Abstract
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 μg 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 μg, 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.