Clinical development of CAR T cells-challenges and opportunities in translating innovative treatment concepts

Abstract

Chimeric antigen receptor (CAR) T cell therapy, together with checkpoint inhibition, has been celebrated as a breakthrough technology due to the substantial benefit observed in clinical trials with patients suffering from relapsed or refractory B-cell malignancies. In this review, we provide a comprehensive overview of the clinical trials performed so far worldwide and analyze parameters such as targeted antigen and indication, CAR molecular design, CAR T cell manufacturing, anti-tumor activities, and related toxicities. More than 200 CAR T cell clinical trials have been initiated so far, most of which aim to treat lymphoma or leukemia patients using CD19-specific CARs. An increasing number of studies address solid tumors as well. Notably, not all clinical trials conducted so far have shown promising results. Indeed, in a few patients CAR T cell therapy resulted in severe adverse events with fatal outcome. Of note, less than 10% of the ongoing CAR T cell clinical trials are performed in Europe. Taking lead from our analysis, we discuss the problems and general hurdles preventing efficient clinical development of CAR T cells as well as opportunities, with a special focus on the European stage.

Keywords: ATMPs; cancer; immunotherapy; regulatory issues; toxicities.

Figure 1. CAR T cell therapy—principle and clinical trial overview

(A) The CAR T cell therapy process. T cells are isolated from blood of the patient or a donor, activated, and then genetically engineered to express the CAR construct (an example shown in gray above the vector particle in violet). After ex vivo expansion of the CAR T cells, they are formulated into the final product. The patient undergoes either a conditional chemotherapy or the CAR T cell product is directly infused. (B) Schematic representation of a T cell receptor (TCR) and four types of chimeric antigen receptors (CARs) being displayed on the surface of a T cell while contacting their antigen (red) on the tumor cell. The single‐chain variable fragment (scFv) as ligand‐binding domain mediating tumor cell recognition in CARs is shown in light blue with the VH and VL domains being connected via a long flexible linker and transmembrane domain to intracellular signaling domains. Pro‐inflammatory cytokines or co‐stimulatory ligands expressed by the CAR T cells are depicted for the 4^th generation. (C) Overview of so‐called smart CAR T cells products. Pooled CAR T cell products consist of two or more single‐targeting CAR T cell types with distinct antigen specificities. Multi‐CAR T cells harbor several CAR molecules with different antigen specificities. A tandem CAR T cell expresses a CAR construct harboring two ligand‐binding domains with different antigen specificities. In a conditional CAR T cell activation and co‐stimulation are separated on two CAR constructs recognizing different target antigens. In the split CAR construct the ligand‐binding or signaling domain is physically separated allowing controlled CAR T cell activation. iCAR T cells additionally express a receptor engineered to recognize an antigen expressed on normal tissue to provide an inhibitory signal in turn. In addition CAR T cells can be equipped with suicide genes or switches (e.g., iCasp9) allowing ablation of CAR T cells. (D) Left, status of published CAR T cell gene therapy trials or trials registered at ClinicalTrials.gov including long‐term follow‐up studies. The status of one trial is unknown and not listed. The total number of clinical trials (dark blue bars) is compared to published clinical trials (light blue bars). The asterisk indicates zero trials. Right, phases of CAR T cell gene therapy trials. Long‐term follow‐up studies are not included. For nine trials, the phase classification is unknown. The asterisk indicates zero trials.

Figure 2. CAR T cell trials over time and geographical distribution

(A) Timeline of cancer CAR T cell trials as listed in Datasets EV1 and EV2 distinguishing between ongoing number (dark blue bars) and newly initiated trials in the indicated year (light blue bars). (B) Geographical distribution of worldwide ongoing CAR T cells clinical trials (left) and distribution of trial sites of the ongoing European studies (right). Five studies are multi‐centric, of which four are multi‐country trials in Europe (Dataset EV5). Long‐term follow‐up studies are not included. Color code indicates the prevalence of trials from low (green) to high (red).

Figure 3. Indication, age, CAR generation, and targeted antigen in clinical trials

(A) Solid tumors versus tumors of the hematopoietic and lymphoid system. The number of ongoing trials (dark blue bar) is compared to the number of non‐active trials (light blue bar). (B) Patient age distribution for solid tumors (light blue bars) and hematological malignancies (dark blue bars). (C) Generation of the CAR constructs applied. (D) Targeted antigens separated for tumors of hematopoietic or lymphoid origin (upper panel) and for solid tumors (lower panel).

Figure 4. CAR T cell dose and percentages of CAR‐positive cells within CAR T cell products

(A, B) CAR T cell dose indicated in the study description of published CAR T cell gene therapy trials or trials registered at ClinicalTrials.gov. The CAR T cell dose is normalized to 75 kg or 1.72 m² per dose. CAR T cells are either administered as a fixed dose (A) or in a dose escalation regimen (B). Each dot represents a single trial. (C, D) The reported number of CAR‐positive cells as given in Datasets EV3 and EV4 in column “%CAR⁺ cells (median; range)” was used to identify the median amount of CAR‐positive T cells in the various cell products within one clinical trial (C) and the range of variability between cell products within one clinical trial (lowest (min) and highest (max) percentage CAR‐positive T cells per trial) (D).

Figure 5. Clinical outcome

(A, B) Best clinical outcome for hematological malignancies (A) and solid tumors (B) dependent on the targeted antigen. The number of treated patients is provided in brackets below the targeted antigen. CR, complete response; PR, partial response; SD, stable disease; PD, progressive disease; NR, no response; NE, not evaluable.

Figure 6. Important drivers in CAR T cell trials