In a record year for health-care acquisitions, some of the largest pharmaceutical companies tried to sell their portfolios of older drugs - and failed.

Drug companies have agreed to about $260 billion of deals in 2014, the most since at least 2002 and more than twice the volume last year, according to data compiled by Bloomberg.

Fueled by cheap debt, buyers such as Actavis P.L.C. and Merck & Co. paid substantial premiums amid competition for new drug pipelines.

Meanwhile, sales of mature-drug portfolios - made up of older medicines such as Sanofi's blood thinner Plavix and GlaxoSmithKline P.L.C.'s antidepressant Paxil - were unsuccessful.

Faced with declining sales and expiring patents on certain portfolios, some pharmaceutical companies have been dividing assets into units that can then be split off, or beginning divestment processes for the drugs. While the strategy may help potential investors assess products in isolation, it can also create complicated structures that are difficult to value.

The selling companies, which use the revenues of established products to fund research and development, may assign more value to them than buyers, such as private-equity firms, said Emmanuel Papadakis, an analyst at MainFirst Bank AG in London.

"There are obstacles to arriving at a price for those kinds of assets," Papadakis said.

While the portfolios can be tricky to package and price, their cash flow and brand names can be valuable in growing emerging markets, and deal-makers say potential suitors may come back in 2015.

"Private-equity firms and trade buyers are still very interested in these assets." said Nigel Jones, a partner at Linklaters in London who specializes in health care. "It's not a matter of lack of funding - there's still money around."

In Sanofi's case, the products considered for sale were made in separate plants involving hundreds of employees, many of whom are in France, where job cuts are a sensitive topic. In addition, former chief executive officer Chris Viehbacher considered selling the mature portfolio - valued at about $8 billion - without consulting the board. That sale was scrapped.

Glaxo said it ended the sale of its North American and European established products, with annual sales of about one billion pounds ($1.6 billion), because the offers weren't high enough. Potential buyers found it difficult to value the portfolio given the diversity of treatments being offered, said a person with knowledge of the sale, asking not to be named as the process is private.

Sanofi and Glaxo aren't the only ones with deals halted. This week, Brussels, Belgium-based UCB SA's sale of its U.S. generic-drug business to two private-equity firms for $1.53 billion fell apart after U.S. regulators asked for an additional study on one of the unit's products.

Still, there's speculation that sales may be revived next year, with Merck also among drugmakers that may dispose of established drugs, people familiar with the industry said.