Positron emission tomography molecular imaging to monitor anti-tumor systemic response for immune checkpoint inhibitor therapy

Progress in the application of molecular imaging technology in immunological tolerance and immune metabolism visualization research

Immunological tolerance and immune metabolism play crucial roles in maintaining immune homeostasis and the immune response to diseases. The advancement of molecular imaging technologies, particularly optical molecular imaging, nuclear medicine imaging, and magnetic resonance imaging, has led to a significant progress in the visualization of immune tolerance and immune metabolism. Molecular imaging technologies enable real-time monitoring and analysis of dynamic changes in immune tolerance mechanisms and immune metabolism in living organisms, allowing the development of new strategies for early disease diagnosis, targeted therapy, and immunotherapy. This article reviews the latest advancements in the application of molecular imaging technologies in the fields of immunological tolerance and immune metabolism, with a focus on their applications in the regulation of immune tolerance regulation, immune metabolism, and immunotherapy.

Development and characterisation of [18F]TTDP, a novel T cell immunoglobulin and ITIM domain tracer, in humanised mice and non-human primates

PURPOSE

The T cell immunoglobulin and ITIM domain (TIGIT) blockade immunotherapy response is directly associated with individual differences of TIGIT expression on tumour-infiltrating lymphocytes (TILs) in tumour immune microenvironment (TIME) of non-small cell lung cancer (NSCLC). Here, we developed a TIGIT-targeted PET tracer to evaluate its feasibility in predicting immunotherapy efficacy, aiming to manage NSCLC patients accurately.

METHODS

We synthesised a 18F-labeled TIGIT-targeted D-peptide, [18F]TTDP, and investigated the specificity of [18F]TTDP both to murine TIGIT and human TIGIT by a series of in vitro and in vivo assays. [18F]TTDP PET imaging was performed in humanised immune system (HIS) mice models bearing NSCLC patient-derived xenografts (PDXs) to evaluate the predictive value of FDA-approved combination immunotherapy of atezolizumab plus tiragolumab. Lastly, rhesus macaque was applied for [18F] TTDP PET to explore the tracer's in vivo distribution and translational potential in non-human primates.

RESULTS

[18F]TTDP showed high specificity for both murine TIGIT and human TIGIT in vitro and in vivo. The HIS NSCLC PDX platform was successfully established for [18F]TTDP PET imaging, and tumour uptake of [18F]TTDP was significantly correlated with the TIGIT expression of TILs in the TIME. [18F]TTDP PET imaging, in predicting treatment response to the combination immunotherapy in NSCLC HIS-PDX models, showed a sensitivity of 83.33% and a specificity of 100%. In addition, [18F]TTDP PET also showed cross-species consistency of the tracer biodistribution between non-human primate and murine animals, and no adverse events were observed.

CONCLUSION

The combined implementation of the [18F]TTDP and HIS-PDX model creates a state-of-the-art preclinical platform that will impact the identification and validation of TIGIT-targeted PET image-guided diagnosis, treatment response prediction, beneficial patient screening, novel immunotherapies, and ultimately the outcome of NSCLC patients. We first provided in vivo biodistribution of [18F]TTDP PET imaging in rhesus macaque, indicating its excellent translational potential in the clinic.

Targeting of immune checkpoint regulator V-domain Ig suppressor of T-cell activation (VISTA) with 89Zr-labelled CI-8993

BACKGROUND

CI-8993 is a fully human IgG1κ monoclonal antibody (mAb) that binds specifically to immune checkpoint molecule VISTA (V-domain Ig suppressor of T-cell activation). Phase I safety has been established in patients with advanced cancer (NCT02671955). To determine the pharmacokinetics and biodistribution of CI-8993 in patients, we aimed to develop 89Zr-labelled CI-8993 and validate PET imaging and quantitation in preclinical models prior to a planned human bioimaging trial.

METHODS

CI-8993 and human isotype IgG1 control were conjugated to the metal ion chelator p-isothiocyanatobenzyl-desferrioxamine (Df). Quality of conjugates were assessed by SE-HPLC, SDS-PAGE, and FACS. After radiolabelling with zirconium-89 (89Zr), radioconjugates were assessed for radiochemical purity, immunoreactivity, antigen binding affinity, and serum stability in vitro. [89Zr]Zr-Df-CI-8993 alone (1 mg/kg, 4.6 MBq) or in combination with 30 mg/kg unlabelled CI-8993, as well as isotype control [89Zr]Zr-Df-IgG1 (1 mg/kg, 4.6 MBq) were assessed in human VISTA knock-in female (C57BL/6 N-Vsirtm1.1(VSIR)Geno, huVISTA KI) or control C57BL/6 mice bearing syngeneic MB49 bladder cancer tumours; and in BALB/c nu/nu mice bearing pancreatic Capan-2 tumours.

RESULTS

Stable constructs with an average chelator-to-antibody ratio of 1.81 were achieved. SDS-PAGE and SE-HPLC showed integrity of CI-8993 was maintained after conjugation; and ELISA indicated no impact of conjugation and radiolabelling on binding to human VISTA. PET imaging and biodistribution in MB49 tumour-bearing huVISTA KI female mice showed specific localisation of [89Zr]Zr-Df-CI-8993 to VISTA in spleen and tumour tissues expressing human VISTA. Specific tumour uptake was also demonstrated in Capan-2 xenografted BALB/c nu/nu mice.

CONCLUSIONS

We radiolabelled and validated [89Zr]Zr-Df-CI-8993 for specific binding to huVISTA in vivo. Our results demonstrate that 89Zr-labelled CI-8993 is now suitable for targeting and imaging VISTA expression in human trials.

