A Phase 2a Study of the LSD1 Inhibitor Img-7289 (bomedemstat) for the Treatment of Myelofibrosis

Ruxolitinib (Jakafi^®) is the one approved therapy for myelofibrosis (MF) based on reduction of splenomegaly and symptoms but JAK inhibition has not proven to significantly modify disease progression. There remains the need for novel therapies with distinct modes of action that can improve the patient experience of MF and impact progression. Lysine-specific demethylase, LSD1, is an epigenetic enzyme critical for self-renewal of malignant myeloid cells and differentiation of myeloid progenitors. LSD1 bound to GFI1b permits maturation of progenitors to megakaryocytes and enables their normal function. IMG-7289 (bomedemstat) is an orally available LSD1 inhibitor. In mouse models of myeloproliferative neoplasms (MPN), IMG-7289 reduced elevated peripheral cell counts, spleen size, inflammatory cytokines, mutant allele frequencies, and marrow fibrosis (Jutzi et al. 2018) supporting its clinical development.

IMG-7289-CTP-102 is an ongoing, multi-center, open-label study that recently transitioned from a Phase 1/2a dose-range finding study to a Phase 2b study of IMG-7289 administered orally once-daily in adult patients with intermediate-2 or high-risk MF resistant to or intolerant of ruxolitinib. The key objectives are safety, PD, changes in spleen volume (MRI/CT) and total symptoms scores (TSS) using the MPN-SAF instrument. Inclusion criteria included a platelet count ≥100K/μL. Bone marrow (BM) biopsies and imaging studies (both centrally-read) were conducted at baseline and during washout (post-Day 84). The MPN-SAF was self-administered weekly. Phase 1/2a patients were treated for 84 days followed by a washout of up to 28 days. Patients demonstrating clinical benefit could resume treatment for additional 12 week cycles. Dosing was individually tailored using platelet count as a biomarker of effective thrombopoiesis. Patients were started at a presumed sub-therapeutic dose of 0.25 mg/kg/d and up-titrated weekly until the platelet count rested between 50 and 100K/μL.

This preliminary analysis includes 20 patients; 18 enrolled in the Phase 1/2a study, 2 in the Phase 2b portion. 50% had PMF, 35% Post-ET-MF, 15% Post-PV-MF. The median age was 65 (48-89) with 70% males. The median baseline platelet count was 197 k/μL (102-1309k/μL). 12 patients (56%) were transfusion-dependent at baseline. Sixty percent were IPSS-classified as high risk, the remainder, intermediate risk-2. 71% had more than 1 mutation of the 261 AML/MPN genes sequenced of which 63% were high molecular risk (ASXL1, U2AF1, SRSF2) mutations; 31% had abnormal karyotypes.

Sixteen patients completed the first 12 weeks; 4 patients withdrew, one due to fatigue (Day 33), one for progressive disease (Day 39), one due to physician decision (Day 76), one for an unrelated SAE of cellulitis (Day 83). All patients were up-titrated from the starting dose 0.25 mg/kg to an average daily dose of 0.89 mg/kg ± 0.20 mg/kg, the dose needed to achieve the target platelet count range; 17 achieved the target platelet range in a mean time of 45 days.

Of patients evaluable for response after cycle 1 in Phase1/2a (N=14), 50% had a reduction in spleen volume from baseline (median SVR: -14%; -2% to -30%). Further, 79% (N=11) recorded a reduction in TSS (mean change -28%; -13% to -69%); for 21% of patients (N=3), the change was >-50%. Improved BM fibrosis scores at Day 84 were observed in 2/13 patients. Two patients had improvement in transfusion requirements. Plasma IL-8 levels were significantly elevated in 6/14 patients at baseline and dropped in a dose-dependent manner over 21 days in 5/6 patients.

The mean duration of treatment is 166 days (14-539) at the census point in this ongoing study.

Nineteen patients (95%) reported 358 AEs of which 22 were SAEs. Of the SAEs, 2 were deemed by investigators as possibly related: painful splenomegaly and heart failure. There have been no safety signals, DLTs, progression to AML, or deaths.

This is the first clinical study of an LSD1 inhibitor in patients with MPNs. Once-daily IMG-7289 was well-tolerated in a heterogeneous population of patients with advanced MF and limited therapeutic options. Despite under-dosing and slow dose escalation, IMG-7289 improved symptom burdens in most patients and modestly reduced spleen volumes in a subset of patients. The Phase 2b 24-week expansion study with more aggressive dosing aimed at preserving safety and enhancing efficacy is open for enrollment in the US, UK and EU.

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