Abstract 5156: Targeting N-myristoylation in B-cell lymphomas as a therapeutic strategy

Myristoylation is the N-terminal modification of proteins with the fatty acid myristate. This process is mediated by two ubiquitously expressed N-myristoyltransferases, NMT1 and NMT2, and is critical for membrane targeting and cell signaling. Because NMT expression is increased in some cancers, we used three robotic screens to evaluate the potential of the potent pan-NMT inhibitor PCLX-001 on 300 cancer cell lines spanning the spectrum of human cancers. We discovered a marked increase in the sensitivity of hematological cancer cell lines, including B-cell lymphomas, to myristoylation inhibition. PCLX-001 consistently reduced both lymphoma cell proliferation and viability at concentrations lower than those needed to inhibit the growth of or to kill benign immortalized B cells. In lymphoma cell lines, PCLX-001 treatment inhibited early B-cell receptor (BCR) signaling events by disrupting membrane targeting of several myristoylated Src family kinases and promoted their ubiquitin-mediated degradation. Unexpectedly, PCLX-001 also promoted the degradation of non-myristoylated transcriptional activators P-ERK, c-Myc, NFκB and CREB downstream in the BCR signaling cascade, leading to loss of survival signals and apoptosis. Furthermore, compared to clinically approved drugs dasatinib and ibrutinib, PCLX-001 was more potent in vitro at inhibiting B-cell signaling, had a wider breadth of efficacy, and had greater selectivity thus sparing normal B cells. PCLX-001 treatment reduced tumor size in a time and concentration dependent manner in three B-cell lymphoma xenograft models and resulted in complete disease regression in two of these models, including an R-CHOP refractory lymphoma patient-derived xenograft. To investigate the potential mechanisms responsible for the sensitivity of hematological cancers to PCLX-001, we examined the NMT expression levels in cancer cells using publically available databases. Contrary to the reported NMT overexpression in some cancers, we found that hematological cancer cell lines and tumors both display significant reduction in NMT2 expression. The decreased NMT2 expression is significantly correlated with lower EC50 and poorer patient prognosis. Using the CRISPR-based genetic alteration Cancer Dependency Map, we discovered that cancer cells are highly dependent on functional NMT1, and that NMT1 dependency increases with low NMT2 expression. PCLX-001 treatment may mimic the effect of genetic alteration of NMT1 in hematological cancer ce