Promising potential of a 18F-labelled small-molecular radiotracer to evaluate PD-L1 expression in tumors by PET imaging

Synthesis and immunotherapy efficacy of a PD-L1 small-molecule inhibitor combined with its 131I-iodide labelled isostructural compound

Although antibody-based immune checkpoint blockades have been successfully used in antitumor immunotherapy, the low response rate is currently the main problem. In this work, a small-molecule programmed cell death-ligand (PD-L1) inhibitor, LG-12, was developed and radiolabeled with 131I to obtain the chemically and biologically identical radiopharmaceutical [131I]LG-12, which aimed to improve the antitumor effect by combination of LG-12 and [131I]LG-12. LG-12 showed high inhibitory activity to PD-1/PD-L1 interaction. The results of cell uptake and biodistribution studies indicated that [131I]LG-12 could specifically bind to PD-L1 in B16-F10 tumors. It could induce immunogenic cell death and the release of high mobility group box 1 and calreticulin. The combination of [131I]LG-12 and LG-12 could significantly inhibit tumor growth and resulted in enhanced antitumor immune response. This PD-L1 small-molecule inhibitor based combination strategy has great potential for tumor treatment.

Evaluating immunotherapeutic outcomes in triple-negative breast cancer with a cholesterol radiotracer in mice

Evaluating the response to immune checkpoint inhibitors (ICIs) remains an unmet challenge in triple-negative breast cancer (TNBC). The requirement for cholesterol in the activation and function of T cells led us to hypothesize that quantifying cellular accumulation of this molecule could distinguish successful from ineffective checkpoint immunotherapy. To analyze accumulation of cholesterol by T cells in the immune microenvironment of breast cancer, we leveraged the PET radiotracer, eFNP-59. eFNP-59 is an analog of cholesterol that our group validated as an imaging biomarker for cholesterol uptake in preclinical models and initial human studies. In immunocompetent mouse models of TNBC, we found that elevated uptake of exogenous labeled cholesterol analogs functions as a marker for T cell activation. When comparing ICI-responsive and -nonresponsive tumors directly, uptake of fluorescent cholesterol and eFNP-59 increased in T cells from ICI-responsive tumors. We discovered that accumulation of cholesterol by T cells increased in ICI-responding tumors that received anti-PD-1 checkpoint immunotherapy. In patients with TNBC, tumors containing cycling T cells had features of cholesterol uptake and trafficking within those populations. These results suggest that uptake of exogenous cholesterol analogs by tumor-infiltrating T cells allows detection of T cell activation and has potential to assess the success of ICI therapy.

Development and In Vivo Evaluation of Small-Molecule Ligands for Positron Emission Tomography of Immune Checkpoint Modulation Targeting Programmed Cell Death 1 Ligand 1

A substantial portion of patients do not benefit from programmed cell death protein 1/programmed cell death 1 ligand 1 (PD-1/PD-L1) checkpoint inhibition therapies, necessitating a deeper understanding of predictive biomarkers. Immunohistochemistry (IHC) has played a pivotal role in assessing PD-L1 expression, but small-molecule positron emission tomography (PET) tracers could offer a promising avenue to address IHC-associated limitations, i.e., invasiveness and PD-L1 expression heterogeneity. PET tracers would allow for improved quantification of PD-L1 through noninvasive whole-body imaging, thereby enhancing patient stratification. Here, a large series of PD-L1 targeting small molecules were synthesized, leveraging advantageous substructures to achieve exceptionally low nanomolar affinities. Compound 5c emerged as a promising candidate (IC50 = 10.2 nM) and underwent successful carbon-11 radiolabeling. However, a lack of in vivo tracer uptake in xenografts and notable accumulation in excretory organs was observed, underscoring the challenges encountered in small-molecule PD-L1 PET tracer development. The findings, including structure-activity relationships and in vivo biodistribution data, stand to illuminate the path forward for refining small-molecule PD-L1 PET tracers.

