MADRID – A novel Dedee F. Murrell, MD, said at the annual congress of the European Academy of Dermatology and Venereology.
Bruce Jancin/MDedge News
Dr. Dedee F. Murrell
“In pemphigus, we have a considerable unmet medical need. We could do with a treatment that has rapid onset, is steroid sparing or avoiding, safe for chronic administration, avoids chronic B cell depletion – which is an issue with rituxumab – is efficacious in both newly diagnosed as well as in our commonly relapsing patients, and is convenient to administer,” observed Dr. Murrell, professor of dermatology at the University of New South Wales in Sydney.
In the phase 2 BELIEVE trial, the BTK inhibitor known for now only as PRN1008 appeared to check all the boxes. However, definitive evidence of the drug’s efficacy and safety must await the results of the ongoing, double-bind, placebo-controlled, pivotal phase 3 PEGASUS trial, which is enrolling a planned 120 patients with pemphigus vulgaris or foliaceus in 19 countries.
Pemphigus is driven by autoantibodies against desmogleins 1 and 3. Even in contemporary practice, this blistering disease has roughly a 5% mortality rate. Current management of the disease with high-dose corticosteroids at 1 mg/kg per day or more with or without rituximab (Rituxan) is challenging because of the associated pronounced toxicities. And even when rituximab is utilized, patients need to be on high-dose steroids for at least 3-6 months before a rituximab response is achieved, Dr. Murrell said.
PRN1008 has three mechanisms of action targeting the drivers of pemphigus and other immune-mediated diseases, she explained. The drug blocks inflammatory B cells, neutrophils, and macrophages; eliminates downstream signalling by antidesmoglein autoantibodies; and prevents production of new autoantibodies. The drug has a double lock-and-key mechanism which makes it highly specific for its target, so treated patients are much less likely to experience bruising, diarrhea, and other off-target effects than is the case with other tyrosine kinase inhibitors.
“Also, PRN1008 is reversible. It comes off its target receptor after about 12 hours, at which point serum levels become low. So if any side effects do develop, the patient can recover quickly, unlike with rituximab, which involves ongoing inhibition of B cells for a long period of time,” the dermatologist noted.
BELIEVE was a phase 2 dose-ranging study of 27 patients with pemphigus treated open-label with PRN1008 for 12 weeks. The primary endpoint was control of disease activity, meaning no new lesions while established lesions showed some evidence of healing on no more than 0.5 mg/kg per day of prednisone. This outcome was achieved in 27% of participants at 2 weeks, 54% at 4 weeks, and 73% at 12 weeks. Autoantibody levels dropped by a mean of 65% at 12 weeks, with a median 70% reduction in Pemphigus Disease Activity Index scores while patients were on an average of just 12 mg of prednisone per day.
Phase 2b of BELIEVE included a separate group of 15 patients on PRN1008 for 24 weeks. Nine achieved a Pemphigus Disease Activity Index score of 0 or 1. Six patients had a complete response, meaning an absence of both new and established lesions while on no or a very low dose of prednisone. Another five patients were unable to achieve a complete response, and the jury was still out on another four still on treatment.
The side effect profile was benign in comparison with that of current standard therapies, she said. It consisted of a handful of cases of mild, transient nausea, headache, or upper abdominal pain and a few Grade 1 infections. There have been no severe treatment-related adverse events in BELIEVE participants.
Patients enrolled in the ongoing phase 3 PEGASUS trial start with a short course of high-dose corticosteroids, followed by double-blind randomization to PRN1008 at 400 mg twice a day or placebo, with a corticosteroid taper. The primary endpoint is durable complete remission at week 37, defined as no lesions being present for at least the previous 8 weeks while on no more than 5 mg/day of prednisone.
Secondary endpoints include cumulative corticosteroid dose through 36 weeks and patient-reported quality of life measures assessed out to 61 weeks. The trial is scheduled for completion in the spring of 2022.
Dr. Murrell reported serving as a consultant to the study sponsor, Principia Biopharma, as well as numerous other pharmaceutical companies.