Development of small-molecular-based radiotracers for PET imaging of PD-L1 expression and guiding the PD-L1 therapeutics

Exploration of Bicyclic Peptide Ligands for Immune-Specific PET Imaging: Targeting Tumor PD-L1 with [18F]AlF-BCY10959

As a new modality of ligands, bicyclic peptides hold great promise in the discovery of novel programmed death ligand 1 (PD-L1) targeted radiotracers, which have not yet been reported. In this study, first-in-class bicyclic peptide-based radiotracers [18F]AlF-BCY509 and [18F]AlF-BCY10959 were developed and evaluated for PET imaging of tumor PD-L1 expression. The automatic radiosynthesis was achieved with robust radiochemical yields (55.1-90.2%) and high molar activity (42.5-90.8 GBq/μmol). Cell-based assays demonstrated high specificity and affinity of [18F]AlF-BCY509 and [18F]AlF-BCY10959 with IC50 values of 9.36 ± 1.35 and 7.12 ± 1.24 nM and KD values of 11.41 ± 1.04 and 8.09 ± 0.85 nM. In PET imaging, the accumulation of [18F]AlF-BCY10959 in PD-L1-positive tumors with moderate retention over 120 min was discovered, with the tumor uptake of 14.74 ± 1.67%ID/cc and tumor-to-muscle ratio of 12.41 ± 1.07 at 30 min. The in vivo specificity was strictly verified by PD-L1-knockout and PD-L1-positive tumors with blocking. The biodistribution manifested a rapid distribution and fast clearance from the body, supporting the favorable pharmacokinetics of [18F]AlF-BCY10959. [18F]AlF-BCY10959 was excreted through the urinary and hepatobiliary systems, indicating the doomed radiation exposure organs. The effective doses of [18F]AlF-BCY10959 and [18F]-FDG were comparable, highlighting its safety for human use. In conclusion, [18F]AlF-BCY10959 provides an attractive option to detect PD-L1 expression and lays the groundwork to further develop promising bicyclic peptide tracers for clinical use.

Radiosynthesis and Evaluation of a Novel 68Ga-Labeled Peptide for PD-L1-Targeted PET Imaging

Although programmed death-ligand 1 (PD-L1)-targeted immunotherapy has demonstrated favorable therapeutic effects, concern regarding a low response rate persists. Our study aimed to develop a novel peptide probe for PD-L1 targeting positron emission tomography (PET)/computed tomography (CT) imaging as an alternative for assessing PD-L1 expression and exploring its potential role in guiding PD-L1 immunotherapy in vivo. The probe targeting PD-L1 was obtained by modifying 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid (DOTA) with the peptide CCC, followed by labeling with 68Ga. Radiochemical purity, log P, pharmacokinetics, and stability of the probe were evaluated in vitro and in vivo. The systematic evaluation of the probe performance included microPET/CT imaging, biodistribution, biosafety, and an investigation of its use in immunotherapy monitoring. 68Ga-DOTA-CCC was successfully synthesized with over 99% radiochemical purity, eliminating the need for purification. The probe exhibited good hydrophilicity and stability and was rapidly metabolized with a short blood clearance half-life of 16.1 ± 0.5 min. Additionally, the probe exhibited an excellent PD-L1 targeting ability, with tumor uptake positively correlating with PD-L1 expression levels in both cellular experiments and microPET/CT imaging. Moreover, the dynamic expression of PD-L1 was assessed using 68Ga-DOTA-CCC during atezolizumab administration. 68Ga-DOTA-CCC accurately reflects PD-L1 expression and holds promise for precisely guiding PD-L1-targeted immunotherapy.

Synthesis and preclinical evaluation of small molecule-based radiotracers for PET imaging of PD-L1 expression and dynamics

PURPOSE

Small molecule-based radiotracers offer several potential advantages in positron emission tomography (PET) imaging, and are therefore a promising approach for non-invasively and accurately monitoring of programmed death ligand 1 (PD-L1) expression in vivo. In this study, two small-molecule radiotracers were developed to assess PD-L1 expression and dynamics during treatments.

METHODS

[18F]LG-2 and [18F]LG-3 were designed based on a phenoxymethyl-biphenyl scaffold with a tris-(hydroxymethyl)-aminomethane terminal group. The radiolabeling was achieved by a two-step method through the "click" chemistry. Cellular uptake assays in different tumor cells were performed to determine the specificity of the two tracers to PD-L1. The ability of [18F]LG-2 and [18F]LG-3 to detect PD-L1 expression in vivo as well as to monitor PD-L1 dynamics during chemotherapy and immunotherapy was investigated via PET imaging.

