89Zr-Labeled Anti-PD-L1 Antibody Fragment for Evaluating In Vivo PD-L1 Levels in Melanoma Mouse Model

PET Imaging of Peptide Probe Al[18F]F-NOTA-PCP1 for Monitoring the Engagement of PD-L1 Antibodies in Tumors

Immune checkpoint inhibitors (ICIs) are a powerful treatment modality for various types of cancer. The effectiveness of ICIs is intimately connected to the binding status of antibodies to receptors. However, validated means to accurately evaluate target specificity and predict antibody efficacy in vivo are lacking. A novel peptide-based probe called Al[18F]F-NOTA-PCP1 was developed and validated for its specificity to PD-L1 in A549, U87MG, GL261, and GL261-iPDL1 cell lines, as well as in xenograft models. Then the probe was used in PET/CT scans to determine the binding status of PD-L1 antibodies (atezolizumab, avelumab, and durvalumab) in U87MG xenograft model mice. Moreover, Al[18F]F-NOTA-PCP1 was used to evaluate the impact of different treatment times and doses. Al[18F]F-NOTA-PCP1 PET/CT can be used to evaluate the interaction between PD-L1 and antibodies to determine the effectiveness of immunotherapy. By quantifying target engagement, the probe has the potential to predict the efficacy of immunotherapy and optimize the dose and treatment schedules for PD-L1 immunotherapy. This imaging agent could be a valuable tool in guiding personalized treatment strategies and improving cancer patient outcomes.

Noninvasive PET imaging of tumor PD-L1 expression with 64Cu-labeled Durvalumab

Breast cancer (BrCa) ranks as the most prevalent malignant neoplasm affecting women worldwide. The expression of programmed death-ligand 1 (PD-L1) in BrCa has recently emerged as a biomarker for immunotherapy response, but traditional immunohistochemistry (IHC)-based methods are hindered by spatial and temporal heterogeneity. Noninvasive and quantitative PD-L1 imaging using appropriate radiotracers can serve to determine PD-L1 expression in tumors. This study aims to demonstrate the viability of PET imaging with 64Cu-labeled Durvalumab (abbreviated as Durva) to assess PD-L1 expression using a murine xenograft model of breast cancer. Durvalumab, a human IgG1 monoclonal antibody against PD-L1, was assessed for specificity in vitro in two cancer cell lines (MDA-MB-231 triple-negative breast cancer cell line and AsPC-1 pancreatic cancer cell line) with positive and negative PD-L1 expression by flow cytometry. Next, we performed the in vivo evaluation of 64Cu-NOTA-Durva in murine models of human breast cancer by PET imaging and ex vivo biodistribution. Additionally, mice bearing AsPC-1 tumors were employed as a negative control. Tumor uptake was quantified based on a 3D region-of-interest (ROI) analysis of the PET images and ex vivo biodistribution measurements, and the results were compared against conventional IHC testing. The radiotracer uptake was evident in MDA-MB-231 tumors and showed minimal nonspecific binding, corroborating IHC-derived results. The results of the biodistribution showed that the MDA-MB-231 tumor uptake of 64Cu-NOTA-Durva was much higher than 64Cu-NOTA-IgG (a nonspecific radiolabeled IgG). In Conclusion, 64Cu-labeled Durvalumab PET/CT imaging offers a promising, noninvasive approach to evaluate tumor PD-L1 expression.

Evaluation of 177Lu-Labeled Pertuzumab F(ab')2 Fragments for HER2-Positive Cancer Targeting: A Comparative In Vitro and In Vivo Study

