Pyrazolopyrimidinone derivatives for the treatment of impotence

A compound represented by the following formula (I) or a pharmaceutically acceptable salt thereof (formula not shown),

• wherein
• R1 is an alkyl group of C1-C3,
• R2 is a substituted or unsubstituted alkyl group of C2-C6,
• R3 is a substituted or unsubstituted alkyl group of C2-C6, and
• R4 is a substituted or unsubstituted alkyl group of C1-C6, a substituted or unsubstituted cycloalkyl group of C3-C6, a substituted or unsubstituted benzene, a substituted or unsubstituted pyridine, or a substituted or unsubstituted pyrrole, wherein the substituent for the substituted groups is selected from the following group:
• C1-C10-alkyl group, C3-C6 cycloalkyl, halogen,
• C1-C6 fluoroalkyl group, C1-C10 alkoxy group, substituted or unsubstituted benzene or heterocycle, wherein the heterocycle is selected from the group consisting of pyridine, isoxazole, thiazole, pyrimidine, indan, benzthiazole, pyrazole, thiadiazole, oxazole, piperidine, morpholine, imidazole, pyrrolidine, thienyl, triazole, pyrrol and furyl.