Targeting CD19 May Yield Paradigm-Altering Technology

Figure 1. CD19 as a Regulator in BCR Signaling

BCR indicates B-cell receptor; BTK, Bruton tyrosine kinase.

Source: GeneCards, www.genecards.org./p>

For the past two decades, researchers have been exploring B-cell specific antigens in hopes of developing a new anticancer target that would mirror the success of the CD20-targeting rituximab (Rituxan). Now, strategies aimed at CD19 are proving particularly promising.

CD19 is serving as the target not only of antibody-based therapies but also in a potentially paradigm-altering approach to cancer immunotherapy that continues to yield impressive clinical outcomes. The complexities of developing CD19-targeting immunotherapy were highlighted recently when several clinical trials were suspended for a safety review in response to the deaths of two patients during a study.

CD19 Makes an Attractive Target

CD19 is a type I transmembrane glycoprotein belonging to the immunoglobulin superfamily, which plays a critical role in establishing an optimal immune response by regulating both B-cell receptor (BCR)- dependent and BCR—independent signaling pathways. It regulates both antigen-independent development and immunoglobulin- induced activation of B cells. (Figure 1).

CD19 makes an attractive target for cancer therapy since its expression on normal cells is limited to those of B-cell lineage. Furthermore, it is expressed on the vast majority of B-cell malignancies, including 80% of acute lymphoblastic leukemias (ALLs), 88% of B-cell lymphomas, and 100% of B-cell leukemias.

Thus, CD19 is a suitable tumor-associated antigen (TAA) against which to target anticancer agents. In contrast to CD20, CD19 is expressed throughout B-cell development, from B-cell precursors through to mature B cells, before expression is lost when they become plasma cells.

This wider range of expression potentially gives CD19-targeted agents an advantage over their CD20 counterparts, since they could be more useful in treating early B-cell neoplasms like ALL, which cannot be treated with rituximab.

Antibody-Based Strategies Under Study

A number of monoclonal antibodies (mAbs) targeting the CD19 protein are in various stages of development for the treatment of B-cell malignancies. The murine B4 antibody was one of the earliest CD19-targeted agents and was subsequently humanized to form HuB4. In order to boost efficacy and reduce toxicity, several novel CD19 antibody designs that build upon HuB4 are currently being explored, including both engineered and conjugated antibodies.

Engineered Antibodies

MEDI-551 and MOR-208 are antibodies that have been genetically engineered so that they have optimized fragment crystallizable (Fc) regions to enhance binding to the Fc-gamma receptor on the surface of target cells and, ultimately, the cytotoxic efficacy of the antibody.

Phase I data for MEDI-551 demonstrated an objective response rate of 26.5% among 34 evaluable patients with advanced B-cell malignancies, including non-Hodgkin lymphoma (NHL), chronic lymphocytic leukemia (CLL), and multiple myeloma (MM), who were treated with doses of 0.5 to 12 mg/kg. In early clinical trial results for MOR208, an 11% overall response rate (ORR), all of which were partial responses (PRs), was observed among 27 evaluable patients with CLL or small lymphocytic leukemia who participated in the phase I study.

Blinatumomab (AMG103) is a bispecific T-cell engaging antibody (BiTE), an artificial bispecific antibody generated through the fusion of the single-chain variable fragments of two different antibodies: one that binds to T cells through the CD3 receptor and one that binds to CD19 on the surface of tumor cells. It is designed to bring cytotoxic T cells into close proximity with tumor cells expressing CD19.

The first trial of blinatumomab was a doseescalation study in 38 patients with indolent NHL, and an overall response rate (ORR) of 28.9% was observed. The focus has subsequently switched to B-cell ALL. In a phase II study of patients with persistent or relapsed minimal residual disease, 80% of patients experienced complete molecular remission. Two phase III studies and various phase II studies of blinatumomab are currently ongoing.

