Eszopiclone [Lunesta™, Estorra™] is a short-acting hypnotic agent that is a stereoselective isomer of the agent zopiclone, which has been available in Europe since 1992. Eszopiclone is structurally unrelated to the benzodiazepines, and Sepracor (the originator of eszopiclone) has stated that the drug acts rapidly, with the duration of effect lasting up to 6 hours. This may result in improved sleep maintenance, with less nocturnal awakening.Originally, racemic zopiclone was developed and marketed by Rhône-Poulenc Rorer, which merged with Hoechst Marion Roussel to form Aventis.Sepracor anticipates that eszopiclone will have equivalent efficacy to the racemic version with potential for an improved side effect profile.In October 1999, Sepracor exclusively licensed Aventis Pharma's preclinical, clinical and postmarketing surveillance data package for zopiclone, its isomers and metabolites. The company intends to use this information in addition to data from Sepracor's own studies as part of the regulatory package to gain approval of eszopiclone in the US. In July 2004, Sepracor announced terms of an additional agreement with Aventis under which it would have the right to read and reference Aventis' regulatory filings related to zopiclone outside the US for the purpose of development and regulatory registration of eszopiclone outside the US. Additionally, Aventis would assign Sepracor the foreign counterparts to the US patent covering eszopiclone and its therapeutic use.[1]In August 2004, Paul Royalty Fund II, an affiliate of Paul Capital Partners, purchased from Sanofi-Aventis the royalty rights on US sales of eszopiclone. In exchange for the rights, Sanofi-Aventis will receive fixed and milestone payments totalling up to US$115 million.[2]In December 2004 the US FDA approved eszopiclone (Lunesta™) for the treatment of insomnia. It is indicated for patients who experience difficulty falling asleep as well as for patients who have sleep maintenance difficulty, and is approved for long-term treatment. The recommended dosing to improve sleep onset and/or maintenance is 2mg or 3mg for adult patients (aged 18–64 years) and 2mg for older adult patients (aged ≥65 years). The 1mg dose is for sleep onset in older adult patients whose primary complaint is difficulty falling asleep. The launch of eszopiclone in the US is expected to take place in the first quarter of 2005.[3,4]The approval follows an NDA submission in January 2003, an approvable letter in February 2004, and a resubmission of the NDA in June 2004. The NDA contained data from 24 clinical trials that included >2700 adult and elderly subjects, as well as data from >60 preclinical studies. Six phase III trials in adult and elderly patients with chronic or transient insomnias were also included in the data submission.[5-7]Preliminary results from a completed phase IIIB/IV trial report that eszopiclone in combination with fluoxetine significantly improved sleep parameters among patients with insomnia and co-existing major depressive disorder. Furthermore the combination of eszopiclone and fluoxetine resulted in greater improvement in HAM-D17 scores in patients than the fluoxetine-placebo group. This trial and three other phase IIIB/IV were initiated in late 2003 to evaluate the efficacy of eszopiclone in the treatment of insomnia in patients with depression, rheumatoid arthritis, chronic insomnia, and in women who experience symptoms of perimenopause.[8-10]Sepracor has been granted a US patent for eszopiclone [S-zopiclone, (+)-zopiclone, Lunesta™, Estorra™], a single isomer of zopiclone.US patents (Nos. 6,319,926 and 6,444,673) have been issued covering the use of eszopiclone for the treatment of insomnia, eszopiclone and pharmaceutical compositions comprising eszopiclone.