DNA compaction by mononuclear platinum cancer drug cisplatin and the trisplatinum anticancer agent BBR3464: Differences and similarities.

Cisplatin, a mononuclear platinum compound, which is known as a cancer drug for long time, can exhibit considerable side effects and is also not effective in many types of cancer. Therefore, the alternative platinum anticancer agents that can act at a much lower dose limit compared to the dose relevant for cisplatin treatment have been searched for. BBR3464, a trinuclear platinum compound, is found to exhibit cytotoxic effects at 10 to 1000 times lower dose limit, even in cisplatin-resistant cancer cells. The primary cellular target for cisplatin and BBR3464 is thought to be DNA. Herein, we report the nature of DNA structural changes that are induced by cisplatin and BBR3464, considering the same DNA sequence and similar sample deposition methods for comparison purpose. We have applied high-resolution atomic force microscopy (AFM) in order to obtain an idea about the molecular basis of BBR3464's effectiveness at the lower dose limit. We show from the molecularly resolved AFM images that both the compounds can compact the whole dsDNA molecules, though the degree of compaction in case of BBR3464 treatment is significantly higher. Furthermore, local compaction in terms of loop structure formation could be induced by both BBR3464 and cisplatin, though BBR3464 generated microloops and macroloops both, whereas cisplatin could generate primarily the microloops. It is a significant observation that BBR3464 could induce relatively drastic DNA structural changes in terms of loop formation as well as overall DNA compaction at a molar ratio, which is 50 times less than that applied for cisplatin treatment. Implications of such structural changes in cytotoxic effects of the platinum anticancer agents will be mentioned.