Upregulation of Key Molecules for Targeted Imaging and Therapy.

Targeted diagnosis and therapy enable precise tumor detection and treatment. Successful examples for precise tumor targeting are diagnostic and therapeutic radioligands. However, patients with tumors expressing low levels of the relevant molecular targets are deemed ineligible for such targeted approaches. METHODS: We performed a screen for drugs that upregulate the somatostatin receptor subtype 2 (sstr2). Then, we characterized the effects of these drugs on transcriptional, translational, and functional levels in vitro and in vivo. RESULTS: We identified 9 drugs that act as epigenetic modifiers, including the inhibitor of DNA methyltransferase decitabine as well as the inhibitors of histone deacetylase tacedinaline and romidepsin. In vitro, these drugs upregulated sstr2 on transcriptional, translational, and functional levels in a time- and dose-dependent manner. Thereby, their combinations revealed synergistic effects. In vivo, drug-based sstr2 upregulation improved the tumor-to-background and tumor-to-kidney ratios, which are the key determinants of successful sstr2-targeted imaging and radiopeptide therapy. CONCLUSION: We present an approach that uses epigenetic modifiers to improve sstr2 targeting in vitro and in vivo. Translation of this method into the clinic may potentially convert patients ineligible for targeted imaging and therapy to eligible candidates.

Evaluation of three different families of bombesin receptor radioantagonists for targeted imaging and therapy of gastrin releasing peptide receptor (GRP-R) positive tumors.

Two new classes of radiolabeled GRP receptor antagonists are studied and compared with the well-established statine-based receptor antagonist DOTA-4-amino-1-carboxymethylpiperidine-d-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2 (RM2, 1; DOTA:1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid; Sta:(3S,4S)-4-amino-3-hydroxy-6-methylheptanoic acid). The bombesin-based pseudopeptide DOTA-4-amino-1-carboxymethylpiperidine-d-Phe-Gln-Trp-Ala-Val-Gly-His-Leuψ(CHOH-CH2)-(CH2)2-CH3 (RM7, 2), and the methyl ester DOTA-4-amino-1-carboxymethylpiperidine-d-Phe-Gln-Trp-Ala-Val-Gly-His-Leu-OCH3 (ARBA05, 3) analogues are labeled with (111)In and evaluated in vitro in PC-3 cell line and in vivo in PC-3 tumor-bearing nude mice. Antagonist potency was assessed by immunofluorescence-based receptor internalization and Ca(2+) mobilization assays. The conjugates showed good binding affinity, the IC50 value of 2 (3.2 ± 1.8 nM) being 2 and 10 times lower than 1 and 3. Compared to (111)In-1, (111)In-2 showed higher uptake in target tissues such as pancreas (1.5 ± 0.5%IA/g and 39.8 ± 9.3%IA/g at 4 h, respectively), whereas the compounds had similar tumor uptake (11.5 ± 2.4%IA/g and 11.8 ± 3.9%IA/g at 4h, respectively). The displacement of the radioligand in vivo was different in different receptor positive organs and depended on the displacing peptide.

[111In-DOTA]LTT-SS28, a first pansomatostatin radioligand for in vivo targeting of somatostatin receptor-positive tumors.

Radiolabeled pansomatostatin-like analogues are expected to enhance the diagnostic sensitivity and to expand the clinical indications of currently applied sst2-specific radioligands. In this study, we present the somatostatin mimic [DOTA]LTT-SS28 {[(DOTA)Ser1,Leu8,D-Trp22,Tyr25]SS28} and its 111In radioligand. [DOTA]LTT-SS28 exhibited a pansomatostatin-like profile binding with high affinity to all five hsst1-hsst5 subtypes (IC50 values in the lower nanomolar range). Furthermore, [DOTA]LTT-SS28 behaved as an agonist at hsst2, hsst3, and hsst5, efficiently stimulating internalization of the three receptor subtypes. Radioligand [111In-DOTA]LTT-SS28 showed good stability in the mouse bloodstream. It displayed strong and specific uptake in AR42J tumors 4 h postinjection (9.3±1.6% ID/g vs 0.3±0.0% ID/g during sst2 blockade) in mice. Significant and specific uptake was also observed in HEK293-hsst2-, HEK293-hsst3-, and HEK293-hsst5-expressing tumors (4.43±1.5, 4.88±1.1, and <3% ID/g, respectively, with values of <0.5% ID/g during receptor blockade). In conclusion, the somatostatin mimic [111In-DOTA]LTT-SS28 specifically localizes in sst2-, sst3-, and sst5-expressing xenografts in mice showing promise for multi-sst1-sst5 targeted tumor imaging.

[DOTA]Somatostatin-14 analogs and their (111)In-radioligands: effects of decreasing ring-size on sst1-5 profile, stability and tumor targeting.

