THE EFFECTS OF PH AND INTRALIPOSOMAL BUFFER STRENGTH ON THE RATE OF LIPOSOME CONTENT RELEASE AND INTRACELLULAR DRUG-DELIVERY

Targeted liposomal drug formulations may enter cells by receptor-media ted endocytosis and then traffick by membrane flow into acidic intrace llular compartments. In order to understand the impact of these intrac ellular pH changes on liposomal drug unloading, the effect of pH on th e release from folate-targeted liposomes of three model compounds with distinct pH dependencies was examined. 5(6)-carboxyfluorescein, which titrates from its anionic to uncharged form following internalization by KB cells, displays strong endocytosis-dependent release, since onl y its uncharged (endosomal) form is membrane permeable. Endocytosis-tr iggered unloading of drugs of this sort is enhanced by encapsulating t he drug in a weak buffer at neutral pH, so that acidification of the i ntraliposomal compartment following cellular uptake can occur rapidly. Sulforhodamine B, in contrast, retains both anionic and cationic char ges at endosomal pH (similar to pH 5), and consequently, escapes the e ndosomes only very slowly. Doxorubicin, which is commonly loaded into liposomes in its membrane-impermeable (cationic) form using an acidic buffer, still displays endocytosis-triggered unloading, since sufficie nt uncharged doxorubicin remains at endosomal pi-Is to allow rapid re- equilibration of the drug according to the new proton gradient across the membrane. In this case, when the extraliposomal [H+] increases 250 -fold from 4 x 10(-8) M (pH 7.4, outside the cell) to 10(-5) M (pH 5, inside the endosome), the ratio of doxorubicin inside to outside the l iposome must decrease by st factor of 250. Therefore, the collapse of the transliposomal pH gradient indirectly drives an efflux of the drug molecule from the liposome. Since a change in intraliposomal pH is no t required to unload drugs of this type, the intraliposomal compartmen t can be buffered strongly at acidic pH to prevent premature release o f the drug outside the cell. In summary, pH triggered release of lipos ome-encapsulated drugs can be achieved both with drugs that increase a s well as decrease their membrane permeabilities upon acidification, a s long as the intraliposomal buffer strength and pH is rationally sele cted.