GlaxoSmithKline has sold its entire stake in Innoviva back to the US-based biopharmaceutical company for about $392 million, the British drugmaker said on Thursday, as it simplifies operations ahead of a split into two businesses.

The London-listed company said it sold roughly 32 million shares in the Nasdaq-listed Innoviva for $12.25 per share, a marginal discount to the stock's closing price of $12.29 on Wednesday.

GSK will give details in June on its plan to separate next year into an over-the-counter products business and another for prescription drugs and vaccines. Preparations have hurt earnings, but the company hopes the streamlining will pay off in the long term.

Shares of GSK were 0.3% lower at 1,361.3 pence by 1427 GMT, while Innoviva gained 5.1% to about $13.

An existing collaboration between the companies on respiratory treatments is not affected by the share sale, GSK said, adding the disposal frees up capital to help make more investments in line with its strategic priorities.

Under the respiratory deal, GSK pays royalties to Innoviva on lung drugs Trelegy Ellipta, Relvar/Breo Ellipta and Anoro Ellipta that the companies co-developed.