Dasatinib Once-Daily Dose Regimen Provides Robust Anti-Leukemic Activity While Avoiding Suppression of T Cell Activation.
Authors: Schieven Gary L , Zhang Rosemary , Pitt Sidney , McGlinchey Kelly , Menard Krista , Smykla Richard , Susulic Vojkan , Wen Mei-Li , Wiebesiek Amy , Townsend Robert , Lee Francis Y. 
Published: October 2019

Abstract
Abstract 2198
Poster Board II-175
Dasatinib (SPRYCEL®), a small molecule tyrosine kinase inhibitor is 325-fold more potent against BCR-ABL than imatinib. Targeting BCR-ABL in chronic myeloid leukemia (CML), dasatinib offers the most favorable benefit-risk ratio with the dose regimen of 100-mg once daily (in comparison with 3 other treatment arms: 50 mg BID, 140 mg QD and 70 mg BID. Duration of cytogenetic response and progression-free survival were similar across all 4 arms, but there was significantly less frequent grade 3-4 neutropenia, thrombocytopenia, anemia and pleural effusion in the 100-mg QD arm compared to the other 3 arms combined (Shah et al, J Clin Oncol 26:3204, 2008). The undiminished efficacy of once daily dosing occurs despite the fact that orally administered dasatinib has a pronounced peak-to-trough plasma pharmacokinetics profile and a relatively short half-life (∼3-5 h), which allows for complete recovery of BCR-ABL kinase activity within 8-12 hrs after a daily dose. The clinical efficacy of once daily dasatinib supports the notion that continuous target (BCR-ABL) inhibition is not required for anti-leukemic activity although the truncated exposure may help to ameliorate other side-effects of “on” or “off” targets inhibition. In addition to BCR-ABL, dasatinib potently inhibits SRC-family kinases (SFKs) with an IC50 in the low single digit nM range; SFKs play key roles in T cell activation. Based on continuous exposure studies, it had been suggested that dasatinib may act as an immunomodulator in vivo. We investigated the effects of variation in exposure duration in vivo and in vitro on the anti-leukemic and T cell activation inhibition activities of dasatinib in preclinical models.

Methods.
Anti-leukemic activity was determined in vitro in K562 and KU812 CML cell lines, and in vivo as xenografts in K562. BCR-ABL kinase activity was monitored with a phospho-specific CRKL antibody. Human T cells were isolated from PBMC by rosetting with sheep red blood cells and stimulated with anti-CD3/CD28 antibodies for 48 h. Cytokines production were measured by ELISA. T cell proliferation was determined at 3 days by 3H-thymidine incorporation. Immunocompetency of dasatinib treated mice were determined using the mouse cardiac allograft model and the in vivo MLR T cell proliferation model.

Results.
Transient (1-6 h) dasatinib exposure of CML cells that caused >80% inhibition of phospho-CRKL is highly cytotoxic. Degree of cytotoxicity directly correlates with the magnitude of BCR-ABL kinase inhibition. In vivo single dose of 30 mg/kg dasatinib administered IV was highly cytotoxic to K562 xenografts as determined by in vivo-in vitro colony formation assay. Intermittent IV dosing regimens of dasatinib (Q4D or Q7D) were effective against K562 xenografts. Dosing regimen in mice (5 mg/kg, PO) that closely mimic the pharmacokinetics of 100 mg oral dose in human was equally efficacious as administering the same dose in 2 split doses (BID, 2.5 mg/kg, PO). In terms of effects on T cell activation, a linear relationship was observed between serum concentration and in vitro T cell proliferation IC50 values. Modeling the human PK profile, delayed dasatinib treatment of T cells after T cell stimulation in vitro led to a time dependent decrease in potency as measured by both IC50 and Emax values. Comparison of serum adjusted IC50 values from these studies to the human PK profile suggests that dasatinib at the approved 100-mg once-daily dose would permit T cell activation on a daily basis. A similar pattern was observed in preclinical in vivo models. Dasatinib was found to be completely protective in a mouse model of cardiac allograft rejection at a dose of 25 mg BID, whereas a dose of 15 mg BID was not protective. In the MLR model, dasatinib inhibits T cell proliferation at 50 mg/kg but at the clinically relevant dose of 5 mg/kg was completely devoid of T cell inhibitory effects. Taken together, these results suggest that dasatinib may be able to provide anti-leukemic activity while avoiding suppression of T cell activation at clinically relevant doses.

Disclosures:
Schieven: Bristol-Myers Squibb Co: Employment. Zhang:Bristol-Myers Squibb Co: Employment. Pitt:Bristol-Myers Squibb Co: Employment. McGlinchey:Bristol-Myers Squibb Co: Employment. Menard:Bristol-Myers Squibb Co: Employment. Smykla:Bristol-Myers Squibb Co: Employment. Susulic:Bristol-Myers Squibb Co: Employment. Wen:Bristol-Myers Squibb Co: Employment. Wiebesiek:Bristol-Myers Squibb Co: Employment. Townsend:Bristol-Myers Squibb Co: Employment. Lee:Bristol-Myers Squibb Co: Employment.