The Swiss drugmaker said on Sunday it would pay $74.00 a share through a tender offer for InterMune, representing a premium of 38 percent to the closing price on Aug. 22.
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 group Genentech for around $47 billion.
The sale of InterMune will not come as a huge surprise to investors as people familiar with the matter said on Aug. 13 that InterMune was working with financial advisers to evaluate strategic options, due to takeover interest from big drugmakers.
Shares in the California-based biotechnology company rose sharply on that news, so the $74 offered by Roche marks a 63 percent premium to the stock's level on Aug. 12.
Such a hefty premium is not unusual in biotech takeovers, reflecting intense competition for promising new drugs among larger companies, which rely on small innovative firms for a growing proportion of their products.
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. Pirfenidone is approved for so-called idiopathic pulmonary fibrosis (IPF) 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 United States and has other experimental respiratory products in clinical development, including another severe asthma drug called lebrikizumab.
However, the Swiss group's efforts to produce successful non-cancer drugs from its own labs have been mixed, with setbacks in recent years for experimental drugs against heart disease, diabetes and schizophrenia.