Topophore C: a liposomal nanoparticle formulation of topotecan for treatment of ovarian cancer

Abstract

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.

Topotecan and Doxorubicin Combination to Treat Recurrent Ovarian Cancer: The Influence of Drug Exposure Time and Delivery Systems to Achieve Optimum Therapeutic Activity

“…Although the 20 mg/kg free topotecan dose was tolerated, 15 mg/kg was used for the drug combination studies with Doxil because it was therapeutically superior. As noted previously (30), the MTD of Topophore C was 7.5 mg/kg, at least 2-fold lower than the dose that can be achieved with free drug. Treatment with Topophore C, however, provided significantly better activity than free topotecan when administered at equitoxic doses.…”

“…Liposomes were prepared using DSPC and cholesterol (CH; 55:45 molar ratio) by film hydration extrusion as described previously (30). Briefly, DSPC and CH were weighed, dissolved in chloroform, and then mixed such that the final mole ratio of the 2 lipids was 55:45, respectively.…”

“…Lipid concentrations were measured by scintillation counting of the lipid marker 3 H-CHE. Topotecan concentrations were determined by measuring the absorbance at 370 nm on UV-Vis spectrophotometer (Agilent/Hewlett Packard, model: 8453, Agilent Technologies) as described previously (30). Briefly, a portion of the sample collected from the spin columns was adjusted to 100 mL followed by addition of 900 mL Triton X-100 (1% v/v).…”

“…Doxil, an LNP formulation of doxorubicin, is already approved for use in relapsed ovarian cancer. Topophore C, an LNP formulation of topotecan that was recently described (30,31), was selected for the in vivo studies summarized below. Before evaluating efficacy, it was important to assess whether co-administration of the various drug combinations selected affected the pharmacokinetic of the drugs when compared with their use alone.…”

“…This concept envisioned development of novel combination products (2 drugs formulated in a single LNP composition) or the combination of 2 different LNP formulations. A number of groups, including our own, have been pursuing the development of optimized LNP formulations for topotecan (27)(28)(29)(30), and many of these exhibit promising therapeutic potential based on preclinical studies. The formulation recently developed in our laboratory uses pH gradient encapsulation methods combined with drug complexing ability of encapsulated copper to prepare a formulation that exhibits improved drug retention in vivo and improved therapeutic activity when compared with the free drug (30).…”