MEASUREMENT OF THE REACTION BETWEEN THE FUNGICIDES CAPTAN OR FOLPET AND BLOOD THIOLS

Abstract

Captan and folpet are two protectant fungicides that react with thiols. Current dermal risk assessment paradigms consider that 100% of the absorbed material is available systemically. Both fungicides are associated with duodenal tumors in mice after dietary administration and NOELs for effects leading to tumor formation have been established at approximately 400 ppm (60 mg/kg/day). Thus, it is important to quantify systemic exposure so as to establish a meaningful risk characterization. This study simultaneously mixed 14 C-captan and 14 C-folpet with blood at approximately 1 μg/mL (1 ppm) at 37°C, which served to refine earlier pre-GLP measurements made with nonradiolabeled material at 22°C. A series of experimental procedures was employed that progressively refined the ability to arrest and measure the chemical reaction from 0 to 31.1 s. Acetone (10 μL) containing 14 C-captan and 14 C-folpet, both at approximately 100 μg/mL, was delivered into 1 mL human blood contained in a 3-mL Hamilton syringe. The final captan concentration was 0.94 μg/mL and that of folpet was 1.06 μg/mL. The reaction was stopped by expressing the blood mixture into a centrifuge tube that contained 5 mL acetone and 80 μL 7.3 M phosphoric acid. Reaction vessels were placed on a vortex mixer at high speed. Unlabeled captan, folpet, (THPI; captan's main ring degradate), and (PI; folpet's main ring degradate) were contained in the acetone and served as ultraviolet markers during (HPLC). Extracts were passed through a Florisil silica gel column and analyzed by HPLC. Radioactivity in HPLC fractions was measured by liquid scintillation. The chemical reaction with thiols followed the first-order rate equation A t = A 0 e- kt, with rate kinetic constants of 0.718 (captan) and 0.141 (folpet). The resulting half-lives in blood were 0.97 s for captan and 4.9 s for folpet. Mass spectrometry confirmed that the respective imide rings (THPI and PI) were the only degradation products. Risk assessments that ignore this rapid degradation do not accurately estimate systemic exposure and are unnecessarily conservative.