RESULTS

The radiolabeling of [18F]LG-2 and [18F]LG-3 was achieved with overall radiochemical yield of 15 ± 3% for [18F]LG-2 and 18 ± 5% for [18F]LG-3. In vitro cell uptake studies in tumor cells with varying PD-L1 levels demonstrated the specific binding of these tracers to PD-L1. PET imaging in mice bearing B16-F10 tumors displayed comparable tumor uptake of 6.45 ± 0.38%ID/mL for [18F]LG-2 and 5.64 ± 0.02%ID/mL for [18F]LG-3, while [18F]LG-3 showed nearly a 50% reduction in uptake in the liver and intestines compared to [18F]LG-2. PET signals of [18F]LG-3 in A375-hPD-L1, A375-hPD-L1/A375 and A375 tumor-bearing mice demonstrated a strong and linear correlation with PD-L1 expression levels. The dynamic of PD-L1 status in tumors after cisplatin and PD-L1 inhibitor treatments were accurately evaluated with [18F]LG-3 PET imaging.

CONCLUSION

The small-molecule radiotracer [18F]LG-3 is a promising candidate for evaluating PD-L1 expression and monitoring the dynamic of PD-L1 status during the treatment process.

Preclinical and first‑in‑human evaluation of [68Ga]Ga-DOTA-PEG2-Asp2-PDL1P PET imaging to assess tumor PD-L1 expression

PURPOSE

PD-L1 PET imaging can provide a non-invasively and real-time assessment of PD-L1 expression status at tumor sites. This study aimed to evaluate the targeting efficacy and biodistribution of a novel peptide-based PD-L1 PET agent, [68Ga]Ga-DOTA-PEG2-Asp2-PDL1P, in preclinical studies and human participants.

METHODS

[68Ga]Ga-DOTA-PEG2-Asp2-PDL1P was synthesized and the probe stability was analyzed in vitro and in vivo. Cellular uptake of the probe was evaluated using tumor cell lines with different PD-L1 expression levels. Small animal PET imaging and semi-quantitative studies were conducted in PC3, H1975 and A549 tumor-bearing mice models, with tumor PD-L1 expression confirmed through immunofluorescence and immunohistochemistry. Furthermore, [68Ga]Ga-DOTA-PEG2-Asp2-PDL1P PET imaging was performed in 1 healthy volunteer and 14 lung cancer patients to assess biodistribution and PD-L1 expression at tumor sites.

RESULTS

[68Ga]Ga-DOTA-PEG2-Asp2-PDL1P exhibited a radiochemical purity of > 99% and had good stability both in vitro and in vivo. In vitro cellular uptake and in vivo small animal PET imaging revealed the probe binding to PD-L1 with high affinity and specificity, consistent with the results of immunofluorescence and immunohistochemistry. In the clinical study involving 15 participants, [68Ga]Ga-DOTA-PEG2-Asp2-PDL1P was proven safe with demonstrating low uptake in normal organs and physiologically excreting via the urinary system. Lung cancer patients with high PD-L1 expression (TPS 70-90%) exhibited higher tumor uptake and tumor-to-background ratios than those with negative or low PD-L1 expression (TPS < 1-10%), with SUVmax of 1.89-2.27 vs. 0.87-1.01, tumor-to-lung ratios of 4.73-7.68 vs. 1.61-2.35, and tumor-to-muscle ratios of 6.73-12.61 vs. 4.35-5.61.

CONCLUSION

[68Ga]Ga-DOTA-PEG2-Asp2-PDL1P showed promising as a PET agent to assess tumor PD-L1 expression in preclinical and first-in-human studies, offering a non-invasive, real-time and accurate tool to address clinical challenges in predicting and assessing the efficacy of immunotherapy.

Development and First-in-Human evaluation of a Site-Specific [18F]-Labeled PD-L1 nanobody PET radiotracer for noninvasive imaging in NSCLC

Immunohistochemistry (IHC) for PD-L1 detection is limited by its invasiveness and heterogeneity of tumors. To address these challenges, a new PD-L1-targeted nanobody-based immune-PET radiotracer [18F]AlF-APN09 was developed using the site-specific radiolabeling method with the complexing agent (Mal-RESCA) under mild conditions. [18F]AlF-APN09 was prepared at room temperature (pH 4.6-4.8) within 20 min with satisfactory radiochemical yields (45.8 ± 4.48 %, non-decay corrected), high radiochemical purity (>98 %) and moderate apparent molar activity (15-35 GBq/μmol), and remained stable in both PBS and 5 % HSA after 4 h (>90 %). Cell uptake studies indicated variable levels of surface PD-L1 expression in the following order: A549PD-L1 > H1975 > A549. In micro-PET/CT imaging, A549PD-L1 and H1975 tumors were distinctly visualized in a 6.0:1 and 3.2:1 ratios over PD-L1-negative A549 tumors in vivo. Ex vivo biodistribution studies showed tumor uptake values of 6.47 ± 1.06 %ID/g (A549PD-L1) and 2.27 ± 0.19 %ID/g (H1975), significantly higher than 0.90 ± 0.28 %ID/g in A549 tumors. The estimated effective radiation dose in humans was 8.65E-03 mSv/MBq, lower than that of conventional [18F]FDG. First-in-human imaging was conducted on a single resectable non-small cell lung cancer (NSCLC) subject without any adverse reactions. The radiotracer exhibited renal excretion with minimal hepatobiliary clearance. Tumor uptake reached SUVmax 4.20 at 2 h post-injection, demonstrating high contrast and rapid clearance. After PD-1 inhibitor immunotherapy and chemotherapy, the subject showed a therapeutic response and postoperative pathological specimens confirmed a major pathological response (MPR). These results suggest that we have successfully developed a new PD-L1-targeted nanobody PET tracer using the site-specific labeling method with the complexing agent (Mal-RESCA) within 20 min under mild conditions and [18F]AlF-APN09 is a promising noninvasive PET radiotracer for visualizing PD-L1 expression in tumors, offering rapid tumor targeting, excellent signal-to-noise ratios, and favorable clearance properties.

