A Drug That Boosts Naturally Occurring Opioids Eases Migraine Pain in an Animal Model
PL37, which inhibits the enzymes that degrade enkephalins, reduces migraine pain in rats.
The endogenous opioid system – the body’s innate pain-relieving system – has a powerful effect on pain. Naturally occurring opioid peptides like enkephalins sometimes produce pain relief so powerful that it even allows soldiers and athletes to ignore serious injuries in the heat of the moment.
What if there were medications that could amplify the endogenous opioid system to treat migraine pain, but without the notoriously problematic side effects of exogenous opioids like fentanyl and morphine? A new study now takes a hopeful step toward that future.
The study from the Neuro-Dol laboratory, Inserm/Université Clermont Auvergne, and Pharmaleads, a drug company based in Paris, France, shows that PL37, a so-called dual enkephalinase inhibitor that inhibits the two enzymes that degrade enkephalins, reduces migraine pain in rats.
The drug was successful both at treating and preventing migraine pain, possibly through effects on neurons of the trigeminocervical complex, which receives synaptic input from periorbital trigeminal sensory neurons.
“This paper is exciting, as it shows that a dual enkephalinase inhibitor can relieve migraine-associated, pain-like effects, specifically cephalic hypersensitivity. The advantage with this approach is that you’re utilizing the endogenous opioid system without the adverse effects of giving exogenous opioids,” said Amynah Pradhan, a migraine and pain researcher at the University of Illinois at Chicago, US, who was not part of the study.
The research appeared online April 12, 2022, in Brain.
Inhibiting the enzymes that degrade enkephalins
For several decades, Pharmaleads has been working on a class of drugs called Dual ENKephalinase Inhibitors (DENKIs) to treat pain. Normally, two enzymes called neutral endopeptidase (NEP) and aminopeptidase N (APN) break down enkephalins, limiting their pain inhibitory effects.
“After a painful stimulation there is a release of enkephalins near the site of pain. Our aim is to protect enkephalins from degradation during pain states by inhibiting these two enkephalinase enzymes with DENKIs. We have seen promising results in preclinical studies showing that PL265, a DENKI we synthesized, is effective at treating other pain types,” said Hervé Poras, Chief Scientific Officer at Pharmaleads.
With those promising early results from mouse models of ocular pain and neuropathic pain, Pharmaleads aimed to test if a different DENKI the company is developing, PL37, could treat migraine pain. For this, they brought in the expertise of Radhouane Dallel’s group at Université Clermont Auvergne, with the groups working together to test the drug in an animal model.
Putting PL37 to the test
The researchers used a rat model of migraine where they injected isosorbide dinitrate (ISDN), a drug that reliably produces headache in people with migraine and results in periorbital mechanical hypersensitivity in rodents, into the intraperitoneal (abdominal cavity) area in the rats. The group then assessed mechanical hypersensitivity by measuring the animals’ withdrawal thresholds in response to mechanical stimulation of the cephalic skin between the eyes. For this, the group used thin filaments called von Frey hairs to poke the animals.
As expected, a single injection of ISDN caused a rapid lowering of mechanical thresholds (that is, increased sensitivity to the von Frey hairs) for two hours – what is called acute cephalic cutaneous mechanical hypersensitivity. But the investigators found that oral administration of PL37 had a powerful effect, reducing the mechanical hypersensitivity in response to acute ISDN, as shown by increased withdrawal thresholds.
Next, to test whether PL37 could reduce pain during chronic migraine, the team injected ISDN each day for five days, modeling a more chronic migraine-like disorder. Oral PL37 on the fifth day of ISDN treatment failed to reduce chronic, cutaneous, ictal mechanical hypersensitivity in the animals. By contrast, when administered intravenously, PL37 effectively reduced that same hypersensitivity.
Can PL37 also prevent migraine pain?
The next question was whether PL37 could prevent migraine pain, rather than only treat it once the pain was already present.
