A safety switch based on inducible caspase 9 (iCasp9) was developed to enhance the safety of adoptive cell therapy. The iCasp9 system uses a modified FK-binding protein fused to caspase 9, which becomes activated by a dimerizing drug, AP1903, leading to rapid cell death. This system was tested in five patients who received genetically modified T cells following haploidentical stem-cell transplantation for relapsed leukemia. The T cells were detected in peripheral blood and increased over time. When GVHD developed, patients received a single dose of AP1903, which eliminated over 90% of the modified T cells within 30 minutes, effectively stopping GVHD without recurrence. The iCasp9 system allows for rapid and controlled cell death, reducing the risk of adverse effects. The study demonstrated that the system is safe and effective, with no significant immune response against the transduced cells. The iCasp9-based safety switch offers advantages over traditional suicide genes, including reduced immunogenicity and faster cell death. However, further research is needed to evaluate its long-term safety and efficacy in broader clinical applications. The study highlights the potential of inducible apoptosis as a safe and effective method for managing adverse events in cellular therapies.A safety switch based on inducible caspase 9 (iCasp9) was developed to enhance the safety of adoptive cell therapy. The iCasp9 system uses a modified FK-binding protein fused to caspase 9, which becomes activated by a dimerizing drug, AP1903, leading to rapid cell death. This system was tested in five patients who received genetically modified T cells following haploidentical stem-cell transplantation for relapsed leukemia. The T cells were detected in peripheral blood and increased over time. When GVHD developed, patients received a single dose of AP1903, which eliminated over 90% of the modified T cells within 30 minutes, effectively stopping GVHD without recurrence. The iCasp9 system allows for rapid and controlled cell death, reducing the risk of adverse effects. The study demonstrated that the system is safe and effective, with no significant immune response against the transduced cells. The iCasp9-based safety switch offers advantages over traditional suicide genes, including reduced immunogenicity and faster cell death. However, further research is needed to evaluate its long-term safety and efficacy in broader clinical applications. The study highlights the potential of inducible apoptosis as a safe and effective method for managing adverse events in cellular therapies.