Specialty pharma Allergan (NYSE:AGN) announced yesterday that it is acquiring privately held Esprit Pharma for $370 million in an all-cash transaction.

Esprit is best known for holding the U.S. marketing rights to overactive-bladder drug SANCTURA XR, an extended-release medicine that was approved for sale by the FDA in August. Esprit acquired the rights from drugmaker Pliva in 2005.

Pliva had received the marketing rights to SANCTURA XR and its twice-a-day sibling from Idevus Pharmaceuticals (NASDAQ:IDEV), which had licensed the drug from another company. Follow the SANCTURA XR marketing rights musical chairs, eh?

SANCTURA XR currently has at least three years of market exclusivity, but Allergan expects pending patents will extend its generics-free life through 2023. Allergan plans to launch SANCTURA XR in the first quarter next year after it completes reimbursement negotiations with insurance companies.

While Esprit Pharma is not well known to many investors, its management team headed another privately held specialty drugmaker, ESP Pharma, that was acquired by PDL BioPharma (NASDAQ:PDLI) back in 2005 for $500 million, and which PDL is now trying to get rid of after its grand plans for ESP crashed earlier in the year.

Esprit also markets the original nonextended release version of SANCTURA. Combined sales of this drug and Esprit's drug for treating menopause symptoms (ESTRASORB) are expected to be $40 million to $48 million this year.

Hopefully Allergan's management didn't put too much stock into Esprit's sales numbers for SANCTURA, though. PDL's co-founder and others have accused ESP of possible "channel stuffing" to inflate its sales and earnings figures ahead of its sale to PDL. But since Allergan's main reason for acquiring Esprit was to get access to SANCTURA XR and its sales force, worries about channel stuffing may not be a factor.

Allergan estimates that with its increased detailing of SANCTURA XR, it will be able to reach peak sales of the drug between $300 million and $400 million. The deal also provides a ready-made sales force for Allergan's own potential overactive-bladder treatment, Botox (yes, that Botox), which is in phase 2 and 3 trials.

Only time will tell if SANCTURA XR can meet Allergan's peak sales guidance, but with the twice-a-day version of the compound on the market since mid-2004, it's hard to see how Allergan will be able to reach this aggressive sales target for SANCTURA XR.