What I’m going to be talking to you today is an extension of the work that you saw Steve Grupp present. This is data from the initial phase I trial of CTL019 showing relapse free survival in this population of children and young adults with relapsed and refractory ALL. We can see that in responding patients the majority of patients will remain disease free at 12 months with a 60% relapse free survival indicating very good outcomes which you saw replicated on the global study of tisagenlecleucel. We have seen that in these patients who maintain disease response the vast majority will have persistence of their CD19 CAR T-cells and we can measure that by seeing that their normal B-cells which also express CD19 will be cleared from the circulation by the effect of the CD19 directed CAR T-cells. But for a fraction of patients shown here we can see loss of that B-cell aplasia. So when we see recovery of B-cells, that is a marker of loss of persistence of the CD19 directed CAR T-cells and we have observed a higher rate of CD19 positive relapse in that group of patients who lose their CD19 CAR T-cells prior to six months after infusion. So in this group of patients who recovered their B-cells, indicating loss of CAR T-cells, we started to think about how could we potentially improve on the persistence and surveillance for leukaemia and hopefully prevent relapse.

Our hypothesis was that T-cells upon activation may become exhausted through activation of immune checkpoint pathways and that one such pathway, PD-1, may be involved in early loss of CD19 CAR T-cells, therefore, that the combination with PD-1 checkpoint blockade may improve the function of the CAR T-cells and their persistence. So in the group of patients who had poor persistence or a partial or no response to prior CAR T-cell therapy we employed a combination approach with PD-1 blockade. With the schematic of the rationale being here, this is the tumour cell with the CD19 antigen and the CAR T-cell with a CD19 directed CAR. The T-cell activation upon this antigen engagement results in cytotoxic effect in killing of the tumour cell but what can potentially happen with T-cell activation is that there is checkpoint activation as well, either through ligand on the tumour cell or on cells in the microenvironment and that that can then bind to the checkpoint receptor on T-cells and block the effect of the T-cells.

So one potential way to block that blunting of activation is through monoclonal antibodies directed against the ligand and another method is through monoclonal antibodies binding the PD-1 receptor on the T-cell. So this is the approach we took in three groups of patients where we hypothesised that this pathway may play a role. So the first setting was in patients who had partial or no response to CD19 directed CAR T-cells where our hypothesis was that the activation of the T-cells may lead to activation of the checkpoint pathway and block a full response. In that group of patients we added PD-1 blockade and saw that we unfortunately did not see an effect in this small group where four of the four patients treated had progression of their disease. One patient this progression was marked by reduced CD19 expression which was probably the mode of escape from CD19 CAR T-cells.

But in the second setting, in patients who responded to CAR T-cells but had poor persistence marked by early B-cell recovery we saw that patients who were reinfused with a CAR T-cell product followed by infusion with PD-1 blockade, three out of six patients had a return of B-cell aplasia and sustained CR with B-cell aplasia showing continued persistence of their CAR T-cells.

Then in a third situation, this was a little bit different than the other two, in this situation we had patients who had bulk extramedullary disease and we hypothesised that the PD-1 checkpoint pathway may be activated through the microenvironment in that extramedullary situation and added PD-1 blockade for patients who did not respond in their extramedullary disease and saw that two out of four patients had a sustained CR and two out of four patients had a partial response with the addition of PD-1 blockade.

We saw that this combination resulted in few adverse events and it was well tolerated. A few patients did have fever or grade 2 CRS as well as cytopenias but overall it was tolerated very well without serious adverse events.
So, in summary, we show that PD-1 checkpoint inhibitors can be safely combined with CD19 CAR T-cell therapy and that this mechanism may be useful to improve CAR T-cell persistence. We found that in half the patients PD-1 checkpoint blockade can improve that persistence and that this patient may particularly benefit patients in that situation with poor persistence marked by early B-cell recovery as well as those with bulky extramedullary disease.