CAR Chase: Where Do Engineered Cells Go in Humans?

Abstract

Chimeric antigen receptor (CAR) – and T-cell receptor (TCR) – modified T-cells are rapidly emerging as a viable treatment option for cancer patients. While initial clinical trials for these CAR T cells showed response rates of over 90% in some cases, retrospective studies have revealed a wide variability in patient responses as well as a significant proportion of patients relapsing after an initial response. In addition, patients often have severe adverse reactions to this therapy (e.g., cytokine release and neurologic syndromes). As a result, much research is still needed to be able to predict both therapeutic outcomes and possible toxicities. Furthermore, little success has been seen in treating solid tumors with engineered T cells and uncovering modes of failure is a topic of much research. Finally, little is known about the T cells’ pharmacokinetics after infusion into the patient, as standard methods of tracking the cells analyze peripheral blood and tumor biopsies – both of which lack spatiotemporal information. Herein, we propose that reporter gene-based imaging of engineered T cells in humans would be tremendously valuable in elucidating the fate of the transplanted T cells and would greatly facilitate clinical translation of new CAR and TCR technologies. Currently, there are no FDA-approved reporter genes and few methods have advanced to human studies. Herein, we outline current reporter gene approaches to track engineered cells in vivo , analyze why current reporter genes have not progressed into the clinic, and propose “rules” for designing a widely applicable reporter gene for use in humans.

Non-invasive cell-tracking methods for adoptive T cell therapies

“…In a pre-clinical setting, imaging can be used to track ACTs and to aid the development and optimization of new T cell engineering strategies by elucidating (un)intended effects on their migration patterns, local expansion and redistribution. [22][23][24] For example, longitudinal tracking of CAR-T cells revealed the superior targeting, potency and safety of CAR-T cells that had target-affinities mimicking those of native T cell receptors. 30 In a clinical setting, in vivo cell tracking could potentially be implemented in the clinical workflow to optimize treatment protocols.…”

“…In a clinical context, PET/ SPECT imaging is favored over optical imaging techniques (bioluminescence and fluorescence imaging) because of the high sensitivity, penetration depth and absolute quantification provided by PET/SPECT imaging. 23,81 High spatial anatomical resolution can be achieved by co-registration with MRI or computed tomography (CT, based on the attenuation of X-rays). Consequently, PET/SPECT reporters are the most promising gene reporter systems for clinical therapy.…”

“…127,128 Several clinical trials are currently using HSV1-tk to track CAR-T cells in glioma and prostate cancer patients (NCT00730613, NCT01082926, and NCT01140373, Table 1). 23 For example, PET imaging with 18 F-FHBG has been used successfully to track HSV-1tk + CAR-T cells, visualizing their homing to the tumor site in high-grade glioma (Fig. 7).…”

“…18 Imaging the longitudinal kinetics of CAR-T cells during the entire treatment can help the development and optimization of new T cell engineering strategies. Image reprinted from Krebs et al, 23 with permission. ACT, adoptive cell therapy; TCR, T cell receptor.…”

“…Therefore, sci-511 entific advancements in the applied intracellular delivery meth-512 ods might be required before direct MRI cell-labeling for T cell 513 tracking can be integrated into the clinical workflow. 514 Indirect (T-) cell-labeling methods 515Indirect cell labeling requires the genetic engineering of T cells so 516 that they express specific reporter genes 23,26. These reporter517 genes may encode a receptor, enzyme or symporter that enables 518 cell detection upon administration of a corresponding tracer for 519 imaging.…”