It has been surprisingly found that very accurate measurements of
mass transfer can be made rapidly by permitting
diffusion of an agent desired to be measured into a small, known volume of
receiver or out of a known volume of donor, then rapidly pumping or flushing (“pulsing”) the
receiver with a known volume of fluid. More specifically, a novel method of transferring small quantities of a contained material (either dissolved or suspended) between two media, based on such pulsing, hereinafter called pulsatile
microdialysis (PMD), is disclosed. In a preferred embodiment, one medium (the dialysate) is inside a small, permeable tube (
microdialysis probe window) and the other (external medium) is outside. The transfer of material between the two media can be utilized, for example, to sample
drug concentrations in the external medium, or the release of drugs from systems within the dialysate, or for other measurements as disclosed herein. In PMD, a dialysate fluid is pumped into a
microdialysis probe window, allowed to occupy the probe window while at rest for some
resting time, and then flushed at a
high rate out as a
single pulse. A model that is based on a Fick's Laws was solved, and equations were derived to calculate the effects of various experimental parameters. The models were verified against experimental data using methazolamide,
warfarin and
benzocaine as test drugs. The data followed the mathematical models. For cases in which the concentration of
free drug in the medium outside the probe was constant or changed very slowly, the concentration calibration plots were linear. In simulated first order uptake studies, the PMD and direct donor sampling data were in nearly exact agreement with the theoretical values of k=0.09 min−1. In another experiment, the free concentration of
warfarin sodium in the medium outside the probe was made to decline rapidly in a known first order manner, with rate constants as high as 0.077 sec−1. The concentration in the external medium calculated from the PMD data was in nearly exact agreement with the known concentration at various times, and the experimental rate constants were in nearly exact agreement with the theoretical rate constants. For binding of methazolamide to
activated charcoal, and for the binding of
sodium warfarin to
bovine serum albumin, PMD was able to generate sufficient data points to accurately characterize the rapid initial binding. This invention demonstrates that PMD is an accurate method of sampling
drug concentrations and measuring rates and extents of a number of processes, including
protein binding, adsorption to binding agents such as
activated charcoal, release from
microemulsion drug delivery systems, and the determination of drug
diffusion coefficients, and for various other purposes which will occur to those skilled in the art. Compared to known methods such as traditional (continuous) microdialysis, the present invention offers the ability to sample more frequently, and over much shorter time intervals, thereby accurately obtaining data not heretofore available.