CARs on Track in the Clinic: Workshop of the Blood and Marrow Transplant Clinical Trials Network Subcommittee on Cell and Gene Therapy Washington DC, 18 May 2010 (original) (raw)

Mol Ther. 2011 Mar; 19(3): 432–438.

Workshop of the Blood and Marrow Transplant Clinical Trials Network Subcommittee on Cell and Gene Therapy Washington DC, 18 May 2010

Donald B Kohn

1Department of Microbiology, Immunology, and Molecular Genetics and Department of Pediatrics, University of California, Los Angeles, Los Angeles, California, USA

Gianpietro Dotti

2Center for Cell and Gene Therapy, Baylor College of Medicine, The Methodist Hospital and Texas Childrens Hospital, Houston, Texas, USA

Renier Brentjens

3Department of Medicine and Center for Cell Engineering, Memorial Sloan-Kettering Cancer Center, New York, New York, USA

Barbara Savoldo

2Center for Cell and Gene Therapy, Baylor College of Medicine, The Methodist Hospital and Texas Childrens Hospital, Houston, Texas, USA

Michael Jensen

4Center for Immunity and Immunotherapies, Seattle Children's Research Institute, Seattle, Washington, USA

Laurence JN Cooper

5Division of Pediatrics, MD Anderson Cancer Center, Houston, Texas, USA

Carl H June

6Department of Pathology and Laboratory Medicine and Abramson Family Cancer Research Institute, University of Pennsylvania, Philadelphia, Pennsylvania, USA

Steven Rosenberg

7Surgery Branch, National Cancer Institute, National Institutes of Health, Bethesda, Maryland, USA

Michel Sadelain

3Department of Medicine and Center for Cell Engineering, Memorial Sloan-Kettering Cancer Center, New York, New York, USA

Helen E Heslop

2Center for Cell and Gene Therapy, Baylor College of Medicine, The Methodist Hospital and Texas Childrens Hospital, Houston, Texas, USA

1Department of Microbiology, Immunology, and Molecular Genetics and Department of Pediatrics, University of California, Los Angeles, Los Angeles, California, USA

2Center for Cell and Gene Therapy, Baylor College of Medicine, The Methodist Hospital and Texas Childrens Hospital, Houston, Texas, USA

3Department of Medicine and Center for Cell Engineering, Memorial Sloan-Kettering Cancer Center, New York, New York, USA

4Center for Immunity and Immunotherapies, Seattle Children's Research Institute, Seattle, Washington, USA

5Division of Pediatrics, MD Anderson Cancer Center, Houston, Texas, USA

6Department of Pathology and Laboratory Medicine and Abramson Family Cancer Research Institute, University of Pennsylvania, Philadelphia, Pennsylvania, USA

7Surgery Branch, National Cancer Institute, National Institutes of Health, Bethesda, Maryland, USA

*Department of Microbiology, Immunology, and Molecular Genetics, University of California, Los Angeles, 290D BSRB, 615 Charles E. Young Drive South, Los Angeles, California 90095, USA. E-mail: ude.alcu.tendem@nhokd

Copyright © 2011 The American Society of Gene & Cell Therapy

Chimeric antigen receptors (CARs) are fusion proteins between single-chain variable fragments (scFv) from monoclonal antibodies recognizing tumor-associated antigens and intracellular signaling domains such as the CD3 ζ-chain cytoplasmic tail. Transduction of peripheral blood T lymphocytes with these targeting moieties confers specific cytotoxic immunoreactivity against cells expressing the target epitope. These agents have shown high activity in preclinical models of immunotherapy, and several sites are studying them clinically. The Blood and Marrow Transplant Clinical Trials Network (BMT CTN) Subcommittee on Cell and Gene Therapy convened a meeting of the investigators in the United States performing phase I clinical trials using CARs targeting CD19 on B-lineage malignancies, as well as representatives from the National Institutes of Health (NIH), to review the state of the science and to discuss the challenges to be overcome to advance these agents to multicenter trials. This article summarizes those discussions and highlights the promises of this approach to immunotherapy.

