Interesting possibilities to improve the safety of ipilimumab.

Abstract

e13102 Background: The problem of the ipilimumab therapy is that a common molecular target (CTLA-4 receptor) is expressed on the targeted and non-targeted T cells, respectively. We suggest that the frequency of immune-related adverse events can be reduced by treating conditions in which the number of the targeted cells is larger than the non-targeted ones and/or by improving the accuracy of the targeting. Methods: The first approach was tested in a phase I clinical trial in patients with relapsed malignancy following allogeneic hematopoietic stem cell transplantation. An ipilimumab blockade was used against CTLA-4 positive allogeneic T cells (responding patients had greater than 80% donor T-cell chimerism). The graft-versus-malignancy (GVM) effects were augmented without a significant impact on graft-versus-host disease (GVHD). A feasible approach could also be pretargeting that has been worked out for radioimmunodetection and radioimmunotherapy. To this end we suggest a testable thought experiment for the treatment of chronic lymphocytic leukemia (CLL). First a streptavidin (StAv) modified anti-CD19 mAb is administered, which is followed by the subsequent delivery of biotin labeled anti-CTLA-4 mAb. The latter endows T cells with the ability to travel to tumor sites without prematurely succumbing to apoptosis, while streptavidin’s ultra-high affinity for biotin (10-15M) ensures a tight bond to the biotin labeled anti-CTLA-4. Results: Using the law of mass action we calculated that above 1 mg/L ipilimumab concentration (i.e. about 5 mg/patient ~70kg) more than 80% of anti-CD19-StAv sensitized B cells will have been bound to anti-CTLA-4-biotin sensitized T cells. Conclusions: This way, the forces of the immune system liberated by the anti-CTLA-4 antibody blockade could be focused with laser sharp accuracy on CLL cells without collateral damage to normal host cells. One should note that 0.3 mg/kg ipilimumab alone already caused mild autoimmunity.