High-Resolution PET Imaging with Therapeutic Antibody-based PD-1/PD-L1 Checkpoint Tracers

Evaluation of response to immune checkpoint inhibitors: Is there a role for positron emission tomography?

Strategies targeting intracellular negative regulators such as immune checkpoint inhibitors (ICPIs) have demonstrated significant antitumor activity across a wide range of solid tumors. In the clinical practice, the radiological effect of immunotherapeutic agents has raised several more relevant and complex challenges for the determination of their imaging-based response at single patient level. Accordingly, it has been suggested that the conventional Response Evaluation Criteria in Solid Tumors assessment alone, based on dimensional evaluation provided by computed tomography (CT), tends to underestimate the benefit of ICPIs at least in a subset of patients, supporting the need of immune-related response criteria. Different from CT, very few data are available for the evaluation of immunotherapy by means of ¹⁸F-fluoro-2-deoxy-D-glucose positron emission tomography (FDG-PET). Moreover, since the antineoplastic activity of ICPIs is highly related to the activation of T cells against cancer cells, FDG accumulation might cause false-positive findings. Yet, discrimination between benign and malignant processes represents a huge challenge for FDG-PET in this clinical setting. Consequently, it might be of high interest to test the complex and variegated response to ICPIs by means of PET and thus it is worthwhile to ask if a similar introduction of immune-related PET-based criteria could be proposed in the future. Finally, PET might offer a new insight into the biology and pathophysiology of ICPIs thanks to a growing number of non-invasive immune-diagnostic approaches based on non-FDG tracers.

Precision medicine in immune checkpoint blockade therapy for non-small cell lung cancer

Immune checkpoint blockade therapy by targeting the programmed death protein 1/programmed death ligand 1 (PD-L1) axis using antibodies has yielded promising clinical responses in patients with non-small cell lung cancer (NSCLC). However, owing to the dynamic expression of PD-L1, degree of mutational/neoantigen load, intratumoral heterogeneity, infiltrated immune cells of tumor microenvironment of NSCLC, the response rates to these agents are limited, despite several companion diagnostic assays by detecting PD-L1 in tumor cells have been introduced into clinical practice. Therefore, in this era of precision medicine, there is an urgent need for predictive biomarkers to identify NSCLC patients likely to benefit from this novel therapy.

Rapid PD-L1 detection in tumors with PET using a highly specific peptide

Molecular imaging can report on the status of the tumor immune microenvironment and guide immunotherapeutic strategies to enhance the efficacy of immune modulation therapies. Imaging agents that can rapidly report on targets of immunomodulatory therapies are few. The programmed death ligand 1 (PD-L1) is an immune checkpoint protein over-expressed in several cancers and contributes to tumor immune suppression. Tumor PD-L1 expression is indicative of tumor response to PD-1 and PD-L1 targeted therapies. Herein, we report a highly specific peptide-based positron emission tomography (PET) imaging agent for PD-L1. We assessed the binding modes of the peptide WL12 to PD-L1 by docking studies, developed a copper-64 labeled WL12 ([⁶⁴Cu]WL12), and performed its evaluation in vitro, and in vivo by PET imaging, biodistribution and blocking studies. Our results show that [⁶⁴Cu]WL12 can be used to detect tumor PD-L1 expression specifically and soon after injection of the radiotracer, to fit within the standard clinical workflow of imaging within 60 min of administration.

PD-L1 Detection in Tumors Using [(64)Cu]Atezolizumab with PET

The programmed death protein 1 (PD-1) and programmed death-ligand 1 (PD-L1) pair is a major immune checkpoint pathway exploited by cancer cells to develop and maintain immune tolerance. With recent approvals of anti-PD-1 and anti-PD-L1 therapeutic antibodies, there is an urgent need for noninvasive detection methods to quantify dynamic PD-L1 expression in tumors and to evaluate the tumor response to immune modulation therapies. To address this need, we assessed [(64)Cu]atezolizumab for the detection of PD-L1 expression in tumors. Atezolizumab (MPDL3208A) is a humanized, human and mouse cross-reactive, therapeutic PD-L1 antibody that is being investigated in several cancers. Atezolizumab was conjugated with DOTAGA and radiolabeled with copper-64. The resulting [(64)Cu]atezolizumab was assessed for in vitro and in vivo specificity in multiple cell lines and tumors of variable PD-L1 expression. We performed PET-CT imaging, biodistribution, and blocking studies in NSG mice bearing tumors with constitutive PD-L1 expression (CHO-hPD-L1) and in controls (CHO). Specificity of [(64)Cu]atezolizumab was further confirmed in orthotopic tumor models of human breast cancer (MDAMB231 and SUM149) and in a syngeneic mouse mammary carcinoma model (4T1). We observed specific binding of [(64)Cu]atezolizumab to tumor cells in vitro, correlating with PD-L1 expression levels. Specific accumulation of [(64)Cu]atezolizumab was also observed in tumors with high PD-L1 expression (CHO-hPD-L1 and MDAMB231) compared to tumors with low PD-L1 expression (CHO, SUM149). Collectively, these studies demonstrate the feasibility of using [(64)Cu]atezolizumab for the detection of PD-L1 expression in different tumor types.

