Development of Peptide-Based Probes for Molecular Imaging of the Postsynaptic Density in the Brain.

The postsynaptic density (PSD) comprises numerous scaffolding proteins, receptors, and signaling molecules that coordinate synaptic transmission in the brain. Postsynaptic density protein 95 (PSD-95) is a master scaffold protein within the PSD and one of its most abundant proteins and therefore constitutes a very attractive biomarker of PSD function and its pathological changes. Here, we exploit a high-affinity inhibitor of PSD-95, AVLX-144, as a template for developing probes for molecular imaging of the PSD. AVLX-144-based probes were labeled with the radioisotopes fluorine-18 and tritium, as well as a fluorescent tag. Tracer binding showed saturable, displaceable, and uneven distribution in rat brain slices, proving effective in quantitative autoradiography and cell imaging studies. Notably, we observed diminished tracer binding in human post-mortem Parkinson's disease (PD) brain slices, suggesting postsynaptic impairment in PD. We thus offer a suite of translational probes for visualizing and understanding PSD-related pathologies.

Poly(ADP-ribose) polymerase (PARP)-targeted PET imaging in non-oncology application: a pilot study in preclinical models of nonalcoholic steatohepatitis.

Poly(ADP-ribose) polymerase (PARP) activation often indicates a disruptive signal to lipid metabolism, the physiological alteration of which may be implicated in the development of non-alcoholic fatty liver disease. The objective of this study was to evaluate the capability of [68Ga]DOTA-PARPi PET to detect hepatic PARP expression in a non-alcoholic steatohepatitis (NASH) mouse model. In this study, male C57BL/6 mice were subjected to a choline-deficient, L-amino acid-defined, high-fat diet (CDAHFD) for a 12-week period to establish preclinical NASH models. [68Ga]DOTA-PARPi PET imaging of the liver was conducted at the 12-week mark after CDAHFD feeding. Comprehensive histopathological analysis, covering hepatic steatosis, inflammation, fibrosis, along with blood biochemistry, was performed in both NASH models and control groups. Despite the induction of severe inflammation, steatosis and fibrosis in the liver of mice with the CDAHFD-NASH model, PET imaging of NASH with [68Ga]-DOTA-PARPi did not reveal a significantly higher uptake in NASH models compared to the control. This underscores the necessity for further development of new chelator-based PARP1 tracers with high binding affinity to enable the visualization of PARP1 changes in NASH pathology.

Radiochemistry for positron emission tomography.

Positron emission tomography (PET) constitutes a functional imaging technique that is harnessed to probe biological processes in vivo. PET imaging has been used to diagnose and monitor the progression of diseases, as well as to facilitate drug development efforts at both preclinical and clinical stages. The wide applications and rapid development of PET have ultimately led to an increasing demand for new methods in radiochemistry, with the aim to expand the scope of synthons amenable for radiolabeling. In this work, we provide an overview of commonly used chemical transformations for the syntheses of PET tracers in all aspects of radiochemistry, thereby highlighting recent breakthrough discoveries and contemporary challenges in the field. We discuss the use of biologicals for PET imaging and highlight general examples of successful probe discoveries for molecular imaging with PET - with a particular focus on translational and scalable radiochemistry concepts that have been entered to clinical use.

Structure-activity relationship of pyrazol-4-yl-pyridine derivatives and identification of a radiofluorinated probe for imaging the muscarinic acetylcholine receptor M4.

There is an accumulating body of evidence implicating the muscarinic acetylcholine receptor 4 (M4) in schizophrenia and dementia with Lewy bodies, however, a clinically validated M4 positron emission tomography (PET) radioligand is currently lacking. As such, the aim of this study was to develop a suitable M4 PET ligand that allows the non-invasive visualization of M4 in the brain. Structure-activity relationship studies of pyrazol-4-yl-pyridine derivates led to the discovery of target compound 12 - a subtype-selective positive allosteric modulator (PAM). The radiofluorinated analogue, [18F]12, was synthesized in 28 ± 10% radiochemical yield, >37 GBq/µmol and an excellent radiochemical purity >99%. Initial in vitro autoradiograms on rodent brain sections were performed in the absence of carbachol and showed moderate specificity as well as a low selectivity of [18F]12 for the M4-rich striatum. However, in the presence of carbachol, a significant increase in tracer binding was observed in the rat striatum, which was reduced by >60% under blocking conditions, thus indicating that orthosteric ligand interaction is required for efficient binding of [18F]12 to the allosteric site. Remarkably, however, the presence of carbachol was not required for high specific binding in the non-human primate (NHP) and human striatum, and did not further improve the specificity and selectivity of [18F]12 in higher species. These results pointed towards significant species-differences and paved the way for a preliminary PET study in NHP, where peak brain uptake of [18F]12 was found in the putamen and temporal cortex. In conclusion, we report on the identification and preclinical development of the first radiofluorinated M4 PET radioligand with promising attributes. The availability of a clinically validated M4 PET radioligand harbors potential to facilitate drug development and provide a useful diagnostic tool for non-invasive imaging.

