Gastro-intestinal absorption and in vitro release of boric acid from water-emulsifying ointments

Boric acid taken orally by six male volunteers in a cross-over study was absorbed to equal extents from a water solution and a 3% waterless, water-emulsifying ointment, though with a slight initial delay in the latter case. Virtually complete gastro-intestinal absorption and renal excretion were indicated by the 96-hr urinary recovery, amounting to 89.1-98.3% (mean 93.9%) and 89.2-97.5% (mean 92.4%) of the dose ingested as solution and ointment, respectively, normal daily boron excretion having been taken into account. The in vitro release of boric acid, measured for 24 hr by dialysis in water at 37 degrees C, reached 95% from a purely water-based jelly but only about 5% from the water-emulsifying ointment. The low boric acid release from the ointment was not significantly influenced when the ointment was dialysed against buffer solutions of pH 2.5 and 9.6 instead of water, or when the maximum possible amount of water (26.9% w/w) was incorporated into the ointment before dialysis. The 24-hr boric acid release from a number of other oil-based ointments, either hydrophobic or water-emulsifying and containing 1-3% boric acid and 0-28.5% water, was also low (0.9-18.3% of the boric acid content). This indicates that the formulation of the ointment is an important factor in determining the extent of release of boric acid when the ointment is applied externally, but that it does not alter the absorption of boric acid should the ointment be ingested.