Study on potentiation of antitumor agent by MDR1 inhibitors in malignant tumor cells. Monitoring multidrug resistance using 99mTc-MIBI and comparison with expression of MDR1 mRNA

The aim of this study was to explore whether 99mTc-methoxyisobutylisonitrile (MIBI) is suitable for elucidation of multidrug resistance and prediction of potentiation of antitumor agents by MDR1 inhibitors in malignant tumor cells. Tumor cells (RG2 and C6 gliomas, Walker 256 mammary carcinoma) were continuously incubated with low dose vincristine to induce and maintain multidrug resistance. MTT assays demonstrated significant increases of surviving fractions in all vincristine (VCR)-resistant sublines as compared with those of drug-naive cell lines. Double-label accumulation studies (99mTc-MIBI, 14C-TdR) were performed in all drug-naive cell lines and VCR-resistant sublines. In all VCR-resistant sublines, RT-PCR revealed higher expression of MDR1 mRNA as compared with drug-naive cell lines. 99mTc-MIBI accumulation in VCR-resistant sublines expressing higher levels of MDR1 mRNA was significantly lower than in drug-naive cell lines expressing lower levels of MDR1 mRNA. However, there were no significant differences in cell proliferation as measured by 14C-TdR accumulation rate. 99mTc-MIBI accumulation is negatively correlated with MDR1 mRNA levels among drug-naive cell lines and VOR-resistant sublines. These findings indicated that the development of drug resistance was associated with enhanced 99mTc-MIBI extrusion. After pretreatment with MDR1 inhibitors (verapamil, cyclosporin A, FK506), surviving fractions of all VCR-resistant sublines significantly decreased as compared with those of non-treated VCR-resistant sublines. MTT assays revealed enhanced effects on VOR cytotoxity following pretreatment with MDR1 inhibitors. 99mTc-MIBI accumulation significantly increased after one-hour pretreatment with MDR1 inhibitors in all VOR-resistant sublines. In contrast, there were no significant differences in MDR1 mRNA levels between non-treated and MDR1 inhibitor-treated VOR-resistant sublines. MDR1 inhibitors had no significant effects on cell proliferation as measured by 14C-TdR accumulation rate. These findings indicated that MIBI-dependent mechanisms allowing VCR extrusion were functionally inhibited by MDR1 inhibitors. 99mTc-MIBI may be a suitable imaging agent for detecting MDR1-mediated drug resistance and for monitoring therapeutic effects of MDR1 inhibitors in malignant tumors. 99mTc-MIBI SPECT is expected to provide more definitive criteria for monitoring multidrug resistance and for predicting potentiation of antitumor agents by MDR1 inhibitors in patients with malignant brain tumors. (author)