Although cell therapies are showing great clinical promise, they will not have a broader impact in medical practice until several obstacles are overcome. These include the complexity of producing patient-specific cellular products (locally at multiple sites or centrally with distribution to clinical sites), the need for rigorous oversight, identification of funding sources to support the stages of product (and vector) manufacture, and the complex and fragmented regulatory and legal barriers, including investigational new drug (IND) sponsorship, indemnification, and the performance of the clinical trial itself.

The BMT CTN was established in 2001 with a primary mission of testing candidate hematopoietic cell therapies from single-center phase I and II trials in larger, definitive multicenter trials (https://web.emmes.com/study/bmt2). The BMT CTN Subcommittee on Cell and Gene Therapy has met several times over the past three years to discuss which emerging new biological therapies are sufficiently mature to allow consideration of multi-institutional trials.1 The subcommittee sought to balance the desire for consensus between investigators to achieve a single common approach with their continued need to explore individual approaches based on their own specific expertise. One area of focus was gene modification of T lymphocytes to target them to specific tumor-associated antigens. Investigators have studied both tumor antigen–directed receptors derived from T-cell receptors (TCRs) cloned from tumor-reactive cytotoxic T lymphocytes (CTLs) and CARs. Transduction of peripheral blood T lymphocytes with these targeting moieties confers specific cytotoxic immunoreactivity against cells expressing the target epitope. These agents have shown high activity in preclinical models, and several sites are studying them clinically.2,4,5 One advantage of CARs over TCRs is that CAR-transduced T cells recognize targets in a manner not restricted to the major histocompatibility complex, opening eligibility to a wider group of patients. ClinicalTrials.gov lists trials at seven sites in the United States in which patients are given infusions of T cells modified to express CARs against the CD19 antigen present on most B-lineage leukemias and lymphomas (Table 1).

Table 1

Clinical trials in the United States using chimeric antigen receptor (CAR)-modified T lymphocytes for immunotherapy of B-lineage malignancies

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We convened a small workshop immediately preceding the annual meeting of the American Society of Cell and Gene Therapy in Washington, DC, on 18 May 2010, to work toward achieving a consensus on the elements needed for a multi-institutional trial of CD19 CAR+ T cells for immunotherapy of B-cell malignancies. The meeting was organized by the subcommittee's cochairs, Helen Heslop and Donald Kohn, and chaired by Michel Sadelain. Attendees included all the principal investigators of the US clinical trials and their associates, as well as project officers from the National Heart, Lung, and Blood Institute (NHLBI) and the National Cancer Institute (NCI), which cosponsor the BMT CTN. The contemporaneous discussions of the state of the field at the time of the workshop are presented here, although several of the trials have yielded further advances, including treatment of several patients with evidence of clinical efficacy.

Clinical trial presentations

Michel Sadelain emphasized in his opening remarks that CD19 is an excellent tumor-associated antigen for immunotherapy. CD19 is present on B-lineage cells of almost all stages, from the pro-B-cell stage to mature B cells, absent only from plasma cells. CD19 is expressed at high levels on essentially all B-lineage leukemias and lymphomas, including pre-B acute lymphoblastic leukemias (pre-B ALLs), chronic lymphocytic leukemia (CLL), and lymphomas. Critically, CD19 is not expressed on hematopoietic stem cells (or other tissues), so there should not be myelosuppressive effects or other organ toxicities from targeting cells that express it.

There were presentations of individual clinical trials (Renier Brentjens, Memorial Sloan-Kettering Cancer Center; Gianpietro Dotti, Baylor College of Medicine; Steven Rosenberg, NCI; Carl June, University of Pennsylvania; Laurence Cooper, M.D. Anderson Cancer Center; and Michael Jensen, City of Hope National Medical Center), and it was evident that there were disparate components in each investigator's approach, in terms of the B-lineage malignancies being treated, the design of the anti-CD19 CAR molecule, the CAR gene delivery method, the preparative regimens, and the type of T-cell type targeted for transduction (Table 1). A total of 18 treated subjects were tallied as of May 2010, with accelerating enrollment projected over the next few years. Excellent early clinical responses were reported anecdotally for several of the trials.6

