PET/CT Imaging of 89Zr-N-sucDf-Pembrolizumab in Healthy Cynomolgus Monkeys

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.

99mTc-Labeled Cyclic Peptide Targeting PD-L1 as a Novel Nuclear Imaging Probe

Recent cancer therapies have focused on reducing immune suppression in the tumor microenvironment to prevent cancer progression and metastasis. PD-1 is a checkpoint protein that stops the immune response and is expressed on immune T cells. Cancer cells express a PD-1 ligand (PD-L1) to bind to the T-cell surface and activate immunosuppressive pathways. This study aimed to design, synthesize, and evaluate a 99mTc-labeled PD-L1-targeting cyclic peptide inhibitor (99mTc-iPD-L1) as a novel SPECT radiopharmaceutical for PD-L1 expression imaging. AutoDock software (version 1.5) was used to perform molecular docking for affinity calculations. The chemical synthesis was based on the coupling reaction of 6-hydrazinylpyridine-3-carboxylic acid with a 14-amino-acid cyclic peptide. iPD-L1 was prepared for 99mTc labeling. Radio-HPLC was used to verify radiochemical purity. The stability of the radiopeptide in human serum was evaluated by HPLC. iPD-L1 specificity was assessed by SDS-PAGE. [99mTc]Tc-iPD-L1 cellular uptake in PD-L1-positive cancer cells (HCC827 and HCT116) and biodistribution in mice with induced tumors were also performed. One patient with advanced plantar malignant melanoma received [99mTc]Tc-iPD-L1. The iPD-L1 ligand (AutoDock affinity: -6.7 kcal/mol), characterized by UPLC mass, FT-IR, and UV-Vis spectroscopy, was obtained with a chemical purity of 97%. The [99mTc]Tc-iPD-L1 was prepared with a radiochemical purity of >90%. In vitro and in vivo analyses demonstrated [99mTc]Tc-iPD-L1 stability (>90% at 24 h) in human serum, specific recognition for PD-L1, high uptake by the tumor (6.98 ± 0.89% ID/g at 1 h), and rapid hepatobiliary and kidney elimination. [99mTc]Tc-iPD-L1 successfully detected PD-L1-positive lesions in a patient with plantar malignant melanoma. The results obtained in this study warrant further dosimetric and clinical studies to determine the sensitivity and specificity of [99mTc]Tc-iPD-L1/SPECT for PD-L1 expression imaging.

Spontaneous, naturally occurring cancers in non-human primates as a translational model for cancer immunotherapy

The complexity of cancer immunotherapy (CIT) demands reliable preclinical models to successfully translate study findings to the clinics. Non-human primates (NHPs; here referring to rhesus and cynomolgus macaques) share broad similarities with humans including physiology, genetic homology, and importantly also immune cell populations, immune regulatory mechanisms, and protein targets for CIT. Furthermore, NHP naturally develop cancers such as colorectal and breast cancer with an incidence, pathology, and age pattern comparable to humans. Thus, these tumor-bearing monkeys (TBMs) have the potential to bridge the experimental gap between early preclinical cancer models and patients with human cancer.This review presents our current knowledge of NHP immunology, the incidence and features of naturally-occurring cancers in NHP, and recent TBM trials investigating CIT to provide a scientific rationale for this unique model for human cancer.

Recent Trends in Diagnostic Biomarkers of Tumor Microenvironment

The tumor microenvironment (TME) play critical roles in tumor survival, progression, and metastasis and can be considered potential targets for molecular imaging of cancer. The targeting agents for imaging of TME components (e.g., fibroblasts, mesenchymal stromal cells, immune cells, extracellular matrix, blood vessels) provide a promising strategy to target these biomarkers for the early diagnosis of cancers. Moreover, various cancer types have similar tumor immune microenvironment (TIME) features that targeting those biomarkers and offer clinically translatable molecular imaging of cancers. In this review, we categorize and summarize the components in TME which have been targeted for molecular imaging. Moreover, this review updated the recent progress in targeted imaging of TIME biological molecules by various modalities for the early detection of cancer.

Isoniazid and rifapentine treatment effectively reduces persistent M. tuberculosis infection in macaque lungs

A once-weekly oral dose of isoniazid and rifapentine for 3 months (3HP) is recommended by the CDC for treatment of latent tuberculosis infection (LTBI). The aim of this study is to assess 3HP-mediated clearance of M. tuberculosis bacteria in macaques with asymptomatic LTBI. Twelve Indian-origin rhesus macaques were infected with a low dose (~10 CFU) of M. tuberculosis CDC1551 via aerosol. Six animals were treated with 3HP and 6 were left untreated. The animals were imaged via PET/CT at frequent intervals. Upon treatment completion, all animals except 1 were coinfected with SIV to assess reactivation of LTBI to active tuberculosis (ATB). Four of 6 treated macaques showed no evidence of persistent bacilli or extrapulmonary spread until the study end point. PET/CT demonstrated the presence of significantly more granulomas in untreated animals relative to the treated group. The untreated animals harbored persistent bacilli and demonstrated tuberculosis (TB) reactivation following SIV coinfection, while none of the treated animals reactivated to ATB. 3HP treatment effectively reduced persistent infection with M. tuberculosis and prevented reactivation of TB in latently infected macaques.

