, according to experts attempting to put these puzzle pieces together at the 2023 Scottsdale Headache Symposium.
As the details of the complex chain of molecular events become better understood, there is reason to believe that the targets for aborting events early in the process are not necessarily therapeutic at later stages or vice versa, according to Peter Goadsby, MBBS, MD, PhD, director of the National Institute for Health Research and professor of neurology, King’s College, London.
Dr. Peter J. Goadsby
“I think this explains some of the frustration at trigger modulation. I think we are often trying to modulate a process that has already got started,” he said. The analogy might be closing the barn door after the animals have escaped.
Migraine phases might explain pathology
Given the progress in understanding each step that leads from one phase to the next in migraine onset, this premise is not surprising. Increased blood flow, trigeminal activation, and release of calcitonin gene–related peptide (CGRP) are early events in this process, according to Dr. Goadsby, but there is still uncertainty about the triggers of brainstem stimulation and cranial blood flow that precedes these events.
In his talk about the advances that led to the development of CGRP-targeted therapy, Dr. Goadsby explained how and why CGRP inhibition, along with triptans, pituitary adenylate cyclase–activating polypeptide (PACAP) inhibitors, and ditans, have been developed as treatment targets in migraine, while other once-promising targets, such as substance P inhibition and inducible nitric oxide synthase (INOS) inhibition, have not.
Much of this progress has been by trial and error through clinical studies in which efficacy has or has not been seen. Despite the progress in mapping the release of CGRP and its activity, Dr. Goadsby acknowledged that there is still much about its participation in migraine pathophysiology that remains poorly defined.
“Anyone who tells you that they know how CGRP works I think is blowing smoke, frankly,” Dr. Goadsby said. “Clearly these things are complex.”
CGRP is active in the CSF
This includes the site of action. Dr. Goadsby said that it is widely believed that CGRP inhibitors are active in the dura and not in the cerebrospinal fluid (CSF). However, Dr. Goadsby said that a study undertaken with monoclonal antibodies targeting CGRP have produced compelling evidence that CGRP is reduced in the CSF.
“They clearly get into the CSF,” said Dr. Goadsby, noting that the barrier between peripheral blood and the CSF “is different from the blood-brain barrier.”
Widely regarded as playing a pivotal role in the development of CGRP as a therapeutic target in migraine, Dr. Goadsby spent some time speculating about its potential for preventing the earliest steps in the process that leads from the premonitory state to allodynia, prodromal symptoms, migraine, and postdromal recovery.
Of triggers, “light is my favorite example,” he said. He noted that many patients are convinced that light initiates the subsequent steps that end in a migraine. This is fair assumption for those who have seen a sequence of events in which light in the absence of any other symptom always precedes prodromal symptoms and migraine.
“Why would you not think that?” he asked. “Unless you point out that the attack had already started and the reason that you are noticing the light is because of photophobia that started during the premonitory phase.”
It is increasingly clear that CGRP inhibition does have clinical benefit when started at early signs of a coming migraine, according to Dr. Goadsby. He cited a phase 3 study published just days before he spoke at the Scottsdale Headache Symposium. Called PRODROME, the study associated the CGRP receptor antagonist ubrogepant, which is already approved for treatment of migraine, with a significant reduction in the risk of moderate to severe headache relative to placebo when measured 24 hours after randomization (46% vs. 29%; P < .0001).