Population Cellular Kinetics of Lisocabtagene Maraleucel, an Autologous CD19-Directed Chimeric Antigen Receptor T-Cell Product, in Patients with Relapsed/Refractory Large B-Cell Lymphoma

Lisocabtagene maraleucel (liso-cel; JCAR017) is a CD19-directed, genetically modified, defined composition, autologous cellular immunotherapy administered at equal target doses of CD8⁺ and CD4⁺ chimeric antigen receptor (CAR)-positive T cells [1]. The CAR comprises an FMC63 monoclonal antibody-derived single-chain variable fragment, immunoglobulin G4 hinge region, CD28 transmembrane domain, 4-1BB (CD137) costimulatory domain, and CD3ζ activation domain [2]. In addition, liso-cel includes a nonfunctional truncated epidermal growth factor receptor that is coexpressed on the cell surface with the CD19-specific CAR and can serve as a surrogate for CAR expression [3‐5]. CAR binding to CD19 expressed on the cell surface of tumors and normal B cells induces activation and proliferation of CAR T cells, release of proinflammatory cytokines, and cytotoxic killing of target cells [6].

TRANSCEND NHL 001 (TRANSCEND; NCT02631044) is a multicenter, multicohort, open-label, seamless design (i.e. consisting of dose-finding, dose-expansion, and dose-confirmation phases) study to determine the safety, antitumor activity, and cellular kinetics of liso-cel in patients with relapsed/refractory large B-cell lymphoma (LBCL; LBCL cohort) and mantle cell lymphoma (mantle cell lymphoma cohort). TRANSCEND is the largest clinical study reported to date of CD19-directed CAR T-cell therapy in relapsed/refractory LBCL [1, 3, 7‐9]. Eligible patients underwent leukapheresis for collection of autologous peripheral blood mononuclear cells for manufacture of liso-cel followed by lymphodepleting chemotherapy (LDC; fludarabine 30 mg/m² and cyclophosphamide 300 mg/m² for 3 days) [1]. During the liso-cel manufacturing process (between leukapheresis and LDC), bridging therapy with systemic and/or radiation therapy was allowed at the discretion of the treating physician. Liso-cel was administered as two sequential infusions of CD8⁺ and CD4⁺ CAR⁺ T cells at one of three dose levels (50 × 10⁶, 100 × 10⁶, or 150 × 10⁶ CAR⁺ T cells). The 269 patients who received liso-cel had a median age of 63 years and a median of three previous lines of systemic therapy; 67% had chemotherapy-refractory disease, 33% had previous autologous hematopoietic stem cell transplantation (HSCT), and 59% received bridging therapy. In efficacy-evaluable patients (n = 256), treatment with liso-cel resulted in a high rate of durable complete response (CR), with an objective response rate of 73%, CR rate of 53%, and an estimated duration of CR at 1 year of 65% [1]. Liso-cel treatment was associated with a low incidence of severe cytokine release syndrome (CRS; 2%) and neurological events (NEs; 10%). CRS of any grade was reported in 42% of patients, with median time to onset of 5 days and median time to resolution of 5 days. NEs of any grade were reported in 30% of patients, with median time to onset of 9 days and median time to resolution of 11 days. Additional safety outcomes of interest included severe neutropenia (60%), anemia (37%), and thrombocytopenia (27%), with prolonged cytopenia (not resolved at Day 29) in 37% of patients. Severe infections, including bacterial, fungal, and viral, were reported in 12% of patients.

Cellular kinetics of two other CAR T-cell therapies (tisagenlecleucel and axicabtagene ciloleucel [axi-cel]) were characterized in B-cell acute lymphoblastic leukemia (B-ALL; tisagenlecleucel) [10‐12] and relapsed/refractory LBCL (tisagenlecleucel [13] and axi-cel [8]). These CAR T-cell therapies generally showed rapid in vivo expansion within 2 weeks post-infusion followed by subsequent biexponential decline [8, 12]. In TRANSCEND, median time to liso-cel peak expansion was 12 days, and higher liso-cel expansion was associated with higher rates of CR and partial response (maximum transgene levels [C_max], 3.55-fold) and higher incidence of CRS (C_max, 2.29-fold) and NEs (C_max, 3.34-fold) in patients with relapsed/refractory LBCL [1]. Liso-cel was present in peripheral blood for up to 2 years [1]. Large between-subject variability (BSV) was noted in all three CAR T-cell therapies (e.g. percentage coefficient of variation [%CV] of > 100% for tisagenlecleucel) [12, 13], as expected for biologic products that expand in vivo; patient characteristics might also contribute to the variability of CAR T-cell expansion. Tisagenlecleucel expansion was lower in LBCL than B-ALL [13]. Patient demographics such as age and sex had no significant impact on the expansion of both tisagenlecleucel and axi-cel [14, 15]; however, these findings might be because of the relatively small sample size given the large BSV. TRANSCEND is the largest clinical study among CD19-directed CAR T-cell therapies, and in this study the association of baseline intrinsic and disease factors with liso-cel kinetics was investigated. This study describes a population cellular kinetic model of liso-cel that was developed to characterize the kinetics of the liso-cel transgene in relapsed/refractory LBCL, as assessed by quantitative polymerase chain reaction (qPCR) after intravenous infusion of liso-cel, and to understand covariates that might influence liso-cel kinetics in individual patients.

