Neuropeptide FF receptors as novel targets for limbic seizure attenuation.

Neuropeptide Y (NPY) is a well established anticonvulsant and first-in-class antiepileptic neuropeptide. In this study, the controversial role of NPY1 receptors in epilepsy was reassessed by testing two highly selective NPY1 receptor ligands and a mixed NPY1/NPFF receptor antagonist BIBP3226 in a rat model for limbic seizures. While BIBP3226 significantly attenuated the pilocarpine-induced seizures, neither of the highly selective NPY1 receptor ligands altered the seizure severity. Administration of the NPFF1/NPFF2 receptor antagonist RF9 also significantly attenuated limbic seizure activity. To further prove the involvement of NPFF receptors in these seizure-modulating effects, low and high affinity antagonists for the NPFF receptors were tested. We observed that the low affinity ligand failed to exhibit anticonvulsant properties while the two high affinity ligands significantly attenuated the seizures. Continuous NPFF1 receptor agonist administration also inhibited limbic seizures whereas bolus administration of the NPFF1 receptor agonist was without effect. This suggests that continuous agonist perfusion could result in NPFF1 receptor desensitization and mimic NPFF1 receptor antagonist administration. Our data unveil for the first time the involvement of the NPFF system in the management of limbic seizures.

PEGylation, increasing specific activity and multiple dosing as strategies to improve the risk-benefit profile of targeted radionuclide therapy with 177Lu-DOTA-bombesin analogues.

BACKGROUND: Radiolabelled bombesin (BN) conjugates are promising radiotracers for imaging and therapy of breast and prostate tumours, in which BN2/gastrin-releasing peptide receptors are overexpressed. We describe the influence of the specific activity of a 177Lu-DOTA-PEG5k-Lys-B analogue on its therapeutic efficacy and compare it with its non-PEGylated counterpart. METHODS: Derivatisation of a stabilised DOTA-BN(7-14)[Cha13,Nle14] analogue with a linear PEG molecule of 5 kDa (PEG5k) was performed by PEGylation of the ϵ-amino group of a ß3hLys-ßAla-ßAla spacer between the BN sequence and the DOTA chelator. The non-PEGylated and the PEGylated analogues were radiolabelled with 177Lu. In vitro evaluation was performed in human prostate carcinoma PC-3 cells, and in vivo studies were carried out in nude mice bearing PC-3 tumour xenografts. Different specific activities of the PEGylated BN analogue and various dose regimens were evaluated concerning their therapeutic efficacy. RESULTS: The specificity and the binding affinity of the BN analogue for BN2/GRP receptors were only slightly reduced by PEGylation. In vitro binding kinetics of the PEGylated analogue was slower since steady-state condition was reached after 4 h. PEGylation improved the stability of BN conjugate in vitro in human plasma by a factor of 5.6. The non-PEGylated BN analogue showed favourable pharmacokinetics already, i.e. fast blood clearance and renal excretion, but PEGylation improved the in vivo behaviour further. One hour after injection, the tumour uptake of the PEG5k-BN derivative was higher compared with that of the non-PEGylated analogue (3.43 ± 0.63% vs. 1.88 ± 0.4% ID/g). Moreover, the increased tumour retention resulted in a twofold higher tumour accumulation at 24 h p.i., and increased tumour-to-non-target ratios (tumour-to-kidney, 0.6 vs. 0.4; tumour-to-liver, 8.8 vs. 5.9, 24 h p.i.). In the therapy study, both 177Lu-labelled BN analogues significantly inhibited tumour growth. The therapeutic efficacy was highest for the PEGylated derivative of high specific activity administered in two fractions (2 × 20 MBq = 40 MBq) at day 0 and day 7 (73% tumour growth inhibition, 3 weeks after therapy). CONCLUSIONS: PEGylation and increasing the specific activity enhance the pharmacokinetic properties of a 177Lu-labelled BN-based radiopharmaceutical and provide a protocol for targeted radionuclide therapy with a beneficial anti-tumour effectiveness and a favourable risk-profile at the same time.

PEGylation of (99m)Tc-labeled bombesin analogues improves their pharmacokinetic properties.

