NEW YORK – Two studies, one recently published and one just kicking off, are part of growing efforts to determine why CAR T-cell therapy leads to long-lasting responses in some multiple myeloma patients and early relapses in others.
In a study published in Blood Cancer Discovery, researchers from Emory University zeroed in on elements of the tumor microenvironment, including T-cell receptor diversity and T-cell gene expression in bone marrow samples, that influence how long patients respond to B-cell maturation antigen (BCMA)-directed autologous CAR T-cell therapy. Meanwhile, researchers at Stanford University are gearing up to sequence patients' peripheral blood cell-free DNA, hoping to shed light on molecular alterations that correlate with CAR T-cell responses.
Ultimately, both groups are hoping that by understanding the differences between patients who relapse early and those who remain in remission for months or years, they'll be able to better select patients for CAR T-cell therapy; counteract resistance mechanisms in those likely to relapse; and develop next-generation CAR T-cell therapies or combination treatments that stave off resistance.
Most multiple myeloma patients who receive BCMA-targeted autologous CAR T-cell therapy do respond, at least initially. In the clinical trials that the US Food and Drug Administration used to approve two such treatments, Bristol Myers Squibb/2seventy Bio's Abecma (idecabtagene vicleucel) and Legend Biotech/Janssen's Carvykti (ciltacabtagene autoleucel), 72 percent and 97 percent of patients, respectively, responded.
But disease relapses are also common after initial response. In the Abecma pivotal trial, dubbed KarMMa, for instance, the median duration of response was 10.7 months, and while it is too early to report the median duration of response for Carvykti, late relapses are still occurring, wrote Charlotte Graham and Marcela Maus from Massachusetts General Hospital in a recent commentary simultaneously published with the Emory University study in Blood Cancer Discovery. The frequent recurrences despite encouraging initial responses have some researchers asking why some patients' responses are more durable than others.
Unpacking TME differences
In the Blood Cancer Discovery study published Monday, researchers including Madhav Dhodapkar, the director of the Center for Cancer Immunology at Emory's Winship Cancer Institute, used myriad single-cell techniques to analyze 28 bone marrow samples taken from multiple myeloma patients who participated in a Phase I clinical trial of the University of Pennsylvania's investigational BCMA-targeted autologous CAR T-cell therapy. While the study didn't technically use one of the two FDA-approved CAR T-cell therapies, Dhodapkar said that the treatment construct for the therapy used in the Penn study was quite similar to that of the marketed CAR T-cell therapies, including in terms of the costimulatory domain used. "Because it's a similar construct, the shared findings, in principle, should be very similar," he said.
To conduct the study, Dhodapkar and colleagues took pre- and post-therapy bone marrow specimens and performed CITE-seq, single-cell transcriptomics, mass cytometry, and T-cell-receptor sequencing. The researchers then looked at data from these analyses alongside patients' clinical response data, splitting patients into two response categories: those who lived fewer than six months without their disease progressing, and those who lived more than six months without their disease progressing.
Dhodapkar highlighted a number of differences that came to light from these single-cell analyses. For example, patients whose T-cell repertoires are more diverse at baseline tended to respond for longer periods of time. Patients with longer progression-free survival were likely to have an increase in the proportion of T cells in the bone marrow and a decrease in myeloid cell proportions immediately after treatment. And the T cells from patients with shorter versus longer progression-free survival times expressed distinct gene signatures differentially both at baseline and immediately after treatment.
For example, among patients who responded for a longer period, tumor cells expressed higher levels of interferon response genes and mature plasma genes at baseline, whereas tumor cells from patients with shorter response times had higher expression of genes associated with epithelial-to-mesenchymal transition. After treatment, T cells from patients with shorter responses tended to express greater levels of immune checkpoint genes and T-cell exhaustion genes, while longer responders expressed greater levels of genes associated with bone marrow retention.
While each nuanced observation may not be immediately actionable, the "take-home" message from the experiment, according to Dhodapkar, is that the response differences could all be attributed to patients' biological variances rather than to the elements in the CAR T-cell products.
"While these CAR T-cell therapies have been studied extensively as sort of mediators of the response, it is the properties of the non-CAR T cells — the other immune cells — in the microenvironment that seem to have a major impact or correlation with how long the responses are," he said.
