HG-67: PRECLINICAL EFFICACY OF ALK2 INHIBITORS IN ACVR1 MUTANT DIPG

We recently reported somatic mutations in ACVR1, encoding the serine/threonine kinase ALK2 receptor, in diffuse intrinsic pontine glioma (DIPG) and have explored its utility as a novel drug target in these incurable childhood brainstem tumours. 50/206(24%) DIPG patients harboured ACVR1 mutations, with the extended survival times (p = 0.0041) and H3.1K27M associations (p = 0.046) of this genotype restricted to G328E/V/W substitutions; the common germline variant in fibrodysplasia ossificans progressiva (FOP), R206H, instead frequently co-segregated with H3.3K27M and a shorter overall survival. This mutation is reported to confer abnormal responsiveness to activin A in FOP, a phenomenon recapitulated in vitro in both R206H and G328V primary patient-derived DIPG models assessed by Western blot for p-Smad1/5/8 and ID1/2. Unlike classical BMP ligands, the spatiotemporal expression of activin A in neurodevelopment correlates with the suggested origins of DIPG. Screening mutant and wild-type DIPG cultures with a range of quinazolinone- and pyrazolo[1,5-a]pyrimidine-based ALK2 inhibitors, both published and newly-derived in-house, demonstrated differential effects on cell viability down to sub-micromolar concentrations, recapitulating genetic knockdown with shRNA, and corresponding to sustained time- and concentration-dependent reduction in signaling. Preclinical studies showed LDN-193189 and LDN-214117 to be orally bioavailable and tolerated, with penetration of the CNS at potentially active concentrations. Treatment of mice bearing orthotopic xenografts of H3.3K27M, ACVR1 R206H mutant HSJD-DIPG-007 cells with 25mg/kg LDN-193189 for 28 days significantly extended animal survival compared to vehicle control (p = 0.0053). Continued development of novel ALK2 inhibitors with increased potency, selectivity and pharmacokinetic properties may play an important role in future trials of DIPG patients.