Abstracts

It has been suggested that patients taking higher doses of oxcarbazepine (OXC) may have measureable levels of carbamazepine (CBZ). This is a important clinical issue when patients taking OXC present to the emergency room with breakthough seizures. Since many emergency room physicians are still not familiar with OXC, this may lead to the erroneous overdosing of OXC as [quot]compensation[quot] for the observed low, but measureable CBZ level. We sought to make concurrent measurements of OXC and CBZ in patients taking either drug in an attempt to determine how likely such cross reactivity might be. A retrospective review of adult epilepsy and trigeminal neuralgia patients (2001-2004) was performed to identify those taking OXC or CBZ in monotherapy. From this list, we noted simultaneous steady state measurements of the mono-hydroxy deriviative (MHD) of OXC and CBZ in 15 different patients (12 epilepsy; 3 trigeminal neuralgia). Some had more than one concurrent level performed; there were a total of 19 concurrent levels. CBZ levels were performed locally in the hospital lab. MHD levels were sent out to a commercial lab (Quest). With two exceptions (1- MHD, 1- CBZ), levels represented troughs. Due to the relatively small sample, data were analyzed descriptively. No obvious differences in doses or levels between the epilepsy and trigeminal neuralgia patients were noted, so these data were collapsed. The median age of OXC patients was 42 (27-69). The median dose was 1200mg/ day (525- 2550mg, n=14). The median MHD level was 19.5 [mu]g/ml (8.2-65). The concomitant median CBZ in these same patients was [lt] 0.6 [mu]g/ml ([lt]0.2- [lt]2.0).
The median age of CBZ patients was 31(27-69). The median dose was 1100 mg/ day (600- 1500mg; n=5). The median CBZ level was 8.9 [mu]g/ml (7.3- 17.7). The concomitant median MHD level in these same patients was 0 [mu]g/ml (0-0). Simultaneous measurement of MHD and CBZ levels in OXC treated patients, and in CBZ treated patients, did not reveal evidence of cross reactivity at doses typically used in clinical practice. Although the sample sizes were small, they did include a number of patients treated with high doses of OXC ([gt] 1200mg/day) and CBZ ([gt] 1000 mg/day); those in which assay cross reactivity might be expected to occur. While confusion between the two drugs might still exist on the basis of similarities of both the trade and generic names; at least on the basis of this series, cross measurement by their respective assays is probably not common.