Sobi wins FDA Fast Track for pacritinib in VEXAS syndrome
Sobi North America has secured FDA Fast Track designation for pacritinib in VEXAS syndrome, a rare systemic condition that overlaps haematological and inflammatory features and for which no approved therapy currently exists. The designation, which is intended to expedite development and regulatory review of medicines targeting serious unmet needs, applies to the company's ongoing Phase 2 PAXIS study.
VEXAS, an acronym for Vacuoles in blood-cell precursors, E1 enzyme, X-linked, Autoinflammatory, Somatic, was first characterised in 2020. It is driven by somatic mutations in the UBA1 gene and can be progressive and life-threatening, with patients at risk of multi-organ involvement. Most immunomodulatory agents fail to control the disease adequately, leaving clinicians reliant on moderate to high-dose glucocorticoids that carry their own significant toxicity burden.
"There is a critical need for new treatment options for people living with this debilitating condition, who often face serious complications and limited therapeutic choices," said Lydia Abad-Franch, Chief Medical Officer at Sobi. "With this designation, we look forward to working even more closely with the FDA as we accelerate the clinical evaluation of pacritinib."
The PAXIS trial
Pacritinib, marketed in the US as Vonjo for a subset of myelofibrosis patients with very low platelet counts, is a dual IRAK1 and JAK2 inhibitor. Sobi has been evaluating it in VEXAS since 2024 through PAXIS (NCT06782373), which the company describes as the first randomised trial ever initiated in the condition. The study is a double-blind, placebo-controlled, dose-finding Phase 2 trial followed by an open-label period; it is designed to test whether pacritinib can prevent disease flares as glucocorticoids are tapered. Topline data are expected in 2027.
Fast Track status gives Sobi more frequent interactions with the FDA during development and makes the programme eligible for rolling review of a future marketing application, both of which can compress the regulatory timeline meaningfully.
Market and competitive context
VEXAS occupies an unusual position in rare-disease drug development. It sits at the intersection of haematology and rheumatology, making patient identification and trial enrolment challenging: the condition is almost certainly underdiagnosed, and epidemiological estimates vary considerably. That ambiguity cuts both ways commercially, as a precise market size is difficult to project, but the complete absence of approved agents means any validated treatment would face limited direct competition at launch.
Pacritinib's existing US approval in myelofibrosis provides Sobi with a manufacturing, safety and regulatory reference point that pure de novo development programmes would lack. The drug's IRAK1 inhibitory activity is thought to be particularly relevant in VEXAS, where innate immune pathway dysregulation plays a central role, though the precise mechanistic rationale has not been fully published from the PAXIS protocol to date.
Broader interest in JAK and IRAK inhibition for inflammatory haematological conditions is growing, with several companies active in adjacent myeloid inflammatory disease spaces. Whether any of those programmes will move into VEXAS specifically remains to be seen. For now, Sobi holds a notable first-mover advantage in randomised evidence generation, and the Fast Track designation reinforces that positioning ahead of the 2027 readout.