Preclinical evaluation of [99mTc]Tc-labeled anti-EpCAM nanobody for EpCAM receptor expression imaging by immuno-SPECT/CT

ImmunoPET imaging of EpCAM in solid tumours with nanobody tracers: a preclinical study

PURPOSE

Epithelial cell adhesion molecule (EpCAM) is a potential therapeutic target and anchoring molecule for circulating and disseminated tumour cells (CTC/DTC) in liquid biopsy. In this study, we aimed to construct EpCAM-specific immuno-positron emission tomography (immunoPET) imaging probes and assess the diagnostic abilities in preclinical cancer models.

METHODS

By engineering six single-domain antibodies (e.g., EPCD1 - 6) targeting EpCAM of different binding properties and labelling with 68Ga (T1/2 = 1.1 h) and 18F (T1/2 = 110 min), we developed a series of EpCAM-targeted immunoPET imaging probes. The probes' pharmacokinetics and diagnostic accuracies were investigated in cell-derived human colorectal (LS174T) and esophageal cancer (OE19) tumour models.

RESULTS

Based on in vitro binding affinities and in vivo pharmacokinetics of the first three tracers ([68Ga]Ga-NOTA-EPCD1, [68Ga]Ga-NOTA-EPCD2, and [68Ga]Ga-NOTA-EPCD3), we selected [68Ga]Ga-NOTA-EPCD3 for tumour imaging which showed an average tumour uptake of 2.06 ± 0.124%ID/g (n = 3) in LS174T cell-derived tumour model. Development and characterisation of [18F]AIF-RESCA-EPCD3 showed comparable tumour uptake of 1.73 ± 0.0471%ID/g (n = 3) in the same tumour model. Further validation of [68Ga]Ga-NOTA-EPCD3 in OE19 cell-derived tumour model showed an average tumour uptake of 4.27 ± 1.16%ID/g and liver uptake of 13.5 ± 1.30%ID/g (n = 3). Near-infrared fluorescence imaging with Cy7-EPCD3 confirmed the in vivo pharmacokinetics and relatively high liver accumulation. We further synthesized another three 18F-labeled nanobody tracers ([18F]AIF-RESCA-EPCD4, [18F]AIF-RESCA-EPCD5, and [18F]AIF-RESCA-EPCD6) and found that [18F]AIF-RESCA-EPCD6 had the best pharmacokinetics with low background. [18F]AIF-RESCA-EPCD6 showed explicit uptake in the subcutaneously inoculated OE19 tumour model with an average uptake of 4.70 ± 0.26%ID/g (n = 3). In comparison, the corresponding tumour uptake (0.17 ± 0.25%ID/g, n = 3) in the EPCD6 blocking group was substantially lower (P < 0.001), indicating the targeting specificity of the tracer.

CONCLUSIONS

We developed a series of 68Ga/18F-labeled nanobody tracers targeting human EpCAM. ImmunoPET imaging with [18F]AIF-RESCA-EPCD6 may facilitate better use of EpCAM-targeted therapeutics by noninvasively displaying the target's expression dynamics.

Recent Advances in Radiotracers Targeting Novel Cancer-Specific Biomarkers in China: A Brief Overview

Radiopharmaceuticals play a critical role in nuclear medicine, providing novel tools for specifically delivering radioisotopes for the diagnosis and treatment of cancers. As the starting point for developing radiopharmaceuticals, cancer-specific biomarkers are important and receive worldwide attention. This field in China is currently experiencing a rapid expansion, with multiple radiotracers targeting novel targets being developed and translated into clinical studies. This review provides a brief overview of the exploration of novel imaging targets, preclinical evaluation of their targeting ligands, and translational research in China from 2020 to 2023, for detecting cancer, guiding targeted therapy, and visualizing the immune microenvironment. We believe that China will play an even more important role in the development of nuclear medicine in the world in the future.

