Angiotensin III (Ang III, also designated Ang 2-8 or des-Asp1-angiotensin II) is a naturally occurring heptapeptide (Arg-Val-Tyr-Ile-His-Pro-Phe) generated by the enzymatic removal of the N-terminal aspartate residue from angiotensin II by aminopeptidase A (APA). Once considered merely a degradation product of Ang II with reduced potency, Ang III is now recognized as a bioactive peptide in its own right — particularly within the brain, where accumulating evidence positions it as the primary effector of the central renin-angiotensin system (RAS), and in the adrenal gland, where it is equipotent to Ang II in stimulating aldosterone release.

Overview

Angiotensin III occupies a distinctive position within the RAAS cascade. As the immediate metabolite of angiotensin II, it retains activity at both AT1 and AT2 receptors but with a pharmacological profile that differs meaningfully from its parent peptide. In peripheral tissues, Ang III has approximately 25-50% of the vasoconstrictive potency of Ang II at AT1 receptors. However, two properties distinguish it from Ang II and have driven significant research interest:

First, Ang III is equipotent to Ang II in stimulating aldosterone secretion from the adrenal zona glomerulosa. This observation, first reported by Campbell and colleagues, suggested that Ang III — not Ang II — may be the principal adrenal secretagogue in the intact RAAS, particularly in tissues where aminopeptidase A rapidly converts Ang II to Ang III [1].

Second, and more consequentially, a series of landmark studies by Bhargava, Bhatt, Reaux-Le Goazigo, and others demonstrated that Ang III, rather than Ang II, is the primary effector peptide of the brain RAS controlling vasopressin release, sympathetic outflow, and central blood pressure regulation. This was established using EC33, a selective aminopeptidase A inhibitor that blocks the conversion of Ang II to Ang III in the brain, and showed that central pressor responses attributed to Ang II were actually mediated by Ang III [2, 3].

This discovery has led to the development of aminopeptidase A inhibitors — most notably firibastat (QGC006) — as a novel class of centrally acting antihypertensive agents that reduce brain Ang III formation to lower blood pressure.

Ang III is further metabolized by aminopeptidase N (APN) to produce angiotensin IV (Ang 3-8), which acts at the AT4 receptor (IRAP) to influence cognition and renal function, extending the RAAS cascade beyond its classical boundaries.

Mechanism of Action

Angiotensin III exerts its effects through the same AT1 and AT2 receptors as angiotensin II, but with a distinct pharmacological profile shaped by receptor selectivity differences and tissue-specific distribution:

AT1 Receptor Activity

Ang III activates AT1 receptors with approximately 25-50% of the affinity and efficacy of Ang II in peripheral vasculature. This produces vasoconstriction, but with reduced potency compared to the parent peptide. However, in specific tissues — particularly the adrenal zona glomerulosa — Ang III is fully equipotent to Ang II in AT1-mediated aldosterone secretion. This tissue-selective potency likely reflects differences in receptor density, coupling efficiency, and local peptide concentrations [1].

AT2 Receptor Activity (Preferential)

Ang III exhibits preferential activity at AT2 receptors compared to Ang II, particularly within the central nervous system and kidney. AT2 activation by Ang III produces:

• Natriuresis: AT2-mediated inhibition of proximal tubular sodium reabsorption, promoting sodium excretion. This is a primary mechanism of Ang III's renal effects and may represent an intrinsic counter-regulatory function within the RAAS [4].
• Vasodilation: AT2-dependent NO and bradykinin release, partially offsetting AT1-mediated vasoconstriction.
• Anti-proliferative signaling: Growth inhibition and apoptosis in certain cell types through AT2-dependent mechanisms.

