Adult patients with relapsed B-ALL have an overall poor prognosis. Those patients who relapse following multi-agent chemotherapy protocols have an even more dismal prognosis. The standard of care for these patients is salvage chemotherapy followed by allo-HSCT if in remission and clinically eligible to undergo allo-HSCT. However, many such patients fail to achieve a second CR and/or are ineligible for additional transplantation therapy (1
Treatment of patients with relapsed indolent B cell malignancies, including CLL and B cell follicular lymphomas, using autolgous T cells targeted to the CD19 antigen through the retroviral or lentiviral introduction of a CD19 specific CAR have demonstrated very promising clinical responses (8
). Despite a growing body of clinical evidence for efficacy using CAR T cell technology in these indolent B cell tumors, it remains unknown whether this same technology may similarly exhibit significant anti-tumor efficacy in adults with far more aggressive and often fatal relapsed B-ALL.
The limitations of our study include the short-follow up post T cell infusion as well as the relatively small number of patients treated to date with the 19-28z T cells. The treatment of these patients with allo-SCT, the standard of care for relapsed B-ALL patients, after 19-28z T cell infusion, makes it indeed difficult to discern the curative or long-term remission potential of CD19 CAR-targeted T cells. However, it is unequivocal that patients who were ineligible for all-SCT and had no further therapeutic option in the face of rapidly progressing refractory disease, were promptly brought into profound molecular remission by a single infusion of 19-28z T cells.Despite these limitations, our work thus strongly supports the efficacy of 19-28z T cells in adults with acute leukemia.
In this report we summarize the outcomes of 5 adults with relapsed B-ALL, 4 which demonstrated persistent disease following salvage chemotherapy at the time of CAR T cell infusion. Overall, we report that in all patients, ranging from overt morphologic disease to MRD, treatment with autologous T cells uniformly resulted in MRD-CRs independent of tumor burden at the time of T cell therapy. These results demonstrate a potent anti-tumor efficacy of autologous T cells genetically modified to target the CD19 antigen expressed on adult B-ALL tumor cells. These consistent clinical outcomes are meaningful for several reasons: First, these data demonstrate the potential and rapid kinetics of 19-28z CD19-targeted T cells in eradicating chemotherapy resistant adult B-ALL; and second, the ability to rapidly and reliably achieve MRD−
status dramatically improves the prognosis of these patients by providing a bridge to allo-HSCT under optimal conditions, the standard of care for these patients. Specifically, in the absence of CAR modified T cell therapy, two patients treated on this cohort would not have been eligible for allo-HSCT, while an additional two patients would have undergone this therapy in the MRD+
setting with predicted poor clinical outcomes (3
Additionally, our studies demonstrate that cytokine-mediated toxicities, which are a significant previously published impediment to the application of this therapy, correlates to tumor bulk at the time of modified T cell infusion. Specifically, we demonstrate an improved side-effect profile with CAR modified T cells if infused in the context of a lower disease burden. These findings suggest that optimal safety of modified T cell infusions is either in the MRD+ setting or perhaps soon after initial salvage chemotherapy in the context of a largely aplastic bone marrow. Importantly, we find that a T cell-mediated cytokine release syndrome is not requisite to achieving optimal CAR T cell mediated anti-tumor efficacy resulting in MDR− complete remissions of disease.
Sufficient persistence of CD19-targeted CAR-modified cells to achieve tumor eradication (MRD−
status) is, in theory, essential to optimally cancer anti-tumor efficacy. However, optimal anti-CD19 targeted T cell therapy as presented by collaborators at UPenn and other institutions present a balanced analysis with respect to the risks of potentially life threatening cytokine storms and lifelong B cell aplasias (9
). In the present study, we observed a rapid onset of cytokine elevations in parallel with in vivo
T cell expansion correlating to T cell clearance 3-8 weeks after infusion as assessed by FACS, RT-PCR, and in vitro
expansion assays. Steroid therapy to ameliorate cytokine driven toxicity in 2 patients markedly reduced modified T cell populations at a time of maximum detectable CAR modified T cells () similar to a previously reported CLL patient at UPenn treated with high dose steroid therapy to control T cell mediated cytokine toxicities following infusion with CD19 targeted T cells (9
). Nonetheless, in our studies, despite the steroid limited duration of 19-28z CAR modified T cells, CD19 targeted modified T cell persistence was sufficient to convert all patients with detectable disease to an MRD−
status (). Further studies of modified T cell persistence in patients moving on to allo-HSCT were precluded by this additional standard of care therapy. Patient MSK-ALL04, of note, was managed expectantly due to co-morbidities prohibiting allo-HSCT and infusion of additional 19-28z CAR modified T cells. The patient, followed by modified CAR T cell infusion required additional lymphotoxic steroid therapy soon after modified T cell infusion (day 6) curtailing 19-28z+
T cell persistence and ultimately resulting in the relapse of the patient disease. Significantly, in vitro
studies demonstrate that the relapsed clone retained CD19 expression and sensitivity to 19-28z+
T cells ex vivo
. These results are consistent with the potential of lengthening molecular remissions, converting relapsed patients back to an MRD-status, or ultimately fully eradicating CD19+
malignant tumor clones in those patients otherwise ineligible for allo-HSCT through additional infusions of CAR modified T cells. Considering the decreased tumor burden at the time of second line therapy with CAR modified T cell infusion, the latter is likely to induce more modest cytokine elevations and therefore to be better tolerated.
