IIt’s a great question. Currently， our armamentarium for CAR-T cells are dominated by CD19 directed CAR-T cells in lymphoma， and BCMA directed CAR-T cells in multiple myeloma， but these represent only two of potentially limitless targets to treat malignancies. Currently we’re looking at multiple other targets as well. Selecting a target is a critical element of CAR-T cell treatment. And in some ways， IT’s the most challenging. We have to select a target that is sufficiently specific for the cancer cell and won’t destroy a healthy cell that the patient requires to live healthy. So the anti-CD19 CAR-T cells are not entirely selective for B cell lymphoma. In fact， CD19 is expressed on all mature， healthy B lymphocytes as well. And so with anti CD19 CAR-T cells， we eliminate all healthy b lymphocytes in the human body of our patients. Now， the fact is that patients can live without their B cells for an extended period of time. And we see recovery of these cells happening usually nine to twelve months after their CAR-T cell treatment. And occasionally we can supplement antibodies to allow patients of fight off infection while their immune system is recovering， while their B cells recur， so the B cell， in that sense is a sacrificial target we can eliminate it for a period of time.
There are other B cell targets that are also under investigation. CD22， it was a target on the surface of the B lymphocyte is a very appealing target for immunotherapy， similar to CD19， CD22 is not yet been targeted with other agents in our patients likes a Rituximab has targeted CD20. And so it is a naïve antigen to direct against. And anti CD22 CAR-T cells have also shown significant evidence in small trials. In fact， what current clinical trials are evaluating in both lymphoma and in acute lymphoblastic leukemia is trying to target multiple antigens at the same time. And so early evidence suggests that not only targeting CD22， but also targeting CD19 at the same time might work better than either a single CD19 or a single CD22 directed CAR-T cell. So both CD22 CAR-T cells， and bi-specific CAR-T cells against multiple B cell antigens are under investigation.
We’re also looking at other lymphoid antigens. That includes CD37， which is expressed on a number of B cells and T cells. And so we’re going to be looking at this target in both B and T cell lymphoma that expresses CD37. We’re very interested in CD30. CD30 is widely expressed in classical hodgkin lymphoma， EBV-associated large B cell lymphomas， and on anaplastic large T cell lymphomas， as well as with other lymphomas with B cell differentiation， often times including things like primary affusion lymphoma. And so CD30 is an appealing target for those patients. And we previously have recorded of using a CD30 directed therapy， the antibody drug conjugate Brentuximab vedotin， and for the treatment of patients with CD30 positive B cell and T cell lymphomas. CD30 directed CARs are currently in development with early data demonstrating activity in both B cell lymphoma， T cell lymphoma， and hodgkin lymphoma， but those require much further investigation on a larger number of patients.
Beyond that， there are still additional B cell antigens that could be targeted. I think we have to continue identify additional T cell antigens. Unlike the B cell， which the human body can live without for a period of time， we can’t live without T lymphocytes due to the severe infectious risk. So finding a target among T cell lymphomas means finding one that won’t eliminate all healthy T cells in the body. Beyond lymphomas， there are CARs currently being directed for acute myelogenous leukemia， as well as newer drugs for acute lymphoblastic leukemia. So I think over the next five to ten years， I hope to see an explosion of CAR-T cells directed numerous antigens that broaden the availability of this technology to multiple diseases， which we can’t currently target with anti CD19 cells.
Resistance of CAR-T cell therapy is a critically important topic today. Clearly， we see the majority of patients with diffuse large B cell lymphoma will have a response to CAR-T cells， but that only about forty percent of them will have durable， long term remissions， so ultimately the majority of patients with diffuse large B cell lymphoma will be resistant to CAR-T cell therapy.
Why might that be? Well， one potential explanation is loss of the CD19 antigen. So we see this and other diseases targeted with monoclonal antibodies， for example， that the cell can down regulate the target antigen and thus be resistant to an immune directed target. That’s been a very common mechanism of resistance to CAR-T cells in acute lymphoblastic leukemia.
Interestingly， it appears to be less common in diffuse large B cell lymphoma， where only about 10% of patients who are resistant to CAR-T cells have lost the CD19 antigen. In fact， if you look at patients relapsing on CAR-T cells， we find that the majority of them still express the CD19 antigen and still have the CAR-T cell detected in their blood. Why isn’t it working? Well， if the target is there and the cell is there， then something is preventing it from attacking the lymphoma cell.
And so it seems to me that the primary mechanism of resistance is immune mediated， that the cells are finding a way to elude the immune response. And so targeting the immune response and restoring activity of the CAR-T cell is likely the way we’re going to overcome most mechanisms of resistance. How might we do that? Well， we have immune checkpoint inhibitors， which have shown the ability in lymphomas and multiple other cancers to restore an effective host immune attack against cancer cells. We’ve seen some patients can respond to immune checkpoint inhibitors and reactivate their CAR-T cells. However， most patients just treated with an immune checkpoint inhibitor alone will not restore sensitivity to CAR-T cells. I think we need to understand that better， and test patients biopsies for PD-L1 expression. And I think we’re likely to find a subset of patients with resistant therapy on CAR-T cells who up-regulated PD-L1 and are a good candidate for a PD-L1 targeted antibody， an immune checkpoint inhibitor.
But there are multiple other immune checkpoints beyond PD-L1. So we also have to look at other immune checkpoints， and targeting， and they’re over a dozen different immune checkpoints. And if we survey every patient for the increasing levels of expression of these checkpoints， we may be able to identify a cocktail of inhibitors that will help restore immune function for our patients. And it might be personalized to it by patient level. We can also look at other drugs that might help improve the immune response. We have immune-modulating drugs， such as Thalidomide. Thalidomide we know has been used in follicular lymphoma and other lymphomas and works in part by helping restore T cell immune response against lymphoma. We have early evidence in multiple different lymphomas that giving Thalidomide can down-regulate exhaustion markers in the CAR-T cell， up-regulate activity of the CAR-T cell and may restore effectiveness of that CAR-T cell.
We can then look at other drugs such as BDK and IDK inhibitors， and studies in chronic lymphoblastic leukemia have shown that patients treated with a BDK or IDK inhibitor called ibrutinib have shown down regulation of exhaustion markers， and also increased activation markers with better outcomes for those patients after treated with CAR-T cells.
And then finally， we can make better CAR-T cells. So currently， our CAR-T cells are what we call second generation. They target CD19， and they have a co-simulation domain. But are there other mechanisms we can use to make this CAR-T cell work better? Well， there are multiple potential options. We can target the CAR-T cell against multiple cancer antigens at once， and thus make it more powerful. We can use more powerful or additional co-stimulation domains. We can use CRISPR or gene editing technology to delete immune checkpoints and thus prevent the CAR-T cell from becoming exhausted in the first place， and potentially even use allogenetic healthy CAR-T cell donors， and create T cells from healthy donors that have not been previously exposed to chemotherapy. And again， use gene editing technology to delete the T cell receptor and immunogenic proteins and use effectively now and off the shelf CAR-T cell product， which is resistant to immune exhaustion to treat a patient without the necessary turnaround time required to create an autologous CAR-T cell product.
So I think both thinking about the CAR-T cell trial designs the CAR-T cell products themselves and using combination strategies with a different additional treatments be the immune-therapy immuno-modulating agents or small molecule inhibitors will hopefully help us overcome the primary mechanism of immune evasion on T cell exhaustion， for CAR-T cell patients.