Overview

CGRP was discovered in 1982 by Amara, Jonas, Rosenfeld, and colleagues through alternative RNA processing of the calcitonin gene (CALCA) [1]. The CALCA gene on chromosome 11p15.2 produces calcitonin in thyroid C-cells but, through tissue-specific alternative splicing, produces α-CGRP in neurons. β-CGRP is encoded by a separate gene (CALCB) on the same chromosome and differs from α-CGRP by three amino acids in humans.

CGRP signals through a heterodimeric receptor complex consisting of the calcitonin receptor-like receptor (CLR, a class B GPCR) and receptor activity-modifying protein 1 (RAMP1). This CLR/RAMP1 complex is essential for CGRP binding and signaling — neither component alone forms a functional CGRP receptor. A third component, receptor component protein (RCP), couples the receptor to intracellular Gs-cAMP signaling. The requirement for RAMP1 is pharmacologically significant: the same CLR pairs with RAMP2 to form the adrenomedullin-1 receptor (AM1) or with RAMP3 to form the adrenomedullin-2 receptor (AM2), allowing selective targeting of CGRP signaling without disrupting adrenomedullin pathways. McLatchie LM et al. (1998) — Nature 393, 333-339.

CGRP is widely distributed throughout the central and peripheral nervous systems. In the trigeminovascular system, CGRP-containing sensory fibers innervate cerebral and meningeal blood vessels. Release of CGRP from these perivascular nerve endings causes potent vasodilation, promotes mast cell degranulation, and drives neurogenic inflammation — the cascade now understood to underlie migraine headache.

Mechanism of Action

CGRP exerts its effects through the CLR/RAMP1 receptor complex, primarily via Gs-cAMP-PKA signaling:

Vasodilation: CGRP is the most potent vasodilator peptide known. It acts directly on vascular smooth muscle cells through CLR/RAMP1 receptors, increasing cAMP and activating PKA, which opens KATP channels and promotes smooth muscle relaxation. In cerebral and meningeal arteries, CGRP released from trigeminal sensory fibers produces potent vasodilation that contributes to the throbbing headache of migraine. CGRP also causes vasodilation indirectly through endothelial NO release. Brain SD et al. (1985) — Nature 313, 54-56.

Trigeminovascular Activation and Migraine: During a migraine attack, activation of the trigeminovascular system leads to release of CGRP from trigeminal sensory nerve endings surrounding meningeal blood vessels. CGRP causes meningeal vasodilation, promotes plasma protein extravasation, drives mast cell degranulation, and sensitizes trigeminal nociceptors — collectively producing neurogenic inflammation. The resulting pain signals are transmitted through the trigeminal ganglion to the trigeminal nucleus caudalis in the brainstem, then to higher pain-processing centers. Jugular venous CGRP levels are elevated during migraine attacks and normalize with successful triptan treatment. Goadsby PJ et al. (1990) — Ann Neurol. 28, 183-187.

Neurogenic Inflammation: CGRP released from sensory nerve terminals promotes neurogenic inflammation through vasodilation, increased vascular permeability, and immune cell activation. This mechanism is relevant not only to migraine but also to inflammatory conditions of the skin, joints, and airways.

Pain Modulation: In the spinal cord dorsal horn, CGRP released from primary afferent terminals facilitates nociceptive transmission by enhancing glutamate and substance P signaling. CGRP contributes to central sensitization — the amplification of pain signaling that underlies chronic pain conditions.

Cardiovascular Protection: CGRP has cardioprotective effects including coronary vasodilation, positive inotropic and chronotropic effects, and protection against ischemia-reperfusion injury. CGRP inhibits vascular smooth muscle proliferation and may have anti-hypertensive effects. These protective roles raise theoretical safety considerations for chronic CGRP blockade in migraine therapy.

Wound Healing: CGRP-containing sensory fibers innervate the skin and play a role in wound healing through promoting angiogenesis, keratinocyte proliferation, and immune cell recruitment. Denervation impairs wound healing partly through loss of CGRP-mediated signaling.

