99mTc-methoxyisobutylisonitrile (99mTc-MIBI) has been suggested as a tracer for the scintigraphic detection of multidrug resistance (MDR). The aim of this study was to compare MDR characteristics in vitro and in vivo by immunohistochemic and functional uptake assays in established tumor cell lines cultured and grown in severe combined immunodeficient (SCID) mice. METHODS: The presence of MDR was assessed in vitro in drug-resistant HT-29(mdr1) colon carcinoma cells and in nonresistant HT-29(par) cells by JSB-1 immunohistochemistry, uptake of the fluorescent dye Rhodamine 123, and quantitative measurement of 99mTc-MIBI accumulation. For in vivo imaging, SCID mice bearing subcutaneous xenografts of these cell lines were injected with 99mTc-MIBI and 18F-FDG for scintigraphic and PET examination. After imaging, tumors were analyzed by immunohistochemistry and electron microscopy. RESULTS: All HT-29(mdr1) cells cultured in vitro exhibited distinct JSB-1 immunoreactivity, although to a variable degree, whereas HT-29(par) cells were completely devoid of JSB-1 staining. Rhodamine 123 accumulated poorly in HT-29(mdr1) cells but strongly in HT-29(par) cells. Accumulation of 99mTc-MIBI was 0.05% +/- 0.01% of the activity of the external medium in HT-29(mdr1) cells, but about eight times higher in HT-29(par) cells (0.40% +/- 0.09%), a very low percentage compared with other tumor cell lines. No difference in 201TlCl accumulation was observed between both cell lines. In vivo, neither HT-29(par) nor HT-29(mdr1) tumors grown in SCID mice could be detected by 99mTc-MIBI scintigraphy. In FDG PET, both HT-29(mdr1) and HT-29(par) tumors were clearly visible. FDG uptake was, however, markedly higher in HT-29(par) than in HT-29(mdr1) tumors. Both tumor types were poorly vascularized, as shown histologically. JSB-1 immunoreactivity was absent in all HT-29(par) tumors examined, whereas the majority of HT-29(par) tumor cells were stained. Electron microscopy showed that HT-29(par) tumors contained significantly less mitochondria than hepatocytes of the SCID mouse liver, which displayed high 99mTc-MIBI uptake in our scintigraphy studies. CONCLUSION: Sufficient 99mTc-MIBI uptake is the major prerequisite for distinguishing successfully between drug-resistant and sensitive cells. Negative 99mTc-MIBI scintigrams are not necessarily associated with MDR expression. In some tumors, FDG may be an in vivo marker for MDR as suggested by PET.
