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Immune checkpoint inhibitor (ICI) therapy has emerged as a major treatment option for a variety of cancers. Among the immune checkpoints addressed, the programmed death receptor 1 (PD-1) and its ligand PD-L1 are the key targets for an ICI. PD-L1 has especially been proven to be a reproducible biomarker allowing for therapy decisions and monitoring therapy success. However, the expression of PD-L1 is not only heterogeneous among and within tumor lesions, but the expression is very dynamic and changes over time. Immunohistochemistry, which is the standard diagnostic tool, can only inadequately address these challenges. On the other hand, molecular imaging techniques such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) provide the advantage of a whole-body scan and therefore fully address the issue of the heterogeneous expression of checkpoints over time. Here, we provide an overview of existing PET, SPECT, and optical imaging (OI) (radio)tracers for the imaging of the upregulation levels of PD-1 and PD-L1. We summarize the preclinical and clinical data of the different molecule classes of radiotracers and discuss their respective advantages and disadvantages. At the end, we show possible future directions for developing new radiotracers for the imaging of PD-1/PD-L1 status in cancer patients.

Background/Objectives: Positron emission tomography (PET) and single-photon emission computed tomography (SPECT) are highly sensitive clinical imaging modalities, frequently employed in conjunction with magnetic resonance imaging (MRI) or computed tomography (CT) for diagnosing a wide range of disorders. Efficient and robust radiolabeling methods are needed to accommodate the increasing demand for PET and SPECT tracer development. Copper-mediated radiohalogenation (CMRH) reactions enable rapid late-stage preparation of radiolabeled arenes, yet synthetic challenges and radiolabeling precursors’ instability can limit the applications of CMRH approaches. Methods: A series of aryl-boronic acids were converted into their corresponding aryl-boronic acid 1,1,2,2-tetraethylethylene glycol esters [ArB(Epin)s] and aryl-boronic acid 1,1,2,2-tetrapropylethylene glycol esters [ArB(Ppin)s] as stable and versatile precursor building blocks for radiolabeling via CMRH. General protocols for the preparation of 18F-labeled and 123I-labeled arenes utilizing CMRH of these substrates were developed and applied. The radiochemical conversions (RCC) were determined by radio-(U)HPLC. Results: Both ArB(Epin)s and ArB(Ppin)s-based radiolabeling precursors were prepared in a one-step synthesis with chemical yields of 49–99%. Radiolabeling of the aryl-boronic esters with fluorine-18 or iodine-123 via CMRH furnished the corresponding radiolabeled arenes with RCC of 7–99% and 10–99%, respectively. Notably, a radiohalogenated prosthetic group containing a vinyl sulfone motif was obtained with an activity yield (AY) of 18 ± 3%, and applied towards the preparation of two clinically relevant PET tracers. Conclusions: This approach enables the synthesis of stable radiolabeling precursors and thus provides increased versatility in the application of CMRH, thereby supporting the development of novel PET and SPECT radiotracers.

Background/Objectives: The cell surface proteoglycan glypican-3 (GPC3) is reportedly overexpressed in hepatocellular carcinoma (HCC) tissues, but not in benign liver tissues, rendering this protein a potential target for radionuclide theranostic approaches. Peptides are generally a promising class of targeting molecules for the development of radioligands because they combine straightforward synthetic access with favorable pharmacokinetics. Among the published peptides with disclosed structures, one of the most promising radioligands is [18F]AlF-NOTA-TJ12P2, which has a reported comparably high binding affinity to GPC3 and a high hydrophilicity. In this study, we aimed to design novel GPC3-targeting radioligands based on the TJ12P2 peptidic scaffold. Methods: Peptides were synthesized on solid phase using an Fmoc protecting group strategy. For comparative investigations, the reference nanobody HN3 was expressed in E. coli, isolated and subsequently modified with NODA-GA or SulfoCy3. The binding of native peptides, scrambled variants and reference nanobodies to GPC3 was investigated by surface plasmon resonance (SPR) interaction analysis, and fluorescently labeled versions of peptides and nanobodies were used for fluorescence microscopy in HepG2 (GPC3+) or SK Hep1 (GPC3−) cells. The chelator-bearing peptides were radiolabeled with gallium-67 and their stability towards radiolysis and in human serum was investigated. The binding of radiolabeled peptides and nanobodies to HepG2 cells was assessed in real-time ligand binding experiments. Results: The synthesized native peptides did not exhibit binding towards GPC3 in SPR interaction analyses, and the observed response was comparable to that of the scrambled variants at equal concentrations. Additionally, no binding to or uptake of the fluorescent constructs into cells was observed with fluorescence microscopy regardless of cellular GPC3 expression level. In real-time radioligand binding experiments, very fast association and dissociation of the gallium-67 labeled peptides to GPC3 positive HepG2 cells was observed, suggesting either extremely fast binding kinetics or unspecific binding of the peptides. Conclusions: Taken together, these findings suggest that the peptide TJ12P2 lacks specific binding to GPC3 in vitro and might not serve as a basis for the development of radioligands targeting GPC3.

