A preliminary investigation of the potential anticancer properties of 8-hydroxyquinoline derivatives

Abstract

Derivatives of the quinoline moiety have been shown to exert a range of biological activities, including anti-neoplastic activity. The clinical application of quinoline derivatives in the treatment of malignancies has been limited due to non-selectivity. Four novel hydroxyquinoline derivatives were synthesised as potential anticancer agents. Each of these compounds contains the characteristic quinoline nucleus: a heterocyclic moiety containing a nitrogen atom. A hydroxyl group is present on C-8 and an azo bond links the heterocyclic quinoline core to a monocyclic benzene ring with various substituents. The aim of this study was to investigate the potential anticancer properties of the four hydroxyquinolines as pertains to their in vitro cytotoxicity, ability to circumvent multidrug resistance and possible mechanism of action. The acute in vivo toxicity profile of the two most promising compounds was also investigated. The tumour specificity of a compound is an indicator of the selective cytotoxicity of a compound towards cancer cells, while maintaining minimal toxicity towards normal cells. To this end the four hydroxyquinolines were screened on a range of commercially available cancer cell lines and primary (normal) cultures. The cancer cell lines used included human cervical adenocarcinoma (HeLa), human estrogen receptor positive breast cancer (MCF-7) and several resistant cancer cell lines. Chicken embryo fibroblasts and human lymphocytes were included as primary cell cultures. From the results the cancer cell lines most sensitive to each compound were identified: breast (MCF-7) and leukaemia (Jurkat) cells were most sensitive to HQ5, a resistant colon cancer cell line (COLO 320DM) was most susceptible to HQ6 and HQ7, and HQ10 was most effective against cervical cancer (HeLa). Data indicated that HQ5 and HQ10 displayed the highest tumour specificities and these two promising hydroxyquinoline derivatives were selected for further investigation. As quinoline derivatives have been reported to modulate multidrug resistance through the inhibition of the P-glycoprotein (P-gp) efflux pump, the effect of HQ5 and HQ10 on P-gp was evaluated in two experimental models. Firstly the experimental hydroxyquinolines were used in combination with the P-gp substrates doxorubicin, vinblastine and paclitaxel on three P-gp expressing cell lines to ascertain whether a synergistic combination could be observed. Secondly the direct effect of HQ5 and HQ10 on the function of P-gp was determined through the rhodamine 123 retention assay. According to the results obtained from the combination therapy, the combinations of the experimental compounds and the known chemotherapeutic agents tested were at most additive. Data obtained from the rhodamine 123 accumulation assay revealed that HQ5 and HQ10 did not inhibit P-gp in the three P-gp expressing cell lines tested but appeared to enhance P-gp activity. The mechanism of action of the two selected hydroxyquinolines was further investigated through flow cytometric analysis. The effect of HQ5 and HQ10 on the induction of apoptosis or necrosis in MCF-7 cells was determined. Results indicated that the two experimental compounds induced apoptosis in a dose and time dependent manner. After investigating the effect of the hydroxyquinolines on the cell cycle progression of MCF-7 cells, it was observed that HQ5 and HQ10 arrested the cell cycle at the G1 checkpoint. Results suggest that at higher concentrations of HQ5 inhibition resembled a G2 inhibitor. In an acute in vivo cytotoxicity study the tolerability and safety profile of HQ5 and HQ10 were investigated. After daily intraperitoneal administration of either of the two compounds at two concentrations, no obvious histological signs of toxicity were reported. However a dose of 2 mg/kg per day of HQ10 caused a significant reduction in body weight. Haematological analysis revealed that administration of 0.1 mg/kg of HQ5 resulted in a significant decrease in white cell count. No other haematological parameters studied showed any difference between the animals in the control and experimental groups. It was thus concluded that daily dosing of HQ5 and HQ10 was well-tolerated and caused no severe toxicity. Chronic in vivo toxicity profiles were not determined in this study. The in vitro studies suggested that HQ5 and HQ10 displayed promising anticancer properties. However, further investigation revealed several unfavourable characteristics, with regards to the solubility, purity and stability of these experimental hydroxyquinolines. In addition in vivo studies added further doubt on the success of these compounds in a clinical setting and it was concluded that these compounds were unsuitable for further development. Copyright