Intravenous pretreatment with empty pH gradient liposomes alters the pharmacokinetics and toxicity of doxorubicin through in vivo active drug encapsulation

Liposomes have been used widely to improve the therapeutic activity of phar maceutical agents. The traditional approach for such applications has been to formulate the pharmaceutical agent in liposomes prior to administration in vivo. In this report we demonstrate that liposomes exhibiting a transmem brane pH gradient injected intravenously (iv) can actively encapsulate doxo rubicin in the circulation after iv administration of free drug. Small (110 nm) liposomes composed of phosphatidylcholine (PC)/cholesterol (Chol, 55:4 5 mol: mel) exhibiting a pH gradient (inside acidic) were administered iv 1 h prior to free doxorubicin, and plasma drug levels as well as toxicity an d efficacy were evaluated. Predosing with egg PC/Chol pH gradient liposomes increased the plasma concentration of doxorubicin as much as 200-fold comp ared to free drug alone as well as to predosing with dipalmitoyl PC/Chol pH gradient liposomes or EPC/Chol liposomes without a pH gradient. The abilit y of the liposomes to alter the pharmacokinetics of doxorubicin was depende nt on the presence of a transmembrane pH gradient and correlated with the e xtent of doxorubicin uptake into the liposomes at 37 degrees C in pH 7.5 bu ffer, indicating that doxorubicin was being actively accumulated in the cir culating liposomes. This in vivo drug loading was achieved over a range of doxorubicin doses (5 mg/kg-40 mg/kg) and was dependent on the dose of EPC/C hol liposomes administered prior to free doxorubicin injection. The altered pharmacokinetic properties of doxorubicin associated with in vivo doxorubi cin encapsulation were accompanied by a decrease in drug toxicity and maint ained antitumor potency. These results suggest that pretreatment with empty liposomes exhibiting a pH gradient may provide a versatile and straightfor ward method for enhancing the pharmacological properties of many drugs that can accumulate into such vesicle systems at physiological temperatures.