Background Immune checkpoint inhibitor therapy addressing the PD-1/PD-L1 axis is a promising approach in cancer treatment. A clinically suitable radiotracer would allow molecular imaging of the temporospatial changes in tumor PD-L1 expression. This could enable the clinicians to select eligible patients for checkpoint therapy and monitor therapeutic efficacy. Results Four biphenyl-based small-molecule PD-L1 ligands were synthesized using a convergent synthetic route, with a linear sequence of up to eleven steps. Two candidates were covalently labeled with 18 F via either an azido glycosyl or PEG2 moiety, while the other two were modified with a RESCA chelator for Al[ 18 F]F 2+ -labeling. The lipophilicity was assessed through determination of log  D 7.4 values. In vitro binding affinities (inhibition constant, K i ) toward PD-L1 were determined in competition with one of our previously published biphenyl-based small-molecule ( K D  =  ~ 21 nM). Compared to this compound, both covalently labeled 18 F-ligands exhibited decreased water solubility (log  D 7.4  ~ − 2.5 and − 2.7), along with a markedly reduced ( K i  = 200‒500 nM) affinity. This was in line with in vivo small animal PET, where both compounds were characterized by a negligible tumor uptake, lack of contrast between target-positive/negative tumors and exclusively unfavorable hepatobiliary excretion. Similar results were observed for the chelator-modified ligands with slightly increased hydrophilicity (log  D 7.4  ~ − 2.8 and − 2.9), showing a binding affinity of 150 nM for one compound, while binding was lost completely for the other. Again, a poor in vivo performance was observed, characterized by hepatobiliary clearance and lack of specific tumor uptake in the PD-L1 positive tumor. Conclusion Four biphenyl-based, 18 F-labeled PD-L1 radioligands were developed using prosthetic groups (azido glycosyl or PEG2) for covalent fluorination and Al[ 18 F]F 2 ⁺-complexation with the RESCA chelator. Despite limited in vitro and in vivo performance, these fluorination approaches offer a foundation for developing improved PD-L1 radioligands after increasing the hydrophilicity and the spacing between the radiolabel and binding motif.