Unlike systemic therapies, CAR T-cell therapies are administered once and the kinetics depend on CAR T-cell expansion, contraction, and persistence after infusion into the patient. Patient and disease characteristics are anticipated to play a role in the variability of cellular kinetics observed with CAR T-cell therapy. Liso-cel transgene observations were well described by a piecewise model of cellular growth kinetics that featured lag, exponential growth, and biphasic decay phases in treated patients. Unless otherwise specified, the following summaries are for a typical patient who was 63 years of age, had SPD before LDC (per IRC) of 22.5 cm², had LDH before LDC of 269 U/L, and did not use tocilizumab and/or corticosteroids for the treatment of CRS and/or NEs. Liso-cel transgene levels were stable during the lag phase (T_lag) and doubled approximately eight times during the growth phase (T_gro), reaching a C_max of 23,600 copies/μg (91.5% BSV) at 9.29 days (T_max = T_lag + T_gro). After peak levels, the liso-cel transgene decay phase was biphasic, with α-phase (HL_α) and β-phase (HL_β) half-life estimates of 5.00 (97.7% BSV) and 352 days, respectively. The fraction of peak liso-cel transgene levels appearing in the β-phase (F_β) was estimated at 0.659%. In this empirical cellular kinetic model, the fractions of α-phase and β-phase were assumed as effector and memory T cells, respectively.

A covariate search using stepwise forward addition (α = 0.01) and backward elimination (α = 0.001) demonstrated that baseline age, SPD per IRC before LDC, LDH before LDC, and the use of tocilizumab and/or corticosteroids for the treatment of CRS and/or NEs were associated with liso-cel kinetics. Univariate effect sizes should be interpreted with caution as the covariates may be correlated. Therefore, the association of patient covariates was instead assessed by simulation of C_max, T_max, and AUC_0–28d using the final model. Observed patient covariates were used to condition these simulations, thus preserving any collinearity in the covariates. Treatment with tocilizumab and/or corticosteroids was associated with higher C_max and AUC_0–28d. Higher C_max and AUC_0–‍28d were also associated with CRS and/or NEs [1], which triggered the therapeutic intervention with tocilizumab and/or corticosteroids. In TRANSCEND, the median time to onset of CRS and NEs was 5.0 and 9.0 days, respectively [1], and the median time from onset of CRS to first administration of tocilizumab was 1.5 days [1]. Therefore, the relationship of tocilizumab/corticosteroid use with cellular kinetic parameters should be interpreted with caution. Examination of these cellular kinetic parameters across the covariate range (quartiles, in the case of continuous covariates) showed the expected association pattern described in Fig. 4. The magnitude of effect on expansion metrics demonstrated that the covariate associations were smaller than the residual BSV in the population. Particularly, BSV not explained by covariates for C_max was estimated as 91.5%. Thus, the covariate effects were not considered meaningful.

Recent work with physiologically based pharmacokinetic models of exogenous T-cell administration found that the administered dose did not contribute to initial T-cell blood concentrations [22]. In this effort, a similar lack of dose dependence was noted with liso-cel transgene levels. Dose was examined as a covariate on model parameters but was not found to be statistically significant. This suggests that the dose was not associated with expansion quantities in the tested administered dose range (44–156 × 10⁶ CAR⁺ T cells). Thus, no dose adjustment is recommended in specific populations [23]. Tisagenlecleucel also exhibited a flat relationship between dose and the cellular kinetic parameters (0.2–5.0 × 10⁶ CAR⁺ T cells/kg in patients weighing ≤ 50 kg, and 0.1–2.5 × 10⁸ CAR⁺ T cells in patients weighing > 50 kg) [12].

