Japan’s Shionogi has rather mysteriously pulled out of a planned acquisition of the USA’s Victory Pharma.

In May, Shionogi’s Sciele Pharma unit, which was acquired in a $1.4 billion deal last year, was going to pay $150 million to get hold of Victory, a privately-held firm. The latter’s lead product is the pain drug Naprelan, the only branded, once-daily sustained release formulation of naproxen.

Now Sciele and Victory Pharma say they have “mutually agreed” to terminate their previously-announced merger “due to the occurrence of an unforeseen development that occurred after the agreement was signed”. Neither firm is offering any details as to what this unforeseen development is.

Shionogi said that the termination will have little effect on its earnings and a is expected to have “a small impact on Sciele’s 2009 revenue and earnings guidance”. It added that Sciele will continue to focus on its “growth and diversification strategy”, having completed 10 transactions since 2007, and is looking to license and acquire products.

When the deal was originally announced, Ed Schutter, chief operating officer at Sciele, said that the acquisition, and getting hold of Naprelan, would provide “an immediate expansion into the pain market” for the company. Elan Corp developed the drug and manufactures Naprelan at its facility in Athlone, Ireland.