The discovery and evaluation of [18F]BMS-986229, a novel macrocyclic peptide PET radioligand for the measurement of PD-L1 expression and in-vivo PD-L1 target engagement

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.

Synthesis and preclinical evaluation of an Al18F radio-fluorinated bivalent PD-L1 nanobody

Immunotherapy targeting the programmed death 1/programmed death ligand 1 (PD-1/PD-L1) pathway has achieved remarkable clinical success, but there is a shortage of effective approaches for screening suitable patients. Recently developed PD-L1 nanobody probes have limitations, including limited availability of radionuclides, short tumor retention times, and accumulation in non-target organs. To enhance tumor retention and improve tumor-to-normal tissue contrast, we herein report the synthesis and preclinical evaluation of two Al18F-labeled bivalent PD-L1 nanobody probes ([18F]TzTCO-BINb109 and [18F]RESCA-BINb109). Preliminary results indicated that [18F]TzTCO-BINb109 had a greater affinity for PD-L1 and better stability than [18F]RESCA-BINb109. Micro-PET/CT revealed that [18F]TzTCO-BINb109 uptake in A549-PDL1 tumors peaked at 240 min post-injection (3.19 ± 0.49 %ID/g) and demonstrated sustained retention without in vivo defluorination. In contrast, [18F]RESCA-BINb109 exhibited shorter tumor retention (at 60 and 240 min, 2.08 ± 0.22 and 1.37 ± 0.26 %ID/g, respectively) and significant defluorination in vivo. Ex vivo biodistribution studies revealed that the tumor uptake of [18F]TzTCO-BINb109 was consistent with the PET results, with the highest uptake by A549-PDL1 tumor cells (3.43 ± 0.94 %ID/g) compared with H1975 (0.93 ± 0.18 %ID/g) and A549 (0.68 ± 0.12 %ID/g) cells observed at 240 min post-injection. Compared with the previously reported monomeric PD-L1-targeting nanobody probe, [68Ga]NOTA-Nb109, [18F]TzTCO-BINb109 demonstrated enhanced tumor uptake, prolonged retention, and superior tumor-to-normal tissue contrast, contributing to higher imaging quality. These results confirmed that the bivalent PD-L1 nanobody radioligand, [18F]TzTCO-BINb109, was a promising diagnostic probe for PD-L1 detection, efficacy evaluation, and prescription optimization of immune checkpoint inhibitor therapies.

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.

Peptide-based immuno-PET/CT monitoring of dynamic PD-L1 expression during glioblastoma radiotherapy

Real-time, noninvasive programmed death-ligand 1 (PD-L1) testing using molecular imaging has enhanced our understanding of the immune environments of neoplasms and has served as a guide for immunotherapy. However, the utilization of radiotracers in the imaging of human brain tumors using positron emission tomography/computed tomography (PET/CT) remains limited. This investigation involved the synthesis of [18F]AlF-NOTA-PCP2, which is a novel peptide-based radiolabeled tracer that targets PD-L1, and evaluated its imaging capabilities in orthotopic glioblastoma (GBM) models. Using this tracer, we could noninvasively monitor radiation-induced PD-L1 changes in GBM. [18F]AlF-NOTA-PCP2 exhibited high radiochemical purity (>95%) and stability up to 4 h after synthesis. It demonstrated specific, high-affinity binding to PD-L1 in vitro and in vivo, with a dissociation constant of 0.24 nM. PET/CT imaging, integrated with contrast-enhanced magnetic resonance imaging, revealed significant accumulation of [18F]AlF-NOTA-PCP2 in orthotopic tumors, correlating with blood-brain barrier disruption. After radiotherapy (15 Gy), [18F]AlF-NOTA-PCP2 uptake in tumors increased from 9.51% ± 0.73% to 12.04% ± 1.43%, indicating enhanced PD-L1 expression consistent with immunohistochemistry findings. Fractionated radiation (5 Gy × 3) further amplified PD-L1 upregulation (13.9% ± 1.54% ID/cc) compared with a single dose (11.48% ± 1.05% ID/cc). Taken together, [18F]AlF-NOTA-PCP2 may be a valuable tool for noninvasively monitoring PD-L1 expression in brain tumors after radiotherapy.

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.

The coming of age of cyclic peptide drugs: an update on discovery technologies

INTRODUCTION

Cyclic peptides are an established class of pharmaceuticals, with the ability to bind to a broader range of protein targets than traditional small molecules while also being capable of oral availability and cell penetration. Historically, cyclic peptide drugs have been discovered almost exclusively through natural product mining approaches; however, the last two decades have seen the development of display screening approaches capable of rapidly identifying de novo (i.e. not natural product derived) cyclic peptide ligands to targets of interest.

AREAS COVERED

In this review, the authors describe the current clinical landscape for cyclic peptide pharmaceuticals. This article focuses on the discovery approaches that have led to the development of different classes of molecules and how the development of newer technologies, particularly phage and mRNA display, has broadened the clinical applicability of such molecules.

