Antifolate pseudo-resistance due to elevated levels of thymidine and hypoxanthine in a commercial serum preparation.

BACKGROUND: Batch variability of sera used for cell culture is of considerable experimental concern. A novel fetal calf serum product, FCS Gold, was claimed to be the first defined fetal calf serum free of batch variation. MATERIALS AND METHODS: The efficacy of methotrexate (MTX) and LY231514 (multitargeted antifolate, MTA) in CCRF-CEM cells and KB cells was compared using media supplemented with FCS Gold or conventional fetal bovine serum. RESULTS: IC50 values from tests using conventional serum corresponded to published data. FCS Gold fully protected the cells from antifolate drug cytotoxicity. Dialysis of FCS Gold restored responsiveness to antifolate drugs. Elevated levels of hypoxanthine and thymidine were present in FCS Gold. They were approximately 10-fold greater than the concentrations required to overcome growth arrest mediated by 2 microM MTX. CONCLUSION: FCS Gold or identical products, e.g. FBS Gold, should not be used in studies on antifolate drug action.