Kinetics of degradation differ between drugs. (A) Levels of IKZF3, ZFP91, ZNF692, CK1α, RNF166, and GSPT2 protein as measured by the iMRM assay following treatment of MM1S cells for the indicated period of time with vehicle (orange), 10 µM thalidomide (dark green), 1 µM lenalidomide (red), 1 µM pomalidomide (blue), 1 µM avadomide (light green), or 0.1 µM CC-885 (purple). Protein levels were normalized to housekeeping proteins (β-actin and GAPDH) and the level at the start of the experiment was normalized to 1. Data are mean ± SEM. Curves represent the nonlinear regression (n ≥ 3 replicates). (B) Levels of CK1α and IKZF1 protein as measured by the iMRM assay in MM1S (red) and KG-1 (blue) cells after treatment with 1 µM lenalidomide. Curves represent the nonlinear regression. n ≥ 3 replicates; P values are from a sum of squares F test. NS, nonsignificant (corrected for multiple hypothesis testing).