Long-lasting migraine pain from daily ISDN for five days results in persistent mechanical hypersensitivity in between ISDN injections (the interictal phase). In their hands, the researchers saw that the animals developed interictal hypersensitivity after three days of daily ISDN.
The authors then tested whether PL37 could prevent the interictal hypersensitivity. They tried oral PL37 daily for five days, which reduced mechanical hypersensitivity during the interictal phases. This suggests that the drug could be effective as a preventive treatment for migraine.
Central or peripheral?
The authors also wanted to understand how PL37 reduces hypersensitivity. Here there is still a lot of work ahead. For example, it’s unclear if PL37 acts mainly in the peripheral nervous system or in the central nervous system for its therapeutic effect.
“If we know more about the mechanism of action, we can then work to improve the efficacy of the compound. But first of all, we need more information about where in the nervous system the drug is acting,” said co-author Philippe Luccarini, Université Clermont Auvergne.
So, in a final set of experiments, the authors asked whether PL37 reduces central sensitization, the phenomenon where nociceptive neurons in the central nervous system show increased responsiveness to sensory input.
To find out, the investigators looked at expression of c-Fos, a protein that serves as a marker of neuronal activation in the trigeminocervical complex. In this case, the researchers measured c-Fos expression in the trigeminocervical complex after mechanical stimulation of the periorbital area. They did this in animals that received ISDN and PL37, comparing them to control animals receiving only ISDN.
In the controls, daily treatment with ISDN for five days produced strong c-Fos expression in the superficial layers of the trigeminocervical complex. However, c-Fos expression was cut roughly in half in animals that received PL37 before the fifth injection of ISDN.
These findings provide early indications that PL37 might reduce central sensitization, at least in an animal model of migraine. However, these findings don’t discount effects that PL37 might have in reducing peripheral sensitization.
“We’re currently doing more experiments to validate the peripheral versus central question, but it seems to me that there’s a partly central and partly peripheral effect,” Luccarini said.
A convergence of ideas
What is encouraging about the new findings, the researchers believe, is that the adverse consequences seen with exogenous opioids might be avoidable. One reason why is that drugs like PL37 may act locally, to prevent the degradation of enkephalin only where these opioid peptides are produced, rather than throughout the body.
“We think PL37 works by helping to increase local activation of opioid receptors by enkephalins, whether it be centrally or peripherally. What’s really exciting is that we don’t see opioid side effects in our clinical Phase 1 or 2 trials with DENKIs as a result. This means no tolerance or addiction, but also no drowsiness or constipation,” said Poras.
A related reason why DENKIs may be safer is that enkephalins attach mainly to a type of opioid receptor called the delta opioid receptor. In contrast, exogenous opioids primarily bind to another receptor, the mu opioid receptor, and do so throughout the body rather than locally, which is thought to explain the side effects of exogenous opioids.
Further, research in animals has shown that drugs that activate the delta opioid receptor may be effective at treating migraine, and the beneficial effects of vagal nerve stimulation on migraine may also work through the delta opioid receptor.
“The strength of this paper is that it’s a convergence of ideas,” according to Pradhan, who studies how delta opioid receptors contribute to migraine pain. “If you increase endogenous opioid signaling with DENKIs, you get anti-migraine effects, and if you give delta receptor agonists you also get anti-migraine effects. Moreover, the problematic side effects of exogenous opioids are primarily mediated by mu opioid receptors, not delta opioid receptors. It brings it all together nicely.”
So why not just directly activate delta opioid receptors? One problem is that drugs that do so have the potential to produce side effects like seizures. That may not be the case with DENKIs.
“The strength of DENKIs is that they help to regulate the endogenous opioid system but without overactivating it,” said Pradhan.
Dual enkephalinase inhibitor PL37 as a potential novel treatment of migraine: evidence from a rat model. Descheemaeker et al. Brain. 2022 Apr 12; awac139.
This work is licensed under a Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 International License.
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