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(MIBn is suitable for elucidation of multidrug resistance and prediction ofpotentiation of antitumor agents by MDRI inhibitors in malignant tumor cells.Tumor cells (RG2 and C6 gliomas, Walker 256 mammary carcinoma) were continuouslyincubated with low dose vincristine to induce and maintain multidrug resistance.MTT assays demonstrated significant increases of surviving fractions in all vincristne(VCR) -resistant sublines as compared with those of drug-naive cell lines. Double-labelaccumulation studies (99mTc-MIBI, 14C-TdR) were performed in all drug-naive celllines and VCR-resistant sublines. In all VCR-resistant sublines, RT-PCR revealed higherexpression of MDRI mRNA as compared with drug-naive cell lines. 99mTc-MIBIaccumulation in VCR-resistant sublines expressing higher levels of MDRI mRNA wassignificantly lower than in drug-naive cell lines expressing lower levels of MDRI mRNA.However, there were no significant differences in cell proliferation as measured by 14CTdRaccumulation rate. 99mTc-MIBI accumulation is negatively correlated with MDRImRNA levels among drug-naive cell lines and VCR-resistant sublines. These findingsindicated that the development of drug resistance was associated with enhanced 99mTcMIBIextrusion. After pretreatment with MDRI inhibitors (verapamil, cyclosporin A,FK506), surviving fractions of all VCR-resistant sublines significantly decreased ascompared with those of non-treated VCR-resistant sublines. MTT assays revealedenhanced effects on VCR cytotoxity following pretreatment with MDRI inhibitors.ggmTc-MIBI accumulation significantly increased after one-hour pretreatment withMDRI inhibitors in all VCR-resistant sublines. In contrast, there were no significantdifferences in MDRI mRNA levels between non-treated and MDRI inhibitor-treatedVCR-resistant sublines. MDRI inhibitors had no significant effects on cell proliferationas measured by 14C-TdR accumulation rate. These findings indicated that MIBI-dependentmechanisms allowing VCR extrusion were functionally inhibited by MDRI inhibitors.ggmTc-MIBI may be a suitable imaging agent for detecting MDRI-mediated drugresistance and for monitoring therapeutic effects of MDRI inhibitors in malignanttumors. ggmTc-MIBI SPECT is expected to provide more definitive criteria for monitoringmultidrug resistance and for predicting potentiation of antitumor agents by MDRI inhibitors in patients with malignant brain tumors