New technology can potentially overcome CAR T-cell limitations
Dr Philip Low - Purdue University, West Lafayette, USA
What we’re doing is building on the shoulders of giants in this field. There has been a tremendous amount of terrific work aimed at harnessing the power of T-cell killing and targeting that to cancer cells to eliminate tumour tissue. What we have done is looked at a few of the limitations that still exist with this terrific technology and we’ve tried to solve them, those limitations. One of the limitations is the inability to control the rate of tumour lysis, in some cases it occurs so rapidly that a tumour lysis syndrome ensues or a cytokine storm arises and this activates the patient’s immune system systemically leading to often very serious side effects and even death and we’ve discovered a way to control this.
A second limitation that we have undertaken to address is the inability to terminate the tumour lysing response after the malignant mass has been entirely eliminated. We have used our same technology to allow this rapid termination of the response. The third limitation that we have addressed is the inability of the current CAR T-cell technology to kill tumours where they’re antigenically very heterogeneous. That is right now the CAR T-cells are designed with an antibody on the outside of the engineered T-cell surface that recognises a tumour specific antigen. If the tumour mutates, and tumours constantly mutate, they have very unstable genomes, so they often will mutate not to express that particular antigen in which case the engineered CAR T-cell will not recognise or kill that particular clone of T-cells. After you kill those that you can kill, those that you cannot kill then take over and recur. So we have developed the capability to use one single engineered chimeric antigen receptor T-cell to kill essentially virtually all tumour cells, regardless of their antigenic specificity.
So these three things can improve the already terrific CAR T-cell technology that’s been developed by many others.
What was your method?
What we have done is we have taken the standard CAR T-cell technology and instead of using an antibody on the outside surface to recognise the tumour specific antigen we have attached an antibody to fluorescein which is a simple yellow dye. We have a very high affinity femtomolar antibody that recognises fluorescein. Now, that doesn’t do any good because that’s not going to enable it to bind to a cancer cell so what we do is we link fluorescein to another small molecular weight ligand that is very specific for a subset of cancers. We form a bridge between fluorescein, usually a couple or three atoms, and this tumour specific ligand so the two then enable, the fluorescein binds to the engineered T-cell, the tumour specific ligand binds to the cancer cell surface and the bridging molecule then forces the docking of the two cell types together. Once one docks the T-cell to the cancer cell that triggers the killing response that eliminates the tumour.
Now, how do we solve our three limitations? First of all just by administering the right concentration at the right rate of this bridging molecule we can control the rate of tumour lysis and cytokine release. So just controlling the rate of administration solves that problem. How do we terminate the response when the tumour has been eliminated? We simply discontinue administration of the bridging molecule. So that then no longer allows the residual CAR T-cells to attack any other cells, the healthy cells that may express the same tumour specific antigen. Then how do we address the heterogeneity in the tumour? We actually have tumour specific ligands for, I would say, over 95% of human tumours. So we make a cocktail, the fluorescein linked to, let’s say, a ligand that targets all GI tract cancers and then we make fluorescein link to a ligand that targets another set of cancers and fluorescein linked to a ligand that targets another set of cancers. We have found that four of these fluorescein ligand conjugates in a cocktail can bind and force the contact between the engineered CAR T-cell and virtually every cancer clone that we have been able to identify. There may be some that will still escape our ability to kill but nevertheless the capability is there to use a single engineered CAR T-cell with these diversity of adapter molecules, small molecular weight adapter molecules that are very easy to synthesise and are very stable during storage, use these adapters to kill essentially all tumour cells in the body.
How does it work?
What will happen is when you inject this the tumour specific ligand will bind to all the tumour cells and paint the tumour cells yellow with this very high affinity [?? 6:03] that the antibody on the CAR T-cell recognises. By derivatising all tumour cells with this paint and leaving the healthy cells unmarked you have this specificity that allows tumour cell specific killing.
It must make for some very interesting looking scans.
Let me tell you, it’s not part of this particular technology but the same strategy is being used to assist surgeons in removal of all malignant disease during surgery. That is in human clinical trials, the surgeons inject the tumour targeted fluorescein very shortly before surgery, they go in and the way the clinical trials have been set up to date, they remove all the malignant disease they can see, he or she can see with the naked eye and then they turn on the fluorescent lamp. To date they’re finding that they can remove five times more malignant lesions with the aid of our tumour targeted fluorescent dye than without it. Virtually 100% of the fluorescent lesions have turned out to be cancer so it really enables… We believe this is going to revolutionise the whole field of cancer surgery. But the same technology works to target the CAR T-cell to these same cancer cells so they can clean up afterwards.
Are there currently any clinical trials?
No, we haven’t started the CAR T-cell clinical trials but, again, we have used the same bridging molecule for fluorescence guided surgery in human clinical trials. Those are being done at the Mayo Clinic, at the University of Pennsylvania Medical Centre, the University of Leiden in Holland, the University of Groningen in Holland, Moffat Cancer Centre, Indiana University near us, a number of places. It’s working out very well, to date we have a sensitivity of 98% and a positive predictive value or specificity of 96% which is phenomenal in the clinical trials finished to date. So it’s encouraging technology, both of them are.
What’s the next step?
Yes, we’re going to move as quickly as possible to bring the T-cell technology, again using basically the engineered characteristics of the current CAR T-cells except we’re placing fluorescein, an anti-fluorescein, SCFV, on the outside for recognition of our painted cancer cells. All of this was done by a graduate student, Yong Gu Lee, who is standing over there and he should be here, not me, because he did all of the work.