Decreased phenytoin level during antineoplastic therapy: a case report

Area of Science:

• Oncology
• Pharmacology
• Neurology

Background:

• A male patient with brain tumor-related seizures was undergoing treatment with anticonvulsants phenytoin (PHT) and phenobarbital (PB).
• The patient also received a combination antineoplastic regimen including carmustine (BCNU), vinblastine (VLB), methotrexate (MTX), and radiotherapy.

Observation:

• Plasma concentrations of PHT significantly decreased from 9.4 to 5.6 micrograms/ml within 24 hours of VLB administration.
• These reduced PHT levels persisted for at least 10 days post-VLB treatment.
• The patient experienced partial seizures during the period of low PHT plasma concentrations.

Findings:

• Phenobarbital (PB) plasma concentrations remained unaffected throughout the observation period.
• The observed interaction suggests a potential reduction in PHT absorption.
• This impaired absorption is hypothesized to be caused by vinblastine (VLB), methotrexate (MTX), or a combination of both.

Implications:

• Highlights a critical drug-drug interaction between antineoplastic agents (VLB, MTX) and the anticonvulsant phenytoin (PHT).
• Underscores the importance of therapeutic drug monitoring for phenytoin during concurrent chemotherapy.
• Suggests potential need for dose adjustments or alternative anticonvulsant strategies in patients receiving VLB or MTX.