Lipophilic platinum complexes entrapped in liposomes: improved stability and preserved antitumor activity with complexes containing linear alkyl carboxylato leaving groups.

Lipophilic diaminocyclohexane (DACH) platinum complexes have shown significant promise in preclinical studies. One of these compounds, cis-bis-neodecanoato-trans-R,R-1,2-diaminocyclohexaneplatinum++ +(II) (NDDP), which contains two branched leaving groups of 10 carbons, showed a favorable toxicity profile in a liposomal formulation in early clinical trials. However, like many other DACH platinum compounds with branched leaving groups, it is unstable within the liposomes, thus preventing its widespread clinical evaluation. We studied the effect of the configuration of leaving groups on intraliposomal complex stability by studying a series of DACH platinum complexes containing linear alkyl carboxylato leaving groups of 5-18 carbons. The entrapment efficiency was greater than 90% for all liposomal preparations of the complexes and was independent of lipid composition and length of the leaving group. The drug leakage from the liposomes was minimal, but was directly related to the length of the leaving group. Intraliposomal stability was inversely related to the length of the leaving group and the content of DMPG (dimyristoyl phosphatidylglycerol) in the liposomes. The effect of length of leaving group on intraliposomal stability was minimal in compounds with leaving groups smaller than 10 carbons, but very pronounced in compounds with longer leaving groups. Stable liposomal formulations of selected compounds with leaving groups of 6 and 10 carbons had significant in vivo antitumor activity against both L1210/S and L1210/PDD leukemias. The results indicate (1) that compounds with linear leaving groups are much more stable within DMPG-containing liposomes than compounds with branched leaving groups and (2) that DMPG is required for in vivo antitumor activity. Stable and active liposomal formulations of selected compounds with linear leaving groups have been identified. These formulations are candidates for clinical development.