Small-Molecule Radiotracers for Visualization of V-Domain Immunoglobulin Suppressor of T Cell Activation

V-domain immunoglobulin suppressor of T cell activation (VISTA) plays a critical role in regulating innate and adaptive immune responses within the tumor immune microenvironment. Quantifying VISTA expression is necessary to determine whether patients respond to a related combination immunotherapy. This study developed two 68Ga-labeled small-molecule probes ([68Ga]Ga-DCA and [68Ga]Ga-DNCA) for visualizing and differentiating VISTA expression. These probes exhibited excellent targeting capabilities for multiple tumor types (including B16-F10, 4T1, MC38, and CT26 tumors), consistent with the levels of VISTA expression determined by immunoblotting. Co-injection of inhibitor CA-170 led to decreased tumor uptake of both [68Ga]Ga-DCA and [68Ga]Ga-DNCA. [68Ga]Ga-DCA was used to verify the feasibility of monitoring VISTA expression in lung metastasis models. In summary, this study describes the use of 68Ga-labeled CA-170 analogues as small-molecule probes for imaging VISTA. This could provide a visual method and enable personalized immunotherapy in patients.

MHC-I and PD-L1 Expression is Associated with Decreased Tumor Outgrowth and is Radiotherapy-inducible in the Murine Head and Neck Squamous Cell Carcinoma Model MOC1

INTRODUCTION

Combined radiotherapy and immune checkpoint inhibition is a potential treatment option for head and neck squamous cell carcinoma (HNSCC). Immunocompetent mouse models can help to successfully develop radio- immunotherapy combinations and to increase our understanding of the effects of radiotherapy on the tumor microenvironment for future clinical translation. Therefore, the aim of this study was to develop a homogeneous, reproducible HNSCC model originating from the Mouse Oral Cancer 1 (MOC1) HNSCC cell line, and to explore the radiotherapy-induced changes in its tumor microenvironment, using flow cytometry and PD-L1 microSPECT/CT imaging.

MATERIALS AND METHODS

In vivo growing tumors originating from the parental MOC1 line were used to generate single cell derived clones. These clones were screened in vitro for their ability to induce programmed cell death ligand 1 (PD-L1) and major histocompatibility complex class I (MHC-I) following IFNγ exposure. Clones with different IFNγ sensitivity were inoculated in C57BL/6 mice and assessed for tumor outgrowth. The composition of the tumor microenvironment of a stably growing (non)irradiated MOC1-derived clone was assessed by immunohistochemistry, flow cytometry and PD-L1 microSPECT/CT.

RESULTS

Low in vitro inducibility of MHC-I and PD-L1 by IFNγ was associated with increased tumor outgrowth of MOC1 clones in vivo. Flow cytometry analysis of cells derived from a stable in vivo growing MOC1 clone MOC1.3D5low showed expression of MHC-I and PD-L1 on several cell populations within the tumor. Upon irradiation, MHC-I and PD-L1 increased on leukocytes (CD45.2+) and cancer associated fibroblasts (CD45.2-/EpCAM-/CD90.1+). Furthermore, PD-L1 microSPECT/CT showed increased tumor uptake of radiolabeled PD-L1 antibodies with a heterogeneous spatial distribution of the radio signal, which co-localized with PD-L1+ and CD45.2+ areas.

DISCUSSION

PD-L1 and MHC-I inducibility by IFNγ in vitro is associated with tumor outgrowth of MOC1 clones in vivo. In tumors originating from a stably growing MOC1-derived clone, expression of these immune-related markers was induced by irradiation shown by flow cytometry on several cell populations within the tumor microenvironment such as immune cells and cancer associated fibroblasts. PD-L1 microSPECT/CT showed increased tumor uptake following radiotherapy, and autoradiography showed correlation of uptake with areas that are heavily infiltrated by immune cells. Knowledge of radiotherapy-induced effects on the tumor microenvironment in this model can help optimize timing and dosage for radio- immunotherapy combination strategies in future research.