Design, Synthesis, and Biological Evaluation of Small-Molecule-Based Radioligands with Improved Pharmacokinetic Properties for Imaging of Programmed Death Ligand 1

Small molecules offer some advantages for developing positron emission tomography (PET) tracers and are therefore a promising approach for imaging and therapy monitoring of programmed death ligand 1 (PD-L1) positive tumors. Here, we report six biphenyl PD-L1 radioligands using the NODA-GA-chelator for efficient copper-64 complexation. These radioligands contain varying numbers of sulfonic and/or phosphonic acid groups, serving as hydrophilizing units to lower the log D7.4 value down to -4.28. The binding affinities of compounds were evaluated using saturation binding and a real-time binding assay, with a highest binding affinity of 21 nM. Small-animal PET imaging revealed vastly different pharmacokinetic profiles depending on the quantity and type of hydrophilizing units. Of the investigated radioligands, [64Cu]Cu-3 showed the most favorable kinetics in vitro. This was also found in vivo, with a predominantly renal clearance and a specific uptake in the PD-L1-overexpressing tumor. With further modifications, this compound could be a promising candidate for the imaging of PD-L1 in the clinical setting.

Synthesis and preliminary biological evaluation of a novel 99mTc-labeled small molecule for PD-L1 imaging

In recent years, PD-1/PD-L1 checkpoint blockade immunotherapy with remarkable efficacy has set off a heat wave. The expression level of PD-L1, which plays a predictive role in anti-PD-1/PD-L1 therapy, could be quantified by noninvasive imaging with radiotracers. Herein, we introduced the synthesis and preliminary biological evaluation of a novel 99mTc-labeled small molecule radiotracer [99mTc]G3C-CBM for PD-L1 imaging. [99mTc]G3C-CBM was achieved with high radiochemical purity (>96 %) and remained good stability in PBS and FBS. In competitive combination experiment, [99mTc]G3C-CBM was displaced by increasing concentrations of unlabeled G3C-CBM, resulting in an IC50 value of 41.25±2.23 nM for G3C-CBM. The uptake of [99mTc]G3C-CBM in A375-hPD-L1 cells (17.51±2.08 %) was approximately 6.47 folds of that in A375 cells (2.71±0.36 %) after co-incubation for 2 h. The biodistribution results showed that the radioactivity uptake in A375-hPD-L1 tumor reached the maximum (0.35±0.01 %ID/g) at 2 h post injection, and the optimum tumor/muscle ratio of 2.94±0.29 occurred at the same time. In addition, [99mTc]G3C-CBM was quickly cleared from the blood with a clearance half-life of just 119.25 min. These results indicate that [99mTc]G3C-CBM is a potential SPECT PD-L1 imaging agent and is worthy of further study.

A novel 68Ga-labeled cyclic peptide molecular probe based on the computer-aided design for noninvasive imaging of PD-L1 expression in tumors

Programmed death-ligand 1 (PD-L1) serves as a crucial biomarker for guiding the screening of cancer patients and the stratification of immunotherapy. However, due to the high heterogeneity of tumors, the current gold standard for detecting PD-L1 expression (immunohistochemistry) fails to comprehensively evaluate the overall PD-L1 expression levels in the body. Fortunately, the use of PD-L1 targeted radiotracers enables quantitative, real-time, and noninvasive assessment of PD-L1 expression levels and dynamics in tumors. Notably, analyzing the binding mode between the precursor and the target protein to find linker binding sites that do not affect the activity of the target molecule can greatly enhance the successful development of molecular probes. This study introduced a groundbreaking cyclic peptide molecular probe called 68Ga-DOTA-PG1. It was derived from the BMS-71 cyclic peptide and was specifically designed to evaluate the expression of PD-L1 in tumors. The radiolabeling yield of 68Ga-DOTA-PG1 surpassed 97% while maintaining a radiochemical purity of over 99%. In vitro experiments demonstrated the effective targeting of PD-L1 in tumor cells by 68Ga-DOTA-PG1, with significantly higher cellular uptake observed in A375-hPD-L1 cells (PD-L1 + ) compared to A375 cells (PD-L1-). Biodistribution and PET imaging studies consistently showed specific accumulation of 68Ga-DOTA-PG1 in A375-hPD-L1 tumors, with a maximum uptake of 11.06 ± 1.70% ID/g at 2 h, significantly higher than the tumor uptake in A375 cells (1.70 ± 0.17% ID/g). These results strongly indicated that 68Ga-DOTA-PG1 held great promise as a PET radiotracer for imaging PD-L1-positive tumors.

