Cytotoxic activity of difluoromethylornithine compared with fenretinide in neuroblastoma cell lines

Background Maintenance therapy with 13-cis-retinoic acid and immunotherapy (given after completion of intensive cytotoxic therapy) improves outcome for high-risk neuroblastoma patients. The synthetic retinoid fenretinide (4-HPR) achieved multiple complete responses in relapse/refractory neuroblastoma in early-phase clinical trials, has low systemic toxicity, and has been considered for maintenance therapy clinical trials. Difluoromethylornithine (DFMO, an irreversible inhibitor of ornithine decarboxylase with minimal single-agent clinical response data) is being used for maintenance therapy of neuroblastoma. We evaluated the cytotoxic activity of DFMO and fenretinide in neuroblastoma cell lines. ProcedureResultsWe tested 16 neuroblastoma cell lines in bone marrow-level hypoxia (5% O-2) using the DIMSCAN cytotoxicity assay. Polyamines were measured by HPLC-mass spectrometry and apoptosis by transferase dUTP nick end labeling (TUNEL) using flow cytometry. At clinically achievable levels (100M), DFMO significantly decreased (P<0.05) polyamine putrescine and achieved modest cytotoxicity (<1 log (90% cytotoxicity). Prolonged exposures (7 days) or culture in 2% and 20% O-2 did not enhance DFMO cytotoxicity. However, fenretinide (10M) even at a concentration lower than clinically achievable in neuroblastoma patients (20M) induced1 log cell kill in 14 cell lines. The average IC90 and IC99 of fenretinide was 4.71M and 9.9 +/- 1.8M, respectively. DFMO did not induce a significant increase (P>0.05) in apoptosis (TUNEL assay). Apoptosis by fenretinide was significantly higher (P<0.001) compared with DFMO or controls. ConclusionsDFMO as a single agent has minimal cytotoxic activity for neuroblastoma cell lines.