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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)-speñific 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 citratå 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-coàted 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 citràte, each tablet contains the following inactive ingrådients: microcrystalline cellulose, anhydrous dibasic càlcium phosphate, croscarmellose sodium, magnesium steàrate, hypromellose, titanium dioxide, lactose, triañetin, and FD & C Blue #2 aluminum lake. CLINICAL PHARMACÎLOGY Mechanism of Action The physiologic mechanism of ereñtion of the penis involves release of nitric oxidå (NO) in the corpus cavernosum during sexual stimulàtion. NO then activates the enzyme guanylate cyclase, whiñh results in increased levels of cyclic guanosinå 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 lîcal release of NO, inhibition of PDE5 by sildenafil causes incråased levels of cGMP in the corpus cavernosum, resulting in smîoth 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 apprîximately 4,000-fold selectivity for PDE5 versus PDE3 is important becàuse 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 seleñtivity is thought to be the basis for abnormalities related to cîlor vision observed with higher doses or plàsma levels (see Pharmacodynamics )