Preliminary results from part one of a phase II study of blinatumomab in patients with relapsed/refractory NHL were presented at the 55th Annual Meeting of the American Society of Hematology (ASH) in New Orleans in December 2013. Among the 7 patients evaluable for response, the ORR was 57%. A recommended stepwise dosage of 9, 28, and 112 μg/day will be evaluated in the second part of the trial.

Conjugated Antibodies

Conjugation of monoclonal antibodies to toxins, radionuclides, and cytotoxic drugs has proved to be a successful strategy for combining the potent cytotoxicity of traditional anticancer agents and the specificity of monoclonal antibodies. The FDA has approved several conjugated antibodies targeting CD20 and CD30 for the treatment of hematologic malignancies.

CD19 makes an even more attractive target for a conjugated antibody than CD20 since it is internalized more efficiently. Two such agents targeting CD19, SAR3419 and SGN-CD19A, conjugate anti-CD19 antibodies to different cytotoxic drugs. Development of SAR3419 is more advanced, with several phase II studies ongoing. In a phase I trial, among 35 patients with relapsed/refractory B-cell lymphoma treated with escalating doses of SAR3419, 6 achieved a PR or complete response (CR) and 74% of patients experienced tumor shrinkage, including 47% with rituximab-refractory disease.

CD19-Targeted Therapies in Clinical Trials

Description

Ongoing Trials

CAR T cells

Various research institutions

Immunotherapeutic approach in which the patient’s T cells are genetically engineered to express CARs designed to recognize CD19 on the surface of B-cell malignancies

• More than 20 clinical trials
• Phase I or II in patients with a variety of B-cell malignancies, including ALL, CLL, and DLBCL
• Study populations include patients who have not undergone stem cell transplantation and patients who relapsed after transplant

SAR3419

Sanofi

ADC composed of the humanized monoclonal IgG1 anti-CD19 antibody huB4 attached to the potent cytotoxic maytansine derivative DM4

• 3 phase II trials
• Monotherapy in patients with ALL
• Single-agent and in combination with rituximab in patients with DLBCL

Blinatumomab (AMG103) Amgen

Bispecific T-cell engaging (BiTE) antibody designed to bind T cells through the CD3 receptor and tumor cells through CD19, to direct cytotoxic T cells to kill B cells expressing this protein

• 2 phase III studies in combination with chemotherapy in patients with newly diagnosed BCR-ABL—negative ALL, or vs investigator’s choice of chemotherapy in patients with relapsed/refractory ALL
• 6 phase II studies in patients with ALL or DLBCL

MOR-208

(Formerly XmAb 5574) MorphoSys AG

Anti-CD19 antibody genetically engineered to have an optimized fragment crystallizable (Fc) region, via changes in the protein sequence of the Fc domain, to enhance binding to the Fc-gamma receptor and subsequent antibodydependent cellular cytotoxicity

• 3 phase II trials
• Monotherapy in patients with NHL and ALL
• In combination with lenalidomide in patients with relapsed/refractory CLL, SLL, or PLL

MEDI-551 MedImmune, LLC

An Fc-engineered (via afucosylation) humanized antibody against CD19

• Phase II trial of MEDI-551 in combination with bendamustine at 2 dose levels vs bendamustine plus rituximab in patients with relapsed/refractory CLL
• Phase II study of MEDI-551 + ICE/DHAP at 2 dose levels vs rituximab + ICE/DHAP in relapsed/refractory DLBCL
• Phase I dose escalation study in patients with advanced B-cell malignancies

Combotox Montefiore Medical Center

Mixture of immunotoxins targeting CD19 and CD22, composed of single-chain variable fragment antibodies fused to a deglycosylated form of the ricin A chain

• Phase I in combination with cytarabine in patients with relapsed/refractory ALL

DT2219ARL Masonic Cancer Center, University of Minnesota

Bispecific immunotoxin targeting CD19 and CD22, composed of single-chain variable fragment antibodies fused to a truncated form of the diphtheria toxin

