Recharacterization of HS Pathology Underway

NEW YORK, NY — Work over the past several years has reframed hidradenitis suppurativa (HS) as a large-field inflammatory process, a recharacterization that has immediate relevance to treatment strategies and goals, according to two investigators intimately involved in much of the recent progress.

“Success is being achieved by targeting multiple inflammatory axes in HS, and therapeutics are evolving rapidly,” reported James G. Krueger, MD, PhD, head of the Laboratory of Investigative Dermatology, Rockefeller University, New York, NY.

The activity of targeted anti-inflammatory therapies — bimekizumab just joined adalimumab and secukinumab as a third approved biologic for HS — is not news, but the degree to which inflammation is upregulated systemically, not just at areas of skin involvement, has changed the conceptualization of HS.

HS Is a Systemic Inflammatory Disease

Relative to psoriasis, for which there are many parallels, “HS is hugely more inflammatory in the systemic circulation,” Krueger said at the 27th Annual Winter Symposium — Advances in Medical and Surgical Dermatology (MSWS) 2src24. Yet, HS is also more complex involving additional pathways that appear to include dysbiosis. The concept of follicular occlusion, once a common explanation for HS, has been left far behind.

“Unlike psoriasis, which we can treat really well by inhibiting a single pathway target, HS is just not that simple,” Krueger said. Although largely an inflammatory process, the cascade of inflammatory factors for specific manifestations, such as tunnels, means that optimal therapy in one case might have little benefit in another.

The relatively new evidence that HS activity is not confined to lesional skin might be the most important recent step toward new strategies to target disease. These studies were performed by Kristina Navrazhina, MD, PhD, now a resident in dermatology at the Icahn School of Medicine at Mount Sinai, New York. She received her PhD while studying HS activity in non-lesional skin. Her work has led her to conclude that the best chance for better outcomes in HS is early diagnosis and treatment. Although this is generally true of any pathology, the changes in the HS phenotype once fistulae form includes a poor response to conventional therapies. 

In fact, based on her work in evaluating HS activity in non-lesional skin, Navrazhina has shown that “many patients with modest lesions already have advanced disease.” Consistent with the premise that HS is a deeply systemic inflammatory process, nodules, considered an early manifestation, turn out to be “the tip of the iceberg.”

Non-Lesional HS Skin Is Inflamed

When she has employed RNA sequencing based on tape strip sampling from completely normal skin away from nodules, interleukin (IL)-17 and a broad array of other inflammatory markers were found to be upregulated. When she performed ultrasound to look for disease activity under the normal skin, she has often found tunnels already formed. Doppler ultrasound showed some of these tunnels were actively draining.

This might provide a partial explanation for why therapies are not always effective even when clinical signs of disease are modest. 

“Are we missing the opportunity for intervening?” Navrazhina asked, noting that early intervention has been limited traditionally by extremely long diagnostic delays. Citing the literature, Navrazhina said the average delay is 7 years for HS versus 1 year for psoriasis. Patients often cycle through 3 or 4 providers before the diagnosis is made, she said.

Awakening first-line clinicians to the signs and symptoms of HS, whether in the emergency room or primary care, is a critical message because of the incrementally difficult task to control disease once fistulae have formed. 

Krueger made the same appeal. For the neutrophilic inflammation that characterizes nodules, targeted therapies are often effective, but he agreed that available therapies are generally far less so once tunnels form.

Role Seen for Bacteria in HS Pathogenesis

One reason might be an interaction between anaerobic bacteria and the keratinocytes that form the tunnel walls, according to Krueger. Although HS is not typically considered an infectious disease, he reported that the interaction of these bacteria with keratinocytes is associated with expression of approximately 1srcsrcsrc inflammatory gene products. The process of tunnel formation is traced to how factors recruited by upregulated inflammation, such as chemokines, coordinate.

He described recent work pursing novel strategies such as highly targeted antibiotics or inhibitors of complement factor C5a, which has been proposed as a biomarker for HS, to intervene in preventing or reversing HS tunnels. 

While this work progresses, one of the most Important unmet needs in HS is an accepted measure of clinically meaningful improvement in advanced disease, particularly the impact of therapy on HS tunnels, according to Krueger. 

“There is no measure of tunnel activity that the FDA accepts in evaluating drugs,” he noted, which will be essential for approving therapies that offer this benefit. 

A phase 3 trials program for one of the promising drugs, sonelokimab, was announced early in 2src24. A nanobody that targets IL-17A/A, IL-17A/F, and IL-17F/F, the small size of this molecule permits exceptional tissue penetration while the broad anti-IL-17 activity has a high degree of theoretical potential in late-stage HS, according to Krueger. 

There are numerous pieces of the HS puzzle that are still missing, but both Krueger and Navrazhina are enthusiastic about new targets and opportunities for disease control that are stemming from a better understanding of the underlying pathophysiology. Not least, both indicated that testing for inflammatory phenotypes will allow for individualized therapeutic choices with a maximum likelihood of response, particularly if earlier diagnosis permits earlier treatment. 

“Due to the heterogeneity of HS, it is hard to know who will respond to which treatment or which treatment should be started first,” Navrazhina said. She thinks that early measures of the inflammatory profile in nodules or even non-lesional skin might provide that guidance. 

Both Krueger and Navrazhina reported no financial relationships relevant to this work. 

Ted Bosworth is a medical journalist based in New York City.