Photoactivatable Platinum Complexes

The development of photoactivatable prodrugs of platinum-based antitumor agents is aimed at increasing the selectivity and hence lowering toxicity of this important class of antitumor drugs. These drugs could find use in treating localized tumors accessible to laser-based fiber-optic devices. Pt-IV complexes appeared attractive because these octahedral complexes are usually substitution inert and require reduction to the Pt-II species to become cytotoxic. Based on the knowledge of Pt-IV photochemistry, Pt-IV analogs of cisplatin, [Pt(en)Cl-2] and transplatin were designed, synthesized and investigated for their ability to be photoreduced to cytotoxic Pt-II species. Two classes of photoactivatable Pt complexes have been looked at thus far: diiodo-Pt-IV and diazido-Pt-IV diam(m)ine complexes. The first generation, diiodo-Pt-IV complexes, represented by cis, trans-[Pt(en)(I)(2)(OAc)(2)], react to visible light by binding irreversibly to DNA and forming adducts with 5'-GMP in the same manner as [Pt(en)Cl-2]. Furthermore, the photolysis products are cytotoxic to human cancer cells in vitro. However, these complexes are too reactive towards biological thiols (i.e.,glutathione), which rapidly reduced them to cytotoxic Pt-II species, thus making them unsuitable as drugs. The second generation, diazido-Pt-IV complexes, represented by cis, trans, cis-[Pt(N-3)(2)(OH)(2)(NH3)(2)] and cis, trans-[Pt(en)(N-3)(2)(OH)(2)], are also photosensitive, binding irreversibly to DNA and forming similar products with DNA and 5'-GMP in the presence of light as the respective Pt-II complexes. However, they are stable to glutathione and thus show very low dark cytotoxicity. Light of lambda(irr) = 366 nm activates both complexes to cytotoxic species that effectively kill cancer cells by destroying their nuclei, leaving behind shrunken cell ghosts. Interestingly, the all-trans analog, trans, trans, trans-[Pt(N-3)(2)(OH)(2)(NH3)(2)] is non-toxic to HaCaT keratinocytes in the dark, but as active as cisplatin in the light. These studies show that photoactivatable Pt-IV antitumor agents represent a promising area for new drug development.