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

Abstract

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.SIGNIFICANCE STATEMENT 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.

Keywords: calcitonin gene-related peptide; cortical spreading depression; headache; migraine; pain; trigeminovascular.

Figure 1.

Effect of CSD on activity of Aδ meningeal primary afferent nociceptors in animals treated with isotype. A, CSD recording (top trace) and plot of firing rate (bottom trace) for one Aδ neuron that was activated by CSD. B, Here and in the next 3 figures, box plots of neuronal firing rate before and after CSD illustrate the median and interquartile range of 25th–75th percentile. C, Plot of baseline and post-CSD firing rates (see Materials and Methods) for the isotype-treated Aδ neurons that were activated by CSD (n = 6). D, E, Plots of the latency to the onset of activation (D) and the duration of activation (E) for the neurons shown in B. F, Example of one Aδ-fiber that was not activated by CSD. G, Box plots of neuronal firing rate before and after CSD. H, Plot as in B, for the isotype-treated Aδ neurons that were not activated by CSD (n = 3). Asterisks in Figs. 1B, 3B, and 4B indicate statistically significant difference (p < 0.05).

Figure 2.

Effect of CSD on activity of Aδ meningeal primary afferent nociceptors in animals treated with CGRP-mAb. A, CSD recording (top trace) and plot of firing rate (bottom trace) for one Aδ neuron that was activated by CSD. B, Plot of baseline and post-CSD firing rates for the CGRP-mAb-treated Aδ neurons that were activated by CSD (n = 2). C, D, Plots of the latency to the onset of activation (C) and the duration of activation (D) for the neurons shown in B. E, Example of one Aδ fiber that was not activated by CSD. F, Box plots of neuronal firing rate before and after CSD. G, Plot as in B, for the mAb-treated Aδ neurons that were not activated by CSD (n = 8).

Figure 3.

Effect of CSD on activity of C-fiber meningeal primary afferent nociceptors in animals treated with isotype. A, CSD recording (top trace) and plot of firing rate (bottom trace) for one C-fiber neuron that was activated by CSD. B, Box plots of neuronal firing rate before and after CSD. C, Plot of baseline and post-CSD firing rates for the isotype-treated C-fiber neurons that were activated by CSD (n = 7). D, E, Plots of the latency to the onset of activation (D) and the duration of activation (E) for the neurons shown in B. F, Example of one C-fiber neuron that was not activated by CSD. G, Box plots of neuronal firing rate before and after CSD. H, Plot as in B, for the isotype-treated C-fiber neurons that were not activated by CSD (n = 9).

Figure 4.

Effect of CSD on activity of C-fiber meningeal primary afferent nociceptors in animals treated with CGRP-mAb. A, CSD recording (top trace) and plot of firing rate (bottom trace) for one C-fiber neuron that was activated by CSD. B, Box plots of neuronal firing rate before and after CSD. C, Plot of baseline and post-CSD firing rates for the CGRP-mAb-treated C-fiber neurons that were activated by CSD (n = 8). D, E, Plots of the latency to the onset of activation (D) and the duration of activation (E) for the neurons shown in B. F, Example of one Aδ fiber that was not activated by CSD. G, Box plots of neuronal firing rate before and after CSD. H, Plot as in B, for the mAb-treated Aδ neurons that were not activated by CSD (n = 6).

Figure 5.

Proposed mechanisms for prevention of migraine by CGRP-mAbs. CSD induces brief constriction, brief dilatation, and prolonged constriction of pial arteries (Step 1), as well as immediate and delayed activation of C-fiber meningeal nociceptors containing CGRP (Step 2). Upon their CGRP-independent activation, meningeal C-fibers release CGRP in the dura and by doing so; mediate a CGRP-dependent activation of the nearby Aδ-fibers (Step 3). Once activated, C-fiber meningeal nociceptors converge on and activate WDR neurons in the spinal trigeminal nucleus, whereas Aδ-fibers converge on and activate both WDR and HT neurons (Step 4) that eventually transmit the nociceptive signals from the dura to the thalamus (Step 5). The absence of CGRP receptors from the meningeal C-fibers renders the activation of the C-WDR pathway CGRP-independent (red), and thus, unresponsive to the CGRP-mAb. In contrast, the presence of CGRP receptors on meningeal Aδ-fibers renders the activation of the Aδ-HT pathway CGRP-dependent (blue), and thus, responsive to the CGRP-mAb.