In vivo somatostatin receptor scintigraphy using Octreoscan is a valuable method for the visualisation of human endocrine tumours and their metastases. Recently, several new, alternative somatostatin radioligands have been synthesised for diagnostic and radiotherapeutic use in vivo. Since human tumours are known to express various somatostatin receptor subtypes, it is mandatory to assess the receptor subtype affinity profile of such somatostatin radiotracers. Using cell lines transfected with somatostatin receptor subtypes sst1, sst2, sst3, sst4 and sst5, we have evaluated the in vitro binding characteristics of labelled (indium, yttrium, gallium) and unlabelled DOTA-[Tyr3]-octreotide, DOTA-octreotide, DOTA-lanreotide, DOTA-vapreotide, DTPA-[Tyr3]-octreotate and DOTA-[Tyr3]-octreotate. Small structural modifications, chelator substitution or metal replacement were shown to considerably affect the binding affinity. A marked improvement of sst2 affinity was found for Ga-DOTA-[Tyr3]-octreotide (IC50 2.5 nM) compared with the Y-labelled compound and Octreoscan. An excellent binding affinity for sst2 in the same range was also found for In-DTPA-[Tyr3]-octreotate (IC50 1.3 nM) and for Y-DOTA-[Tyr3]-octreotate (IC50 1.6 nM). Remarkably, Ga-DOTA-[Tyr3]-octreotate bound at sst2 with a considerably higher affinity (IC50 0.2 nM). An up to 30-fold improvement in sst3 affinity was observed for unlabelled or Y-labelled DOTA-octreotide compared with their Tyr3-containing analogue, suggesting that replacement of Tyr3 by Phe is crucial for high sst3 affinity. Substitution in the octreotide molecule of the DTPA by DOTA improved the sst3 binding affinity 14-fold. Whereas Y-DOTA-lanreotide had only low affinity for sst3 and sst4, it had the highest affinity for sst5 among the tested compounds (IC50 16 nM). Increased binding affinity for sst3 and sst5 was observed for DOTA-[Tyr3]-octreotide, DOTA-lanreotide and DOTA-vapreotide when they were labelled with yttrium. These marked changes in subtype affinity profiles are due not only to the different chemical structures but also to the different charges and hydrophilicity of these compounds. Interestingly, even the coordination geometry of the radiometal complex remote from the pharmacophoric amino acids has a significant influence on affinity profiles as shown with Y-DOTA versus Ga-DOTA in either [Tyr3]-octreotide or [Tyr3]-octreotate. Such changes in sst affinity profiles must be identified in newly designed radiotracers used for somatostatin receptor scintigraphy in order to correctly interpret in vivo scintigraphic data. These observations may represent basic principles relevant to the development of other peptide radioligands.

Abstract.Earlier studies have shown that modification of the octapeptide octreotide in positions 3 and 8 may result in compounds with increased somatostatin receptor affinity that, if radiolabelled, display improved uptake in somatostatin receptor-positive tumours. The aim of a recent research study in our laboratory was to employ the parallel peptide synthesis approach by further exchanging the amino acid in position 3 of octreotide and coupling the macrocyclic chelator DOTA(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) to these peptides for labelling with radiometals like gallium-67 or -68, indium-111, yttrium-90 and lutetium-177. The purpose was to find radiopeptides with an improved somatostatin receptor binding profile in order to extend the spectrum of targeted tumours. In]DOTA-OC). The internalized radiopeptides were externalized intact upon 2 h of internalization followed by an acid wash. After 2-3 h of externalization a plateau is reached, indicating a steadystate situation explained by reactivation of the receptors followed by re-endocytosis. Biodistribution studies in CA 20948 tumour-bearing rats showed rapid clearance from all sstr-negative tissues except the kidneys. At 4 h the uptake of [ 111 In]DOTA-NOC in the tumour and sstr-positive tissues, such as adrenals, stomach and pancreas, was three to four times higher than that of [ 111 In]DOTA-TOC. Differential blocking studies indicate that this is at least partially due to the uptake mediated by sstr3 and sstr5. These very promising preclinical data justify the use of this new radiopeptide for imaging and potentially internal radiotherapy studies in patients.