Multiple somatostatin receptor (sst)-subtype expression has been manifested in several human tumors. Hence, the availability of radiopeptides retaining the full pansomatostatin profile of the native hormone (SS14) is expected to increase the sensitivity and broaden the clinical indications of currently applied sst2-preferring cyclic octapeptide radioligands, like OctreoScan(®) ([(111)In-DTPA]octreotide). On the other hand, SS14 has been excluded from clinical use due to its rapid in vivo degradation. We herein present a small library of seven novel cyclic SS14-mimics carrying at their N-terminus the universal chelator DOTA (1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid) for stable binding of medically useful radiometals, like (111)In. By decreasing the number of amino acids composing the ring in their structure from 12 up to 6 AA, we induced important changes in key-biological parameters in vitro and in vivo. In particular, we observed unexpected changes and even total loss of sst1-5-affinity (6AA-ring), as well as weaker sst2-internalization efficacy as the ring size decreased. In contrast, in vivo stability increased with decreasing ring size, reaching its maximum in the 6AA-ring analogs. Interestingly, only the 12AA- and 9AA-ring members of this series showed sst2-specific uptake in AR4-2J tumors in mice revealing the prominent role of ring size on the biological response of tested SS14-derived radioligands.

Syntheses, receptor bindings, in vitro and in vivo stabilities and biodistributions of DOTA-neurotensin(8-13) derivatives containing ß-amino acid residues - a lesson about the importance of animal experiments.

Neurotensin(8-13) (NTS(8-13)) analogs with C- and/or N-terminal ß-amino acid residues and three DOTA derivatives thereof have been synthesized (i.e., 1-6). A virtual docking experiment showed almost perfect fit of one of the 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) derivatives, 6a, into a crystallographically identified receptor NTSR1 (Fig.1). The affinities for the receptors of the NTS analogs and derivatives are low, when determined with cell-membrane homogenates, while, with NTSR1-exhibiting cancer tissues, affinities in the single-digit nanomolar range can be observed (Table 2). Most of the ß-amino acid-containing NTS(8-13) analogs (Table 1 and Fig.2), including the (68) Ga complexes of the DOTA-substituted ones (6; Figs.2 and 5), are stable for ca. 1 h in human serum and plasma, and in murine plasma. The biodistributions of two (68) Ga complexes (of 6a and 6b) in HT29 tumor-bearing nude mice, in the absence and in the presence of a blocking compound, after 10, 30, and 60 min (Figs. 3 and 4) lead to the conclusion that the amount of specifically bound radioligand is rather low. This was confirmed by PET-imaging experiments with the tumor-bearing mice (Fig.6). Comparison of the in vitro plasma stability (after 1 h) with the ex vivo blood content (after 10-15 min) of the two (68) Ga complexes shows that they are rapidly cleaved in the animals (Fig.5).

[(99m)Tc]Demomedin C, a radioligand based on human gastrin releasing peptide(18-27): synthesis and preclinical evaluation in gastrin releasing peptide receptor-expressing models.

The synthesis and preclinical evaluation of [(99m)Tc]Demomedin C in GRPR-expressing models are reported. Demomedin C resulted by coupling a Boc-protected N(4)-chelator to neuromedin C (human GRP(18-27)), which, after (99m)Tc-labeling, afforded [(99m)Tc]Demomedin C. Demomedin C showed high affinity and selectivity for the GRPR during receptor autoradiography on human cancer samples (IC(50) in nM: GRPR, 1.4 ± 0.2; NMBR, 106 ± 18; and BB(3)R, >1000). It triggered GRPR internalization in HEK-GRPR cells and Ca(2+) release in PC-3 cells (EC(50) = 1.3 nM). [(99m)Tc]Demomedin C rapidly and specifically internalized at 37 °C in PC-3 cells and was stable in mouse plasma. [(99m)Tc]Demomedin C efficiently and specifically localized in human PC-3 implants in mice (9.84 ± 0.81%ID/g at 1 h pi; 6.36 ± 0.85%ID/g at 4 h pi, and 0.41 ± 0.07%ID/g at 4 h pi block). Thus, human GRP-based radioligands, such as [(99m)Tc]Demomedin C, can successfully target GRPR-expressing human tumors in vivo while displaying attractive biological features--e.g. higher GRPR-selectivity--vs their frog-homologues.

Unexpected sensitivity of sst2 antagonists to N-terminal radiometal modifications.

