Inhibition of phosphodiesterase type 5 in cardiovascular disease
Oliver, James John
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Nitric oxide is released from the endothelium and causes relaxation of vascular smooth muscle by stimulating guanylate cyclase to produce guanosine 3’,5’-cyclic monophosphate (cGMP) which, in turn, is degraded by phosphodiesterase type 5 (PDE5). Inhibition of PDE5, with drugs like sildenafil citrate, promotes NOstimulated relaxation of vascular smooth muscle. The overall aim of the work contained within this thesis was to further characterise the systemic vascular effects of PDE5 inhibition. Four clinical studies were performed. The aims of the first study were to investigate in healthy men the effect of smoking on endothelium-dependent vasomotor function measured as the change in peripheral arterial wave reflection with inhaled salbutamol, and the effect of acute sildenafil 100 mg on this response. Smokers (n=12) exhibited a reduced response to inhaled salbutamol compared to non-smokers (n=11) [mean(standard deviation) area under the curve of the change in central augmentation index following salbutamol 400 μg: -29(143) AU in smokers vs -159(124) AU in non-smokers, P=0.03]. In the smokers, there was a trend to an improvement in the response to salbutamol following sildenafil [-96(266) AU vs -29(143) AU with matched placebo; P=0.2]. The co-administration of glyceryl trinitrate (GTN) and sildenafil is absolutely contraindicated because of the potential for profound hypotension. The aim of the second study was to characterise the time course of this interaction. Twenty men with stable angina, maintained on their usual medicines, were administered sublingual GTN 400 μg 1, 4, 6 and 8 hours after sildenafil 100 mg or matched placebo. Compared to the combination of GTN and placebo, the combination of GTN and sildenafil resulted in greater mean maximum reductions from baseline in sitting systolic blood pressure (BP) at 1, 4 and 8 hours, and in sitting diastolic BP at all time points (all P<0.05). Compared to placebo, sildenafil alone reduced systolic BP at 1, 4, 6 and 8 hours (P<0.01 at 1 hour and P<0.05 at 4, 6, and 8 hours) and diastolic BP at 4, 6, and 8 hours (all P<0.01). Analysis of the change in BP from the measures taken before each GTN challenge suggested that the interaction on BP might be synergistic at 1 hour after sildenafil, but no more than additive at 6 and 8 hours after sildenafil. Symptoms consistent with hypotension occurred following GTN in 6 subjects at 1 hour and 3 subjects at 4 hours after sildenafil, but in no subjects at 6 and 8 hours after sildenafil or at any time after placebo. In the third study, 25 otherwise untreated hypertensives were given sildenafil 50 mg or matched placebo three times daily for 16 days and the effects on ambulatory BP, clinic BP, arterial wave reflection, carotid-femoral pulse wave velocity and brachial artery flow-mediated dilatation were measured. Three subjects were withdrawn because of side effects and the data from the remaining 22 subjects were analysed. Sildenafil reduced ambulatory BP [change from baseline in average daytime BP: systolic -8(9) mmHg vs 2(9) mmHg with placebo, P<0.01; diastolic -6(5) mmHg vs 0(6) mmHg, P<0.01] and clinic BP [change from baseline to 1 hour after drug administration on day 16: systolic -5(11) mmHg vs 4(10) mmHg, P<0.01; diastolic -5(5) mmHg vs 2(7) mmHg, P<0.01]. Sildenafil, but not placebo, reduced arterial wave reflection [central augmentation index from 32(9)% at baseline to 30(10)% at 1 hour after administration on day 16, P<0.05; radial augmentation index from 88(13)% to 84(13)%, P<0.01], but the change in arterial wave reflection was not statistically significant compared to the change with placebo. Sildenafil did not affect pulse wave velocity or flow-mediated dilatation. The fourth study investigated the potential of combined PDE5 inhibition and organic nitrate for the management of treatment-resistant hypertension (TRH). In 6 patients with TRH, maintained on their usual antihypertensives sildenafil 50 mg alone, isosorbide mononitrate (ISMN) 10 mg alone and co-administered sildenafil and ISMN all acutely reduced systolic BP and diastolic BP compared to placebo (quantified as the area under the curve of the change from baseline to 4 hours after drug administration; all P≤0.01). The combination produced a greater reduction in systolic BP than did either sildenafil alone (P=0.03) or ISMN alone (P=0.01) and a greater reduction in diastolic BP than did sildenafil alone (P=0.02). Compared to placebo, from 1 to 3 hours after drug administration BP was on average 13/10 mmHg lower with sildenafil alone, 18/14 mmHg lower with ISMN alone and 26/18 mmHg lower with the combination. The following conclusions were made. (1) Smokers exhibit impaired vascular responsiveness to inhaled salbutamol, indicating systemic endothelial dysfunction, which may be improved by sildenafil. (2) In men with stable angina there is an interaction on BP reduction between sildenafil 100 mg and sublingual GTN 400 μg for at least 8 hours after sildenafil administration, but this interaction is no more than additive from 6 hours after sildenafil administration. (3) Regular sildenafil monotherapy reduces BP in hypertension. (4) In patients with TRH maintained on their usual antihypertensives sildenafil alone and ISMN alone both acutely reduce BP and there is additional BP reduction when these drugs are given in combination.