Carbamazepine pharmacokinetics in obese and lean subjects

OBJECTIVE: To compare carbamazepine pharmacokinetic parameters between obese and lean subjects following the administration of a single 200-mg tablet. DESIGN: Single-dose intervention, open study. SETTING: Teaching university hospital. SUBJECTS: Eighteen obese (group A) otherwise healthy subjects, referred to the metabolic outpatient clinic, and 13 healthy lean (group B) volunteers. Inclusion criterion for the obese subjects was a body mass index (BMI = weight/height²) of more than 30 kg/m². In the obese group, mean ± SD total body weight (TBW), BMI, and percent of ideal body weight (IBW) were 111.4 ± 19.9 kg, 38.8 ± 6.0 kg/m², and 182.7% ± 30.7%, respectively. These values were significantly greater than the respective values of 63.2 ± 8.3 kg, 22.4 ± 1.6 kg/m², and 105.8% ± 5.8% obtained in the lean group (p < 0.001). INTERVENTION: Single-close oral administration of carbamazepine 200-ms tablet (Teril, Taro, Israel). OUTCOME: Carbamazepine elimination half life (t( 1/2 )), apparent volume of distribution (V(area)/F), and its oral clearance (CI(po)/F) were derived from the drug concentration- time curves. RESULTS: Carbamazepine V(area)/F and t( 1/2 ) were significantly greater in group A than in group B (98.4 ± 26.9 vs. 60.7 ± 8.5 L, respectively, p < 0.001; and 59.4 ± 14.7 vs. 31.0 ± 5.0 h, respectively, p < 0.001), but its CI(po)/F was reduced only slightly in obese as compared with lean subjects (19.8 ± 5.2 vs. 23.0 ± 4.6 mL/min, respectively, p = 0.07). Correction for IBW yielded similar results for V(area)/F and t( 1/2 ), but CI(po)/F per kg of IBW was significantly smaller in the obese than in the lean subjects (0.32 ± 0.07 vs. 0.39 ± 0.06 mL/min/kg of IBW, respectively, p < 0.02). Linear correlations were observed between V(area)/F and TBW for both group A (r = 0.92, p < 0.001) and group B (r = 0.77, p < 0.002). CONCLUSION: In comparison with lean subjects, carbamazepine V(area)/F is significantly greater in obese subjects and its t( 1/2 ) is markedly prolonged. The minor nonsignificant effect of obesity on carbamazepine CI(po)/F suggests that in obese subjects the carbamazepine daily dose should be based on IBW, not on TBW.