Differences in the pharmacokinetics and mechanisms of action between cytotoxic chemotherapy and adoptive T cell therapies have important implications for phase I clinical trials and determination of therapeutic range. (a) Patient-to-patient variation in drug levels (orange line) is small for chemotherapeutics. Drug exposure (the area under the curve) is predictable and, in the case of toxicity, can be terminated by stopping drug dosing (each dose is indicated by an arrowhead). The efficacy of chemotherapy derives from the greater sensitivity of tumors than vital normal tissues to the cytotoxic actions of the drug. This difference in tissue sensitivity creates a therapeutic window (gray shaded area) between the minimum effective dose (MED) (the dose at which tumor regression occurs) and the maximum tolerated dose (MTD) (the dose at which intolerable toxicities occur) of the drug. (b-c) Because engineered T cells used in ACT are produced on a patient-to-patient basis their pharmacokinetics fluctuate substantially (blue shaded area). Given this variability, it is not clear that the MTD from one cohort of patients can be broadly applied to others. Furthermore, safe phase I testing is challenging because infused T cells do not degrade predictably but rather – unlike any other type of cancer treatment – increase in quantity (due to T cell proliferation) and can persist indefinitely (due to formation of long-lived memory T cell populations) following infusion. Therefore, in the case of adverse events, exposure to T cell therapies cannot be terminated reliably by the standard approach of stopping drug dosing ,. (b) A therapeutic window (red shaded area) in which durable regression of high-tumor-burden disease occurs without debilitating autoimmunity has thus far not been identified for any cellular therapy directed against antigens expressed by tumors and essential healthy tissues–,,. (c) Targeting an antigen that is not expressed by healthy tissue opens the therapeutic window (gray shaded area, by increasing the MTD) for ACT because direct cytotoxicity to normal tissues does not occur, even at high doses of cells.