Evaluation of Short-Term Efficacy of PD-1 Monoclonal Antibody Immunotherapy for Lymphoma by Positron Emission Tomography/Computed Tomography Imaging with Convolutional Neural Network Image Registration Algorithm

The objective of this study was to investigate the value of PET/CT imaging based on CNN image registration algorithm in evaluating the short-term efficacy of PD-1 monoclonal antibody immunotherapy for lymphoma. 36 patients with advanced lymphoma confirmed by histology or cytochemistry and treated with PD-1 monoclonal antibody admitted to hospital were included. In addition, 38 normal controls were from healthy volunteers. All patients were treated with PD-1 monoclonal antibody intravenous infusion, and the image data were processed by CT with intelligent segmentation algorithm. Medication method: nivolumab 3 mg/kg for 2 weeks; pembrolizumab 2 mg/kg for 3 weeks; and continuous medication until tumor progression or unacceptable adverse reactions. Efficacy was evaluated every 3 cycles. Imaging examinations were performed after 3 weeks of medication to evaluate the therapeutic effect. The concentrations of IL-2, IL-7, basic fibroblast growth factor (FGF), granulocyte-macrophage colony-stimulating factor (GM-CSF), platelet-derived growth factor (PDGF-bb), and other factors in the normal control group were significantly higher than those in the advanced lymphoma patients, and the differences were statistically significant (P < 0.05). The PET/CT imaging automatic segmentation accuracy of the CNN image registration algorithm was greater than 81%, and 27 patients were treated for more than 3 cycles, including 1 case of partial remission (PR) (3.7%), 16 cases of stable disease (SD) (59.3%) after 3 cycles of treatment, and 10 cases of progressive disease (PD) (37%). After 6 cycles of treatment, 1 case was PR (8.3%), 7 cases were SD (58.4%), and 4 cases were PD (33.3%). Adverse reactions included fever, fatigue, gastrointestinal reactions, hypothyroidism, and interstitial pneumonia. PD-1 monoclonal antibody immunotherapy had a certain short-term effect on lymphoma.

Development of Radiotracers for Imaging of the PD-1/PD-L1 Axis

Immune checkpoint inhibitor (ICI) therapy has emerged as a major treatment option for a variety of cancers. Among the immune checkpoints addressed, the programmed death receptor 1 (PD-1) and its ligand PD-L1 are the key targets for an ICI. PD-L1 has especially been proven to be a reproducible biomarker allowing for therapy decisions and monitoring therapy success. However, the expression of PD-L1 is not only heterogeneous among and within tumor lesions, but the expression is very dynamic and changes over time. Immunohistochemistry, which is the standard diagnostic tool, can only inadequately address these challenges. On the other hand, molecular imaging techniques such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT) provide the advantage of a whole-body scan and therefore fully address the issue of the heterogeneous expression of checkpoints over time. Here, we provide an overview of existing PET, SPECT, and optical imaging (OI) (radio)tracers for the imaging of the upregulation levels of PD-1 and PD-L1. We summarize the preclinical and clinical data of the different molecule classes of radiotracers and discuss their respective advantages and disadvantages. At the end, we show possible future directions for developing new radiotracers for the imaging of PD-1/PD-L1 status in cancer patients.

ImmunoPET: harnessing antibodies for imaging immune cells

Dramatic, but uneven, progress in the development of immunotherapies for cancer has created a need for better diagnostic technologies including innovative non-invasive imaging approaches. This review discusses challenges and opportunities for molecular imaging in immuno-oncology and focuses on the unique role that antibodies can fill. ImmunoPET has been implemented for detection of immune cell subsets, activation and inhibitory biomarkers, tracking adoptively transferred cellular therapeutics, and many additional applications in preclinical models. Parallel progress in radionuclide availability and infrastructure supporting biopharmaceutical manufacturing has accelerated clinical translation. ImmunoPET is poised to provide key information on prognosis, patient selection, and monitoring immune responses to therapy in cancer and beyond.

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.