Noninvasive Evaluation of Tumoral PD-L1 Using a Novel 99mTc-Labeled Nanobody Tracer with Rapid Renal Clearance

The expression level of PD-L1 in tumor tissue is considered one of the effective biomarkers to guide PD-1/PD-L1 therapy. Quantifying whole-body PD-L1 expression by SPECT imaging may help in selecting patients that potentially respond to PD-1/PD-L1 therapy. Nanobody is the smallest antibody fragment with antigen-binding ability that is well suited for radionuclide imaging. Nevertheless, high retention of radioactivity in the kidney may limit its clinical translation. The present study aimed to screen, design, and prepare a nanobody-based SPECT probe with rapid renal clearance to evaluate the PD-L1 expression level in vivo noninvasively. A phage library was constructed by immunizing alpaca with recombinant human PD-L1 protein, and 17 anti-PD-L1 nanobodies were screened by the phage display technique. After sequence alignment and flow cytometry analysis, APN09 was selected as the candidate nanobody, and a GGGC chelator was attached to its C-terminus for 99mTc labeling to prepare a SPECT imaging probe. The affinity and specificity of 99mTc-APN09 were evaluated by protein and cell-binding experiments, and SPECT imaging and biodistribution were performed in a mouse model with bilateral transplantation of A549 and A549PD-L1 tumors. The ability of 99mTc-APN09 to quantify the PD-L1 expression level in vivo was validated in tumor models with different PD-L1 expression levels. 99mTc-APN09 had a radiochemical purity higher than 99% and a binding equilibrium dissociation constant of 21.44 ± 1.65 nM with hPD-L1, showing high affinity. SPECT imaging results showed that 99mTc-APN09 could efficiently detect PD-L1-positive tumors within 0.5 h, and the quantitative results of SPECT were well correlated with the expression level of PD-L1 in cell lines. SPECT imaging and biodistribution results also showed that 99mTc-APN09 was rapidly cleared from the kidney in 2 h postinjection. 99mTc-APN09 was a simple and stable tool for visualizing PD-L1 expression in the whole body. In addition, due to its significant reduction in renal retention, it has better prospects for clinical translation.

Promising Diagnostic and Therapeutic Approaches Based on VHHs for Cancer Management

The discovery of the distinctive structure of heavy chain-only antibodies in species belonging to the Camelidae family has elicited significant interest in their variable antigen binding domain (VHH) and gained attention for various applications, such as cancer diagnosis and treatment. This article presents an overview of the characteristics, advantages, and disadvantages of VHHs as compared to conventional antibodies, and their usage in diverse applications. The singular properties of VHHs are explained, and several strategies that can augment their utility are outlined. The preclinical studies illustrating the diagnostic and therapeutic efficacy of distinct VHHs in diverse formats against solid cancers are summarized, and an overview of the clinical trials assessing VHH-based agents in oncology is provided. These investigations demonstrate the enormous potential of VHHs for medical research and healthcare.

Immuno-PET of colorectal cancer with a CEA-targeted [68 Ga]Ga-nanobody: from bench to bedside

PURPOSE

An accurate diagnosis of colorectal carcinoma (CRC) can assist physicians in developing reasonable therapeutic regimens, thereby significantly improving the patient's prognosis. Carcinoembryonic antigen (CEA)-targeted PET imaging shows great potential for this purpose. Despite showing remarkable abilities to detect primary and metastatic CRC, previously reported CEA-specific antibody radiotracers or pretargeted imaging are not suitable for clinical use due to poor pharmacokinetics and complicated imaging procedures. In contrast, radiolabeled nanobodies exhibit ideal characteristics for PET imaging, for instance, rapid clearance rates and excellent distribution profiles, allowing same-day imaging with sufficient contrast. In this study, we developed a novel CEA-targeted nanobody radiotracer, [68 Ga]Ga-HNI01, and assessed its tumor imaging ability and biodistribution profile in preclinical xenografts and patients with primary and metastatic CRC.