Background: Radiolabeled antibody fragments present a promising opportunity as theranostic agents, offering distinct advantages over whole antibodies. In this study, the authors investigate the potential of [177Lu]Lu-DTPA-F(ab')2-pertuzumab as a theranostic agent for precise targeting of human epidermal growth factor receptor 2 (HER2)-positive cancers. Additionally, the authors aim to quantitatively assess the binding synergism in the presence of cold trastuzumab. Materials and Methods: F(ab')2-pertuzumab was prepared by pepsin digestion and conjugated with a bifunctional chelator. The immunoconjugate was radiolabeled with 177Lu and characterized by chromatography techniques. Binding parameters (affinity, specificity, and immunoreactivity) and cellular binding enhancement studies were evaluated in HER2-overexpressing and triple-negative cell lines. The in vivo enhancement in tumor uptake of the radiolabeled immunoformulation was assessed in severe combined immunodeficient (SCID) mice bearing tumors, both in the presence and absence of unlabeled trastuzumab. Results: The formulation of [177Lu]Lu-DTPA-F(ab')2-pertuzumab could be prepared in high yields and with consistent radiochemical purity, ensuring reproducibility. Comprehensive in vitro and in vivo evaluation studies confirmed high specificity and immunoreactivity of the formulation toward HER2 receptors. Binding synergism of radiolabeled pertuzumab fragments in the presence of trastuzumab to HER2 receptors was observed. Conclusions: The radioformulation of [177Lu]Lu-DTPA-F(ab')2-pertuzumab holds great promise as a targeted approach for addressing HER2-positive cancers. A potentially effective strategy to amplify therapeutic efficacy involves dual epitope targeting by combining radiolabeled pertuzumab with cold trastuzumab.

Image-based response assessment during immunotherapy in skin cancer

Immune-checkpoint inhibitors and further immunotherapeutic treatment strategies have significantly extended therapy options for melanoma and other skin cancer entities over the last decade. In the context of a broader application of immunotherapeutic approaches, sufficient ways to monitor the course of the disease during therapy are required. Immunotherapies are based on different ways of modulating the immune system. This leads to complex clinical response patterns including pseudoprogression and others, requiring an adaptation of conventional diagnostic imaging tools or the introduction of novel technologies. In this review, current non-invasive imaging approaches for response assessment during immunotherapies in skin cancers as well as their limitations are discussed. To overcome present hurdles, promising alternatives to better address novel imaging features during immunotherapy are depicted giving an outlook on what can be expected in the future.

Impact of glycoengineering and antidrug antibodies on the anticancer activity of a plant-made lectin-Fc fusion protein

Plants are an efficient production platform for manufacturing glycoengineered monoclonal antibodies and antibody-like molecules. Avaren-Fc (AvFc) is a lectin-Fc fusion protein or lectibody produced in Nicotiana benthamiana, which selectively recognizes cancer-associated high-mannose glycans. In this study, we report the generation of a glycovariant of AvFc that is devoid of plant glycans, including the core α1,3-fucose and β1,2-xylose residues. The successful removal of these glycans was confirmed by glycan analysis using HPLC. This variant, AvFcΔXF , has significantly higher affinity for Fc gamma receptors and induces higher levels of luciferase expression in an antibody-dependent cell-mediated cytotoxicity (ADCC) reporter assay against B16F10 murine melanoma cells without inducing apoptosis or inhibiting proliferation. In the B16F10 flank tumour mouse model, we found that systemic administration of AvFcΔXF , but not an aglycosylated AvFc variant lacking affinity for Fc receptors, significantly delayed the growth of tumours, suggesting that Fc-mediated effector functions were integral. AvFcΔXF treatment also significantly reduced lung metastasis of B16F10 upon intravenous challenge whereas a sugar-binding-deficient mutant failed to show efficacy. Lastly, we determined the impact of antidrug antibodies (ADAs) on drug activity in vivo by pretreating animals with AvFcΔXF before implanting tumours. Despite a significant ADA response induced by the pretreatment, we found that the activity of AvFcΔXF was unaffected by the presence of these antibodies. These results demonstrate that glycoengineering is a powerful strategy to enhance AvFc's antitumor activity.