Quantitative PET imaging of the CD4 pool in nonhuman primates

PURPOSE

Previous SPECT and PET semi-quantitative in vivo imaging studies in monkeys have demonstrated specific uptake of radiolabeled rhesus recombinant anti-CD4 monoclonal antibody fragment CD4R1-F(ab΄)₂ in the spleen and clusters of lymph nodes (LNs) but yielded conflicting results of imaging the gut CD4 + T-cell pool. Here, using PET dynamic imaging with kinetic analysis, we performed a fully quantitative CD4 imaging in rhesus macaques.

METHODS

The biodistributions of [⁸⁹Zr]Zr-CD4R1-F(ab΄)₂ and/or of [⁸⁹Zr]Zr-ibalizumab were performed with static PET scans up to 144 h (6 days) post-injection in 18 rhesus macaques with peripheral blood CD4 + T cells/μl ranging from ~ 20 to 2400. Fully quantitative analysis with a 4-h dynamic scan, arterial sampling, metabolite evaluation, and model fitting was performed in three immunocompetent monkeys to estimate the binding potential of CD4 receptors in the LNs, spleen, and gut.

RESULTS

The biodistributions of [⁸⁹Zr]Zr-CD4R1-F(ab΄)₂ and [⁸⁹Zr]Zr-ibalizumab were similar in lymphoid tissues with a clear delineation of the CD4 pool in the LNs and spleen and a significant difference in lymphoid tissue uptake between immunocompetent and immunocompromised macaques. Consistent with our previous SPECT imaging of [^99mTc]Tc-CD4R1-F(ab΄)₂, the [⁸⁹Zr]Zr-CD4R1-F(ab΄)₂ and [⁸⁹Zr]Zr-Ibalizumab uptakes in the gut were low and not different between uninfected and SIV-infected CD4-depleted monkeys. Ex vivo studies of large and small intestines confirmed the in vivo images.

CONCLUSION

The majority of specific binding to CD4 + tissue was localized to LNs and spleen with minimal uptake in the gut. Binding potential derived from fully quantitative studies revealed that the contribution of the gut is lower than the spleen's contribution to the total body CD4 pool.

89Zr-pembrolizumab imaging as a non-invasive approach to assess clinical response to PD-1 blockade in cancer

BACKGROUND

Programmed cell death protein-1 (PD-1) antibody treatment is standard of care for melanoma and non-small cell lung cancer (NSCLC). Accurately predicting which patients will benefit is currently not possible. Tumor uptake and biodistribution of the PD-1 antibody might play a role. Therefore, we performed a positron emission tomography (PET) imaging study with zirconium-89 (⁸⁹Zr) labeled pembrolizumab before PD-1 antibody treatment.

PATIENTS AND METHODS

Patients with advanced or metastatic melanoma or NSCLC received 37 MBq (1 mCi) ⁸⁹Zr-pembrolizumab (∼2.5 mg antibody) intravenously plus 2.5 or 7.5 mg unlabeled pembrolizumab. After that, up to 3 PET scans were performed on days 2, 4, and 7. Next, PD-1 antibody treatment was initiated. ⁸⁹Zr-pembrolizumab tumor uptake was calculated as maximum standardized uptake value (SUV_max) and expressed as geometric mean. Normal organ uptake was calculated as SUV_mean, and expressed as a mean. Tumor response was assessed according to (i)RECIST v1.1.

RESULTS

Eighteen patients, 11 with melanoma and seven with NSCLC, were included. The optimal dose was 5 mg pembrolizumab, and the optimal time-point for PET scanning was day 7. The tumor SUV_max did not differ between melanoma and NSCLC (4.9 and 6.5, P = 0.49). Tumor ⁸⁹Zr-pembrolizumab uptake correlated with tumor response (Ptrend = 0.014) and progression-free (P = 0.0025) and overall survival (P = 0.026). ⁸⁹Zr-pembrolizumab uptake at 5 mg was highest in the spleen with mean SUV_mean 5.8 (±SD 1.8). There was also ⁸⁹Zr-pembrolizumab uptake in Waldeyer's ring, normal lymph nodes, and at sites of inflammation.

CONCLUSION

⁸⁹Zr-pembrolizumab uptake in tumor lesions correlated with treatment response and patient survival. ⁸⁹Zr-pembrolizumab also showed uptake in lymphoid tissues and at sites of inflammation.