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Research

Safety Profile

As an endogenous neuropeptide, CGRP itself is not administered therapeutically. Safety considerations relate to both CGRP excess (migraine) and CGRP blockade (anti-CGRP therapeutics). For anti-CGRP monoclonal antibodies, the most common adverse effects are injection site reactions and constipation. Erenumab has been associated with constipation and, rarely, severe constipation requiring hospitalization, likely reflecting CGRP's role in gastrointestinal motility. Theoretical cardiovascular concerns (given CGRP's vasodilatory and cardioprotective roles) have not been borne out in clinical trials, though patients with recent cardiovascular events were excluded from pivotal studies. Long-term post-marketing surveillance is ongoing. No hepatotoxicity has been observed with monoclonal antibodies, distinguishing them from early gepants (telcagepant). The American Headache Society position statement (2019) notes that anti-CGRP therapies have favorable safety profiles compared with traditional migraine preventives. Raynaud's phenomenon and hypertension have been reported rarely with erenumab. Russell FA et al. (2014) — PMID: 25287861

Clinical Research Protocols

CGRP measurement and provocation testing are used in migraine research:

CGRP Provocation Testing: Intravenous CGRP infusion (1.5-2.0 μg/min for 20 minutes) reliably triggers migraine-like headache in migraine patients (within 1-12 hours) but produces only mild, non-migraine headache in healthy controls. This provocation model is used for proof-of-concept testing of anti-CGRP therapeutics.

Plasma CGRP Measurement: Jugular venous or peripheral blood CGRP measured by ELISA or radioimmunoassay. Ictal (during attack) levels: significantly elevated vs interictal. Interictal levels may also be elevated in chronic migraine. Challenges: CGRP is rapidly degraded, requiring aprotinin-containing collection tubes and immediate cold centrifugation.

Anti-CGRP Antibody Protocols:

• Erenumab: 70 mg or 140 mg SC monthly via autoinjector. No titration required. Allow 3 months to assess efficacy.
• Fremanezumab: 225 mg SC monthly or 675 mg SC quarterly. Both regimens show similar efficacy.
• Galcanezumab: 240 mg SC loading dose, then 120 mg SC monthly. Also approved for episodic cluster headache (300 mg SC monthly).
• Eptinezumab: 100-300 mg IV infusion quarterly. Fastest onset — significant efficacy demonstrated within 1 day of first infusion.

Pharmacokinetic Profile

Ongoing & Future Research

• Active areas of CGRP research include:

• CGRP and Cluster Headache: Galcanezumab is approved for episodic cluster headache. Ongoing research is evaluating anti-CGRP therapies for chronic cluster headache, where treatment options remain limited.

• CGRP in Post-Traumatic Headache: Elevated CGRP levels have been demonstrated after traumatic brain injury, and anti-CGRP therapies are being evaluated for post-traumatic headache (NCT04684381).

• Cardiovascular Safety of Long-Term CGRP Blockade: Given CGRP's cardioprotective roles, long-term registries and studies are monitoring cardiovascular outcomes in patients receiving chronic anti-CGRP therapy, particularly in those with pre-existing cardiovascular risk factors.

• CGRP in Non-Headache Pain: CGRP's role in neurogenic inflammation extends beyond headache. Research is exploring anti-CGRP approaches for inflammatory pain conditions, neuropathic pain, and complex regional pain syndrome (CRPS).

• Biomarker-Guided Therapy: Interictal plasma and salivary CGRP levels are being investigated as predictive biomarkers for response to anti-CGRP therapy, aiming to personalize migraine treatment selection.

• Dual CGRP/Amylin Receptor Targeting: Given the structural relationship between CGRP and amylin receptors (shared CLR and RAMP components), research is investigating the role of amylin receptor activation in migraine pathophysiology and whether dual-targeted therapies might offer advantages.