<bold>Introduction:</bold> Prostate specific membrane antigen (PSMA), highly expressed in metastatic castration-resistant prostate cancer (mCRPC), is an established therapeutic target. Theranostic PSMA-targeting agents are widely used in patient management and has shown improved outcomes for mCRPC patients. Earlier, we optimized a urea-based probe for radionuclide visualization of PSMA-expression in vivo using computer modeling. With the purpose to develop a targeting agent equally suitable for radionuclide imaging and therapy, the agent containing DOTA chelator was designed (BQ7876). The aim of the study was to test the hypothesis that Lu-177-labeled BQ7876 possesses target binding and biodistribution properties potentially enabling its use for radiotherapy.<bold>Methods:</bold> BQ7876 was synthesized and labeled with Lu-177. Specificity and affinity of [Lu-177]Lu-BQ7876 to PSMA-expressing PC3-pip cells was evaluated and its processing after binding to cells was studied. Animal studies in mice were performed to assess its biodistribution in vivo, target specificity and dosimetry. [Lu-177]Lu-PSMA-617 was simultaneously evaluated for comparison.<bold>Results:</bold> BQ7876 was labeled with Lu-177 with radiochemical yield >99%. Its binding to PSMA was specific in vitro and in vivo when tested in antigen saturation conditions as well as in PSMA-negative PC-3 tumors. The binding of [Lu-177]Lu-BQ7876 to living cells was characterized by rapid association, while the dissociation included a rapid and a slow phase with affinities K-D1 = 3.8 nM and K-D2 = 25 nM. The half-maximal inhibitory concentration for Lu-nat-BQ7876 was 59 nM that is equal to 61 nM for Lu-nat-PSMA-617. Cellular processing of [Lu-177]Lu-BQ7876 was accompanied by slow internalization. [Lu-177]Lu-BQ7876 was cleared from blood and normal tissues rapidly. Initial elevated uptake in kidneys decreased rapidly, and by 3 h post injection, the renal uptake (13 +/- 3%ID/g) did not differ significantly from tumor uptake (9 +/- 3%ID/g). Tumor uptake was stable between 1 and 3 h followed by a slow decline. The highest absorbed dose was in kidneys, followed by organs and tissues in abdomen.<bold>Discussion:</bold> Biodistribution studies in mice demonstrated that targeting properties of [Lu-177]Lu-BQ7876 are not inferior to properties of [Lu-177]Lu-PSMA-617, but do not offer any decisive advantages.

Gastrin-releasing peptide receptor (GRPR) and prostate-specific membrane antigen (PSMA) are overexpressed in most prostate cancers. GRPR expression is higher in early stages while PSMA expression increases with progression. The possibility of targeting both markers with a single theranostics radiotracer could improve patient management. Three GRPR/PSMA-targeting bispecific heterodimers (urea derivative PSMA-617 and bombesin-based antagonist RM26 linked via X-triazolyl-Tyr-PEG2, X = PEG2 (BO530), (CH2)(8) (BO535), none (BO536)) were synthesized by solid-phase peptide synthesis. Peptides were radio-iodinated and evaluated in vitro for binding specificity, cellular retention, and affinity. In vivo specificity for all heterodimers was studied in PC-3 (GRPR-positive) and LNCaP (PSMA-positive) xenografts. [I-125]I-BO530 was evaluated in PC-3pip (GRPR/PSMA-positive) xenografts. Micro single-photon emission computed tomography/computed tomography (microSPECT/CT) scans were acquired. The heterodimers were radiolabeled with high radiochemical yields, bound specifically to both targets, and demonstrated high degree of activity retention in PC-3pip cells. Only [I-125]I-BO530 demonstrated in vivo specificity to both targets. A biodistribution study of [I-125]I-BO530 in PC-3pip xenografted mice showed high tumor activity uptake (30%-35%ID/g at 3 h post injection (pi)). Activity uptake in tumors was stable and exceeded all other organs 24 h pi. Activity uptake decreased only two-fold 72 h pi. The GRPR/PSMA-targeting heterodimer [I-125]I-BO530 is a promising agent for theranostics application in prostate cancer.

Radionuclide imaging using radiolabeled inhibitors of prostate-specific membrane antigen (PSMA) can be used for the staging of prostate cancer. Previously, we optimized the Glu-urea-Lys binding moiety using a linker structure containing 2-napththyl-L-alanine and L-tyrosine. We have now designed a molecule that contains mercaptoacetyl-triglutamate chelator for labeling with Tc-99m (designated as BQ0413). The purpose of this study was to evaluate the imaging properties of [Tc-99m]Tc-BQ0413. PSMA-transfected PC3-pip cells were used to evaluate the specificity and affinity of [Tc-99m]Tc-BQ0413 binding in vitro. PC3-pip tumor-bearing BALB/C nu/nu mice were used as an in vivo model. [Tc-99m]Tc-BQ0413 bound specifically to PC3-pip cells with an affinity of 33 +/- 15 pM. In tumor-bearing mice, the tumor uptake of [Tc-99m]Tc-BQ0413 (38 +/- 6 %IA/g in PC3-pip 3 h after the injection of 40 pmol) was dependent on PSMA expression (3 +/- 2 %IA/g and 0.9 +/- 0.3 %IA/g in PSMA-negative PC-3 and SKOV-3 tumors, respectively). We show that both unlabeled BQ0413 and the commonly used binder PSMA-11 enable the blocking of [Tc-99m]Tc-BQ0413 uptake in normal PSMA-expressing tissues without blocking the uptake in tumors. This resulted in an appreciable increase in tumor-to-organ ratios. At the same injected mass (5 nmol), the use of BQ0413 was more efficient in suppressing renal uptake than the use of PSMA-11. In conclusion, [Tc-99m]Tc-BQ0413 is a promising probe for the visualization of PSMA-positive lesions using single-photon emission computed tomography (SPECT).

Previously, we designed the EuK-based PSMA ligand BQ0413 with an maE3 chelator for labeling with technetium-99m. It showed efficient tumor targeting, but our preclinical data and preliminary clinical results indicated that the renal excretion levels need to be decreased. We hypothesized that this could be achieved by a decrease in the ligand's total negative charge, achieved by substituting negatively charged glutamate residues in the chelator with glycine. The purpose of this study was to evaluate the tumor targeting and biodistribution of two new PSMA inhibitors, BQ0411 and BQ0412, compared to BQ0413. Conjugates were radiolabeled with Tc-99m and characterized in vitro, using PC3-pip cells, and in vivo, using NMRI and PC3-pip tumor-bearing mice. [99mTc]Tc-BQ0411 and [99mTc]Tc-BQ0412 demonstrated PSMA-specific binding to PC3-pip cells with picomolar affinity. The biodistribution pattern for the new conjugates was characterized by rapid excretion. The tumor uptake for [99mTc]Tc-BQ0411 was 1.6-fold higher compared to [99mTc]Tc-BQ0412 and [99mTc]Tc-BQ0413. [99mTc]Tc-BQ0413 has demonstrated predominantly renal excretion, while the new conjugates underwent both renal and hepatobiliary excretion. In this study, we have demonstrated that in such small targeting ligands as PSMA-binding EuK-based pseudopeptides, the structural blocks that do not participate in binding could have a crucial role in tumor targeting and biodistribution. The presence of a glycine-based coupling linker in BQ0411 and BQ0413 seems to optimize biodistribution. In conclusion, the substitution of amino acids in the chelating sequence is a promising method to alter the biodistribution of [99mTc]Tc-labeled small-molecule PSMA inhibitors. Further improvement of the biodistribution properties of BQ0413 is needed.

