Comparative SL profile analysis upon drug withdrawal. R10 cells were incubated in 4-HPR-free medium for 48 h (WD = without drug) or long-term (~3 weeks) and endogenous SL profiles analyzed by LC-MS. Significance was based on R10 values. Data are average ± SD of 2-4 independent values. *P < 0.05; **P < 0.01; ANOVA plus Tamhane or Bonferroni post-hoc test.