BackgroundProgrammed cell death ligand 1 (PD-L1) plays a critical role in the tumor microenvironment and overexpression in several solid cancers has been reported. This was associated with a downregulation of the local immune response, specifically of T-cells. Immune checkpoint inhibitors showed a potential to break this localized immune paralysis, but only 30% of patients are considered responders. New diagnostic approaches are therefore needed to determine patient eligibility. Small molecule radiotracers targeting PD-L1, may serve as such diagnostic tools, addressing the heterogeneous PD-L1 expression between and within tumor lesions, thus aiding in therapy decisions.ResultsFour biphenyl-based small-molecule PD-L1 ligands were synthesized using a convergent synthetic route with a linear sequence of up to eleven steps. As a chelator NODA-GA, CB-TE2A or DiAmSar was used to allow radiolabeling with copper-64 ([64Cu]Cu-14–[64Cu]Cu-16). In addition, a dimeric structure based on DiAmSar was synthesized ([64Cu]Cu-17). All four radioligands exhibited high proteolytic stability (> 95%) up to 48 h post-radiolabeling. Saturation binding yielded moderate affinities toward PD-L1, ranging from 100 to 265 nM. Real-time radioligand binding provided more promising KD values around 20 nM for [64Cu]Cu-14 and [64Cu]Cu-15. In vivo PET imaging in mice bearing both PC3 PD-L1 overexpressing and PD-L1-mock tumors was performed at 0–2, 4–5 and 24–25 h post injection (p.i.). This revealed considerably different pharmacokinetic profiles, depending on the substituted chelator. [64Cu]Cu-14, substituted with NODA-GA, showed renal clearance with low liver uptake, whereas substitution with the cross-bridged cyclam chelator CB-TE2A resulted in a primarily hepatobiliary clearance. Notably, the monomeric DiAmSar radioligand [64Cu]Cu-16 demonstrated a higher liver uptake than [64Cu]Cu-15, but was still renally cleared as evidenced by the lack of uptake in gall bladder and intestines. The dimeric structure [64Cu]Cu-17 showed extensive accumulation and trapping in the liver but was also cleared via the renal pathway. Of all tracer candidates and across all timepoints, [64Cu]Cu-17 showed the highest accumulation at 24 h p.i. in the PD-L1-overexpressing tumor of all timepoints and all radiotracers, indicating drastically increased circulation time upon dimerization of two PD-L1 binding motifs.ConclusionsThis study shows that chelator choice significantly influences the pharmacokinetic profile of biphenyl-based small molecule PD-L1 radioligands. The NODA-GA-conjugated radioligand [64Cu]Cu-14 exhibited favorable renal clearance; however, the limited uptake in tumors suggests the need for structural modifications to the binding motif for future PD-L1 radiotracers.

Successful treatment of solid cancers relies on precise diagnosis, e.g. using noninvasive molecular imaging, followed by surgical removal and/or chemo/immunotherapy. Despite advances in pre-operative imaging, real-time intraoperative tools remain limited, which often results in high rates of tumor-positive margins and recurrence after tumor resection. To address this limitation, we aimed to develop multifunctional fibroblast activation protein alpha (FAP) targeting tracers for bimodal medical imaging, enabling both pre-operative noninvasive molecular imaging via positron emission tomography (PET) and optical visualization during intraoperative fluorescence-guided surgery. NODAGA-FAP647 and NODAGA-FAP800 targeting human FAP (hFAP) were synthesized bearing a (R)-NODAGA chelator and a fluorophore (AlexaFluor647 or IRDye800CW, respectively). Binding affinities and binding kinetics of both unlabeled and Ga-labeled compounds were evaluated in vitro using HT1080 cells (hFAP-expressing and wild type, WT) along with respective frozen xenograft tissue sections. Using real-time binding, both compounds exhibited picomolar binding affinities to hFAP via radioactive/fluorescent detection. This was primarily driven by low dissociation rate constants in vitro. Pharmacokinetics and tumor uptake were evaluated via PET and fluorescence imaging in mice bearing xenografts from the same cells. In vivo, both compounds were rapidly distributed and accumulated in hFAP-expressing but not WT-HT1080 tumors within 10-20 min post-injection. Fluorescence imaging showed a similarly good and selective tumor uptake in the first two hours and a qualitatively visible difference compared to WT-HT1080 beyond 24 h. Both compounds were quickly cleared from normal tissue and excreted renally. Two FAP-targeting bimodal ligands were synthesized and evaluated in vitro and in vivo, showing high specificity and selectivity, along with rapid and selective tumor accumulation. Their long tumor retention and high imaging contrast make them promising candidates for clinical translation.

