PRTX‐100 and methotrexate in patients with active rheumatoid arthritis: A Phase Ib randomized, double‐blind, placebo‐controlled, dose‐escalation study

PRTX-100 is a highly-purified preparation of staphylococcal protein A (SpA), with immunologic activity in vitro and in animal models of immune-mediated inflammation. Following single-dose healthy volunteer studies of safety and pharmacokinetics (PK), a multicenter, double-blind, placebo-controlled, sequential dose-escalation, repeated-dose phase I trial was conducted in patients with active rheumatoid arthritis (RA) on methotrexate therapy. Patients were randomized to receive either weekly intravenous PRTX-100 (0.15, 0.45, 0.90, or 1.50 µg/kg) or placebo for 4 weeks. Safety and disease activity were assessed over 16 weeks. Pharmacokinetic profiles were obtained after the first and fourth doses. The most common treatment-related adverse events were nausea, muscle spasms, dizziness, flushing, fatigue, RA flare, and headache. No serious adverse events were considered related to PRTX-100, and none occurred in the highest dose group. Geometric mean values for plasma Cmax (ng/mL) were 4.1, 15.7, 26.5, and 51.2 for doses of 0.15, 0.45, 0.90, and 1.5 µg/kg, respectively. Anti-drug antibodies (ADAs) developed in most PRTX-100 patients, but incidence and titer were not dose-dependent. At the two highest doses, data suggest PRTX-100 may have an effect on RA disease activity, even in patients with ADAs.