Pharmacokinetics of methotrexate after intravenous and intramuscular injection of methotrexate‐bearing positively charged liposomes to rats

The pharmacokinetics and tissue distribution of methotrexate (MTX) were investigated after intravenous (IV) and intramuscular (IM) injection of free MTX (treatment I), freshly prepared MTX‐bearing positively charged liposomes (large unilamellar vesicles), PLUVs (treatment II), and empty PLUVs mixed manually with free MTX (treatment III), 4 mg kg⁻¹ as free MTX to rats, using HPLC assay. After 1 min IV infusion, the plasma concentrations of MTX (C_p), the area under the plasma concentration—time curve (AUC, 173 against 314 μg mL min⁻¹), the terminal half‐life (t_1/2, 24.0 against 412 min), the mean residence time (MRT, 13.0 against 324 min), and the apparent volume of distribution at steady state (V_SS, 289 against 3370 mL kg⁻¹) were significantly larger, but the total body clearance (CL, 23.1 against 12.8 mL min⁻¹ kg⁻¹), the renal clearance (CL_R, 8.38 against 3.09 mL min⁻¹ kg⁻¹), the non‐renal clearance (CL_NR, 14.6 against 9.56 mL min⁻¹ kg⁻¹), and the amount of MTX excreted in urine (X_u, 415 against 275 μg) were significantly lower in treatment II than in treatment I. This could be due to the fact that some of the MTX‐bearing PLUVs were entrapped in tissues and the rest were present in plasma (larger MRT and V_ss in treatment II), and MTX is slowly released from MTX‐bearing PLUVs (longer t_1/2 in treatment II). In the present HPLC assay, the concentrations of MTX represent the sum of free MTX and MTX in MTX‐bearing PLUVs (larger C_p and AUC and slower CL in treatment II). Saturable formation of 7‐hydroxymethotrexate from MTX was reported in rabbit blood and non‐linear disposition of MTX was also reported in rats and rabbits (lower X_u and CL_R in treatment II). After 1 min IV infusion, some pharmacokinetic parameters of MTX, such as AUC, CL, CL_R, CL_NR, and X_u, were significantly different between treatments I and III, but nonetheless the differences were smaller than those between treatments I and II. After both IV and IM administration, the amount of MTX remaining per gram of tissue or organ in the kidney, stomach, small intestine, and large intestine was significantly smaller in treatment II than in treatment I. Such tissue results imply that the side‐effects of MTX on kidney and GI tract could be reduced after both IV and IM administration of MTX‐bearing PLUVs rather than free MTX. The encapsulation efficiency of MTX in MTX‐bearing PLUVs was 5.47%, and MTX was released slowly from MTX‐bearing PLUVs when incubated in phsophate buffered saline, rat plasma and rat liver homogenates.