The inhibition of insulin-regulated aminopeptidase (IRAP, EC 3.4.11.3) by angiotenesin IV is known to improve memory and learning in rats. Screening 10 500 low-molecular-weight compounds in an enzyme inhibition assay with IRAP from Chinese Hamster Ovary (CHO) cells provided an arylsulfonamide (N-(3-(1H-tetrazol-5-yl)phenyl)-4-bromo-5-chlorothiophene-2-sulfonamide), comprising a tetrazole in the meta position of the aromatic ring, as a hit. Analogues of this hit were synthesized, and their inhibitory capacities were determined. A small structure-activity relationship study revealed that the sulfonamide function and the tetrazole ring are crucial for IRAP inhibition. The inhibitors exhibited a moderate inhibitory potency with an IC50=1.1±0.5 μm for the best inhibitor in the series. Further optimization of this new class of IRAP inhibitors is required to make them attractive as research tools and as potential cognitive enhancers.

Angiotensin IV (Ang IV) and related peptide analogues, as well as non-peptide inhibitors of insulin-regulated aminopeptidase (IRAP), have previously been shown to enhance memory and cognition in animal models. Furthermore, the endogenous IRAP substrates oxytocin and vasopressin are known to facilitate learning and memory. In this study, the two recently synthesized 13-membered macrocylic competitive IRAP inhibitors HA08 and HA09, which were designed to mimic the N-terminal of oxytocin and vasopressin, were assessed and compared based on their ability to bind to the IRAP active site, and alter dendritic spine density in rat hippocampal primary cultures. The binding modes of the IRAP inhibitors HA08, HA09 and of Ang IV in either the extended or γ-turn conformation at the C-terminal to human IRAP were predicted by docking and molecular dynamics (MD) simulations. The binding free energies calculated with the linear interaction energy (LIE) method, which are in excellent agreement with experimental data and simulations, have been used to explain the differences in activities of the IRAP inhibitors, both of which are structurally very similar, but differ only with regard to one stereogenic center. In addition, we show that HA08, which is 100-fold more potent than the epimer HA09, can enhance dendritic spine number and alter morphology, a process associated with memory facilitation. Therefore, HA08, one of the most potent IRAP inhibitors known today, may serve as a suitable starting point for medicinal chemistry programs aided by MD simulations aimed at discovering more drug-like cognitive enhancers acting via augmenting synaptic plasticity.

Inhibition of Insulin-regulated Aminopeptidase (IRAP) has been shown to improve cognitive functions in several animal models. Recently, we performed a screening campaign identifying novel small-molecule based compounds acting as inhibitors of the enzymatic activity IRAP. Here we report on the chemical synthesis, structure-activity relationships (SAR) and initial characterization of physicochemical properties of a series of imidazo[1,5-α]pyridine-based inhibitors, including delineation of their mode of action as non-competitive inhibitors with a small L-leucine-based IRAP substrate. The best compound displays an pIC50 values of 6.0. We elucidate the importance of two chiral sites in these molecules and find they have little impact on the compound´s metabolic stability or physicochemical properties. The carbonyl group of a central urea moiety was initially believed to mimic substrate binding to a catalytically important Zn²⁺ ion in the active site, although the plausibility of this binding hypothesis is challenged by observation of excellent selectivity versus the closely related aminopeptidase N (APN). Taken together with the non-competitive inhibition pattern, we also consider an alternative model of allosteric binding.

Insulin‐regulated aminopeptidase (IRAP) is a new potential macromolecular target for drugs aimed for treatment of cognitive disorders. Inhibition of IRAP by angiotensin IV (Ang IV) improves the memory and learning in rats. The majority of the known IRAP inhibitors are peptidic in character and suffer from poor pharmacokinetic properties. Herein, we present a series of small non‐peptide IRAP inhibitors derived from a spiro‐oxindole dihydroquinazolinone screening hit (pIC50 5.8). The compounds were synthesized either by a simple microwave (MW)‐promoted three‐component reaction, or by a two‐step one‐pot procedure. For decoration of the oxindole ring system, rapid MW‐assisted Suzuki‐Miyaura cross‐couplings (1 min) were performed. A small improvement of potency (pIC50 6.6 for the most potent compound) and an increased solubility could be achieved. As deduced from computational modelling and MD simulations it is proposed that the S‐configuration of the spiro‐oxindole dihydroquinazolinones accounts for the inhibition of IRAP.

Intracerebroventricular injection of angiotensin IV, a ligand of insulin-regulated aminopeptidase (IRAP), has been shown to improve cognitive functions in several animal models. Consequently, IRAP is considered a potential target for treatment of cognitive disorders. To identify nonpeptidic IRAP inhibitors, we adapted an established enzymatic assay based on membrane preparations from Chinese hamster ovary cells and a synthetic peptide-like substrate for high-throughput screening purposes. The 384-well microplate-based absorbance assay was used to screen a diverse set of 10,500 compounds for their inhibitory capacity of IRAP. The assay performance was robust with Z-values ranging from 0.81 to 0.91, and the screen resulted in 23 compounds that displayed greater than 60% inhibition at a compound concentration of 10M. After hit confirmation experiments, purity analysis, and promiscuity investigations, three structurally different compounds were considered particularly interesting as starting points for the development of small-molecule-based IRAP inhibitors. After resynthesis, all three compounds confirmed low M activity and were shown to be rapidly reversible. Additional characterization included activity in a fluorescence-based orthogonal assay and in the presence of a nonionic detergent and a reducing agent, respectively. Importantly, the characterized compounds also showed inhibition of the human ortholog, prompting our further interest in these novel IRAP inhibitors.

A fast and convenient synthetic route towards spiro-oxindole dihydroquinazolinones as novel and drug-like insulin-regulated aminopeptidase (IRAP) inhibitors is reported. The synthesis is performed using a MW heated continuous flow system employing 200 mm X 3 mm i MW absorbing silicon carbide (SiC) or MW transparent borosilicate tubular reactors. A three-component MW-flow reaction to build up the spiro compounds (9 examples, 4087% yield), using the SiC reactor, as well as a SuzukiMiyaura cross-coupling reaction (71%), employing the borosilicate reactor, are presented with residence times down to 168 s. The continuous MW-flow routes provide a smooth and scalable synthetic methodology towards this class of IRAP inhibitors.

AIMS: Radiolabeled Exendin-4 has been proposed as suitable imaging marker for pancreatic beta cell mass quantification mediated by Glucagon-like peptide-1 receptor (GLP-1R). However, noticeable species variations in basal pancreatic uptake as well as uptake reduction degree due to selective beta cell ablation were observed.

METHODS: -Exendin4 Positron Emission Tomography (PET) in the same species. In vitro, ex vivo, and in vivo data formed the basis for calculating the theoretical in vivo contribution of each pancreatic compartment.

RESULTS: -Exendin4.

CONCLUSIONS: IPR as well as the exocrine GLP-1R density is the main determinants of the species variability in pancreatic uptake. Thus, the IPR in human is an important factor for assessing the potential of GLP-1R as an imaging biomarker for pancreatic beta cells.

