Drug diffusion and bioavailability: tetracycline metallic chelation.

The effects of chelation on the in vitro diffusion rate of tetracycline at variable pH values and concentrations was studied. A two-compartment diffusion cell using a semipermeable cellophane membrane was used to determine diffusion rates. Tetracycline in solution was mixed with aluminum, magnesium, copper, calcium and cobalt ions and analyzed by ultraviolet spectrophotometry. Diffusion rates were determined at pH 1, 5.5 and 8. Tetracycline chelate formation is pH-dependent. The diffusion rate of tetracycline chelates is concentration-dependent and is reduced even if the chelate is water soluble.