Mechanisms of Resistance for CAR T Therapy

Transcript:

Krishna V. Komanduri, MD: The other general point that I want to talk about that goes across platforms is the possibility of developing resistance. There are some mechanisms of developing resistance. Steve, do you want to talk about that, some of your work in the clinical settings?

Stephen J. Schuster, MD: Yes, we’ve done some work in this area and obviously one concern is that the tumor cells can lose the antigen to which the CAR is targeted, and indeed that is a mechanism of resistance. It seems to be more common, the mechanism of resistance, in ALL. Probably, it’s those cells being closer to stem cells that are more plastic. But in lymphomas, interestingly, in my own experience, probably it’s 20% or less where there’s antigen loss. But we see upregulation of inhibitory ligands, PD-L1, TIM3, LAG3. We looked at all of them and it’s either more inhibitory ligands being upregulated, and that has led to some clinical trials of using checkpoint inhibitors together with CAR T cells, or at least rescuing some patients who seem to be progressing after CAR T-cell therapy.

Krishna V. Komanduri, MD: Great points. And I think, obviously, another thing that we’re starting to see is the notion that we might treat patients with either multiple CARs targeting different antigens to prevent that escape, or, again, in sequential fashion if we do have escape. So, these are all great questions. When escape happens or when relapse happens, can a patient receive multiple infusions? What are your experiences with patients when they relapse? We have a problem with either response or no response. Patients who respond have durable responses that last for years. And we’ve been doing this for about 4 years now. I can tell you that the patients who don’t respond, despite the fact that they have in vivo expansion of the CAR cells, that happens early and you recognize it right away. So, it doesn’t make sense if cells are expanding and a patient is not responding to give them more of the same cells. It has been tried, it just doesn’t seem to work. And in those patients, we try giving checkpoint inhibitor. Probably works, I’d say, about 20% to 25% of the time in a rescue situation.

Obviously, other CARs, other targets, may be another approach. But it seems that if it’s antigen loss, perhaps another CAR altogether. If it’s upregulation of inhibitory ligands, a checkpoint inhibitor, and then at least some of the pathology we’ve seen, and this is I think unique to lymphoma, we’ve seen an inability of the CAR cells to actually get into a tumor mass. So, we’ve taken excisional biopsies and seen tumors surrounded by CAR cells but not the cells unable to egress from the circulation into the tumors. So, maybe even one-third mechanism of resistance. And I think we don’t understand enough about that kind of trafficking yet to manipulate it therapeutically.

David Maloney, MD, PhD: I’d like to add that. Our experience has been a little different. And so, when we’ve had patients who relapsed and still retained the CD19 antigen and they’ve generally had a loss of CAR T cells, they, in general, do not respond to a second infusion. And in those who don’t respond, we’ve been able to show that they generate a T-cell response, again, the antibody portion of the CAR. Remember, almost all of the CARs to date for CD19 use the same antibody. And so, that’s a mooring antibody, and so that you can generate immune responses. So, if you were immunized to one, you’re most likely not going to respond to the other.

Now, there are ways around that. One way around it that’s the most interesting is to humanize the CAR, use a human. I know there are studies ongoing in all of our institutions. And so, that is one approach.

The second thing I’d just comment on is that if you’re stressing the system by targeting CD19, you’re going to increase the risk of escape. And so, I actually like using blinatumomab beforehand because I think it increases the chance of having immunologic escape by antigen-negative escape after CARs. And obviously, if you’re negative afterwards, then you’re not going to respond to blinatumomab as a subsequent therapy, but you could switch antigens. So, a couple minor points.

Stephen J. Schuster, MD: It has also been described that there’s antigen splicing. So, you can have a salvage still seeing 19-positive, but there’s alternative splicing that happens in the epitope to which the antibody is directed and is lost. So, it’s an area that needs a lot more work.

Krishna V. Komanduri, MD: I hate to add more anecdotes, but I have had a couple of patients who have had relapses, who remained CD19-positive, and who did respond to a second infusion. But in those cases, while we saw response, we did not expect that response to be durable and, therefore, we went to an allogeneic transplantation and consolidation.

Michael Pulsipher, MD: One thing to add to that is in the pediatric leukemia experience, we’ve seen that early loss of CARs or early relapses don’t respond very well at all. However, if patients lose their CARs 6 months or 9 months later, they have responded to reinfusion with more durable CARs.

David Maloney, MD, PhD: Yes, we have the same experience as well.

Michael Pulsipher, MD: Exactly. And that perhaps has something to do with the T cells. Some T cells may be more long lived than others, and it’s reasonable to think about that.

Transcript Edited for Clarity