NEW ORLEANS – Patients taking enzyme-inducing antiepileptic drugs (AEDs) may require a clinically meaningful increase in their vitamin D doses to achieve the same 25-hydroxyvitamin D (25[OH]D) plasma levels as patients taking nonenzyme-inducing AEDs, based on a retrospective chart review presented at the annual meeting of the American Epilepsy Society.
While patients receiving either type of AED had similar average 25(OH)D levels in the study (32.0 ng/mL in the enzyme-inducing AED group and 33.2 ng/mL in the noninducing AED group), those in the enzyme-inducing group required 1,587 U/day to meet the goal – a 409-unit increase in dose, compared with the 1,108 U/day dose taken by patients in the nonenzyme-inducing group.
“Patients taking enzyme-inducing AEDs may benefit from more intensive monitoring of their vitamin D supplementation, and clinicians should anticipate this likely pharmacokinetic interaction,” said Barry E. Gidal, PharmD, professor of pharmacy and neurology at the University of Wisconsin–Madison, and his colleagues.
Researchers have suggested that enzyme-inducing AEDs may affect CYP450 isoenzymes, increase vitamin D metabolism, and reduce 25(OH)D plasma levels. “It follows … that a potential pharmacokinetic interaction could exist between enzyme-inducing AEDs and oral formulations of vitamin D used for supplementation,” the investigators said.
To test the hypothesis, Dr. Gidal and his colleagues reviewed the charts of patients with epilepsy who were on any AED regimen and were prescribed vitamin D at William S. Middleton Memorial Veterans Hospital in Madison, Wisconsin, between January 2013 and September 2017.
The researchers grouped patients by those using enzyme-inducing AEDs and those taking noninducing AEDs. Patients who were taking AEDs in both categories were placed in the enzyme-inducing AED group. Patients with malabsorptive conditions and patients using calcitriol were excluded from the analysis.
Data included AEDs used, prescription and over-the-counter vitamin D use, 25(OH)D plasma concentration, renal function, age, gender, and ethnicity. Patients’ 25(OH)D levels were measured using a chemiluminescence immunoassay, and a minimum 25(OH)D plasma level of 30 ng/mL was the therapeutic goal.
The multivariant analysis was adjusted for potentially confounding variables including 25(OH)D concentration, over-the-counter vitamin D use, chronic kidney disease, age, gender, and ethnicity.
The analysis included 1,113 observations from 315 patients, and 263 of the observations (23.6%) were in the enzyme-inducing AED group. The enzyme-inducing group and noninducing groups were mostly male (90.5% and 91.8%, respectively) and similar in average age (65.9 and 61.4 years, respectively). Variables were evenly distributed between the groups, with the exceptions of chronic kidney disease, which was less common in the enzyme-inducing group (6.1% vs. 13.8%), and ethnicity (78.7% Caucasian in the enzyme-inducing group vs. 87.7% Caucasian in the noninducing group). The most common enzyme-inducing AED was phenytoin (50.6%), followed by carbamazepine (31.9%), phenobarbital (14.1%), oxcarbazepine (6.8%), primidone (1.9%), and eslicarbazepine (0.8%).
Dr. Gidal reported honoraria from Eisai, Sunovion, Lundbeck, and GW Pharmaceuticals.
SOURCE: Gidal BE et al. AES 2018, Abstract 1.315.