Abstract 707: Personalization of dexamethasone in acute lymphoblastic leukemia

Synthetic glucocorticoids, such as dexamethasone (Dex), are pivotal in the treatment of childhood acute lymphoblastic leukemia (ALL) but are associated with significant variability, both in terms of toxicity and efficacy. We aimed to investigate three key variables to better understand how Dex personalization may be achieved: pharmacokinetics (PK), intracellular Dex accumulation, and cellular response, following Dex binding to the glucocorticoid receptor (GR) in ALL cells. For Dex PK studies, blood samples were collected post oral administration on one of the first three days of induction chemotherapy in 99 patients on the UKALL 2011 trial receiving either 6mg/m2 for28 days (standard arm) or 10mg/m2 for 14 days (short arm). Plasma Dex levels were analysed using a validated LC/MS method, and a non-compartmental pharmacokinetic analysis. To assess intracellular Dex levels, cell lines, primagraft (n=9) and primary patient samples (n=6) were studied. The plasma Dex LC/MS method was optimized to quantify Dex in ALL cell lysates. Dex accumulation was also assessed using flow cytometric analysis of Dex conjugated to FITC. Cellular Dex sensitivity was assessed using Alamar Blue assays. There was a wide Dex PK variability, with AUC0-12h, and Cmax significantly higher on the short compared to the standard arm; 564 (202-1606) versus 408 (142-1009), median (range), p=0.0003 and 0.0006, respectively. However there was substantial overlap between the two arms, with a number of patients on the standard arm exhibiting higher exposures than those on short therapy. Dex GI50 values ranged from 37 - > 1000 nM and 2 - > 1000 nM in cell lines and patient samples respectively. Western blotting indicated wildtype GR in all samples, with R3D11 and REH cell lines serving as hemizygous deleted and GR negative controls. Dex accumulation in cell lines was 2.1 and 1.8 (range 1.2 - 2.1) pmol/106 cells in PreB697 and Dex resistant sub-lines, respectively. While patient samples exhibited greater variability, Dex accumulation was not significantly different between sensitive and resistant cells; mean of 1.0 versus 1.4 (range 0.1-2.3, 0.4-4.4 pmol/ 106 cells, p=0.17). Flow cytometry Dex FITC accumulation confirmed these data, with a mean fluorescence intensity of 4.2 versus 4.1 (range 1.5-5.9, 2.0 - 9.1, respectively; p=0.97). These data suggest that while PK and cellular response are hugely variable, variations in drug accumulation do not appear to play a key role in Dex response in ALL ce