Chelated somatostatin agonists have been shown to be sensitive to N-terminal radiometal modifications, with Ga-DOTA agonists having significantly higher binding affinity than their Lu-, In-, and Y-DOTA correlates. Recently, somatostatin antagonists have been successfully developed as alternative tracers to agonists. The aim of this study was to evaluate whether chelated somatostatin antagonists are also sensitive to radiometal modifications and how. We have synthesized 3 different somatostatin antagonists, DOTA-p-NO(2)-Phe-c[D-Cys-Tyr-D-Aph(Cbm)-Lys-Thr-Cys]-D-Tyr-NH(2), DOTA-Cpa-c[D-Cys-Aph(Hor)-D-Aph(Cbm)-Lys-Thr-Cys]-D-Tyr-NH(2) (DOTA-JR11), and DOTA-p-Cl-Phe-c[D-Cys-Tyr-D-Aph(Cbm)-Lys-Thr-Cys]-D-Tyr-NH(2), and added various radiometals including In(III), Y(III), Lu(III), Cu(II), and Ga(III). We also replaced DOTA with 1,4,7-triazacyclononane,1-glutaric acid-4,7-acetic acid (NODAGA) and added Ga(III). The binding affinity of somatostatin receptors 1 through 5 was evaluated in all cases. In all 3 resulting antagonists, the Ga-DOTA analogs were the lowest-affinity radioligands, with a somatostatin receptor 2 binding affinity up to 60 times lower than the respective Y-DOTA, Lu-DOTA, or In-DOTA compounds. Interestingly, however, substitution of DOTA by the NODAGA chelator was able to increase massively its binding affinity in contrast to the Ga-DOTA analog. The 3 NODAGA analogs are antagonists in functional tests. In vivo biodistribution studies comparing (68)Ga-DOTATATE agonist with (68)Ga-DOTA-JR11 and (68)Ga-NODAGA-JR11 showed not only that the JR11 antagonist radioligands were superior to the agonist ligands but also that (68)Ga-NODAGA-JR11 was the tracer of choice and preferable to (68)Ga-DOTA-JR11 in transplantable HEK293-hsst(2) tumors in mice. One may therefore generalize that somatostatin receptor 2 antagonists are sensitive to radiometal modifications and may preferably be coupled with a (68)Ga-NODAGA chelator-radiometal complex.

[111In-DOTA]Somatostatin-14 analogs as potential pansomatostatin-like radiotracers - first results of a preclinical study.

BACKGROUND: In this study, we report on the synthesis, radiolabeling, and biological evaluation of two new somatostatin-14 (SS14) analogs, modified with the universal chelator DOTA. We were interested to investigate if and to what extent such radiotracer prototypes may be useful for targeting sst1-5-expressing tumors in man but, most importantly, to outline potential drawbacks and benefits associated with their use. METHODS: AT1S and AT2S (DOTA-Ala1-Gly2-c[Cys3-Lys4-Asn5-Phe6-Phe7-Trp8/DTrp8-Lys9-Thr10-Phe11-Thr12-Ser13-Cys14-OH], respectively) were synthesized on the solid support and labeled with 111In. The sst1-5 affinity profile of AT1S/AT2S was determined by receptor autoradiography using [Leu8,dTrp22,125I-Tyr25]SS28 as radioligand. The ability of AT2S to stimulate sst2 or sst3 internalization was qualitatively analyzed by an immunofluorescence-based internalization assay using hsst2- or hsst3-expressing HEK293 cells. Furthermore, the internalization of the radioligands [111In]AT1S and [111In]AT2S was studied at 37 °C in AR4-2J cells endogenously expressing sst2. The in vivo stability of [111In]AT1S and [111In]AT2S was tested by high-performance liquid chromatography analysis of mouse blood collected 5 min after radioligand injection, and biodistribution was studied in normal mice. Selectively for [111In]AT2S, biodistribution was further studied in SCID mice bearing AR4-2J, HEK293-hsst2A+, -hsst3+ or -hsst5+ tumors. RESULTS: The new SS14-derived analogs were obtained by solid phase peptide synthesis and were easily labeled with 111In. Both SS14 conjugates, AT1S, and its DTrp8 counterpart, AT2S, showed a pansomatostatin affinity profile with the respective hsst1-5 IC50 values in the lower nanomolar range. In addition, AT2S behaved as an agonist for sst2 and sst3 since it stimulated receptor internalization. The 111In radioligands effectively and specifically internalized into rsst2A-expressing AR4-2J cells with [111In]AT2S internalizing faster than [111In]AT1S. Ex vivo mouse blood analysis revealed a rapid degradation of both radiopeptides in the bloodstream with the DTrp8 analog showing higher stability. Biodistribution results in healthy mice were consistent with these findings with only [111In]AT2S showing specific uptake in the sst2-rich pancreas. Biodistribution of [111In]AT2S in tumor-bearing mice revealed receptor-mediated uptake in the AR4-2J (1.82 ± 0.36 %ID/g - block 0.21 ± 0.17 %ID/g at 4 h post injection (pi)), the HEK293-hsst2A+ (1.49 ± 0.2 %ID/g - block 0.27 ± 0.20 %ID/g at 4 h pi), the HEK293-hsst3+ (1.24 ± 0.27 %ID/g - block 0.32 ± 0.06 %ID/g at 4 h pi), and the HEK293-hsst5+ tumors (0.41 ± 0.12 %ID/g - block 0.22 ± 0.006 %ID/g at 4 h pi). Radioactivity washed out from blood and background tissues via the kidneys. CONCLUSIONS: This study has revealed that the native SS14 structure can indeed serve as a motif for the development of promising pansomatostatin-like radiotracers. Further peptide stabilization is required to increase in vivo stability and, consequently, to enhance in vivo delivery and tumor targeting.

