The decision by Swiss drugmaker Roche in August to buy US biotech company InterMune for $8.3 billion was "exceptional" and not a sign of more ambitious plans for major acquisitions, the Swiss drugmaker's chief executive, Severin Schwan, said in an interview with the news agency Reuters.

Damping down expectations that Roche could embark on a series of such multibillion-dollar deals to bolster its presence in the treatment of rare diseases, Schwan said there was no change in the company's M&A strategy or its likely pace of deal-making.

"A transaction of this size is really an exceptional thing," he said, adding that the company's last deal of this dimension was the $3.4 billion acquisition of US diagnostics firm Ventana in 2007.

Schwan also played down rumors that the company might want to buy the shares in Japan's Chugai Pharmaceutical Co that it does not already own, saying management is "happy" with the current relationship and ownership structure.

Buying InterMune brings Roche a promising new drug, pirfenidone, for treating progressive idiopathic pulmonary fibrosis and also helps diversify the world's leading maker of cancer drugs beyond oncology by expanding its interests in respiratory medicine, as well as taking it into the high-priced arena of rare diseases.

There has been persistent speculation that Roche might be plotting a determined assault on the rare-disease space, with talk last year linking it to both Alexion Pharmaceuticals and BioMarin Pharmaceuticals.

But Schwan said his deal-making was "very agnostic" when it came to both the number of patients targeted and disease area. Instead, the key consideration was finding promising science in an area of high unmet medical need to bring innovative - and commercially successful - products to market.