such as tryptase and histamine, a new study has found.
“Whether cluster headache attacks provoked by CGRP and PACAP38/VIP are mediated by distinct signaling pathways will be worth investigating in forthcoming studies,” wrote Lanfranco Pellesi, MD, of the Danish Headache Center at the University of Copenhagen, and his coauthors. The study was published in Cephalalgia.
To assess how these biochemical variables might contribute to cluster headache attacks, the researchers launched a randomized, double-blind trial of data from 44 Danish participants with cluster headache. The average age of the patients was 38 years; 14 had active episodic cluster headache, 15 had episodic cluster headache in remission, and 15 had chronic cluster headache.
All patients received a continuous infusion of either PACAP38 (10 pmol/kg per minute) or VIP (8 pmol/kg per minute) over a 20-minute period, using a time- and volume-controlled infusion pump. Blood was collected for analysis at fixed time points, including at baseline, at the end of the infusion, 10 minutes after the infusion, and 70 minutes after the infusion. Technical problems led to missing values in 285 out of 1,144 planned plasma samples.
PACAP38 infusion resulted in a cluster headache attack in 13 of the 44 participants and VIP induced an attack in 12 of the 44. No differences in plasma CGRP (P = .7074), tryptase (P = .6673), and histamine (P = .4792) levels were found between patients who developed attacks and those who did not, and the plasma concentrations did not differ among the various blood-drawing time points.
There was also no difference in plasma CGRP levels between patients with active episodic cluster headache, those with episodic cluster headache in remission, and those with chronic cluster headache. After post hoc analysis, plasma tryptase and plasma histamine levels were similar among the three cluster headache patient groups.
The final link to the cluster headache puzzle has not yet been found
“We know a lot about cluster headache: how it presents, how we can stop it acutely, and how we can stop it preventively. But we don’t know everything about all the neurotransmitters involved, the triggers that start an attack, or the causes of pain,” Alan Rapoport, MD, professor of neurology at the University of California, Los Angeles (UCLA), and past president of the International Headache Society, said in an interview. “This study was performed to find the answer to a small piece of the puzzle. Is CGRP the missing link for patients who begin a cluster attack, or should we be looking elsewhere?
“I would be cautious and say it appears that it doesn’t seem to be related, but further studies may show something different,” he added. “The reason for my qualification: There is a monoclonal antibody [galcanezumab], which grabs CGRP and prevents it from docking on its receptor, that has been approved for preventive treatment of episodic cluster headache. When you have episodic cluster, go into a cluster period, and take galcanezumab, it could and should decrease the number of attacks that you would ordinarily have had. That means it is related somewhat. But it certainly doesn’t work for everyone, so more investigations like this are needed.”
“What’s important about this study is that it opens up the possibility that there is another way into the cluster attack that could be operationalized for therapeutic purposes,” Peter Goadsby, MD, PhD, professor of neurology at UCLA and president of the American Headache Society, said in an interview.
When asked about the authors’ stated interest in investigating “if monoclonal antibodies targeting the CGRP pathway prevent PACAP38- or VIP-induced cluster headache attacks” as a follow-up, Dr. Goadsby strongly backed the idea. “If I sound excited about actually exploring whether that was a useful treatment or not, it’s because cluster headache is a dreadful condition. And the sooner you could work out whether it was useful or put the money into something else, well, that’s where I’d go.
“I think the principle here of doing experimental medicine, getting into human work with targets like this at the earliest possible time, is something that is not done as often as would be appropriate,” he added. “There is not enough investment, in my view, in early phase experimental work, which really just gets to that next step. Broadly speaking, the encouragement and support of experimental medicine is crucial to developing new therapies.”
The authors recognized their study’s potential limitations, including it’s being an exploratory study with results that should be interpreted cautiously. They acknowledged discrepancies with previous studies of plasma CGRP during cluster headache attacks, offering “different methodologies, including intra-assay differences and the location of blood sampling” as a possible reason. They also explained that some of the data are missing “completely at random” due to their policy of discarding all observations with incomplete laboratory measurements, adding that the impact on their sample size was “only modest.”
“In spite of these limitations,” Dr. Rapoport said, “this is an excellent study that shows us that PACAP38- and VIP-induced cluster headache attacks are not associated with alterations in plasma CGRP or in histamine and tryptase.”
Regarding potential conflicts of interest, one author reported being employed at the testing lab where the histamine measurements were conducted, as did another author who serves as the lab’s scientific adviser. A third author reported receiving personal fees from various pharmaceutical companies.