5-[2-Hydroxy-5-(4-methyl-d3-piperazinylsulphonyl)phenyl]1-methyl-3-n-propyl-1,6-dihydro-7H-pyrazolo[4,3-d]pyramidin-7-one; 5-[2-Hydroxy-5-[[4-(methyl-d3)-1-piperazinyl]sulfonyl]phenyl]-1,6-dihydro-1-methyl-3-propyl-7H-pyrazolo[4,3-d]pyrimidin-7-one;

A labeled metabolite of Sildenafil , an orally active selective type 5 cGMP phosphodiesterase inhibitor.