5-[2-Ethoxy-5-[(4-(methyl-d3)-1-piperazinyl)sulfonyl]phenyl]-2,6-dihydro-2-methyl-3-propyl-7H-pyrazolo[4,3-d]pyrimidin-7-one; 1-[[3-(4,7-Dihydro-2-methyl-7-oxo-3-|propyl-2H-pyrazolo[4,3-d]pyrimidin-5-yl)-4-ethoxyphenyl]sulfonyl]-4-(methyl-d3)piperazine;

Labeled Iso Sildenafil. Isosildenafil is the isomer of Sildenafil methylated at N-2 position of the pyrazolo[4,3-d]pyrimidin-7-one nucleus.