Phosphorylation of sst2 receptors in neuroendocrine tumors after octreotide treatment of patients.

Somatostatin analogues, which are used to treat neuroendocrine tumors, target the high levels of somatostatin receptor subtype 2 (SSTR1; alias sst2) expressed in these cancers. However, some tumors are resistant to somatostatin analogues, and it is unknown whether the defect lies in sst2 activation or downstream signaling events. Because sst2 phosphorylation occurs rapidly after receptor activation, we examined whether sst2 is phosphorylated in neuroendocrine tumors. The sst2 receptor phosphorylation was evaluated by IHC and Western blot analysis with the new Ra-1124 antibody specific for the sst2 receptor phosphorylated at Ser341/343 in receptor-positive neuroendocrine tumors obtained from 10 octreotide-treated and 7 octreotide-naïve patients. The specificity, time course, and subcellular localization of sst2 receptor phosphorylation were examined in human embryo kinase-sst2 cell cultures by immunofluorescence and confocal microscopy. All seven octreotide-naïve tumors displayed exclusively nonphosphorylated cell surface sst2 expression. In contrast, 9 of the 10 octreotide-treated tumors contained phosphorylated sst2 that was predominantly internalized. Western blot analysis confirmed the IHC data. Octreotide treatment of human embryo kinase-sst2 cells in culture demonstrated that phosphorylated sst2 was localized at the plasma membrane after 10 seconds of stimulation and was subsequently internalized into endocytic vesicles. These data show, for the first time to our knowledge, that phosphorylated sst2 is present in most gastrointestinal neuroendocrine tumors from patients treated with octreotide but that a striking variability exists in the subcellular distribution of phosphorylated receptors among such tumors.

Evaluation of 177Lu-DOTA-sst2 antagonist versus 177Lu-DOTA-sst2 agonist binding in human cancers in vitro.

UNLABELLED: Somatostatin receptor targeting of neuroendocrine tumors using radiolabeled somatostatin agonists is today an established method to image and treat cancer patients. However, in a study using an animal tumor model, somatostatin receptor antagonists were shown to label sst(2)- and sst(3)-expressing tumors in vivo better than agonists, with comparable affinity even though they are not internalized into the tumor cell. In the present study, we evaluated the in vitro binding of the antagonist (177)Lu-DOTA-pNO(2)-Phe-c (DCys-Tyr-DTrp-Lys-Thr-Cys) DTyrNH(2) ((177)Lu-DOTA-BASS) or the (177)Lu-DOTATATE agonist to sst(2)-expressing human tumor samples. METHODS: Forty-eight sst(2)-positive human tumor tissue samples (9 ileal carcinoids, 10 pheochromocytomas, 7 breast carcinomas, 10 renal cell carcinomas, and 12 non-Hodgkin lymphomas) were analyzed by in vitro receptor autoradiography for the expression of sst(2), comparing the binding capacity of (177)Lu-DOTA-BASS and (177)Lu-DOTATATE in successive tissue sections. The autoradiograms were quantitated using an electronic autoradiography detection system. RESULTS: In all cases, the radiolabeled antagonist bound to more receptor sites than did the agonist. The mean ratios of the antagonist (177)Lu-DOTA-BASS to the agonist (177)Lu-DOTATATE were 4.2 ± 0.5 in the 9 ileal carcinoids, 12 ± 3 in the 10 pheochromocytomas, 11 ± 4 in the 7 breast carcinomas, 5.1 ± 0.6 in the 10 renal cell carcinomas, and 4.8 ± 0.7 in the 12 non-Hodgkin lymphomas. CONCLUSION: The present in vitro human data, together with previous in vivo animal tumor data, are strong arguments indicating that sst(2) antagonists may be worth testing in vivo in patients in a wide range of tumors including nonneuroendocrine tumors.

Bombesin antagonist-based radioligands for translational nuclear imaging of gastrin-releasing peptide receptor-positive tumors.

