Whole-body PET tracking of a d-dodecapeptide and its radiotheranostic potential for PD-L1 overexpressing tumors

Biomarker-driven molecular imaging probes in radiotherapy

Background: Biomarker-driven molecular imaging has emerged as an integral part of cancer precision radiotherapy. The use of molecular imaging probes, including nanoprobes, have been explored in radiotherapy imaging to precisely and noninvasively monitor spatiotemporal distribution of biomarkers, potentially revealing tumor-killing mechanisms and therapy-induced adverse effects during radiation treatment. Methods: We summarized literature reports from preclinical studies and clinical trials, which cover two main parts: 1) Clinically-investigated and emerging imaging biomarkers associated with radiotherapy, and 2) instrumental roles, functions, and activatable mechanisms of molecular imaging probes in the radiotherapy workflow. In addition, reflection and future perspectives are proposed. Results: Numerous imaging biomarkers have been continuously explored in decades, while few of them have been successfully validated for their correlation with radiotherapeutic outcomes and/or radiation-induced toxicities. Meanwhile, activatable molecular imaging probes towards the emerging biomarkers have exhibited to be promising in animal or small-scale human studies for precision radiotherapy. Conclusion: Biomarker-driven molecular imaging probes are essential for precision radiotherapy. Despite very inspiring preliminary results, validation of imaging biomarkers and rational design strategies of probes await robust and extensive investigations. Especially, the correlation between imaging biomarkers and radiotherapeutic outcomes/toxicities should be established through multi-center collaboration involving a large cohort of patients.

Navigating the landscape of PD-1/PD-L1 imaging tracers: from challenges to opportunities

Immunotherapy targeted to immune checkpoint inhibitors, such as the program cell death receptor (PD-1) and its ligand (PD-L1), has revolutionized cancer treatment. However, it is now well-known that PD-1/PD-L1 immunotherapy response is inconsistent among patients. The current challenge is to customize treatment regimens per patient, which could be possible if the PD-1/PD-L1 expression and dynamic landscape are known. With positron emission tomography (PET) imaging, it is possible to image these immune targets non-invasively and system-wide during therapy. A successful PET imaging tracer should meet specific criteria concerning target affinity, specificity, clearance rate and target-specific uptake, to name a few. The structural profile of such a tracer will define its properties and can be used to optimize tracers in development and design new ones. Currently, a range of PD-1/PD-L1-targeting PET tracers are available from different molecular categories that have shown impressive preclinical and clinical results, each with its own advantages and disadvantages. This review will provide an overview of current PET tracers targeting the PD-1/PD-L1 axis. Antibody, peptide, and antibody fragment tracers will be discussed with respect to their molecular characteristics and binding properties and ways to optimize them.

Recent Progress in Peptide-Based Molecular Probes for Disease Bioimaging

Recent strides in molecular pathology have unveiled distinctive alterations at the molecular level throughout the onset and progression of diseases. Enhancing the in vivo visualization of these biomarkers is crucial for advancing disease classification, staging, and treatment strategies. Peptide-based molecular probes (PMPs) have emerged as versatile tools due to their exceptional ability to discern these molecular changes with unparalleled specificity and precision. In this Perspective, we first summarize the methodologies for crafting innovative functional peptides, emphasizing recent advancements in both peptide library technologies and computer-assisted peptide design approaches. Furthermore, we offer an overview of the latest advances in PMPs within the realm of biological imaging, showcasing their varied applications in diagnostic and therapeutic modalities. We also briefly address current challenges and potential future directions in this dynamic field.

Preclinical development of novel PD-L1 tracers and first-in-human study of [68Ga]Ga-NOTA-RW102 in patients with lung cancers

BACKGROUND

The programmed cell death protein-1 (PD-1)/programmed death receptor ligand 1 (PD-L1) axis critically facilitates cancer cells' immune evasion. Antibody therapeutics targeting the PD-1/PD-L1 axis have shown remarkable efficacy in various tumors. Immuno-positron emission tomography (ImmunoPET) imaging of PD-L1 expression may help reshape solid tumors' immunotherapy landscape.

METHODS

By immunizing an alpaca with recombinant human PD-L1, three clones of the variable domain of the heavy chain of heavy-chain only antibody (VHH) were screened, and RW102 with high binding affinity was selected for further studies. ABDRW102, a VHH derivative, was further engineered by fusing RW102 with the albumin binder ABD035. Based on the two targeting vectors, four PD-L1-specific tracers ([68Ga]Ga-NOTA-RW102, [68Ga]Ga-NOTA-ABDRW102, [64Cu]Cu-NOTA-ABDRW102, and [89Zr]Zr-DFO-ABDRW102) with different circulation times were developed. The diagnostic efficacies were thoroughly evaluated in preclinical solid tumor models, followed by a first-in-human translational investigation of [68Ga]Ga-NOTA-RW102 in patients with non-small cell lung cancer (NSCLC).

