Targeting polyamines for treatment of malignant pleural mesothelioma in xenograft models

Abstract

Introduction: Inhaling asbestos fibres is the commonest cause of malignant pleural mesothelioma (MPM). Although the use of asbestos has been restricted, the incidence of MPM is still rising due to a long lag time in malignant transformation. In 2004, the United States Food and Drug Administration approved a combination of pemetrexed with cisplatin for treatment of unresectable MPM. However, overall prognosis is still extremely poor. As such, development of novel therapeutic options is urgently needed. Ornithine decarboxylase (ODC) is highly expressed in 211H and H226 MPM xenografts and clinical tumour samples. Upregulation of ODC increases polyamine production and enhances tumour growth. Alpha-difluoromethylornithine (DFMO) is a specific ODC inhibitor which can suppress polyamines production. This study aimed to disclose the therapeutic effect of DFMO in MPM xenograft models. Methods: Nude mice were subcutaneously inoculated with human MPM cells (211H or H226). Mice were treated with DFMO in drinking water when tumour size reached 50 to 100 mm3 . Mice with tumour size >600 mm3 were considered reaching humane endpoint. Spermidine levels, protein expression, cytokines concentrations and apoptosis were investigated by Dot plot, western blot, enzyme-linked immunosorbent assay and terminal deoxynucleotidyl transferase dUTP nick end labelling assay, respectively. Nitrated protein was identified using Dionex UltiMate 3000 RSLCnano system coupled to Thermo Fisher Orbitrap Fusion Lumos Tribrid mass spectrometer (Centre for PanorOmic Sciences, The University of Hong Kong). Results: Alpha-difluoromethylornithine–suppressed tumour growth in both xenografts. DFMO increased median survival from 29 days in control arm to 41 days in treatment arm in mice with 211H xenografts (P=0.0234), while from 30 days to 43.5 days in those with H226 xenografts (P=0.0050). There was no synergism when combining DFMO with either cisplatin or pemetrexed. The tumour suppressive effect of DFMO was more effective when compared with cisplatin or pemetrexed alone. Upon DFMO treatment, decrease in spermidine level, increase in nitrotyrosine content (nitration), and activation of apoptosis were observed in both xenografts. In addition, increase in nitrosocysteine level, increase in intratumoural interleukin 6, keratinocyte chemoattractant and tumour necrosis factor alpha as well as elevation of DNA lesion and downregulation of pAkt were induced by DFMO in H226 xenografts only. Moreover, anti-cancer effect of DFMO was partially reversed by supplement of spermidine during DFMO treatment in both xenografts. Nitration was found at the Tyr53 of the human actin sequence in both xenografts which might affect actin polymerisation. Conclusion: Alpha-difluoromethylornithine may have a potential role in treating MPM. Acknowledgement: This research was supported by Hong Kong Pneumoconiosis Compensation Fund Board.