Pharmacokinetics and Biodistribution of Pegylated Methotrexate after IV Administration to Mice

The efficacy of methotrexate (MTX) as an antimetabolite chemotherapeutic agent highly depends on its blood circulation half-life. In our previous study, different conjugates of MTX (MTX-PEG) were synthesized, their physicochemical properties were investigated and MTX-PEG5000 was finally selected as optimum drug-conjugate for further investigations. In the current work, first the stability of MTX-PEG5000 was studied at 37 °C and the results indicated its high stability in plasma (T_1/2 = 144 h) and a relatively rapid degradation in tissue homogenate (T_1/2 = 24 h). The study of protein binding pointed out that the conjugate was highly protein-bound (95%). The results of pharmacokinetic studies in mice indicated that MTX-PEG5000 had longer plasma distribution and elimination half-lives compared to free MTX (T_{1/2 α} 9.16 min for MTX-PEG5000 versus 2.45 min for MTX and T_{1/2 β} 88.44 for MTX-PEG5000 versus 24.33 min for MTX). Pharmacokinetic parameters also showed higher area under the curve (AUC) of conjugate compared to parent drug (12.33 mg.mL^-1.min for MTX-PEG5000 versus 2.64 mg.mL^-1.min for MTX). The biodistribution studies demonstrated that MTX-PEG5000 did not highly accumulate in liver and intestine and had a mild and balanced distribution to other organs. Also, the conjugate was measurable in tissues up to 48 h after injection and was detected in the brain, suggesting the possibility of delivering drug to brain tumors.