Global biotech merger volume has reached levels not seen in four years as big pharmaceutical companies pursue deals to get access to new drugs, with bankers saying therapeutic areas such as cancer, inflammation and autoimmune diseases are proving to be especially attractive.

Large pharmaceutical companies that have expired patents are looking for products to supplement their drug development efforts and sometimes to also give their primary-care sales forces more drugs to sell, according to healthcare bankers at Credit Suisse Group.

"There seems to be a lot of interest in select oncology names, as well as the inflammatory and autoimmune spaces," Charles Newton, head of biotech investment banking at Credit Suisse, said in an interview last week.

"While it is difficult to generalize across the industry, those are the key therapeutic areas where we see large pharma business development teams spending a lot of time," he said.

Big drugmakers are facing one of the worst patent cliffs in history. They are also flush with cash and have easy access to debt, allowing them to make aggressive bids for promising biotech companies and other targets.

They have driven the volume of biotech M&A to more than $25 billion so far this year, compared with nearly $10 billion during the same period last year, according to Thomson Reuters data as of Thursday. The volume is the highest since 2008, when deal volumes surged to $54 billion due to Roche's $46.7 billion takeover of Genentech, the data shows.

"Their patents have expired, and as a result they have holes in their revenue line they are trying to fill," said Scott Lindsay, global head of mergers and acquisitions at Credit Suisse. "The other part is just shoring up the pipeline they have. So even if they don't have a major patent problem, they're all looking for growth."

Credit Suisse's healthcare team advised on $16 billion of this year's biotech deals, taking the top spot in the global rankings of deal advisers in the sector, the data shows.

The bank recently advised Human Genome Sciences on its $3 billion sale to GlaxoSmithKline, and Amylin Pharmaceuticals on its $5.3 billion sale to Bristol-Myers Squibb. Credit Suisse also advised Inhibitex on its $2.5 billion takeover by Bristol.

The past several months have seen a robust takeover appetite for hepatitis C drugmakers such as Inhibitex, and saw Pharmasset's near $11 billion sale to Gilead Sciences. Regulatory filings showed that both Pharmasset and Inhibitex drew several other bidders in the auction processes, underscoring the interest in the makers of hepatitis C drugs.

Sometimes competition for these assets means that the buyers pay hefty premiums.

Bristol-Myers paid Amylin a 101% premium to the price before its interest was first reported. Bristol also paid a lofty 163% premium to Inhibitex. GSK's $3 billion takeover was a 99% premium to Human Genome's stock before its bid was first made public.

Inhibitex, Amylin and Human Genome Sciences are the three highest premiums paid for multibillion-dollar M&A transactions in the history of the biotech industry, Newton said.

"Some might ask whether targets are getting too expensive because of these premiums, but we don't believe acquirers look exclusively through that lens as the premium paid is only one variable in the equation," he added.

Big pharma's preference for commercial-stage assets, rather than clinical trial-stage drugs carrying regulatory approval risk, means that companies tend to pursue multibillion-dollar transactions rather than going for smaller deals.

"Given the scale of large-cap pharmas, (deal size) becomes a factor at much higher numbers than it would in other industries," Lindsay said.