Radiorhodium Agents

The utility of radiation in the destruction of cancerous tissues was recognized and exploited since the time of M. Sklodowska-Curie. Radiometals possess a wider range of nuclear properties and show diverse chemistries; therefore they are very valuable pharmaceuticals.90 Desirable radionuclide characteristics for radiotherapy are high specific activity, low 7-emission, a decay half-life similar to the half-life of residence at the target, high linear energy transfer and ranges of particles comparable with the site of deposition. The radionuclide 105Rh [(35.4h, 0.567MeV ¡3~ (70%), 0.247MeV ¡3~ (30%), 0.320 MeV 7)] is an attractive ^^-emitter for radiotherapy because of its advantageous properties. Complexes used for radiotherapy should be inert, to minimize loss of the rhodium complex from its carrier molecule. The rhodium(III) complexes of d6 low-spin electronic configuration belong to the most inert complexes. The complexes with multidentate, chelating ligands are more inert than the complexes with appropriate monodentate ligands. Therefore very frequently, the complexes with multidentate ligands are used as radiotherapeutic agents. In order to bind a metallic complex with a carrier, it is important to prepare a suitable bifunctional chelate that will act both as a ligand to the radionuclide and as a linker to the carrier molecule. Examples of such molecules are thiobis[ethylenenitrilo(tetra-acetic acid)], S{CH2CH2N(CH2COOH)2}2 (L)91 and 1,5,9,13-tetrathiacyclo-hexadecane-3,11-diol (L0)92 which form octahedral Rh(III) complexes [RhL] and [RhCl2L']+, respectively. The 105Rh(III) complexes with HOOCCH2S (CH2CH2S)3CH2COOH (2,5,8,11-tetrathiadodecane-1,12-dicarboxylic acid) and analogous tetrathiatridecane- and tetrathiatetradecane-dicarboxylic acids were evaluated for their stability under physiological conditions and in vivo in normal mice. The results indicated that the cis-[RhCl2{HOO CCH2(SCH2CH2)3SCH2COOH}]+ complex showed the most promising in vivo characteristics. Thus, this complex is a potential radiopharmaceutical.93 The stability of the 105Rh-bleomycin complex in plasma is sufficient to allow targeted delivery of the radioisotope.94 The biodistribution of 105Rh complexes with trithiaazacyclotetradecanes and tetrathiacyclohexadecanes (1,4,7-trithia-11-azacyclotetradecane, 1,5,9-trithia-13-azacyclohexadecane, 1,5,9,13-tetrathia-cyclohexadecane-3-ol, etc.) has been investigated in rats. The results revealed that the cis-[RhCl2{14}ane]+ complexes exhibited less kidney retention than the trans-[RhCl2{16}ane]+ complexes.95 The 103mRh complexes have also been proposed for targeted radiotherapy of small tumors.96 Rhodium trichloride was used as catalyst for regiospecific deuteriation and tritiation of various drugs.97 The 106Ru 106Rh radioisotopes are used for the treatment of chor-oidal melanoma.7 Brachytherapy with 106Ru-106Rh plaques can be used for small medium-sized choroidal melanomas.98,99 The treatment using 106Ru-106Rh radionuclides gives frequently satisfactory results.

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