UNLABELLED: Bombesin receptors are overexpressed on a variety of human tumors. In particular, the gastrin-releasing peptide receptor (GRPr) has been identified on prostate and breast cancers and on gastrointestinal stromal tumors. The current study aims at developing clinically translatable bombesin antagonist-based radioligands for SPECT and PET of GRPr-positive tumors. METHODS: A potent bombesin antagonist (PEG(4)-D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH(2) [AR]) was synthesized; conjugated to the chelators DOTA, 6-carboxy-1,4,7,11-tetraazaundecane (N4), 1,4,7-triazacyclononane, 1-glutaric acid-4,7 acetic acid (NODAGA), and 4,11-bis(carboxymethyl)-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (CB-TE2A); and radiolabeled with (111)In, (99m)Tc, (68)Ga, and (64)Cu, respectively. The radioconjugates were evaluated in vitro and in vivo in PC-3 tumor-bearing nude mice. Antagonist potency was determined by Ca(2+)-flux measurements and immunofluorescence. RESULTS: All the conjugates showed high binding affinity to GRPr (inhibitory concentration of 50% [IC(50)], 2.5-25 nmol/L). The immunofluorescence and Ca(2+)-flux assays confirmed the antagonist properties of the conjugates. Biodistribution revealed high and specific uptake in PC-3 tumor and in GRPr-positive tissues. Tumor uptake of (64)Cu-CB-TE2A-AR (31.02 ± 3.35 percentage injected activity per gram [%IA/g]) was higher than (99m)Tc-N4-AR (24.98 ± 5.22 %IA/g), (111)In-DOTA-AR (10.56 ± 0.70 %IA/g), and (68)Ga-NODAGA-AR (7.11 ± 3.26 %IA/g) at 1 h after injection. Biodistribution at later time points showed high tumor-to-background ratios because of the fast washout of the radioligand from normal organs, compared with tumor. High tumor-to-background ratios were further illustrated by PET and SPECT images of PC-3 tumor-bearing nude mice acquired at 12 h after injection showing high tumor uptake, clear background, and negligible or no radioactivity in the abdomen. CONCLUSION: The chelators do influence the affinity, antagonistic potency, and pharmacokinetics of the conjugates. The promising preclinical results warrant clinical translation of these probes for SPECT and PET.

N-imidazolebenzyl-histidine substitution in somatostatin and in its octapeptide analogue modulates receptor selectivity and function.

Despite 3 decades of focused chemical, biological, structural, and clinical developments, unusual properties of somatostatin (SRIF, 1) analogues are still being uncovered. Here we report the unexpected functional properties of 1 and the octapeptide cyclo(3-14)H-Cys-Phe-Phe-Trp(8)-Lys-Thr-Phe-Cys-OH (somatostatin numbering; OLT-8, 9) substituted by imBzl-l- or -d-His at position 8. These analogues were tested for their binding affinity to the five human somatostatin receptors (sst(1-5)), as well as for their functional properties (or functionalities) in an sst(3) internalization assay and in an sst(3) luciferase reporter gene assay. While substitution of Trp(8) in somatostatin by imBzl-l- or -d-His(8) results in sst(3) selectivity, substitution of Trp(8) in the octapeptide 9 by imBzl-l- or -d-His(8) results in loss of binding affinity for sst(1,2,4,5) and a radical functional switch from agonist to antagonist.

PET of somatostatin receptor-positive tumors using 64Cu- and 68Ga-somatostatin antagonists: the chelate makes the difference.

UNLABELLED: Somatostatin-based radiolabeled peptides have been successfully introduced into the clinic for targeted imaging and radionuclide therapy of somatostatin receptor (sst)-positive tumors, especially of subtype 2 (sst2). The clinically used peptides are exclusively agonists. Recently, we showed that radiolabeled antagonists may be preferable to agonists because they showed better pharmacokinetics, including higher tumor uptake. Factors determining the performance of radioantagonists have only scarcely been studied. Here, we report on the development and evaluation of four (64)Cu or (68)Ga radioantagonists for PET of sst2-positive tumors. METHODS: The novel antagonist p-Cl-Phe-cyclo(D-Cys-Tyr-D-4-amino-Phe(carbamoyl)-Lys-Thr-Cys)D-Tyr-NH(2) (LM3) was coupled to 3 macrocyclic chelators, namely 4,11-bis(carboxymethyl)-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane (CB-TE2A), 1,4,7-triazacyclononane,1-glutaric acid-4,7-acetic acid (NODAGA), and DOTA. (64/nat)Cu- and (68/nat)Ga-NODAGA-LM3 were prepared at room temperature, and (64/nat)Cu-CB-TE2A-LM3 and (68/nat)Ga-DOTA-LM3 were prepared at 95°C. Binding affinity and antagonistic properties were determined with receptor autoradiography and immunofluorescence microscopy using human embryonic kidney (HEK)-sst2 cells. In vitro internalization and dissociation was evaluated using the same cell line. Biodistribution and small-animal PET studies were performed with HEK-sst2 xenografts. RESULTS: All metallopeptides demonstrated antagonistic properties. The affinities depend on chelator and radiometal and vary about 10-fold; (68/nat)Ga-NODAGA-LM3 has the lowest half maximal inhibitory concentration (1.3 ± 0.3 nmol/L). The biodistribution studies show impressive tumor uptake at 1 h after injection, particularly of (64)Cu- and (68)Ga-NODAGA-LM3 (∼40 percentage injected dose per gram of tissue [%ID/g]), which were proven to be specific. Background clearance was fast and the tumor washout relatively slow for (64)Cu-NODAGA-LM3 (∼15 %ID/g, 24 h after injection) and almost negligible for (64)Cu-CB-TE2A-LM3 (26.9 ± 3.3 %ID/g and 21.6 ± 2.1 %ID/g, 4 and 24 h after injection, respectively). Tumor-to-normal-tissue ratios were significantly higher for (64)Cu-NODAGA-LM3 than for (64)Cu-CB-TE2A-LM3 (tumor-to-kidney, 12.8 ± 3.6 and 1.7 ± 0.3, respectively; tumor-to-muscle, 1,342 ± 115 and 75.2 ± 8.5, respectively, at 24 h, P < 0.001). Small-animal PET shows clear tumor localization and high image contrast, especially for (64)Cu- and (68)Ga-NODAGA-LM3. CONCLUSION: This article demonstrates the strong dependence of the affinity and pharmacokinetics of the somatostatin-based radioantagonists on the chelator and radiometal. (64)Cu- and (68)Ga-NODAGA-LM3 and (64)Cu-CB-TE2A-LM3 are promising candidates for clinical translation because of their favorable pharmacokinetics and the high image contrast on PET scans.

