Methotrexate (MTX), a structural analogue of folic acid and a known developmental toxicant, is used clinically as a chemotherapeutic agent and as a treatment for psoriasis and rheumatoid arthritis. MTX inhibits dihydrofolate reductase, preventing the formation of folinic acid (FA; tetrahydrofolate) and stopping one carbon metabolism needed for the synthesis of DNA in rapidly dividing cells. To determine if exogenous FA could mitigate the developmental toxicity of MTX in developing fruit flies, the effect of co-administration of FA on the incidence of wing notches induced by MTX was evaluated. Drosophila were exposed throughout development (egg through third instar larva) in culture vials to medium containing 10 ug/vial MTX with or without FA. Each vial contained 1g of powdered medium and 5ml of distilled deionized water or a solution of test chemical in water. A mated, untreated, Oregon- R wild-type female (Carolina Biological Supply Co.) was added to each culture vial and allowed to oviposit for 20 hours, then removed. Emerging offspring were collected over 10 days, and examined microscopically (25 X) for wing blade notches, a morphological defect shown to occur with an increased incidence in flies exposed to developmental toxicants. The incidence of wing notches was statistically increased (64/137; p<0.001) by MTX alone and by MTX plus 10 ug/vial FA (103/208; p<0.001). However, no wing notches were found in flies treated with MTX plus 50 ug/vial FA (0/271) nor in the concurrent control (0/218). Mortality was significantly decreased (p<0.001) in both groups of flies treated with MTX plus FA compared to the MTX alone group. These results parallel the protective effects of folinic acid analogues on the developmental toxicity of MTX reported in mammals. Furthermore, they demonstrate an addition capability of this Drosophila-based assay to investigate mechanisms of developmental toxicity in addition to its proposed role as a prescreen.
The Toxicologist. Society of Toxicology 37th Annual Meeting, March 1-5,1998, Seattle, Washington