Formation and disposition of 7-hydroxymethotrexate in rabbits

Area of Science:

• Pharmacokinetics
• Drug Metabolism
• Toxicology

Background:

• Methotrexate (MTX) is a crucial chemotherapy agent.
• Understanding MTX metabolism is vital for optimizing therapeutic efficacy and minimizing toxicity.
• Rabbit models offer insights into mammalian drug disposition.

Purpose of the Study:

• To investigate the metabolic profile of methotrexate in rabbits.
• To identify and quantify major methotrexate metabolites.
• To characterize the pharmacokinetic behavior of methotrexate and its metabolites.

Main Methods:

• Intravenous infusion of radiolabeled methotrexate ([3',5',7-(3)H]methotrexate) in rabbits.
• Analysis using enzyme kinetics, reverse-phase high-performance liquid chromatography (RP-HPLC), and scintillation counting.
• In vitro studies to assess tissue-specific conversion of methotrexate.

Main Results:

• 7-Hydroxymethotrexate (7-OH-MTX) identified as a major metabolite in plasma, urine, bile, and tissues.
• Cumulative 7-OH-MTX production reached 31.8% of the total dose within 6 hours.
• Peak plasma 7-OH-MTX concentrations exceeded those of MTX, with slower, biphasic clearance.
• Kidneys exhibited the highest MTX concentration, significantly exceeding plasma levels.
• Higher 7-OH-MTX/MTX ratios observed in liver, small intestine, kidney, and testis compared to plasma.
• In vitro studies confirmed significant MTX to 7-OH-MTX conversion in lung, kidney, and liver.

Conclusions:

• 7-Hydroxymethotrexate is a significant metabolite of methotrexate in rabbits.
• Tissue accumulation and slower clearance of 7-OH-MTX contribute to methotrexate's overall disposition.
• Understanding these metabolic pathways is crucial for predicting methotrexate toxicity and efficacy.