Impact of isothiazolinone biocides on gonadal steroid production: Focus on 3β-hydroxysteroid dehydrogenase inhibition

Isothiazolinone biocides have been widely used as disinfectants during the COVID-19 pandemic. However, their effects on gonadal steroidogenesis remain unclear. This study investigated the endocrine-disrupting potential of isothiazolinone biocides, focusing on their inhibition of gonadal 3β-hydroxysteroid dehydrogenases (3β-HSDs). Enzymatic assays, molecular docking, and 3D-QSAR analyses evaluated the impact of seven isothiazolinones on human KGN cell 3β-HSD2 (h3β-HSD2) and rat testicular 3β-HSD1 (r3β-HSD1). Dichlorooctylisothiazolinone, butylbenzo[d]isothiazol-3(2 H)-one, octylisothiazolinone, and benzisothiazolinone inhibited h3β-HSD2 with IC₅₀ values of 4.02, 15.35, 12.12, and 30.55 μM, respectively. These compounds also inhibited r3β-HSD1 with IC₅₀ values of 1.67, 16.51, 13.25, and 29.28 μM, respectively. Whereas, methylchloroisothiazolinone selectively inhibited r3β-HSD1 with IC₅₀ value of 40.11 μM. The compounds acted as mixed/noncompetitive inhibitors, targeting the NAD⁺ binding pocket. Inhibitory potency correlated with compound hydrophobicity, heavy atom count, and molecular weight. A 3D-QSAR pharmacophore model achieved high predictive accuracy (R² = 0.98) for h3β-HSD2 inhibition. These biocides suppressed progesterone production in human KGN granulosa tumor cells. The binding free energies were ranged from -6.52 to -7.19 kcal/mol for h3β-HSD2 and -6.0 to -7.04 kcal/mol for r3β-HSD1. The results underscore the endocrine-disrupting capabilities of isothiazolinones via inhibition of critical steroidogenic enzymes, offering insights into their structure-activity relationships and potential side effects.