Measurement and clinical and pharmacokinetic implications of diffusion coefficients of antibiotics in tissues.

A method for determining diffusion coefficients of four antibiotics in extracellular tissue space according to Fick's law is described. This new method was applied to rat brain tissue and to agar. After diffusion of the antibiotic in one axis, the gradient concentration was determined with microvoltammetric electrodes. These microelectrodes (1 micron at the extreme tip) measured the free diffusible form of electroactive antibiotics in the extracellular brain space. Metronidazole, chloramphenicol succinate, cefsulodin, and piperacillin gave diffusion coefficients ranging from 0.1 x 10(-6) to 0.2 x 10(-6) cm2 . s-1 in tissue; chloramphenicol base, which is positively charged, gave a coefficient of 0.04 x 10(-6) cm2 . s-1. The coefficient ranged from 0.6 x 10(-6) to 1.2 x 10(-6) cm2 . s-1 in agar. These coefficients were used to simulate antibiotic concentrations in infectious sites and between capillaries by using a simple model of plane diffusion.