On the terminal homologation of physiologically active peptides as a means of increasing stability in human serum--neurotensin, opiorphin, B27-KK10 epitope, NPY.

The terminal homologation by CH(2) insertion into the peptides mentioned in the title is described. This involves replacement of the N-terminal amino acid residue by a ß(2) - and of the C-terminal amino acid residue by a ß(3) -homo-amino acid moiety (ß(2) hXaa and ß(3) hXaa, resp.; Fig. 1). In this way, the structure of the peptide chain from the N-terminal to the C-terminal stereogenic center is identical, and the modified peptide is protected against cleavage by exopeptidases (Figs. 2 and 3). Neurotensin (NT; 1) and its C-terminal fragment NT(8-13) are ligands of the G-protein-coupled receptors (GPCR) NT1, NT2, NT3, and NT analogs are promising tools to be used in cancer diagnostics and therapy. The affinities of homologated NT analogs, 2b-2e, for NT1 and NT2 receptors were determined by using cell homogenates and tumor tissues (Table 1); in the latter experiments, the affinities for the NT1 receptor are more or less the same as those of NT (0.5-1.3 vs. 0.6 nM). At the same time, one of the homologated NT analogs, 2c, survives in human plasma for 7 days at 37° (Fig. 6). An NMR analysis of NT(8-13) (Tables 2 and 4, and Fig. 8) reveals that this N-terminal NT fragment folds to a turn in CD(3) OH. - In the case of the human analgesic opiorphin (3a), a pentapeptide, and of the HIV-derived B27-KK10 (4a), a decapeptide, terminal homologation (→3b and 4b, resp.) led to a 7- and 70-fold half-life increase in plasma (Fig. 9). With N-terminally homologated NPY, 5c, we were not able to determine serum stability; the peptide consisting of 36 amino acid residues is subject to cleavage by endopetidases. Three of the homologated compounds, 2b, 2c, and 5c, were shown to be agonists (Fig. 7 and 11). A comparison of terminal homologation with other stability-increasing terminal modifications of peptides is performed (Fig. 5), and possible applications of the neurotensin analogs, described herein, are discussed.

Development of a potent DOTA-conjugated bombesin antagonist for targeting GRPr-positive tumours.

PURPOSE: Radiolabelled somatostatin-based antagonists show a higher uptake in tumour-bearing mouse models than agonists of similar or even distinctly higher receptor affinity. Very similar results were obtained with another family of G protein-coupled receptor ligands, the bombesin family. We describe a new conjugate, RM2, with the chelator DOTA coupled to D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH(2) via the cationic spacer 4-amino-1-carboxymethyl-piperidine for labelling with radiometals such as (111)In and (68)Ga. METHODS: RM2 was synthesized on a solid support and evaluated in vitro in PC-3 cells. IC(50) and K(d) values were determined. The antagonist potency was evaluated by immunofluorescence-based internalization and Ca(2+) mobilization assays. Biodistribution studies were performed in PC-3 and LNCaP tumour-bearing mice with (111)In-RM2 and (68)Ga-RM2, respectively. PET/CT studies were performed on PC-3 and LNCaP tumour-bearing nude mice with (68)Ga-RM2. RESULTS: RM2 and (111)In-RM2 are high-affinity and selective ligands for the GRP receptor (7.7 ± 3.3 nmol/l for RM2; 9.3 ± 3.3 nmol/l for (nat)In-RM2). The potent antagonistic properties were confirmed by an immunofluorescence-based internalization and Ca(2+) mobilization assays. (68)Ga- and (111)In-RM2 showed high and specific uptake in both the tumour and the pancreas. Uptake in the tumour remained high (15.2 ± 4.8%IA/g at 1 h; 11.7 ± 2.4%IA/g at 4 h), whereas a relatively fast washout from the pancreas and the other abdominal organs was observed. Uptake in the pancreas decreased rapidly from 22.6 ± 4.7%IA/g at 1 h to 1.5 ± 0.5%IA/g at 4 h. CONCLUSION: RM2 was shown to be a potent GRPr antagonist. Pharmacokinetics and imaging studies indicate that (111)In-RM2 and (68)Ga-RM2 are ideal candidates for clinical SPECT and PET studies.

