PURPOSE: The goal of this study is to understand the activation processes that take place within the liposomal formulation of lipophilic diaminocyclohexane platinum (DACH-Pt) complexes, to identify the activated species of this class of compounds, and to use that information to develop a reproducible liposomal formulation of DACH-Pt complexes. MATERIALS AND METHODS: Liposomal DACH-Pt complexes were prepared by lyophilization-rehydration method using PC, PG and PA. Their intraliposomal stability and biological activity were determined by HPLC and in vitro/in vivo experiments. RESULTS: DACH-Pt complexes in a liposomal formulation have shown significant promise in preclinical studies and clinical phase I, II trials. Interestingly, they are prodrugs which converts into one or more undetennined activated platinum species within the liposomes ex vivo. Our studies have shown that the stability of liposomal DACH-Pt complexes is inversely related with the antitumor activity of those complexes. The configuratian of leaving group in the complexes and pH of the liposome suspension affect significantly the degradation/activation process that takes place within the liposomes. DACH-Pt complexes with linear (L10) leaving groups are more stable than complexes with branched ones (B10 and NDDP), but also significantly less potent. The presence of PG and PA in the liposome is a prerequisite for the degradation/activation process of DACH-Pt complexes. As PG and PA formulation gave more dramatic changes of the original complexes than PC alone due to lower pH, the cytotoxicity and antitumor activity at those fonnulations increased against PC alone. DACH-Pt complexes are very stable in liposomes containing PC alone but inactive in vitro/in vivo experiments. CONCLUSION: These results also support that the active species produced within the liposomal DACH-Pt complexes is DACH-Pt-Cl2.