Exploring Hydrophilic PD-L1 Radiotracers Utilizing Phosphonic Acids: Insights into Unforeseen Pharmacokinetics

Immune checkpoint inhibitor therapy targeting the PD-1/PD-L1 axis in cancer patients, is a promising oncological treatment. However, the number of non-responders remains high, causing a burden for the patient and the healthcare system. Consequently, a diagnostic tool to predict treatment outcomes would help with patient stratification. Molecular imaging provides said diagnostic tool by offering a whole-body quantitative assessment of PD-L1 expression, hence supporting therapy decisions. Four PD-L1 radioligand candidates containing a linker-chelator system for radiometalation, along with three hydrophilizing units-one sulfonic and two phosphonic acids-were synthesized. After labeling with 64Cu, log D7.4 values of less than -3.03 were determined and proteolytic stability confirmed over 94% intact compound after 48 h. Binding affinity was determined using two different assays, revealing high affinities up to 13 nM. µPET/CT imaging was performed in tumor-bearing mice to investigate PD-L1-specific tumor uptake and the pharmacokinetic profile of radioligands. These results yielded an unexpected in vivo distribution, such as low tumor uptake in PD-L1 positive tumors, high liver uptake, and accumulation in bone/bone marrow and potentially synovial spaces. These effects are likely caused by Ca2+-affinity and/or binding to macrophages. Despite phosphonic acids providing high water solubility, their incorporation must be carefully considered to avoid compromising the pharmacokinetic behavior of radioligands.

Predicting efficacy of immunotherapy in mice with triple negative breast cancer using a cholesterol PET radiotracer

Predicting the response to cancer immunotherapy remains an unmet challenge in triple-negative breast cancer (TNBC) and other malignancies. T cells, the major target of current checkpoint inhibitor immunotherapies, accumulate cholesterol during activation to support proliferation and signaling. The requirement of cholesterol for anti-tumor functions of T cells led us to hypothesize that quantifying cellular accumulation of this molecule could distinguish successful from ineffective checkpoint immunotherapy. To analyze accumulation of cholesterol by T cells in the immune microenvironment of breast cancer, we leveraged a novel positron emission tomography (PET) radiotracer, FNP-59. FNP-59 is an analog of cholesterol that our group has validated as an imaging biomarker for cholesterol uptake in pre-clinical models and initial human studies. In immunocompetent mouse models of TNBC, we found that elevated uptake of exogenous labeled cholesterol analogs functions as a marker for T cell activation. When comparing immune checkpoint inhibitor (ICI)-responsive EO771 tumors to non-responsive AT-3 tumors, we found significantly higher uptake of a fluorescent cholesterol analog in T cells of the ICI-responsive tumors both in vitro and in vivo. Using the FNP-59 radiotracer, we discovered that accumulation of cholesterol by T cells increased further in ICI-responding tumors that received ant-PD-1 checkpoint immunotherapy. We verified these data by mining single cell sequencing data from patients with TNBC. Patients with tumors containing cycling T cells showed gene expression signatures of cholesterol uptake and trafficking. These results suggest that uptake of exogenous cholesterol analogs by tumor-infiltrating T cells predict T cell activation and success of ICI therapy.

