We have recently developed a liposomal nanoparticle (LNP) formulation of irinotecan based on loading method that involves formation of a complex between copper and the water soluble camptothecin. The loading methodology developed for irinotecan was evaluated to develop a LNP topotecan formulation (referred to herein as Topophore C) and test its activity in pre-clinical model of ovarian carcinoma. Topotecan was encapsulated into preformed liposomes containing 300 mM copper sulfate and the divalent metal ionophore A23187. Formulation optimization studies included assessments of loading efficiency, influence of temperature on drug loading and in vitro stability of the resulting formulation. In vivo assessments included drug and liposome pharmacokinetics, drug levels within plasma and the peritoneal cavity following intravenous (i.v.) administration in mice and efficacy studies on ES2 ovarian cancer model. Topotecan loading into liposomes was optimized with encapsulation efficiency of >98 % at a final drug-to-lipid (D/L) mole ratio of 0.1. Higher D/L ratios could be achieved, but the resulting formulations were less stable as judged by in vitro drug release studies. Following Topophore C administration in mice the topotecan plasma half-life and AUC were increased compared to free topotecan by 10-and 22-fold, respectively. Topophore C was 2-to 3-fold more toxic than free topotecan, however showed significantly better anti-tumor activity than free topotecan administered at doses with no observable toxic effects. Topophore C is a therapeutically interesting drug candidate and we are particularly interested in developing its use in combination with liposomal doxorubicin for treatment of platinum refractory ovarian cancer.

Purpose: To provide proof-of-concept data to support use of Doxil-liposomal topotecan (Topophore C) combinations to treat ovarian cancer.Experimental Design: ES-2, OVCAR-3, and SKOV-3 ovarian cancer cell lines were treated with doxorubicin-topotecan combinations by exposing the cells to drugs from 1 to 72 hours. Pharmacokinetic analysis was conducted following administration of liposomal formulations of these drugs alone and in combination. Efficacy assessments were completed in ES-2 and SKOV-3 ovarian cancer models.Results: On the basis of drug doses capable of achieving 50% reduction in cell viability over 72 hours, doxorubicin-topotecan combinations were additive in SKOV-3 but highly synergistic in ES-2 and OVCAR-3 cells. Favorable drug-drug interactions increased with increased drug exposure time. Topophore C pharmacokinetic remained unaffected when co-administered with Doxil. In the ES-2 model, Doxil at maximum tolerated dose (MTD 7.5 mg/kg) in combination with free topotecan (MTD 15 mg/kg) did not enhance median survival time (MST) over that achieved with topotecan alone. In contrast, MST was increased to 52 days with combination of Topophore C (MTD 2.5 mg/kg) and Doxil (7.5 mg/kg) compared with untreated animals (MST 18 days) or those treated with Topophore C alone (MTD 5 mg/kg, MST 40 days). In the SKOV-3 model, combination treatments showed better therapeutic efficacy than the individual drugs.Conclusions: Topotecan-doxorubicin combinations produced additive or synergistic effects which were best achieved when the tumor cells were exposed to drugs over extended time. Doxil-Topophore C combinations are therapeutically superior as judged in two ovarian cancer models.