INTRODUCTION: Radiolabeled bombesin (BN) conjugates are promising radiotracers for imaging and therapy of breast and prostate tumors in which BN(2)/gastrin-releasing peptide (GRP) receptors are overexpressed. However, the low in vivo stability of BN conjugates may limit their clinical application. In an attempt to improve their pharmacokinetics and counteract their rapid enzymatic degradation, we prepared a series of polyethylene glycol (PEG)-ylated BN(7-14) analogues for radiolabeling with (99m)Tc(CO)(3) and evaluated them in vitro and in vivo. METHODS: Derivatization of a stabilized (N(α)His)Ac-BN(7-14)[Cha(13),Nle(14)] analogue with linear PEG molecules of various sizes [5 kDa (PEG(5)), 10 kDa (PEG(10)) and 20 kDa (PEG(20))] was performed by PEGylation of the ɛ-amino group of a ß(3)hLys-ßAla-ßAla spacer between the stabilized BN sequence and the (N(α)His)Ac chelator. The analogues were then radiolabeled by employing the (99m)Tc-tricarbonyl technique. Binding affinity and internalization/externalization studies were performed in vitro in human prostate carcinoma PC-3 cells. Stability was investigated in vitro in human plasma and in vivo in Balb/c mice. Finally, single photon emission computed tomography (SPECT)/X-ray computed tomography studies were performed in nude mice bearing PC-3 tumor xenografts. RESULTS: PEGylation did not affect the binding affinity of BN analogues, as the binding affinity for BN(2)/GRP receptors remained high (K(d)<0.9 nM). However, in vitro binding kinetics of the PEGylated analogues were slower. Steady-state condition was reached after 4 h, and the total cell binding was 10 times lower than that for the non-PEGylated counterpart. Besides, PEGylation improved the stability of BN conjugates in vitro and in vivo. The BN derivative conjugated with a PEG(5) molecule showed the best pharmacokinetics in vivo, i.e., faster blood clearance and preferential renal excretion. The tumor uptake of the (99m)Tc-PEG(5)-Lys-BN conjugate was slightly higher compared to that of the non-PEGylated analogue (3.91%±0.44% vs. 2.80%±0.28% injected dose per gram 1 h postinjection, p.i.). Tumor retention was also increased, resulting in a threefold higher amount of radioactivity in the tumor at 24 h p.i. Furthermore, decreased hepatobiliary excretion and increased tumor-to-nontarget ratios (tumor-to-blood: 17.1 vs. 2.1; tumor-to-kidney: 1.1 vs. 0.4; tumor-to-liver: 5.8 vs. 1.0, 24 h p.i.) were observed and further confirmed via small-animal SPECT images 1 h p.i. CONCLUSION: PEGylation proved to be an effective strategy to enhance the tumor-targeting potential of (99m)Tc-labeled BN-based radiopharmaceuticals and probably other radiolabeled peptides.

Synthesis and evaluation of bombesin analogues conjugated to two different triazolyl-derived chelators for (99m)Tc labeling.

Overexpression of the gastrin-releasing peptide receptor (GRPR) in a variety of human carcinomas has provided a means of diagnosis and treatment. Previously we reported a metabolically stable (N(α)His)Ac-ßAla-ßAla-[Cha(13),Nle(14)]BBS(7-14) analogue with high affinity for the GRPR. We have also shown that the biodistribution pattern of this fairly lipophilic, radiolabeled peptide can be enhanced by glycation, which is easily carried out by Cu(I)-catalyzed cycloaddition. Herein, we further elaborate this "click approach" in the synthesis of a new series of triazole-based chelating systems as alternatives to the (N(α)His)Ac chelator for labeling with the (99m)Tc(CO)(3) core. The bombesin analogues, containing these new chelating systems, were evaluated with regard to their synthesis and in vitro and in vivo properties, and were compared with their (N(α)His)Ac counterparts. The influence of the chelator on biodistribution properties was less than that of glycation, which clearly improved the tumor-to-background ratios.

Bis(thiosemicarbazones) as bifunctional chelators for the room temperature 64-copper labeling of peptides.

A range of new carboxylate functionalised bis(thiosemicarbazone) ligands and their Cu(II) complexes have been prepared, fully characterised and radiolabeled in high yield with both (64)Cu and (99m)Tc. Conjugation to a bombesin derivative was achieved using standard solid phase synthetic methodologies and the (64)Cu-labeled conjugate was shown to have good tumour uptake in mice with xenografted PC-3 tumours.

