Swiss pharma giant Roche has offered to buy out the remaining 44 per cent of shares in US biotech Genentech for US$43.7 billion (A$45 billion).

Roche acquired a majority in Genentech in 1990 and currently owns 55.9 per cent of the San Francisco company, set up by biotech pioneer Herb Boyer in 1976. Genentech manufactures the blockbuster cancer drugs Herceptin and Avastin, among others.

Roche is offering US$89 per share in cash and is promising to respect Genentech's 'unique research culture' and headquarters.

Roche estimates that the combined entity will be the seventh largest US pharmaceuticals company in terms of market share and will generate more than US$15 billion in annual revenues.