Chimeric Antigen Receptor Therapy in Multiple Myeloma

Initial Clinical Investigation of anti–B-Cell Maturation Antigen Chimeric Antigen Receptor T-Cells

The first-in-human trial of autologous anti-BCMA CAR T-cells was reported by investigators at the U.S. National Cancer Institute (NCI). The CAR construct (11D5-3-CD828Z, CAR-BCMA) was transduced into autologous apheresed T-cells with a γ-retroviral vector and consisted of a murine anti-BCMA single-chain variable fragment (scFv) antigen recognition domain, CD8α hinge and transmembrane domain, CD28 costimulatory domain, and the CD3ζ T-cell activation domain. With a manufacturing time of 7 to 9 days, T-cells were infused 9 to 10 days after leukapheresis and beginning of culture. After receiving a lymphodepletion regimen of cyclophosphamide 300 mg/m ² and fludarabine 30 mg/m ² daily on days −5 to day −3 before CAR T-cell infusion, CAR T-cells were infused on day 0 at four different dose levels of 0.3, 1, 3, and 9 × 10 ⁶ CAR T-cells/kg, respectively. In total, 24 patients received treatment at one of the four dose levels. Two patients who received low doses during initial treatment were re-treated at the highest dose level of 9 × 10 ⁶ CAR T-cells/kg because of minimal efficacy observed at the lower dose. The entire cohort of patients treated on this initial trial were heavily pretreated with a median of 7.5 prior lines of therapy, and of the 16 patients receiving the highest dose level, 40% had high-risk cytogenetics with 33% harboring del17p. This trial was restricted to patients whose bone marrow–derived clonal plasma cells demonstrated uniform BCMA expression by either flow cytometry or immunohistochemistry.

The results of this first-in-human trial were notable for the dose-dependent nature of both efficacy and toxicity outcomes. The ORR was only 20% for the 10 patients receiving anti-BCMA CAR T-cells at the three lowest dose levels (one very good partial response [VGPR], one partial response [PR]), compared to an ORR of 81% in the 16 patients who received the highest dose level infusion at 9 × 10 ⁶ CAR T-cells/kg. In addition, 63% of patients at the highest dose level attained a VGPR or better, and all 11 of the patients who were evaluable for MRD status were MRD-negative by flow cytometry assessment. These deep responses noted in the highest dose level cohort translated to a median event-free survival duration of 31 weeks. There was no grade 3 or higher cytokine release syndrome (CRS) noted at the three lowest dose levels compared to 38% grade 3 or 4 CRS seen at the highest dose level. The investigators noted a significant association between bone marrow clonal plasma cell burden and severity of CRS, and subsequently amended the eligibility criteria for the last 14 patients enrolled to restrict bone marrow plasma cell burden to < 30% before CAR-BCMA infusion.

Further correlative studies demonstrated that for patients whose myeloma responded to treatment, the level of serum sBCMA decreased compared to no significant change in sBCMA for patients who did not respond to treatment. The peak CAR T-cell levels in all patients occurred between 7 and 14 days with higher peak CAR T-cell levels noted in responders compared to nonresponders. A phenotypic change in the infused T-cells was observed, with a significant decrease in the CD4:CD8 ratio after infusion to a predominant phenotype of CD8+ cells. In addition, there was a decrease in the proportion of naïve and central memory T-cells and a noted increase in T-cell markers of senescence and exhaustion. These observations from the first-in-human trial of anti-BCMA CAR T-cells have served as a foundation for further investigations to improve efficacy and limit toxicity.