• Phase I in CD19/CD22-positive B-cell leukemia or lymphoma

SGN-CD19A

Seattle Genetics

ADC composed of a humanized anti-CD19 monoclonal antibody conjugated to the microtubuledisrupting agent monomethyl auristatin F (MMAF)

• 2 phase I trials in patients with relapsed/ refractory B-lineage NHL, including DLBCL and lymphoblastic leukemia

ADC indicates antibody-drug conjugate; ALL, acute lymphoblastic leukemia; CAR, chimeric antigen receptor; CLL, chronic lymphocytic leukemia; DHAP, dexamethasone-cytarabine; DLBCL, diffuse large B-cell lymphoma; ICE, Ifosfamide-carboplatin-etoposide; NHL, non-Hodgkin lymphoma; PLL, prolymphocytic leukemia; SLL, small lymphocytic leukemia.

Phase II data presented at the ASH meeting showed a modest ORR of 31.1% among patients with relapsed/refractory diffuse large B-cell lymphoma (DLBCL) treated with SAR3419 in combination with rituximab. However, the study population was of poor prognosis, with 60% refractory to first-line therapy and, among patients with relapsed DLBCL, a higher ORR of 58.3% was observed.

Preliminary results of a phase I dose-escalation study of SGN-CD19A were also reported at ASH.

The best responses to date were 1 CR at a dose of 1.3 mg/kg among 8 evaluable patients with relapsed/refractory ALL and 2 patients with stable disease among 4 participants with lymphoma.

Dramatic Results for Cellular Immunotherapy

As researchers have come to appreciate the complexCARs capable of recognizing a variety of TAAs have been explored, but the focus of clinical development has been on CD19. Five academic centers have been involved in the evaluation of CD19-CAR T cells: Baylor College of Medicine in Texas, City of Hope Comprehensive Cancer Center in California, Memorial Sloan Kettering Cancer Center (MSK) in New York, the National Cancer Institute (NCI) in Maryland, and the Abramson Cancer Center at the University of Pennsylvania (UPenn) in Philadelphia.

The results of clinical trials performed thus far, particularly those emerging from the NCI, UPenn, and MSK, have demonstrated the potent biological activity of this approach. Complete remissions have been observed, several of them durable, with an absence of minimal residual disease. The first evidence of clinical activity came from studies at the NCI in 8 patients with follicular lymphoma (FL). These patients experienced dramatic regression, with 2 CRs, 4 PRs, 1 stable disease (SD), and 1 not evaluable.

In general, however, the first generation of CD19-CAR T cells showed disappointing clinical activity, mostly due to a lack of activation or persistence, or to a failure to overcome tumor-induced mechanisms of immune suppression. Furthermore, they induced significant acute toxicity. Researchers have therefore focused on improving CAR design, production processes, and administration protocols to overcome these issues.

For example, since the proper activation of T cells is a two-step process that requires both the TCR and a costimulatory signal, second- and third-generation CARs have been designed to include T-cell costimulatory domains, such as CD28. To reduce toxicity, efforts are under way to design CARs that elicit lower levels of inflammatory cytokines and to introduce “suicide genes” into CARs so that the infused T cells can be rapidly and specifically eliminated if severe toxicity is experienced.

Numerous updates to ongoing clinical trials at NCI, UPenn, and MSK were presented at ASH. NCI researchers reported dramatic results in patients with treatment-resistant chronic lymphocytic leukemia (CLL): Among the 10 patients treated to date, 1 patient with CLL experienced a CR that is ongoing after 1 year of follow-up and another patient had disease regression in bone marrow, blood, and lymph nodes. In addition, 2 patients with CLL or mantle cell lymphoma experienced PRs and 1 patient with DLBCL achieved SD greater than 11 months. Highlighting the resistant nature of disease among the patients treated in this trial, one patient was described as having previously undergone numerous chemotherapy regimens, three bone marrow transplants, and five donor lymphocyte infusions. The team at UPenn also reported success among 14 patients with relapsed/refractory CLL with CTL019 cells. The CR rate was 21% and the PR rate was 36%, for an ORR of 57%. The most impressive results with CTL019 were reported in patients with ALL. Among 20 patients (16 pediatric and 4 adult), the CR rate was 82%.