Novel neurotensin analogues for radioisotope targeting to neurotensin receptor-positive tumors.

The increased expression of the neurotensin (NT) receptor NTS1 by different cancer cells, such as pancreatic adenocarcinoma and ductal breast cancer cells, as compared to normal epithelium, offers the opportunity to target these tumors with radiolabeled neurotensin analogues for diagnostic or therapeutic purposes. The aim of the present study was to design and synthesize new neurotensin radioligands and to select a lead molecule with high in vivo tumor selectivity for further development. Two series of neurotensin analogues bearing DTPA were tested: a series of NT(8-13) analogues, with DTPA coupled to the α-NH(2), sharing the same peptide sequence with analogues previously developed for radiolabeling with technetium or rhenium, as well as an NT(6-13) series in which DTPA was coupled to the ε-NH(2) of Lys(6). Changes were introduced to stabilize the bonds between Arg(8)-Arg(9), Pro(10)-Tyr(11), and Tyr(11)-Ile(12) to provide metabolic stability. Structure-activity studies of NT analogues have shown that the attachment of DTPA induces an important loss of affinity unless the distance between the chelator and the NT(8-13) sequence, which binds to the NTS1 receptor, is increased. The doubly stabilized DTPA-NT-20.3 exhibits a high affinity and an elevated stability to enzymatic degradation. It shows specific tumor uptake and high tumor to blood, to liver, and to intestine activity uptake ratios and affords high-contrast planar and SPECT images in an animal model. The DTPA-NT-20.3 peptide is a promising candidate for imaging neurotensin receptor-positive tumors, such as pancreatic adenocarcinoma and invasive ductal breast cancer. Analogues carrying DOTA are being developed for yttrium-90 or lutetium-177 labeling.

A stable neurotensin-based radiopharmaceutical for targeted imaging and therapy of neurotensin receptor-positive tumours.

PURPOSE: Neurotensin (NT) and its high affinity receptor (NTR1) are involved in several neoplastic processes. Thus, NT-based radiopharmaceuticals are potential tracers for targeted diagnosis and therapy of NTR-positive tumours. A new analogue based on NT(8-13), NT-XIX, with the three enzymatic cleavage sites stabilised, was synthesised and tested. METHODS: The synthesis was performed by Boc strategy. Labelling with (99m)Tc/(188)Re was performed using the tricarbonyl technique. Metabolic stability was tested in vitro and in vivo. NT-XIX was further characterised in vitro in HT-29 cells and in vivo in nude mice with HT-29 xenografts. RESULTS: NT-XIX showed much longer half-lives than non-stabilised analogues. Binding to NTR1 was highly specific, although the affinity was lower than that of natural NT. Bound activity rapidly internalised into HT-29 cells and 50% remained trapped after 24 h. In the time-course biodistribution, the highest uptake was found in the tumour at all p.i. times. In vivo uptake was specific, and accumulation of activity in the kidneys was low. Radioactivity clearance from healthy organs was faster than that from the tumour, resulting in improved tumour-to-tissue ratios and good SPECT/CT imaging. Treatment with (188)Re-NT-XIX (30 MBq, in three or four fractions) decreased tumour growth by 50% after 3 weeks. CONCLUSION: The high in vivo stability and the favourable in vivo behaviour makes NT-XIX an excellent candidate for the imaging and therapy of NTR1-positive tumours.

Novel glycated [99mTc(CO)3]-labeled bombesin analogues for improved targeting of gastrin-releasing peptide receptor-positive tumors.

