Development of novel liposomal and receptor tyrosine kinase-targeted immunoliposomal vinorelbine formulations with improved drug retention

Proc Amer Assoc Cancer Res, Volume 46, 2005 4394 Following the successful clinical development of liposomal anthracyclines (doxorubicin, daunorubicin), vinca alkaloids are the next most likely candidates to join the list of clinically approved liposomal anticancer agents. However, in contrast to anthracyclines, liposome-encapsulated vincas suffer from increased drug leakage rates in vivo due to the inherently higher hydrophilicity of these drugs. We have developed a novel liposome drug loading strategy employing transmembrane gradients of the salts formed by polyalkyl-substituted amines with highly charged anions that allowed for improved loading capacity, loading efficiency, and greatly improved in vivo drug encapsulation stability of anticancer vinca alkaloids. This strategy was applied to vinorelbine, an attractive candidate for liposomal delivery due to its favorable toxicity profile, positive clinical experience in breast and lung carcinomas, and a chemical structure amenable to active loading into liposomes via transmembrane gradient approach. A series of liposomal vinorelbine formulations using various drug loads and intraliposomal drug-stabilizing anions were prepared, with drug-to-lipid ratios of up to 0.5 g/mmol phospholipid, and encapsulation efficiency over 90%. In vivo drug release rates in the blood of rats varies from drug release half-life of 1.8 hours to over 24 hours. These formulations showed prominent antitumor activity in several human and syngeneic tumor models in mice. The most active of these formulations is currently in Phase I clinical trials. The suitability of these novel vinorelbine liposome formulations for antibody-directed targeting (immunotargeting) was also studied. HER2- or EGFR- targeted immunoliposomal vinorelbines formulations were prepared from the amphiphilic PEG-DSPE conjugated of anti-HER2 scFv or anti-EGFR Fab’ and preformed vinorelbine liposomes using a micellar insertion method. The immunoliposomal vinorelbines showed target-specific cytototoxicity in cell cultures and good in vivo stability. Antitumor efficacy of these formulations tested in HER2-overexpressing breast tumor and EGFR-overexpressing brain tumor xenografts in nude mice was target specific and dependent on the rate of drug release from the liposome, with more stable constructs showing greater efficacy. Thus, better anticancer properties of liposomally formulated vinca alkaloids were achieved through improved drug encapsulation stability and cancer cell-specific targeting.