry and Application of Dyes", Topics in Applied Chemistry, Plenum Press, New York, N.Y., 1990. Additonal dyes can be found listed in Haugland, R. P., "Handbook of Fluorescent Probes and Research Chemicals", Sixth Edition, Molecular Probes, Inc., Eugene OR, 1996. Such chormophores and fluorophores may be covalently linked either directly to the vector or to or within a linker structure. Once again linkers of the type described above in connection with the metal reporters may be used for organic chromophores or fluorophores with the chromophores/fluorophores taking the place of some or all of the chelant groups. As with the metal chelants discussed above chromophores/ fluorophores may be carried in or on a particulate linker-moieties, eg. in or on a vesicle or covalently bonded to inert matrix particles that can also function as a light scattering reporter. Particulate Reporters or Linker-Reporters The particulate reporters and linker-reporters generally fall into two categories - those where the particle comprises a matrix or shell which carries or contains the reporter and those where the particle matrix is itself the reporter. Examples of the first category are: vesicles (eg. micelles, liposomes, microballoons and microbubbles) containing a liquid, gas or solid phase which contains the contrast effective reporter, eg. an echogenic gas or a precursor therefor (see for example GB 9700699.3), a chelated paramagnetic metal or radionuclide, or a water-soluble iodinated X-ray contrast agent; porous particles loaded with the reporter, eg. paramagnetic metal loaded molecular sieve particles; and solid particles, eg. of an inert biotolerable polymer, onto which the reporter is bound or coated, eg. dye- loaded polymer particles. Examples of the second category are: light scattering organic or inorganic particles; magnetic particles (ie. superparamagnetic, ferromagnetic or ferrimagnetic particles); and dye particles. Preferred particulate reporters or reporter-linkers include superparamagnetic particles (see US-A-4770183 , WO97/25073, WO96/09840, etc.), echogenic vesicles (see W092/17212, W097/29783, etc.), iodine-containing vesicles (see WO95/26205 and GB9624918.0) , and dye- loaded polymer particles (see W096/23524) . The particulate reporters may have one or more vectors attached directly or indirectly to their surfaces. Generally it will be preferred to attach a plurality (eg. 2 to 50) of vector moieties per particle. Particularly conveniently, besides the desired targeting vector, one will also attached flow decelerating vectors to the particles, ie . vectors which have an affinity for the capillary lumen or other organ surfaces which is sufficient to slow the passage of the contrast agent through the capillaries or the target organ but not sufficient on its own to immobilise the contrast agent. Such flow decelerating vectors (described for example in GB9700699.3) may moreover serve to anchor the contrast agent once it has bound to its target site. The means by which vector to particle attachment is achieved will depend on the nature of the particle surface. For inorganic particles, the linkage to the particle may be for example by way of interaction between a metal binding group (eg. a phosphate, phosphonate or oligo or polyphosphate group) on the vector or on a linker attached to the vector. For organic (eg. polymeric) particles, vector attachment may be by way of direct covalent bonding between groups on the particle surface and reactive groups in the vector, eg. amide or ester bonding, or by covalent attachment of vector and particle to a linker. Linkers of the type discussed above in connection with chelated metal reporters may be used although in general the linkers will not be used to couple particles together. For non-solid particles, eg. droplets (for example of water insoluble iodinated liquids as described in US-A- 5318767, US-A-5451393, US-A-5352459 and US-A-5569448 ) and vesicles, the linker may conveniently contain hydrophobic "anchor" groups, for example saturated or unsaturated C 12 _ 30 chains, which will penetrate the particle surface and bind vector to particle. Thus for phospholipid vesicles, the linker may serve to bind the vector covalently to a phospholipid compatible with the vesicle membrane. Examples of linker binding to vesicles and inorganic particles are described