PURPOSE: The cytotoxicity potential of PHMB was determined using colorimetric in vitro assays with murine fibroblast cells (L929) and an immortalized human corneal epithelial cell line (HCE-T). These colorimetric assays are useful for the quantitative factor-induced cytotoxicity within a 24 to 72 hour period of cell culture. Soft contact lens care products containing PHMB corresponding soft contact lens care product excipient solutions, without PHMB, and PHMB in PBS were tested.

METHODS: Lens care solutions with and without PHMB were diluted 1:3 with growth medium. PHMB in PBS was diluted at 10, 5, 2.5, 1.25 and 0.625 ppm in growth medium. Tests used were the cell viability assays, MTS/PES (MTS/PES) and Alamar Blue (AB), and the cell membrane integrity assay using neutral red uptake and release, (NRUR). The endpoint was spectrophotometric measurement using a microplate reader for MTS/PES at 490 nm and NRUR at 540 nm and a cytofluorometer for AB at 530/580 excitation/emission. The data were expressed as the ratio of the test solution’s absorbance value to the absorbance value of the negative control and the final percentage calculated. These percentage values were evaluated by their IC50 (inhibition of control by 50%) and compared using ANOVA/Tukey HSD test and t - test for statistical significance at a p<0.05 to the negative control and between samples.

RESULTS: Based on these studies, the following lens care solutions were noncytotoxic at 48 hours exposure using the MTS/PES, AB and NRUR: SOLOcare® PLUS (CIBA Vision) and the corresponding excipient solution without PHMB and AQuify™ MPS( SOLOcare®AQUA) (CIBA Vision) and the corresponding excipient solution without PHMB, and PHMB solutions of 5, 2.5, 1.25 and 0.625 ppm. The following solutions were cytotoxic using the same assays: ReNu MultiPlus® (Bausch & Lomb), ReNu MultiPlus®(Bausch & Lomb) excipient solution without PHMB and PHMB at 10 ppm.

CONCLUSIONS: The PHMB concentration of the marketed contact lens solutions is approximately 1 ppm. PHMB at 1ppm was noncytotoxic using the three in vitro assays, MTS/PES, AB and NRUR with L929 and HCE-T cells. The presence of PHMB in the ReNu solution did not increase the cytotoxicity indicating that the excipients are responsible for the cytotoxicity observed with ReNu MultiPlus®. SOLOcare® PLUS and AQuify™ MPS were both noncytotoxic. These results suggest that not all PHMB-containing solutions are equal.