A novel target, B-cell maturation antigen, has been identified for future therapeutic development as chimeric antigen receptor T-cell therapy for patients with multiple myeloma.
Gregor B Adams
Chimeric antigen receptor (CAR)-modified T-cell therapies have shown immense promise for the treatment of patients with B-cell malignancies, prompting their exploration across various other blood disorders. These agents have primarily targeted CD19, with 2 currently being evaluated for approval by the FDA for types of non-Hodgkin lymphoma.
For those with multiple myeloma, early results showed some promising signals of efficacy for CD19-targeted CAR T cells; however, there were biologic limitations to this approach, as CD19 is typically only present during B-cell development and not in plasma or myeloma cells. To this end, a novel target, B-cell maturation antigen (BCMA), was identified for future therapeutic development.
BCMA is broadly expressed on both cell lines and primary myeloma cells. Moreover, the antigen is restricted solely to plasma cells and myeloma cells, with >98% of all myeloma cells testing positive for the BCMA protein or RNA.
Several CAR T-cell therapies have emerged that target BCMA, with the most recent coming from Kite Pharma. This agent, KITE-585, consists of a fully human anti-BCMA IgG single-chain variable fragment (scFv; clone RD-1). The hinge/transmembrane portion and costimulatory domain are both composed of CD28.
"This is cellular therapy," noted Gregor B Adams, from Kite Pharma, who presented research on the new agent at the 2017 AACR Annual Meeting. "There are multiple different things that are going to be able to differentiate one therapy from the other, which includes all of the manufacturing steps. When you take a cell from aphaeresis all the way through the manufacturing process to infusion of cells back into the patient, all of these steps are going to have an impact on the efficacy for the patients."
According to findings presented at the AACR meeting,1,2 as a first step to developing the novel scFv for KITE-585, CAR specificity for BCMA was elucidated using a unique screening process. In this step, various molecules of interest were tested on microarray technology to select for an optimal level of BCMA specificity. These microarrays expressed over 4542 vectors, which accounted for approximately 70% of all proteins in the human plasma membrane.
After testing the agent on cell microarrays, the agent moved into tissue microarray testing. These microarrays were derived from 36 samples of non-diseased tissues from 3 donors. This step, which was required by the FDA, allowed researchers to test for cross reactivity. In general, BCMA was the only target hit, representing a low likelihood of off-target toxicity.
In experiments conducted on cell lines with various BCMA expression, KITE-585 showed high selectivity for BCMA at low effector and target cell ratios. Moreover, in the 2 low BCMA expressing models (MV4;11 and Jurkat) the T cells did not proliferate; however, in higher expressing cell lines, there was a high relative cell count.
“CAR T cells are fully active in the presence of soluble BCMA protein at concentrations found in the serum of multiple myeloma patients with active disease,” said Adams.
Cytokine release in response to targets demonstrated polyfunctionality of the modified T cells, with no evidence of toxic signaling. Specifically, KITE-585 was associated with increases in Th1 and Th2 cytokines along with increases in lytic and other factors. The ability to perform multiple functions along with proliferation has been shown to be a strong signal of CAR T cell efficacy, according to findings using the CD19-targeted agents.3
Overall, treatment with KITE-585 led to a near eradication of the tumors in the mouse models as compared with untransfused mice. Mice were treated at day 6, when their approximate tumor volume was 100 mm3. Those in the experimental arm had a rapid decline and elimination of the tumor whereas untreated mice continued to have rapidly growing tumors that eventually surpassed 500 mm3.
"When we infuse KITE-585 cells into these animals, we get complete ablation of these tumors cells," Adams said.
CAR T cells were found to be persistent in the study, with an initial spike that plateaued by week 2. The persistence of KITE-585 in the peripheral blood was similar to what was seen with a single copy of the human gene RPP30, showing that there was not a reduction in the cell's ability to proliferate normally.
Following these early preclinical findings, Kite plans to initiate a phase I study to explore the CAR T-cell therapy in patients with multiple myeloma. This study is anticipated to begin enrolling in 2017, according to the company.
"KITE-585 is a fully human anti-BCMA CAR construct that potentially targets multiple myeloma cell lines in vitro and in vivo," said Adams. "KITE-585 is specific for BCMA and demonstrates a polyfunctional, target dependent response, with no evidence of tonic signaling." Following these laboratory experiments, KITE-585 was explored in animal models. The agent was administered following a conditioning regimen to mice with subcutaneous myeloma cell lines.