Argenx has posted phase 2 data on efgartigimod in primary immune thrombocytopenia (ITP). The results hint at the efficacy of the antibody Fc fragment in patients with the autoimmune bleeding disorder, leading Argenx to commit to running a phase 3 in the indication.

Patients with ITP suffer from low platelet counts. In mild cases, the condition manifests in spontaneous bruising. If platelet levels fall particularly low, bleeding inside the brain or skull can cause serious, potentially fatal, complications. Physicians have a number of ways of managing the disease, from widely used corticosteroids to Rigel Pharmaceuticals’ recently approved SYK inhibitor Tavalisse. But Argenx thinks there is a need for a safer, more broadly effective therapy.

"Efgartigimod has the potential to address ITP in a novel way across patient types, targeting disease at the source by eliminating IgGs and restoring platelet numbers," Argenx Chief Medical Officer Nicolas Leupin said in a statement.

To assess whether efgartigimod can deliver such outcomes by binding to FcRn, blocking antibody recycling, Argenx tested the drug in a 38-patient clinical trial. All the subjects had low platelet counts despite receiving standard of care. Participants received four weekly administrations of the drug, at one of two doses, or placebo, after which their platelet levels were tested weekly for 21 weeks. 

Argenx designed the trial to assess the safety and tolerability of efgartigimod. In keeping with trials of efgartigimod in other indications, the phase 2 suggested the antibody Fc fragment has a clean safety profile. One patient in the trial suffered a serious adverse event, but it was deemed to be unrelated to the experimental therapy. Most adverse events were mild and considered unrelated to efgartigimod.

The bigger, still largely unresolved question is whether efgartigimod is efficacious. Argenx performed post-hoc analyses of secondary endpoint measures to assess the magnitude of effect. The analyses found 46% of patients in the two treatment arms had significantly improved platelet counts at two or more assessments. One-quarter of patients in the placebo arm achieved such an improvement. 

Argenx also reported improved platelet counts in patients who joined an open-label extension trial that assessed the higher of the two efgartigimod doses, and posted data suggesting the responses triggered by efgartigimod are durable.

The Belgian biotech thinks the analyses support further development of efgartigimod in ITP. A phase 3 trial in the indication, plus a phase 2 assessment of a subcutaneous formulation, are now in the works. Argenx needs the data to be compelling enough to differentiate efgartigimod from existing treatments and other experimental assets, including rozanolixizumab and RVT-1401, rival FcRn drugs in development at UCB and Roivant, respectively.

Efgartigimod is already in phase 3 in patients with generalized myasthenia gravis—another disease targeted by rozanolixizumab—and a phase 2 trial in pemphigus vulgaris. Argenx just revealed it is adding chronic inflammatory demyelinating polyneuropathy to its list of target indications. A phase 2 trial in the new indication is due to start in the first half of next year. 

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