Two pentapeptides, Ac-Tyr-Ile-His-Pro-Phe/Ile, were synthesized and shown to have angiotensin II AT(2) receptor affinity and agonistic activity. Based on these peptides, a new series of 13 pseudopeptides was synthesized via introduction of five different turn scaffolds replacing the Tyr-Ile amino acid residues. Pharmacological evaluation disclosed subnanomolar affinities for some of these compounds at the AT(2) receptor. Substitution of Phe by Ile in this series of ligands enhanced the AT(2) receptor affinity of all compounds. These results suggest that the C-terminal amino acid residues can be elaborated on to enhance the AT(2) receptor affinity in truncated Ang II analogues.

Gastrin releasing peptide receptor (GRPR) is overexpressed in prostate cancer (PC-3) and can be used for diagnostic purposes. We herein present the design and preclinical evaluation of two novel NOTA/NODAGA-containing peptides suitable for labeling with the positron emission tomography (PET) radionuclide Ga-68. These analogs are based on the previously reported GRPR-antagonist DOTAGA-PEG2-[Sar¹¹]RM26, developed for targeted radiotheraostic applications. Both NOTA-PEG2-[Sar¹¹]RM26 and NODAGA-PEG2-[Sar¹¹]RM26 were successfully labeled with Ga-68 and evaluated in vitro and in vivo using PC-3 cell models. Both, [⁶⁸Ga]Ga-NOTA-PEG2-[Sar¹¹]RM26 and [⁶⁸Ga]Ga-NODAGA-PEG2-[Sar¹¹]RM26 displayed high metal-chelate stability in phosphate buffered saline and against the EDTA-challenge. The two [⁶⁸Ga]Ga-labeled conjugates demonstrated highly GRPR-mediated uptake in vitro and in vivo and exhibited a slow internalization over time, typical for radioantagonistis. The [^natGa]Ga-loaded peptides displayed affinity in the low nanomole range for GRPR in competition binding experiments. The new radiotracers demonstrated biodistribution profiles suitable for diagnostic imaging shortly after administration with fast background clearance. Their high tumor uptake (13 ± 1 and 15 ± 3% IA/g for NOTA and NODAGA conjugates, respectively) and high tumor-to-blood ratios (60 ± 10 and 220 ± 70, respectively) 3 h pi renders them promising PET tracers for use in patients. Tumor-to-normal organ ratios were higher for [⁶⁸Ga]Ga-NODAGA-PEG2-[Sar¹¹]RM26 than for the NOTA-containing counterpart. The performance of the two radiopeptides was further supported with the PET/CT images. In conclusion, [⁶⁸Ga]Ga-NODAGA-PEG2-[Sar¹¹]RM26 is a promising PET imaging tracer for visualization of GRPR-expressing lesions with high imaging contrast shortly after administration.

Prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPR) are promising targets for molecular imaging of prostate cancer (PCa) lesions. Due to the heterogenic overexpression of PSMA and GRPR in PCa, a heterodimeric radiotracer with the ability to bind to both targets could be beneficial. Recently, our group reported the novel heterodimer BQ7800 consisting of a urea-based PSMA inhibitor, the peptide-based GRPR antagonist RM26 and NOTA chelator. The study reported herein, aimed to improve the affinity of BQ7800 towards PSMA by changing the composition of the two linkers connecting the PSMA- and GRPR-targeting motifs. Three novel heterodimeric analogues were synthesized by incorporation of phenylalanine in the functional linker of the PSMA-binding motif and/or shortening the PEG-linker coupled to RM26. The heterodimers were labeled with indium-111 and evaluated in vitro. In the competitive binding assay, BQ7812, featuring phenylalanine and shorter PEG-linker, demonstrated a nine-fold improved affinity towards PSMA. In the in vivo biodistribution study of [In-111]In-BQ7812 in PC3-pip tumor-bearing mice (PSMA and GRPR positive), the activity uptake was two-fold higher in the tumor and three-fold higher in kidneys than for [In-111]In-BQ7800. Herein, we showed that the affinity of a bispecific PSMA/GRPR heterodimer towards PSMA could be improved by linker modification.

Simple Summary Prostate cancer continues to be the most frequently diagnosed form of cancer and the leading cause of cancer-related deaths among men. For a successful treatment plan and outcome, an early diagnosis, correct staging, and monitoring of treatment response are crucial. To improve this, a radiotracer could be used to target the two most abundant proteins overexpressed in prostate cancer: prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPR). To date, no such heterodimeric radiotracer is used in the clinic. In this study, we have preclinically characterized and evaluated a galium-68 labelled PSMA/GRPR-targeting radiotracer for PET imaging of prostate cancer. We hope that the findings from this study will be able to contribute to designing better heterodimeric ligands, promote clinical translation of a heterodimer, and serve as a step towards a first-in-human study. The development of radioligands targeting prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPR) has shown promising results for the imaging and therapy of prostate cancer. However, studies have shown that tumors and metastases can express such targets heterogeneously. To overcome this issue and to improve protein binding, radioligands with the ability to bind both PSMA and GRPR have been developed. Herein, we present the preclinical characterization of [Ga-68]Ga-BQ7812; a PSMA/GRPR-targeting radioligand for the diagnostic PET imaging of prostate cancer. This study aimed to evaluate [Ga-68]Ga-BQ7812 to promote the translation of such imaging probes into the clinic. [Ga-68]Ga-BQ7812 demonstrated rapid and specific binding to both targets in a PSMA/GRPR-expressing PC3-pip cell line. Results from the biodistribution study in PC3-pip xenografted mice showed specific binding to both targets, with the highest activity uptake at 1 h pi in tumor (PSMA+/GRPR+, 10.4 +/- 1.0% IA/g), kidneys (PSMA+, 45 +/- 16% IA/g), and pancreas (GRPR+, 5.6 +/- 0.7% IA/g). At 3h pi, increased tumour-to-organ ratios could be seen due to higher retention in the tumor compared with other PSMA or GRPR-expressing organs. These results, together with low toxicity and an acceptable estimated dosimetry profile (total effective dose = 0.0083 mSv/MBq), support the clinical translation of [Ga-68]Ga-BQ7812 and represent a step towards its first clinical trial.

Prostate-specific membrane antigen (PSMA) is overexpressed in the majority of prostate cancer cells and is considered to be an important target for the molecular imaging and therapy of prostate cancer. Herein, we present the design, synthesis, and evaluation of 11 PSMA-binding radioligands with modified linker structures, focusing on the relationship between molecular structure and targeting properties. The linker design was based on 2-naphthyl-L-alanine-tranexamic acid, the linker structure of PSMA-617. X-ray crystal-structure analysis of PSMA and structure-based design were used to generate the linker modifications, suggesting that substitution of tranexamic acid could lead to interactions with Phe546, Trp541, and Arg43 within the binding cavity. After synthesis through SPPS, analogues were labelled with indium-111 and evaluated in vitro for their specific binding, affinity, and cellular retention. Selected compounds were further evaluated in vivo in PSMA-expressing tumour-bearing mice. Based on the results, 2-naphthyl-L-alanine appears to be crucial for good targeting properties, whereas tranexamic acid could be replaced by other substituents. [¹¹¹In]In-BQ7859, consisting of a 2-naphthyl-L-alanine-L-tyrosine linker, demonstrated favourable targeting properties. The substitution of tranexamic acid for L-tyrosine in the linker led to an improved tumour-to-blood ratio, highlighting [¹¹¹In]In-BQ7859 as a promising PSMA-targeting radioligand.

