X4 Pharmaceuticals, Inc. announced new positive data from its Phase 1b clinical trial evaluating the ability of its lead clinical candidate, mavorixafor, to increase the absolute neutrophil count (ANC) in people with idiopathic, cyclic, or congenital chronic neutropenia (CN) as monotherapy or concurrently with injectable granulocyte colony-stimulating factor (G-CSF). Key highlights from the Phase 1b trial are as follows: 100% of study participants (n=25) responded to treatment with a single dose of 400 mg of mavorixafor, alone or dosed concurrently with G-CSF: Participants achieved a mean ANC increase at peak of >3,000 cells/microliter; Consistent responses were seen across all of the CN disorders studied – idiopathic, cyclic, and congenital neutropenias. All neutropenic participants (n=14) reached normalized ANC levels (>1,500 cells/microliter): When assessed as a monotherapy in participants withsevere chronic neutropenia who were not being treated with G-CSF (n=6), a single dose of mavorixafor led to normalized ANC levels in all participants within 2 hours, with a mean ANC increase at peak of 2,500 cells/microliter; When assessed in participants with moderate or severe neutropeniadespite being treated with G-CSF (n=8), 100% reached normalized ANC levels, suggesting the potential of mavorixafor to both normalize the neutrophil counts in patients with partial response to G-CSF and also to potentially enable the reduction or elimination of G-CSF dosing.

When assessed in participants with CN with normalized ANC counts on chronic G-CSF (n=11), all participants experienced a consistent and sustained increase in ANC, suggesting mavorixafor's potential to reduce or possibly eliminate G-CSF treatment in these patients. Mavorixafor was well tolerated in the study; all treatment-related adverse events were deemed to be low grade, consistent with previous clinical studies in WHIM syndrome, and no treatment-related serious adverse events were reported.