Cytotoxic Activity of Novel GnRH Analogs Conjugated with Mitoxantrone in Ovarian Cancer Cells

The gonadotropin-releasing hormone (GnRH) receptor (GnRH-R) is highly expressed in ovarian cancer cells (OCC), and it is an important molecular target for cancer therapeutics. To develop a new class of drugs targeting OCC, we designed and synthesized Con-3 and Con-7 which are novel high-affinity GnRH-R agonists, covalently coupled through a disulfide bond to the DNA synthesis inhibitor mitoxantrone. We hypothesized that Con-3 and Con-7 binding to the GnRH-R of OCC would expose the conjugated mitoxantrone to the cellular thioredoxin, which reduces the disulfide bond of Con-3 and Con-7. The subsequent release of mitoxantrone leads to its intracellular accumulation, thus exerting its cytotoxic effects. To test this hypothesis, we determined the cytotoxic effects of Con-3 and Con-7 using the SKOV-3 human OCC. Treatment with Con-3 and Con-7, but not with their unconjugated GnRH counterparts, resulted in the accumulation of mitoxantrone within the SKOV-3 cells, increased their apoptosis, and reduced their proliferation, in a dose- and time-dependent manner, with half-maximal inhibitory concentrations of 0.6-0.9 µM. It is concluded that Con-3 and Con-7 act as cytotoxic "prodrugs" in which mitoxantrone is delivered in a GnRH-R-specific manner and constitute a new class of lead compounds for use as anticancer drugs targeting ovarian tumors.