Pfizer announced it has stopped trying to discover new drugs for Alzheimer’s disease. About 300 employees in Massachusetts will be laid off. The firm has spent heavily on its past attempts. In 2012, Pfizer and its partner Johnson & Johnson halted development of a drug after field trials showed it failed to slow memory loss. Its compounds have focused on the “evil protein” concept of Alzheimer’s cause. The “infectious” possibility remains largely unexplored to date.