ACE Inhibitors - Evaluation of Disposition Characteristics and Concentration Effect Relationships

Harrigan, Janet R (1990) ACE Inhibitors - Evaluation of Disposition Characteristics and Concentration Effect Relationships. PhD thesis, University of Glasgow.

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Extensive study of the active site of angiotensin converting enzyme (ACE) has led to the development of specific inhibitors which are now successfully used in the treatment of hypertension and cardiac failure. The work carried out within this thesis has been concerned with the evaluation of the disposition characteristics of the ACE inhibitors and their concentration effect relationships. Characterisation of the pharmacokinetics of any compound is dependent upon the availability of accurate, precise, sensitive and specific assay methodology. Three groups of methods have been compared within this thesis, two different methods to measure plasma ACE activity, three methods to measure plasma enalaprilat levels and three methods to measure plasma benazeprilat levels. The methods to measure plasma ACE activity yielded results in good agreement. One of the three methods to measure plasma enalaprilat levels gave results that were significantly lower than the remaining two methods. No obvious explanation could be found for this. Of the three methods to measure benazeprilat, the specific method gave significantly lower results than the two non specific methods, the possible existence of unstable glucuronides contributing to the discrepancy between the methods. These results highlight the need to know the specificity of any assay technique being used. The concentration effect relationship of two different series of ACE inhibitors was assessed in plasma from individual rabbits and volunteers. Differences were found in both the rank order of potency of the compounds between rabbit and man, and also, there were significant differences in the potency of single compounds between rabbit and man. These differences may reflect variations in the tertiary structure of ACE between rabbit and man and indicate that different doses would be needed to obtain the same degree of ACE inhibition. The in vivo/in vitro relationship for plasma ACE inhibition was assessed for perindoprilat and quinaprilat. For both compounds the in vivo values were significantly lower than those found in vitro, indicating greater inhibition of ACE in vivo. Any calculations based upon the in vitro values would overestimate the amount of drug needed to elicit the same response in vivo. The third group of in vitro studies examined the effect of the presence of parent compound on the in vitro potency of the metabolite. In vitro dose response curves for five metabolites were characterised in the presence and absence of parent compound. The presence of parent compound brought about a significant decrease in potency for S-10211 (the active metabolite of S-9650) and quinaprilat in rabbit, and for enalaprilat and perindoprilat in man. The decrease in potency seen for some of the metabolites studied may be due to the parent compound binding to a second active site sequence on the ACE molecule where it causes steric hindrance but not ACE inhibition. The rabbit was used as a model to characterise the effect of saturation of ACE binding sites on the pharmacokinetics of radiolabelled spiraprilat. Rabbits were pretreated with either placebo or unlabelled spiraprilat and the pharmacokinetics of a tracer radiolabelled intravenous dose of spiraprilat characterised. The pharmacokinetics of radiolabelled spiraprilat were best described by a three compartment model with a terminal elimination half life of the order of 2.5 hours. The effect of saturation of plasma and tissue ACE binding sites caused an increase in the rate of elimination during the second phase, the effect was small and did not contribute to a change in total clearance of the radiolabelled dose. Despite higher plasma ACE activity values in rabbit, the long terminal half life, a characteristic feature of ACE inhibitor drugs in man, was not observed in rabbit for radiolabelled spiraprilat. Thus, the rabbit would appear not to be a good model for the disposition of ACE inhibitors in man. The pharmacokinetics of enalaprilat, formed after an oral dose of enalapril, were characterised in the presence and absence of saturated plasma and tissue ACE binding sites. Captopril was the agent used to saturate ACE binding sites. Eight volunteers received a single 10mg dose of enalapril. A washout period ensued then the volunteers were pretreated with captopril, 50mg twice daily, followed by a second 10mg dose of enalapril. Analysis of enalaprilat pharmacokinetics revealed no differences in the presence and absence of captopril. Analysis of the plasma ACE activity data indicated that induction of plasma ACE had occurred during the pretreatment with captopril. Thus, the lack of any detectable change in enalaprilat pharmacokinetics could have been due to the induction of ACE by pretreatment with captopril.

Item Type: Thesis (PhD)
Qualification Level: Doctoral
Keywords: Molecular biology
Date of Award: 1990
Depositing User: Enlighten Team
Unique ID: glathesis:1990-78065
Copyright: Copyright of this thesis is held by the author.
Date Deposited: 28 Feb 2020 12:09
Last Modified: 28 Feb 2020 12:09

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