The purpose of the dose escalation part of the study is to characterize the safety and tolerability of ALN-BCAT as monotherapy and in combination with pembrolizumab; and to determine the recommended dose(s) for expansion (RDFE) of ALN-BCAT as monotherapy and in combination with pembrolizumab. The purpose of the dose expansion part of the of the study is to evaluate the antitumor activity of ALN-BCAT as monotherapy and in combination with pembrolizumab; to characterize the safety and tolerability of ALN-BCAT as monotherapy and in combination with pembrolizumab.