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Australian Journal of Pharmacy : October 2005
ucation dysfunction.15 In addition, sildenafil has shown efficacy in treating erectile dysfunction in men with diabetes, depression, and spinal cord injury.15 Selectivity The second generation of PDE-5s include vardenafil and tadalafil, which may provide more versatility in rate of onset, selectivity of activity and duration when compared with sildenafil.11,12,15 It has been purported that both vardenafil and tadalafil are more selective for PDE-5 in the testes and, thus, will be more effective than sildenafil. However, no efficacy trials comparing sildenafil, tadalafil and vardenafil have been undertaken. PDE-6 plays an important role in the conversion of light impulses into nerve impulses in the retina. Sildenafil has been shown to cause certain visual disturbances in patients taking sildenafil.10 Patients involved in trials where doses were escalated reported seeing a blue haze and transient increased brightness when taking sildenafil.16 This mainly occurred where doses were over 200mg. These changes were reported as being mild and transitory. Other patients have reported transient abnormal vision or blurred vision. This occurred in 3 to 11 per cent of patients and once again appeared to be dose-related with these visual changes more common at the 100mg doses than at lower doses. Only one patient was reported to have dropped out of the trials due to problems with visual disturbances. Tadalafil has higher selectivity for PDE-6 than does sildenafil and therefore may be more likely to cause visual disturbances.16 However, it is not clear whether tadalafil reaches high enough serum levels to be able to inhibit PDE-6 and create visual problems.17 In assessment of five randomised trials conducted at 74 centres, tadalafil did not show any clinically significant effects on vision and only one patient reported an episode of abnormal colour vision.17 In trials with vardenafil, some patients were found to have mild and transient impairment of colour discrimination in the blue/green range and in the purple range one hour after dosing.12 These changes had improved by six hours and no changes were present at 24 hours. Tadalafil is a potent inhibitor of PDE-11 which is found in the smooth muscle of internal organs, cardiac muscle, skeletal muscle, the pituitary gland and germ cells of the testes. It is not clear what the implications of inhibition of this enzyme mean clinically, but it may be responsible for the back pain reported in clinical trials—6 per cent of patients in trials reported back Drug Table One: Onset and duration of PDE-5 inhibitors Onset Sildenafil (Viagra) Vardenafil (Levitra) Tadalafil (Cialis) 30–60 mins 25–30 mins 16–60 mins Duration 4 hours 4–5 hours Up to 36 hours Studies show that sildenafil also improves sexual functioning in men with chronic health conditions who are at increased risk of either developing erectile dysfunction or having erectile dysfunction. pain.12 It is not clear what potency that vardenafil has for PDE-11 but clinical trials have reported that vardenafil can cause abdominal pain, chest pain and back pain. Pharmacokinetics Sildenafil has a short duration of action and an onset of action between 30 and 60 minutes.10 Vardenafil has a similar duration of action but a shorter onset of action. An advantage of tadalafil over sildenafil and vardenafil is its long half-life, allowing patients more flexibility and spontaneity with their partners over time frames reportedly up to 36 hours.11,12 Efficacy of tadalafil Tadalafil was studied in five randomised, double-blind, placebo- controlled, parallel group trials that were conducted at 74 centres.17 They researched a total of 1,112 men with a mean age of 59 years. Patients on 20mg of tadalafil were most successful with 75 per cent of intercourse attempts being successfully completed compared to 67 per cent in the 10mg group and 35 per cent on placebo. Tadalafil was also reported to be effective across all age groups and over differing severities of sexual dysfunction, with about 80 per cent of study participants reporting a positive response on a global assessment questionnaire.17 A number of randomised studies show that tadalafil is effective for treating erectile dysfunction in men regardless of severity of comorbid conditions, such as diabetes, hypertension, and cardiovascular disease. In an analysis of patients enrolled in eight randomised, double-blind, placebo controlled trials, 46 per cent of the men randomised to receive 10mg tadalafil and 57 per cent of men randomised to receive 20mg tadalafil reported ability to achieve successful intercourse compared with 20 per cent of the placebo group.11 In terms of quality of erections, the men treated with tadalafil had significantly improved erections compared with the placebo group: 59 per cent for the 10mg group and 66 per cent for the 20mg group compared to 28 per cent for the placebo group. In an analysis of men taking antihypertensive medications, 54 per cent and 65 per cent of sexual attempts made by men taking tadalafil 10 and 20mg, respectively, were successfully completed THE AUSTRALIAN JOURNAL OF PHARMACY VOL.86 OCTOBER 2005 ? 841