Fremanezumab—A Humanized Monoclonal Anti-CGRP Antibody—Inhibits Thinly Myelinated (Aδ) But Not Unmyelinated (C) Meningeal Nociceptors

Calcitonin gene-related peptide (CGRP), the most abundant neuropeptide in primary afferent sensory neurons, is strongly implicated in the pathophysiology of migraine headache, but its role in migraine is still equivocal. As a new approach to migraine treatment, humanized anti-CGRP monoclonal antibodies (CGRP-mAbs) were developed to reduce the availability of CGRP, and were found effective in reducing the frequency of chronic and episodic migraine. We recently tested the effect of fremanezumab (TEV-48125), a CGRP-mAb, on the activity of second-order trigeminovascular dorsal horn neurons that receive peripheral input from the cranial dura, and found a selective inhibition of high-threshold but not wide-dynamic range class of neurons. To investigate the basis for this selective inhibitory effect, and further explore the mechanism of action of CGRP-mAbs, we tested the effect of fremanezumab on the cortical spreading depression-evoked activation of mechanosensitive primary afferent meningeal nociceptors that innervate the cranial dura, using single-unit recording in the trigeminal ganglion of anesthetized male rats. Fremanezumab pretreatment selectively inhibited the responsiveness of Aδ neurons, but not C-fiber neurons, as reflected in a decrease in the percentage of neurons that showed activation by cortical spreading depression. These findings identify Aδ meningeal nociceptors as a likely site of action of fremanezumab in the prevention of headache. The selectivity in its peripheral inhibitory action may partly account for fremanezumab's selective inhibition of high-threshold, as a result of a predominant A-δ input to high-threshold neurons, but not wide dynamic-range dorsal horn neurons, and why it may not be effective in all migraine patients.<b>SIGNIFICANCE STATEMENT</b> Recently, we reported that humanized CGRP monoclonal antibodies (CGRP-mAbs) prevent activation and sensitization of high-threshold (HT) but not wide-dynamic range trigeminovascular neurons by cortical spreading depression (CSD). In the current paper, we report that CGRP-mAbs prevent the activation of Aδ but not C-type meningeal nociceptors by CSD. This is the first identification of an anti-migraine drug that appears to be selective for Aδ-fibers (peripherally) and HT neurons (centrally). As the main CGRP-mAb site of action appears to be situated outside the brain, we conclude that the initiation of the headache phase of migraine depends on activation of meningeal nociceptors, and that for selected patients, activation of the Aδ-HT pain pathway may be sufficient for the generation of headache perception.