Clinical trials of CD19-targeted CAR-modified T cell therapy; a complex and varied landscape

CD19 chimeric antigen receptors (CARs) comprise a CD19specific single chain variable fragment (scFv), fused to an extracellular spacer, a transmembrane domain, and one or more intracellular T cell stimulatory or costimulatory sequences [1]. First-generation CARs contain only a T cellstimulatory sequence (most often CD3ζ), whereas second or third-generation CARs also contain one or two costimulatory sequences (e.g. 4-1BB, CD28) in addition to CD3ζ, respectively. When introduced into T cells by genetic modification, CD19 CARs redirect the specificity of the T cells to target cells expressing CD19. T cells are generally infused after chemotherapy given for lymphodepletion and the robust proliferation and accumulation of CAR-T cells in the first 2–3 weeks appear important for lysis of CD19-expressing tumor cells and clinical efficacy. Patients may develop cytokine release syndrome (CRS), characterized by fever, hypotension, capillary leak, coagulopathy, organ dysfunction and neurologic toxicity, associated with elevated concentrations of serum cytokines such as IL-6 and IFN-γ. CD19 CAR-T cell immunotherapy has shown remarkable efficacy in refractory B-cell acute lymphoblastic leukemia (B-ALL), and encouraging responses have also been observed in B-cell non-Hodgkin lymphoma (NHL) and chronic lymphocytic leukemia (CLL) [2–9]. CD19 CAR-T cell immunotherapy is still in its infancy; however, a number of centers have begun to accumulate and report significant experience in the manufacturing and delivery of effective CAR-T cell immunotherapy to patients with B-cell malignancies [2–9]. Inevitably, because the rates of efficacy and toxicity reported in clinical trials vary, comparisons between studies have been made in an effort to identify critical factors that govern outcomes of CAR-T cell immunotherapy. Although a comprehensive review of the differences in approaches at each center is beyond the scope of this editorial, it is important to note that manufacturing strategies, including procedures for isolation and stimulation of T cells, the design of CAR constructs, CAR gene transfer, and expansion and formulation of the cell product for infusion widely differ between centers. Many of the details of manufacturing are proprietary, rendering comparisons between studies difficult. In addition, study participants and eligibility criteria vary, as do the selections of lymphodepletion regimens, the dose and timing of CAR-T cell infusions, and the approaches to evaluation and restaging. Patterns identified from analyses of correlative laboratory data and clinical outcomes are emerging and beginning to guide the field. However, caution is warranted when comparing data obtained in studies conducted in different centers using different CAR-T cell products. Preclinical studies suggest, not surprisingly, that the design of CAR constructs can have a profound impact on function. The selection of scFv, extracellular spacer length and composition, and the presence and selection of intracellular costimulatory sequences have been shown in model systems to impact the efficacy of CAR-T cell immunotherapy [10–13]. In a clinical trial, CD19 CAR-T cells incorporating a secondgeneration CAR with CD28 costimulation appeared more effective compared to their first-generation counterparts [14]; and the recent success of clinical trials incorporating secondgeneration CD28or 4-1BB-costimulated CD19 CARs compared to previous studies using first-generation CARs validates this observation [2–6,8,9]. A more difficult question is if there are outcome differences specific to the costimulatory sequence incorporated into the second-generation CAR constructs. Minimal residual disease (MRD)-negative complete response (CR) rates appear higher and CAR-T cell persistence appears longer in B-ALL patients treated in two clinical trials using different CAR-T cell products incorporating 4-1BBcostimulated CARs compared to two other trials in which the CAR constructs incorporated CD28-mediated costimulation [2,5,6,9]. However, differences in other facets of the CAR constructs used in these trials (e.g. scFv, spacer and transmembrane domains), as well as in CAR-T cell manufacturing, clinical eligibility, lymphodepletion chemotherapy, the schedule of treatments, and selection of additional postremission therapy render comparisons between the studies and identification of the optimal approaches difficult. In NHL patients, the data regarding relative efficacy of 4-1BBor CD28-costimulation is even more limited, and at this stage does not provide