Abstract 4199: Inhibition of NF-kB inducing kinase (NIK) selectively abrogates NIK and TRAF3 mutant multiple myeloma tumor growth

Enhanced NF-kB signaling is a hallmark of aggressive lymphoid malignancies, including multiple myeloma (MM), mantle cell lymphoma (MCL), and diffuse large B-cell lymphoma (DLBCL). Non-canonical NF-kB signaling involves NIK-dependent activation of IKKα, which triggers nuclear accumulation of p52/RelB heterodimers. NIK is a highly unstable protein and degradation is mediated by a ubiquitin ligase complex consisting of TRAF2, TRAF3 and c-IAP1/2 (encoded by BIRC2/3). In a subset of MM, NIK is stabilized by mutations in NIK, TRAF2/3 or BIRC2/3 (Annunziata et al./Keats et al., Canc. Cell 2007). Similar activating mutations in the non-canonical NF-kB signaling pathway were recently found in ibrutinib-refractory MCL cell lines (Rahal et al., Nat. Med. 2014), and in DLBCL (Zhang et al., Cell Rep. 2015). In many other cases of B-cell malignancies, NIK is stabilized by high level expression of the upstream TNF receptors (BAFFR, CD40, LTβR) or high abundance of their ligands in the bone marrow or the lymph nodes. To date, bioavailable NIK inhibitors have not been reported, and it has remained unclear whether NIK inhibitors are effective and tolerated in mouse models of B-cell malignancies associated with activation of NIK. Here, we report for the first time on a potent orally bioavailable NIK kinase inhibitor, TRC694. TRC694 selectively inhibits NIK enzymatic activity, translating into inhibition of phospho-IKKα in NIK and in TRAF3 mutant cell lines with single digit nM IC50. TRC694 prevents nuclear accumulation of p52/RelB (but not canonical NF-kB) and represses the associated NF-kB gene program selectively in MM cell lines with genetic activation of the non-canonical NF-kB pathway. Proliferation of NIK translocated, TRAF3 or BIRC3 mutant MM cell lines is inhibited by low nM concentrations of TRC694, whereas MM cell lines which lack genetic activation of non-canonical NF-kB are much less sensitive to TRC694. Consistently, elevated expression of a previously described 11-gene NFkB signature in MM (Annunziata et al., Canc. Cell 2007) is predictive of sensitivity to TRC694 in a 21-MM cell line panel. A single, oral dose of 10 to 40 mg/kg of TRC694 to mice bearing a NIK-translocated MM tumor (JJN-3), was sufficient to inhibit phospho-IKKα and repress P52-mediated transcription of NFkB regulated genes in the tumors. Consistently, once-daily, oral dosing of TRC694 to mice bearing subcutaneous NIK translocated (JJN-3) or TRAF3 (RPMI-8226, MM.1S) mutant MM tumors, completely