Background: Limited data are available about the effect of lopinavir and low-dose ritonavir on glucuronidation. Lamotrigine undergoes glucuronidation. We studied the effect of lopinavir/ritonavir on the pharmacokinetics of lamotrigine and vice versa.
Methods: Twenty-four healthy subjects received 50 mg lamotrigine once daily on days 1 and 2 and 100 mg twice daily on day 3 through day 23. Lopinavir (400 mg twice daily)/ritonavir (100 mg twice daily) was added on day 11. Depending on the decrease in lamotrigine trough level between days 10 and 20, either the study was stopped (<20% decrease) or a dose increase was applied from day 23 to day 31, as follows: increase to 150 mg lamotrigine twice daily if there was a 20% to 33% decrease, increase to 200 mg twice daily if there was a 34% to 66% decrease, and increase to 300 mg twice daily if there was a greater than 66% decrease. On days 10, 20, and 31, 12-hour pharmacokinetic curves were drawn.
Results: The mean decrease in lamotrigine trough level between days 10 and 20 was 55.4% (n = 18). A dose increment to 200 mg lamotrigine twice daily was used in all subjects. The area under the plasma concentration-time curve (AUC) values of lamotrigine on day 20 (with lopinavir/ritonavir) and day 10 (without lopinavir/ritonavir) were bioinequivalent, with a point estimate of 0.50 (90% confidence interval, 0.47-0.54). After dose adjustment of lamotrigine to 200 mg twice daily, the AUC on day 31 (n = 15) was bioequivalent to that on day 10, with a point estimate of 0.91 (90% confidence interval, 0.82-1.02). The median AUC ratios of lamotrigine 2N-glucuronide to lamotrigine on day 10 and day 20 were 0.57 (interquartile range, 0.39-0.75) and 1.12 (interquartile range, 0.87-1.31). Pharmacokinetic parameters for lopinavir/ritonavir were similar to historical controls.
Conclusion: Lopinavir/ritonavir decreases the AUC of lamotrigine, probably by induction of glucuronidation. A dose increment to 200% of the initial lamotrigine dose is needed to achieve concentrations similar to those with lamotrigine alone. Lamotrigine does not appear to affect the pharmacokinetics of lopinavir/ritonavir.