Formulation development for the orexin receptor antagonist almorexant: assessment in two clinical studies

Almorexant, a dual orexin receptor antagonist, was investigated for the treatment of insomnia. The following observations initiated further formulation development: the active pharmaceutical ingredient (API) was sticking to the apparatus used during tablet compression; almorexant has an absolute bioavailability of 11.2%; and almorexant modestly decreased the latency to persistent sleep by 10.4 minutes in patients. Two randomized crossover studies were performed to investigate the pharmacokinetics of several new formulations in healthy subjects. In study I, the old "sticky" tablet was compared to two new formulations developed to prevent sticking: a qualitatively similar tablet but with a larger API crystal size and a tablet with 30% more excipients as well as a larger API crystal size. This latter formulation was available in two strengths. The geometric mean ratios and 90% confidence interval of the area under the curve (AUC) were within the bioequivalence range of 0.80-1.25 for the different comparisons between formulations. In study II, 100 mg of the reference tablet was compared to 25 and 50 mg of a liquid-filled hard gelatin capsule developed to increase the bioavailability of almorexant. The geometric mean ratios of the maximum concentration and AUC comparing the new 25 and 50 mg capsule formulations to the reference tablet did not exceed 0.25 and 0.50, respectively, indicating that the new capsule formulation did not increase the maximum concentration of or the total exposure to almorexant. In conclusion, a new tablet was developed but formulation development aimed at increasing the bioavailability of almorexant failed.