Four Am Law 100 firms are facing off in a takeover fight involving Endo International’s unsolicited $2.2 billion cash-and-stock bid to buy Auxilium Pharmaceuticals.

The proposed transaction would scuttle Auxilium’s proposed $345 million acquisition earlier this summer of Canadian biotechnology firm QLT, a deal that would allow Auxilium to redomicile itself north of the border. On Wednesday, the Chesterbrook, Pa.-based Auxilium adopted a poison pill to fend off Endo.