Small-Molecule Radiotracers for Visualization of V-Domain Immunoglobulin Suppressor of T Cell Activation

V-domain immunoglobulin suppressor of T cell activation (VISTA) plays a critical role in regulating innate and adaptive immune responses within the tumor immune microenvironment. Quantifying VISTA expression is necessary to determine whether patients respond to a related combination immunotherapy. This study developed two 68Ga-labeled small-molecule probes ([68Ga]Ga-DCA and [68Ga]Ga-DNCA) for visualizing and differentiating VISTA expression. These probes exhibited excellent targeting capabilities for multiple tumor types (including B16-F10, 4T1, MC38, and CT26 tumors), consistent with the levels of VISTA expression determined by immunoblotting. Co-injection of inhibitor CA-170 led to decreased tumor uptake of both [68Ga]Ga-DCA and [68Ga]Ga-DNCA. [68Ga]Ga-DCA was used to verify the feasibility of monitoring VISTA expression in lung metastasis models. In summary, this study describes the use of 68Ga-labeled CA-170 analogues as small-molecule probes for imaging VISTA. This could provide a visual method and enable personalized immunotherapy in patients.

Validation of the C-X-C chemokine receptor 3 (CXCR3) as a target for PET imaging of T cell activation

PURPOSE

CXCR3 is expressed on activated T cells and plays a crucial role in T-cell recruitment to the tumor microenvironment (TME) during cell-based and immune checkpoint inhibitor (ICI) immunotherapy. This study utilized a 64Cu-labeled NOTA-α-CXCR3 antibody to assess CXCR3 expression in the TME and validate it as a potential T cell activation biomarker in vivo.

PROCEDURES

CXCR3+ cells infiltrating MC38 tumors (B57BL/6 mice, untreated and treated with αPD-1/αCTLA-4 ICI) were quantified using fluorescence microscopy and flow cytometry. A commercial anti-mouse CXCR3 antibody (α-CXCR3) was site-specifically conjugated with 2,2,2-(1,4,7-triazacyclononane-1,4,7-triyl)triacetic acid (NOTA) and radiolabeled with 64Cu. Saturation binding of [64Cu]Cu-NOTA-α-CXCR3 was investigated using CHO cells stably transfected with murine CXCR3. Biodistribution and PET imaging studies both at baseline and after 1 to 3 cycles of ICI, respectively, were carried out using different molar activities (10 GBq/µmol to 300 GBq/µmol) of [64Cu]Cu-NOTA-α-CXCR3.

RESULTS

Flow cytometry analysis at baseline confirmed the presence of CXCR3 + T-cells in MC38 tumors, which was significantly increased at day five after ICI (treated 33.8 ± 17.4 vs. control 8.8 ± 6.2 CD3+CXCR3+ cells/mg). These results were qualitatively and quantitatively confirmed by immunofluorescence of tumor cryoslices. In vivo PET imaging of MC38 tumor bearing mice before, during and after ICI using [64Cu]Cu-NOTA-α-CXCR3 (Kd = 3.3 nM) revealed a strong dependence of CXCR3-specificity of tracer accumulation in secondary lymphoid organs on molar activity. At 300 GBq/µmol (1.5 µg of antibody/mouse), a specific signal was observed in lymph nodes (6.33 ± 1.25 control vs. 3.95 ± 1.23%IA/g blocking) and the spleen (6.04 ± 1.02 control vs. 3.84 ± 0.79%IA/g blocking) at 48 h p.i. Spleen-to-liver ratios indicated a time dependent systemic immune response showing a steady increase from 1.08 ± 0.19 (untreated control) to 1.54 ± 0.14 (three ICI cycles).

CONCLUSIONS

This study demonstrates the feasibility of in vivo imaging of CXCR3 upregulation under immunotherapy using antibodies. However, high molar activities and low antibody doses are essential for sensitive detection in lymph nodes and spleen. Detecting therapy-induced changes in CXCR3+ T cell numbers in tumors was challenging due to secondary antibody-related effects. Nonetheless, CXCR3 remains a promising target for imaging T cell activation, with anticipated improvements in sensitivity using alternative tracers with high affinities and favorable pharmacokinetics.

Light on Alzheimer's disease: from basic insights to preclinical studies

Alzheimer's disease (AD), referring to a gradual deterioration in cognitive function, including memory loss and impaired thinking skills, has emerged as a substantial worldwide challenge with profound social and economic implications. As the prevalence of AD continues to rise and the population ages, there is an imperative demand for innovative imaging techniques to help improve our understanding of these complex conditions. Photoacoustic (PA) imaging forms a hybrid imaging modality by integrating the high-contrast of optical imaging and deep-penetration of ultrasound imaging. PA imaging enables the visualization and characterization of tissue structures and multifunctional information at high resolution and, has demonstrated promising preliminary results in the study and diagnosis of AD. This review endeavors to offer a thorough overview of the current applications and potential of PA imaging on AD diagnosis and treatment. Firstly, the structural, functional, molecular parameter changes associated with AD-related brain imaging captured by PA imaging will be summarized, shaping the diagnostic standpoint of this review. Then, the therapeutic methods aimed at AD is discussed further. Lastly, the potential solutions and clinical applications to expand the extent of PA imaging into deeper AD scenarios is proposed. While certain aspects might not be fully covered, this mini-review provides valuable insights into AD diagnosis and treatment through the utilization of innovative tissue photothermal effects. We hope that it will spark further exploration in this field, fostering improved and earlier theranostics for AD.