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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 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. Disclosures Bozik: Knopp Biosciences: Employment. Brown:Leidos Biomedical Research Inc.: Employment. Demarco:Knopp Biosciences: Employment. Komarov:Knopp Biosciences: Employment. Magee:Leidos Biomedical Research Inc.: Employment. Prussin:Knopp Biosciences: Employment. Signore:Knopp Biosciences: Employment. Sullivan:Knopp Biosciences: Employment. Wetzler:Leidos Biomedical Research Inc.: Employment. Dunbar:Novartis: Research Funding. Dworetzky:Knopp Biosciences: Employment.

Introduction Idiopathic CD4+ Lymphocytopenia (ICL) is a rare, likely heterogeneous syndrome characterized by persistent CD4+ lymphopenia ( Methods We analyzed bone marrow core biopsies of 12 ICL patients in a phase I/IIA NIH clinical trial designed to evaluate recombinant human Interleukin-7 (rhIL-7) as a potential therapy for ICL . Subjects received 3 injections of rhIL-7 over two 24-week periods. Bone marrow biopsies were performed at week 1 and week 24 to analyze numbers of T-cells and T-cell precursors at baseline and post rhIL-7 therapy. Peripheral blood T-cell counts were assessed in parallel. Double chromogenic immunohistochemical staining using anti-terminal deoxynucleotidyl transferase (TdT) and anti-CD3 antibodies was performed on fixed, paraffin-embedded samples using an automated stainer (Ventana). Single CD3-positive T-cells and double-positive TdT/CD3 precursor T-cells were counted in at least ten consecutive fields under a light microscope and results expressed as mean positive cell number per field before and after treatment with rhIL-7. Similar staining procedure using TdT and CD79a was performed to evaluate precursor B-cells in the same patient cohort. In addition, bone marrow biopsies from 10 healthy control subjects were analyzed in parallel. Results Compared to healthy controls, ICL patients showed no significant differences in the number of bone marrow precursor T-cells (p=0.069) but had lower levels of mature T-cells (p Conclusion Novel histological technique for double chromogenic staining allows for the visualization of T-cell precursors in bone marrow biopsies. Our study revealed no evidence that lymphopenia in ICL patients is associated with lack of T-cell precursors in bone marrow, suggesting downstream defects in T-cell differentiation, proliferation or survival. Administration of rhIL-7 was associated with an increase in peripheral blood T-cells and increase in bone marrow precursor T-cells, while B-cell lineage cells remained unchanged. Disclosures No relevant conflicts of interest to declare.