Swiss pharmaceutical company Roche is buying U.S. cancer therapy company Ignyta for 1.7 billion dollars, in a deal that will broaden Roche's oncology product portfolio, both companies announced Friday.

Roche will offer 27 dollars per share to Ignyta share holders, a premium of 74 per cent over Ignyta's closing price on Thursday, according to the companies that are based in Basel and San Diego, respectively.

Ignyta has focused on patients with cancers harboring specific rare mutations, and currently has a treatment against certain tumors in a phase II trial that Roche said interim data showed to be promising.

Phase II trials of up to several hundred patients are designed to test the effectiveness of drugs and medical devices, and if successful are followed up by phase III trials on a larger number of patients before possible approval by regulators for use by patients.

"Cancer is a highly complex disease and many patients suffer from mutations which are difficult to detect and treat," Roche Pharmaceuticals' chief executive Daniel O'Day said in a statement.

"The agreement with Ignyta builds on Roche's strategy of fitting treatments to patients and will allow Roche to broaden and strengthen its oncology portfolio globally."

Ignyta will remain based in San Diego.

Roche has been seeking to expand its product portfolio as several of its most lucrative treatments are going off patent which opens them up to competition from cheaper generics.

Roche plans to close the transaction in the first half of next year.