Impact of Radiolabeling Strategies on the Pharmacokinetics and Distribution of an Anti-PD-L1 PET Ligand

Molecular imaging with PET offers an alternative method to quantify programmed-death-ligand 1 (PD-L1) to accurately select patients for immunotherapies. More and more clinical and preclinical trials involve radiolabeling of antibody fragments for their desirably fast clearance and high tumor penetration. As the radiolabeling strategy can significantly impact pharmacokinetics and biodistribution, we explored in this work a site-specific radiofluorination strategy on an anti-PD-L1 fragment antigen-binding (Fab) and compared the pharmacokinetic and biodistribution properties with the same Fab labeled using stochastic radiolabeling chemistry. We applied an enzymatic bioconjugation mediated by a variant of the lipoic acid ligase (LplA) that promotes the formation of an amide bond between a short peptide cloned onto the C terminus of the Fab. A synthetic analogue of the enzyme natural substrate, lipoic acid, was radiolabeled with fluorine-18 for site-specific conjugation by LplA. We compared the biodistribution of the site-specifically labeled Fab with a stochastically labeled Fab on lysine side chains in tumor-bearing mice. The two methods of fluorination demonstrate a comparable whole-body biodistribution. The ⁸⁹Zr-labeled Fab had different biodistribution compared to either ¹⁸F-labeled Fab. We attribute the difference to [⁸⁹Zr] metabolism. Fab-LAP-[¹⁸F]FPyOctA therefore reflects better the true pharmacokinetic profile of the Fab.

Anti-PD-L1 immunoconjugates for cancer therapy: Are available antibodies good carriers for toxic payload delivering?

Immune checkpoint mechanisms are important molecular cell systems that maintain tolerance toward autoantigens in order to prevent immunity-mediated accidental damage. It is well known that cancer cells may exploit these molecular and cellular mechanisms to escape recognition and elimination by immune cells. Programmed cell death protein-1 (PD-1) and its natural ligand programmed cell death ligand-1 (PD-L1) form the PD-L1/PD-1 axis, a well-known immune checkpoint mechanism, which is considered an interesting target in cancer immunotherapy. In fact, the expression of PD-L1 was found in various solid malignancies and the overactivation of PD-L1/PD-1 axis results in a poor patient survival rate. Breaking PD-L1/PD-1 axis, by blocking either the cancer side or the immune side of the axis, is currently used as anti-cancer strategy to re-establish a tumor-specific immune response. For this purpose, several blocking antibodies are now available. To date, three anti-PD-L1 antibodies have been approved by the FDA, namely atezolizumab, durvalumab and avelumab. The main advantages of anti-PD-L1 antibodies arise from the overexpression of PD-L1 antigen by a high number of tumor cells, also deriving from different tissues; this makes anti-PD-L1 antibodies potential pan-specific anti-cancer molecules. Despite the good results reported in clinical trials with anti-PD-L1 antibodies, there is a significant number of patients that do not respond to the therapy. In fact, it should be considered that, in some neoplastic patients, reduced or absent infiltration of cytotoxic T cells and natural killer cells in the tumor microenvironment or presence of other immunosuppressive molecules make immunotherapy with anti-PD-L1 blocking antibodies less effective. A strategy to improve the efficacy of antibodies is to use them as carriers for toxic payloads (toxins, drugs, enzymes, radionuclides, etc.) to form immunoconjugates. Several immunoconjugates have been already approved by FDA for treatment of malignancies. In this review, we focused on PD-L1 targeting antibodies utilized as carrier to construct immunoconjugates for the potential elimination of neoplastic cells, expressing PD-L1. A complete examination of the literature regarding anti-PD-L1 immunoconjugates is here reported, describing the results obtained in vitro and in vivo. The real potential of anti-PD-L1 antibodies as carriers for toxic payload delivery is considered and extensively discussed.

