Background: Polyamine synthesis is an oncogenic pathway deregulated by MYC. Ornithine decarboxylase (Odc) is a MYC-target encoding the rate-limiting enzyme in polyamine synthesis. DFMO is an FDA-approved Odc inhibitor being studied in neuroblastoma trials at doses from 1,000-9,000 mg/m2/day, and as monotherapy in first response or with chemotherapy following relapse. Better understanding its mechanisms of activity may identify testable responder hypotheses and guide subsequent trial design.
Methods: We used qPCR and SNP-arrays to define MYCN and ODC1 gene copy-number, and correlated this with DFMO sensitivity and global protein translation (governed by MYC-polyamine signaling). DFMO was tested in tumor xenografts and transgenic TH-MYCN models at exposures equivalent to ~7,500 mg/m2/day (high-dose); exposures of ~2,000 and ~4,000 mg/m2/day were tested in TH-MYCN mice. As MYC influences the tumor microenvironment (TME) via arginine-polyamine signaling we characterized tumor infiltrating leukocytes (TILs) by flow-cytometry.
Results: Of 23 cell lines, 13 were MYCN-amplified and 4/13 (31%) had ODC1 co-amplification. Of 256 MYCN-amplified primary tumors, 33 (13%) had ODC1 co-amplification. Inhibition of global protein translation (>50%) by DFMO correlated with MYC/ODC1 signaling. In treating MYCN, ALK and TP53 mutant xenografts, and in TH-MYCN mice, high-dose DFMO extended survival (p<0.05) on 5 chemotherapy backbones. TH-MYCN tumors treated with high-dose DFMO have significantly increased (up to 3-fold) NK, DC, CD4- NKT and G-MDSC cells defining an inflammatory TME. Lower DFMO exposures (≤4,000 mg/m2/day) did not inhibit tumor progression but elicited intermediate TIL changes.
Conclusions: ODC1 is transcriptionally activated by MYCN and by genomic amplification, and expression has been independently correlated with poor survival. We show DFMO inhibits Odc to deplete tumor polyamines, impede MYC-driven protein translation and tumor progression, and antagonize the tumor-permissive TME. Ongoing studies are defining the exposures required for cell autonomous and immune effects, relative sensitivity of MYCN/ODC1 co-amplified tumors, and testing for synergy with immunotherapy.