eFT508, a Potent and Selective Mitogen-Activated Protein Kinase Interacting Kinase (MNK) 1 and 2 Inhibitor, Is Efficacious in Preclinical Models of Diffuse Large B-Cell Lymphoma (DLBCL)

Dysregulated translation of messenger RNA (mRNA) plays a role in the pathogenesis of multiple solid tumors and hematological malignancies. MNK1 and MNK2 integrate signals from several oncogenic and immune signaling pathways, including RAS, p38, and Toll-like receptor (TLR) pathways, by phosphorylating eukaryotic initiation factor 4E (eIF4E) and other key effector proteins including hnRNPA1 and PSF.  Through phosphorylation of these regulatory proteins MNK1 and MNK2 selectively regulate the stability and translation of a subset of cellular mRNA.  eFT508 is a potent, highly selective, and orally bioavailable MNK1 and MNK2 inhibitor.  eFT508 has a half-maximal inhibitory concentration (IC₅₀) of 1-2 nM against both MNK isoforms in enzyme assays and inhibits the kinase through a reversible, ATP-competitive mechanism of action.  Treatment of tumor cell lines with eFT508 led to a dose-dependent reduction in eIF4E phosphorylation at serine 209 (IC₅₀ = 2-16 nM), consistent with previous findings that phosphorylation of this site is solely dependent upon MNK1/MNK2.  In a panel of ~50 hematological cancers, eFT508 showed anti-proliferative activity against multiple DLBCL cell lines.  Sensitivity to eFT508 in TMD8, OCI-Ly3 and HBL1 DLBCL cell lines was associated with dose-dependent decreases in production of pro-inflammatory cytokines including TNFα, IL-6, IL-10 and CXCL10.  Further evaluation eFT508 mechanism of action demonstrated that decreased TNFα production correlated with a 2-fold decrease in TNFα mRNA half-life.  These findings are consistent with MNK1 phosphorylation of specific RNA-binding proteins, eg, hnRNPA1, that regulate the stability and translation of mRNA containing specific AU-rich elements (ARE) in their 3’-untranslated regions (UTR).  Pro-inflammatory cytokines are drivers of key hallmarks of cancer including tumor cell survival, migration and invasion, angiogenesis, and immune evasion, while also driving drug resistance.  Therefore, eFT508 was tested in vivo in 7 subcutaneous human lymphoma xenograft models.  Significant anti-tumor activity was observed in the TMD8 and HBL-1 ABC-DLBCL models, both of which harbor activating MyD88 mutations.  In addition, eFT508 combined effectively with components of R-CHOP and with novel targeted agents, including ibrutinib and venetoclax, in human lymphoma models.  These results underscore the potential of eFT508 for the treatment of DLBCL.  eFT508 has also been characterized in nonclinical safety pharmacology and toxicology studies. Clinical trials in patients with hematological and other malignancies are planned.