Preparation and Bioevaluation of 18F-Labeled Small-Molecular Radiotracers via Sulfur(VI) Fluoride Exchange Chemistry for Imaging of Programmed Cell Death Protein Ligand 1 Expression in Tumors

Nowadays, one of the most effective methods of tumor immunotherapy is blocking programmed cell death protein 1/programmed cell death protein ligand 1 (PD-1/PD-L1) immune checkpoints. However, there is still a significant challenge in selecting patients to benefit from immune checkpoint therapies. Positron emission tomography (PET), a noninvasive molecular imaging technique, offers a new approach to accurately detect PD-L1 expression and allows for a better prediction of response to PD-1/PD-L1 target immunotherapy. Here, we designed and synthesized a novel group of aryl fluorosulfate-containing small-molecule compounds (LGSu-1, LGSu-2, LGSu-3, and LGSu-4) based on the phenoxymethyl-biphenyl scaffold. After screening by the time-resolved fluorescence resonance energy transfer (TR-FRET) assay, the most potent compound LGSu-1 (half maximal inhibitory concentration (IC50): 15.53 nM) and the low-affinity compound LGSu-2 (IC50: 189.70 nM) as a control were selected for 18F-radiolabeling by sulfur(VI) fluoride exchange chemistry (SuFEx) to use for PET imaging. [18F]LGSu-1 and [18F]LGSu-2 were prepared by a one-step radiofluorination reaction in over 85% radioconversion and nearly 30% radiochemical yield. In B16-F10 melanoma cell assays, [18F]LGSu-1 (5.00 ± 0.06%AD) showed higher cellular uptake than [18F]LGSu-2 (2.55 ± 0.04%AD), in which cell uptake could be significantly blocked by the nonradioactivity LGSu-1. In vivo experiments, micro-PET imaging of B16-F10 tumor-bearing mice and radiographic autoradiography of tumor sections showed that [18F]LGSu-1 was more effectively accumulated in the tumor due to the higher binding affinity with PD-L1. The above experimental results confirmed the potential of the small-molecule probe LGSu-1 as a targeting PD-L1 imaging tracer in tumor tissues.

An overview of current advances of PD-L1 targeting immuno-imaging in cancers

The programmed death protein 1/programmed cell death ligand 1 (PD-1/PD-L1) pathway plays a significant role in immune evasion. PD-1 or PD-L1 immune checkpoint inhibitors (ICIs) have become a standard treatment for multiple types of cancer. To date, PD-L1 has served as a biomarker for predicting the efficacy of ICIs in several cancers. The need to establish an effective detection method that could visualize PD-L1 expression and predict the efficacy of PD-1/PD-L1 ICIs has promoted a search for new imaging strategies. PD-L1-targeting immuno-imaging could provide a noninvasive, real-time, repeatable, dynamic, and quantitative assessment of the characteristics of all tumor lesions in individual patients. This study analyzed the existing evidence in the literature on PD-L1-based immuno-imaging (2015-2022). Original English-language articles were searched using PubMed and Google Scholar. Keywords, such as "PD-L1," "PET," "SPECT," "PET/CT," and "SPECT/CT," were used in various combinations. A total of nearly 50 preclinical and clinical studies of PD-L1-targeting immuno-imaging were selected, reviewed, and included in this study. Therefore, in this review, we conducted a study of the advances in PD-L1-targeting immuno-imaging for detecting the expression of PD-L1 and the efficacy of ICIs. We focused on the different types of PD-L1-targeting agents, including antibodies and small PD-L1-binding agents, and illustrated the strength and weakness of these probes. Furthermore, we summarized the trends in the development of PD-L1-targeting immuno-imaging, as well as the current challenges and future directions for clinical workflow.

Design and Biological Evaluation of Small-Molecule PET-Tracers for Imaging of Programmed Death Ligand 1

Noninvasive molecular imaging of the PD-1/PD-L1 immune checkpoint is of high clinical relevance for patient stratification and therapy monitoring in cancer patients. Here we report nine small-molecule PD-L1 radiotracers with solubilizing sulfonic acids and a linker-chelator system, designed by molecular docking experiments and synthesized according to a new, convergent synthetic strategy. Binding affinities were determined both in cellular saturation and real-time binding assay (LigandTracer), revealing dissociation constants in the single digit nanomolar range. Incubation in human serum and liver microsomes proved in vitro stability of these compounds. Small animal PET/CT imaging, in mice bearing PD-L1 overexpressing and PD-L1 negative tumors, showed moderate to low uptake. All compounds were cleared primarily through the hepatobiliary excretion route and showed a long circulation time. The latter was attributed to strong blood albumin binding effects, discovered during our binding experiments. Taken together, these compounds are a promising starting point for further development of a new class of PD-L1 targeting radiotracers.