METHODS

The novel nanobody HNI01 was acquired by immunizing the llama with CEA proteins. [68 Ga]Ga-HNI01 was synthesized by site-specifically conjugating [68 Ga]Ga with tris(hydroxypyridinone) (THP). Small-animal PET imaging and biodistribution studies were performed in CEA-overexpressed LS174T and CEA-low-expressed HT-29 tumor models. Following successful preclinical assessment, a phase I study was conducted on 9 patients with primary and metastatic CRC. Study participants received 151.21 ± 25.25 MBq of intravenous [68 Ga]Ga-HNI01 and underwent PET/CT scans at 1 h and 2 h post injection. Patients 01-03 also underwent whole-body dynamic PET imaging within 0-40 min p.i. All patients underwent [18F]F-FDG PET/CT imaging within 1 week after [68 Ga]Ga-HNI01 imaging. Tracer distribution, pharmacokinetics, and radiation dosimetry were calculated.

RESULTS

[68 Ga]Ga-HNI01 was successfully synthesized within 10 min under mild conditions, and the radiochemical purity was more than 98% without purification. Micro-PET imaging with [68 Ga]Ga-HNI01 revealed clear visualization of LS174T tumors, while signals from HT-29 tumors were significantly lower. Biodistribution studies indicated that uptake of [68 Ga]Ga-HNI01 in LS174T and HT-29 was 8.83 ± 3.02%ID/g and 1.81 ± 0.87%ID/g, respectively, at 2 h p.i. No adverse events occurred in all clinical participants after the injection of [68 Ga]Ga-HNI01. A fast blood clearance and low background uptake were observed, and CRC lesions could be visualized with high contrast as early as 30 min after injection. [68 Ga]Ga-HNI01 PET could clearly detect metastatic lesions in the liver, lung, and pancreas and showed superior ability in detecting small metastases. A significant accumulation of radioactivity was observed in the kidney, and normal tissues physiologically expressing CEA receptors showed slight uptakes of [68 Ga]Ga-HNI01. An interesting finding was that strong uptake of [68 Ga]Ga-HNI01 was found in non-malignant colorectal tissues adjacent to the primary tumor in some patients, suggesting abnormal CEA expression in these healthy tissues.

CONCLUSION

[68 Ga]Ga-HNI01 is a novel CEA-targeted PET imaging radiotracer with excellent pharmacokinetics and favorable dosimetry profiles. [68 Ga]Ga-HNI01 PET is an effective and convenient imaging tool for detecting CRC lesions, particularly for identifying small metastases. Furthermore, its high specificity for CEA in vivo makes it an ideal tool for selecting patients for anti-CEA therapy.

Non-invasive molecular imaging for precision diagnosis of metastatic lymph nodes: opportunities from preclinical to clinical applications

Lymph node metastasis is an indicator of the invasiveness and aggressiveness of cancer. It is a vital prognostic factor in clinical staging of the disease and therapeutic decision-making. Patients with positive metastatic lymph nodes are likely to develop recurrent disease, distant metastasis, and succumb to death in the coming few years. Lymph node dissection and histological analysis are needed to detect whether regional lymph nodes have been infiltrated by cancer cells and determine the likely outcome of treatment and the patient's chances of survival. However, these procedures are invasive, and tissue biopsies are prone to sampling error. In recent years, advanced molecular imaging with novel imaging probes has provided new technologies that are contributing to comprehensive management of cancer, including non-invasive investigation of lymphatic drainage from tumors, identifying metastatic lymph nodes, and guiding surgeons to operate efficiently in patients with complex lesions. In this review, first, we outline the current status of different molecular imaging modalities applied for lymph node metastasis management. Second, we summarize the multi-functional imaging probes applied with the different imaging modalities as well as applications of cancer lymph node metastasis from preclinical studies to clinical translations. Third, we describe the limitations that must be considered in the field of molecular imaging for improved detection of lymph node metastasis. Finally, we propose future directions for molecular imaging technology that will allow more personalized treatment plans for patients with lymph node metastasis.

