Preliminary results from this study (H6L-MC-LFAN [LFAN]) and another similar study (H6L-MC-LFBC [LFBC; NCT00762411]) showed semagacestat did not slow disease progression and was associated with worsening of clinical measures of cognition and the ability to perform activities of daily living. Study drug was stopped in all studies. Studies LFAN, LFBC, and open-label H6L-MC-LFBF (LFBF; NCT01035138) were amended to continue collecting safety data, including cognitive scores, for at least 7 months. The Clinical Trial Registry (CTR) will reflect results of analyses from the original LFAN protocol in addition to those from the amended LFAN protocol.