Two serious adverse events in clinical trials using CARs were discussed. One occurred after infusion of an anti-CD19 CAR in a patient with advanced CLL and the second after adoptive transfer of an anti-HER-2/neu CAR in a patient with metastatic colon cancer. Both were quickly and responsibly published in the scientific literature.7,8,9 The treatment-related death with the anti-CD19 CAR occurred shortly after the patient received cyclophosphamide for lymphodepletion and infusion of CAR-transduced cells.8 Although the precise etiology of this patient's death remains uncertain, the picture was consistent with an inflammatory cytokine cascade after cyclophosphamide administration, which worsened after infusion of T cells, to give a clinical picture of acute sepsis, renal failure and resultant shock, and adult respiratory distress syndrome. Importantly, this patient's death did not appear to be directly caused by the cellular product, and this trial has been reopened. In contrast, toxicity with the Her2/neu CAR-modified cells may have been a direct effect of infusion of a large number of T cells recognizing the target antigen expressed at low levels on normal cells in the lung.7 This demonstrates that clinical trials of T-cell immunotherapy evaluating new CARs with novel specificities may pose risks that were not previously identified using small-animal models. T cells that are genetically modified to express a CAR can distinguish between the same antigen expressed on normal and malignant cells only if there are differences in overall avidity, which may be affected by variables such as the number of antigen molecules expressed per target cell and their density. Such deleterious “on-target” effects may not be well tolerated in medically fragile patients enrolled in phase I trials who have bulky malignant disease. However, this patient population urgently needs new therapeutic options, and, although caution is warranted, the potential benefits of CAR therapy should not be abandoned as some toxicities occur in early-phase trials. Although effective CAR immunotherapy against B-cell malignancies may inadvertently eliminate a patient's normal B lymphocytes, this side effect can be ameliorated with intravenous gamma globulin replacement as needed.

Scientific and organizational issues presentations

We addressed specific key questions regarding the development and implementation of a multicenter trial (Table 2), including (i) which CAR design is best?; (ii) which T-cell subpopulations are most efficacious?; (iii) what is the best way to transduce and expand the cells to obtain the most therapeutically active product?; (iv) what form of preinfusion conditioning (none, lymphodepletion, myeloablation) most effectively supports T-cell expansion, activity, and persistence in vivo?; (v) which specific B-lineage malignancies should be studied?; and (vi) what are the logistic, regulatory, legal, and financial challenges in developing and performing a multicenter trial of cell and gene therapy?

Table 2

Variables to be considered for chimeric antigen receptor (CAR) therapy and some options

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Which CAR design is best? Michel Sadelain (Memorial Sloan-Kettering Cancer Center) discussed the design and function of CAR proteins. The CD19 antigen-binding moieties of CARs are derived from scFv of light and heavy chains of monoclonal antibodies.2 These may differ in their affinity and ability to access the target epitope owing to the steric constraints of being tethered to the T cell. Additionally, because the single-chain monoclonal antibody components of most CARs are currently derived from murine antibodies, they have the potential to be immunogenic and engender immune responses against the gene-modified T cells.10 Generating CARs using human scFV elements may be beneficial if human antimurine antibody responses are observed to limit the persistence of CAR-modified T cells. CARs use antibodies as the component that recognizes the target antigen, in contrast to approaches using TCR-targeting elements, which will recognize processed peptides of antigens presented by human leukocyte antigen (HLA) molecules. Hence, CARs do not require peptide processing or HLA expression on the target cell for the epitope to be accessible to the genetically modified T cell. Thus, signaling through the CAR has inherent advantages over activation through αβ-TCRs because a given CAR is not limited to working only in a subset of patients with a specific HLA type. A limitation of CARs, however, is that at present they can be targeted only against extracellular (surface) antigens, which represent only a subset of potential tumor-associated antigens.

Most CARs consist of an antigen-recognizing single-chain antibody domain that is connected to the _trans_-membrane and cytoplasmic tail by a “stalk” derived from a human immunoglobulin Fc region, or CD8. The length of the stalk may influence the ability of the CAR to bind to target antigens, depending on their conformation and accessibility, and it may allow T cells to overcome steric hindrance and attain sufficient proximity to achieve a cytotoxic reaction. Although the immunoglobulin Fc domain has been used most often as a stalk, a recent report indicated that it may lead to nonspecific activation of effector cells through interactions with their Fc receptors.11 _Trans_-membrane domains from different integral membrane costimulatory molecules, such as CD8 or CD28, have been used in CARs.

