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Biological Consequences of Trinuclear Platinum Complexes: Comparison of [{trans-PtCl(NH₃)₂}₂µ-(trans-Pt(NH₃)₂(H₂N(CH₂)₆-NH₂)₂)]⁴⁺ (BBR 3464) with Its Noncovalent Congeners

Departments of Chemistry (A.L.H., N.F.) and Biology (J.J.R.), Virginia Commonwealth University, Richmond, Virginia

[{trans-PtCl(NH₃)₂}₂µ-(trans-Pt(NH₃)₂(H₂N(CH₂)₆NH₂)₂)]⁴⁺ (BBR 3464) is a 4+ cationic trinuclear platinum drug that has undergone phase II clinical trials in the treatment of ovarian and lung cancers. The chemical structure of BBR 3464 is distinct from that of clinically used agents such as cisplatin and oxaliplatin. As a consequence, the modes of DNA binding and the structures of BBR 3464 DNA adducts are also structurally distinct from those formed by cisplatin and oxaliplatin. Previous chemical and spectroscopic measurements on BBR 3464 had elucidated a significant noncovalent contribution to DNA binding. To examine this effect further, the biological activity of two BBR 3464 analogs that bind DNA only through noncovalent interactions was investigated in this study, and their cellular effects were compared with those caused by the "parent" drug. The compounds were [{trans-PtL(NH₃)₂}₂µ-(trans-Pt(NH₃)₂(H₂N(CH₂)₆NH₂)₂)]ⁿ⁺, with L = NH₃, n = 6 for compound I, and L = H₂N(CH₂)₆NH₃, n = 8 for compound II. All compounds induce caspase-dependent apoptosis in both primary mast cells and transformed mastocytomas, although with a smaller IC₅₀ value in the transformed cells. In cells deficient in either the tumor suppressor proteins p53 or Bax, apoptosis was least affected in the case of II, but in all cases the effect of p53 deficiency was greater than that of Bax. Surprisingly, cellular uptake was actually enhanced for the more highly charged compounds, resulting in significant (micromolar) cyotoxicity for II. Cellular accumulation was enhanced in mastocytomas over primary mast cells, suggesting a mechanism for enhancement of tumor cell selectivity.

Address correspondence to: Dr. Nicholas Farrell, Department of Chemistry, Virginia Commonwealth University, 1001 W. Main St., Richmond, VA 23284. E-mail: nfarrell{at}vcu.edu

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