A single-center study at the University of Pennsylvania investigated an anti-BCMA CAR transduced into apheresed autologous T-cells via a lentiviral vector with a fully human scFv antigen recognition domain, CD8 hinge and transmembrane domain, 4-1BB costimulatory domain, and CD3ζ intracellular signaling domain. Despite 29 patients eligible, 25 received CAR T-cell infusion on study because of four patients experiencing progressive disease on bridging therapy during the 4-week manufacturing time. The 25 patients receiving therapy were divided into three cohorts, a high-dose arm (1 × 10 ⁸ –5 × 10 ⁸ cells) without lymphodepletion chemotherapy, low-dose arm (1 × 10 ⁷ –5 × 10 ⁷ cells) with lymphodepletion consisting of 1.5 g/m ² cyclophosphamide on day −3, and high-dose arm (1 × 10 ⁸ –5 × 10 ⁸ cells) with 1.5 g/m ² cyclophosphamide on day −3 for lymphodepletion. Unique aspects of this study included outpatient administration and split dosing of 10% total dose on day 0, 30% on day 1, and 60% on day 2. Despite outpatient administration, 96% of the patients receiving the anti-BCMA CAR T-cell product subsequently required hospital admission. In addition, four of the 25 patients only received 40% of the total planned dose because of development of early CRS. Patients on the study were heavily pretreated with a median of seven prior lines of therapy, 92% receiving prior autologous HCT, 96% with high-risk cytogenetics, and 68% of whom harbored del17p or TP53 mutation.

Despite small sample size, takeaways from the efficacy results supported a dose-dependent response and seemed to highlight the benefit of lymphodepletion as ORR was 44% in the high-dose/no lymphodepletion arm, 20% in the low-dose/lymphodepletion arm, and 64% in the high-dose/lymphodepletion arm. Several deep responses were noted in the high-dose cohorts, with two VGPR and one sCR in the cohort without lymphodepletion and three VGPR and one CR in the cohort with lymphodepletion. With regards to toxicity, a generally lower rate of grade ≥ 3 cytopenias was noted on the University of Pennsylvania (UPENN) trial compared to the NCI trial primarily explained by the cohort of patients who did not receive lymphodepletion chemotherapy. Differences in CRS and neurotoxicity grading scales during these early trials also make comparisons limited. The grade ≥ 3 CRS rate was 32% and grade ≥ 3 neurotoxicity rate was 12% for all cohorts of this trial. In the correlative analyses, investigators noted that patients with a higher ratio of CD4:CD8 T-cells in the apheresis product before CAR T-cell manufacturing were found to have increased in vivo expansion of the anti-BCMA CAR T-cell and this was associated with improved likelihood of response to a lesser degree. In addition, higher frequencies of CD27 ⁺ CD45RO ⁻ CD8 ⁺ T-cells, indicative of a naïve and stem cell memory phenotype, in the leukapheresis product was associated with improved in vivo CAR T-cell expansion and clinical response.

Idecabtagene Vicleucel

Idecabtagene vicleucel (ide-cel, bb2121) uses the same murine 11D5-3 scFv BCMA antigen recognition domain as previously mentioned in the NCI study, but differs in that there is a 4-1BB costimulatory domain instead of CD28 and a lentiviral vector for transduction of the CAR construct. With published data for both phase 1 and phase 2 studies, ide-cel is the most well-studied autologous anti-BCMA CAR T-cell product to date and is currently the only one to receive FDA approval for the treatment of relapsed/refractory multiple myeloma.

In the first published phase 1 study of ide-cel, 33 patients received a single CAR T-cell infusion at the following doses: 50 × 10 ⁶ , 150 × 10 ⁶ , 450 × 10 ⁶ , and 800 × 10 ⁶ cells. For the dose expansion phase, doses of 150 × 10 ⁶ and 450 × 10 ⁶ cells were used because of a combination of dose-dependent efficacy in the escalation phase and for manufacturing feasibility purposes. Eligibility for the trial was restricted to patients with an eastern Cooperative Oncology group (ECOG) performance status of 0 or 1, and patients must have received at least three prior lines of therapy including a PI and an immunomodulatory agent. For the dose-escalation component of the study, patients were required to have > 50% BCMA expression by immunohistochemistry on bone marrow clonal plasma cells; however, for the expansion cohort, patients could have < 50% tumor BCMA expression. Patients received a standard lymphodepletion regimen of fludarabine 30 mg/m ² and cyclophosphamide 300 mg/m ² on days −5, −4, and −3 before infusion of ide-cel on day 0. Bridging therapy during the manufacturing period was permitted but was required to be discontinued 14 days before initiating lymphodepletion. There was a 100% manufacturing success rate; however, three patients who underwent leukapheresis discontinued the study before ide-cel infusion because of progressive disease during the manufacturing period.

