The PI3kinase pathway and the susceptibility of its inhibitors to ABC transporters

Abstract

The PI3kinase pathway and ABC transporters play a crucial role in normal growth, survival and death of a cell. However in cancer cells, disturbances in these pathways lead to uncontrolled growth even under treatment of various anticancer drugs. Two major efflux pumps i.e. p-glycoprotein (Pgp) and multidrug resistance protein 1 (MRP1) are found to be involved in conferring resistance to vast variety of compounds. Similarly, activated PI3kinse pathway controls many other downstream signal transduction elements that in the end prevent the cancer cells from undergoing apoptosis. This thesis characterises the interaction between PI3kinase pathway and its inhibitors towards major efflux pumps by focussing on susceptibility and structure activity relationship of many compounds. Several clinically available antineoplastic agents and various PI3kinase inhibitors developed in Auckland Cancer Society Research Centre (ACSRC) were used for understanding these two pathways. The first aim was to determine the susceptibility of existing standard compounds in different cell lines. IC50 values of vinblastine and etoposide showed that thymidine incorporation assay had worked effectively. It also showed that the parent CEM, and its derived CEM/E1000 (MRP1 over-expression) and CEM/VLB100 (Pgp over-expression) subline were working properly. After validation of this assay, the next objective was to investigate the susceptibility of pan-PI3kinase isoform inhibitors against resistance. Most of the compounds did not show any significant sign of vulnerability towards both resistant cell lines. Only SN32078 showed susceptibility to resistance in CEM/VLB100 subline. It was observed that conversion of 5-bromo to 5-cyanopyrazolo and methyl to {2-methyl (2-methylamino (ethylamine)} at 5-nitrobenzene had brought this significant change in resistance. The next main aim was to focus on a novel series of compounds synthesized in ACSRC. Most of these compounds were derivatives of PIk-75, a p110 alpha isoform specific inhibitor. These compounds displayed better potency and aqueous stability than PIk-75. Both CEM/E1000 and CEM/VLB100 sublines showed resistance against only few compounds such as SN33070 and SN32932 respectively among this series. Most structural changes were focused on bigger functional groups such as morpholinyl; imidazol-methyl amino; pyridinyl-ethylamino; dimethylaminopropylmethyl amino; methyl sulphonyl piperazinyl amino. SN31842 showed susceptibility towards both Pgp and MRP1 resistant sublines. The other aim was to understand the effect of physicochemical property on resistance profile of cell line. It was observed that no direct or indirect relation could be established between resistance of cell lines and this series of compounds. Structure activity relationship provided us information about various modifications that could increase the potency while reducing the resistance factor. SN31302 was selected based on its low resistance factor in both sublines for investigating its role as a Pgp inhibitor like verapamil. It did reduce IC50 value of vinblastine in CEM/VLB100 subline but on very small scale and hence was not considered as a direct Pgp inhibitor. It must be following some different mechanism to reverse the Pgp overexpression. This shows that these novel compounds have different mode of action on ABC transporter and it could be through protein kinase C and p53 gene.