Metabolism of 8-Cl-cAMP to 8-Cl-ADO: effect of IBMX and serum-free media on extracellular drug metabolism. A, Cells in 96-well culture plates were incubated with increasing concentrations (0–30 μm) of 8-Cl-ADO, 8-Cl-cAMP, or 8-Cl-cAMP plus IBMX (50 μm) in media containing 10% FBS (Hyclone). Cells were also incubated with 8-Cl-cAMP in serum-free medium. All cultures were incubated for 4 d. B, List of IC50 values from [3H]thymidine and MTS assays. C, Cells in 96-well culture plates were incubated (4 d) with increasing concentrations of 8-Cl-cAMP in media containing serum from GIBCO/Invitrogen or from Hyclone. D, Cells in 96-well culture plates were incubated (4 d) with increasing concentrations of IBMX (0–800 μm) and/or 8-Cl-cAMP (25 μm). In A, C, and D, Cell Titer 96 AQ solution was added for 3.5 h before plates were read using an ELISA plate reader at 460 nm. Results are expressed as percentage of control absorbance and are mean ± sem of six experiments (A) and mean ± sd of two experiments (C and D). *, P = 0.01; **, P < 0.0001 (A).