Drugmaker Pfizer Inc. has launched an all-out assault on defensive measures interwoven in rival American Home Product’s plan for a $71 billion friendly merger with pharmaceutical maker Warner-Lambert Co., according to an amended lawsuit filed in Delaware.

Pfizer, which has launched a $75 billion hostile takeover bid for Warner-Lambert, contends in its revised Delaware Chancery Court suit that American Home officials are using “unprecedented” and “unlawful” tactics to ward off Pfizer’s bid, including a $2 billion breakup fee.