Assessing migraine therapeutics

To the Editor,

We read with great interest the recent article by Bhakta et al. (1) “Migraine therapeutics differentially modulate the CGRP pathway” in Cephalalgia describing the in vitro comparison of various migraine therapeutics. One of the findings reported by the authors was the effect of CGRP receptor antagonists on AMY1 receptor binding and amylin signaling. However, the clinical translatability and impact of these finding remains unclear due to several limitations of the in vitro data presented.

While useful for mechanistic studies, model systems using recombinant expression of receptors in carrier systems have limitations when extrapolating to normal physiological conditions. The pharmacology of the recombinantly generated HEK293-AMY1 receptor cell line used in this study does not agree with that previously reported in the literature. Specifically, in Supplementary Figure 5, the agonists are ∼100–1000× more potent than what has been established in the published literature (Table 1) (2,3). Such increased agonist potency can often result from overexpressed and/or mis-configured receptors in these cells (4). Data generated, including binding, cAMP signaling, and receptor trafficking, only reflects what is happening in that specific system; normalizing to physiological conditions or comparing to previously published data in other cell systems should be done with caution.

Table 1.

Comparison of agonist potency at the AMY1 receptor.

	Bhakta et al. 2021 (Supplemental Figure 5) (1)	Walker et al. 2015 (Table 2) (3)	Hong et al. 2012 (Table 3) (2)
pEC50 at the AMY1 receptor
h/rCGRP	11.12 ± 0.05	9.63 ± 0.12	8.70 ± 0.17
h/rAdrenomedullin	9.66 ± 0.06	–	6.48 ± 0.28
h/rIntermedin	10.27 ± 0.05	–	8.07 ± 0.19, 8
h/rCalcitonin	11.56 ± 0.07	–	8.93 ± 0.09, 9
h/rAmylin	11.13 ± 0.05	9.46 ± 0.05	–

Of further note, the authors did not assess and/or report the inhibitory effect of the test compounds against CGRP signaling at the AMY1 receptor. Figure 5(c) only shows the inhibitory effect of the compounds against amylin signaling at the AMY1 receptor. However, since CGRP and amylin are equally potent at the AMY1 receptor, comparison should be made with both agonists to differentiate the impact of CGRP receptor antagonists versus anti-CGRP ligand agents. Without this comparison, the impact of an anti-CGRP agent, including fremanezumab, to the signaling of CGRP at the AMY1 receptor cannot be fully characterized, limiting the conclusions that can be made about the physiological or clinical impact of such agents on signaling via the AMY1 receptor.

Thus, the authors’ claim that “the diverse mechanisms of action of CGRP ligand versus receptor agents may differentially affect efficacy, safety, and/or tolerability in migraine patients” is not substantially justified by the non-clinical data presented in their article, and a more balanced interpretation of their findings is warranted. Beyond the issues raised above regarding the artificial and potentially biased cell systems, the study as a whole was conducted in vitro with no direct evidence linking the findings to clinical efficacy, safety, and/or tolerability in migraine patients. For example, the authors imply that blocking the effects of amylin on AMY1 has the potential for metabolism-related side effects, but this was not observed in clinical trials with erenumab (Aimovig® package insert). Caution should be taken when extrapolating clinical impact from in vitro data.