Design, Synthesis, and Biological Evaluation of a Small-Molecule PET Agent for Imaging PD-L1 Expression

Immunotherapy blocking programmed cell death protein 1/programmed death ligand 1 (PD-1/PD-L1) pathway has achieved great therapeutic effect in the clinic, but the overall response rate is not satisfactory. Early studies showed that response to treatment and overall survival could be positively related to PD-L1 expression in tumors. Therefore, accurate measurement of PD-L1 expression will help to screen cancer patients and improve the overall response rate. A small molecular positron emission tomography (PET) probe [18F]LP-F containing a biphenyl moiety was designed and synthesized for measurement of PD-L1 expression in tumors. The PET probe [18F]LP-F was obtained with a radiochemical yield of 12.72 ± 1.98%, a radiochemical purity of above 98% and molar activity of 18.8 GBq/μmol. [18F]LP-F had good stability in phosphate buffer saline (PBS) and mouse serum. In vitro assay indicated that [18F]LP-F showed moderate affinity to PD-L1. Micro-PET results showed that the tumor accumulation of [18F]LP-F in A375 tumor was inferior to that in A375-hPD-L1 tumor. All the results demonstrated that [18F]LP-F could specifically bind to PD-L1 and had a potential application in non-invasive evaluation of PD-L1 expression in tumors.

Development of a radiolabeled site-specific single-domain antibody positron emission tomography probe for monitoring PD-L1 expression in cancer

Despite advances in immunotherapy for the treatment of cancers, not all patients can benefit from programmed cell death ligand 1 (PD-L1) immune checkpoint blockade therapy. Anti-PD-L1 therapeutic effects reportedly correlate with the PD-L1 expression level; hence, accurate detection of PD-L1 expression can guide immunotherapy to achieve better therapeutic effects. Therefore, based on the high affinity antibody Nb109, a new site-specifically radiolabeled tracer, ⁶⁸Ga-NODA-cysteine, aspartic acid, and valine (CDV)-Nb109, was designed and synthesized to accurately monitor PD-L1 expression. The tracer ⁶⁸Ga-NODA-CDV-Nb109 was obtained using a site-specific conjugation strategy with a radiochemical yield of about 95% and radiochemical purity of 97%. It showed high affinity for PD-L1 with a dissociation constant of 12.34 ± 1.65 nM. Both the cell uptake assay and positron emission tomography (PET) imaging revealed higher tracer uptake in PD-L1-positive A375-hPD-L1 and U87 tumor cells than in PD-L1-negative A375 tumor cells. Meanwhile, dynamic PET imaging of a NCI-H1299 xenograft indicated that doxorubicin could upregulate PD-L1 expression, allowing timely interventional immunotherapy. In conclusion, this tracer could sensitively and dynamically monitor changes in PD-L1 expression levels in different cancers and help screen patients who can benefit from anti-PD-L1 immunotherapy.

Molecular imaging of immune checkpoints in oncology: Current and future applications

Immune checkpoint (IC) blockade therapy has become the first-line treatment for various cancers. However, the low response rate and acquired drug resistance severely restrict the clinical application of immune checkpoint inhibitors (ICIs). Nuclide molecular imaging of ICs can provide non-invasive and whole-body visualization of in vivo IC dynamic biodistribution. Therefore, molecular imaging of ICs can predict and monitor responses to ICIs as a complementary tool to existing immunohistochemical techniques. Herein, we outlined the current status and recent advances in molecular imaging of the "first-generation" and "next-generation" ICs in preclinical and clinical studies.

Development of Radiotracers for Imaging of the PD-1/PD-L1 Axis

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.