Preclinical antibody-PET imaging of PD-L1

Programmed cell death protein-1/ligand-1 (PD-1/PD-L1) blockade, including antibody therapeutics, has transformed cancer treatment. However, a major challenge in the field relates to selecting patients who are likely to respond to immune checkpoint inhibitors. Indeed, biopsy-based diagnostic tests to determine immune checkpoint protein levels do not accurately capture the inherent spatial and temporal heterogeneity of PD-L1 tumor expression. As a result, not all PD-L1-positive tumors respond to immunotherapies, and some patients with PD-L1-negative tumors have shown clinical benefits. In 2018, a first-in-human study of the clinically-approved anti-PD-L1 antibody Atezolizumab labeled with the positron emitter zirconium-89 validated the ability of positron emission tomography (PET) to visualize PD-L1 expression in vivo and predict tumor response to immunotherapy. These studies have triggered the expansion of PD-L1-targeted immunoPET to assess PD-L1 protein levels and PD-L1 expression heterogeneity in real time and across the whole tumor. First, this mini-review introduces new PD-L1 PET imaging studies of the last 4 years, focusing on the expansion of preclinical tumor models and anti-PD-L1 antibodies/antibody fragments in development. Then, the review discusses how these preclinical models and targeting agents can be utilized to study spatial and temporal heterogeneity of PD-L1 expression.

Radiopharmaceuticals as Novel Immune System Tracers

Immune checkpoint inhibitors (ICIs) have transformed the treatment paradigms for multiple cancers. However, ICI therapy often fails to generate measurable and sustained antitumor responses, and clinically meaningful benefits remain limited to a small proportion of overall patients. A major obstacle to development and effective application of novel therapeutic regimens is optimized patient selection and response assessment. Noninvasive imaging using novel immunoconjugate radiopharmaceuticals (immuno–positron emission tomography and immuno-single-photon emission computed tomography) can assess for expression of cell surface immune markers, such as programmed cell death protein ligand-1 (PD-L1), akin to a virtual biopsy. This emerging technology has the potential to provide clinicians with a quantitative, specific, real-time evaluation of immunologic responses relative to cancer burden in the body. We discuss the rationale for using noninvasive molecular imaging of the programmed cell death protein-1 and PD-L1 axis as a biomarker for immunotherapy and summarize the current status of preclinical and clinical studies examining PD-L1 immuno–positron emission tomography. The strategies described in this review provide insight for future clinical trials exploring the use of immune checkpoint imaging as a biomarker for both ICI and radiation therapy, and for the rational design of combinatorial therapeutic regimens.

Translating Molecules into Imaging—The Development of New PET Tracers for Patients with Melanoma

Melanoma is a deadly disease that often exhibits relentless progression and can have both early and late metastases. Recent advances in immunotherapy and targeted therapy have dramatically increased patient survival for patients with melanoma. Similar advances in molecular targeted PET imaging can identify molecular pathways that promote disease progression and therefore offer physiological information. Thus, they can be used to assess prognosis, tumor heterogeneity, and identify instances of treatment failure. Numerous agents tested preclinically and clinically demonstrate promising results with high tumor-to-background ratios in both primary and metastatic melanoma tumors. Here, we detail the development and testing of multiple molecular targeted PET-imaging agents, including agents for general oncological imaging and those specifically for PET imaging of melanoma. Of the numerous radiopharmaceuticals evaluated for this purpose, several have made it to clinical trials and showed promising results. Ultimately, these agents may become the standard of care for melanoma imaging if they are able to demonstrate micrometastatic disease and thus provide more accurate information for staging. Furthermore, these agents provide a more accurate way to monitor response to therapy. Patients will be able to receive treatment based on tumor uptake characteristics and may be able to be treated earlier for lesions that with traditional imaging would be subclinical, overall leading to improved outcomes for patients.