Radiolabeled bombesin (BBS) analogues are promising pharmaceuticals for imaging of cancer cells expressing gastrin-releasing peptide receptors (GRPR). However, most of the radiolabeled BBS derivatives show a high accumulation of activity in the liver and a strong hepatobiliary excretion, both unfavorable for imaging and therapy of abdominal lesions. For this reason, we introduced hydrophilic carbohydrated linker moieties into our BBS analogues to reduce the abdominal accumulation and to improve the tumor-to-background ratios. A stabilized BBS(7-14) sequence bearing the (NalphaHis)Ac-chelator was modified with amino acid linkers containing a lysine or propargylglycine residue. The epsilon-amino group of a lysine was either coupled to shikimic acid or reacted with glucose to form the Amadori conjugate. Alternatively, a glucose was attached to the peptide via "click" chemistry with the propargylglycine side chain. The peptides were synthesized on Rink amide resin using solid-phase peptide synthesis and labeled with 99mTc using the tricarbonyl technique. Binding and degradation were tested in Vitro in GRPR-expressing PC-3 cells. Biodistribution and SPECT/CT imaging studies were performed in nude mice bearing PC-3 tumor xenografts. The new peptides showed a log D between -0.2 and -0.5 and kept the high affinity for GRPR with Kd values of <0.5 nM. In Vitro, they were rapidly internalized into the tumor cells and showed an increased cellular retention and stability (t(1/2 )>35 min). In ViVo, all new compounds exhibited higher tumor-to-background ratios compared to the nonglycated reference. Thus, the best results were obtained with the triazole coupled glucose with a 4-fold increased uptake and retention in tumor tissue (3.6 and 2.5%ID/g at 1.5 h and 5 h p.i, respectively) and a significantly reduced accumulation in the liver (0.6 vs 2.4%ID/g, 1.5 h p.i., respectively). Apart from higher tumor-to-liver ratios (17-fold, 1.5 h p.i.), both tumor-to-kidney and tumor-to-blood ratios could be significantly improved by a factor of 1.5 and 2.7, respectively (1.5 h p.i., P<0.05). The imaging studies proved the reduction of abdominal background, and tumor xenografts could clearly be visualized. In conclusion, the introduction of a carbohydrated linker substantially improved the biodistribution properties of BBS analogues labeled with the 99mTc-tricarbonyl core.

Glycation methods for bombesin analogs containing the (NalphaHis)Ac chelator for 99mTc(CO)3 radiolabeling.

The overexpression of peptide receptors in a variety of human carcinomas has generated considerable interest in peptide-based radiopharmaceuticals for peptide receptor imaging and peptide receptor radiotherapy. The gastrin-releasing peptide receptor is overexpressed in human prostate-, breast-, colon- and small cell lung carcinoma cells. We have developed metabolically stable (99m)Tc-radiolabeled bombesin ([Cha(13), Nle(14)]BBS(7-14)) analogs, which bind with high affinity to the gastrin-releasing peptide receptors. However, because of their lipophilicity, they showed unfavorable biodistribution with high hepatic accumulation and hepatobiliary excretion. We now report a study of different glycation methods for [Cha(13), Nle(14)]BBS(7-14) analogs to improve their biodistribution profile. Whereas the glycation using the Maillard reaction was problematic, resulting in low yields, selective introduction of the glycomimetic shikimic acid to the side chain of a Lys residue was possible. A chemoselective ligation of alpha-D-glucose to an amino-oxyacetylated [Cha(13), Nle(14)]BBS(7-14) analog could be achieved, but was complicated by the co-elution of starting peptide and glycopeptide. The best procedure consisted of the [1,3]-cycloaddition of N(3)-beta-D-glucose to a propargylglycine-containing [Cha(13), Nle(14)]BBS(7-14) analog, using a catalytic amount of Cu(I)I. All glycated [Cha(13), Nle(14)]BBS(7-14) analogs showed high affinity for the gastrin-releasing peptide receptor and rapid accumulation into PC-3 tumor cells.

Influence of the molecular charge on the biodistribution of bombesin analogues labeled with the [99mTc(CO)3]-core.

The overexpression of Bombesin (BBS) receptors on a variety of human cancers make them interesting targets for tumor imaging and therapy. Analogues of the neuropeptide BBS have been functionalized with the (NalphaHis)- chelator for labeling with the 99mTc-tricarbonyl core. The introduction of a betaAla-betaAla linker between the stabilized BBS binding sequence and the chelator led to increased tumor uptake but still rather unfavorable in ViVo properties. Novel polar linkers, with different charge, have been introduced in the molecule and tested for their influence on the biodistribution. The new analogues showed a shift in hydrophilicity from a Log D=0.9 to Log D values between 0.4 and -2.2. All compounds kept the increased stability in both human plasma (t(1/2)>16 h) and in tumor cells (t(1/2)=30-40 min). The compounds with Log D values between +1 and -1 showed the highest binding affinities with Kd values of <0.5 nM, as well as the highest cellular uptake. However, higher hydrophilicity (Log D < -1.8) led to lower affinity and a substantial decrease of internalization. The introduction of a positive charge (beta3hLys) resulted in unfavorable biodistribution, with increased kidney uptake. The introduction of an uncharged hydroxyl group (beta3hSer) improved the biodistribution, resulting in significantly better tumor-to-tissue ratios. The compound with one single negative charge (beta3hGlu) showed a significant increase in the tumor uptake (2.1+/-0.6% vs 0.80+/-0.35% ID/g in comparison to the betaAla-betaAla analogue) and also significantly higher tumor-to-tissue ratios. The specificity of the in ViVo uptake was confirmed by coinjection with natural BBS. Moreover, the analogue provided a much clearer image of the tumor xenografts in the SPECT/CT studies. The introduction of a single negative charge may be useful in the development of new BBS analogues to obtain an improved biodistribution profile, with increased tumor uptake and better imaging.