The intracellular cytoplasmic domains of CARs provide the genetically modified T cell with the appropriate signal(s) to activate their cytolytic activity and other properties related to cell physiology, survival, and so on. “First-generation” CARs typically had the intracellular domain from the CD3 ζ-chain, which is the primary transmitter of signals from endogenous TCRs. “Second-generation” CARs add intracellular signaling domains from various costimulatory protein receptors (e.g., CD28, 41BB, ICOS) to the cytoplasmic tail of the CAR to provide additional signals to the T cell. Preclinical studies have indicated that the second generation of CAR designs improves the antitumor activity of T cells.12,13 More recent, “third-generation” CARs combine multiple signaling domains, such as CD3ζ-CD28-41BB or CD3ζ-CD28-OX40, to further augment potency.14,15,16 These may lead to higher levels of antiapoptotic signaling elements (e.g., bcl-xL, pAKT), extending the survival of the modified T cells. Potentially, “too much of a good thing” could become problematic if the T cells proliferate autonomously or become activated too readily.

Several vector types have been used for introducing CAR into T cells, including γ-retroviral vectors, lentiviral vectors, and electroporated plasmids, the last of which have contained components from transposon systems (such as Sleeping Beauty) to increase the efficiency of stable gene integration.17,18,19,20 All are effective, allowing a relatively high percentage of T cells to be modified; additional studies will define which is best overall in terms of efficacy for gene delivery and expression, cost, ease of production, and safety profile. Vectors have been made to coexpress a CAR with a second transgene conferring an additional property on the genetically modified T cells, such as a selectable marker/reporter, a T cell–stimulating cytokine, a specific chemokine receptor, or a suicide gene to allow the T cells to be ablated in the event of serious toxicity.19,21,22,23,24

Which T cells are the most efficacious, and how should they be prepared? Barbara Savoldo (Baylor College of Medicine) provided an overview of the list of T-cell populations that can be used for immunotherapy with CARs: bulk peripheral blood mononuclear cells (PBMCs), CD8+ (CD4-depleted PBMCs), PBMCs that are selectively depleted of T-regulatory cells (Tregs), isolated central memory T (Tcm) cells, Epstein–Barr virus (EBV)-specific CTLs, and tri-virus-specific CTLs.25,26,27,28 PBMCs used directly after phlebotomy or leukopheresis contain heterogeneous CD8+ and CD4+ T cells fractions that may cooperate for more effective immune responses. However, the CD4+ population may harbor Treg cells that might be countertherapeutic, suppressing the antitumor activity of CD8+ CTLs. Depletion of CD4+ cells by immunomagnetic separation results in a mostly CD8+ T-cell population depleted of Tregs but also depleted of potential CD4+ helpers. More specific depletion of Tregs from PBMCs may be undertaken by negative selection of CD25+ cells, which eliminates many Tregs. Although CD25 depletion also removes proliferating T cells, it may still preserve helper function better than broad CD4+ removal. The value of T cell–subset depletion in terms of its impact on therapeutic benefit remains an open question.

Tcm cells are critical cells that lead to effective and more enduring immune responses, and they can be enriched before gene transfer via preparative fluorescence-activated cell sorting.28 CTL populations with specific responses to EBV or multiple viruses (e.g., EBV, adenovirus, cytomegalovirus) have been produced and shown to mediate highly effective antiviral effects in the post–hematopoietic stem cell transplant (HSCT) setting.29 Adoptively transferred virus-specific CTLs remain in patients for years, probably driven by chronic antigenic stimulation from the viruses; transducing these antiviral CTLs with CARs could harness this antigen-driven persistence to provide long-lasting antitumor effectors.17 Mouse studies have recently shown that naive T cells may also offer a pool of lymphocytes with the potential for improved in vivo survival after infusion. Although these manipulated cell populations may possess greater activity than the heterogeneous PBMC pool, their production for clinical use is more cumbersome and requires special expertise in the cell-processing laboratory. It remains to be determined whether the extra effort to use these more specialized T cells for immunotherapy is worthwhile.

