Etoposide encapsulated in positively charged liposomes: Pharmacokinetic studies in mice and formulation stability studies

Etoposide is an antineoplastic agent which acts by forming a ternary comple x with topoisomerase II and DNA, causing DNA breaks and cell death. In rece nt studies we have demonstrated that encapsulation in liposomes increases t he antitumour efficacy and reduces the adverse effects associated with etop oside. The present study was thus conducted to evaluate whether encapsulati on in cationic liposomes altered the pharmacokinetics of etoposide and to s tudy the effect of cholesterol incorporation on the stability of the liposo mes. Etoposide-encapsulated unilammellar liposomes were synthesized by thin film hydration followed by extrusion. The drug was administered to Swiss albino mice at a dose of 10 mg kg(-1) The concentration of the drug in plasma was analysed at different time points till 360 min after injection, using a h. p.l.c, method. The terbium chloride-dipicolinic acid interaction method was applied to study the stability of the formulation in mouse serum and also following storage at 0 degreesC over a period of time. The effect of the fr ee and liposomal drug on myelosuppression was evaluated at 10 mg m(-2) and 40 mg m(-2) dose levels by quantifying blood cell counts on day 15 and day 21 following a 5 day course of therapy. Encapsulation in cationic liposomes increased the area under the concentrat ion vs time curve to 42.98 mug h ml(-1) from 24.18 mug h ml(-1) in the case of the free drug. Half-life (beta) was 58.62 and 186 min in the case of fr ee and liposomal etoposide. respectively. In the stability studies, incorpo ration of cholesterol progressively stabilized the formulation in serum. Th e use of sucrose at increasing concentrations as a cryoprotectant also incr eased the shelf stability of the formulation at 0 degreesC. Toxicity studie s using a dose of pure drug revealed that though myelosuppression was evide nt in both liposomal- and free drug-treated groups on day 15 it was reverse d by day 21 following initiation of therapy. The present findings suggest t hat liposomes could serve as an alternative mode of delivery for etoposide. (C) 2000 Academic Press.