IN-VITRO AND IN-VIVO STUDIES OF 3 RADIOLABELED SOMATOSTATIN ANALOGS -I-123-OCTREOTIDE (OCT), I-123-TYR-3-OCT AND IN-111-DTPA-D-PHE-1-OCT

Scintigraphy with long-acting somatostatin (SST) analogues may be usef ul for the localization of tumours expressing receptors (R) for SST. I n this study we have analysed the in vitro and in vivo binding propert ies of three SST analogues, I-123-octreotide (OCT), I-123-Tyr-3-OCT an d In-111-DTPA-D-Phe-1-OCT. In vitro binding studies performed with a v ariety of primary rumours (n=48) as well as with several tumour cell l ines (A431 HT29, PANC1, COLO320, HMC1, KU812) indicated significant in vitro binding of these three radiolabelled SST analogues to two subpo pulations of SSTR, high (K-d 0.2-2.0 nhl) and low (K-d 5-15 nM) affini ty ones. The number of SSTR on tumour cells was at least a 1000-fold h igher as compared with normal peripheral blood cells. Comparative scin tigraphic studies using I-123-OCT and/or I-123-Tyr-3-OCT and/or In-111 -DTPA-D-Phe-1-OCT were performed in 21 patients with histologically ve rified intestinal carcinoid rumours, Corresponding scintigraphic resul ts were obtained in 18 of 21 patients investigated with two different SSTR ligands, either I-123-OCT/I-123-Tyr-3-OCT (four of five), I-123-O CT/(111)InDTPA-D-Phe-1-OCT (eight of nine), or I-123-Tyr-3-OCT/In-111- DTPA-D-Phe-1-OCT (six of seven). We conclude that various rumours expr ess high amounts of SSTR which are recognized by three radiolabelled S ST analogues: I-123-OCT, I-123-Tyr-3-OCT and In-111-DTPA-D-Phe-1-OCT. Differences between these SST analogues in their in vitro binding and/ or in vivo scanning properties are observed in a minority of patients, Thus, the labelling of OCT with iodine may be an alternative approach for those nuclear medicine departments for which In-111-DTPA-D-Phe-1- OCT is not easily available, or is too expensive.