Carl June (University of Pennsylvania) coined June's Law: the T-cell type and method for its activation are the most important factors in the efficacy of immunotherapy. This was stated rhetorically as “Do we want an orchestra or a soloist?” He observed that a mixture of CD8+ and CD4+ T cells works better than either T-cell subtype alone in preclinical models.30 In addition, mixtures of CD4+ and CD8+ CAR T cells have greater persistence in HIV-infected patients compared with infusion of CD8+ T cells.31,32

There are several ways to propagate T cells for transduction and expansion, and the culture conditions dictate the outcome. For example, Bonini's group compared T cells expanded using antibody to CD3 and recombinant interleukin 2 (IL-2) with T cells expanded using anti-CD3 and anti-CD28 and found that the latter method produced more T cells with a central memory immunophenotype. The addition of IL-7 and IL-15 to culture medium provides a further augmentation of potency.33 As June and colleagues first observed, bead-bound antibodies to CD3 and CD28 provide activation and costimulation that produces multifunctional T cells with high proliferative potential.27 Artificial antigen-presenting cells (APCs) have also been developed to provide a more physiological context for antigen presentation to T cells.34,35,36 Engineering a cell line such as K562 or NIH 3T3 to express a tumor antigen (e.g., CD19), costimulatory molecules, and cytokines can provide an “off-the-shelf” reagent that can be used for in vitro expansion of CAR+ T-cell products for multiple subjects.35,37,38,39,40 Artificial APCs may be particularly needed for stimulation of isolated T-cell subsets (e.g., Tcm cells) that are devoid of the autologous APCs present in PBMC leukopheresis products. Longer duration of culture produces higher numbers of gene-modified T cells, but it is not certain whether there is a progressive decline in their specific effector activity and replicative capacity as they expand in number. Different culture conditions may also modify the content of Tregs, the trafficking patterns of the reinfused cells, and other properties that may reflect the potency and endurance of their antitumor activity, such as telomere length.

The day of infusion of engineered T cells in relation to the preinfusion conditioning is also important; for example, T cells given on the second day after autologous HSCT have greater expansion and persist longer than those given on the day 12 after transplant, even though the latter corresponds to the nadir of lymphocyte counts.41 Boosting infused T cells in vivo by systemic treatment with cytokines such as IL-2 or antigen-pulsed dendritic cells may also affect the activity of the T cells. Given these many variables, the cell dose range for maximal efficacy and minimal toxicity will probably be quite different for different cell products and different CARs, and also may vary as a function of the intensity and duration of host conditioning. Current clinical trials use a wide variety of approaches in terms of T-cell types, culture conditions, and cell doses.

Which is the most effective conditioning regimen? Steven Rosenberg firmly stated his observation that increasing the intensity of preparative conditioning improves the efficacy of T-cell therapies.42 He reminded us that every model of adoptive immunotherapy in mice requires immunodepletion and addition of soluble cytokines. There are several hypotheses to explain how lymphodepletion may increase the efficacy of T-cell-mediated immunotherapy, including elimination of competing lymphoid cell sinks for cytokines (e.g., IL-7 and IL-15) that are essential for survival and function of the infused T cells; eradication of regulatory elements, such as Tregs; and enhancement of endogenous host APC activity.43 He reviewed the experience of the NCI Surgery Branch using tumor-infiltrating lymphocytes expanded from patients' tumor samples, without gene engineering, to treat patients with advanced melanoma.42 The investigators observed increasing objective response rates and survival as the intensity of preinfusion conditioning was increased, from no prior conditioning to lymphodepleting regimens of cyclophosphamide+fludarabine–based up to cytoablative regimens, including 1,200-cGy total body irradiation with autologous peripheral blood stem cell rescue. Because these doses of chemotherapy or radiation are not by themselves sufficient to elicit antitumor responses, their benefits result from the favorable environment they produce for T-cell persistence and expansion in vivo. The pretransplant conditions typically used for HSCT are at this level of intensity in terms of the lymphopenia they produce and thus may provide a suitable setting for clinical trials.