RESULTS

While RW102 has a high binding affinity to PD-L1 with an excellent KD value of 15.29 pM, ABDRW102 simultaneously binds to human PD-L1 and human serum albumin with an excellent KD value of 3.71 pM and 3.38 pM, respectively. Radiotracers derived from RW102 and ABDRW102 have different in vivo circulation times. In preclinical studies, [68Ga]Ga-NOTA-RW102 immunoPET imaging allowed same-day annotation of differential PD-L1 expression with specificity, while [64Cu]Cu-NOTA-ABDRW102 and [89Zr]Zr-DFO-ABDRW102 enabled longitudinal visualization of PD-L1. More importantly, a pilot clinical trial shows the safety and diagnostic value of [68Ga]Ga-NOTA-RW102 immunoPET imaging in patients with NSCLCs and its potential to predict immune-related adverse effects following PD-L1-targeted immunotherapies.

CONCLUSIONS

We developed and validated a series of PD-L1-targeted tracers. Initial preclinical and clinical evidence indicates that immunoPET imaging with [68Ga]Ga-NOTA-RW102 holds promise in visualizing differential PD-L1 expression, selecting patients for PD-L1-targeted immunotherapies, and monitoring immune-related adverse effects in patients receiving PD-L1-targeted treatments.

TRIAL REGISTRATION NUMBER

NCT06165874.

Linear Peptide-Based PET Tracers for Imaging PD-L1 in Tumors

Programmed cell death receptor 1 (PD-1) and its ligand PD-L1 are particularly interesting immune checkpoint proteins for human cancer treatment. Positron emission tomography (PET) imaging allows for the dynamic monitoring of PD-L1 status during tumor progression, thus informing patients' response index. Herein, we report the synthesis of two linear peptide-based radiotracers, [64Cu]/[68Ga]HKP2201 and [64Cu]/[68Ga]HKP2202, and validate their utility for PD-L1 visualization in preclinical models. The precursor peptide HKP2201 was derived from a linear peptide ligand, CLP002, which was previously identified by phage display and showed nanomolar affinity toward PD-L1. Appropriate modification of CLP002 via PEGylation and DOTA conjugation yielded HKP2201. The dimerization of HKP2201 generated HKP2202. The 64Cu and 68Ga radiolabeling of both precursors was studied and optimized. PD-L1 expression in mouse melanoma cell line B16F10, mouse colon cancer cell line MC38, and their allografts were assayed by immunofluorescence and immunohistochemistry staining. Cellular uptake and binding assays were conducted in both cell lines. PET imaging and ex vivo biodistribution studies were employed in tumor mouse models bearing B16F10 and MC38 allografts. [64Cu]/[68Ga]HKP2201 and [64Cu]/[68Ga]HKP2202 were obtained with satisfactory radiocharacteristics. They all showed lower liver accumulation compared to [64Cu]/[68Ga]WL12. B16F10 and MC38 cells and their tumor allografts sections were verified to express PD-L1. These tracers demonstrated a concentration-dependent cell affinity and a comparable half-maximal effect concentration (EC50) with radiolabeled WL12. Competitive binding and blocking studies demonstrated the specific target of these tracers to PD-L1. PET imaging and ex vivo biodistribution studies revealed notable tumor uptake in tumor-bearing mice and rapid clearance from blood and major organs. Importantly, [64Cu]/[68Ga]HKP2202 showed higher tumor uptake compared to [64Cu]/[68Ga]HKP2201. Of note, [64Cu] labeled tracers showed longer retention in tumors than [68Ga] labeled traces, indicating advantages in the long-term tracking of PD-L1 dynamics. In comparison, [68Ga]HKP2201 and [68Ga]HKP2202 showed lower liver accumulation, enabling its great potential in the fast detection of both primary and metastatic tumors, including hepatic carcinoma. [64Cu]/[68Ga]HKP2201 and [64Cu]/[68Ga]HKP2202 are promising PET tracers for visualizing PD-L1 status. Notably, their combination would cooperate in rapid diagnosis and subsequent treatment guidance. Future assessment of the radiotracers in patients is needed to fully evaluate their clinical value.

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.

Ultrasmall, elementary and highly translational nanoparticle X-ray contrast media from amphiphilic iodinated statistical copolymers

To expand the single-dose duration over which noninvasive clinical and preclinical cancer imaging can be conducted with high sensitivity, and well-defined spatial and temporal resolutions, a facile strategy to prepare ultrasmall nanoparticulate X-ray contrast media (nano-XRCM) as dual-modality imaging agents for positron emission tomography (PET) and computed tomography (CT) has been established. Synthesized from controlled copolymerization of triiodobenzoyl ethyl acrylate and oligo(ethylene oxide) acrylate monomers, the amphiphilic statistical iodocopolymers (ICPs) could directly dissolve in water to afford thermodynamically stable solutions with high aqueous iodine concentrations (>140 mg iodine/mL water) and comparable viscosities to conventional small molecule XRCM. The formation of ultrasmall iodinated nanoparticles with hydrodynamic diameters of ca. 10 nm in water was confirmed by dynamic and static light scattering techniques. In a breast cancer mouse model, in vivo biodistribution studies revealed that the 64Cu-chelator-functionalized iodinated nano-XRCM exhibited extended blood residency and higher tumor accumulation compared to typical small molecule imaging agents. PET/CT imaging of tumor over 3 days showed good correlation between PET and CT signals, while CT imaging allowed continuous observation of tumor retention even after 10 days post-injection, enabling longitudinal monitoring of tumor retention for imaging or potentially therapeutic effect after a single administration of nano-XRCM.