Imaging immunity in patients with cancer using positron emission tomography

Immune checkpoint inhibitors and related molecules can achieve tumour regression, and even prolonged survival, for a subset of cancer patients with an otherwise dire prognosis. However, it remains unclear why some patients respond to immunotherapy and others do not. PET imaging has the potential to characterise the spatial and temporal heterogeneity of both immunotherapy target molecules and the tumor immune microenvironment, suggesting a tantalising vision of personally-adapted immunomodulatory treatment regimens. Personalised combinations of immunotherapy with local therapies and other systemic therapies, would be informed by immune imaging and subsequently modified in accordance with therapeutically induced immune environmental changes. An ideal PET imaging biomarker would facilitate the choice of initial therapy and would permit sequential imaging in time-frames that could provide actionable information to guide subsequent therapy. Such imaging should provide either prognostic or predictive measures of responsiveness relevant to key immunotherapy types but, most importantly, guide key decisions on initiation, continuation, change or cessation of treatment to reduce the cost and morbidity of treatment while enhancing survival outcomes. We survey the current literature, focusing on clinically relevant immune checkpoint immunotherapies, for which novel PET tracers are being developed, and discuss what steps are needed to make this vision a reality.

124I-Labeled Monoclonal Antibody and Fragment for the Noninvasive Evaluation of Tumor PD-L1 Expression In Vivo

Lung cancer is a highly heterogeneous cancer and is divided broadly into small and nonsmall cell lung cancer (SCLC or NSCLC). In all NSCLC patients, it is estimated that 50%-60% are programmed cell death ligand 1 (PD-L1) positive, and anti-PD-1/PD-L1 therapies have shown their clinical application prospects in advanced NSCLC. To avoid unnecessary adverse effects and provide anti-PD-1/PD-L1 therapy to the most appropriate patient population, the PD-L1 expression in patients preparing for treatment must be evaluated accurately and in real time. In this study, we noninvasively evaluate the PD-L1 expression in an NSCLC xenograft using ¹²⁴I-labeled F(ab')₂ fragments of durvalumab (Durva) and compared it with the ¹²⁴I-labeled intact antibody in terms of the biodistribution and dosimetry. The aim is to develop a nuclide labeled molecular probe with better performance for PD-L1 immunoPET imaging. After cleaving using IdeS protease, the F(ab')₂ fragments of Durva were labeled with ¹²⁴I. The radioligand showed a high radiochemical purity (>96%) and outstanding stability. Western blot, quantitative real-time polymerase chain reaction, and flow cytometry were performed on the two selected NSCLC cell lines to measure the in vitro PD-L1 expression. The H460 cells showed a much higher PD-L1 expression than the A549 cells, both at the protein level and the mRNA level. In the following cell binding experiment and binding specificity assay, the labeled radioligand showed good affinity to high PD-L1 expression cells and could be blocked with excess unlabeled intact Durva. The results of the biodistribution and the positron emission tomography (PET) image showed that the peak tumor uptake of ¹²⁴I-Durva-F(ab')₂ was close to ¹²⁴I-Durva, but much earlier (5.29 ± 0.42% ID/g for ¹²⁴I-Durva-F(ab')₂ at 12 h vs 5.18 ± 0.73% ID/g for ¹²⁴I-Durva at 48 h). Compared with ¹²⁴I-Durva, an accelerated blood clearance was observed for ¹²⁴I-Durva-F(ab')₂. The faster blood clearance allowed for a higher tumor-to-background ratio, which was reflected on the image in contrast. The H460 tumors showed excellent contrast as early as 4 h after injection with ¹²⁴I-Durva-F(ab')₂, and for ¹²⁴I-Durva, the xenograft could not be distinguished clearly until 24 h after injection. Interestingly, ¹²⁴I-Durva-F(ab')₂ showed lower accumulations compared to other metal isotopes labeled PD-L1 antibodies in bone, liver, spleen etc., which will be beneficial for metastasis detection. Another benefit of accelerated blood clearance was a reduction in the radiation dose. According to the results of the OLINDA/EXM, the effective dose for the total body of ¹²⁴I-Durva was 4.25-times greater than that of ¹²⁴I-Durva-F(ab')₂ (186 μSv/MBq vs 43.8 μSv/MBq). All of these data indicated that ¹²⁴I-Durva-F(ab')₂ is a promising immunoPET tracer for evaluating the in vivo PD-L1 levels in an NSCLC model and is expected to be successful in future clinical application.

