NPS Pharmaceuticals ( NPSP)and Enzon ( ENZN) have called off their merger, the companies announced after the bell Wednesday.

The two companies announced their merger more than three months ago. Rumors of a breakup began circulating after NPS Pharma pulled out of an appearance at Thursday's UBS Warburg specialty pharma investment conference.

In a joint statement, the companies said the merger failed after NPS asked to renegotiate and Enzon refused. NPS will end up paying a termination fee of 1.5 million shares of its stock, according to the statement. A possible reason for NPS Pharmaceutical's decision to renegotiate: Reported weak sales of Enzon's hepatitis C drug in the first quarter.

NPS Pharmaceutical ended regular trading up $2.48, or 11.7%, to $23.75, while Enzon closed higher by 40 cents, or 2.7%, to $15.