Characterisation of the pharmacological actions in humans of multiple vasoactive enzyme inhibitors
with therapeutic potential in heart failure.
MD thesis, University of Glasgow.
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The work described in this thesis looks particularly but not exclusively at two recently developed molecules which have dual enzyme inhibitor activity. Omapatrilat, a molecule which inhibits both angiotensin converting enzyme (ACE) and neutral endopeptidase (NEP), and SLV 306 (active metabolite KC12615) a compound with both NEP and endothelin converting enzyme (ECE) inhibiting properties.
Neurohumoral activation characterises the complex chronic heart failure syndrome. Clearly there is value in antagonizing neurohumoral systems likely to have detrimental effects in patients with heart failure, while simultaneously augmenting potentially desirable neurohumoral mediators. However enzyme inhibitors which act on multiple vasoactive mediators with complex interactions have unpredictable effects.
Omapatrilat has received much attention following demonstration of a powerful hypotensive effect but a higher than expected incidence of angioedema in patients with hypertension or heart failure. GW660511X is another dual ACE/NEP inhibitor at an earlier stage in development. The pharmacodynamic profile of neither the ACE inhibitor activity nor the NEP inhibitor activity of GW 660511X has been fully described in humans. SLV 306 is the first orally available molecule of its kind and its NEP and ECE inhibitory properties have not previously been demonstrated in humans either in vitro or in vivo.
The intention of this thesis is further characterisation of these molecules and their therapeutic potential while utilising their inhibitory properties in further investigation of the human neurohumoral system. I specifically consider the possible mediators of the impressive hypotensive effects of these molecules and of the unexpected and potentially life threatening side effects associated with them. Having demonstrated the complex neurohumoral substrate of these molecules I go on to report, for the first time in heart failure patients, the benefits of a more specific approach to neurohumoral manipulation using a recently developed renin inhibitor, aliskiren. Aliskiren has been shown to offer haemodynamic benefit in patients with hypertension but has not previously been given to patients with chronic heart failure.
1) Small resistance arteries from patients (n=89) with coronary artery disease but normal left ventricular function were studied using wire myography. The vasopressor response to various neurohormones in the presence of omapatrilat, KC12615 (the active metabolite of SLV 306) and other vasoactive enzyme inhibitors is reported.
2) Following pilot studies of the pressor response to intravenous infusion of big ET-1 (n=6) and pharmacokinetic modeling of orally dosed SLV 306 in healthy volunteers (n=29), the effect of 3 doses of SLV 306 and placebo given at four separate visits one week apart, on the pressor and neurohumoral response to intravenous infusion of big endothelin-1 in healthy volunteers (n=15) is compared.
3) The effect of 2 oral doses of GW660511X and a single dose of the ACE inhibitor ramipril, given on three separate visits one week apart, on the pressor and neurohumoral response to an intravenous infusion of angiotensin I in healthy volunteers (n=16) is compared.
4) Finally, the neurohumoral and blood pressure response to aliskiren an orally active, long acting renin inhibitor is compared with placebo for one week and the ACE inhibitor ramipril for 6 weeks, in patients with left ventricular systolic dysfunction (n=27).
1) In patients with coronary artery disease but normal left ventricular systolic dysfunction; the vasodilator response to bradykinin was augmented by omapatrilat, KC 12615, phosphoramidon (NEP/ECE inhibitor), captopril (ACE inhibitor), and thiorphan (NEP inhibitor). Of note the augmentation is no greater with omapatrilat than captopril and in arteries taken from patients prescribed ACE inhibitor, KC 12615 does not augment the response. The vasodilator response to adrenomedullin was augmented by omapatrilat, KC 12615 and phosphoramidon. The vasoconstrictor response to angiotensin I was inhibited by omapatrilat and captopril and the vasoconstrictor response to endothelin-1 was inhibited by KC 12615 and phosphoramidon.
2) In healthy volunteers, SLV 306 caused a dose dependent attenuation of the hypertensive and reflex bradycardia response to big ET-1. There was also a dose dependent increase in ANP, big ET-1 and the ratio big ET-1: ET-1 but no increase in ET-1 following big ET-1 infusion.
3) In healthy volunteers, there was no notable change in blood pressure (pre angiotensin I infusion) and no significant inhibition of pressor response to angiotensin I following administration of GW660511X. Ramipril 10mg was associated with a reduction in blood pressure (pre angiotensin I infusion) and inhibition of the response to angiotensin I. There was significantly greater reduction in ACE activity with ramipril than GW660511X. GW660511X but not ramipril led to a dose dependent increase in plasma ANP concentration.
4) In patients with chronic heart failure, aliskiren suppressed plasma renin activity and reduced plasma angiotensin II. Ramipril in comparison caused an increase in renin activity and no change in angiotensin II. There were no significant changes in blood pressure with either treatment.
I have demonstrated the ACE and NEP inhibitory properties of omapatrilat and for the first time in humans, the ECE and NEP inhibitory properties of SLV 306, in vitro in patients with coronary artery disease but normal left ventricular dysfunction. I found no additional augmentation of bradykinin by omapatrilat or SLV 306 over and above that offered by ACE inhibition but significant augmentation by both dual inhibitors of adrenomedullin. This contradicts the suggestion that bradykinin has a role in the incidence of angioedema offers adrenomedullin as an alternative mediator. Adrenomedullin augmentation may also contribute significantly to the hypotensive effects of these molecules.
I have demonstrated for the first time in humans the ECE and NEP inhibitory properties of SLV 306 in vivo.
GW660511X is shown to inhibit NEP but to a much lesser extent ACE. Of note the comparison made is with full dose of a powerful pure ACE inhibitor. Any inhibition of ACE activity in contrast to the study of pure NEP inhibitors is consistent with the belief that dual inhibition offers additional benefit.
Finally I have demonstrated for the first time in patients with chronic heart failure the renin inhibitor activity of aliskiren, confirming attenuation of the renin angiotensin aldosterone pathway consistently from its origin and in contrast the rise in renin activity seen with ACE inhibitors.
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