Potentials of Non-Invasive 18F-FDG PET/CT in Immunotherapy Prediction for Non-Small Cell Lung Cancer

The immune checkpoint inhibitors (ICIs), by targeting cytotoxic-T-lymphocyte-associated protein 4, programmed cell death 1 (PD-1), or PD-ligand 1, have dramatically changed the natural history of several cancers, including non-small cell lung cancer (NSCLC). There are unusual response manifestations (such as pseudo-progression, hyper-progression, and immune-related adverse events) observed in patients with ICIs because of the unique mechanisms of these agents. These specific situations challenge response and prognostic assessment to ICIs challenging. This review demonstrates how 18F-FDG PET/CT can help identify these unusual response patterns in a non-invasive and effective way. Then, a series of semi-quantitative parameters derived from 18F-FDG PET/CT are introduced. These indexes have been recognized as the non-invasive biomarkers to predicting the efficacy of ICIs and survival of NSCLC patients according to the latest clinical studies. Moreover, the current situation regarding the functional criteria based on 18F-FDG PET/CT for immunotherapeutic response assessment is presented and analyzed. Although the criteria based on 18F-FDG PET/CT proposed some resolutions to overcome limitations of morphologic criteria in the assessment of tumor response to ICIs, further researches should be performed to validate and improve these assessing systems. Then, the last part in this review displays the present status and a perspective of novel specific PET probes targeting key molecules relevant to immunotherapy in prediction and response assessment.

PET imaging of immune checkpoint proteins in oncology

Despite the remarkable clinical successes of immune checkpoint inhibitors (ICIs) in various advanced cancers, response is still limited to a subset of patients that generally exhibit tumoral expression of immune checkpoint (IC) proteins. Development of biomarkers assessing the expression of such ICs is therefore a major challenge nowadays to refine patient selection and improve therapeutic benefits. Positron emission tomography (PET) imaging using IC-targeted radiolabeled monoclonal antibodies (immunoPET) provides a non-invasive and whole-body visualization of in vivo IC biodistribution. As such, PET imaging of ICs may serve as a robust biomarker to predict and monitor responses to ICIs, complementing the existing immunohistochemical techniques. Besides monoclonal antibodies, other PET radioligand formats, ranging from antibody-derived fragments to small proteins, have gained increasing interest owing to their faster pharmacokinetics and enhanced imaging characteristics. We provide an overview of the various strategies investigated so far for PET imaging of ICs in preclinical and clinical studies, emphasizing their benefits and limitations. Moreover, we discuss various parameters to consider for designing optimized and best-suited PET radioligands.

Molecular imaging in immuno-oncology: current status and translational perspectives

Introduction: Only 20-40% of patients respond to therapy with immune checkpoint inhibitors (ICIs). Therefore, the early identification of subjects that can benefit from such therapeutic regimen is mandatory. Areas covered: The immunobiological mechanisms of ICIs are briefly illustrated. Furthermore, the limitations of traditional radiological approaches are covered. Then, the pros and cons of molecular imaging through positron emission computed tomography (PET/CT) are reviewed, with a particular focus on ¹⁸f-fluorodeoxyglucose (¹⁸F-FDG) and PET-derived metabolic parameters. Lastly, translational perspective of radiopharmaceuticals others than ¹⁸F-FDG such as ⁸⁹zirconium (⁸⁹Zr) or fluorine-18 (¹⁸F) labeled monoclonal antibodies (e.g.⁸⁹Zr-atezolizumab, ⁸⁹Zr-nivolumab) binding to specific biomarkers are discussed. Expert opinion: Molecular imaging presents a prominent role for the management of oncological patients treated with ICIs. Preliminary clinical data indicate that PET/CT with ¹⁸F-FDG is useful for assessing the response to treatment and for the imaging of immune-related adverse effects. Nevertheless, the methodological approach (iPERCIST, PERCIMT, or others) to be used for an optimal diagnostic accuracy and patients' evaluation is still a debated issue. PET/CT with radioligands directed toward ICIs biomarkers, although is still in a translational phase, holds the promise of accurately predicting the response to treatment and revealing the acquired resistance to immunotherapy.