Relationships between liposome properties, cell membrane binding, intracellular processing, and intracellular bioavailability.

Positive surface charge enhances liposome uptake into cells. Pegylation, used to confer stealth properties to enable in vivo applications of cationic liposomes, compromises internalization. The goal of this study was to determine the quantitative relationships between these two liposome properties (separately and jointly), liposomes binding to cell membrane, and the subsequent internalization and residence in intracellular space (referred to as intracellular bioavailability). The results, obtained in pancreatic Hs-766T cancer cells, revealed nonlinear and inter-dependent relationships, as well as substantial qualitative and quantitative differences. The proportionality constant K of intracellular and membrane-bound liposomes at equilibrium (i.e., I(eq) and B(eq)) showed a positive triphasic relationship with surface charge and a negative biphasic relationship with pegylation. Near-neutral liposomes showed little internalization of the membrane-bound moiety, increasing to a constant K value for medium charge liposomes (+15 to +35 mV zeta potential), followed by a further increase for highly charged liposomes (greater than or equal to +46 mV). The decline of pegylation with K value showed a breakpoint at 2%. The negative consequences of pegylation (%PEG) were partially offset by increasing charge (ZP). The best-fitting regression equations are: B(eq) = -1.36 × %PEG + 0.33 × ZP; I(eq) = -1.52 × %PEG + 0.34 × ZP. It suggested that 1% pegylation increase can be offset with 4 mV ZP. The differences are such that it may be possible to balance these parameters to simultaneously maximize the stealth property and intracellular bioavailability of cationic liposomes.