Abstract 3679: Targeting drug-resistant GIST cells with a quadruplex-binding small molecule

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Abstract Gastrointestinal tumours (GIST) originate, at least in part, from disregulation and consequent aberrant expression of the c-kit oncogene. Imatinib is an effective inhibitor of the ATP-binding activity of the kinase domain in the c-kit protein, and is used clinically to treat GIST patients. However acquired resistance to Imatinib is a severe problem hindering long-term patient survival. We have previously reported on an alternative approach to c-kit therapy that involves targeting and stabilising quadruplex DNA sequences with a small molecule, (i) in two sequences within the c-kit promoter so that c-kit expression is down-regulated, and (ii) in the single-stranded overhang of telomeric DNA, inhibiting telomerase activity in GIST cells. We have previously reported that both of these are targeted by a potent (IC50 ≪ 1μM) tetra-substituted naphthalene diimide compound in the Imatinib-sensitive GIST882 patient-derived cell line. This compound is equally potent in the patient-derived GIST48 Imatinib-resistant line; we report here that it has no effect on c-kit expression, telomerase activity, or the expression of other oncogenes in which promoter quadruplexes have been identified. Instead we find that the translation of the apoptosis-related bcl-2 gene is profoundly affected, but not its transcription. The bcl-2 gene has a potential quadruplex-forming region in the 5′-UTR sequence, and we report here that this forms a stable RNA quadruplex structure, which is further stabilised by the binding of the naphthalene diimide ligand. A reporter assay approach using a dual luciferase psiCHECK-2 vector, containing the bcl-2 5′-UTR sequence, is being used to validate the key concept that this G-rich sequence is involved in modulating translation in vitro, and in vivo through transfection into mammalian cells. Our current experiments in an Imatinib-resistant GIST cell line in the presence of the compound are enabling us to substantiate this cellular target and characterize the mode of action. Note: This abstract was not presented at the AACR 101st Annual Meeting 2010 because the presenter was unable to attend. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 101st Annual Meeting of the American Association for Cancer Research; 2010 Apr 17-21; Washington, DC. Philadelphia (PA): AACR; Cancer Res 2010;70(8 Suppl):Abstract nr 3679.