Conference Coverage

BTK inhibitor calms pemphigus vulgaris with low-dose steroids



– An investigational molecule that blocks the downstream proinflammatory effects of B cells controlled disease activity and induced clinical remission in patients with pemphigus by 12 weeks.

At the end of a 24-week, open-label trial, patients taking the oral inhibitor of Bruton tyrosine kinase (BTK) plus very-low-dose prednisone also experienced a mean 65% reduction in antidesmoglein antibodies, a key driver of the sometimes-fatal blistering disease, Deedee Murrell, MD, said at the annual meeting of the American Academy of Dermatology.

The clinical efficacy plus a favorable safety profile supports the further development of the molecule, designed and manufactured by Principia Biopharma in San Francisco. The company is currently recruiting for a pivotal phase 3 trial of PRN1008 in 120 patients with moderate to severe pemphigus vulgaris.

Despite the recent approval of rituximab (Rituxan) for moderate to severe pemphigus, there remains an unmet need for a quick-acting, steroid-sparing, anti-inflammatory treatment, said Dr. Murrell, professor and head of the department of dermatology at the University of New South Wales, Sydney.

“We need something to use instead of high-dose steroids while we are waiting for rituximab to kick in, which can take 3 months,” and rituximab, which depletes B cells, puts patients at risk for infection, she said. “We need something that has rapid onset, is steroid sparing, safe for chronic administration, avoids B-cell depletion, and is convenient.”

Blocking the BTK receptor on B cells puts the brakes on the B-cell mediated inflammatory pathway, preventing activation of monocytes, macrophages, mast cells, basophils, and neutrophils. At the same time, however, it does not deplete the B-cell population, said Dr. Murrell, the lead investigator.

The BELIEVE study comprised 27 patients with mild to severe pemphigus of an average 6 years’ duration. Most (18) had relapsing disease; the remainder had newly diagnosed pemphigus. A majority (16) had severe disease, as measured by a score of 15 or more on the Pemphigus Disease Activity Index (PDAI). Almost all (23) were positive for antidesmoglein antibodies. Only one patient was negative for antibodies.

The mean corticosteroid dose at baseline was 14 mg/day, although that ranged from no steroids to 30 mg/day.

The study consisted of a 12-week treatment phase and a 12-week follow-up phase. During treatment, patients could take no more than 0.5 mg/kg of prednisone daily, although with 400 mg PRN1008 twice a day. They were allowed to undertake rescue immunosuppression if they experienced a disease flare.

The primary endpoint was disease control by day 29 as evidenced by no new lesions. Secondary endpoints were complete remission, minimization of prednisone, quality of life, antibody levels, and clinician measures including the PDAI and the Autoimmune Bullous Skin Disorder Intensity Score.

By the end of week 4, 54% of patients had achieved the primary endpoint. The benefit continued to expand, with 73% reaching that response by the end of week 12. During this period, the mean prednisone dose was 12 mg/day.

Among the 24 patients who completed the study, complete remission occurred in 17% by week 12. However, patients continued to respond through the follow-up period, even after the study medication was stopped. By week 24, 25% of these patients experienced a complete remission. At the point of remission, the mean steroid dose was 8 mg/day. The median duration of remission was 2 months after stopping PRN1008.

The PDAI fell by a median of 70% by week 12 and was maintained at that level by the end of week 24. The median level of antidesmoglein autoantibodies fell by up to 65%. Again, the improvement continued throughout the off-drug follow-up period. In subgroup analyses, PRN1008 was more effective in patients with moderate to severe disease than those with mild disease (80% response vs. 64%). It was equally effective in those with newly diagnosed disease (75% vs. 72%) and regardless of antibody level at baseline.

The adverse event profile was relatively benign. Most side effects were mild and transient, and included upper abdominal pain, headache, and nausea. There were two mild infections and one serious infection, which presented in a patient with a long-standing localized cellulitis that activated and was associated a high fever. It was culture negative and PRN1008 was restarted without issue.

There was also one serious adverse event and one death, both unrelated to the study drug. One patient developed a pancreatic cyst that was discovered on day 29. The patient dropped out of the study to have elective surgery. The death occurred in a patient who developed acute respiratory failure on day 8 of treatment, caused by an undiagnosed congenital pulmonary sequestration. The patient died of a brain embolism shortly after lung surgery.

Dr. Murrell designed the study and was an investigator. She reported a financial relationship with Principia, as well as with numerous other pharmaceutical companies.

SOURCE: Murrell D et al. AAD 2019, Session S034.

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