Assessment of reperfused myocardium using a new ischaemia-avid imaging agent, technetium-99m HL91: comparison with myocardial glucose uptake

Accumulation of Tc-99m HL91 in tumor hypoxia: in vitro cell culture and in vivo tumor model

Hypoxic cells within a tumor can account, in part, for resistance to radiotherapy and chemotherapy. Indeed, the oxygenation status has been shown to be a prognostic marker for the outcome of therapy. The purpose of this study was to determine whether Tc-99m HL91 (HL91), a noninvasive imaging tracer, detects tumor hypoxia in vitro in cell culture and in vivo in a tumor model. Uptake of HL91 in vitro into human lung cancer cells (A549) and murine Lewis lung cancer cells (LL2) was investigated at oxygen concentrations of 20% O2 (normoxia), and 1% O2 (hypoxia). HL91 biodistribution was studied in four groups: severe combined immune deficiency (SCID) mice bearing A549 tumors, C57BL/6NCrj (B6) mice bearing LL2 tumors, SCID controls, and B6 controls. Accumulation of the tracer was compared between tumors treated with hydralazine or phosphate-buffered saline (PBS). Scintigraphic images were obtained for hydralazine-treated mice and PBS-treated mice in each of the four study groups. Autoradiography of tumor slices was also acquired. In vitro studies identified hypoxia-selective uptake of HL91, with significantly increased uptake in the hypoxic state than in the normoxic state. Biodistribution and scintigraphy showed increased HL91 uptake during tumor hypoxia at 0.5 hours, and there was progressively increased activity for up to 4 hours after tracer administration. HL91 accumulation in tumor hypoxia was markedly increased in mice treated with hydralazine compared with those treated with PBS. Autoradiography revealed high HL91 uptake in the peripheral areas around the necrotic regions of the tumor, which were identified by histologic examination. HL91 exhibits selectivity for tumor hypoxia both in vitro and in vivo and provides a successful imaging modality for the detection of tumor hypoxia in vivo.

Investigation into 64Cu-labeled Bis(selenosemicarbazone) and Bis(thiosemicarbazone) complexes as hypoxia imaging agents

BACKGROUND

Cu-diacetyl-bis(N4-methylthiosemicarbazone) [Cu-ATSM], although excellent for oncology applications, may not be suitable for delineating cardiovascular or neurological hypoxia. For this reason, new Cu hypoxia positron emission tomography (PET) imaging agents are being examined to search for a higher selectivity for hypoxic or ischemic tissue at higher oxygen concentrations found in these tissues. Two approaches are to increase alkylation or to replace the sulfur atoms with selenium, resulting in the formation of selenosemicarbazones.

METHODS

Three 64Cu-labeled selenosemicarbazone complexes were synthesized and one was screened for hypoxia selectivity in vitro using EMT-6 mouse mammary carcinoma cells. Rodent biodistribution and small animal PET images were obtained from BALB/c mice implanted with EMT-6 tumors. One alkylated thiosemicarbazone was synthesized and examined.

RESULTS

Of the three bis(selenosemicarbazone) ligands synthesized and examined, only 64Cu-diacetyl-bis(selenosemicarbazone) [64Cu-ASSM] was isolated in high-enough radiochemical purity to undertake cell uptake experiments where uptake was shown to be independent of oxygen concentration. The bis(thiosemicarbazone) complex synthesized, 64Cu-diacetyl-bis(N4-ethylthiosemicarbazone) [64Cu-ATSE], showed hypoxia selectivity similar to 64Cu-ATSM although at a higher oxygen concentration. Biodistribution studies for 64Cu-ASSM and 64Cu-ATSE showed high tumor uptake at 20 min (64Cu-ASSM, 10.33+/-0.78% ID/g; 64Cu-ATSE, 7.71+/-0.46% ID/g). PET images of EMT-6 tumor-bearing mice visualized the tumor with 64Cu-ATSE and revealed hypoxia selectivity consistent with the in vitro data.

CONCLUSION

Of the compounds synthesized, only 64Cu-ASSM and 64Cu-ATSE could be examined in vitro and in vivo. Although the stability of bis(selenosemicarbazone) complexes increased upon addition of methyl groups to the diimine backbone, the fully alkylated species, 64Cu-ASSM, demonstrated no hypoxia selectivity. However, the additional alkylation present in Cu-ATSE modifies the hypoxia selectivity and in vivo properties when compared with Cu-ATSM.

Increased uptake of 99mTc-HL91 in tumor cells exposed to X-ray radiation

99mTc-HL91, a hypoxic marker, may be a predictor of tumor response to radiotherapy and an indicator of tumor oxygenation in the course of treatment. In this study, serial changes in 99mTc-HL91 uptake were observed in the normoxic condition in a human bladder cancer cell line exposed to a single dose or a fractionated dose of 10 Gy with an x-ray beam. The uptake per cell increased during cell growth retardation induced by the irradiation. This finding indicates that 99mTc-HL91 uptake is affected by injury to cells due to radiation; it may therefore be difficult to correctly assess the tissue oxygenation status during radiotherapy with 99mTc-HL91.