Early detection and treatment of cancers can significantly increase patient prognosis and enhance the quality of life of affected patients. The emerging significance of the tumor microenvironment (TME) as a new frontier for cancer diagnosis and therapy may be exploited by radiolabeled tracers for diagnostic imaging techniques such as positron emission tomography (PET). Cancer-associated fibroblasts (CAFs) within the TME are identified by biomarkers such as fibroblast activation protein alpha (FAPα), which are expressed on their surfaces. Targeting FAPα using small-molecule 18F-labeled inhibitors (FAPIs) has recently garnered significant attention for non-invasive tumor visualization using PET. Herein, two potent aryl-fluorosulfate-based FAPIs, 12 and 13, were synthetically prepared, and their inhibition potency was determined using a fluorimetric FAP assay to be IC50 9.63 and 4.17 nM, respectively. Radiofluorination was performed via the sulfur [18F]fluoride exchange ([18F]SuFEx) reaction to furnish [18F]12 and [18F]13 in high activity yields (AY) of 39–56% and molar activities (Am) between 20–55 GBq/µmol. In vitro experiments focused on the stability of the radiolabeled FAPIs after incubation with human serum, liver microsomes and liver cytosol. Preliminary PET studies of the radioligands were performed in healthy mice to investigate the in vivo biodistribution and 18F defluorination rate. Fast pharmacokinetics for the FAP-targeting tracers were retained and considerable bone uptake, caused by either 18F defluorination or radioligand accumulation, was observed. In summary, our findings demonstrate the efficiency of [18F]SuFEx as a radiolabeling method as well as its advantages and limitations with respect to PET tracer development.

Radiolabeled fluorescent dyes are decisive for bimodal imaging as well as highly in demand for nuclear- and optical imaging. Silicon-rhodamines (SiRs) show unique near-infrared (NIR) optical properties, large quantum yields and extinction coefficients as well as high photostability. Here, we describe the synthesis, characterization and radiolabeling of novel NIR absorbing and emitting fluorophores from the silicon-rhodamine family for use in optical imaging (OI) combined with positron emission tomography (PET) or single photon emission computed tomography (SPECT), respectively. The presented photostable SiRs were characterized using NMR-, UV-Vis-NIR-spectroscopy and mass spectrometry. Moreover, the radiolabeling conditions using fluorine-18 or iodine-123 were extensively explored. After optimization, the radiofluorinated NIR imaging agents were obtained with radiochemical conversions (RCC) up to 70% and isolated radiochemical yields (RCY) up to 54% at molar activities of g.t. 70 GBq/µmol. Radioiodination delivered RCCs over 92% and allowed to isolate the 123I-labeled product in RCY of 54% at a molar activity of g.t. 7.6 TBq/µmol. The radiofluorinated SiRs exhibit in vitro stabilities g.t. 70% after two hours in human serum. The first described radiolabeled SiRs are a promising step toward their further development as multimodal PET/SPECT-NIR imaging agents for planning and subsequent imaging-guided oncological surgery.

Natural polyphenols like oligomeric catechins (procyanidins) derived from green tea and herbal medicines are interesting compounds for pharmaceutical research due to their ability to protect against carcinogenesis in animal models. It is nevertheless still unclear how intracellular pathways are modulated by polyphenols. Monomeric polyphenols were shown to affect the activity of some protein phosphatases (PPs). The three phosphatases of regenerating liver (PRLs) are close relatives and promising therapeutic targets in cancer. In the present study we show that several procyanidins inhibit the activity of all three members of the PRL family in the low micromolar range, whereas monomeric epicatechins show weak inhibitory activity. Increasing the number of catechin units in procyanidins to more than three does not further enhance the potency. Remarkably, the tested procyanidins showed selectivity in vitro when compared to other PPs, and over 10-fold selectivity toward PRL-1 over PRL-2 and PRL-3. As PRL overexpression induces cell migration compared to control cells, the effect of procyanidins on this phenotype was studied. Treatment with procyanidin C2 led to a decrease in cell migration of PRL-1- and PRL-3-overexpressing cells, suggesting the compound-dependent inhibition of PRL-promoted cell migration. Treatment with procyanidin B3 led to selective suppression of PRL-1 overexpressing cells, thereby corroborating the selectivity toward PRL-1- over PRL-3 in vitro. Together, our results show that procyanidins negatively affect PRL activity, suggesting that PRLs could be targets in the polypharmacology of natural polyphenols. Furthermore, they are interesting candidates for the development of PRL-1 inhibitors due to their low cellular toxicity and the selectivity within the PRL family.