Targeting polyamines as potential adjuvant therapy in malignant pleural mesothelioma xenograft models

Abstract

Introduction: Inhaling asbestos fibres is a well-known common cause of malignant pleural mesothelioma (MPM). Although the import and use of asbestos have been restricted, the incidence of MPM is not expected to decline due to a long lag time in malignant transformation. In 2004, the United States Food and Drug Administration approved a combination of cisplatin with pemetrexed for treatment of unresectable MPM. Concurrently, development of novel adjuvant therapeutic agents for resected early-stage disease is also urgently desired. Ornithine decarboxylase (ODC) is highly expressed in 211H and H226 MPM xenografts and clinical tumour samples. Ornithine decarboxylase upregulation increases polyamine production and enhances tumour growth. Recent preclinical investigations have revealed the adjuvant effect of α-difluoromethylornithine (DFMO), a well-known specific ODC inhibitor, in colon cancers using xenograft model. Nevertheless, adjuvant effect of DFMO in MPM has not yet been disclosed. The aim of this study was to investigate the adjuvant effect of DFMO in MPM xenograft models. Methods: Nude mice were fed with DFMO (in drinking water) 7 days before subcutaneous inoculation of 200 000 tumour cells (211H [biphasic] or H226 [epithelioid]). Mice with tumour size >600 mm3 were considered reaching humane endpoint. Protein expression, spermidine levels, cytokine concentrations, and apoptosis were investigated by western blot, dot plot, enzyme-linked immunosorbent assay, and TUNEL (terminal deoxynucleotidyl transferase dUTP nick end labeling) assay, respectively. Results: Tumour growth was suppressed by DFMO in both xenografts. Median survival increased from 49.5 days in the control arm to 65 days in the DFMO treatment arm in mice with 211H xenograft (P=0.08), and from 44 days to 120 days in mice with H226 xenograft (P=0.0002). In the H226 xenograft model, 43% of treated mice did not reach humane endpoint, mimicking long-term survival. Upon DFMO treatment, decrease in spermidine level, increase in nitrotyrosine content (reactive nitrogen species; RNS), and activation of apoptosis were observed in both xenografts. In addition, increase in RNS, elevation of intratumoralinterleukin-6, keratinocyte chemoattractant and tumour necrosis factor-α, upregulation of DNA lesion and inhibition of Akt/mTOR pathway were induced by DFMO in H226 xenograft, which may explain higher potency of DFMO in this xenograft. Conclusion:There is a potential role for DFMO as adjuvant therapy in MPM; especially epithelioid mesothelioma, which is partially mediated by spermidine depletion, induction of RNS, and apoptosis.The findings from this study will provide scientific foundation for future design of clinical trials of DFMO for adjuvant therapy in early disease for advanced MPM . Acknowledgement: This research was supported by the Hong Kong Pneumoconiosis Compensation Fund Board .