Birinapant (TL32711), a Small Molecule Smac Mimetic, Induces Regressions in Childhood Acute Lymphoblastic Leukemia (ALL) Xenografts That Express TNFα and Synergizes with TNFα in Vitro – A Report From the Pediatric Preclinical Testing Program (PPTP)

Birinapant is a small molecule mimetic of Smac that potently and specifically antagonizes multiple inhibitors of apoptosis proteins (IAPs). Birinapant rapidly degrades cIAPs and enables cytokines (TNFα, TRAIL) to activate the extrinsic apoptosis pathway, while it rapidly turns off the canonical NF-κB survival pathway, causing cancer cell death. Preclinical studies using adult cancer models have shown that birinapant causes tumor regressions as a single agent in selected models and that it has potent antitumor activity when combined with chemotherapies and death receptor ligands.

Birinapant was evaluated against the 23 cell lines of the PPTP in vitro panel (including 1 AML and 5 ALL lines) using 96 hour exposure at concentrations from 1.0 nM to 3.0 μM, both as a single agent and in combination with TNFα (10 ng/mL) or TRAIL (10 ng/mL). Birinapant was tested against 2 PPTP solid tumor xenografts (rhabdomyosarcoma, Rh30; Ewing, CHLA-258), an anaplastic large cell lymphoma (ALCL) xenograft (Karpas-299), and 3 B-precursor ALL xenografts (ALL-2, ALL-17, & ALL-19) at a dose of 30 mg/kg administered by the intraperitoneal route using a Q3 day × 5 schedule. Gene expression data for the PPTP cell lines and xenografts was available using both Affymetrix U133 Plus 2.0 and Agilent SurePrint G3 arrays.

Birinapant demonstrated limited single agent activity (median relative IC₅₀ (rIC₅₀) > 3 μM), with only the AML cell line Kasumi-1 showing Relative In/Out% (Rel I/O%) values < 0% with rIC₅₀ of 37 nM. Marked potentiation of birinapant was observed for a subset of cell lines with the addition of TNFα or TRAIL. The 5 ALL cell lines showed a median rIC₅₀ value of 3.6 nM for birinapant in combination with TNFα, with Rel I/O% values between −50% and −100% (indicative of a profound cytotoxic effect). Four of 5 ALL cell lines showed little or no potentiation of birinapant effect with the addition of TRAIL. Among solid tumor cell lines, potentiation of birinapant effect was observed for selected rhabdomyosarcoma, rhabdoid tumor, Ewing sarcoma, and neuroblastoma cell lines with the addition of either TNFα or TRAIL.

Birinapant was well tolerated in vivo. Birinapant induced significant differences in event-free survival (EFS) distribution compared to control in 3 of 3 (100%) of the B-precursor ALL xenografts, but in none of the solid tumor or ALCL xenografts. Objective responses were not observed for the solid tumor and ALCL xenografts, whereas for the ALL panel one xenograft (ALL-17) achieved a complete response (CR) and another (ALL-2) achieved a maintained CR, with treated animals remaining in remission at day 42, approximately 30 days after their last treatment with birinapant. Given the mechanism of action of Smac mimetics, TNFα expression was examined. TNFα expression was significantly higher for the PPTP ALL xenografts compared to the PPTP solid tumor xenografts and to 15 normal tissues. TNFα expression in ALL clinical specimens was examined using publicly available datasets, with the observation that its expression is significantly higher for high-risk B-precursor ALL compared to a set of normal tissues, but with a wide range of TNFα expression among ALL cases. Lymphotoxin A and B also show significantly elevated expression in ALL compared to normal tissues. Among the ALL xenografts tested with birinapant, the best responding xenograft (ALL-2) showed the highest TNFα expression. Karpas-299 also showed high TNFα expression, but the two solid tumor xenografts did not. Unlike the ALL cell lines for which exogenous TNFα potentiated birinapant in vitro activity, exogenous TNFα did not potentiate birinapant in vitro activity against Karpas-299.

Birinapant showed little single agent in vitro activity against ALL cell lines, though it markedly potentiated the activity of exogenously added TNFα for these cell lines. In vivo, birinapant showed remission-inducing activity against 2 of 3 ALL xenografts, with one of these showing a maintained CR. TNFα is mechanistically associated with the activity of Smac mimetics, and the initial PPTP in vivo data for ALL xenografts are consistent with a relationship between TNFα expression and responsiveness to birinapant. The PPTP results suggest that birinapant may show high level activity against a subset of childhood ALL, and additional in vivo testing is ongoing to better identify predictive markers that can reliably select responsive cases.

Chunduru:TetraLogic Pharmaceuticals: Employment, Equity Ownership. Graham:TetraLogic Pharmaceuticals: Employment, Equity Ownership. Geier:TetraLogic Pharmaceuticals: Honoraria. Houghton:TetraLogic Pharmaceuticals: Honoraria.