The feasibility of targeted imaging and therapy using radiolabeled albumin-binding domain-derived affinity proteins (ADAPTs) has been demonstrated. However, high renal uptake of radioactivity limits the maximum tolerated dose. Successful reduction of renal retention of radiolabeled Fab fragments has been demonstrated by incorporating a cleavable linker between the targeting agent and the radiometal chelator. The present study investigated if the introduction of a glycine-leucine-glycine-lysine (GLGK)-linker would reduce the kidney uptake of radiolabeled ADAPT6 and also compared it with the non-residualizing [125I]I-[(4-hydroxyphenyl)ethyl]maleimide ([125I]I-HPEM) labeling strategy. GLGK was site-specifically coupled to human epidermal growth factor receptor 2 (HER2)-targeting ADAPT6. Conjugates without the cleavable linker were used as controls and all constructs were labeled with lutetium-177 (177Lu). [125I]I-HPEM was coupled to ADAPT6 at the C-terminus. Biodistribution of all constructs was evaluated in NMRI mice 4 h after injection. Specific binding to HER2-expressing cells in vitro was demonstrated for all constructs. No significant difference in kidney uptake was observed between the [177Lu]Lu-2,2 ',2",2"'-(1,4,7,10-tetraazacyclododecane-1,4,7,10-tetrayl)tetraacetic acid-GLGK-conjugates and the controls. The renal activity of [125I]I-HPEM-ADAPT6 was significantly lower compared with all other constructs. In conclusion, the incorporation of the cleavable GLGK-linker did not result in lower renal retention. Therefore, the present study emphasized that, in order to achieve a reduction of renal retention, alternative molecular design strategies may be required for different targeting agents.

Gastrin-releasing peptide receptors (GRPRs) are overexpressed in prostate cancer and are suitable for targeted radionuclidetherapy (TRT). We optimized the bombesin-derived GRPR-antagonist PEG2-RM26 for labeling with 177Lu and further determinedthe effect of treatment with 177Lu-labeled peptide alone or in combination with the anti-HER2 antibody trastuzumab in amurine model. The PEG2-RM26 analog was coupled to NOTA, NODAGA, DOTA and DOTAGA chelators. The peptide-chelatorconjugates were labeled with 177Lu and characterized in vitro and in vivo. A preclinical therapeutic study was performed in PC-3xenografted mice. Mice were treated with intravenous injections (6 cycles) of (A) PBS, (B) DOTAGA-PEG2-RM26, (C) 177LuDOTAGA-PEG2-RM26, (D) trastuzumab or (E) 177Lu-DOTAGA-PEG2-RM26 in combination with trastuzumab. 177Lu-DOTAGA-PEG2-RM26 demonstrated quantitative labeling yield at high molar activity (450 GBq/μmol), high in vivo stability (5 min pi >98% ofradioligand remained when coinjected with phosphoramidon), high affinity to GRPR (KD = 0.4 0.2 nM), and favorablebiodistribution (1 hr pi tumor uptake was higher than in healthy tissues, including the kidneys). Therapy with 177Lu-DOTAGAPEG2-RM26 induced a significant inhibition of tumor growth. The median survival for control groups was significantly shorterthan for treated groups (Group C 66 days, Group E 74 days). Trastuzumab together with radionuclide therapy significantlyimproved survival. No treatment-related toxicity was observed. In conclusion, based on in vitro and in vivo characterization ofthe four 177Lu-labeled PEG2-RM26 analogs, we concluded that 177Lu-DOTAGA-PEG2-RM26 was the most promising analog forTRT. Radiotherapy using 177Lu-DOTAGA-PEG2-RM26 effectively inhibited tumor growth in vivo in a murine prostate cancermodel. Anti-HER2 therapy additionally improved survival.

Gastrin-releasing peptide receptors (GRPRs) are promising targets in oligometastatic prostate cancer. We have recently used ⁵⁵Co (T_1/2 =17.5 h) as a label for next day PET imaging of GRPR expression obtaining high imaging contrast. The radionuclide-chelator combination can significantly influence the biodistribution of radiopeptides. Therefore, in this study, we hypothesized that the properties of ⁵⁵Co-labeled PEG₂-RM26 can be improved by identifying the optimal macrocyclic chelator. All analogues (X-PEG₂-RM26, X = NOTA,NODAGA,DOTA,DOTAGA) were successfully labeled with radiocobalt with high yields and demonstrated high stability. The radiopeptides bound specifically and with picomolar affinity to GRPR and their cellular processing was characterized by low internalization. The best binding capacity was found for DOTA-PEG₂-RM26. Ex vivo biodistribution in PC-3 xenografted mice was characterized by rapid blood clearance via renal excretion. Tumor uptake was similar for all conjugates at 3 h pi, exceeding the uptake in all other organs. Higher kidney uptake and longer retention were associated with N-terminal negative charge (DOTAGA-containing conjugate). Tumor-to-organ ratios increased over time for all constructs, although significant chelator-dependent differences were observed. Concordant with affinity measurements, DOTA-analog had the best retention of activity in tumors, resulting in the highest tumor-to-blood ratio 24 h pi, which translated into high contrast PET/CT imaging (using ⁵⁵Co).

High gastrin releasing peptide receptor (GRPR) expression is associated with numerous cancers including prostate and breast cancer. The aim of the current study was to develop a Co-55-labeled PET agent based on GRPR antagonist RM26 for visualization of GRPR-expressing tumors. Labeling with Co-57 and Co-55, stability, binding specificity, and in vitro and in vivo characteristics of Co-57-NOTA-PEG(2)-RM26 were studied. NOTA-PEG(2)-RM26 was successfully radiolabeled with Co-57 and Co-55 with high yields and demonstrated high stability. The radiopeptide showed retained binding specificity to GRPR in vitro and in vivo. Co-57-NOTA-PEG(2)-RM26 biodistribution in mice was characterized by rapid clearance of radioactivity from blood and normal non-GRPR-expressing organs and low hepatic uptake. The clearance was predominantly renal with a low degree of radioactivity reabsorption. Tumor-to-blood ratios were approximately 200 (3 h pi) and 1000 (24 h pi). The favorable biodistribution of cobalt-labeled NOTA-PEG(2)-RM26 translated into high contrast preclinical PET/CT (using Co-55) and SPECT/CT (using Co-57) images of PC-3 xenografts. The initial biological results suggest that Co-55-NOTA-PEG(2)-RM26 is a promising tracer for PET visualization of GRPR-expressing tumors.

Simultaneous targeting of the prostate-specific membrane antigen (PSMA) and gastrin-releasing peptide receptor (GRPR) could improve the diagnostic accuracy in prostate cancer (PCa). The aim of this study was to develop a PSMA/GRPR-targeting bispecific heterodimer for SPECT and positron emission tomography (PET) diagnostic imaging of PCa. The heterodimer NOTA-DUPA-RM26 was produced by manual solid-phase peptide synthesis. NOTA-DUPA-RM26 was labeled with ¹¹¹In and ⁶⁸Ga, with yields >98%, and demonstrated a high stability and binding specificity to PSMA and GRPR. IC₅₀ values for ^natIn-NOTA-DUPA-RM26 were 4 ± 1 nM towards GRPR and 824 ± 230 nM towards PSMA. An in vivo binding specificity 1 h pi of ¹¹¹In-NOTA-DUPA-RM26 in PC3-PIP-xenografted mice demonstrated partially blockable tumor uptake when co-injected with an excess of PSMA- or GRPR-targeting agents. Simultaneous co-injection of both agents induced pronounced blocking. The biodistribution of ¹¹¹In-NOTA-DUPA-RM26 and ⁶⁸Ga-NOTA-DUPA-RM26 revealed fast activity clearance from the blood and normal organs via the kidneys. Tumor uptake exceeded normal organ uptake for both analogs 1 h pi. ⁶⁸Ga-NOTA-DUPA-RM26 had a significantly lower tumor uptake (8 ± 2%ID/g) compared to ¹¹¹In-NOTA-DUPA-RM26 (12 ± 2%ID/g) 1 h pi. Tumor-to-organ ratios increased 3 h pi, but decreased 24 h pi, for ¹¹¹In-NOTA-DUPA-RM26. MicroPET/CT and microSPECT/CT scans confirmed biodistribution data, suggesting that ⁶⁸Ga-NOTA-DUPA-RM26 and ¹¹¹In-NOTA-DUPA-RM26 are suitable candidates for the imaging of GRPR and PSMA expression in PCa shortly after administration. 