in GB9622368.0 and WO97/25073. Besides the vectors, other groups may be bound to the particle surface, eg. stabilisers (to prevent aggregation) and biodistribution modifiers such as PEG. Such groups are discussed for example in WO97/25073, WO96/09840, EP-A-284549 and US-A-4904479. Preferably the V-L-R agents of the invention will have the non-peptidic endothelin receptor targetting vectors (such as bosentan or BMS 182874) coupled directly or indirectly to a reporter, eg. with covalently bound iodine radioisotopes, with metal chelates attached directly or via an organic linker group or coupled to a particulate reporter or linker-reporter, eg. a superparamagnetic crystals (optionally coated, eg. as in WO97/25073), or a vesicle, eg. a gas containing or iodinated contrast agent containing micelle, liposome or microballoon. All of the publications referred to herein are incorporated herein by reference. The agents of the invention may be administered to patients for imaging in amounts sufficient to yield the desired contrast with the particular imaging technique. Where the reporter is a metal, generally dosages of from 0.001 to 5.0 mmoles of chelated imaging metal ion per kilogram of patient bodyweight are effective to achieve adequate contrast enhancements. For most MRI applications preferred dosages of imaging metal ion will be in the range of from 0.02 to 1.2 mmoles/kg bodyweight while for X-ray applications dosages of from 0.05 to 2.0 mmoles/kg are generally effective to achieve X-ray attenuation. Preferred dosages for most X-ray applications are from 0.1 to 1.2 mmoles of the lanthanide or heavy metal compound/kg bodyweight. Where the reporter is a radionuclide, dosages of 0.01 to 100 mCi, preferably 0.1 to 50 mCi will normally be sufficient per 70 kg bodyweight. Where the reporter is a superparamagnetic particle, the dosage will normally be 0.5 to 30 mg Fe/kg bodyweight. Where the reporter is a gas or gas generator, eg. in a microballoon, the dosage will normally be 0.05 to 100 μL gas per 70 kg bodyweight . The dosage of the compounds of the invention for therapeutic use will depend upon the condition being treated, but in general will be of the order of from 1 pmol/kg to 1 mmol/kg bodyweight. The compounds of the present invention may be formulated with conventional pharmaceutical or veterinary aids, for example emulsifiers, fatty acid esters, gelling agents, stabilizers, antioxidants , osmolality adjusting agents, buffers, pH adjusting agents, etc., and may be in a form suitable for parenteral or enteral administration, for example injection or infusion or administration directly into a body cavity having an external escape duct, for example the gastrointestinal tract, the bladder or the uterus. Thus the compounds of the present invention may be in conventional pharmaceutical administration forms such as tablets, capsules, powders, solutions, suspensions, dispersions, syrups, suppositories etc. However, solutions, suspensions and dispersions in physiologically acceptable carrier media, for example water for injections, will generally be preferred. The compounds according to the invention may therefore be formulated for administration using physiologically acceptable carriers or excipients in a manner fully within the skill of the art. For example, the compounds, optionally with the addition of pharmaceutically acceptable excipients, may be suspended or dissolved in an aqueous medium, with the resulting solution or suspension then being sterilized. For imaging of some portions of the body the most preferred mode for administering contrast agents is parenteral, e.g., intravenous administration. Parenterally administrable forms, e.g. intravenous solutions, should be sterile and free from physiologically unacceptable agents, and should have low osmolality to minimize irritation or other adverse effects upon administration, and thus the contrast medium should preferably be isotonic or slightly hypertonic . Suitable vehicles include aqueous vehicles customarily used for administering parenteral solutions such as Sodium Chloride Injection, Ringer's Injection, Dextrose Injection, Dextrose and Sodium Chloride Injection, Lactated Ringer's Injection and other solutions such as are described in Remington's Pharmaceutical Sciences, 15th ed. , Easton: Mack Publishing Co., pp. 1405-1412 and 