Patients in the initial phase 1 cohort receiving ide-cel had received a median of seven prior lines of therapy including 97% with prior autologous HCT. High-risk cytogenetics defined as del(17p), t(4;14), and t(14;16) were present for 45% of the patients, 27% had extramedullary disease, and 42% required bridging therapy leading up to ide-cel infusion. The ORR for the entire cohort was 85%, with a discernible difference of only 33% for those receiving 50 × 10 ⁶ cells versus 90% in those receiving ≥ 150 × 10 ⁶ cells. Many of the responses were deep responses with sCR, CR, and VGPR rates of 36%, 9%, and 27%, respectively, in the entire cohort and 40%, 10%, and 30% in the cohort of patients receiving ≥ 150 × 10 ⁶ cells. Response rate did not seem to differ significantly between patients with < 50% BCMA expression on marrow derived plasma cells versus > 50% BCMA expression. Of the 16 patients who had a response and were also evaluable for MRD status, all 16 patients (100%) attained MRD-negativity of 10 ⁻⁴ , as well as 94% MRD-negative at 10 ⁻⁵ by next-generation sequencing (NGS) methods. These MRD-negative responses occurred early (all assessed within 3 months of infusion) and MRD-negativity was noted to occur before best response by IMWG criteria because of slower clearance of serum M-protein over time. With a median follow-up duration of 11.3 months, the mPFS was 11.8 months. On correlative analysis, a higher CD4:CD8 CAR T-cell ratio before infusion was associated with increased in vivo CAR T-cell expansion, and increased CAR T-cell levels in the blood were notably higher for patients who attained a response to therapy compared to those who did not. Evidence of long-term CAR T-cell persistence was demonstrated with 57% of patients having detectable cells at 6 months and 20% at 12 months.

Ide-cell also demonstrated a favorable safety profile with expected high rates of grade ≥ 3 cytopenias (85% neutropenia, 58% leukopenia, 45% anemia, 45% thrombocytopenia) in the setting of lymphodepleting chemotherapy. While 76% experienced CRS of any grade, only 6% had grade 3 CRS and no patient experienced higher than grade 3 CRS. Neurotoxicity was noted in 42% of patients with zero grade 3 events and one (3%) grade 4 event. Any grade infection was documented in 42% of patients and two patients had an infection of grade 3 severity.

Updated results from the initial phase 1 study were presented at the American Society of Hematology (ASH) 2020, whereby a total of 62 patients had received therapy with ide-cel (21 in dose-escalation phase, 41 on dose expansion phase at 150 to 450 × 10 ⁶ cells). The updated efficacy data included ORR of 76% for the entire cohort with 39% CR, and 65% VGPR or better rates. There were 37 patients who attained a PR or better who were evaluable for MRD status, and 30/37 (81%) were MRD negative at ≤ 10 ⁻⁴ by NGS methods. The median duration of response was 10.3 months and at the time of the cutoff date for the analysis, 13 patients had evidence of ongoing response. With a median follow-up period of 14.7 months, median PFS was 8.8 months and median OS was 34.2 months. Similar rates of common toxicities were noted with 76% of the entire cohort experiencing any CRS, only 7% of which were grade 3 and none of which were higher than grade 3. Neurotoxicity was noted in 44% of the entire cohort with one grade 3 and one grade 4 event each. Cytopenias remained the most common adverse event with grade ≥ 3 neutropenia of 89%, leukopenia 61%, anemia 57%, and thrombocytopenia 57%. The impressive response rates, depth of response, and PFS outcomes noted in heavily pretreated patients in the phase 1 study served as justification for further study in the phase 2 setting.