Bombesin (BN) analogs bind with high affinity to gastrin-releasing peptide receptors (GRPRs) that are up-regulated in prostate cancer and can be used for the visualization of prostate cancer. The aim of this study was to investigate the influence of radionuclide-chelator complexes on the biodistribution pattern of the 111In-labeled bombesin antagonist PEG2-D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH2 (PEG2-RM26) and to identify an optimal construct for SPECT imaging. A series of RM26 analogs N-terminally conjugated with NOTA, NODAGA, DOTA and DOTAGA via a PEG2 spacer were radiolabeled with 111In and evaluated both in vitro and in vivo. The conjugates were successfully labeled with 111In with 100% purity and retained binding specificity to GRPR and high stability. The cellular processing of all compounds was characterized by slow internalization. The IC50 values were in the low nanomolar range, with lower IC50 values for positively charged natIn-NOTA-PEG2-RM26 (2.6±0.1 nM) and higher values for negatively charged natIn-DOTAGA-PEG2-RM26 (4.8±0.5 nM). The kinetic binding studies showed KD values in the picomolar range that followed the same pattern as the IC50 data. The biodistribution of all compounds was studied in BALB/c nu/nu mice bearing PC-3 prostate cancer xenografts. Tumor targeting and biodistribution studies displayed rapid clearance of radioactivity from the blood and normal organs via kidney excretion. All conjugates showed similar uptake in tumors at 4 h p.i. The radioactivity accumulation in GRPR-expressing organs was significantly lower for DOTA- and DOTAGA-containing constructs compared to those containing NOTA and NODAGA. 111In-NOTA-PEG2-RM26 with a positively charged complex showed the highest initial uptake and the slowest clearance of radioactivity from the liver. At 4 h p.i., DOTA- and DOTAGA-coupled analogs showed significantly higher tumor-to-organ ratios compared to NOTA- and NODAGA-containing variants. The NODAGA conjugate demonstrated the best retention of radioactivity in tumors, and, at 24 h p.i., had the highest contrast to blood, muscle and bones.

Multiple endocrine neoplasia type 1 (MEN1) is an endocrine tumor syndrome caused by heterozygous mutations in the MEN1 tumor suppressor gene. The MEN1 pancreas of the adolescent gene carrier frequently contain diffusely spread pre-neoplasias and microadenomas, progressing to macroscopic and potentially malignant pancreatic neuroendocrine tumors (P-NET), which represents the major death cause in MEN1. The unveiling of the molecular mechanism of P-NET which is not currently understood fully to allow the optimization of diagnostics and treatment. Glucagon-like peptide 1 (GLP-1) pathway is essential in islet regeneration, i.e. inhibition of β-cell apoptosis and enhancement of β-cell proliferation, yet involvement of GLP-1 in MEN1 related P-NET has not yet been demonstrated. The objective of this work was to investigate if normal sized islets of Men1 heterozygous mice have increased Glucagon-like peptide-1 receptor (GLP-1R) expression compared to wild type islets, and if this increase is detectable in vivo with positron emission tomography (PET) using [68Ga]Ga-DO3A-VS-Cys40-Exendin-4 (68Ga-Exendin-4). 68Ga-Exendin-4 showed potential for early lesion detection in MEN1 pancreas due to increased GLP1R expression.

Peptides mimicking the C-terminus of the small subunit (R2) of Mycobacterium tuberculosis ribonucleotide reductase (RNR) can compete for binding to the large subunit (R1) and thus inhibit RNR activity. Moreover, it has been suggested that the binding of the R2 C-terminus is very similar in M. tuberculosis and Salmonella typhimurium. Based on modeling studies of a crystal structure of the holocomplex of the S. typhimurium enzyme, a benzodiazepine-based turn mimetic was identified and a set of novel compounds incorporating the benzodiazepine scaffold was synthesized. The compounds were evaluated in a competitive fluorescence polarization assay and in an RNR activity assay. These studies revealed that the compounds incorporating the benzodiazepine scaffold have the ability to compete for the M. tuberculosis R2 binding site with low-micromolar affinity.

Radiolabelled antagonistic bombesin analogues are successfully used for targeting of gastrin-releasing peptide receptors (GRPR) that are overexpressed in prostate cancer. Internalization of antagonistic bombesin analogues is slow. We hypothesized that the use of a non-residualizing radioiodine label might not affect the tumour uptake but would reduce the retention in normal organs, where radiopharmaceutical would be internalized. To test this hypothesis, tyrosine was conjugated via diethylene glycol linker to N-terminus of an antagonistic bombesin analogue RM26 to form Tyr-PEG(2)-RM26. [In-111]In-DOTA-PEG(2)-RM26 was used as a control with a residualizing label. Tyr-PEG(2)-RM26 was labelled with I-125 with 95% radiochemical purity and retained binding specificity to GRPR. The IC50 values for Tyr-PEG(2)-RM26 and DOTA-PEG(2)-RM26 were 1.7 +/- 0.3 nM and 3.3 +/- 0.5 nM, respectively. The cellular processing of [I-125]I-Tyr-PEG(2)-RM26 by PC-3 cells showed unusually fast internalization. Biodistribution showed that uptake in pancreas and tumour was GRPR-specific for both radioconjugates. Blood clearance of [I-125]I-Tyr-PEG(2)-RM26 was appreciably slower and activity accumulation in all organs was significantly higher than for [In-111]In-DOTA-PEG(2)-RM26. Tumor uptake of [In-111]In-DOTA-PEG(2)-RM26 was significantly higher than for [I-125]I-Tyr-PEG(2)-RM26, resulting in higher tumour-to-organ ratio for [In-111]In-DOTA-PEG(2)-RM26 at studied time points. Incorporation of amino acids with hydrophilic side-chains next to tyrosine might overcome the problems associated with the use of tyrosine as a prosthetic group for radioiodination.

Important information on the bioactive conformation of biologically active peptides may be obtained by studies of rigid peptides or well-defined secondary structure mimetics incorporated into pseudopeptides. The structural requirements for the interaction of angiotensin II (Ang II, Asp-Arg-Val-Tyr-Ile-His-Pro-Phe) with its AT₁ and AT₂ receptors were the subject of this study.

The main objectives of this work were to synthesize secondary structure mimetics and incorporate these into Ang II. Ang II has been suggested to adopt a turn conformation around Tyr⁴ when interacting with its AT₁ receptor. Therefore, two γ- and one β-turn mimetic scaffolds based on the benzodiazepine structure were synthesized and decorated with side chains. The scaffolds replaced the turn region around Tyr⁴. Most of the pseudopeptides obtained after incorporation into Ang II exhibited high AT₂/AT₁ selectivity and nanomolar affinity to the AT₂ receptor. One pseudopeptide encompassing a β-turn mimetic also displayed AT₁ receptor affinity.

