In reports that emerged at Easter, U.S. pharmaceutical giant Pfizer is said to have held informal talks with AstraZeneca about buying the London-based maker of asthma and heart drugs. Some said the talks had fizzled and the parties are no longer negotiating.

The discussions were first reported by London's Sunday Times, quoting anonymous bank and industry sources. Pfizer is said to have offered more than $100 billion for the UK drugmaker.

If the deal did take place it would be among the largest ever in the industry, analysts said. Pfizer has reorganized its business over the past three years, shuttering some research projects and emphasizing others. A deal with AstraZeneca would add early-stage drugs in a field of cancer treatments that use the body's own immune cells to recognize and attack cancer.

In January, the London-based company announced an agreement with Immunocore to develop the new cancer treatments that use the body's own immune cells, and it also has several of its own immune-based drugs being tested in multiple cancers.

Buying AstraZeneca would also provide an outlet for Pfizer's $69 billion in untaxed cash overseas as of March 2014, according to data compiled by the news agency Bloomberg.

Reports said a purchase could also potentially allow Pfizer to move its tax domicile to the U.K. through a so-called inversion transaction, a move that has allowed other U.S. drugmakers to lower their tax bill.