Finally, these results can be compared with similar analyses conducted for tisagenlecleucel in pediatric and young adult patients with B-ALL [10] and axi-cel in adult patients with LBCL [19]. Prior literature reports for CAR T-cell therapies were based on a theoretic model developed by De Boer and Perelson [18] describing the murine immune response to an infection by Listeria monocytogenes or lymphocytic choriomeningitis virus. Findings from this present analysis are aligned with previous analyses conducted for other CAR T-cell therapies (expansion of CAR T cells post-infusion followed by biphasic decline). Two new concepts were introduced by this analysis: the introduction of the novel lag phase and the parameterization of the liso-cel transgene population cellular kinetic model. First, previous reports in the literature for CAR T-cell therapies did not report a lag phase after administration, but the general concept of a lag phase as living cells acclimatize to a new environment is well established [21, 24]. Parameterization of the population cellular kinetic model reported here differs from previous efforts, posing the model in more readily interpretable (but still interconvertible) terms. For example, exponents of base 2 were used (instead of base e), reflecting the underlying biology of mitotic doubling of cell populations. Furthermore, the doubling time of CAR T cells is expressed in this model instead of a fold-expansion from baseline ratioed to time to maximum liso-cel transgene levels. The model parameterizations are interconvertible with the previous formulation using the following relationships: fold_x = C_max/C₀; ρ = log(fold_x)/T_gro; α = log(2)/HL_α; β = log(2)/HL_β (definition of terms is available in the study by Stein et al. [10]). Doubling time, T_max, HL_α, and HL_β of tisagenlecleucel were 0.78, 9.3, 4.3, and 220 days, respectively [10], while doubling time, T_max, HL_α, and HL_β of axi-cel were 0.87, 4.9, 3.3, and 173 days, respectively [19]. Doubling time and HL_α of other CD19-directed CAR T-cell therapies were consistent with liso-cel kinetic parameters. T_max of axi-cel was earlier than that of tisagenlecleucel and liso-cel. HL_β of liso-cel was longer than that of tisagenlecleucel or axi-cel (352 vs. 220 or 173 days, respectively), which might be due to longer follow-up with liso-cel; however, the interpretation of these comparisons requires caution because of the limitations of the current analysis stated below.

This analysis was limited by the following factors. Random effects for T_gro, F_β, and HL_β were not estimated in the cellular kinetic model of liso-cel due to their high η shrinkage. T_max (T_lag + T_gro = 9.29 days) in this liso-cel model was slightly earlier than T_max reported by noncompartmental analysis (12 days) [1]. This discrepancy might be in part due to misspecification of the cellular kinetic model by no random effect of T_gro. Furthermore, HL_β had slightly high relative standard error (23.0%). Furthermore, the evaluation period (2 years for liso-cel and 1 year for tisagenlecleucel and axi-cel) was approximately two HL_β, which is relatively short for the robust estimation of HL_β. Therefore, the abovementioned comparison of HL_β among CD19-directed CAR T cells should be interpreted with caution. The following differences among CAR T-cell therapies should also be noted for the comparison of cellular kinetic parameters: defined composition (liso-cel) versus undefined composition (tisagenlecleucel and axi-cel); LBCL (liso-cel and axi-cel) versus B-ALL (tisagenlecleucel); and 4-1BB (liso-cel and tisagenlecleucel) versus CD28 (axi-cel) costimulatory domain. Earlier T_max of axi-cel compared with tisagenlecleucel and liso-cel might be explained in part by the finding that CD28 and 4-1BB co-stimulatory domains are associated with effector T-cell like phenotype and memory T-cell like phenotype, respectively [25]. No apparent relationship has been observed between the CD8⁺:CD4⁺ ratio and cellular kinetic parameters of tisagenlecleucel in LBCL and B-ALL [12, 13], suggesting that defined versus undefined composition might not be a crucial factor to determine cellular kinetics. The eligibility criteria in TRANSCEND is broad and aligns with recommendations for clinical trials of CAR T-cell therapies to maximize generalizability [26]; however, the findings cannot be extrapolated to the broader population (e.g. pediatric patients and patients with severe renal impairment). Nonetheless, the population cellular kinetic model of liso-cel adequately captured the central tendency and variability in observations from patients in TRANSCEND and indicated no systemic biases in model fit in the dimensions of time or predicted values.

Observed liso-cel transgene levels were well described by a piecewise model of cellular growth kinetics that featured lag, growth, and biphasic decay phases in treated patients. Covariates tested were not considered to have a meaningful impact on liso-cel kinetics.