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.

Imaging-guided targeted radionuclide tumor therapy: From concept to clinical translation

Since the first introduction of sodium iodide I-131 for use with thyroid patients almost 80 years ago, more than 50 radiopharmaceuticals have reached the markets for a wide range of diseases, especially cancers. The nuclear medicine paradigm also shifts from solely molecular imaging or radionuclide therapy to imaging-guided radionuclide therapy, which is deemed a vital component of precision cancer therapy and an emerging medical modality for personalized medicine. The imaging-guided radionuclide therapy highlights the systematic integration of targeted nuclear diagnostics and radionuclide therapeutics. Regarding this, nuclear imaging serves to "visualize" the lesions and guide the therapeutic strategy, followed by administration of a precise patient specific dose of radiotherapeutics for treatment according to the absorbed dose to different organs and tumors calculated by dosimetry tools, and finally repeated imaging to predict the prognosis. This strategy leads to significantly enhanced therapeutic efficacy, improved patient outcomes, and manageable adverse events. In this review, we provide an overview of imaging-guided targeted radionuclide therapy for different tumors such as advanced prostate cancer and neuroendocrine tumors, with a focus on development of new radioligands and their preclinical and clinical results, and further discuss about challenges and future perspectives.

Interference With Redox Homeostasis Through a G6PD-Targeting Self-Assembled Hydrogel for the Enhancement of Sonodynamic Therapy in Breast Cancer

Sonodynamics has emerged as a new potential therapy for breast cancer in recent years. However, GSH-mediated redox systems in cancer cells make them tolerable to oxidative stress-related therapy. Herein, in this study, with G6PD, the gatekeeper enzyme of the pentose phosphate pathway, as the regulative target, a self-assembled thermosensitive chitosan-pluronic hydrogel coloaded with ICG (sono-sensitive agent) and RRx-001 (IR@CPGel) was successfully prepared to enhance SDT through interference with redox homeostasis. Both in vitro and in vivo antitumor investigations verified that when integrated with sonodynamic therapy applied in breast cancer treatment, local administration of IR@CPgel could enhance ROS generation under LIFU irradiation and trigger the intrinsic apoptotic pathway of cancer cells, thus effectively inhibiting tumor growth in a safe manner. Moreover, RRx-001 may interfere with redox homeostasis in cancer cells by downregulating G6PD expression. Due to this redox imbalance, proapoptotic signals, such as P21 and P53, were enhanced, and metastasis-related signals, including MMP-2, ZEB1 and HIF-1α, were effectively reduced. Taken together, this work aimed to enhance the efficacy of sonodynamic therapy through local administration of self-assembled IR@CPGel to interfere with redox homeostasis and thus amplify the oxidative stress microenvironment in tumor tissues. In a word, this work provides a new strategy for the SDT enhancement in breast cancer therapy.

Special radionuclide production activities – recent developments at QST and throughout Japan

National Institutes for Quantum Science and Technology (QST), formerly known as the National Institute of Radiological Sciences (NIRS), has been engaged in work on radiopharmaceutical science using cyclotrons since 1974. Eight pioneering researchers founded the basis of this field of research at NIRS, and to the present, many researchers and technicians have accumulated both scientific and technical achievements, as well as inherited the spirit of research. Besides, in recent years, we have developed production systems with AVF-930 cyclotron for various ‘non-standard’ radioisotopes applied in both diagnosis and therapy. Here, we review the past 50 years of our activities on radioisotope and radiopharmaceutical development, as well as more recent activities.

Development of a Stable Peptide-Based PET Tracer for Detecting CD133-Expressing Cancer Cells

CD133 has been recognized as a prominent biomarker for cancer stem cells (CSCs), which promote tumor relapse and metastasis. Here, we developed a clinically relevant, stable, and peptide-based positron emission tomography (PET) tracer, [64Cu]CM-2, for mapping CD133 protein in several kinds of cancers. Through the incorporation of a 6-aminohexanoic acid (Ahx) into the N terminus of a CM peptide, we constructed a stable peptide tracer [64Cu]CM-2, which exhibited specific binding to CD133-positive CSCs in multiple preclinical tumor models. Both PET imaging and ex vivo biodistribution verified the superb performance of [64Cu]CM-2. Furthermore, the matched physical and biological half-life of [64Cu]CM-2 makes it a state-of-the-art PET tracer for CD133. Therefore, [64Cu]CM-2 PET may not only enable the longitudinal tracking of CD133 dynamics in the cancer stem cell niche but also provide a powerful and noninvasive imaging tool to track down CSCs in refractory cancers.