Adrenomedullin

Overview

Adrenomedullin (ADM) is a 52-amino acid peptide originally isolated in 1993 by Kitamura and colleagues from human pheochromocytoma (adrenal medullary tumor) tissue, hence its name. It was identified through a bioassay screening for compounds that elevated cAMP in rat platelets. While initially characterized as a potent vasodilator, subsequent research revealed adrenomedullin to be a multifunctional peptide expressed in virtually every tissue of the body, with roles in vascular tone regulation, angiogenesis, lymphatic development, immune modulation, and organ protection.

Adrenomedullin belongs to the CGRP (calcitonin gene-related peptide) superfamily, which also includes CGRP-alpha, CGRP-beta, amylin, calcitonin, and intermedin (adrenomedullin 2). These peptides share structural features and overlapping receptor pharmacology through the calcitonin receptor-like receptor (CLR) system, where specificity is determined by receptor activity-modifying proteins (RAMPs).

The most translational excitement around adrenomedullin has centered on sepsis and critical care. Circulating adrenomedullin levels rise dramatically in sepsis and correlate with disease severity and mortality. This has led to the development of bio-ADM (biologically active adrenomedullin) as a prognostic biomarker and, more innovatively, to adrecizumab — an anti-adrenomedullin antibody that paradoxically improves outcomes not by blocking adrenomedullin but by redistributing it from interstitial spaces back into the vascular compartment, restoring endothelial barrier function.

Amino Acid Sequence

Human adrenomedullin: YRQSMNNFQGLRSFGCRFGTCTVQKLAHQIYQFTDKDKDNVAPRSKISPQGY-NH₂

• Molecular weight: ~6,028 g/mol
• Gene: ADM (chromosome 11p15.4)
• CAS Number: 137171-69-0
• Receptor: CLR/RAMP2 complex (AM1 receptor); CLR/RAMP3 complex (AM2 receptor)

Structural features:

• 6-amino acid disulfide bond ring — Cys16-Cys21, essential for receptor binding and biological activity
• C-terminal amidation — required for full biological activity
• 52 amino acids — longer than most peptides in the CGRP superfamily
• Structural similarity to CGRP — shares the disulfide ring motif and C-terminal amidation; both signal through CLR but with different RAMP partners

Mechanism of Action

Receptor System

Adrenomedullin signals through the calcitonin receptor-like receptor (CLR), with specificity determined by RAMPs:

AM1 receptor (CLR/RAMP2):

• Primary adrenomedullin receptor
• High affinity for adrenomedullin, low for CGRP
• Couples to Gs — activates adenylyl cyclase, increases cAMP
• RAMP2 is essential for CLR cell-surface expression and adrenomedullin selectivity

AM2 receptor (CLR/RAMP3):

• Secondary adrenomedullin receptor
• Binds both adrenomedullin and CGRP
• Also couples to Gs/cAMP
• May mediate some overlapping functions with CGRP

Vasodilation

• cAMP/PKA signaling in vascular smooth muscle activates KATP channels and reduces intracellular calcium
• Endothelium-dependent component via nitric oxide and prostacyclin release
• One of the most potent endogenous vasodilators known — comparable to CGRP in potency
• Reduces systemic vascular resistance, blood pressure, and pulmonary artery pressure

Endothelial Barrier Protection

This function has become the primary focus of clinical research:

• Adrenomedullin strengthens endothelial cell junctions (VE-cadherin, tight junctions)
• Reduces vascular permeability/leak — critical in sepsis pathophysiology
• Acts via cAMP/Epac/Rap1 pathway to stabilize cortical actin cytoskeleton
• Loss of adrenomedullin signaling → increased vascular leak, edema, organ dysfunction

Immune Modulation

• Reduces pro-inflammatory cytokine production (TNF-alpha, IL-6) from macrophages
• Anti-inflammatory effects partly via cAMP-mediated NF-kappaB suppression
• Bactericidal properties at high local concentrations
• Modulates complement activation

Research Summary

Pharmacokinetics

• Half-life: Approximately 22 minutes in circulation
• Circulating levels: 2-10 pg/mL in healthy adults; rises 5-20x in sepsis, heart failure, and renal failure
• Forms: Circulates as biologically active adrenomedullin (bio-ADM) and inactive mid-regional pro-adrenomedullin (MR-proADM)
• Metabolism: Degraded by neprilysin and other endopeptidases
• Expression: Nearly ubiquitous — vascular endothelium, smooth muscle, heart, lung, kidney, adrenal, brain, GI tract, immune cells
• Regulation: Upregulated by hypoxia, inflammatory cytokines, shear stress, oxidative stress

Clinical Applications

Biomarker: MR-proADM

Mid-regional pro-adrenomedullin (MR-proADM) is a stable surrogate marker for adrenomedullin:

• Used clinically for sepsis prognosis and risk stratification
• Predicts mortality in pneumonia, sepsis, and heart failure
• More stable than active adrenomedullin, making it practical for routine measurement
• Available as a commercial immunoassay (BRAHMS MR-proADM)

Therapeutic: Adrecizumab (Investigational)

Adrecizumab is a non-neutralizing anti-adrenomedullin antibody:

• Mechanism: Binds the N-terminal region of adrenomedullin without blocking receptor interaction
• Proposed action: Redistributes adrenomedullin from interstitial tissue back into the vascular compartment, where it can access endothelial receptors and restore barrier function
• Phase 2 (AdrenOSS-2): Showed trends toward reduced mortality in septic shock patients with high bio-ADM levels
• Represents a novel "redistribute, don't block" antibody therapeutic concept

Common Discussion Topics

1. CGRP family pharmacology — Adrenomedullin and CGRP share the CLR receptor, with specificity determined by RAMPs. This elegant system of receptor-RAMP combinatorics is a paradigm for how a single receptor can serve multiple peptide ligands with distinct biology.

2. Sepsis therapeutic potential — The combination of adrenomedullin as a biomarker (MR-proADM for prognosis) and a therapeutic target (adrecizumab) makes it one of the most complete translational stories in sepsis biology.

3. The redistribution concept — Adrecizumab's mechanism — an antibody that improves the target peptide's function by redirecting it rather than blocking it — is novel in pharmacology and may have implications for other peptide systems.

4. Natriuretic peptide comparison — While adrenomedullin is vasodilatory like ANP and BNP, it signals through a completely different receptor system and has more prominent endothelial barrier effects.

5. Cardiovascular protection — Adrenomedullin's combination of vasodilation, anti-inflammation, and endothelial protection has made it a subject of investigation in heart failure, pulmonary hypertension, and ischemia-reperfusion injury.

Related Compounds

• CGRP — calcitonin gene-related peptide; shares CLR receptor system with different RAMP (RAMP1)
• ANP — atrial natriuretic peptide; vasodilator through a different receptor system (NPR-A)
• BNP — B-type natriuretic peptide; cardiac biomarker and vasodilator
• VIP — vasoactive intestinal peptide; another potent vasodilator with immunomodulatory properties
• Urocortin — CRF-family peptide with complementary cardiovascular protective effects