Central Nervous System Effects

The most important mechanistic distinction of Ang III lies in the brain RAS. When Ang II is administered centrally, it is rapidly converted to Ang III by brain aminopeptidase A. Reaux-Le Goazigo et al. demonstrated that the central pressor effects traditionally attributed to Ang II are actually mediated by Ang III, as shown by:

• APA inhibitors (EC33) blocking the central conversion of Ang II to Ang III abolish the central pressor response [2].
• Direct central administration of Ang III produces vasopressin release and sympathetic activation even when APA-resistant Ang II analogs do not [3].
• These central effects are mediated through AT1 receptors in the hypothalamus and brainstem, but require Ang III — not Ang II — as the ligand.

Aldosterone Secretion

In adrenal glomerulosa cells, Ang III is equipotent to Ang II in stimulating aldosterone synthesis and release. The mechanism involves AT1-mediated activation of phospholipase C, calcium mobilization, and steroidogenic enzyme upregulation. Given that aminopeptidase A is highly expressed in the adrenal gland, locally generated Ang III may be the predominant aldosterone secretagogue under physiological conditions [1, 5].

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Research

Safety Profile

Angiotensin III is an endogenous peptide with a normal physiological role in RAAS signaling. Direct administration of Ang III has been used in research settings with effects similar to, but generally less potent than, angiotensin II. The primary safety considerations relate to pharmacological agents targeting the Ang III pathway:

• Firibastat (APA inhibitor): In clinical trials, firibastat was generally well tolerated. Most adverse events were mild, including headache, nasopharyngitis, and dizziness. No significant hyperkalemia or renal impairment was observed, distinguishing it from peripheral RAAS blockers that can impair potassium excretion.
• Central effects: As Ang III acts primarily within the brain, APA inhibitors that modulate central Ang III levels may theoretically affect thirst, salt appetite, and vasopressin release — though these effects have not been clinically problematic in trials.
• No peripheral RAAS disruption: Because firibastat acts centrally, it does not directly affect peripheral Ang II levels, AT1 signaling, aldosterone, or renal function — providing a different safety profile from ACE inhibitors, ARBs, and mineralocorticoid receptor antagonists.

Clinical Research Protocols

• Intrarenal Ang III infusion: 1-100 pmol/min in animal studies to assess natriuretic responses [4].
• Central (ICV) administration: 10-100 pmol bolus injections in rodent studies for central pressor and AVP release studies [2, 3].
• Firibastat (oral): 250-500 mg twice daily in Phase II/III clinical trials for resistant hypertension [6, 7].
• Duration: Firibastat trials ranged from 8 weeks to 6 months.

Subpopulation Research

• Resistant hypertension: Firibastat showed efficacy in treatment-resistant patients, particularly those with overweight/obesity [6].
• Black/African American patients: The NEW-HOPE trial demonstrated particular efficacy of firibastat in Black patients, a population often undertreated by conventional RAAS blockers due to lower peripheral renin activity [6].
• Salt-sensitive hypertension: Central RAS overactivation is implicated in salt sensitivity; APA inhibitors may address this mechanism.
• Heart failure: Elevated central Ang III-mediated AVP release contributes to volume overload and hyponatremia in CHF; APA inhibition may reduce inappropriate AVP secretion [8].
• Obesity-related hypertension: Adipose tissue RAS activation increases Ang II and Ang III generation; central APA inhibition may particularly benefit obese hypertensive patients.

Pharmacokinetic Profile

Ongoing & Future Research

• Firibastat refinement: Despite the FRESH trial results, subgroup analyses continue to identify populations that may benefit from central APA inhibition, with potential for precision medicine approaches to patient selection.
• Dual APA/APN inhibitors: Combined inhibition of Ang III formation and degradation to simultaneously modulate Ang III and angiotensin IV pathways.
• Aldosterone breakthrough: Investigation of Ang III's role in residual aldosterone secretion during ACE inhibitor therapy, with implications for dual RAAS blockade strategies.
• Brain RAS imaging: Development of PET tracers for APA activity to non-invasively assess brain RAS activation in hypertensive patients.
• Renal AT2 agonism: Exploring Ang III-based peptide analogs with enhanced AT2 selectivity for natriuretic applications in salt-sensitive hypertension.