Imaging of CAR T-Cells in Cancer Patients: Paving the Way to Treatment Monitoring and Outcome Prediction

Abstract

In 2013, the journal Science named cancer immunotherapy as the ''breakthrough of the year'' based on targeted approaches using chimeric antigen receptor (CAR) T-cells. CD19-specific CAR T-cell therapy has revolutionized the treatment landscape for patients with relapsed B-cell acute lymphocytic leukemia (1,2). Similar successes have not been seen in patients with solid tumors, in part because of inconsistent expression of specific tumor antigens and physical impediments to T-cell trafficking, for example, for passing the blood-brain barrier or reaching metastatic disease in bone marrow. Other factors interfering with prolonged response to CAR T-cell therapy include loss of target antigen and T-cell exhaustion. These resistance mechanisms may be alleviated by CARs targeting 2 or more tumor antigens (NCT03019055), more sophisticated T-cell engineering techniques, or combination therapies of CARs with various immune checkpoint inhibitors. So far, it has remained unclear why certain cell therapies succeed and even provide durable clinical responses (1,2) whereas others fail (3). Ideal monitoring of CAR T-cell therapies should include the ability to track T-cell migration, engagement with the antigenbearing tumor cells, as well as T-cell expansion and persistence at the tumor site-all essential steps for therapeutic efficacy. Imaging studies might perhaps also enable timely intervention to avoid potentially lethal systemic toxicity. Current clinical methods to monitor the infused cells include serum profiling of cytokines associated with T-cell activation, direct enumeration of tumorspecific T-cells in peripheral blood, and (repeated) tumor biopsies. Overall, the in vivo activation dynamics of engineered immune cells remain incompletely understood, as no means of real-time monitoring of the intratumoral milieu currently exists. PRINCIPLES OF CELL TRACKING Cell tracking by imaging requires the direct or indirect labeling of cells. Direct labeling is relatively simple and has been used clinically for several decades, for instance, with 99m Tc or 111 Inlabeled leukocytes. More recently, 89 Zr-oxine has been used for cell labeling and PET imaging (4). As a major disadvantage, the

Metabolic radiolabeling and in vivo PET imaging of cytotoxic T lymphocytes to guide combination adoptive cell transfer cancer therapy

“…Notably, ex vivo radiolabeling of T cells followed by in vivo PET imaging as described in this study has certain limitations, i.e., it is difficult to differentiate between alive and dead cells, and to track proliferated cells [ 31 ]. In addition, long-term imaging is challenging because the label becomes diluted as T cells multiply [ 18 , 31 ]. Therefore, PET imaging of radiolabeled T cells can only provide a quick insight into the early-stage migration of T cells after transplantation.…”

“…Currently, the clinically applied methods for monitoring CAR-T cells include serum profiling of the cytokines associated with T cell activation and examination of T cells in circulation or in the tumor using invasive biopsies. However, these methods are either limited in providing accurate whole-body information on the location of T cells, or are invasive procedures that cannot be repeated frequently [ 18 ]. Advanced molecular imaging techniques, such as positron emission tomography (PET) and single-photon emission computed tomography (SPECT), are now paving the way for noninvasive characterization and quantification of biological processes in living subjects [ 19 ].…”

CAR T-Cell Therapy: An Update for Radiologists

“…Due to the nature of CAR T-cell therapy as a "living drug," investigators have shown strong interest in using imaging to study the in vivo nature of the therapy's migration, expansion, and engagement with target cells at the tumor site [54]. Tumor infiltration by CAR T-cells is associated with treatment response, and the gold standard to predict response is tumor biopsy and immunohistochemical staining.…”

“…of direct labelling is that the markers do not get replicated as the cells replicate. As a result, the markers are diluted over time, susceptible to decay, and lost from cells following cell death, thereby limiting the timeframe in which cells can be visualized [54,58]. Indirect, or in vivo, labelling methods use knowledge of cellular processes to deliver radiolabeled reporter genes into the cell's genome [59].…”

Pharmacokinetic and pharmacodynamic studies of CD19 CAR T cell in human leukaemic xenograft models with dual‐modality imaging

“…In comparison with SPECT, PET has higher resolution and higher sensitivity, 23 making positron nuclide labelling an attractive choice for PET imaging cells. PET has been reported to track infused cells 24 with radionuclides 89 Zr, 68 Ga and 52 Mn 23,25–27 Previously, we have successfully used 89 Zr‐ and 68 Ga‐labelled CAR T cells to obtain early in vivo distribution and migration behaviour, but has not yet demonstrated its targeting and efficacy in solid tumours 27 …”