Novel dimeric DOTA-coupled peptidic Y1-receptor antagonists for targeting of neuropeptide Y receptor-expressing cancers.

BACKGROUND: Several peptide hormone receptors were identified that are specifically over-expressed on the cell surface of certain human tumors. For example, high incidence and density of the Y1 subtype of neuropeptide Y (NPY) receptors are found in breast tumors. Recently, we demonstrated that the use of potent radiolabeled somatostatin or bombesin receptor antagonists considerably improved the sensitivity of in vivo imaging when compared to agonists. We report here on the first DOTA-coupled peptidic Y1 receptor affine dimer antagonists. METHODS: Based on a Y1 affine dimeric peptide scaffold previously reported to competitively antagonize NPY-mediated processes, we have developed new dimeric DOTA-coupled Y1 receptor affine antagonists for scintigraphy and radiotherapy. These dimeric peptides were tested for their specific binding to Y1 expressed in SK-N-MC cells and Y2 expressed in SH-SY5Y as well as for their ability to mediate cAMP production in SK-N-MC cells. RESULTS: Introduction of two DOTA moieties at the N-termini of the dimeric NPY analogs as well as the double Asn29 replacement by Dpr(DOTA) or Lys(DOTA) (6 and 10) moiety dramatically reduced binding affinity. However, asymmetric introduction of the DOTA moiety in one segment of the peptidic heterodimer (8 and 11) resulted in suitable antagonists for receptor targeting with high binding affinity for Y1. All compounds were devoid of Y2 binding affinity. CONCLUSIONS: The design and the in vitro characterization of the first DOTA-coupled dimeric NPY receptor antagonist with high affinity and selectivity for Y1 over Y2 are described. This compound may be an excellent candidate for the imaging of Y1-positive tumors and their treatment.

Radiolabeled bicyclic somatostatin-based analogs: a novel class of potential radiotracers for SPECT/PET of neuroendocrine tumors.

UNLABELLED: A variety of radiolabeled somatostatin analogs have been developed for targeting of somatostatin receptor (sst)-positive tumors. Bicyclic somatostatin-based radiopeptides have not been studied yet. Hypothesizing that the introduction of conformational constraints may lead to receptor subtype selectivity or may help to delineate structural features determining pansomatostatin potency, we developed and evaluated first examples of this new class of potential radiotracers for imaging or therapy of neuroendocrine tumors. METHODS: The bicyclic peptides were synthesized by standard solid-phase peptide synthesis. DOTA was coupled to the resin-assembled peptide for labeling with (177)Lu and (68)Ga. Binding affinity and receptor subtype profile were determined using human ssts. Ca(2+) flux, internalization, and efflux were studied in human embryonic kidney (HEK)-sst(2) and HEK-sst(3) cell lines. Biodistribution and PET/CT studies were performed in corresponding nude mice models. RESULTS: Some of the new analogs showed high affinity for sst(2) and sst(3) and moderate affinity for sst(1), sst(4), and sst(5), while exhibiting agonistic properties. The analog AM3, comprising an octreotide ring and a head-to-tail-coupled Arg-diaminobutyric acid(DOTA) cycle, showed the highest receptor affinity and agonist potency. (177)Lu-AM3 showed high and receptor-mediated uptake in vivo in sst(2) and sst(3) tumors with low background. Kidneys were the only other tissue accumulating radioactivity that could be reduced by a preinjection of lysine. PET/CT studies of (68)Ga-AM3 at 1 h after injection were characterized by clear localization of the tumor, visualization of the kidneys, and negligible background. CONCLUSION: The high rigidity of these new bicyclic somatostatin-based radiopeptides led to agonistic ligands with good affinity for all 5 ssts. The pharmacokinetic data of (177)Lu/(68)Ga-AM3 make this peptide an excellent candidate as an imaging--and especially as a PET--radiotracer.

Absence of somatostatin SST(2) receptor internalization in vivo after intravenous SOM230 application in the AR42J animal tumor model.

