SYK inhibition abrogates the protective effect of stroma and counteracts CAM-DR in context with F-ara-A. Relative viability of CLL cells was calculated by defining the viability of the respective DMSO control (with or without stroma) as 100%. (A) CLL samples were treated with R406, F-ara-A, or the combination of R406 and F-ara-A in the presence and absence of stroma. The effect of F-ara-A (20μM) was limited in stromal coculture as indicated by the significant higher relative viability in stromal coculture (P < .01; n = 14, 8 of which were from patients who had obtained previous therapy). The combination of R406 and F-ara-A prevented the protective effect of the stromal cells toward apoptosis, resulting in no change in relative viability of the combined treatment. n.s. indicates not significant. (B) Dose-effect curves of 8 CLL samples for R406, F-ara-A, and the combination of both at a ratio of 1:2.5 without (left) and with stromal coculture. Displayed are the mean effects ± SEM with logarithmic regression. (C) Mcl-1 stained primary CLL samples (n = 8) cultured with or without stroma in the presence or absence of R406 as assessed by flow cytometry (left). Stromal contact resulted in higher Mcl-1 levels compared with CLL cells cultured alone (murine stroma, P < .001; human stroma, P < .01), whereas concomitant SYK inhibition prevented this up-regulation (murine stroma, P < .01; human stroma, P < .001). A representative immunoblot analysis is shown.