Hypoxia-induced alteration of tracer accumulation in cultured cancer cells and xenografts in mice: implications for pre-therapeutic prediction of treatment outcomes with ^99mTc-sestamibi, ²⁰¹Tl chloride and ^99mTc-HL91

Abstract.

Weak visualization of tumours in pre-therapeutic scintigrams with technetium-99m sestamibi (MIBI) is likely a predictive sign of unfavourable tumour response to radiotherapy and chemotherapy. However, factors relating to this scintigraphic finding are not well understood. The presence of hypoxic tumour cells is one of the major reasons for therapeutic failure; consequently, we attempted to determine whether oxygenation status affects ^99mTc-MIBI accumulation in tumour cells. LS180 human colon cancer and T24 human bladder cancer cells were incubated in air or N₂ gas at 37°C. Cellular uptake of ^99mTc-MIBI was subsequently determined at 15, 60 and 120 min. Uptake of thallium-201 chloride was also assessed. Uptake of ^99mTc-HL91 was assessed as a hypoxic marker. Accumulation of the tracers in LS180 xenografts was observed in mice treated with 5 mg/kg hydralazine and compared with that in untreated mice. pO₂ in the medium and tumours was measured with O₂ microelectrodes. N₂ gas flow gradually reduced pO₂ in the cell suspension to 1–2 mmHg in 60 min. Cellular uptake of ^99mTc-MIBI in LS180 cells decreased by approximately 30% in N₂ gas in comparison to that in air throughout the study. Hypoxia had a more prominent influence on ²⁰¹Tl uptake, which displayed a reduction of approximately 60% in N₂ gas at 120 min, than on ^99mTc-MIBI uptake. On the other hand, N₂ gas induced an increase of 170% in ^99mTc-HL91 uptake at 120 min, indicating the hypoxic condition of cells. The results of in vitro assays employing the T24 cell line were similar to those obtained with the LS180 cell line. Hydralazine treatment markedly reduced ^99mTc-MIBI and ²⁰¹Tl accumulation in LS180 xenografts; moreover, intratumoural pO₂ decreased from 14.5±6.6 mmHg to 7.6±6.2 mmHg. ^99mTc-HL91 accumulation in xenografts was markedly increased by hydralazine. In conclusion, hypoxia reduced accumulation of ^99mTc-MIBI and ²⁰¹Tl in tumour cells. Accordingly, hypoxia may be an important factor in terms of the interpretation of scintigraphic findings obtained with these tracers for pre-therapeutic prediction of tumour response to treatment. Furthermore, the enhanced ^99mTc-HL91 accumulation in hypoxic tumour cells indicates the usefulness of this tracer in this regard.