Among clinically relevant somatostatin functions, agonist-induced somatostatin receptor subtype 2 (sst(2)) internalization is a potent mechanism for tumor targeting with sst(2) affine radioligands such as octreotide. Since, as opposed to octreotide, the second generation multi-somatostatin analog SOM230 (pasireotide) exhibits strong functional selectivity, it appeared of interest to evaluate its ability to affect sst(2) internalization in vivo. Rats bearing AR42J tumors endogenously expressing somatostatin sst(2) receptors were injected intravenously with SOM230 or with the [Tyr(3), Thr(8)]-octreotide (TATE) analog; they were euthanized at various time points; tumors and pancreas were analyzed by immunohistochemistry for the cellular localization of somatostatin sst(2) receptors. SOM230-induced sst(2) internalization was also evaluated in vitro by immunofluorescence microscopy in AR42J cells. At difference to the efficient in vivo sst(2) internalization triggered by intravenous [Tyr(3), Thr(8)]-octreotide, intravenous SOM230 did not elicit sst(2) internalization: immunohistochemically stained sst(2) in AR42J tumor cells and pancreatic cells were detectable at the cell surface at 2.5min, 10min, 1h, 6h, or 24h after SOM230 injection while sst(2) were found intracellularly after [Tyr(3), Thr(8)]-octreotide injection. The inability of stimulating sst(2) internalization by SOM230 was confirmed in vitro in AR42J cells by immunofluorescence microscopy. Furthermore, SOM230 was unable to antagonize agonist-induced sst(2) internalization, neither in vivo, nor in vitro. Therefore, SOM230 does not induce sst(2) internalization in vivo or in vitro in AR42J cells and pancreas, at difference to octreotide derivatives with comparable sst(2) binding affinities. These characteristics may point towards different tumor targeting but also to different desensitization properties of clinically applied SOM230.

Switch from antagonist to agonist after addition of a DOTA chelator to a somatostatin analog.

PURPOSE: Peptide receptor targeting has become an increasingly attractive method to target tumors diagnostically and radiotherapeutically. Peptides linked to a variety of chelators have been developed for this purpose. They have, however, rarely been tested for their agonistic or antagonistic properties. We report here on a somatostatin antagonist that switched to an agonist upon coupling to a DOTA chelator. METHODS: Two novel somatostatin analogs, 406-040-15 and its DOTA-coupled counterpart 406-051-20, with and without cold Indium labeling, were tested for their somatostatin receptor subtypes 1-5 (sst(1)-sst(5)) binding affinity using receptor autoradiography. Moreover, they were tested functionally for their ability to affect sst(2) and sst(3) internalization in vitro in HEK293 cells stably expressing the human sst(2) or sst(3) receptor, using an immunofluorescence microscopy-based internalization assay. RESULTS: All three compounds were characterized as pan-somatostatin analogs having a high affinity for all five sst. In the sst(2) internalization assay, all three compounds showed an identical behavior, namely, a weak agonistic effect complemented by a weak antagonistic effect, compatible with the behavior of a partial agonist. Conversely, in the sst(3) internalization assay, 406-040-15 was a full antagonist whereas its DOTA-coupled counterpart, 406-051-20, with and without Indium labeling, switched to a full agonist. CONCLUSION: Adding the DOTA chelator to the somatostatin analog 406-040-15 triggers a switch at sst(3) receptor from an antagonist to an agonist. This indicates that potential radioligands for tumor targeting should always be tested functionally before further development, in particular if a chelator is added.

Internalized somatostatin receptor subtype 2 in neuroendocrine tumors of octreotide-treated patients.

CONTEXT: Somatostatin receptor subtype 2 (sst(2)) is widely expressed in neuroendocrine tumors and can be visualized immunohistochemically at the cell membrane for diagnostic purposes. Recently, it has been demonstrated in animal sst(2) tumor models in vivo that somatostatin analog treatment was able to induce a complete internalization of the tumor sst(2). PATIENTS AND METHODS: In the present study, we evaluated whether sst(2) expressed in neuroendocrine tumors of patients treated with octreotide are also internalized. Tumor samples were assessed in patients that were treated with various octreotide modalities before and during surgery and compared with tumor samples from untreated patients. Sst(2) immunohistochemistry was performed in all samples with three different sst(2) antibodies (R2-88, UMB-1, and SS-800). Sst(2) receptor expression was confirmed by immunoblotting and in vitro receptor autoradiography. RESULTS: Patients receiving a high dose of octreotide showed predominantly internalized sst(2), and patients with a low dose of octreotide had a variable ratio of internalized vs. membranous sst(2), whereas untreated patients had exclusively membranous sst(2). The internalized sst(2) receptor corresponded to a single sst(2) band in immunoblots and to sst(2) receptors in in vitro receptor autoradiography. Although generally found in endosome-like structures, internalized sst(2) receptors were also identified to a small extent in lysosomes, as seen in colocalization experiments. CONCLUSION: It is the first evidence showing that sst(2) receptors can be internalized in sst(2)-expressing neuroendocrine tumors in patients under octreotide therapy, providing clues about sst(2) receptor biology and trafficking dynamics in patients.