Effect of PPARα and fatty acid oxidation inhibitors on DEX-mediated cytotoxicity in vitro and in vivo. (A) CD5⁺ Daudi cells that overexpress PPARα and vector control cells were treated with DEX for 48 hours. Expression of PPARα and the activated phosphorylated GR was then determined by immunoblotting and quantified by densitometry, using β-actin as a loading control. (B) Percentages of viable Daudi cells that excluded 7AAD were determined by flow cytometry after 48 hours. Despite strong GR activation, DEX-treated PPARα^hi cells were resistant to DEX. (C) (Upper) CLL cells from the indicated numbers of patient samples were cultured in the presence or absence of DEX with or without the indicated concentrations of the PPARα antagonist MK886 or the fatty acid oxidation inhibitor CVT-4325. After 48 hours, specific death was determined by the differences of the percentages of viable 7AAD⁻ cells in control and treated cultures measured by flow cytometry. Averages and standard errors of the results for each inhibitor are shown. (Lower) CIs were obtained by treating with MK886 (0.6, 1.2, and 6 μM) or CVT-4325 (0.2, 0.6, 2, and 6 μM) together with DEX (30 μM) and entering the resulting specific death values into the CompuSyn program. Fraction affected (FA)-CI plots are shown for a single patient sample and indicate that the combinations of MK886 or CVT-4325 with DEX are synergistic (CI < 1). Similar results were obtained with 4 other patient samples. (D) NOD-SCIDγ_c^null mice engrafted 6 weeks earlier with CLL splenocytes were treated with 4 consecutive injections of MK886 (10 mg/kg), DEX (4.6 mg/kg), or both MK886 and DEX. Four days after the last injection, human CD5⁺CD19⁺ CLL cells were measured in peritoneal cavities by flow cytometry. The graphs represent the results from 2 separate experiments with spleen cells from different patients. **P < .01; *P < .05.