Most importantly, cellularly, the Pt(IV) complexes remarkably inhibited HDAC activity, reduced the expression of HDAC enzymes, and induced distinct rates of histone H3 hyperacetylation in cisplatin-sensitive (A2780) and cisplatin-resistant (A2780cisR) ovarian cancer cells, resulting in chromatin decondensation. This evidence concerns the gene HDAC9 and ovarian cancer.