1461-1487 (1975) and The National Formulary XIV, 14th ed . Washington: American Pharmaceutical Association (1975) . The solutions can contain preservatives, antimicrobial agents, buffers and antioxidants conventionally used for parenteral solutions, excipients and other additives which are compatible with the chelates and which will not interfere with the manufacture, storage or use of products . The agents of formula I may be therapeutically effective in the treatment effective in the treatment of disease states as well as detectable in in vivo imaging. Thus for example the vector on the reporter moieites may have therapeutic efficacy, eg. by virtue of the radiotherapeutic effect of a radionuclide reporter, the efficacy in photodynamic therapy of a chromophore (or fluorophore) reporter or the chemotherapeutic effect of the vector moiety. Use of the agents of formula I in the manufacture of therapeutic compositions and in methods of therapeutic or prophylactic treatment of the human or non-human animal body are thus considered to represent further aspects of the invention. In one embodiment, the contrast agent of the invention conveniently comprises a gas-containing or gas- generating material, preferably in suspension in an aqueous carrier material and conjugated to one or more vectors of which at least one is a vector V as defined above, eg. an endothelin antagonist. The gas may take the form of microbubbles stabilised by a monolayer of a film-forming surfactant, or stabilised by a matrix material other than a surfactant . The vectors may be for example coupled to such surfactant or matrix and may be bioactive or non-bioactive . The vectors may have different targeting specificities and in one preferred embodiment are such as to interact with their receptors but not to fixedly bind the gas-containing vesicles. The present invention will now be further illustrated by way of the following non-limiting examples. Unless otherwise indicated, all percentages given are by we ight . Example 1 - Endothelin receptor binding contrast agent for MR Imaging Compound 1 Lysine (O.lg, 0.7 mmol) is added to a solution of 27-0- 3- [2- (3 -carboxy-acryloylamino) -5-hydroxyphenyl] - acryloyloxy mycerone (prepared in accordance with EP 628569, 0.5 g, 0.7 mmol) and DCC (N,N'- dicyclohexylcarbodiimide) in dry DMF (N,N- dimethylformamide) . The reaction mixture is stirred at ambient temperature and is followed by TLC. The dispersion is left overnight at +4°C. The dispersion is filtered and the solvent rotary evaporated before the substance is purified by chromatography. Compound 2 Diethylenetriamine-pentaacetic acid dianhydride (17.9 g, 50 mmol) is dissolved in dry DMF and compound 1 (0.45 g, 0.5 mmol) dissolved in dry DMF is added. The reaction mixture is stirred at elevated temperature under nitrogen atmosphere. The reaction is followed by TLC. The solvent is rotary evaporated and the substance purified by chromatography. Gd (III) Chelate of compound 2 To a solution of compound 2 (0.5 g, 0.4 mmol) in water is added gadolinium oxide Gd 2 0 3 (0.1 g, 0.2 mmol) and the mixture is heated at 95°C. After filtration the solution is evaporated and dried in vacuo at 50°C. Example 2 - Endothelin receptor binding contrast agent for nuclear medicine 99m Tc Chelate of compound 2 Compound 2 from Example 1 (1 mg) is dissolved in 0.1 N NaOH. SnCl 2 .2H 2 0 (100 μg) dissolved in 0.05 N HCl and a solution of 10-100 mCi 99m Tc in the form of sodium pertechnetate in saline is added. The pH of the solution is adjusted to pH 7-8 by addition of 0.5 M phosphate buffer (pH 5) after less than one minute. The reaction is followed by TLC and the substance is purified by chromatography. Example 3 - Endothelin receptor binding contrast agent for nuclear medicine An aqueous solution of 131 I 2 (2 equivalents) and sodium perchlorate (1 equivalent) is added to an aqueous solution of 2 -benzo (1 , 3 ) dioxol-5-yl -3 -benzyl-4- (4- methoxyphenyl) -4-oxobut-2-enoate (prepared in accordance with Example 351 of WO 95/05376, 1 equivalent) . The solvent is rotary evaporated and the substance is purified by chromatography. Example 4 - Endothelin receptor binding contrast agent for ultrasound and scattering light imaging Compound 3 DSPE (distearoylphosphatidylethanolamine) (0.5 g, 0.7 mmol) is added to a solution of 27-0-3 - [2 - (3-carboxy- acryloylamino) 