A few years ago, the drug companies dominating mergers-and-acquisitions headlines were largely household names, from Pfizer Inc. and Merck & Co. to Novartis AG. These days, a new generation of deal makers is making news, with names distinctly less familiar: Actavis PLC, Endo International PLC and Valeant Pharmaceuticals International Inc. Within the past few weeks, two other relatively under- the-radar companies, Mylan NV and Teva Pharmaceutical Industries Ltd., also entered the fray. Combined, these lesser-known players have signed or proposed takeovers worth a combined $180 billion in recent months, causing investors and bankers to sit up and take notice. They’ve also become major revenue generators, racking up some $52 billion in combined sales last year, up 86% from 2010.