Drugmaker Shire P.L.C., which is based in the United Kingdom but has operations in Chesterbrook, said Monday it had agreed to buy San Diego-based Lumena Pharmaceuticals Inc., for an up-front payment of $260 million.

Shire might pay more pending the net cash available when Lumena closes the deal and for milestone payments on the clinical trials of some drugs in its pipeline.

Shire has tried to focus on rare diseases and Lumena has products that treat diseases of the liver. Reports suggest that drugmaker Allergan might try to buy Shire as a way to fend off an unwanted takeover bid from Valeant Pharmaceuticals. - David Sell