Preclinical Pharmacokinetics and Biodistribution Studies of 89Zr-Labeled Pembrolizumab

Pembrolizumab is a humanized monoclonal antibody targeting programmed cell death protein 1 (PD-1) found on T and pro-B cells. Pembrolizumab prevents PD-1 ligation by both PD-L1 and PD-L2, preventing the immune dysregulation that otherwise occurs when T-cells encounter cells expressing these ligands. Clinically, PD-1 blockade elicits potent antitumor immune responses, and antibodies blocking PD-1 ligation, including pembrolizumab, have recently received Food and Drug Administration approval for the treatment of advanced melanoma, renal cell cancer, and non-small cell lung cancer.

METHODS

In this study, we evaluated the pharmacokinetics, biodistribution, and dosimetry of pembrolizumab in vivo, accomplished through radiolabeling with the positron emitter ⁸⁹Zr. PET imaging was used to evaluate the whole-body distribution of ⁸⁹Zr-deferoxamine (Df)-pembrolizumab in two rodent models (mice and rats). Data obtained from PET scans and biodistribution studies were extrapolated to humans to estimate the dosimetry of the tracer. As a proof of concept, the biodistribution of ⁸⁹Zr-Df-pembrolizumab was further investigated in a humanized murine model.

RESULTS

The tracer remained stable in blood circulation throughout the study and accumulated the greatest in liver and spleen tissues. Both mice and rats showed similar biodistribution and pharmacokinetics of ⁸⁹Zr-Df-pembrolizumab. In the humanized mouse model, T-cell infiltration into the salivary and lacrimal glands could be successfully visualized.

CONCLUSION

These data will augment our understanding of the pharmacokinetics and biodistribution of radiolabeled pembrolizumab in vivo, while providing detailed dosimetry data that may lead to better dosing strategies in the future. These findings further demonstrate the utility of noninvasive in vivo PET imaging to dynamically track T-cell checkpoint receptor expression and localization in a humanized mouse model.

Molecular Imaging of Immunotherapy Targets in Cancer

Immunotherapy has emerged as a promising alternative in the arsenal against cancer by harnessing the power of the immune system to specifically target malignant tissues. As the field of immunotherapy continues to expand, researchers will require newer methods for studying the interactions between the immune system, tumor cells, and immunotherapy agents. Recently, several noninvasive imaging strategies have been used to map the biodistribution of immune checkpoint molecules, monitor the efficacy and potential toxicities of the treatments, and identify patients who are likely to benefit from immunotherapies. In this review, we outline the current applications of noninvasive techniques for the preclinical imaging of immunotherapy targets and suggest future pathways for molecular imaging to contribute to this developing field.

Checkpoint Antibodies but not T Cell-Recruiting Diabodies Effectively Synergize with TIL-Inducing γ-Irradiation

T cell-recruiting bispecific antibodies (bsAb) show promise in hematologic malignancies and are also being evaluated in solid tumors. In this study, we investigated whether T cell-recruiting bsAbs synergize with hypofractionated tumor radiotherapy (hRT) and/or blockade of the programmed death-1 (PD-1) immune checkpoint, both of which can increase tumor-infiltrating lymphocyte (TIL) numbers. Unexpectedly, large melanomas treated with hRT plus bsAb (AC133×CD3) relapsed faster than those treated with hRT alone, accompanied by massive TIL apoptosis. This fast relapse was delayed by the further addition of anti-PD-1. Mechanistic investigations revealed restimulation-induced cell death mediated by BIM and FAS as an additional cause of bsAb-mediated TIL depletion. In contrast, the double combination of hRT and anti-PD-1 strongly increased TIL numbers, and even very large tumors were completely eradicated. Our study reveals the risk that CD3-engaging bsAbs can induce apoptotic TIL depletion followed by rapid tumor regrowth, reminiscent of tolerance induction by CD3 mAb-mediated T-cell depletion, warranting caution in their use for the treatment of solid tumors. Our findings also argue that combining radiotherapy and anti-PD-1 can be quite potent, including against very large tumors. Cancer Res; 76(16); 4673-83. ©2016 AACR.