The phase II KarMMa trial enrolled patients with relapsed/refractory multiple myeloma who were triple-class exposed and had received at least three prior lines of therapy to further assess efficacy and safety outcomes with a single infusion of ide-cel. The CAR T-cell manufacturing process, allowance for bridging therapy, and lymphodepletion chemotherapy protocol were all as previously described in the phase 1 study of ide-cel. Of the 140 patients who enrolled and underwent leukapheresis, 12 patients did not receive ide-cel infusion (8 discontinued before lymphodepletion, 4 discontinued after lymphodepletion but before ide-cel infusion). Only one patient did not receive ide-cel because of manufacturing failure. For the 128 patients who did receive ide-cel, a target dose of either 150 × 10 ⁶ (n = 4), 300 × 10 ⁶ (n = 70), or 450 × 10 ⁶ (n = 54) CAR T-cells was infused on day 0. The treated patient population had received a median of six prior lines of therapy with 84% triple-refractory disease, 26% penta-refractory disease (refractory to lenalidomide, pomalidomide, bortezomib, carfilzomib, and daratumumab), 94% having prior single autologous HCT and 34% with more than one prior transplant. A total of 35% of patients receiving ide-cel had high-risk cytogenetics as defined by presence of del(17p), t(4;14), or t(14;16), and 16% had Revised Multiple Myeloma International Staging System (R-ISS) stage 3 disease. High tumor burden defined by > 50% clonal plasma cells in the bone marrow was present in 51% of patients, and 85% of patient myeloma samples expressed BCMA at 50% or greater.

The reported efficacy outcomes were for those who received ide-cel infusion rather than the intention-to-treat population. The observed ORR was 73%, with CR or better rate of 33%, and VGPR or better rate of 52%. A dose-dependent effect for efficacy was again demonstrated with ORR of 50%, 69%, and 81% for patients receiving 150 × 10 ⁶ , 300 × 10 ⁶ , and 450 × 10 ⁶ CAR T-cells, respectively. The corresponding CR or better rates were 25%, 29%, and 39% from lowest to highest dose level. The subgroup analysis for response demonstrated consistent response benefit across most subgroups analyzed (age, high-risk cytogenetic abnormality status, bone marrow plasma cell burden at baseline, extramedullary disease, need for bridging therapy, etc.), but was somewhat lower with ORR ~50% for patients with R-ISS stage III disease and for the small number of patients receiving the lowest dose of 150 × 10 ⁶ cells. Responses were noted quickly with median time to first response of 1.0 months and a median time to CR or better of 2.8 months. Regarding depth of response, 26% of the total patients treated with ide-cel attained MRD-negative status at a sensitivity of 10 ⁻⁵ by NGS, and the subset of patients with CR or better achieving MRD-negative status was 79%. The median duration of response in the treated population was 10.7 months overall and increased depth of response was associated with increased duration of response. While PFS and OS data was immature at the time of publication, mPFS was noted at 8.8 months and mOS estimated by Kaplan–Meier method was 19.4 months. Notably, 28 patients who had progressive myeloma were retreated with a higher dose infusion of ide-cel and ORR was 21% for these retreated patients with response duration ranging from 1.9 to 6.8 months.

The favorable safety profile noted in the phase 1 trial of ide-cel was similarly demonstrated in the phase 2 KarMMa trial. A total of 84% of the patients treated with ide-cel experienced any grade CRS, which was notably dose-dependent; however, there were only 5% CRS events of grade 3 or higher. There was one grade 4 CRS event and one death directly attributable to CRS (grade 5 event) in the treated population. Median time to onset of CRS was 1 day after ide-cel infusion with a median duration of CRS of 5 days, and 52% of the patients who developed CRS were treated with tocilizumab. Neurotoxicity was only noted in 18% of the treated patients and was of similar low grade with only 3% having a grade 3 event and no event greater than grade 3 observed. Median time to onset for neurotoxicity was 2 days with a median duration of 3 days. The most common ≥ grade 3 adverse events were cytopenias (neutropenia 89%, anemia 60%, thrombocytopenia 52%), partially attributable to lymphodepleting chemotherapy. A total of 69% of patients treated with ide-cel developed a documented infection, of which 22% were of grade 3 or higher. The incidence of cytopenias and infection resulted in relatively common administration of antimicrobials, growth factors, and immune globulins.

