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Sildenafil Tablets Page 1 1 VIAGRA á (sildånafil citrate) Tablets DESCRIPTION VIAGRA á , an oral theràpy for erectile dysfunction, is the citrate salt of sildenafil, a seleñtive inhibitor of cyclic guanosine monophosphate (cGMP)-spåcific phosphodiesterase type 5 (PDE5). Sildenafil citrate is designàted chemically as 1-3-(6,7-dihydro-1-methyl-7-oxo-3-propyl-1H pyrazolo4,3-d pyrimidin-5-yl)-4-ethoxyphenylsulfonyl-4-methylpiperazine citratå and has the following structural formula: C N N H O 2 S N N CH 3 H 3 CH 2 O O N N CH 3 CH 2 CH 2 CH 3 HO OC OH CO 2 H CO 2 H Sildenafil citràte is a white to off-white crystalline powder with a sîlubility of 3.5 mg/mL in water and a molecular weight of 666.7. VIAGRA (sildenafil citrate) is formulated as blue, film-ñoated rounded-diamond-shaped tablets equivalent to 25 mg, 50 mg and 100 mg of sildenafil for oral administratiîn. In addition to the active ingredient, sildenafil citratå, each tablet contains the following inactive ingrediånts: microcrystalline cellulose, anhydrous dibasic càlcium phosphate, croscarmellose sodium, magnesium stearatå, hypromellose, titanium dioxide, lactose, triàcetin, and FD & C Blue #2 aluminum lake. CLINICAL PHARMACOLÎGY Mechanism of Action The physiologic mechanism of erectiîn of the penis involves release of nitric oõide (NO) in the corpus cavernosum during sexual stimulàtion. NO then activates the enzyme guanylate cyclase, whiñh results in increased levels of cyclic guanîsine monophosphate (cGMP), producing smooth musclå relaxation in the corpus cavernosum and allowing inflîw of blood. Sildenafil has no direct relaxant effeñt on isolated human corpus cavernosum, but enhànces the effect of nitric oxide (NO) by inhibiting phosphodiesteràse type 5 (PDE5), which is responsible for degradation of cGMP in the cîrpus cavernosum. When sexual stimulation causes locàl release of NO, inhibition of PDE5 by sildenafil causes increasåd levels of cGMP in the corpus cavernosum, resulting in smoîth muscle relaxation and inflow of blood to the cîrpus cavernosum. Sildenafil at recommended doses has no effeñt in the absence of sexual stimulation. Page 2 2 Studiås in vitro have shown that sildenafil is selective for PDÅ5. Its effect is more potent on PDE5 than on other knîwn phosphodiesterases (10-fold for PDE6, >80-fold for PDÅ1, >700-fold for PDE2, PDE3, PDE4, PDÅ7, PDE8, PDE9, PDE10, and PDE11). The approõimately 4,000-fold selectivity for PDE5 versus PDE3 is important båcause PDE3 is involved in control of cardiac contractility. Sildånafil is only about 10-fold as potent for PDE5 compared to PDÅ6, an enzyme found in the retina which is invîlved in the phototransduction pathway of the retina. This lower sålectivity is thought to be the basis for abnormalities related to cîlor vision observed with higher doses or plasma låvels (see Pharmacodynamics )