The NCI team also reported significant efficacy of their CD19-CAR T cells in the same patient population, with a 63% CR rate among the 8 patients treated to date, and a 71% CR among ALL patients.

Safety Issues Identified

In April, MSK temporarily suspended five earlyphase clinical trials exploring 19-28z CAR T-cell therapy in response to the deaths of two patients with adult B-cell ALL. It was not clear that the T-cell therapy precipitated the deaths; one person had a history of cardiac failure and the other developed persistent seizures during the second infusion of cells,according to Renier J. Brentjens, MD, PhD, director of Cellular Therapeutics at MSK.

Nevertheless, Brentjens said, MSK halted the trials to review safety protocols in an attempt to minimize the risk of T-cell related toxicities, particularly in the context of cytokine release syndrome (CRS).

CRS is a common phenomenon observed with T-cell therapies. Upon infusion into the patient, the T cells expand and cytokines are released, causing systemic symptoms such as fever, nausea, chills, hypotension, headache and rash, among others.

The MSK trials were resumed with amendments to the treatment protocol that included reducing T-cell dosage, treating patients at an earlier time point, and precluding patients with significant cardiac disease from enrollment, as well as precluding patients from additional infusions if they experience significant central nervous system toxicity at the time of the first infusion.

Translating T-cell Successes Into Commercial Therapies

Many in the field are calling these novel CAR T-cell techniques paradigm altering, with the potential to revolutionize the way B-cell malignancies and potentially other tumor types are treated. This potential is now being recognized by the pharmaceutical industry. In 2012, Novartis stepped in to license the CAR T-cell technology being developed at UPenn and pumped $20 million into a joint venture to fund further research, development, and commercialization.

Late in 2013, Juno Therapeutics was founded in partnership with the Fred Hutchinson Cancer Research Center, MSK, and the Seattle Children’s Research Institute, to focus on the development of CAR T-cell technology, among other novel cancer immunotherapies. The company was founded with an initial investment of $120 million, one of the largest in biotech startup history, which reflects the current excitement about this technology. Among Juno’s founding scientists are Brentjens and Michel Sadelain, MD, PhD, also from MSK.

In separate interviews with OncologyLive, Brentjens and Sadelain noted that while the process of commercialization of these therapies is well under way and receiving significant interest and assistance from the pharmaceutical industry, there are still challenges to commercial translation. According to Sadelain, a significant challenge is how to distribute this type of cancer treatment to large numbers of patients. Brentjens agreed that administration into patients is a significant issue, as is the cost of production, but believes that “the pharmaceutical industry will find a way to make this a profitable endeavor as long as the clinical outcome data remain as positive as it has to date.”

Figure 2. Key Steps in CAR T-Cell Technology

Brentjens R. Cellular therapy with chimeric antigen receptor (CAR) T cells in lymphoid malignancies. 18th Annual International Congress on Hematologic Malignancies: Focus on Leukemias, Lymphomas, and Myeloma; February 14-15, 2014; New York, NY. Reprinted with permission.

Jane de Lartigue, PhD, is a freelance medical writer and editor based in Davis, California

Key Research

Borate U, Fathi AT, Shah BD, et al. A first-in-human phase 1 study of the antibody-drug conjugate SGN-CD19A in relapsed or refractory B-lineage acute leukemia and highly aggressive lymphoma.

2013 Annual Meeting Abstracts. Blood. 2013;122(21;abstr 1437). Brentjens RJ, Curran KJ. Novel cellular therapies for leukemia: CAR-modified T cells targeted to the CD19 antigen. Hematology Am Soc Hematol Educ Program. 2012;143-151.