First-in-human study of 89Zr-pembrolizumab PET/CT in patients with advanced stage non-small-cell lung cancer

Background: Tumor programmed-death ligand-1 (PD-L1) proportion score is the current method to select non-small-cell lung cancer (NSCLC) patients for single agent treatment with pembrolizumab, a programmed cell death-1 (PD-1) monoclonal antibody. However, not all patients respond to therapy. Better understanding of in vivo drug behavior may help to select patients that benefit most. Methods: NSCLC patients eligible for pembrolizumab monotherapy as first or later line therapy were enrolled. Patients received two injections of ⁸⁹Zr-pembrolizumab; one without a preceding dose of pembrolizumab and one with 200 mg pembrolizumab, directly prior to tracer injection. Up to four PET/CT scans were obtained after tracer injection. Post-imaging acquisition, patients were treated with 200 mg pembrolizumab, every three weeks. Tumor uptake and tracer biodistribution were visually assessed and quantified as standardized uptake value (SUV). Tumor tracer uptake was correlated with PD-1 and PD-L1 expression and response to pembrolizumab treatment. Results: Twelve NSCLC patients were included. One patient experienced grade 3 myalgia after tracer injection. ⁸⁹Zr-pembrolizumab was observed in the blood pool, liver and spleen. Tracer uptake was visualized in 47,2% of 72 tumor lesions measuring ≥20 mm long axis diameter, and substantial uptake heterogeneity was observed within and between patients. Uptake was higher in patients with response to pembrolizumab treatment (n = 3) compared to patients without a response (n = 9), although this was not statistically significant (median SUVpeak 11.4 vs 5.7, P = 0.066). No significant correlations were found with PD-L1 or PD-1 immunohistochemistry. Conclusion: ⁸⁹Zr-pembrolizumab injection was safe with only one grade 3 adverse event, possibly immune related, out of 12 patients. ⁸⁹Zr-pembrolizumab tumor uptake was higher in patients with response to pembrolizumab treatment, but did not correlate with PD-L1 or PD-1 immunohistochemistry.

State of the Art in Radiolabeling of Antibodies with Common and Uncommon Radiometals for Preclinical and Clinical Immuno-PET

Inert and stable radiolabeling of monoclonal antibodies (mAb), antibody fragments, or antibody mimetics with radiometals is a prerequisite for immuno-PET. While radiolabeling is preferably fast, mild, efficient, and reproducible, especially when applied for human use in a current Good Manufacturing Practice compliant way, it is crucial that the obtained radioimmunoconjugate is stable and shows preserved immunoreactivity and in vivo behavior. Radiometals and chelators have extensively been evaluated to come to the most ideal radiometal–chelator pair for each type of antibody derivative. Although PET imaging of antibodies is a relatively recent tool, applications with ⁸⁹Zr, ⁶⁴Cu, and ⁶⁸Ga have greatly increased in recent years, especially in the clinical setting, while other less common radionuclides such as ⁵²Mn, ⁸⁶Y, ⁶⁶Ga, and ⁴⁴Sc, but also ¹⁸F as in [¹⁸F]AlF are emerging promising candidates for the radiolabeling of antibodies. This review presents a state of the art overview of the practical aspects of radiolabeling of antibodies, ranging from fast kinetic affibodies and nanobodies to slow kinetic intact mAbs. Herein, we focus on the most common approach which consists of first modification of the antibody with a chelator, and after eventual storage of the premodified molecule, radiolabeling as a second step. Other approaches are possible but have been excluded from this review. The review includes recent and representative examples from the literature highlighting which radiometal–chelator–antibody combinations are the most successful for in vivo application.

The Future of Cancer Diagnosis, Treatment and Surveillance: A Systemic Review on Immunotherapy and Immuno-PET Radiotracers

Immunotherapy is an effective therapeutic option for several cancers. In the last years, the introduction of checkpoint inhibitors (ICIs) has shifted the therapeutic landscape in oncology and improved patient prognosis in a variety of neoplastic diseases. However, to date, the selection of the best patients eligible for these therapies, as well as the response assessment is still challenging. Patients are mainly stratified using an immunohistochemical analysis of the expression of antigens on biopsy specimens, such as PD-L1 and PD-1, on tumor cells, on peritumoral immune cells and/or in the tumor microenvironment (TME). Recently, the use and development of imaging biomarkers able to assess in-vivo cancer-related processes are becoming more important. Today, positron emission tomography (PET) with 2-deoxy-2-[18F]fluoro-D-glucose ([18F]FDG) is used routinely to evaluate tumor metabolism, and also to predict and monitor response to immunotherapy. Although highly sensitive, FDG-PET in general is rather unspecific. Novel radiopharmaceuticals (immuno-PET radiotracers), able to identify specific immune system targets, are under investigation in pre-clinical and clinical settings to better highlight all the mechanisms involved in immunotherapy. In this review, we will provide an overview of the main new immuno-PET radiotracers in development. We will also review the main players (immune cells, tumor cells and molecular targets) involved in immunotherapy. Furthermore, we report current applications and the evidence of using [18F]FDG PET in immunotherapy, including the use of artificial intelligence (AI).