Which B-cell malignancies should be studied? B-cell-lineage malignancies represent a broad class of distinct disorders, including pre-B ALL, CLL, follicular lymphoma, small lymphocytic lymphoma, multiple myeloma, mucosa-associated lymphoid tissue lymphoma, mantle cell lymphoma, diffuse large B-cell lymphoma, Burkitt's lymphoma/leukemia, Waldenstrom's macroglobulinemia, and hairy cell leukemia. Key criteria for deciding which type(s) of B-cell diseases to include in a multicenter clinical trial are (i) their prevalence (to facilitate accrual), (ii) their prognosis and therapeutic alternatives (ethical considerations), and (iii) the relationship between “standard therapy” and the level of medical care required to support the patient following the proposed lymphodepletion conditioning regimen (which affects medical reimbursement). Autologous HSCT would avoid the immunological issues that may complicate allogeneic HSCT, such as risks of graft-vs.-host disease and use of immunosuppressive medications that could blunt the effects of infused T-cell products. Therefore, disorders that may be treated with autologous HSCT may provide more favorable clinical setting for the use of CAR than those conditions best treated with a allogeneic HSCT.

What are the logistic, regulatory, legal, and financial issues? Unlike donor lymphocyte infusions given in the context of allogeneic bone marrow or stem cell transplantation procedures, the US Food and Drug Administration (FDA) will regulate the manufacturing of autologous gene-modified T cells. Bruce Levine (University of Pennsylvania) discussed cell-product manufacturing, drawing from his considerable experience with granulocyte–macrophage precursor cell preparation for numerous clinical trials. One approach would be to have cell products produced at a single central site for distribution to a network of clinical sites. Central processing entails shipping of cells obtained from patients at each participating site and return of the final cell product. Each of these steps must be validated for its effects on the activity of the cell product. Cell products that can be cryopreserved after manufacturing are relatively easy to distribute; cell products that must be administered freshly after culture require more careful timing both of manufacturing and of conditioning of the patient. The alternative to central production is local production of cells at each institution, eliminating the need for shipping. Local production requires that all sites have the necessary cell-processing facilities and personnel trained to follow the prescribed standard operating procedures precisely to ensure uniformity of process. True good laboratory practice and good clinical practice must be followed. Dr. Sadelain stressed the merits of a hybrid approach, with more than one manufacturing center, because this would demonstrate that a cell therapy manufacturing process can be implemented at more than a single center. A larger number of preparation and treatment centers would increase involvement in cell therapy research and accelerate patient accrual. Continued monitoring of both clinical and laboratory operations by a central entity must be intensive. Local cell-product production may also influence the choice of T-cell population to be used, favoring approaches that are rapid, scalable, and exportable.

Legal issues impose major barriers to cooperation among investigators at separate institutions. Concerns over protection of intellectual property and indemnification for responsibility for adverse events associated with the products may make it difficult to finalize cooperation agreements. Success at overcoming these barriers depends on the attitudes and motivation of the sponsor of the IND, the participating institutions, and the funding entities, which may be able to induce cooperation as a requisite for award funding. These considerations will govern site selection, and probably only highly specialized centers may participate.

With respect to financial considerations, Steven Rosenberg estimated that the full costs at the NCI Surgery Branch to produce and release an autologous gene-engineered T-cell product, including all laboratory supplies and reagents, staff salaries, and product certification assays, amount to about 15,000pertreatedpatient(otherssuggestedsomewhathighercostsperpatient,intherangeof15,000 per treated patient (others suggested somewhat higher costs per patient, in the range of 15,000pertreatedpatient(otherssuggestedsomewhathighercostsperpatient,intherangeof20,000 to 25,000,inpartbasedondifferingcostsforvectorproductionandqualification).Thesecostsdonotcovercapitaldepreciation,overhead,andrentalcosts;however,theestimatescomparefavorablywiththeprocurementcostsforunrelatedallogeneicstemcellproductsforclinicaltransplantation.Dr.Rosenbergpointedoutthat,giventhesignificantcompleteresponseraterealizedwiththisapproach,itcomparesfavorablywiththecostsofover25,000, in part based on differing costs for vector production and qualification). These costs do not cover capital depreciation, overhead, and rental costs; however, the estimates compare favorably with the procurement costs for unrelated allogeneic stem cell products for clinical transplantation. Dr. Rosenberg pointed out that, given the significant complete response rate realized with this approach, it compares favorably with the costs of over 25,000,inpartbasedondifferingcostsforvectorproductionandqualification).Thesecostsdonotcovercapitaldepreciation,overhead,andrentalcosts;however,theestimatescomparefavorablywiththeprocurementcostsforunrelatedallogeneicstemcellproductsforclinicaltransplantation.Dr.Rosenbergpointedoutthat,giventhesignificantcompleteresponseraterealizedwiththisapproach,itcomparesfavorablywiththecostsofover100,000 per year for several FDA-approved monoclonal antibody therapies whose benefits are less enduring. Thus, it is imperative that this potentially lifesaving method be supported within current (and future) health-care reimbursement models.