According to Dhodapkar, this could have implications for how patients are selected for CAR T-cell therapy and influence efforts to tweak treatment plans or pursue combination treatments. For instance, since the study showed that patients with more diverse T-cell repertoires had longer responses to the CAR T-cell therapy, he suggested that oncologists might avoid treating patients with earlier-line drugs that can reduce T-cell diversity to mitigate this negative effect. "If we preserve the T cells better in the context of other therapies, we are more likely to get more bang for our buck, if you will, with the CAR T-cell therapy," he said.
Additionally, given the differential proportions of myeloid and dendritic cells observed in long- and short-term responders, Dhodapkar said there might be opportunities "for future combination therapies where we are trying to not just inject the CAR T-cells, but also manipulate the … other immune cells in the microenvironment." This is becoming more feasible as other types of immunotherapies enter the landscape for multiple myeloma.
Finally, the fact that many of the tumor cells that seemed to escape initial CAR T-cell response expressed higher levels of stem-like genes suggested that immediately targeting residual tumor cells could be "the key to turning initial responses into eventual cures."
As a next step, Dhodapkar and colleagues will use preclinical models to test their hypotheses about the impact of combining CAR T-cell therapy with drugs targeting myeloid cells in the tumor microenvironment. After that, they hope to take the validated approaches back to the clinic and see if they improve CAR T-cell therapy response durations.
Differences in cell-free DNA
The Emory researchers relied entirely on sequential pre- and post-treatment bone marrow samples for their study. Dhodapkar said bone marrow sampling was crucial when it came to accessing the tumor microenvironment directly, since multiple myeloma begins in the bone marrow, and peripheral blood samples might not necessarily pick up cells that are present in the bone marrow.
That said, bone marrow taps, while quite common in multiple myeloma, are painful and invasive. That's one reason that researchers at Stanford University are diving headfirst into a study of cell-free DNA differences between multiple myeloma patients who respond durably versus relapse on BCMA-targeted CAR T-cell therapy.
Hitomi Hosoya, a hematology and medical oncology fellow at Stanford, recently received a $100,000 grant from the Society for Immunotherapy of Cancer and Merck for a project, entitled "Cell-free DNA-based Platform to Determine Treatment Response and Resistant Mechanisms to CAR T-cell Therapy in Multiple Myeloma."
The goal of the study, Hosoya said, is to home in on ctDNA differences among patients who relapse early, potentially giving oncologists the option to intervene before the disease progresses. To do this, Hosoya and colleagues are using CAPP-Seq to analyze patients' peripheral blood for genetic changes that correlate with response times. They hope to publish their findings after analyzing data from the first 30 or 40 patients.
"Our preliminary results show that the ctDNA level correlates with disease burden," she said. "When the patient responds to the therapy, the level goes down, and when they relapse, it goes up, [so] we can detect the increase of the ctDNA level before the clinical relapse." According to Hosoya, these increasing and decreasing "levels" reflect allele frequency changes in genetic mutations assessed using an 85-gene panel that her team designed for this purpose. They piloted the test in a cohort of multiple myeloma patients who received Abecma and found that the ctDNA levels following CAR T-cell therapy were associated with response durations.
As Hosoya sees it, these findings could have direct clinical implications if validated further, not only because they would present the opportunity to intervene and prevent relapse, but also because they would allow patients to receive less invasive testing that uses blood samples rather than bone marrow biopsies.
"Right now, we have to get bone marrow biopsies to find out the genes that are mutated and whether patients have any chromosome changes," she said, explaining that not only are bone marrow biopsies painful, inconvenient, and costly, they may not accurately reflect the extent of patients' disease. "Some patients have patchy disease in the bone marrow," she said. "We cannot always get the disease from sampling the bone marrow."
Hosoya's team has banked blood samples from about 60 multiple myeloma patients, and 30 of these patients have already undergone CAR T-cell therapy. The group hopes to report a preliminary analysis of these samples by year-end.
"Multiple myeloma is a very heterogeneous disease, and CAR T-cell therapy is not a curative option yet," Hosoya said. "There are a lot of unknowns in this field." Dhodapkar, Hosoya, and their colleagues are hopeful that analyses like these might chip away at these unknowns and inform strategies for improving multiple myeloma patients' long-term responses to CAR T-cell therapy.