Roche Holding has agreed to buy US biotech company InterMune for $8.3 billion in cash, marking the latest multibillion-dollar deal in a consolidating pharmaceutical sector.

The Swiss drugmaker said it would pay $74 a share through a tender offer for InterMune, representing a premium of 38% to the closing price on Aug. 22.

Although it is paying a big price for InterMune, the deal is seen inside Roche as a "bolt-on" acquisition, given the company's own market value of around $250 billion.

Under the terms of the agreement, Roche will commence a tender offer to acquire all outstanding shares in the U.S. firm no later than Aug. 29.

The acquisition, which has been recommended by the boards of both companies, is the largest by Roche since 2009, when it bought out the remaining stake it did not already own in US biotech group Genentech for around $47 billion.

Roche CEO Severin Schwan said the deal would allow the company to broaden and strengthen its respiratory portfolio, adding that he believed there was a good strategic and cultural fit between Roche and the California-based biotech firm.

"For us at Roche, this transaction is a good example of a value-creating bolt-on acquisition; we focus on targeted acquisitions that really complement our portfolio, rather than trying to diversify or going into mega mergers," Schwan told reporters on a conference call.

Adding InterMune to its portfolio will give Roche a promising new drug, pirfenidone, for treating a progressive and ultimately fatal scarring condition of the lungs, idiopathic pulmonary fibrosis (IPF).

The drug already has been approved in Europe and Canada, and is undergoing US regulatory review.

Industry analysts expect the drug, which is given as a pill, to have sales of $1.04 billion in 2019, according to consensus forecasts compiled by Thomson Reuters Pharma.

The deal is a further step by Roche to diversify away from its reliance on cancer drugs, where it is the world leader, by expanding into other disease areas, such as respiratory medicine.

Roche already markets Pulmozyme for cystic fibrosis and Xolair for severe asthma in the US and has other experimental respiratory products in clinical development, including another severe asthma drug called lebrikizumab.

The FDA is due to give is verdict on whether to approve pirfenidone by Nov. 23 and Roche said it expected the launch the drug in the US this year.