A novel [186/188Re]-labelled porphyrin for targeted radiotherapy

The concept of labelling a porphyrin, a tumour-avid agent, with a radionuclide to evaluate its potential as a therapeutic modality is reported. A novel water-soluble porphyrin, namely meso-tetrakis[3,4-bis(carboxymethyleneoxy)phenyl]porphyrin, with suitable dicarboxylic acid groups as aromatic substituents in the periphery, was synthesized and characterized. The labelling of this porphyrin with 186/188Re, a beta(-) emitter, was optimized by varying the reaction conditions. The complexation yield was >98% as estimated by paper chromatography in acetone and in saline. The radiochemical purity was found to remain at >98% when stored at 4 degrees C for 24 h. Biodistribution studies in Swiss mice bearing fibrosarcomas showed an uptake of approximately 3.5% per gram of tumour at 30 min post-injection. This uptake in the tumour was retained until 24 h post-injection with major activity showing renal clearance; no significant activity was present in other organs of interest. The tumour/blood and tumour/muscle ratios were observed to be 38 and 5, respectively, at 24 h post-injection, thereby indicating a possible therapeutic potential for tumours.