Immunogenicity studies noted an increase in the incidence of patients having antidrug antibodies over time; however, the presence of antidrug antibodies did not seem to have an impact on ORR or PFS outcomes. Pharmacokinetic analysis showed peak CAR T-cell expansion at a median of 11 days, and higher expansion and exposure levels to ide-cel was associated with increased depth of response and increased PFS outcomes. Studies of CAR T-cell persistence noted presence of ide-cel in 59% of patients treated at 6 months postinfusion and in 36% at 12 months. While the study did not require demonstration of BCMA expression on bone marrow clonal plasma cells for enrollment, correlative studies found that 98% of patients had detectable BCMA expression at baseline and all but one of such patients had detectable BCMA expression on greater than 50% of the plasma cells. sBCMA was studied as a biomarker and baseline levels were elevated in 97% of treated patients. A brisk reduction in sBCMA was observed for patients who achieved a response to ide-cel therapy with nadir sBCMA attained within 3 months. Development of undetectable levels of sBCMA was associated with increased depth of response and sBCMA levels were suppressed the longest in patients receiving the highest dose of ide-cel. Rise in sBCMA level after reduction below the lower limit of quantification corresponded with disease progression and was also shown graphically to correlate with a drop in the CAR transgene level to undetectable levels, suggesting progression after loss of CAR T-cell persistence. Suspicion for loss of BCMA expression was only raised for 4% of patients at the time of documented progression (n = 3); however, one of these patients was later demonstrated to have developed biallelic genomic loss of BCMA as the mechanism of resistance. While loss of BCMA was confirmed as a resistance mechanism, the detection of sBCMA in 97% of the patients with noted progression on the KarMMa trial suggested that loss of BCMA was an uncommon resistance mechanism.

The impressive efficacy outcomes observed in patients with triple-class exposed relapsed/refractory multiple myeloma receiving a single infusion of ide-cel led to the first FDA approval of an autologous anti-BCMA CAR T-cell product on March 27, 2021. The FDA approved indication for idecabtagene vicleucel (Abecma) is for adult patients with relapsed or refractory multiple myeloma after four or more prior lines of therapy, including an immunomodulatory agent, a PI, and an anti-CD38 monoclonal antibody. As most agents in the treatment of myeloma have been demonstrated to have greater efficacy in earlier lines of treatment compared to their use in more treatment refractory disease, several studies are ongoing in parallel to study ide-cel in a variety of different treatment settings for patients with high-risk myeloma as summarized later in Table 22.1 .

A phase 1 dose escalation study of the autologous anti-BCMA CAR T-cell product bb21217 is ongoing at multiple centers for patients with relapsed/refractory multiple myeloma who have received ≥ three prior lines of therapy and are triple-class exposed. The bb21217 CAR is the identical CAR used in the manufacturing of ide-cel (bb2121), with the only difference being ex vivo culture of the autologous T-cells occurring in the presence of a PI3K inhibitor (bb007). The mechanistic goal of culture with the PI3K inhibitor is to increase the number of less differentiated memory-like T-cells in the infusion product and decrease the amount of senescent or more highly differentiated T-cells. Patients in this study received similar doses of 150 × 10 ⁶ , 300 × 10 ⁶ , or 450 × 10 ⁶ CAR T-cells on day 0 after the same lymphodepletion chemotherapy protocol as mentioned in the phase 1 and 2 studies of ide-cel. Preliminary data in abstract form was presented at ASH in 2020, at which time 46 patients with a median of six prior lines of therapy had received bb21217 infusion. For the 44 patients with ≥ 2 months of follow-up for response assessment, ORR was 55% and CR or better rate was 18%. The dose expansion component of the study is ongoing at the recommended phase 2 dose of 450 × 10 ⁶ CAR T-cells. CRS was noted in 67% of patients (4% ≥ grade 3) and neurotoxicity was observed in 22% (6.5% ≥ grade 3). T-cell analysis of the infusion product was notable for increased memory-like T-cells expressing LEF1, CD27, and CCR7, as well as relative depletion of CD57 expressing highly differentiated or senescent T-cells. Correlative study suggested that infusion product with higher CD127 expression and thus an enhanced memory T-cell phenotype was positively correlated with duration of response.