Chemical and biological characterization of new Re(CO)3/[99mTc](CO)3 bombesin analogues.

INTRODUCTION: Bombesin, a neuropeptide with potential for breast and prostate tumor targeting, is rapidly metabolized in vivo, and as a result, uptake in tumor xenografts in mice is poor. An improvement can be expected from the introduction of nonnatural amino acids and spacers. Leu13 was replaced by cyclohexylalanine and Met14 by norleucine. Two spacers, -betaAla-betaAla- and 3,6-dioxa-8-aminooctanoic acid, were inserted between the receptor-binding amino acid sequence (7-14) of bombesin (BBS) and the retroN(alpha)-carboxymethyl histidine chelator used for labeling with the [99mTc](CO)3 core and the rhenium (Re) congener. METHODS: The biological characterization of the new compounds was performed both in vitro on prostate carcinoma PC-3 cells (binding affinity, internalization/externalization) and in vivo (biodistribution in nude mice with tumor xenografts). The stability was also investigated in human plasma. The Re analogues were prepared for chemical characterization. RESULTS: The nonnatural amino acids led to markedly slower degradation in human plasma and PC-3 cell cultures. The receptor affinity of the new technetium 99m ([99mTc])-labeled BBS analogues was similar to the unmodified compound with Kd<1 nM. Uptake in the pancreas and in PC-3 tumor xenografts in nude mice was blocked by unlabeled BBS. The best target-to-nontarget uptake ratio was clearly due to the presence of the more polar spacer, -betaAla-betaAla-. CONCLUSIONS: The different spacers did not have a significant effect on stability or receptor affinity but had a clear influence on the uptake in healthy organs and tumors. Uptake in the kidneys was lower than in the liver, which is likely to be due to the lipophilicity of the compounds. A specific, high uptake was also observed in the gastrin-releasing peptide receptor-rich pancreas. Thus, with the introduction of spacers the in vivo properties of the compounds can be improved while leaving the affinity unaffected.

"Click to chelate": synthesis and installation of metal chelates into biomolecules in a single step.

Click chemistry has been employed for the assembly of novel and efficient triazole-based multidentate chelating systems while simultaneously attaching them to molecules of biological interest. The "click-to-chelate" approach offers a powerful new tool for the modification of (bio)molecules with metal chelators for potential diagnostic and therapeutic applications.

Toward stable N4-modified neurotensins for NTS1-receptor-targeted tumor imaging with 99mTc.

A series of Gly-neurotensin(8-13) analogues modified at the N-terminus by acyclic tetraamines (Demotensin 1-4) were obtained by solid-phase peptide synthesis techniques. Strategic replacement of amino acids and/or reduction of sensitive peptide bonds were performed to enhance conjugate resistance against proteolytic enzymes. During 99mTc-labeling, single species radiopeptides, [99mTc]Demotensin 1-4, were easily obtained in high yields and typical specific activities of 1 Ci/micromol. Peptide conjugates displayed a high affinity binding to the human neurotensin subtype 1 receptor (NTS1-R) expressed in colon adenocarcinoma HT-29 or WiDr cells and/or in human tumor sections. [99mTc]Demotensin 1-4 internalized very rapidly in HT-29 or WiDr cells by a NTS1-R-mediated process. [99mTc]Demotensin 3 and 4, which remained stable during 1 h incubation in murine plasma, were selectively studied in nude mice bearing human HT-29 and WiDr xenografts. After injection, [99mTc]Demotensin 3 and 4 effectively and specifically localized in the experimental tumors and were rapidly excreted via the kidneys into the urine, exhibiting overall biodistribution patterns favorable for NTS1-R-targeted tumor imaging in man.

