Drug interactions between antiepileptics and chemotherapeutic drugs: Value of a pre-treatment pharmaceutical review

Abstract

Background: Many cancer patients take medications for preexisting comorbidities that are unrelated to their malignancy. Eliciting an accurate medication history prior to cancer chemotherapy is important to ensure that any drug-drug interactions are detected, assessed and avoided. Aim: To describe a case of potential interaction between antiepileptics and chemotherapeutic drugs. Clinical details: A 24-year-old female with rapidly progressing metastatic rhabdomyosarcoma of the right maxillary sinus was treated with the Children's Oncology Group ARST0431 protocol for high-risk rhabdomyosarcoma. This intensive regimen combines radiation therapy with 7 chemotherapeutic drugs: vincristine, irinotecan, cyclophosphamide, ifosfamide, doxorubicin, etoposide and actinomycin-D. During routine pretreatment medication reconciliation, the pharmacist identified that the patient was also taking carbamazepine, a potent inducer of cytochrome P450 (CYP) enzymes, for left-sided focal epilepsy. Outcome: There was potential for clinically relevant drug interactions because CYP enzymes induced by carbamazepine were responsible for metabolising some of the chemotherapy drugs planned for her treatment. The pharmacist's intervention resulted in carbamazepine being switched to levetiracetam, a non-enzyme inducing antiepileptic, and avoiding the potential interaction. Conclusion: This case highlights the value of clinical pharmacists undertaking medication reconciliation prior to the start of cancer chemotherapy.