Coiffier B, Thieblemont C, de Guibert S, et al. Phase II study of anti-CD19 antibody drug conjugate (SAR3419) in combination with rituximab: clinical activity and safety in patients with relapsed/refractory diffuse large B-cell lymphoma. 2013 Annual Meeting Abstracts. Blood. 2013;122(21;abstr 4395).

Grupp SA, Frey NV, Aplenc R, et al. T cells engineered with a chimeric antigen receptor (CAR) targeting CD19 (CTL019) produce significant in vivo proliferation, complete responses and long-term persistence without GVHD in children and adults with relapsed, refractory ALL. 2013 Annual Meeting Abstracts. Blood. 2013;122(21;abstr 67).

Hammer O. CD19 as an attractive target for antibody-based therapy. mAbs. 2012;4(5):571-577.

Kochenderfer JN, Rosenberg SA. Treating B-cell cancer with T cells expressing anti-CD19 chimeric antigen receptors [published online April 2, 2013]. Nat Rev Clin Oncol. 2013;10(5):267-276.

Kochenderfer JN, Dudley ME, Carpenter RO, et al. Donor-derived anti-CD19 chimeric-antigen-receptor-expressing T cells cause regression of malignancy persisting after allogenic hematopoietic stem cell transplantation. 2013 Annual Meeting Abstracts. Blood. 2013;122(21;abstr 151).

Kochenderfer JN, Dudley ME, Kassim SH, et al. Effective treatment of chemotherapy-refractory diffuse large B-cell lymphoma with autologous T cells genetically-engineered to express an anti-CD19 chimeric antigen receptor. 2013 Annual Meeting Abstracts. Blood. 2013;122(21;abstr 168).

Lee DW, Shah NN, Stetler-Stevenson M, et al. Anti-CD19 chimeric antigen receptor (CAR) T cells produce complete responses with acceptable toxcitiy but without chronic B-cell aplasia in children with relapsed or refractory acute lymphoblastic leukemia (ALL) even after allogenic hematopoietic stem cell transplantiona (HSCT). 2013 Annual Meeting Abstracts. Blood. 2013; 122(21;abstr 68).

Leslie LA, Younes A. Antibody-drug conjugates in hematologic malignancies. Am Soc Clin Oncol Ed Book. 2013; e108-e113. Portell CA, Wenzell CM, Advani AS. Clinical and pharmacologic aspects of blinatumomab in the treatment of B-cell acute lymphoblastic leukemia. Clin Pharmacol. 2013;5(suppl 1):5-11.

Porter DL, Kalos M, Frey NV, et al. Randomized, phase II dose optimization study of chimeric antigen receptor modified T cells directed against CD19 (CTL019) in patients with relapsed, refractory CLL. 2013 Annual Meeting Abstracts. Blood. 2013;122(21;abstr 873).

Porter DL, Kalos M, Frey NV, et al. Chimeric antigen receptor modified T cells directed against CD19 (CTL019 cells) have long-term persistence and induce durable responses in relapsed, refractory CLL. 2013 Annual Meeting Abstracts. Blood. 2013;122(21;abstr 4162).

Viardot A, Goebeler M, Pfreundschuh M, et al. Open-label phase 2 study of the bispecific T-cell engager (BiTE®) blinatumomab in patients with relapsed/refractory diffuse large B-cell lymphoma. 2013 Annual Meeting Abstracts. Blood. 2013;122(21;abstr 1811).

Von Stackelberg A, Zugmaier G, Handgretinger R, et al. A phase 1/2 study of blinatumomab in pediatric patients with relapsed/ refractory B-cell precursor acute lymphoblastic leukemia. Blood. 2013;122(21;abstr 70).

Woyach JA, Awan F, Flinn IW, et al. Final results of a phase I study of the Fc engineered CD19 antibody XmAb®5574 (MOR00208) in patients with relapsed or refractory chronic lymphocytic leukemia (CLL) or small lymphocytic lymphoma (SLL). Blood. 2012;120(21;abstr 2984).