Fluorescein-labeled stable neurotensin derivatives.

Neurotensin(8-13) analogs containing a glycine or 5-aminovaleroyl spacer were labeled with fluorescein through formation of an N-terminal thiourea function. The receptor binding was measured in HT-29 cell cultures and showed a substantial decrease in affinity, especially for the metabolically stabilized [MeArg(9), Tle(11)] analog. Using fluorescence microscopy, the internalization of the fluorescent neurotensin analogs into HT-29 cells was observed.

Double-stabilized neurotensin analogues as potential radiopharmaceuticals for NTR-positive tumors.

INTRODUCTION: Overexpression of neurotensin (NT) receptors in exocrine pancreatic cancer and other neuroendocrine cancers make them interesting targets for tumor imaging and therapy. Modifications at the cleavage bonds 8-9 and 11-12 led to the synthesis of NT-XII, NT-XIII and NT-XVIII, three new stabilized analogues. (NalphaHis)Ac was coupled to the N-terminus for labeling with [(99m)Tc]-tricarbonyl. METHODS: Stability was tested in vitro in human plasma and HT-29 cells. Binding to NT1 receptors and internalization/efflux were analyzed in intact HT-29 cells. Biodistribution studies were performed in nude mice bearing HT-29 xenografts. RESULTS: All analogues were very stable in human plasma, with half-lives of 20-21 days. Degradation in HT-29 cells was more rapid (t(1/2) of 6.5, 5 and 2.5 h for NT-XII, NT-XIII and NT-XVIII, respectively). They also showed high affinity and specificity for NT1 receptors. Bound activity was rapidly internalized at 37 degrees C. The pattern of externalization was different. NT-XII was released more slowly than NT-XIII and NT-XVIII (half of the activity still inside the cells after 24 h). Bigger differences were found in the biodistribution studies. NT-XII showed the highest tumor uptake as well as the best tumor to nontumor ratios. CONCLUSION: The modifications introduced in NT(8-13) increased plasma stability, maintaining unaffected the in vitro binding properties. The best biodistribution corresponded to NT-XII, which shows to be a good candidate for NT1 receptors overexpressing tumors. First clinical trials are ongoing.

Spatial conformation and topography of the tyrosine aromatic ring in substrate recognition by protein tyrosine kinases.

The side chain orientation of the tyrosine residue included in a peptide, which is an excellent substrate of Syk tyrosine kinase, was fixed in different conformations by either incorporating the tyrosine in cyclic structures (6-OH-Tic, 5-OH-Aic, and Hat derivatives) or adding a sterically bulky substituent in the tyrosine side chain moiety (beta-MeTyr). Synthetic peptides containing tyrosine analogues displaying different side chain orientations were analyzed by NMR techniques and tested as potential substrates of the nonreceptor tyrosine kinases Syk, Csk, Lyn, and Fyn. The "rotamer scan" of the phosphorylatable residue generated optimal substrates in terms of both phosphorylation efficiency and selectivity for Syk tyrosine kinase, while the peptidomimetics were not recognized by the other tyrosine kinases. In particular, l-beta-MeTyr and d-Hat containing peptides resulted to be both suitable substrates for the specific monitoring of Syk and consensus sequence scaffolds for the design of potential inhibitors highly selective for this tyrosine kinase.

Novel 99mTc-labeled neurotensin analogues with optimized biodistribution properties.

Two new 99mTc-labeled neurotensin(8-13) analogues containing the retro-N(alpha)-carboxymethyl-histidine ((N(alpha)His)Ac) chelator were synthesized as potential radiopharmaceuticals for visualization of pancreatic carcinoma. To improve the pharmacokinetic properties, (N(alpha)His)Ac-Arg-NMeArg-Pro-Tyr-Tle-Leu (NT-XII), which is metabolically stabilized at two positions, was further modified. Shikimic acid (3,4,5-trihydroxy-1-cyclohexene-1-carboxylic acid) was introduced to obtain a more hydrophilic peptide (NT-XVIII), or Tyr11 was replaced by 2,6-dimethyltyrosine (Dmt) resulting in a triple-stabilized NT(8-13) analogue (NT-XIX). The latter has the best biodistribution profile.