EXPERT OPINION

The field of de novo cyclic peptide drug discovery is reaching maturity, with the first drugs identified through display screening approaches reaching the market in recent years. Many more are in clinical trials; however, significant technical challenges remain. Technological improvements will be required over the coming years to facilitate the identification of membrane permeable cyclic peptides capable of oral availability and targeting intracellular proteins.

18F-BMS-986229 PET to Assess Programmed-Death Ligand 1 Status in Gastroesophageal Cancer

Anti-programmed death 1 (PD-1) inhibitors are the standard of care for advanced gastroesophageal cancer. Although recommendations and approval by regulatory agencies are often based on programmed death ligand 1 (PD-L1) expression, pathologic assessments of PD-L1 status have several limitations. Single-site biopsies do not adequately capture disease heterogeneity within individual tumor lesions or among several lesions within the same patient, the PD-L1 combined positive score is a dynamic biomarker subject to evolution throughout a patient's disease course, and repeated biopsies are invasive and not always feasible. Methods: This was a prospective pilot study of the PD-L1-targeting radiotracer, 18F-BMS-986229, with PET imaging (PD-L1 PET) in patients with gastroesophageal cancer. Patients were administered the 18F-BMS-986229 radiotracer intravenously at a dose of 370 MBq over 1-2 min and underwent whole-body PET/CT imaging 60 min later. The primary objective of this study was to evaluate the safety and feasibility of 18F-BMS-986229. The trial is registered with ClinicalTrials.gov (NCT04161781). Results: Between February 3, 2020, and February 2, 2022, 10 patients with gastroesophageal adenocarcinoma underwent PD-L1 PET. There were no adverse events associated with the 18F-BMS-986229 tracer, and imaging did not result in treatment delays; the primary endpoint was achieved. Radiographic evaluation of PD-L1 expression was concordant with pathologic assessment in 88% of biopsied lesions, and 18F-BMS-986229 uptake on PET imaging correlated with pathologic evaluation by the combined positive score (Spearman rank correlation coefficient, 0.64). Seventy-one percent of patients with 18F-BMS-986229 accumulation on PET imaging also had lesions without 18F-BMS-986229 uptake, highlighting the intrapatient heterogeneity of PD-L1 expression. Patients treated with frontline programmed death 1 inhibitors who had 18F-BMS-986229 accumulation in any lesions on PET imaging had longer progression-free survival than patients without tracer accumulation in any lesions (median progression-free survival, 28.4 vs. 9.9 mo), though the small sample size prevents any definitive conclusions. Conclusion: PD-L1 PET imaging was safe, feasible, and concordant with pathologic evaluation and offers a potential noninvasive tool to assess PD-L1 expression.

Development, Characterization, and Radiation Dosimetry Studies of 18F-BMS-986229, a 18F-Labeled PD-L1 Macrocyclic Peptide PET Tracer

PURPOSE

In cancer immunotherapy, the blockade of the interaction between programmed death-1 and its ligand (PD-1:PD-L1) has proven to be one of the most promising strategies. However, as mechanisms of resistance to PD-1/PD-L1 inhibition include variability in tumor cell PD-L1 expression in addition to standard tumor biopsy PD-L1 immunohistochemistry (IHC), a comprehensive and quantitative approach for measuring PD-L1 expression is required. Herein, we report the development and characterization of an 18F-PD-L1-binding macrocyclic peptide as a PET tracer for the comprehensive evaluation of tumor PD-L1 expression in cancer patients.

PROCEDURES

18F-BMS-986229 was characterized for PD-L1 expression assessment by autoradiography or PET imaging. 18F-BMS-986229 was utilized to evaluate tumor PD-L1 target engagement in competition with a macrocyclic peptide inhibitor of PD-L1 (BMS-986189) over a range of doses using PET imaging. A whole-body radiation dosimetry study of 18F-BMS-986229 in healthy non-human primates (NHPs) was performed.

RESULTS

In vitro autoradiography showed an 8:1 binding ratio in L2987(PD-L1 +) vs. HT-29 (PD-L1-) tumors, more than 90% of which could be blocked with 1 nM of BMS-986189. Ex vivo autoradiography showed that 18F-BMS-986229 detection was penetrant over a series of sections spanning the entire L2987 tumor. In vivo PET imaging in mice demonstrated a 5:1 tracer uptake ratio (at 90-100 min after tracer administration) in L2987 vs. HT-29 tumors and demonstrated 83%-93% specific binding of BMS-986189 within those dose ranges. In a healthy NHP dosimetry study, the resultant whole-body effective dose was 0.025 mSv/MBq.

CONCLUSION

18F-BMS-986229 has been preclinically characterized and exhibits high target specificity, low background uptake, and a short blood half-life supportive of same day imaging in the clinic. As the PET tracer, 18F-BMS-986229 shows promise in the quantification of PD-L1 expression, and its use in monitoring longitudinal changes in patients may provide insights into PD-1:PD-L1 immuno-therapy treatment outcomes.