The rise and fall of Dimebon.

Dimebon (latrepirdine) was developed and used in Russia as an over-the-counter oral antihistamine for allergy treatment. In the early 1990s, Dimebon was characterized as a low-affinity NMDA receptor antagonist by Dr. Sergey Bachurin and his colleagues. An initial small-scale, open-label trial of Dimebon in 14 Alzheimer's disease (AD) patients demonstrated potential efficacy. Dimebon was then patented for the treatment of neurodegenerative disorders and licensed by Medivation. Extremely promising results were obtained in a double-blind, placebo-controlled, phase II AD trial in 183 patients; however, a phase II trial of Dimebon in 91 Huntington's disease patients was much less successful. Recently, a phase III AD trial of Dimebon in 598 patients failed to result in any significant improvement in primary or secondary outcomes. The failure of Dimebon may be in large part due to insufficient understanding of its mechanism of action. The NMDA receptor blocking activity of Dimebon is too weak to be physiologically relevant, while the proposed "novel mitochondrial mechanism of action" lacks credible scientific evidence or a molecular target. Independent studies indicate that the clinical effects of Dimebon most likely result from inhibition of histamine H₁ and serotonin 5-HT₆ receptors. Careful preclinical studies of novel potential therapies are needed to minimize chances of making similar costly mistakes in the future.