[Hyperlactataemia].

It is a common but flawed presumption that blood lactate reflects the lactic acid production in the body's tissues. Lactate is formed directly from pyruvate and functions to dampen reductions in intracellular pH through lactate-H+ cotransport to the extracellular space. Though this may give rise to elevated blood lactate, increased lactate production is not the cause of metabolic acidosis in such instances. "Lactic acidosis" is thus an inappropriate term as it indicates causality and in this review, we suggest that in the future, the term "hyperlactataemia-associated metabolic acidosis" should be used instead.

Fluorine-18 labeled aldehydes as prosthetic groups for oxime coupling with a FVIIa protein.

New 18 F-labeled nonvolatile aldehyde prosthetic groups derived from [18 F]F-Py-TFP and spirocyclic iodonium (III)ylide precursors for late stage 18 F-labeling were developed. These precursors were characterized, 18 F-labeled, and compared in reactivity for oxime coupling. Oxime coupling was performed on an amino-oxy modified inhibited factor VII (FVIIai-ONH2 ) in low concentration to prove the applicability of the proposed method.

Optimization and Evaluation of Al18F Labeling Using a NOTA-or RESCA1-Conjugated AE105 Peptide Antagonist of uPAR.

Fluorine-18 displays almost ideal decay properties for positron emission tomography (PET) and allows for large scale production. As such, simplified methods to radiolabel peptides with fluorine-18 are highly warranted. Chelation of aluminium fluoride-18 toward specific peptides represents one method to achieve this. With the current methods, chelation of aluminium fluoride-18 can be achieved using NOTA-conjugated peptides. However, the heating to 90-100◦C that is required for this chelation approach may be deleterious to the targeting moiety of the probe. Recently, a new chelator, RESCA1, was developed allowing Al18F chelation at room temperature. Here, we optimize the labeling procedure enabling high chelation efficacy of fluoride-18 at 22◦C, even at full batch labeling. The optimized procedure was tested by Al18F-labeling of RESCA1-AE105-a uPAR targeting peptide. NOTA-AE105 was also labeled with Al18F, and the two peptides were compared head-to-head. [18F]AlF-NOTA-AE105 and [18F]AlF-RESCA1-AE105 could be produced in equal radiochemical yields (RCY), radiochemical purities (RCP) and molar activities. Additionally, the two peptides showed comparable binding affinity to uPAR and uptake in cells expressing the uPAR, when evaluated in vitro. Overall, we found that the performances of [18F]AlF-NOTA-AE105 and [18F]AlF-RESCA1-AE105 were grossly comparable, but importantly RESCA1 can be labeled with aluminium fluoride-18 at 22◦C. Consequently, this study showed that RESCA1 is superior to NOTA with respect to Al18F chelation of temperature sensitive molecules, such as thermolabile peptides and proteins as well as that full batch chelation of RESCA1 with fluoride-18 is possible.

Affinity-Guided Conjugation to Antibodies for Use in Positron Emission Tomography.

The radionuclide copper-64 is widely used in combination with biomolecules, such as antibodies, for positron emission tomography (PET). Copper-64 is ideal for the imaging of biomolecules with long circulation times due to its relatively long half-life, and when conjugated to an antibody, specific cells can be targeted in vivo. Here, we have prepared a trastuzumab-chelator conjugate by using affinity-guided conjugation, in which an azide was attached to the antibody prior to a strain promoted azide-alkyne cycloaddition reaction with DBCO-PEG4-NOTA. The conjugate was benchmarked against a standard nonspecific labeled trastuzumab-NOTA conjugate. The conjugates were tested for incorporation of copper-64, stability in buffer and plasma, and tumor targeting in vivo using PET imaging of mice with xenograft tumors expressing HER2. Both conjugates showed good incorporation of copper-64 and a high stability with less than 10% degradation after 36 h. Furthermore, both conjugates showed accumulation at the tumor site with mean uptake of 7.2 ± 2.4%ID/g and 5.2 ± 1.3%ID/g after 40 h for the affinity-guided labeled trastuzumab and the nonspecific labeled trastuzumab, respectively.