5-hydroxyphenyl] -acryloyloxy mycerone (prepared in accordance with EP 628569, 0.5 g, 0.7 mmol) and DCC in dry DMF. The reaction mixture is stirred at ambient temperature and followed by TLC. The dispersion is left overnight at +4°C. The dispersion is filtered and the solvent rotary evaporated before the substance is purified by chromatography. Gas containing microparticles comprising phosphatidylserine and compound 3 To phosphatidylserine (5 mg) is added 5% propylenglycol - glycerol in water (1 ml) . The dispersion is heated to not more than 80°C for 5 minutes and then cooled to ambient temperature. The dispersion (0.8 ml) is transferred to a vial (1 ml) and compound 1 (0.1-1.0 mg) is added. The vial is put on a roller table for some hours. The head space of the vial is flushed with perfluorobutane . The vial is shaken in a cap-mixer for 45 seconds, whereafter the sample is put on a roller table. After centrifugation the infranatant is exchanged with water. The washing procedure is repeated 2-3 times. Example 5 - Gas-filled microbubbles of distearoyl phosphatidylserine comprising a lipopβptide containing a vector with affinity for endothelin receptors for targeted ultrasound imaging a) Synthesis of 4 ' - F3 , 4-dimethyl-5- isoxazolyl ) sulfamoyll succinanilic acid To a solution of sulfisoxazole (Aldrich, 267 mg, 1.00 mmol) in DMF (10 ml) was added succinic anhydride (1.00 g, 10.0 mmol) and 4 -dimethylaminopyridine (122 mg, 1.00 mmol) . The reaction mixture was stirred at 80°C for 2 hours and then concentrated. The residue was taken up in 5% aqueous sodium bicarbonate solution and extracted with ethyl acetate. The aqueous solution was acidified with dilute hydrochloric acid and the organic material was extracted into ethyl acetate . The organic phase was washed with dilute hydrochloric acid, water and brine, treated with active charcoal and dried (MgS0 4 ) . The solution was filtered and concentrated to give 280 mg (76%) of a white solid. The structure was verified by l tϊ (300 MHz) and 13 C (75 MHz) NMR spectroscopy . Further characterisation was carried out using MALDI mass spectrometry (ACH matrix) , giving a M+Na peak at m/z 390 and a M+K peak at m/z 406 as expected. b) Synthesis of a lipopeptide functionalised with sulfisoxazole The structure shown above was synthesised on a manual nitrogen bubbler appaatus starting with Fmoc protected Rink Amide MBHA resin (Novabiochem) on a 0.125 mmol scale, using amino acids from Novabiochem, palmitic acid from Fluka and the compound from a) above. Coupling was carried out using standard TBTU/HOBt/DIEA protocols. Simultaneous removal of the peptide from the resin and deprotection of side-chain protecting groups was carried out in TFA containing 5% EDT and 5% water for 2 hours. Crude material was precipitated from ether. The product was analysed by analytical HPLC (gradient 70-100% B over 20 min, A = 0.1% TFA/water and B = 0.1% TFA/acetonitrile, flow rate 1 ml/min, column Vydac 218TP54, detection UV 214 nm, retention time 27 min) . Further characterisation was carried out using MALDI mass spectrometry, giving a m+H at m/z 1359, expected 1356. c) Preparation of gas-filled microbubbles comprising the compound from b) above A solution of 1.4% propylene glycol/2.4% glycerol (1.0 ml) was added to a mixutre of DSPS (Avanti, 4.5 mg) and the product from b) above (0.5 mg) in a vial. The mixture was sonicated for 5 minutes and then heated at 80°C for 5 minutes (the vial was shaken during warming) and cooled. Head space was flushed with perfluorobutane gas and the vial was shaken in a cap mixer for 45 seconds followed by extensive washing with deionised water. MALDI mass spectrometry showed no detectable level of compound from b) above in the final wash solution. Incorporation of isoxazole-containing lipopeptide into the bubbles was confirmed by MALDI-MS as follows. About 50 μl of microbubbles were transferred to a clean vial containing ca 100 μl of 90% methanol . The mixture was sonicated for 30 seconds and analysed by MALDI-MS (ACH- matrix) , giving a M+H peak at m/z 1359 corresponding to lipopeptide b) . Example 6 131 I labelleled Sulfisoxasole (Compound 4) : Sulfisoxasole was labelled with 131 I according to the procedure supplied with the iodination agent IODO-BEADS iodination agent from Pierce. Labelling yield: 91%. Example 7 - Tc chelate of N- (MAG-3) -Sulfisoxazole The title compound was prepared as outlined in Scheme 1 below according to the description in Nuclear Medicine Communications 16 (1995) 942-957. (the * -marked oxygen may be replaced by a sulph

