The ACE Inhibitor Family

## Overview

Angiotensin-converting enzyme (ACE) inhibitors were the first rationally designed class of antihypertensives based on enzyme structure. ACE is a zinc-dependent metallopeptidase (a carboxypeptidase) that cleaves the C-terminal His-Leu dipeptide from angiotensin I to produce the potent vasoconstrictor angiotensin II. It also inactivates bradykinin. Captopril (1977, Cushman and Ondetti) was the first orally active ACE inhibitor, designed by mimicking the transition state of ACE's substrate. More than a dozen ACE inhibitors are now approved globally.

## Zinc-Binding Pharmacophore

The catalytic site of ACE contains a zinc ion coordinated by His383, His387, and Glu411. Endogenous substrates form a transition-state intermediate in which the zinc stabilizes the developing tetrahedral carbon of the scissile bond. ACE inhibitors exploit this by presenting a zinc-chelating group that coordinates the zinc more tightly than the substrate transition state.

Three zinc-binding groups have been exploited clinically:

1. **Thiol (SH)**: Captopril's thiol provides strong zinc chelation (Ki ~0.0023 nM) but is responsible for rash, taste disturbance, and proteinuria in ~10% of patients.
2. **Carboxylate (COOH)**: All subsequent ACE inhibitors use a carboxylate as the zinc ligand. It is weaker than thiol but avoids thiol-related toxicity. Enalaprilat (active form of enalapril) is the prototype.
3. **Phosphonate**: Fosinopril uses a phosphinyl group, providing moderate zinc chelation. It has a dual elimination pathway (renal and hepatic) useful in renal impairment.

## Prodrug Strategy

The carboxylate-containing active forms (enalaprilat, lisinopril-diacid, ramiprilat) are too hydrophilic for oral absorption. The solution was to mask the carboxylate as an ethyl ester (enalapril, ramipril, benazepril, perindopril). These are absorbed intact in the gut and hydrolyzed by hepatic and intestinal esterases to the active diacid. Lisinopril is the exception—it is an active lysine analog without ester masking and has adequate oral bioavailability (~25%).

## The Proline Moiety

C-terminal proline is a pharmacophoric anchor. The pyrrolidine ring engages a hydrophobic S2' subsite of ACE lined by Tyr520, Phe512, and Thr516. Replacement of proline by alanine reduces ACE inhibitory potency ~100-fold; N-methylalanine is less potent than proline. Some inhibitors use bicyclic prolines (ramipril, perindopril): octahydroindol-2-carboxylic acid or octahydrocyclopenta-pyrrole. These constrained analogs confer tighter binding, longer dissociation from tissue ACE, and extended duration.

## S1 Subsite and Lipophilic P2 Group

Captopril has only a methyl group at P2, which partially occupies the S1 hydrophobic subsite of ACE. Enalapril's first SAR improvement was replacing the methyl with a more extended phenylpropyl (methylbutyryl) chain, increasing potency ~100-fold. Subsequent ACE inhibitors explored indane (indapril), fluorobiphenyl (quinapril), and other lipophilic groups at P2 for further S1 pocket engagement.

## Tissue ACE vs Plasma ACE

Plasma ACE is readily inhibited by all ACE inhibitors. Tissue ACE (vascular, cardiac, renal) dissociates much more slowly from highly lipophilic, tightly binding inhibitors. Ramipril's bicyclic proline confers an in vitro dissociation half-life of ~30 hours from tissue ACE—potentially relevant to cardioprotective benefits beyond blood pressure reduction.

## Key Takeaways

- Zinc chelation via thiol, carboxylate, or phosphonate is the key pharmacophoric element
- Carboxylate-based inhibitors replaced thiol due to improved safety; phosphonate allows dual elimination
- C-terminal proline pyrrolidine is a critical structural anchor for the S2' subsite
- Lipophilic P2 groups dramatically increase potency by filling the S1 hydrophobic pocket
- Prodrug ester masking was needed for oral bioavailability of dicarboxylate-active inhibitors