99mTc-EDDA/HYNIC-TOC:: a new 99Tc-labelled radiopharmaceutical for imaging somatostatin receptor-positive tumours:: first clinical results and intra-patient comparison with 111In-labelled octreotide derivatives

[(111)In-diethylene triamine penta-acetic acid-D-Phe(1)]-octreotide (DTPA-octreotide) scintigraphy has gained widespread acceptance as a diagnostic clinical procedure in oncology for imaging somatostatin receptor-positive tumours. However, indium-lll as a radiolabel has several drawbacks, including limited availability, suboptimal gamma energy and high radiation burden to the patient. We have recently reported on the preclinical development of (99m)Tc-EDDA/HYNIC-TOC. a new octreotide derivative which showed promising results both in vitro and in vivo. We now report our initial clinical experiences with this new radiopharmaceutical in ten oncological patients. The clinical diagnoses were: carcinoid syndrome (n = 5), thyroid cancer (n = 3), pancreatic cancer (n = 1) and pituitary tumour (n = 1). The biodistribution and kinetics of (99m)Tc-EDDA/HYNIC-TOC were compared with those of (111)In-DTPA-octreotide in six cases, and with those of (111)In-DOTA-TOC in five cases. With the new tracer tumours were imaged within 15 min after injection and showed the highest target/non-target ratios 4 h after injection. Tumour uptake persisted up to 20 h p.i. The rate of blood clearance was similar to that of (111)In-DTPA-octreotide but faster than that of (111)In-DOTA-TOC, while urinary excretion was lower compared with the (111)In derivatives. Semi-quantitative region of interest analysis showed that (99m)Tc-EDDA/HYNIC-TOC produced higher tumour/organ (target/non-target) ratios than the (111)In derivatives, especially in relation to heart and muscle. Significantly more lesions could he detected in (99m)Tc images. We conclude that (99m)Tc-EDDA/HYNIC-TOC shows better imaging properties for the identification of somatostatin receptor-positive tumour sites than currently available (111)In-labelled octreotide derivatives.