Claims: Claims 1. A composition of matter of formula I V - L - R (I) where V is a non-peptidic organic group having binding affinity for an endothelin receptor site, L is a linker moiety or a bond, and R is a moiety detectable in in vivo imaging of a human or animal body. 2. A composition of matter as claimed in claim 1 wherein V is the residue of a compound of formula II 0-Bd-Rg (II) where 0 is an optionally substituted phenyl group, optionally part of a fused polycyclic structure containing up to 4 fused rings; Bd is a 1 to 5 atom long optionally unsaturated organic bridge containing backbone atoms selected from C, N, S and 0 optionally carrying side chain substituents and optionally forming part of a fused ring, and containing a 0 II -X- (CH 2 ) n -Y- moiety where X is C or SO, n is 0 or 1 and Y is 0, S or an amine nitrogen; and Rg is a 5- or 6 -membered saturated or unsaturated optionally substituted carbocyclic or heterocyclic ring, optionally part of a fused polycyclic structure containing up to 5 fused rings; wherein the optional substituents are selected from halogen and oxygen atoms, carbon and sulphur oxyacid groups, hydroxy, alkyl, aryl, aralkyl, acyl, alkoxy, alkylenedioxy, alkylthio, alkylamino, acyloxy, aralkylamino, aralkyloxy, acylthio, amino, and acylamino groups and combinations thereof, and wherein ring heteroatoms are selected from 0, N and S, aryl groups are mono or bicyclic carbocyclic or heterocyclic rings containing up to 10 ring atoms, and no more than 7 rings and no more than 6 fused rings are present . 3. A composition of matter as claimed in claim 1 wherein V is the residue of a compound of formula III wherein R 1 and R 2 , which may be the same or different, are hydroxy, sulphur oxyacid, acylamino, or carboxyacylamino groups . 4. A composition of matter as claimed in claim 1 wherein V is the residue of a compound of formula IV wherein each R 3 which may be the same or different is hydrogen or an acyl group; R 4 is a carboxy or acyl group; and R 5 is hydrogen, hydroxy or an acyloxy group. 5. A composition of matter as claimed in claim 1 wherein V is the residue of a compound of formula V (where R 6 is hydrogen or alkyl; R 7 is aryl, aralkyl, arylcarbonyloxy or aralkylcarbonyl and any alkyl moiety may have 1 to 6 carbons; Z is an optionally oxa substituted λ . 6 alkylene group optionally with a CH 2 moiety replaced by an amine nitrogen or an oxygen atom 0 0 0 0 -0-C-, -NH-C-, -0-C-CH 2 - or -NH-C-CH 2 group); Z ,1 1 0" is hydrogen and R 8 is hydrogen or optionally substituted C γ . 6 alkyl group) . 6. A composition of matter as claimed in claim 1 wherein V is the residue of a compound of formula VI (where the isoxazoyl group is attached at the 5 or 3 positions; R 9 is an optionally substituted, optionally fused benzene ring, a fused pyridine ring, a phenyl group, or a carboxyl or amino group where the ring to which it is attached is unsaturated or a fused 2,2- propylidene group or an oxygen atom where the ring to which it is attached is saturated; and R 10 is a straight chain or branched C^g alkyl group or a halogen atom) . 7. A composition of matter as claimed in claim 1 wherein V is the residue of a compound of formula VII (where R 11 is hydrogen or x . 6 alkyl optionally substituted by oxyacid or other reactive substituents for vector linkage; R 12 is C 1 . 6 alkyl optionally substituted by oxyacid or other reactive substituents for vector linkage; R 13 is hydrogen, halogen or C 1 . 6 alkyl and R 14 is hydrogen, C^ 6 alkoxy or methylenedioxy) . 8. A composition of matter as claimed in claim 1 wherein V is the residue of a compound of formula VIII (where Z" is selected from alkylsulphonylaminocarbonyl, arylsulphonylaminocarbonyl, aryl, arylaminocarbonyl, and carboxy; R 15 is hydrogen, alkyl, oxyacid or another reactive group for vector attachment; R 16 is hydrogen, halogen, carboxyl, C^g alkoxy-carbonyl , or C 1 _ 6 alkyl, alkoxyl, hydroxy-alkyl or haloalkyl; and R 17 is hydrogen, halogen, nitro, carboxyl, aminocarbonyl, acetylamino, alkoxycarbonylamino, alkoxy or alkylenedioxy) . 