In this study, immortalized human oral cells (oral epidermoid carcinoma cell line KB) was used as an experimental model to assess the cytotoxicity of exposure to cigarette smoke condensates (CSCs) prepared under ISO and HCI smoking regimens. The dose of the total particulate matter exposed to the cells ranged from 0 to 300 µg/mL, and the MTT tests and BrdU (5-bromo-2-deoxyuridine) incorporation tests were conducted after the cells were exposed to the CSCs for 24 h.

The results indicated that oral cell in vitro proliferation can be inhibited after exposed to CSCs, and exposure to the CSCs has an influence on metabolic activity and nucleic acid replication activity of the cells. In both the MTT and BrdU incorporation tests, the cytotoxicity was decreased from ISO regimen to HCI regimen under the same per unit of TPM, similar to an earlier cytotoxicity report on Chinese hamster ovary (CHO) cells. Compared with the MTT test results, the BrdU incorporation assay showed that the nucleic acid replication activity was inhibited more significantly than metabolic activity with the EC₅₀ ranging from 68(ISO) to 81 (HCI) µg/mL for the BrdU incorporation assay, and 92(ISO) to 123(HCI) µg/mL for the MTT test. Further study on the release of harmful components in mainstream smoke showed that the toxicant output per unit of TPM and Nicotine for the HCI regimen was lower than the ISO regimen for reference cigarette 3R4F. Furthermore, the CSCs under the HCI regimen have higher water content than ISO. Herein, these two factors might contribute to the decrease in cytotoxicity to human oral cells under the HCI regimen than the ISO regimen.