We hypothesized that the position of the guanidino group of the arginine residue and the N-terminal end, in relation to the tyrosine side chain, was critical for AT₂ receptor affinity. Conformational evaluation of the pseudopeptides revealed that in all the compounds with AT₂ receptor affinity the arginine side chain and the N-terminal end could reach common regions, not accessible to the inactive compound. It is proposed that Ang II has a more extended bioactive conformation when binding to the AT₂ receptor than when binding to the AT₁ receptor.

Furthermore, in a Gly scan of Ang II only replacement of the arginine residue reduced the affinity for the AT₂ receptor considerably. Some N-terminal modified Ang II analogues were also synthesized and it was concluded that truncated Ang II analogues interact with the AT₂ receptor differently than Ang II.

Three of the synthesized pseudopeptides were evaluated in AT₂ receptor functional assays and were found to act as agonists.

The present study investigates the importance of the amino acid side chains in the octapeptide angiotensin II (Ang II) for binding to the AT₂ receptor. A Gly scan was performed where each amino acid in Ang II was substituted one-by-one with glycine. The resulting set of peptides was tested for affinity to the AT₂ receptor (porcine myometrial membranes). For a comparison, the peptides were also tested for affinity to the AT₁ receptor (rat liver membranes). Only the substitution of Arg² reduced affinity to the AT₂ receptor considerably (92-fold when compared with Ang II). For the other Gly-substituted analogues the affinity to the AT₂ receptor was only moderately affected. To further investigate the role of the Arg² side chain for receptor binding, we synthesized some N-terminally modified Ang II analogues. According to these studies a positive charge in the N-terminal end of angiotensin III [Ang II (2–8)] is not required for high AT₂ receptor affinity but seems to be more important in Ang II. With respect to the AT₁ receptor, [Gly²]Ang II and [Gly⁸]Ang II lacked binding affinity (K_i > 10 μm). Replacement of the Val³ or Ile⁵ residues with Gly produced only a slight decrease in affinity. Interestingly, substitution of Tyr⁴ or His⁶, which are known to be very important for AT₁ receptor binding, resulted in only 48 and 14 times reduction in affinity, respectively.

Three angiotensin II (Ang II) analogues encompassing a benzodiazepine-based γ-turn-like scaffold have been synthesized. Evaluation of the compounds in a radioligand binding assay showed that they had no affinity to the rat liver AT₁ receptor. However, one of the compounds displayed considerable affinity to the pig uterus AT₂ receptor (K_i = 3.0 nM) while the other two lacked affinity to this receptor. It was hypothesized that the reason for the inactivity of one of these analogues to the AT₂ receptor was that the guanidino group of the Arg² residue and/or the N-terminal end of the pseudopeptide could not interact optimally with the receptor. To investigate this hypothesis, a conformational analysis was performed and a comparison was carried out with the monocyclic methylenedithioether analogue cyclo(S−CH₂−S)[Cys^3,5]Ang II which is known to bind with high affinity to the AT₂ receptor (K_i = 0.62 nM). This comparison showed that, in the compounds with high AT₂ receptor affinity, the guanidino group of the Arg² residue and the N-terminal end could access common regions of space that were not accessible to the inactive compound. To examine the importance of the guanidino group for binding, the Arg side chain was removed by substituting Arg² for Ala² in the analogue having the high affinity. This analogue lacked affinity to AT₂ receptors, which supports the role of the guanidino group in receptor binding.

A benzodiazepine-based beta-turn mimetic has been designed, synthesized, and incorporated into angiotensin II. Comparison of the mimetic with beta-turns in crystallized proteins showed that it most closely resembles a type II beta-turn. The compounds exhibited high to moderate binding affinity for the AT(2) receptor, and one also displayed high affinity for the AT(1) receptor. Molecular modeling showed that the high-affinity compounds could be incorporated into a previously derived model of AT(2) receptor ligands.

New benzodiazepine-based γ-turn mimetics with one or two amino acid side chains were synthesized. The γ-turn mimetics were incorporated into angiotensin II (Ang II) replacing the Val³-Tyr⁴-Ile⁵ or Tyr⁴-Ile⁵ peptide segments. All of the resulting pseudopeptides displayed high AT₂/AT₁ receptor selectivity and exhibited AT₂ receptor affinity in the low nanomolar range. Molecular modeling was used to investigate whether the compounds binding to the AT₂ receptor could position important structural elements in common areas. A previously described benzodiazepine-based γ-turn mimetic with high affinity for the AT₂ receptor was also included in the modeling. It was found that the molecules, although being structurally quite different, could adopt the same binding mode/interaction pattern in agreement with the model hypothesis. The pseudopeptides selected for agonist studies were shown to act as AT₂ receptor agonists being able to induce outgrowth of neurite cells, stimulate p42/p44^mapk, and suppress proliferation of PC12 cells.

Introduction

Pathological formation of fibrosis, is an important feature in many diseases. Fibrosis in liver and pancreas has been associated to metabolic disease including type 1 and 2 diabetes. The current methods for detecting and diagnosing fibrosis are either invasive, or their sensitivity to detect fibrosis in early stage is limited. Therefore, it is crucial to develop non-invasive methods to detect, stage and study the molecular processes that drive the pathology of liver fibrosis. The peptide LRELHLNNN was previously identified as a selective binder to collagen type I with an affinity of 170 nM. Radiolabelled LRELHLNNN thus constitute a potential PET tracer for fibrosis.

Method

LRELHLNNN was conjugated to a DOTA/NOTA moiety via a PEG₂-linker. DOTA-PEG₂-LRELHLNNN was labelled with Gallium-68 and NOTA- PEG₂-LRELHLNNN with aluminium fluoride-18. Biodistribution of [⁶⁸Ga]Ga-DOTA-PEG₂-LRELHLNNN and [¹⁸F]AlF-NOTA-PEG₂-LRELHLNNN was performed in healthy rats ex vivo and in vivo. The ⁶⁸Ga-labelled analogue was evaluated in a mouse model of liver fibrosis by PET/MRI-imaging. The human predicted dosimetry of the tracers was extrapolated from rat ex vivo biodistribution studies at 10, 20, 40, 60, 120, 180 min (only fluoride-18) post-injection.

Results

The peptides were successfully radiolabelled with gallium-68 and aluminium fluoride-18, respectively. The biodistribution of [⁶⁸Ga]Ga-DOTA-PEG₂-LRELHLNNN and [¹⁸F]AlF-NOTA-PEG₂-LRELHLNNN was favorable showing rapid clearance and low background binding in organs where fibrosis may develop. Binding of [⁶⁸Ga]Ga-DOTA-PEG₂-LRELHLNNN to fibrotic liver was higher than surrounding tissues in mice with induced hepatic fibrosis. However, the binding was in the range of SUV 0.3, indicating limited targeting of the tracer to liver. The extrapolated human predicted dosimetric profiles of [⁶⁸Ga]Ga-DOTA-PEG₂-LRELHLNNN and [¹⁸F]AlF-NOTA-PEG₂-LRELHLNNN were beneficial, potentially allowing at least three PET examinations annually.

