Synthesis and X-ray crystal structure of trans,cis-[Pt(OAc)(2)I-2(en)]: A novel type of cisplatin analog that can be photolyzed by visible light to DNA-binding and cytotoxic species in vitro

An original approach intended to facilitate the intratumoral activation of Pt(IV) diamines by illumination with visible light to form photolysis products that irreversibly bind to DNA and are cytotoxic to human cancer cells is reported. The novel Pt(TV) complex trans, cis-[Pt(OAc)(2)I-2(en)] was prepared by the acetylation of trans,cis-[Pt(OH)(2)I-2(en)] with acetic anhydride in CH2Cl2; trans,cis-[Pt(OH)(2)I-2(en)] was synthesized by oxidation of [PtI2(en)] with 30% aqueous H2O2. trans,cis-[Pt(OAc)(2)I-2(en)] crystallized from methanol as deep-red needles with a = 9.029(4) Angstrom, b 11.443(2) Angstrom, c 12.822(2) Angstrom, beta = 95.48(3)degrees, monoclinic space group Cc, and Z = 4. The conformation of the acetato groups around the O-Pt-O axis deviated significantly from the conformation of the acetato groups in the X-ray crystal structure reported for the cis-dichloro analog, which may explain the very different aqueous solubilities of the two compounds. trans,cis-[Pt(OAc)(2)I-2(en)] and trans,cis-[Pt(OH)(2)I-2(en)] displayed broad ligand-to-metal charge-transfer bands centered at lambda = 389 and 384 nm, respectively (epsilon 1372 and 1425 M(-1) cm(-1), respectively), with tailing out to ca. 550 nm. When trans,cis-[Pt(OAc)(2)I-2(en)] was incubated with calf thymus DNA in the absence of light, no covalent binding of Pt to DNA was measurable after 6 h; however, irradiation with light of wavelengths > 375 nm resulted in 63 +/- 13% of the platinum being covalently bound to DNA after 6 h, suggesting that a photoreduction to Pt(II) species took place. Although trans,cis-[Pt(OH)(2)I-2(en)] was also labile to visible light;, only 10 +/- 2% DNA platination was observed after 6 h of illumination; however, covalent binding of Pt to DNA took place quantitatively when a reducing agent such as glutathione was added to the photolyzed incubations. These results provide evidence that the photolysis of the transdihydroxo analog resulted predominately in the substitution of the iodide ligands for water rather than a reduction of Pt(IV) to Pt(II). When protected from light, trans,cis-[Pt(OAc)(2)I-2(en)] and trans,cis-[Pt(OH)(2)I-2(en)], both at a concentration of 10 mu M, had half-lives of 6.6 +/- 0.5 and 46.8 +/- 8.8 h, respectively, at 37 degrees C in Eagle's minimum essential medium (EMEM) containing 5% fetal calf serum. When irradiated with light lambda(irr) > 375 nm, the half-lives were decreased by 24- and 53-fold for the diacetato- and dihydroxoplatinum(IV) complexes, respectively. Compared to the ''dark'' control, the in vitro treatment of TCCSUP human bladder cancer cells with trans,cis-[Pt(OAc)(2)I-2(en)] resulted in 35% greater growth inhibitory activity when during the first 1.5 h of drug exposure the cells were irradiated with light lambda(irr) > 375 nm. The photolysis of trans,cis-[Pt(OH)(2)I-2(en)] with visible light resulted in a 22% enhancement of antiproliferative activity.