[123I]Mtr-TOCA, a radioiodinated and carbohydrated analogue of octreotide: scintigraphic comparison with [111In]octreotide

Purpose Scintigraphy with maltotriose-¹²³I]Tyr³-octreotate (¹²³I]Mtr-TOCA) is compared with ¹¹¹In]DTPA-Phe¹-octreotide (¹¹¹In]OC) to assess the differences in pharmacokinetics and imaging properties as well as to estimate the therapeutic potential of the corresponding ¹³¹I]Mtr-TOCA.Methods Six patients with somatostatin receptor (sstr)-positive tumours were assessed using a dual-head gamma camera. After injection of 137±28 MBq ¹²³I]Mtr-TOCA, dynamic data acquisition of the upper abdomen (30 min) was performed followed by whole-body scans at 0.5 h, 1 h, 3 h and 20 h as well as by SPECT imaging (tumour) at 2 h. ¹¹¹In]OC scintigraphy was performed by acquiring whole-body scans (4 h, 24 h) and SPECT (24 h). Using a region of interest (ROI) method, tissue and tumour bound activity was assessed and dosimetry performed.Results ¹²³I]Mtr-TOCA shows rapid tumour uptake. Up to 4 h, tumour/organ (tu/org) ratios are stable and generally higher than with ¹¹¹In]OC. From 3 h to 20 h, tu/org ratios increase for spleen, remain stable for liver and decrease significantly for all other tissues. In contrast, with ¹¹¹In]OC, tu/org ratios decrease slightly between 4 h and 24 h for liver, spleen and kidney and increase for all other tissues. On ¹²³I]Mtr-TOCA scintigraphy, a total of 27 lesions are detected, whereas 33 lesions are detected on ¹¹¹In]OC scintigraphy (p=0.50). Effective patient absorbed dose is 1.9±0.9 mSv per 100 MBq ¹²³I]Mtr-TOCA.Conclusion Compared with ¹¹¹In]OC, ¹²³I]Mtr-TOCA enables faster imaging of sstr-positive tumours with a lower radiation burden to the patient. ¹²³I]Mtr-TOCA and ¹¹¹In]OC allow for tumour imaging with almost identical contrast and diagnostic yield. As regards peptide receptor radionuclide therapy, radioiodinated Mtr-TOCA is hampered by limited intratumoural activity retention.