Edatrexate (10-ethyl-deazaaminopterin) is a methotrexate analog that has been shown to have greater antitumor activity and improved therapeutic index compared to its parent compound in preclinical systems. We have evaluated the ability of edatrexate to modulate the intrinsic resistance of the lung adenocarcinoma A549 cell line to carboplatin. Concentration effects, exposure time and schedule dependence were assessed. Modulation of resistance was observed with edatrexate treatment (0.2 microM for 1 h) prior to carboplatin. The concentrations of carboplatin to achieve IC50 at the 1-, 3-, and 24-h IC50 were decreased by a mean of 16.8 times (12.2-22.2) with edatrexate preexposure. In contrast, there was little modulation observed of carboplatin resistance when carboplatin was administered prior to edatrexate. In addition, schedule dependency experiments were performed using the method described by Chou and Talalay, in which the ratio of carboplatin to edatrexate was constant or nonconstant, and both the potency of effects and the shapes of the concentration-effect curves were taken into account in a computerized analysis. These experiments also demonstrated schedule dependency. Although both treatments resulted in a reduced IC50 vs. carboplatin alone, the reduction was much greater when edatrexate was added first (12.59 vs. 2.59 times). We conclude that the combination of edatrexate and carboplatin demonstrates schedule-dependent modulation of intrinsic carboplatin resistance in this in vitro model at clinically achievable edatrexate plasma levels (0.01 to 10 microM). The greatest modulatory synergism was observed in the setting of edatrexate treatment before carboplatin. Our findings suggest a potentially useful schedule when combining edatrexate and carboplatin for the treatment of malignant disease.