Proof of concept for the clinical effects of oral rilzabrutinib, the first Bruton tyrosine kinase inhibitor for pemphigus vulgaris: the phase II BELIEVE study

Background Bruton tyrosine kinase (BTK) inhibition targets B-cell and other non-T-cell immune cells implicated in the pathophysiology of pemphigus, an autoimmune disease driven by anti-desmoglein autoantibodies. Rilzabrutinib is a new reversible, covalent BTK inhibitor demonstrating preclinical efficacy as monotherapy in canine pemphigus foliaceus. Objectives To evaluate the efficacy and safety of oral rilzabrutinib in patients with pemphigus vulgaris in a multicentre, proof-of-concept, phase II trial. Methods Patients with Pemphigus Disease Area Index severity scores 8-45 received 12 weeks of oral rilzabrutinib 400-600 mg twice daily and 12 weeks of follow-up. Patients initially received between 0 and <= 0 center dot 5 mg kg(-1) prednisone-equivalent corticosteroid (CS; i.e. 'low dose'), tapered after control of disease activity (CDA; no new lesions, existing lesions healing). The primary endpoints were CDA within 4 weeks on zero-to-low-dose CS and safety. Results In total, 27 patients with pemphigus vulgaris were included: nine newly diagnosed (33%) and 18 relapsing (67%); 11 had moderate disease (41%) and 16 moderate to severe (59%). The primary endpoint, CDA, was achieved in 14 patients (52%, 95% confidence interval 32-71): 11 using low-dose CS and three using no CS. Over 12 weeks of treatment, mean CS doses reduced from 20 center dot 0 to 11 center dot 8 mg per day for newly diagnosed patients and from 10 center dot 3 to 7 center dot 8 mg per day for relapsing patients. Six patients (22%) achieved complete response by week 24, including four (15%) by week 12. Treatment-related adverse events were mostly mild (grade 1 or 2); one patient experienced grade 3 cellulitis. Conclusions Rilzabrutinib alone, or with much lower CS doses than usual, was safe, with rapid clinical activity in pemphigus vulgaris. These data suggest that BTK inhibition may be a promising treatment strategy and support further investigation of rilzabrutinib for the treatment of pemphigus.