Technetium-99m-labeled HL91 to identify tumor hypoxia: correlation with fluorine-18-FDG

Technetium-99m HL91 is a potential agent for imaging hypoxic tissue in vivo. A pilot evaluation using a prototype formulation assessed efficacy in tumor imaging and compared results with 18F-fluorodeoxyglucose (FDG) PET.

Ten patients with malignant tumors were included. Tumors included carcinoma of the bronchus (n = 3), carcinoma of the thyroid (n = 2), lymphoma (n = 2), soft tissue sarcoma (n = 1), carcinoid (n = 1) and carcinoma of the breast (n = 1). Whole-body planar scans and localized SPECT scans were performed at 1 and 4 hr postinjection of 600 MBq 99mTc HL91. Half-body and localized emission/transmission 18F-FDG PET scans were performed 60 min postinjection 370 MBq 18F-FDG in eight patients. Lesion-to-normal tissue background ratios were measured for all 99mTc HL91 and 18F-FDG PET scans and percentage uptake and standardized uptake values measured for 99mTc HL91 and 18F-FDG PET, respectively.

Technetium-99m HL91 showed visible uptake into the tumor area in all seven studies where the tumor was clearly identified by 18F-FDG PET; 99mTc HL91 uptake was not detected in one study (carcinoid) in which 18F-FDG was weakly positive. Two further tumors, in which PET correlation was not available, showed accumulation of 99mTc HL91. In two 99mTc HL91 studies, abnormality was apparent only on SPECT images. Lesion-to-normal tissue background ratios for 99mTc HL91 ranged from 1.22/2.53 at 1 hr and 1.33/2.89 at 4 hr on planar images and 1.23/5.2 and 1.67/14.6 on SPECT images, respectively.

In this study, the use of 99mTc HL91 resulted in uptake in the majority of malignant tumors identified by 18F-FDG-PET. Technetium-99m HL91 exhibits good imaging characteristics, with imaging at 4 hr providing good lesion-to-normal tissue background ratios, that are further enhanced by SPECT.