Oxime Coupling of Active Site Inhibited Factor Seven with a Nonvolatile, Water-Soluble Fluorine-18 Labeled Aldehyde.

A nonvolatile fluorine-18 aldehyde prosthetic group was developed from [18F]SFB, and used for site-specific labeling of active site inhibited factor VII (FVIIai). FVIIai has a high affinity for tissue factor (TF), a transmembrane protein involved in angiogenesis, proliferation, cell migration, and survival of cancer cells. A hydroxylamine N-glycan modified FVIIai (FVIIai-ONH2) was used for oxime coupling with the aldehyde [18F]2 under mild and optimized conditions in an isolated RCY of 4.7 ± 0.9%, and a synthesis time of 267 ± 5 min (from EOB). Retained binding and specificity of the resulting [18F]FVIIai to TF was shown in vitro. TF-expression imaging capability was evaluated by in vivo PET/CT imaging in a pancreatic human xenograft cancer mouse model. The conjugate showed exceptional stability in plasma (>95% at 4 h) and a binding fraction of 90%. In vivo PET/CT imaging showed a mean tumor uptake of 3.8 ± 0.2% ID/g at 4 h post-injection, a comparable uptake in liver and kidneys, and low uptake in normal tissues. In conclusion, FVIIai was labeled with fluorine-18 at the N-glycan chain without affecting TF binding. In vitro specificity and a good in vivo imaging contrast at 4 h postinjection was demonstrated.

Site-Specific 64Cu Labeling of the Serine Protease, Active Site Inhibited Factor Seven Azide (FVIIai-N3), Using Copper Free Click Chemistry.

A method for site-specific radiolabeling of the serine protease active site inhibited factor seven (FVIIai) with 64Cu has been applied using a biorthogonal click reaction. FVIIai binds to tissue factor (TF), a trans-membrane protein involved in hemostasis, angiogenesis, proliferation, cell migration, and survival of cancer cells. First a single azide moiety was introduced in the active site of this 50 kDa protease. Then a NOTA moiety was introduced via a strain promoted azide-alkyne reaction and the corresponding conjugate was labeled with 64Cu. Binding to TF and the stability was evaluated in vitro. TF targeting capability of the radiolabeled conjugate was tested in vivo by positron emission tomography (PET) imaging in pancreatic human xenograft cancer mouse models with various TF expressions. The conjugate showed good stability (>91% at 16 h), an immunoreactivity of 93.5%, and a mean tumor uptake of 2.1 ± 0.2%ID/g at 15 h post injection. In conclusion, FVIIai was radiolabeled with 64Cu in single well-defined position of the protein. This method can be utilized to prepare conjugates from serine proteases with the label at a specific position.

PET Imaging of Tissue Factor in Pancreatic Cancer Using 64Cu-Labeled Active Site-Inhibited Factor VII.

UNLABELLED: Tissue factor (TF) is the main initiator of the extrinsic coagulation cascade. However, TF also plays an important role in cancer. TF expression has been reported in 53%-89% of all pancreatic adenocarcinomas, and the expression level of TF has in clinical studies correlated with advanced stage, increased microvessel density, metastasis, and poor overall survival. Imaging of TF expression is of clinical relevance as a prognostic biomarker and as a companion diagnostic for TF-directed therapies currently under clinical development. Factor VII (FVII) is the natural ligand to TF. The purpose of this study was to investigate the possibility of using active site-inhibited FVII (FVIIai) labeled with (64)Cu for PET imaging of TF expression. METHODS: FVIIai was conjugated to 2-S-(4-isothiocyanatobenzyl)-1,4,7-triazacyclononane-1,4,7-triacetic acid (p-SCN-Bn-NOTA) and labeled with (64)Cu ((64)Cu-NOTA-FVIIai). Longitudinal in vivo PET imaging was performed at 1, 4, 15, and 36 h after injection of (64)Cu-NOTA-FVIIai in mice with pancreatic adenocarcinomas (BxPC-3). The specificity of TF imaging with (64)Cu-NOTA-FVIIai was investigated in subcutaneous pancreatic tumor models with different levels of TF expression and in a competition experiment. In addition, imaging of orthotopic pancreatic tumors was performed using (64)Cu-NOTA-FVIIai and PET/MRI. In vivo imaging data were supported by ex vivo biodistribution, flow cytometry, and immunohistochemistry. RESULTS: Longitudinal PET imaging with (64)Cu-NOTA-FVIIai showed a tumor uptake of 2.3 ± 0.2, 3.7 ± 0.3, 3.4 ± 0.3, and 2.4 ± 0.3 percentage injected dose per gram at 1, 4, 15, and 36 h after injection, respectively. An increase in tumor-to-normal-tissue contrast was observed over the imaging time course. Competition with unlabeled FVIIai significantly (P < 0.001) reduced the tumor uptake. The tumor uptake observed in models with different TF expression levels was significantly different from each other (P < 0.001) and was in agreement with the TF level evaluated by TF immunohistochemistry staining. Orthotopic tumors were clearly visible on the PET/MR images, and the uptake of (64)Cu-NOTA-FVIIai was colocalized with viable tumor tissue. CONCLUSION: (64)Cu-NOTA-FVIIai is well suited for PET imaging of tumor TF expression, and imaging is capable of distinguishing the TF expression level of various pancreatic tumor models.

