cis-[PtCl₂(4,7-H-5-methyl-7-oxo[1,2,4]triazolo[1,5-a]pyrimidine)₂]:  A Sterically Restrictive New Cisplatin Analogue. Reaction Kinetics with Model Nucleobases, DNA Interaction Studies, Antitumor Activity, and Structure−Activity Relationships

The formation and isolation of the antitumor drug cisplatin analogue cis-[PtCl2(Hmtpo-N3)2].2H2O (1) (where Hmtpo = 4,7-H-5-methyl-7-oxo[1,2,4]triazolo[1,5-a]pyrimidine) by reaction of Hmpto with K2[PtCl4] in HCl (0.5 N) is reported. This complex crystallizes in the monoclinic space group P21/c with unit cell dimensions a = 15.215(2) A, b = 9.629(1) A, c = 13.115(3) A, beta = 97.40(2) degrees, and Z = 4. The molecular structure shows that Pt is in an almost square planar environment, PtN2Cl2, which has a cis configuration. The Hmpto ligands show a head to head orientation in the solid state and nonrestricted rotation about the Pt-N bonds in solution. The reactivity of the complex to model nucleobases 9-ethylguanine (9-EtGH) and 1-methylcytosine (1-MeC) has been investigated by 1H NMR spectroscopy at 45 degrees C in aqueous media. The results show that 1 reacts slowly with 9-EtGH (t1/2 approximately 5 days) by displacement of Cl-, producing cis-[Pt(mtpo-N3)2(9-EtGH-N7)2], which is similar to the major cross-link adduct of cisplatin with DNA. However, 1 gives no reaction with 1-MeC. This appears to be due to the lesser reactivity of 1-MeC and to competition between the cross-link reaction and dimerization of 1 to [Pt2(mu-mtpo-N3,N4)4]. Circular dichroism studies of DNA in the presence of 1 show that the platinum complex reacts efficiently after 48 h at a optimum ratio of 0.25 Pt atom/mol of DNA nucleotide. These results and those obtained from reaction of 1 with 9-EtGH suggest that the platinum compound binds the N7 atoms of two guanines of the same strand, forming intrastrand cross-linked adducts. Chelation of DNA bases by 1 causes important conformational changes, bringing the guanines close together. The anticancer activity of complex 1 has been tested against the human cancer cell lines MCF-7 breast carcinoma and A121 ovarian carcinoma. Results indicate a moderate antitumor activity against breast carcinoma and a marked and selective cytotoxic effect against ovarian carcinoma.