Nanobodies in cell-mediated immunotherapy: On the road to fight cancer

The immune system is essential in recognizing and eliminating tumor cells. The unique characteristics of the tumor microenvironment (TME), such as heterogeneity, reduced blood flow, hypoxia, and acidity, can reduce the efficacy of cell-mediated immunity. The primary goal of cancer immunotherapy is to modify the immune cells or the TME to enable the immune system to eliminate malignancies successfully. Nanobodies, known as single-domain antibodies, are light chain-free antibody fragments produced from Camelidae antibodies. The unique properties of nanobodies, including high stability, reduced immunogenicity, enhanced infiltration into the TME of solid tumors and facile genetic engineering have led to their promising application in cell-mediated immunotherapy. They can promote the cancer therapy either directly by bridging between tumor cells and immune cells and by targeting cancer cells using immune cell-bound nanobodies or indirectly by blocking the inhibitory ligands/receptors. The T-cell activation can be engaged through anti-CD3 and anti-4-1BB nanobodies in the bispecific (bispecific T-cell engagers (BiTEs)) and trispecific (trispecific T-cell engager (TriTEs)) manners. Also, nanobodies can be used as natural killer (NK) cell engagers (BiKEs, TriKEs, and TetraKEs) to create an immune synapse between the tumor and NK cells. Nanobodies can redirect immune cells to attack tumor cells through a chimeric antigen receptor (CAR) incorporating a nanobody against the target antigen. Various cancer antigens have been targeted by nanobody-based CAR-T and CAR-NK cells for treating both hematological and solid malignancies. They can also cause the continuation of immune surveillance against tumor cells by stopping inappropriate inhibition of immune checkpoints. Other roles of nanobodies in cell-mediated cancer immunotherapy include reprogramming macrophages to reduce metastasis and angiogenesis, as well as preventing the severe side effects occurring in cell-mediated immunotherapy. Here, we highlight the critical functions of various immune cells, including T cells, NK cells, and macrophages in the TME, and discuss newly developed immunotherapy methods based on the targeted manipulation of immune cells and TME with nanobodies.

Exosomes: A potential tool for immunotherapy of ovarian cancer

Ovarian cancer is a malignant tumor of the female reproductive system, with a very poor prognosis and high mortality rates. Chemotherapy and radiotherapy are the most common treatments for ovarian cancer, with unsatisfactory results. Exosomes are a subpopulation of extracellular vesicles, which have a diameter of approximately 30-100 nm and are secreted by many different types of cells in various body fluids. Exosomes are highly stable and are effective carriers of immunotherapeutic drugs. Recent studies have shown that exosomes are involved in various cellular responses in the tumor microenvironment, influencing the development and therapeutic efficacy of ovarian cancer, and exhibiting dual roles in inhibiting and promoting tumor development. Exosomes also contain a variety of genes related to ovarian cancer immunotherapy that could be potential biomarkers for ovarian cancer diagnosis and prognosis. Undoubtedly, exosomes have great therapeutic potential in the field of ovarian cancer immunotherapy. However, translation of this idea to the clinic has not occurred. Therefore, it is important to understand how exosomes could be used in ovarian cancer immunotherapy to regulate tumor progression. In this review, we summarize the biomarkers of exosomes in different body fluids related to immunotherapy in ovarian cancer and the potential mechanisms by which exosomes influence immunotherapeutic response. We also discuss the prospects for clinical application of exosome-based immunotherapy in ovarian cancer.

Highlight selection of radiochemistry and radiopharmacy developments by editorial board

BACKGROUND

The Editorial Board of EJNMMI Radiopharmacy and Chemistry releases a biannual highlight commentary to update the readership on trends in the field of radiopharmaceutical development.

MAIN BODY

This commentary of highlights has resulted in 21 different topics selected by each coauthoring Editorial Board member addressing a variety of aspects ranging from novel radiochemistry to first in man application of novel radiopharmaceuticals.

CONCLUSION

Trends in radiochemistry and radiopharmacy are highlighted demonstrating the progress in the research field in various topics including new PET-labelling methods, FAPI-tracers and imaging, and radionuclide therapy being the scope of EJNMMI Radiopharmacy and Chemistry.