European Journal of Nuclear Medicine Vol. 29, No. 8, August 2002 Abstract. Weak visualization of tumours in pre-thera- peutic 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; consequent- ly, we attempted to determine whether oxygenation sta- tus affects 99m Tc-MIBI accumulation in tumour cells. LS180 human colon cancer and T24 human bladder can- cer cells were incubated in air or N 2 gas at 37°C. Cellu- lar uptake of 99m Tc-MIBI was subsequently determined at 15, 60 and 120 min. Uptake of thallium-201 chloride was also assessed. Uptake of 99m Tc-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. pO 2 in the medium and tumours was measured with O 2 microelectrodes. N 2 gas flow gradually reduced pO 2 in the cell suspension to 1–2 mmHg in 60 min. Cel- lular uptake of 99m Tc-MIBI in LS180 cells decreased by approximately 30% in N 2 gas in comparison to that in air throughout the study. Hypoxia had a more prominent in- fluence on 201 Tl uptake, which displayed a reduction of approximately 60% in N 2 gas at 120 min, than on 99m Tc- MIBI uptake. On the other hand, N 2 gas induced an in- crease of 170% in 99m Tc-HL91 uptake at 120 min, indi- cating 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 99m Tc-MIBI and 201 Tl accu- mulation in LS180 xenografts; moreover, intratumoural pO 2 decreased from 14.5±6.6 mmHg to 7.6±6.2 mmHg. 99m Tc-HL91 accumulation in xenografts was markedly increased by hydralazine. In conclusion, hypoxia re- duced accumulation of 99m Tc-MIBI and 201 Tl 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 en- hanced 99m Tc-HL91 accumulation in hypoxic tumour cells indicates the usefulness of this tracer in this regard. Keywords: Tumour uptake – Hypoxia – Pre-therapeutic prediction – 99m Tc-sestamibi – 99m Tc-HL91 Eur J Nucl Med (2002) 29:1006–1011 DOI 10.1007/s00259-002-0846-x Introduction A technetium-99m-labelled cationic compound, 99m Tc- sestamibi (MIBI), has been investigated as a tumour-im- Seigo Kinuya ( ✉ ) Department of Biotracer Medicine, Kanazawa University Graduate School of Medical Sciences, 13-1 Takaramachi, Kanazawa, Ishikawa 920-8640, Japan e-mail: kinuya@med.kanazawa-u.ac.jp Tel.: +81-76-2652333, Fax: +81-76-2344257 Original article Hypoxia-induced alteration of tracer accumulation in cultured cancer cells and xenografts in mice: implications for pre-therapeutic prediction of treatment outcomes with 99m Tc-sestamibi, 201 Tl chloride and 99m Tc-HL91 Seigo Kinuya 1 , Kunihiko Yokoyama 1 , Xiao-Feng Li 1 , Jingming Bai 1 , Naoto Watanabe 2 , Noriyuki Shuke 3 , Teruhiko Takayama 4 , Hisashi Bunko 5 , Takatoshi Michigishi 1 , Norihisa Tonami 1 1 Department of Biotracer Medicine, Kanazawa University Graduate School of Medical Sciences, 13-1 Takaramachi, Kanazawa, Ishikawa 920-8640, Japan 2 Department of Radiology, Toyama Medical and Pharmaceutical University, Toyama, Japan 3 Department of Radiology, Asahikawa Medical College, Asahikawa, Japan 4 Department of Radiological Technology, Kanazawa University School of Health Science, Kanazawa, Japan 5 Medical Informatics, Kanazawa University Hospital, Kanazawa, Japan Received 19 December 2001 and in revised form 26 March 2002 / Published online: 17 May 2002 © Springer-Verlag 2002