The required dose of CAR-modified T cells was successfully generated in all patients within 2 weeks. To date, we have enrolled a total of 14 patients to this Phase I protocol. Of these 14 patients, 2 are in a CR and are not eligible for T cell infusion at this time, 3 deferred T cell infusion to go directly to allo-SCT, 3 died during salvage chemotherapy, 5 patients were successfully infused. One patient was lost to follow-up after enrollment. On an intent-to-treat basis we have generated 8 T cell products for enrolled patients including the 5 described in this study and 1 who went straight to allo-SCT and 2 who died during salvage chemotherapy.
The gene-transfer efficiency for 19-28z ranged from 5 to 14.2% (Table S1
). While there is no clear etiology for this modest gene-transfer efficiency, it may be in part due to the nature of acute leukemia or the numerous cycles of high-dose lymphotoxic chemotherapeutic agents received by the patients before T cell harvest. Regardless, it does not appear that the lower CAR-transduction efficiency inhibited the anti-leukemia effect of the 19-28z T cells.
While we and others have previously reported clinical trial outcomes of CAR T cells targeted to either CD19 or CD20 in patients with low grade lymphomas and CLL (8
) and summarized in (16
), herein we are the first to present highly effective clinical outcomes with CD19-targeted CAR T cells in adults in the setting of far more aggressive B-ALL.
Recently published reports from the University of Pennsylvania (UPenn) demonstrate promising clinical outcomes in CLL patients treated with a second generation CAR derived from initial studies published elsewhere (17
) containing the 4-1BB co-stimulatory signaling domain. In these studies, investigators demonstrate marked anti-tumor efficacy, profound cytokine mediated toxicity, and long-term persistence of both B cell aplasias and autologous T cells expressing this 4-1BB containing CAR (9
). Although results in adult ALL have not yet been reported with 4-1BB/CD3ζ CARs, some differences between the kinetics and magnitude of the cytokine response between the two CARs are noteworthy. In CLL patients, the 4-1BB/CD3ζ CAR yielded delayed peak cytokine responses seen greater than 10 days out from infusion, in contrast to a more immediate cytokine release (day 3-5) seen in our CD28/CD3ζ-containing 19-28z CARs. Additionally, chronic B cell aplasias were associated with long-term persistence of the 4-1BB/CD3ζ T cells, in contrast to the recovery of normal B lymphopoeisis we report here (). The more limited expansion and persistence of 19-28z+
T cells enables normal B cell recovery over time in the context of a more modest cytokine mediated toxicity profile. These findings are consistent with murine studies reported so far, which suggest a slower kinetic but ultimately higher accumulation and persistence of 4-1BBz T cells that occurs without antigen. In xenogeneic NSG chimeras, Milone et al (7)
reported similar therapeutic efficacy of CD28/CD3z and 4-1BB/CD3z T cells in treating CD19+
leukemia, with the latter accumulating to higher levels in delayed fashion. In our own preclinical studies (18),
4-1BB/CD3z T cells were not as effective as CD28z T cells in treating B-ALL in scid/beige xenochimeras (19).
Collectively, our studies treating patients with the CD28 containing 19-28z+
CAR modified T cells provides an alternate approach to using CAR technology, one in which multiple (perhaps two or three) infusions of modified T cells induce CRs in the context of lower tumor burdens, which in turn carry a diminished risk for cytokine-mediated toxicities and long-term B cell aplasias.
Adults with relapsed chemotherapy refractory B-ALL have a dismal prognosis outside the context of an allo-HSCT. We report, for the first time, the successful induction of MRD−
remissions in adults with chemotherapy refractory B-ALL by treatment with CD19-targeted CAR-modified T cells. We acknowledge limited post treatment follow-up due to the fact that 4 of 5 patients subsequently received standard of care therapy with allo-HSCT as stipulated in the protocol and were therefore removed from further follow-up on this protocol. The 5th
patient treated was ineligible for allo-HSCT and relapsed 3 months after therapy although this relapse may in part have been precipitated by high dose lymphotoxic steroid therapy to treat T cell mediated cytoline storms soon after modified T cell infusion limiting which in turn limited persistence of 19-28z CAR modified T cells. Further, these results are consistent with the potential of lengthening molecular remissions or alternatively reconverting relapsed patients to MRD-status through additional infusions of 19-28z CAR modified T cells. Whether additional infusions with CAR modified T cells in this particular patient may have changed the clinical outcome remains to be and will be investigated in future relapsed refractory B-ALL patients similarly ineligible for additional therapy with allo-HSCT. The rapid kinetics of 19-28z T cells may prove to be an essential asset in patients with lower tumor burdens but at high risk for an overt relapse. This approach provides a bridge for patients otherwise either ineligible or eligible under very suboptimal conditions (MRD+
) to receive potentially life saving therapy with an allo-HSCT. Furthermore, this therapy has the potential to decrease the relapse rate post allo-HSCT, which can occur in up to one-third of patients (20
). In addition, repeated infusions of 19-28z+
T cells in patients unable to undergo allo-HSCT may prolong remission. Based on the presented data, treatment of relapsed chemotherapy refractory B-ALL in adults with 19-28z+
autologous T cells is a very promising and novel approach worthy of continued study. Finally, given these clinical outcomes, the addition of 19-28z CAR modified T cells for inclusion into currently up-front adult B-ALL treatment protocols warrants serious consideration.