Quantitative PET Imaging of Tissue Factor Expression Using 18F-Labeled Active Site-Inhibited Factor VII.

UNLABELLED: Tissue factor (TF) is upregulated in many solid tumors, and its expression is linked to tumor angiogenesis, invasion, metastasis, and prognosis. A noninvasive assessment of tumor TF expression status is therefore of obvious clinical relevance. Factor VII is the natural ligand to TF. Here we report the development of a new PET tracer for specific imaging of TF using an (18)F-labeled derivative of factor VII. METHODS: Active site-inhibited factor VIIa (FVIIai) was obtained by inactivation with phenylalanine-phenylalanine-arginine-chloromethyl ketone. FVIIai was radiolabeled with N-succinimidyl 4-(18)F-fluorobenzoate and purified. The corresponding product, (18)F-FVIIai, was injected into nude mice with subcutaneous human pancreatic xenograft tumors (BxPC-3) and investigated using small-animal PET/CT imaging 1, 2, and 4 h after injection. Ex vivo biodistribution was performed after the last imaging session, and tumor tissue was preserved for molecular analysis. A blocking experiment was performed in a second set of mice. The expression pattern of TF in the tumors was visualized by immunohistochemistry and the amount of TF in tumor homogenates was measured by enzyme-linked immunosorbent assay and correlated with the uptake of (18)F-FVIIai in the tumors measured in vivo by PET imaging. RESULTS: The PET images showed high uptake of (18)F-FVIIai in the tumor regions, with a mean uptake of 2.5 ± 0.3 percentage injected dose per gram (%ID/g) (mean ± SEM) 4 h after injection of 7.3-9.3 MBq of (18)F-FVIIai and with an average maximum uptake in the tumors of 7.1 ± 0.7 %ID/g at 4 h. In comparison, the muscle uptake was 0.2 ± 0.01 %ID/g at 4 h. At 4 h, the tumors had the highest uptake of any organ. Blocking with FVIIai significantly reduced the uptake of (18)F-FVIIai from 2.9 ± 0.1 to 1.4 ± 0.1 %ID/g (P < 0.001). The uptake of (18)F-FVIIai measured in vivo by PET imaging correlated (r = 0.72, P < 0.02) with TF protein level measured ex vivo. CONCLUSION: (18)F-FVIIai is a promising PET tracer for specific and noninvasive imaging of tumor TF expression. The tracer merits further development and clinical translation, with potential to become a companion diagnostics for emerging TF-targeted therapies.

Synthesis and characterization of (18)F-labeled active site inhibited factor VII (ASIS).

Activated factor VII blocked in the active site with Phe-Phe-Arg-chloromethyl ketone (active site inhibited factor VII (ASIS)) is a 50-kDa protein that binds with high affinity to its receptor, tissue factor (TF). TF is a transmembrane glycoprotein that plays an important role in, for example, thrombosis, metastasis, tumor growth, and tumor angiogenesis. The aim of this study was to develop an (18)F-labeled ASIS derivative to assess TF expression in tumors. Active site inhibited factor VII was labeled using N-succinimidyl-4-[(18)F]fluorobenzoate, and the [(18)F]ASIS was purified on a PD-10 desalting column. The radiochemical yield was 25 ± 6%, the radiochemical purity was >97%, and the pseudospecific radioactivity was 35 ± 9 GBq/µmol. The binding efficacy was evaluated in pull-down experiments, which monitored the binding of unlabeled ASIS and [(18)F]ASIS to TF and to a specific anti-factor VII antibody (F1A2-mAb). No significant difference in binding efficacy between [(18)F]ASIS and ASIS could be detected. Furthermore, [(18)F]ASIS was relatively stable in vitro and in vivo in mice. In conclusion, [(18)F]ASIS has for the first time been successfully synthesized as a possible positron emission tomography tracer to image TF expression levels. In vivo positron emission tomography studies to evaluate the full potential of [(18)F]ASIS are in progress.