Trimethoprim interference in methotrexate assay by an enzyme inhibition assay kit

Area of Science:

• Clinical Chemistry
• Pharmacology
• Oncology

Background:

• Methotrexate (MTX) is a key chemotherapy agent used in leukemia treatment.
• Accurate monitoring of MTX levels in serum and cerebrospinal fluid is essential for therapeutic efficacy and toxicity management.
• Trimethoprim (TMP) is an antibiotic sometimes co-administered with MTX, raising concerns about drug interactions.

Purpose of the Study:

• To investigate and quantify the interference of trimethoprim (TMP) with methotrexate (MTX) assays.
• To determine the impact of TMP on measured MTX concentrations in biological samples from leukemia patients.

Main Methods:

• Utilized a standard MTX assay kit based on dihydrofolate reductase inhibition.
• Analyzed serum and cerebrospinal fluid samples from leukemia patients receiving TMP.
• Quantified apparent MTX concentrations in the presence of known TMP concentrations.

Main Results:

• Confirmed significant interference of TMP with the MTX assay.
• A TMP concentration of 0.17 microgram/ml resulted in an apparent MTX value of 10 microM.
• This indicates a potential for substantial overestimation of MTX levels when TMP is present.

Conclusions:

• Trimethoprim causes spurious elevations in measured methotrexate concentrations.
• This interference can lead to misinterpretation of MTX therapeutic drug monitoring results.
• Clinicians should be aware of this potential assay interference when interpreting MTX levels in patients receiving TMP.