Comparative cytotoxicity of alachlor, acetochlor, and metolachlor herbicides in isolated rat and cryopreserved human hepatocytes.

Noncancerous adverse effects observed at the lowest dose for chloroacetanilide herbicides alachlor [2-chloro-2',6'-diethyl-N-(methoxymethyl)-acetanilide] and acetochlor [2-chloro-2'-methyl-6'-ethyl-N-(ethoxymethyl)acetanilide], but not metolachlor [2-chloro-2'-ethyl-6'-methyl-N-(1-methyl-2-methoxymethyl)acetanilide], are hepatotoxicity in rats and dogs. Liver microsomal N-dealkylation, a step in the putative activating pathway, of acetochlor exceeds that of alachlor and is negligible for metolachlor. In the present investigation, cytotoxicity of the three chloroacetanilides was ranked using isolated rat and cryopreserved human hepatocytes to correlate this endpoint with CYP3A-dependent metabolism. Chloroacetanilide cytotoxicity in rat hepatocyte suspensions was time dependent (e.g., LC(50 - alachlor/2 h) vs. LC(50 - alachlor/4 h) = 765 vs. 325 muM). Alachlor and acetochlor were more potent than metolachlor after 2 and 4 h, times when N-dealkylated alachlor product 2-chloro-N-(2,6-diethylphenyl)acetamide (CDEPA) formation was readily detectable. Alachlor and acetochlor potencies with cryopreserved human hepatocytes at 2 h were comparable to freshly isolated rat hepatocytes, and alachlor metabolism to CDEPA was likewise detectable. Unlike rat hepatocytes, metolachlor potency was equivalent to acetochlor and alachlor in human hepatocytes. Furthermore, chloroacetanilide cytotoxicity from two sources of human hepatocytes varied inversely with CYP3A4 activity. Collectively, while cytotoxicity in rat hepatocytes was consistent with chloroacetanilide activation by CYP3A, an activating role for CYP3A4 was not supported with human hepatocytes.