Zinc-related actions of sublethal levels of benzalkonium chloride: Potentiation of benzalkonium cytotoxicity by zinc

- Zn2+-related cytotoxic actions of benzalkonium (BZK) were examined in rat thymocytes.
- BZK at sublethal levels increased intracellular Zn2+ concentration.
- Low micromolar levels of Zn2+ potentiated the cytotoxicity of sublethal levels of BZK.
- Co-treatment with Zn2+ and BZK accelerated an early process of apoptosis.
- Zn2+ is one of determinants of the cytotoxicity of BZK.

Benzalkonium chloride (BZK) is a common preservative used in pharmaceutical and personal care products. ZnCl2 was recently reported to significantly potentiate the cytotoxicity of some biocidal compounds. In the present study, therefore, we compared the cytotoxic potency of BZK and then further studied the Zn2+-related actions of the most cytotoxic agent among BZK, using flow cytometric techniques with appropriate fluorescent probes in rat thymocytes. Cytotoxicity of benzylcetyldimethylammonium (BZK-C16) was more potent that those of benzyldodecyldimethylammonium and benzyldimethyltetradecylammonium. ZnCl2 (1-10 μM) significantly potentiated the cytotoxicity of BZK-C16 at a sublethal concentration (1 μM). The co-treatment of cells with 3 μM ZnCl2 and 1 μM BZK-C16 increased the population of both living cells with phosphatidylserine exposed on membrane surfaces and dead cells. BZK-C16 at 0.3-1.0 μM elevated intracellular Zn2+ levels by increasing Zn2+ influx, and augmented the cytotoxicity of 100 μM H2O2. Zn2+ is concluded to facilitate the toxicity of BZK. We suggest that the toxicity of BZK is determined after taking extracellular (plasma) and/or environmental Zn2+ levels into account.