? Entecavir hydrate , with CAS number of 209216-23-9, can be called entecavir hydrate ; Entecavir monohydrate ; 2-Amino-1,9-dihydro-9-[(1S,3R,4S)-4-hydroxy-3-(hydroxymethyl)-2-methylenecyclopentyl]-6H-purin-6-one monohydrate ; Entecavir hydrate / 2-Amino-1,9-dihydro-9-[(1S,3R,4S)-4-hydroxy-3-(hydroxymethyl)-2-methylenecyclopentyl]-6H-purin-6-one monohydrate .?Entecavir hydrate (CAS NO.209216-23-9) is an oral antiviral drug used in the treatment of hepatitis B infection.