The present disclosure provides fused 1,4-oxazepines and related analogs represented by Formula (I) and the pharmaceutically acceptable salts, hydrates, and solvates thereof, wherein R1, R2a, R2b, R3a, R3b, R4, R5, A, and Y are as defined as set forth in the specification. The present disclosure is also directed to the use of compounds of Formula (I) to treat a condition or disorder responsive to inhibition of BET bromodomains such as cancer.