Pharmacokinetics and tissue distribution of methotrexate after intravenous injection of differently charged liposome-entrapped methotrexate to rats

Abstract

Liposome-entrapped [ 3H]MTX was prepared by modification of reverse-phase evaporation vesicle (REV) methods. Neutral liposomes were prepared with a mixture of phosphatidylcholine (PC), cholesterol (CH) and α-tocopherol (α-T) (8:4:0.1, molar ratio). Positively and negatively charged liposomes were also prepared by incorporation of stearylamine (SA) (8:4:0.1:1, molar ratio) and dicetyl phosphate (DCP) (8:4:0.1:1, molar ratio) into neutral liposomes, respectively. The release profiles of [ 3H]MTX from liposomes in the presence of rat plasma were the same shape regardless of surface charges, showing initial fast release followed by much slower release. The release profiles of [ 3H]MTX from liposomes were dependent on the surface charge of liposomes. Negatively charged liposomes showed much greater [ 3H]MTX release compared to positively charged and neutral liposomes. The enhanced in vitro release of [ 3H]MTX from negatively charged liposomes in the presence of rat plasma may be due to the interaction of incorporated lipid compositions of liposomes with plasma components, resulting in a change in drug release through the lipid bilayers by destabilization of the liposomal membranes. After intravenous (i.v.) injection of free and liposome-entrapped [ 3H]MTX to rats, the elimination of [ 3H]MTX from the blood stream showed biphasic patterns indicating rapid declining disposition up to 30 min followed by a slower elimination phase. Liposome-entrapped [ 3H]MTX maintained a higher and longer plasma concentration, and mainly intact form in plasma compared to free [ 3H]MTX for 2 h. Liposome-entrapped [ 3H]MTX enhanced bioavailability in plasma due to the retardation of drug release and protection of drug clearance by lipid bilayers of liposomes. Negatively charged liposome-entrapped drug was cleared more rapidly from the blood, resulting from the rapid uptake by liver to a greater extent, possibly via the reticuloendothelial system (RES), since liposome-entrapped drug cannot be eliminated by the kidney, the main eliminating organ for MTX. The tissue distribution of liposome-entrapped [ 3H]MTX was widely different when compared to free [ 3H]MTX. There was a markedly increased uptake of drug in spleen from neutral and negatively charged liposomes. Negatively charged liposomes also increased localization of drug in liver, lung and lymph nodes compared to neutral and positively charged liposomes at 2 h after i.v. injection to rats. From these findings, liposomes containing anticancer drugs would appear to be an effective carrier system for targeting to these sites. Although liposome-entrapped [ 3H]MTX was widely distributed in tissues without the exact role of the liposomal surface charge being known, it was evident that the surface charge of liposomes altered the biodistribution and membrane permeation of drug.