In vitro percutaneous absorption of boron as boric acid, borax, and disodium octaborate tetrahydrate in human skin: a summary

Literature from the first half of this century reports concern for toxicity from topical use of boric acid, but assessment of percutaneous absorption has been impaired by lack of analytical sensitivity. Analytical methods in this study included inductively coupled plasmamass spectrometry which now allows quantitation of percutaneous absorption of¹⁰B in¹⁰B-enriched boric acid, borax and disodium octaborate tetrahydrate (DOT) in biological matrices. In vitro human skin percent doses of boric acid absorbed were 1.2 for a 0.05% solution, 0.28 for a 0.5% solution, and 0.70 for a 5.0% solution. These absorption amounts translated into flux values of, respectively, 0.25, 0.58, and 14.58 μg/cm²/h, and permeability constants (K _p ) of 5.0 x 10^-4, 1.2 x 10^-4, and 2.9 x 10^-4 cm/h for the 0.05%, 0.5%, and 5.0% solutions. The above in vitro doses were at infinite, 1000 μL/cm² volume. At 2 μL/cm² (the in vivo dosing volume), flux decreased some 200-fold to 0.07 μg/cm²/h andK _p of 1.4 x 10^-6 cm/h, while percent dose absorbed was 1.75%. Borax dosed at 5.0%/1000 μL/cm² had 0.41 percent dose absorbed, flux at 8.5 μg/cm²/h, andK _p was 1.7 x 10^-4 cm/h. Disodium octaborate tetrahydrate (DOT) dosed at 10%/1000 μL/cm² was 0.19 percent dose absorbed, flux at 7.9 μg/cm²/h, andK _p was 0.8 x ICH cm/h. These in vitro results from infinite doses (1000 μL/cm²) were a 1000-fold greater than those obtained in the companion in vivo study. The results from the finite (2 μL/cm²) dosing were closer (10-fold difference) to the in vivo results. General application of infinite dose percutaneous absorption values for risk assessment is questioned by these results.