Deuterated fezolinetant (R)-(4-fluorophenyl)-(8-methyl-3-(3-(methyl-d3)-1,2,4-thiadiazol-5-yl)-5,6-dihydro-[1,2,4]trizolo[4,3-a]pyrazin-7(8H)-yl)methanone: or a pharmaceutically acceptable salt or solvate thereof, as an NK-3 antagonist. Also, methods of modulating NK-3 receptor activity including administering an effective amount of the compound or pharmaceutically acceptable salt or solvate thereof. Additionally, a process for manufacturing the compound or pharmaceutically acceptable salt or solvate thereof.