Nancy DiFronzo, from the NHLBI, cosponsor of the BMT CTN, reviewed the relevant NIH funding mechanisms that could provide support for multicenter clinical trials. No extant funding mechanism exactly fits this need. However, several NIH resources offer services that might facilitate a clinical trial evaluating CARs. The NHLBI's Production Assistance for Cellular Therapies program (http://www.pactgroup.net) represents a centralized cell product production model and has been successful in several trials; this resource might be used for a multisite CAR trial. Funding for clinical-grade vector production could be sought from the NHLBI Gene Therapy Resource Program (http://www.gtrp.org), but it would cover only lentiviral vectors. Other academic centers and commercial entities can manufacture clinical-grade γ-retroviral vector stocks as a purchased service. Funding for the development and performance of the clinical trial would need to support the central site for clinical trial management as well as each performance site for cell processing, product certification, and other trial-specific studies. Although a network such as the BMT CTN has a data-coordinating center that could coordinate regulatory reviews and monitor the study, additional funding would probably be needed to cover study expenses that were unique in comparison to a more standard transplant study. This funding would probably be difficult to secure using the traditional RO1 mechanism, and the attendees felt it would be beneficial if the NIH would consider new funding paradigms to enable investigators to implement more complex cell and gene therapy studies.

Summary: next steps

Dr. Sadelain concluded by reviewing the areas that require decision making to be able to move forward to a multisite clinical trial using CARs. Many current trials use second-generation anti-CD19 CARs with the CD3 ζ-chain signaling domain and the CD28 costimulatory domain delivered to T cells using γ-retroviral vectors. It will be interesting to compare these with CARs using the 4-1BB signaling elements in different configurations and using lentiviral vectors for their delivery. The choice of the target T-cell subset will require additional discussions in order to balance the needs for greatest potential efficacy, which may be obtained with a highly selected cell type, with the practicality and ease of adoption by multiple sites of a simpler cell preparation, such as total PBMCs. Autologous HSCT may be a suitable setting for a multi-institutional trial because the transplant conditioning would produce profound lymphodepletion and the cell therapy would fit within the “standard” clinical treatment plan, which may increase third-party coverage for the clinical costs.

The “action plan” that emerged from this workshop included preparation of this document with input from all the stakeholders; exchange of CAR vectors and critical reagents among groups so that they may be compared directly in the different preclinical systems of each laboratory; continuing ongoing discussions; and a second meeting within 6 months to begin to formulate specific plans for protocol development, to decide on the CAR and delivery vector to be used, to outline the laboratory and clinical approaches, and to consider funding applications.

We believe that this gathering represents a solid first step toward achieving the needed consensus for a multi-institutional trial.

Acknowledgments

The co–principal investigators thank NIH representatives Nancy DiFronzo and John Thomas (NHLBI) and Bill Merritt and Toby Hecht (NCI) for their participation in the planning, presentations, and discussions for the workshop. We also thank Adam Mendizabal from the BMT CTN data-coordinating center, staff from the National Marrow Donor Program, and Mary Dean from the American Society of Gene and Cell Therapy for facilitating the meeting.

REFERENCES

Articles from Molecular Therapy are provided here courtesy of The American Society of Gene & Cell Therapy