Somatostatin analogs labeled with radionuclides are of considerable interest in the diagnosis and therapy of SSTRexpressing tumors, such as gastroenteropancreatic, small cell lung, breast and frequently nervous system tumors. In view of the favorable physical characteristics of the Ga isotopes 67 Ga and 68 Ga, enabling conventional tumor scintigraphy, PET and possibly internal radiotherapy, we focused on the development of a Ga-labeled somatostatin analog suit- SSTRs are overexpressed by a variety of neuroendocrine tumors and frequently by tumors of the nervous system, 1-4 making somatostatin analogs, such as OC, 5 attractive candidates for tumor targeting. 111 In-labeled DTPA-OC (OctreoScan) was the first radiopeptide routinely used in the clinic for imaging SSTR-positive tumors by scintigraphy. 6 -10 A new generation of somatostatin analogs, incorporating the macrocyclic chelator DOTA instead of DTPA, have been developed, which ensure better stability of the radiometal-peptide complex. After labeling with 90 Y (␤ --emitter 2.28 MeV, t 1 ⁄2 64.1 hr) or 111 In (␥-emitter, Auger-and conversionelectron emitter, 0.5-245 keV, t 1 ⁄2 67.9 hr), they showed improved biodistribution and tumor uptake in animal models [11][12][13] and their clinical utility as diagnostic and therapeutic tools was confirmed in patients. 14 -18 These findings prompted us and others to develop somatostatin analogs suitable for PET, which offers higher sensitivity and resolution than SPECT, making it possible to visualize very small metastatic lesions, including tumor deposits in regional draining lymph nodes. Several strategies have been studied to develop a somatostatin analog-based PET tracer using different positronemitting radionuclides. Wester et al. 19 successfully labeled OC with 18 F, but despite specific accumulation in the tumor, this radioligand was of limited clinical application, owing to fast tumor washout, high liver uptake and, hence, insufficient visualization of abdominal tumors. Various 64 Cu-labeled somatostatin analogs were synthesized which showed favorable biodistribution in animal models 20,21 and good performance for PET imaging in patients; 22 however, the use of 64 Cu relies on the availability of a cyclotron.Another interesting positron emitter is 68 Ga (t 1 ⁄2 68 min, ␤ ϩ 88%), which is produced by a 68 Ge/ 68 Ga generator available at most PET centers and is not dependent on a cyclotron. Gallium as radiometal is of even broader interest in nuclear medicine because it is also available as 67 Ga (t 1 ⁄2 78 hr), which is not only a ␥-emitter useful for tumor diagnosis (␥-camera scintigraphy, SPECT) but also an emitter of Auger (0.1-8 keV) and conversion (80 -90 keV) electrons, which makes it attractive for internal radiotherapy. 23 The radiotoxicity of 67 Ga has been demonstrated in vitro in a lymphoma cell line 24 and myeloid leukemic blasts. 25 The use of a low-energy emitter might increase the therapeutic index because most of the electrons deposit their energy within the target, i.e., the tumor, thereby min...

This study shows that (177)Lu-labeled [DOTA(0),Tyr(3)]octreotate has therapeutic potential for SCLC. The study also points out the importance of optimal labeling efficiency since the high bone marrow uptake of free lutetium ions can be controlled by a high peptide-bound fraction.

The somatostatin analogue DOTATOC, DOTA-[Tyr(3)]octreotide, is used for in vivo diagnosis and targeted therapy of somatostatin-receptor-positive tumors. DOTATOC consists of a disulfide-bridged octapeptide, d-Phe(1)-Cys(2)-Tyr(3)-d-Trp(4)-Lys(5)-Thr(6)-Cys(7)-Thr(8)-ol, connected to the metal chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). Two metal complexes, Ga(III)- and Y(III)-DOTATOC, were reported to differ significantly in somatostatin receptor affinity and tumor uptake. Our (1)H and (13)C solution NMR data and modeling studies of both compounds are in agreement with a fast conformational equilibrium of the peptide part, as previously reported for octreotide itself. However, the different coordination geometry of Ga(3+) and Y(3+) (6-fold and 8-fold, respectively, as known from model compounds) causes pronounced differences for the d-Phe(1) residue. For Y(III)-DOTATOC this leads to two conformers exchanging slowly on the NMR time scale. From various NMR measurements, they could be identified as cis-trans isomers at the amide bond between DOTA chelator and first residue (d-Phe(1)H(N)) of the peptide.