FLT3 inhibitors, it may be surmised, uniformly bind the inactive receptor with the highest affinity. the relapsed patients. FL levels rose even Tcfec higher with successive courses of chemotherapy, to a mean of 3251 pg/mL after the fourth course. In vitro, exogenous FL at concentrations similar to those observed in patients mitigated FLT3 inhibition and cytotoxicity for each of 5 different FLT3 inhibitors (lestaurtinib, midostaurin, sorafenib, KW-2449, and AC220). The dramatic increase in FL level after chemotherapy represents a possible obstacle to inhibiting FLT3 in this clinical setting. These findings could have important implications regarding the design and outcome of trials of FLT3 inhibitors and furthermore suggest a rationale for targeting FL as a therapeutic strategy. Introduction Acute myeloid leukemia (AML) patients who harbor the FLT3/ITD mutation have an exceptionally poor prognosis.1,2 During Benzyl benzoate the past decade, efforts have been underway to develop FLT3 inhibitors in the hopes of improving outcomes for these patients.3 Several agents have now been studied as monotherapy for this disease, and the results have been only modestly successful. Although there have been a few remissions reported, the responses are usually limited to clearance of peripheral blasts, with persistence of disease in the marrow. In light of this, attention has turned to incorporating these brokers into existing chemotherapy regimens, around the hypothesis that FLT3 inhibition will synergize with chemotherapy in inducing cytotoxicity.4 Alternately, others have postulated that mobilizing the leukemia cells from Benzyl benzoate the marrow might enhance the efficacy of FLT3 inhibition and have therefore tested FLT3 inhibitors in combination with CXCR4 inhibition.5 Several large trials of chemotherapy administered in combination with FLT3 inhibitors are either actively accruing or have recently completed accrual. Because chemotherapy may alter the pharmacokinetics of FLT3 inhibitors and therefore affect target inhibition in vivo, we examined FLT3 inhibition in patients receiving lestaurtinib, an indolocarbazole FLT3 inhibitor, from 2 individual trials in which the agent was combined with chemotherapy. We noted a discrepancy in the degree of FLT3 inhibition, as measured by a plasma inhibitory activity assay, between the 2 groups of patients. We hypothesized that high levels of FLT3 ligand (FL), a cytokine that is known to increase after Benzyl benzoate myelosuppressive therapy, could be interfering with FLT3 inhibition in these trials. We have therefore examined FL levels in response to chemotherapy and FLT3 inhibition, as well as the effect of FL levels on the efficacy of FLT3 inhibitors in vitro and in vivo. Our findings may explain why blasts in the bone marrow are more resistant to FLT3 inhibitors and furthermore have important implications both for FLT3 mutant AML as a disease as well as for efforts to incorporate FLT3 inhibitors into AML therapy. Methods Clinical trials Plasma samples from 4 individual clinical trials of FLT3 inhibitors were used in this study. The Cephalon 204 trial was a randomized trial of lestaurtinib administered in sequence with chemotherapy for AML patients with FLT3 activating mutations in first relapse.6 Chemotherapy consisted of MEC (mitoxantrone, etoposide, and cytarabine) or high-dose cytarabine. There were 123 total plasma samples from 72 patients around the Cephalon 204 trial available for FLT3 ligand analysis, with corresponding lestaurtinib drug levels on all of them. The MRC AML15 trial was a randomized trial of lestaurtinib administered in sequence with chemotherapy (cytarabine, daunorubicin, and etoposide) for newly diagnosed AML patients with FLT3 activating mutations. Patients on AML15 receive additional cycles of chemotherapy, each followed by lestaurtinib, for a total Benzyl benzoate of 4 courses. The chemotherapy regimens have been previously published.7 A total of 155 plasma samples from 69 patients around the AML15 trial were available for analysis of FL levels (62 from course 1, 43 from course 2, 30 from course 3, and 20 from course 4), but corresponding lestaurtinib drug levels on samples from only 18 of these patients. The CP00001 trial is usually a phase 1 dose-escalation trial of AC220 in relapsed or refractory AML patients. 8 Johns Hopkins Protocol J0509 was an open label dose-escalation study of sorafenib for relapsed or refractory AML patients.9 Patient samples (plasma and blasts) Leukemia cell specimens, leukemia plasma samples, and normal donor plasma samples were provided by the Sidney.