Chesterbrook drugmaker Auxilium Pharmaceuticals responded Wednesday to an unsolicited takeover offer from Endo International P.L.C. by saying it would look at the offer, as required by financial regulations, but also would institute a poison-pill stock-dividend plan to ward off Endo. Endo valued the cash-and-stock offer at $28.10 per share, or $2.2 billion, but it did not specify how much of that total was stock and how much was cash.