Docetaxel uptake and modulation of P-gp- mediated docetaxel efflux by tyrosine kinase inhibitors in human lung carcinoma

Treatment with the taxanes, docetaxel and paclitaxel, can result in the emergence of multi-drug resistance (MDR) mediated by P-gp (MDR-1, ABCB1), which is an effective cellular efflux pump for both agents. This thesis was undertaken to examine the contribution of drug transport mechanisms to chemotherapeutic drug resistance, focussing on docetaxel. Sensitive and resistant NSCLC cell lines were used to model docetaxel transport and examine the ability of three tyrosine kinase inhibitors (TKIs), gefitinib, erlotinib and lapatinib, to circumvent resistance to docetaxel, and other chemotherapeutic agents, arising from P-gp over-expression. A HPLC – based method was initially employed to quantify docetaxel levels in cells. The very high taxane levels required rendered this method unreliable for prediction of pharmacologically relevant effects. A more sensitive radiolabel-based technique was then developed to examine lower, pharmacologically achievable concentrations (100-500 nM) of docetaxel. The radiolabel-based assay was then applied to examining docetaxel uptake in the DLKP and A549 NSCLC cell lines and docetaxel accumulation and efflux in the Pgp over-expressing A549-Taxol and DLKP-A cell lines. Passive diffusion is believed to be the mechanism of uptake for docetaxel in most cancer cells due to its lipophilic characteristics. However, evidence was found for an energydependent docetaxel uptake mechanism in DLKP and a non-P-gp energy-dependent efflux mechanism in A549. The contribution of the OATP (organic anion transporting polypeptides) family of transporters to docetaxel uptake in A549 could not be discounted. The existence of transporter-mediated docetaxel uptake in NSCLC cells represents an important new factor in determining the sensitivity of cancer cells to docetaxel. Studies on the TKIs revealed that lapatinib interacted with the ATPase function of P-gp in a manner distinct from gefitinib and erlotinib at clinically achievable concentrations. Lapatinib is most likely a slowly-transported substrate with high affinity for P-gp while erlotinib and gefitinib are most likely transported P-gp substrates. As a result of this, P-gp over-expression may contribute to erlotinib and gefitinib, but not lapatinib, resistance at pharmacological concentrations. Results suggest the three TKIs, particularly lapatinib, have potential clinical utility as MDR modulators capable of augmenting the cytotoxic activity of P-gp substrate chemotherapeutic agents against P-gp positive tumour cells. In addition, each TKI altered EGFR and P-gp protein expression levels.