Figure 2:

Schematic representation of the PK model of both efavirenz and isoniazid. The absorption is described with a series of transit-compartment to capture the delay in absorption, and a rate constant K_a. The hepatic extraction (Eh) is responsible for both first-pass metabolism and the systemic elimination with first-order kinetics. V_c represents the volume of distribution in the central compartment. Drug transfer between the central and peripheral compartment is defined by intercompartmental clearance Q/F, were F represents the oral bioavailability.