Conclusions

We describe the modification, radiolabelling and evaluation of the collagen type I binding peptide LRELHLNNN. The resulting radiotracer analogues demonstrated suitable biodistribution and dosimetry. [⁶⁸Ga]Ga-DOTA-PEG₂-LRELHLNNN exhibited binding to hepatic fibrotic lesions and is a promising tool for PET imaging of fibrosis.

Advances in knowledge

Validation of a new collagen targeting PET tracer.

Implications for patient care

Early, non-invasive diagnosis and stratification of fibrosis in order to improve the diagnosis, staging and treatment of patients with diseases involving fibrosis.

Introduction

Overexpression of gastrin-releasing peptide receptors (GRPR) has been reported in several cancers. Bombesin (BN) analogs are short peptides with a high affinity for GRPR. Different BN analogs were evaluated for radionuclide imaging and therapy of GRPR-expressing tumors. We have previously investigated an antagonistic analog of BN (D-Phe-Gln-Trp-Ala-Val-Gly-His-Sta-Leu-NH₂, RM26) conjugated to NOTA via a PEG₂ spacer (NOTA-PEG₂-RM26) labeled with ⁶⁸Ga, ¹¹¹In and Al¹⁸F. ⁶⁸Ga-labeled NOTA-PEG₂-RM26 showed high tumor-to-organ ratios.

Methods

The influence of different macrocyclic chelators (NOTA, NODAGA, DOTA and DOTAGA) on the targeting properties of ⁶⁸Ga-labeled PEG₂-RM26 was studied in vitro and in vivo.

Results

All conjugates were labeled with generator-produced ⁶⁸Ga with high yields and demonstrated high stability and specific binding to GRPR. The IC₅₀ values of ^natGa-X-PEG₂-RM26 (X = NOTA, DOTA, NODAGA, DOTAGA) were 2.3 ± 0.2, 3.0 ± 0.3, 2.9 ± 0.3 and 10.0 ± 0.6 nM, respectively. The internalization of the conjugates by PC-3 cells was low. However, the DOTA-conjugated analog demonstrated a higher internalization rate compared to other analogs. GRPR-specific uptake was found in receptor-positive normal tissues and PC-3 xenografts for all conjugates. The biodistribution of the conjugates was influenced by the choice of the chelator moiety. Although all radiotracers cleared rapidly from the blood, [⁶⁸Ga]Ga-NOTA-PEG₂-RM26 showed significantly lower uptake in lung, muscle and bone compared to the other analogs. The uptake in tumors (5.40 ± 1.04 %ID/g at 2 h p.i.) and the tumor-to-organ ratios (25 ± 3, 157 ± 23 and 39 ± 4 for blood, muscle and bone, respectively) were significantly higher for the NOTA-conjugate than the other analogs.

Conclusions

Chelators had a clear influence on the biodistribution and targeting properties of ⁶⁸Ga-labeled antagonistic BN analogs. Positively charged [⁶⁸Ga]Ga-NOTA-PEG₂-RM26 provided a low kidney radioactivity uptake, high affinity, high tumor uptake and high image contrast.

The overexpression of gastrin-releasing peptide receptor (GRPR) in cancer can be used for peptide-receptor mediated radionuclide imaging and therapy. We have previously shown that an antagonist analog of bombesin RM26 conjugated to 1,4,7-triazacyclononane-N, N', N ''-triacetic acid (NOTA) via a diethyleneglycol (PEG(2)) spacer (NOTA-PEG(2)-RM26) and labeled with Ga-68 can be used for imaging of GRPR-expressing tumors. In this study, we evaluated if a variation of mini-PEG spacer length can be used for optimization of targeting properties of the NOTA-conjugated RM26. A series of analogs with different PEG-length (n = 2, 3, 4, 6) was synthesized, radiolabeled and evaluated in vitro and in vivo. The IC50 values of Ga-nat-NOTA-PEG(n)-RM26 (n = 2, 3, 4, 6) were 3.1 +/- 0.2, 3.9 +/- 0.3, 5.4 +/- 0.4 and 5.8 +/- 0.3 nM, respectively. In normal mice all conjugates demonstrated similar biodistribution pattern, however Ga-68-NOTA-PEG(3)-RM26 showed lower liver uptake. Biodistribution of Ga-68-NOTA-PEG(3)-RM26 was evaluated in nude mice bearing PC-3 (prostate cancer) and BT-474 (breast cancer) xenografts. High uptake in tumors (4.6 +/- 0.6% ID/g and 2.8 +/- 0.4% ID/g for PC-3 and BT-474 xenografts, respectively) and high tumor-to-background ratios (tumor/ blood of 44 +/- 12 and 42 +/- 5 for PC-3 and BT-474 xenografts, respectively) were found already at 2 h p.i. of Ga-68-NOTA-PEG(3)-RM26. Results of this study suggest that variation in the length of the PEG spacer can be used for optimization of targeting properties of peptide-chelator conjugates. However, the influence of the mini-PEG length on biodistribution is minor when di-, tri-, tetra- and hexaethylene glycol are compared.

Objectives: Fibrosis is involved in many chronic diseases. It affects the functionality of vital organs, such as liver, lung, heart and kidney. Two novel imaging agents for positron emission tomography (PET) imaging of fibrosis have previously pre-clinically demonstrated promising target binding and organ distribution characteristics. However, the relevant disease monitoring in the clinical setup would require multiple repetitive examinations per year. Thus, it is of paramount importance to investigate the absorbed doses and total effective doses and thus, the potential maximum number of examinations per year. Methods: Two cyclic peptide (c[CPGRVMHGLHLGDDEGPC]) analogues coupled via an ethylene glycol linker (EG(2)) to either 2-(4,7-bis(2-(tert-butoxy)-2-oxoethyl)-1,4,7-triazonan-1-yl)acetic acid (NO2A-Col) or 4-(4,7-bis(2-(tert-butoxy)-2-oxoethyl)-1,4,7-triazacyclononan-1-yl)-5-(tert-butoxy)-5-oxopentanoic acid (NODAGA-Col) were labelled with Ga-68. The resulting agents, [Ga-68]Ga-NO2A-Col and [Ga-68]Ga-NODAGA-Col, were administered in the tail vein of male and female Sprague-Dawley rats (N = 24). An ex vivo organ distribution study was performed at the 5-, 10-, 20-, 40-, 60- and 120-min time points. The resulting data were extrapolated for the estimation of human organ and total body absorbed and total effective doses using Organ Level Internal Dose Assessment Code software (OLINDA/EXM 1.1) assuming a similar organ distribution pattern between the species. Time-integrated radioactivity in each organ was calculated by trapezoidal integration followed by a single-exponential fit to the data points extrapolated to infinity. The resulting values were used for the residence time calculation. Results: Ex vivo organ distribution data revealed fast blood clearance and washout from most of the organs. Although the highest organ absorbed dose was found for kidneys (0.1 mGy/MBq), this organ was not the dose-limiting one and would allow for the administration of over 1460 MBq per year for both [Ga-68]Ga-NO2A-Col and [Ga-68]Ga-NODAGA-Col. The total effective dose was the limiting parameter with 0.0155/0.0156 (female/male) mSv/MBq and 0.0164/0.0158 (female/male) mSv/MBq, respectively, for [Ga-68]Ga-NO2A-Col and [Ga-68]Ga-NODAGA-Col. This corresponded to the total amount of radioactivity that could be administered per year of 643 and 621 MBq before reaching the annual limit of 10 mSv. Thus, up to six examinations would be possible. The residence time and organ absorbed doses in liver and spleen were higher for [Ga-68]Ga-NODAGA-Col as compared to [Ga-68]Ga-NO2A-Col. Conclusion: The limiting parameter for the administered dose was the total effective dose that would allow for at least six examinations per year that might be sufficient for adequate disease monitoring in longitudinal studies and a routine clinical setup.

