ID #235 Establishing the therapeutic efficacy of a small molecule inhibitor of B7H3 in mitigating aggressiveness for group 3 medulloblastomas

Abstract

Medulloblastoma (MB) is the most common malignant brain tumor of childhood and a leading cause of cancer-related childhood mortality. Amongst the four primary subgroups, patients with group 3 MB (G3MB) have the most aggressive tumors. B7-H3 is an immune checkpoint molecule whose enrichment in G3MBs is associated with accelerated mortality from heightened metastasis. B7-H3-mediated metastasis has been linked to JAK2/STAT3 signaling. In G3MB, silencing B7-H3 (small-interfering RNA) reduced cancer cell migration, invasion, and the expression of SLUG, a JAK2/STAT3 signaling partner that facilitates metastasis. Given these promising findings, we isolated the first small-molecule inhibitor of B7-H3 (Ni1) that demonstrated low micromolar in vitro potency in 3 G3MB cancer cell lines (IC50∼1.6 uM).

Our overall objective was to demonstrate a mechanistic proof-of-concept for B7-H3 inhibition as an innovative therapeutic approach against G3MB. We hypothesized that Ni1 can suppress G3MB tumor growth and aggressiveness by inhibiting JAK2/STAT3 signaling pathways associated with metastasis. We posited that this strategy could synergize with classical chemotherapy to more effectively kill G3MB cancer cells.

Ni1 exerted a dose-dependent inhibition on cancer cell proliferation, abrogated colony formation and wound healing, and induced late apoptosis, concurrent with an elevation in cleaved caspase 3 and cleaved PARP levels. It also inhibited JAK2/STAT3 expression and phosphorylation and, resultantly, reduced the expression of n-cadherin, vimentin, and SLUG, downstream effectors of JAK2/STAT3 signaling and mediators of metastasis. Ni1 also effectively synergized with cisplatin to induce significant cytotoxicity at lower doses, resulting in a near 50% reduction in cisplatin IC50. In vivo, Ni1 at 10 mg/kg i.p. established 15-fold higher brain than plasma concentrations and had a half-life of 8 hours. At 25 mg/kg, Ni1 exerted an intermediate inhibitory effect on tumor volume (p=0.16) and survival (p=0.08).

Taken together, we provide experimental evidence for the anti-neoplastic properties of the first small-molecule inhibitor of B7-H3.