Dexpramipexole As a Steroid-Sparing Agent in Hypereosinophilic Syndromes (HES): An Open-Label Proof-of-Concept Study

Hypereosinophilic syndromes (HES) are a heterogeneous group of disorders characterized by peripheral eosinophilia and eosinophil-related end organ damage. Whereas most patients respond to steroid therapy, high doses are often necessary and serious side effects are common. Dexpramipexole (KNS-760704) is an orally bioavailable synthetic amino-benzothiazole that was in development for the treatment of amyotrophic lateral sclerosis (ALS). Despite failure to meet the primary efficacy endpoint in a phase 3 trial in ALS patients, dexpramipexole showed an excellent safety profile and was found to significantly decrease absolute eosinophil counts (AEC) in >70% of the study participants. Consequently, a proof-of-concept study was designed to evaluate the safety and efficacy of dexpramipexole as a steroid-sparing agent in 10 HES subjects.

Subjects with HES on steroid monotherapy were eligible for the study if they required ≥10 mg prednisone or equivalent for control of symptoms and eosinophilia. Subjects with AEC <1000/uL and no active symptoms entered a lead-in period, consisting of a standardized steroid taper with weekly assessment of AEC and symptoms to establish a "minimally effective corticosteroid dose (MECD)". Those with AEC ≥1000/uL entered directly into the treatment phase. Once the MECD was established, treatment with dexpramipexole (150 mg twice daily) was initiated. After 12 weeks of treatment, a standardized corticosteroid taper was attempted to determine the "MECD on dexpramipexole". Clinical assessments, including bone marrow and tissue biopsies of affected organs (when possible), were performed prior to and after 12 weeks of dexpramipexole. The primary efficacy endpoint was defined as a ≥50 % change in prednisone dose to maintain AEC at or below baseline levels and control clinical symptoms.

Study enrollment is complete. Median age at enrollment was 51 years (22-72) with 40% women. Baseline clinical manifestations included eosinophilic gastrointestinal disease (60%), pulmonary involvement (50%), skin, muscle, sinus, and cardiac disease. Median baseline AEC was 670/uL (280-2540) and median MECD at baseline was 18.75 mg of prednisone (10-25). To date, the MECD on dexpramipexole has been determined for 7 subjects, and 3 subjects continue on study and have not completed the steroid taper on dexpramipexole. Two of the 7 evaluable subjects met the primary end point. Both subjects reached a MECD of 0 mg of prednisone and had complete symptom resolution with AEC of 0/uL within 3 months of initiating treatment with dexpramipexole. They remain in clinical remission on dexpramipexole monotherapy for 7 and 12 months, respectively. One additional subject who did not meet the primary endpoint at 3 months, was able to maintain a stable dose of 12.5 mg of prednisone with clinical improvement. She has remained on dexpramipexole and has demonstrated a delayed response with AEC 100/uL on 8.75 mg prednisone (50% of her baseline MECD) at 6 months. The remaining 4 evaluable subjects failed to respond and were taken off study. No deaths or drug related adverse events were observed, although 2 subjects reported transient palpitations and insomnia that resolved without drug discontinuation.

Bone marrow biopsies performed at 12 weeks revealed markedly decreased AEC and basophils in responders as compared to baseline samples, and residual eosinophils appeared left-shifted. Other cell lineages were unchanged. Gastrointestinal biopsies in 1 responder demonstrated complete resolution of tissue eosinophilia after 5 months on dexpramipexole. Flow cytometry demonstrated no difference in CD34+ or CD34+IL-5R+ cell numbers after 12 weeks of treatment, but a decrease in eosinophil expression of Siglec 8, an inhibitory receptor expressed only on mature eosinophils. These changes were not observed in the non-responders. CD34+ cell cultures using normal cord blood samples also suggest a delay in maturation of the eosinophil lineage in the presence of dexpramipexole.

Given the frequency of steroid-induced morbidity in patients with HES, alternate therapeutic options are critical. In this pilot study, dexpramipexole showed remarkable efficacy as a steroid-sparing agent without apparent toxicity in a subset of subjects with steroid-responsive HES. Although the mechanism of action is unknown, preliminary data suggest that dexpramipexole may affect eosinophil maturation in the bone marrow.