9. A composition of matter as claimed in claim 1 wherein V is the residue of a compound of formula IX or X (where each R 18 which may be the same or different is an alkoxy or alkylenedioxy group) . 10. A composition of matter as claimed in claim 1 wherein V is the residue of a compound of formula XI (where each R 18 which may be the same or different is an alkoxy or alkylenedioxy group) . 11. A composition of matter as claimed in claim 1 wherein V is the residue of a compound of formula XI (where R 19 is C^g alkyl; R 18 is as defined in claim 10; Z" 1 is O or CH 2 ; and R 20 is alkoxyaryloxy, hydroxyalkoxy, or alkoxyaryl) . 12. A composition of matter as claimed in claim 1 wherein V is the residue of a compound of formula XIV (where R 19 is as defined in claim 11; R 21 is a 5 or 6 ring membered aryl group; and each R 22 , which may be the same or different is a hydrogen or C^g alkyl group) . 13. A composition of matter as claimed in claim 1 wherein V is the residue of a compound of formula XV (where R 18 is as defined in claim 10; R 23 is an aralkyl group; and R 24 is an aralkyl) 14. A composition of matter as claimed in claim 1 wherein V is the residue of a compound of formula XVI (where R 25 is an oxyacid or an ester; R 18 is as defined in claim 10; and R 26 is a hydrogen atom or an optionally substituted C 1 , 6 alkyl group) . 15. A composition of matter as claimed in claim 1 selected from: i) a Gd(III) chelate of a DTPA conjugate of a lysine conjugate of 27-0-3- [2- (3 -carboxy-acryloylamino) -5- hydroxyphenyl] -acryloyloxy mycerone; ii) a Tc chelate of a DTPA conjugate of a lysine conjugate of 27-0-3- [2- (3-carboxy-acryloylamino) -5- hydroxyphenyl] -acryloyloxy mycerone; iii) I 131 -labelled 2-benzo (1 , 3) dioxol-5-yl-3-benzyl-4- (4- methoxyphenyl) -4-oxobut-2-enoate ; iv) a gas containing vesicle carrying 27-0-3- [2- (3- carboxy-acryloylamino) 5-hydroxyphenyl] -acryloyloxy mycerone ; v) a gas containing vesicle carrying a lipopeptide functionalised with sulfisoxazole; vi) I 131 -labelled sulfisozazole ; and vii) a Tc chelate of N- (MAG-3) -sulfisoxazole . 16. A pharmaceutical composition comprising a composition of matter of formula I as defined in any of claims 1 to 15 together with at least one pharmaceutically effective carrier or excipient. 17. The use of a composition of matter of formula I as defined in any of claims 1 to 15 for the manufacture of a contrast medium for use in a method of diagnosis involving administration of said contrast medium to an animate subject and generation of an image of at least part of said subject. 18. A method of generating an image of an animate human or non-human animal subject involving administering a contrast agent to said subject and generating an image of at least a part of said subject to which said contrast agent has distributed, characterised in that as said contrast agent is used a composition of matter of formula I as defined in any of claims 1 to 15. 19. A method as claimed in claim 18 wherein said image is a nuclear imaging image. 20. A method as claimed in claim 18 wherein said image is an ultrasound image. 21. A method of monitoring the effect of treatment of a human or non-human animal subject with a drug to combat or provoke effects associated with endothelin, said method involving administering to said subject a composition of matter of formula I as defined in any of claims 1 to 15 and detecting the uptake of said agent by endothelin receptors, said administration and detection optionally being effected repeatedly. 22. A process for the preparation of a composition of a matter of formula I as defined in any of claims 1 to 15, said process comprising conjugating (i) an organic compound having binding affinity for an endothelin receptor to (ii) a compound detectable in a diagnostic imaging procedure or a chelant compound and if necessary metallating chelant groups in the resultant conjugate with a metal ion detectable in a diagnostic imaging procedure .

Inventors: CUTHBERTSON ALAN, KLAVENESS JO, NAEVESTAD ANNE, SOLBAKKEN MAGNE, CUTHBERTSON, ALAN, KLAVENESS, JO, NAEVESTAD, ANNE, SOLBAKKEN, MAGNE
Applicants: COCKBAIN JULIAN, CUTHBERTSON ALAN, KLAVENESS JO, NAEVESTAD ANNE, NYCOMED IMAGING AS, SOLBAKKEN MAGNE, COCKBAIN, JULIAN, CUTHBERTSON, ALAN, KLAVENESS, JO, NAEVESTAD, ANNE, SOLBAKKEN, MAGNE
Application year: 1997
Main classification: A61K49
IPC codes: A61K47, A61K49, A61K51