Developing treatments for rosacea requires first understanding the many pathways that contribute to its development, says Richard L. Gallo, M.D., Ph.D. The current hypothesis regarding the pathology of rosacea is that “there are certain triggers that we hear about from our patients, or observe ourselves,” Dr. Gallo says. “Then there’s a response,” which includes vascular, inflammatory and histopathologic changes in the skin.
San Diego - Developing treatments for rosacea requires first understanding the many pathways that contribute to its development, says Richard L. Gallo, M.D., Ph.D. The current hypothesis regarding the pathology of rosacea is that “there are certain triggers that we hear about from our patients, or observe ourselves,” Dr. Gallo says. “Then there’s a response,” which includes vascular, inflammatory and histopathologic changes in the skin. Dr. Gallo is professor of medicine and pediatrics and chief of the division of dermatology, University of California, San Diego.
Contributing factors such as UV light may worsen rosacea or sometimes confound the diagnosis, he says. At the same time, “Extravascular fluid accumulates, and as patients develop more chronic disease - the papulopustular and rhinophymatous phases - they develop more edema and dermal inflammation, and the cycle continues.”
Presently, Dr. Gallo says, researchers have a partial understanding of rosacea in the following areas:
“Our journey to our current hypothesis began with understanding the molecules known as antimicrobial peptides (AMPs),” he says. “We discovered over a decade ago that they were in mammalian skin. Since that time, many different types of molecules have been discovered. One of them is cathelicidin - produced by many different cell types in the skin.”
Cathelicidin participates in the innate immune system, which Dr. Gallo describes as multiple layers through which the skin rapidly responds to threats or simply maintains homeostasis. The first layer consists of the physical barrier and chemical surface constituents that can inhibit bacterial proliferation. “The skin must be able to detect when something goes wrong, and able to recruit cells” from elsewhere in the body to mount a response to the threat, he adds. Later, the skin must be able to educate the immune system, “So perhaps the next time you’re exposed, the reaction won’t be as bad.”
AMPs are involved both in the early phase of the cycle, where they act as an antibiotic, and in the later phases. “This is crucial to understanding their role in rosacea,” he says.
Dr. Gallo says cathelicidins can act directly on microbes, as a modestly powerful antibiotic, as well as on their host, to influence the vasculature and the inflammatory process in ways that can be highly potent.
A key development that got researchers thinking about cathelicidins’ role in rosacea is a study in which investigators placed one form of the cathelicidin peptide (LL-37/hCAP-18) in a petri dish with rabbit aortas and compared its impact on blood vessel growth to the effects of one of the most potent angiogenic factors known: basic fibroblast growth factor (bFGF).
In this assay, Dr. Gallo says, the cathelicidin showed a very potent ability to produce direct angiogenesis (Koczulla R, von Degenfeld G, Kupatt C, et al. J Clin Invest. 2003;111(11):1665-1672). “We had this crazy idea - we wondered, if cathelicidin is part of the normal innate immune response, and if cathelicidin can induce vascular changes, is it possible that in rosacea, something has gone wrong with the cathelicidin?” Dr. Gallo says.
Dr. Gallo and colleagues conducted a study in which they biopsied uninvolved skin taken from the nasomalar folds of patients with rosacea. “All of those patients had very high levels of cathelicidin - more than 100-fold more than we saw in similar sites in subjects without rosacea (Yamasaki K, Di Nardo A, Bardan A, et al. Nat Med. 2007;13(8):975-980. Epub 2007 Aug 5),” he says.
When researchers subjected skin samples to mass spectroscopy, they found that in the skin of patients with rosacea, cathelicidin is cleaved into abnormal forms. Researchers found extremely high kallikrein 5 levels in the skin of patients with rosacea.
“Cathelicidin exists as a precursor, and the enzyme kallikrein 5 can cleave it, producing peptides like LL-37” that produce responses such as inflammation and vascular growth, he says. “There’s actually colocalization - the enzyme and the precursor form of cathelicidin both reside in the same place, and levels of both are way too high.”
Also in this study, investigators showed in mouse models that mice lacking cathelicidin experience much less inflammation than normal mice, while those with elevated cathelicidin levels have elevated amounts of inflammation.
“One thing in the literature that helped us understand this is that there are rare subsets of patients with rosacea fulminans who have been described as having a mutation in their vitamin D receptor gene (Jansen T, Krug S, Kind P, et al. J Dermatol. 2004;31(3):244-246),” he says. “This was important to us because in other work, we found that vitamin D is a very important tool for controlling the expression of cathelicidin. With this mutation, the gene may automatically make too much cathelicidin,” much like the mice into which Dr. Gallo and his team injected the kallikrein 5 peptide.
More recent research has lent additional support to Dr. Gallo’s hypothesis that perhaps tetracyclines exert an anti-inflammatory influence by acting on the kallikrein 5-cathelicidin pathway.
“The entity that cleaves and activates kallikrein 5 is a matrix metalloproteinase (MMP),” he says. A few small studies have shown that doxycycline might inhibit MMPs. “We hypothesized that if this was true, it could explain the effect of tetracyclines’ inactivating cathelicidin, and therefore explain the beneficial effects of this therapy.”
But a series of assays showed that “if you just measure kallikrein 5 activity in a biochemical situation where there are no live cells, the addition of doxycycline to the enzyme alone does nothing if the enzyme is already activated. But if you add doxycycline, or other drugs that inhibit metalloproteinases, to live cells and then measure serine protease, we saw that our hypothesis was true. Adding metalloproteinase inhibitor inhibited the serine protease, and the only way that can be true is if you have an indirect effect - doxycycline inhibits MMPs, and then the MMPs inhibit kallikrein 5 (Kanada KN, Nakatsuji T, Gallo RL. J Invest Dermatol. 2012 Feb 16. [Epub ahead of print]).”
Other topical therapies also seem to act upon the cathelicidin pathway. Azelaic acid gel has been shown to reduce kallikrein 5 activity and cathelicidin expression, as well as toll-like receptor (TLR) 2 activity, in a mouse model (Gallo RL. Poster P103. American Academy of Dermatology 68th Annual Meeting. March 5-10, 2010. Miami Beach). TLR-2 is overexpressed in rosacea and stimulates enhanced serine protease production by keratinocytes (Yamasaki K, Kanada K, Macleod DT, et al. J Invest Dermatol. 2011;131(3):688-697).
Presently, Dr. Gallo says, research has established that “a dysfunction in AMP processing occurs in rosacea. There are many pathways that can lead to this. Therefore, no single stimulus is likely to explain the disease in all patients. And I believe that concept fits perfectly with our clinical observations. However, understanding how the innate immune system works may be a critical element in treating patients and offering new targets for therapy,” he says. DT
Disclosures: Dr. Gallo has received research support from the National Rosacea Society, the National Institutes of Health, the Department of Veterans Affairs, Galderma and Bayer (Intendis).