GlaxoSmithKline says that it is ending its involvement in the development of an experimental HIV treatment discovered by US partner Vertex Pharmaceuticals.

The drug major said that "due to insurmountable issues regarding formulation," development of brecanavir, an investigational protease inhibitor which was in Phase II trials, has been discontinued.

Vertex did not say what options there are now for the compound, which came out of an R&D collaboration the firms have been running since 1993, but noted that GSK's decision "does not alter the companies' relationship" with respect to the marketing of Lexiva (fosamprenavir), a protease inhibitor which is sold in Europe as Telzir.

The two firms also have a deal in place, signed last year and potentially worth over $400 million to Vertex, which saw GSK license a novel pain treatment, VX-409, which was still in preclinical development.