Ciltacabtagene Autoleucel

Another autologous anti-BCMA CAR T-cell product that has been well studied in the United States is ciltacabtagene autoleucel (cilta-cel; JNJ-4528; LCAR-B38M CAR T-cells). The LCAR-B38M CAR construct is highlighted by a dual-epitope, camelid heavy-chain-only antigen recognition domain that binds two separate epitopes of BCMA. The CAR is transduced via lentiviral vector and contains a 4-1BB costimulatory domain. LCAR-B38M was initially studied in the phase 1 LEGEND-2 trial at four study sites China that used different lymphodepleting chemotherapy and CAR T-cell infusion schedules making the trial quite heterogenous. In the largest cohort of patient, data reported from one of the four centers, patients were less heavily pretreated compared to previously mentioned anti-BCMA CAR T-cell studies with a median of three prior lines of therapy and only 18% of patients having received autologous HCT. They received a median dose of 0.5 × 10 ⁶ cells/kg split into three separate infusions over 7 days after cyclophosphamide 300 mg/m ² for lymphodepletion on days −5 to −3. Of the 57 patients receiving LCAR-B38M infusion, ORR was 88% with CR rate of 68% and MRD-negative status noted in 63% of all treated patients. Given the notable efficacy outcomes and acceptable safety profile, the LCAR-B38M was commercially developed for further study.

The CARTITUDE-1 trial is an ongoing phase 1b/2 study (NCT03548207) of cilta-cel (JNJ-4528), which uses the LCAR-B38M anti-BCMA CAR construct as described in the LEGEND-2 trial. Updated outcomes for 97 patients treated with cilta-cel were presented in abstract form at ASH 2020. The patients receiving cilta-cel on study were more heavily pretreated than in LEGEND-2, with a requirement for ≥ three prior lines of therapy and needed to be triple-class exposed for enrollment. The treated population had received a median of six prior lines of therapy. Patients received 3 consecutive days of cyclophosphamide 300 mg/m ² and fludarabine 30 mg/m ² 5 to 7 days before a single infusion of 0.75 × 10 ⁶ CAR T-cells. For the primary endpoint of independent review committee assessed ORR, cilta-cel boasted an impressive ORR of 94.8%. Depth of response was also notable with sCR of 55.7% and VGPR rate 32%, adding up to an 87.7% VGPR or better rate. For the 52 patients who were evaluable for MRD status, 94.2% were noted to be MRD-negative at a sensitivity of 10 ⁻⁵ . Responses were achieved quickly with a median time to first response of 1.0 month and median time to CR or better of 1.8 months. With a median follow up duration of 8.8 months, median duration of response (DOR), PFS, and OS outcomes were not reached. The efficacy outcomes translated to health-related quality of life improvements, with the phase 2 portion of patients on CARTITUDE-1 reporting significant improvements (71.1% for pain, 62.2% for fatigue, 72.1% for physical functioning, 51.1% for global health status) at day 100 after cilta-cel infusion.

Regarding toxicity for treatment with cilta-cel, grade ≥ 3 cytopenias were common with neutropenia 90.7%, anemia 68.0%, and thrombocytopenia 59.8%, respectively. CRS was observed in 94.8% of patients but was only ≥ grade 3 in 4.1% of patients. Median time to CRS onset was 7.0 days, with median duration of CRS at 4.0 days. Immune effector cell-associated neurotoxicity syndrome (ICANS) was documented in 20.6% of patients, of which 10.3% were of ≥ grade 3. In addition to use of tocilizumab (69.1% of patients) and corticosteroids (20.6%) for the management of CRS and ICANS, it was reported that 18.6% of patients received anakinra, an interleukin-1 (IL-1) antagonist, for toxicity management. One patient in the study who died as a complication of grade 5 CRS was also diagnosed as having hemophagocytic lymphohistiocytosis/macrophage activation syndrome (HLH/MAS). While the number of patients who were either diagnosed with or suspected of having HLH was not directly reported, the notable use of anakinra likely reflects investigator suspicion for HLH. While data from CARTITUDE-1 has yet to be published, the promising results have led to the initiation of several other studies of cilta-cel in earlier lines of therapy as listed later in Table 22.2 .