Gabapentin does not alter single-dose lithium pharmacokinetics

Lithium (Li) and gabapentin are both exclusively eliminated by renal excret ion. When used in combination, a competitive drug-drug interaction could po ssibly alter Li renal excretion with important clinical implications consid ering the rather narrow therapeutic index of Li. This study examined the si ngle-dose pharmacokinetic profiles of Li in 13 patients receiving placebo a nd then steady-state gabapentin (mean daily dose: 3,646.15 mg). During both phases, a single 600-mg dose of Li was orally administered with serial Li levels obtained at time zero and at 0.25, 0.5, 1, 2, 3, 4, 8, 12, 24, 48, a nd 72 hours. The pharmacokinetic parameters assessed were the following: ar ea under the concentration time curve (AUC) for Li, maximal concentration o f Li (Li C-max), and time to reach peak Li concentration (Li T-max). For pa tients receiving gabapentin, the mean Li AUC at 72 hours was 9.91 +/- 3.54 mmol x hr/mL and did not differ significantly from the mean Li AUC of 10.19 +/- 2.89 mmol x hr/mL for patients receiving placebo. The mean Li C-max wa s 0.69 +/- 0.13 mmol/L for gabapentin patients and did not differ from the mean Li C-max of 0.72 +/- 0.15 mmol/L for placebo patients. The mean serum Li T-max was 1.38 +/- 0.62 hours for gabapentin patients and did not differ significantly from the mean serum Li T-max of 1.5 +/- 0.91 hours for place bo patients. These data indicate that gabapentin treatment at this high the rapeutic dose does not cause clinically significant alterations in short-te rm Li pharmacokinetics in patients with normal renal function. These prelim inary data warrant further controlled study in a larger, more heterogenous patient sample and a longer duration of assessment, but they do suggest tha t these two medications may be administered in combination for the manageme nt of bipolar disorder.