Flip-flop kinetics

In pharmacokinetics, flip-flop phenomenon happens when a drug is released at a sustained rate instead of immediate release, such as sustained-release formulation v.s. immediate-release formulation (tablet, IV).

In flip-flop kinetics, k_a (absorption constant) is much slower than k_e (elimination constant). This apparent difference shift the slope of logCp vs time curve in which now the apparent part of k_e looks much smaller than it is if the drug is administered intravenously or by immediate-release formulation. The part of downward curve becomes a reflection of actual k_a while the upward part of the curve is the actual representation of k_e. That "flip-flop" part of curve is the so-called flip-flop kinetics.

The application of flip-flop kinetics is very important in the development of sustained-release and controlled-release formulation in pharmaceutical manufacturing.