[Ga-68]Ga-DO3A-VS-Cys(40)-Exendin-4/PET-CT targeting glucagon like peptide-1 receptor (GLP-1R) has previously demonstrated its potential clinical value for the detection of insulinomas. The production and accessibility of this radiopharmaceutical is one of the critical factors in realization of clinical trials and routine clinical examinations. Previously, the radiopharmaceutical was prepared manually, however larger scale of clinical trials and healthcare requires automation of the production process in order to limit the operator radiation dose as well as improve tracer manufacturing robustness and on-line documentation for enhanced good manufacturing practice (GMP) compliance. A method for Ga-68-labelling of DO3A-VS-Cys(40)-Exendin-4 on a commercially available synthesis platform was developed. Equipment such as Ge-68/Ga-68 generator, synthesis platform, and disposable cassettes for Ga-68-labelling used in the study was purchased from Eckert & Ziegler. DO3A-VS-Cys(40)-Exendin-4 was synthesized in-house. The parameters such as time, temperature, precursor concentration, radical scavenger, buffer concentration, pH, product purification step were investigated and optimised. Reproducible and GMP compliant automated production of [Ga-68]Ga-DO3A-VS-Cys(40)-Exendin-4 was developed. Exendin-4 comprising methionine amino acid residue was prone to oxidation which was strongly influenced by the elevated temperature, radioactivity amount, and precursor concentration. The suppression of the oxidative radiolysis was achieved by addition of ethanol, dihydroxybenzoic acid and ascorbic acid to the reaction buffer as well as by optimizing heating temperature. The non-decay corrected radiochemical yield was 43 +/- 2% with radiochemical purity of over 90% wherein the individual impurity signals in HPLC chromatogram did not exceed 5%. Automated production and quality control methods were established for paving the pathway for broader clinical use of [Ga-68]Ga-DO3A-VS-Cys(40)-Exendin-4.

Objectives: Fibrosis affecting functionality of vital organs such as liver, lung, heart, and kidney, is involved in many chronic diseases. Positron emission tomography (PET) would not only provide precise localization and extent of the affected tissue but also allow the accurate quantification of the fibrotic process for the subsequent prognosis. Methods: A cyclic peptide c[CPGRVMHGLHLGDDEGPC] conjugated either to 2-(4,7-bis(2-(tert-butoxy)-2-oxoethyl)-1,4,7-triazonan-1-yl)acetic acid (NOTA(tBu)2) or 4-(4,7-bis(2-(tert-butoxy)-2-oxoethyl)-1,4,7-triazacyclononan-1-yl)-5-(tert-butoxy)-5-oxopentanoic acid (NODAGA(tBu)3) via polyethylene glycol link (PEG2) was synthesized and labeled with Ga-68. Non-specific organ distribution, blood clearance, and excretion were investigated ex vivo in healthy rats. The binding specificity of the radioligands was assessed in vitro using autoradiography on cryosections of dog fibrotic heart tissue. Results: The yield of NOTA-PEG(2)-c[CPGRVMHGLHLGDDEGPC] and NODAGA-PEG(2)-c[CPGRVMHGLHLGDDEGPC] was 56% and 41%, respectively. Non-decay-corrected radiochemical yield was 80 +/- 5% with radiochemical purity of 95 +/- 4%. Pharmacokinetic studies in healthy male Sprague-Dawley rats showed fast blood clearance and renal excretion. Lower uptake in liver, spleen, and kidney was found for [[Ga-68]Ga-NOTA](+1)-PEG(2)-c [CPGRVMHGLHLGDDEGPC] as compared to [[Ga-68]Ga-NODAGA](0)-PEG(2)-c[CPGRVMHGLHLGDDEGPC]. Histologic evaluation of the left ventricle (LV) myocardium from a dog with severe mitral regurgitation (MR), revealed mild to moderate perivascular and subendocardial, and mild diffuse interstitial fibrosis. The tracer binding to the cryosections of the tissue was specific with the equilibrium Kd of 2.3 +/- 0.8 mu M and 2.1 +/- 0.9 mu M, respectively for [Ga-68]Ga-NO2A-Col and [Ga-68]Ga-NODAGA-Col. Conclusions: Two novel peptide based agents for the imaging of fibrosis by PET were developed. Moderation of the biodistribution could be achieved by variation of the charge on the complex moiety of the agents. The combination of the fast clearance from non-target organs as well as organs of interest such as lung, heart, and liver and binding specificity to the target tissue suggests the potential of the analogs for the imaging of fibrosis.

There is an unmet medical need for non-invasive, sensitive, and quantitative methods for the assessment of fibrosis. Herein, an improved collagelin analogue labelled with gallium-68 for use with positron emission tomography (PET) is presented. A cyclic peptide, c[CPGRVNleHGLHLGDDEGPC], was synthesized by solid-phase peptide synthesis, conjugated to 2-(4,7-bis(2-(tert-butoxy)-2-oxoethyl)-1,4,7-triazonan-1-yl)acetic acid, and labelled with gallium-68. High performance liquid chromatography (HPLC) was used for the quality and stability assessment of the collagelin analogue. Non-specific organ distribution, blood clearance, and excretion rates were investigated in healthy mice and rats using ex vivo organ distribution analysis and dynamic in vivo PET/CT. Mice with carbon tetrachloride (CCl₄) induced liver fibrosis were used for the investigation of specific binding via in vitro frozen section autoradiography, ex vivo organ distribution, and in vivo PET/CT. A non-decay corrected radiochemical yield (48 ± 6%) of [⁶⁸Ga]Ga-NOTA-PEG₂-c[CPGRVNleHGLHLGDDEGPC] ([⁶⁸Ga]Ga-NO2A-[Nle¹³]-Col) with a radiochemical purity of 98 ± 2% was achieved without radical scavengers. The ⁶⁸Ga-labelling was regioselective and stable at ambient temperature for at least 3 h. The autoradiography of the cryosections of fibrotic mouse liver tissue demonstrated a distinct heterogeneous radioactivity uptake that correlated with the fibrosis scores estimated after Sirius Red staining. The blood clearance and tissue washout from the [⁶⁸Ga]Ga-NO2A-[Nle¹³]-Col was fast in both normal and diseased mice. Dosimetry investigation in rats indicated the possibility for 4–5 PET/CT examinations per year. Radiolytic stability of the collagelin analogue was achieved by the substitution of methionine with norleucine amino acid residue without a deterioration of its binding capability. [⁶⁸Ga]Ga-NO2A-[Nle¹³]-Col demonstrated a safe dosimetry profile suitable for repeated scanning.

In the investigation of the structure–activity relationship of nonpeptide AT₂ receptor agonists, a series of substituted benzamide analogues of the selective nonpeptide AT₂ receptor agonist M024 have been synthesised. In a second series, the biphenyl scaffold was compared to the thienylphenyl scaffold and the impact of the isobutyl substituent and its position on AT₁/AT₂ receptor selectivity was also investigated. Both series included several compounds with high affinity and selectivity for the AT₂ receptor. Three of the compounds were also proven to function as agonists at the AT₂ receptor, as deduced from a neurite outgrowth assay, conducted in NG108-15 cells.