THE PHARMACOKINETICS OF METHOTREXATE AFTER INTRAVENOUS ADMINISTRATION OF METHOTREXATE-LOADED PROLIPOSOMES TO RATS

The pharmacokinetics and tissue distribution of methotrexate (MTX) were investigated after intravenous (i.v.) injection of free MTX (treatment I), MTX-loaded proliposomes (treatment II), and empty proliposomes mixed manually with free MTX (treatment III), 8 mg kg(-1), to rats using an HPLC assay. After i.v. infusion in 1 min, the plasma concentration of MTX (C-p), the area under the plasma concentration-time curve (AUC, 639 versus 913 mu g min mL(-1)), the terminal half-life (t(1/2), 48.8 versus 397 min), the mean residence time (MRT, 8.40 versus 325 min), and the apparent volume of distribution at steady state (V-ss, 98.1 versus 2800 mL kg(-1)) were significantly higher; however, the total body clearance (CL, 12.5 versus 8.76 mL min(-1) kg(-1)), renal clearance (CL(R), 4.49 versus 2.78 mL min(-1) kg(-1)), non-renal clearance (CL(NR), 7.50 versus 5.99 mL min(-1) kg(-1)), and the amount of MTX excreted in urine (X(u), 808 versus 685 mu g, p < 0.0948) were significantly lower from treatment II than from treatment I. This could be due to the fact that some of the MTX-loaded liposomes (formed immediately after hydration of MTX-loaded proliposomes) are entrapped in tissues and the rest are present in the plasma (higher MRT and V-ss from treatment II), and MTX is slowly released from MTX-loaded liposomes (higher t(1/2) from treatment II). In the present HPLC assay, the concentrations of MTX represent the sum of free MTX and MTX loaded in liposomes (higher C-p and AUC, slower CL from treatment II). After i.v. infusion in 1 min, some pharmacokinetic parameters, such as t(1/2), MRT, and V-ss, were significantly different between treatments I and III; however, the differences seemed to be smaller than those between treatments I and II. After 30 min from i.v. infusion, the tissue to plasma (T/P) ratios of MTX in kidney and stomach from treatment II were significantly lower than those from treatment I. This suggested that the i.v. administration of MTX-loaded proliposomes might have fewer side effects in the organs than that of free MTX. The mean amount of MTX loaded in MTX-loaded proliposomes was 2.54 mg/g proliposomes and the MTX was released slowly from hydrated MTX-loaded proliposomes when incubated with phosphate-buffered saline (PBS), rat plasma, or rat liver homogenate.