Estimation of absorption parameters from the non-steady-state phase in the rat gut perfusion model

The aim of the study was to calculate absorption parameters, including permeability coefficients (P(app)), from the non-steady-state portion of the outflow to inflow concentration ratio vs time profiles and compare them with those obtained via the more traditionally used steady-state phase. The rat in-situ intestinal perfusion method was used. The compounds studied, diclofenac and macrogol 4000 (polyethylene glycol (PEG) 4000), were perfused at four different flow rates (0.1-2.0 mL min(-1)). The estimates of P(app) from the non-steady-state data were systematically lower for both compounds. The non-steady-state analysis gave estimates of the intestinal radius, r. The internal diameter of the intestine segment increased as the flow rate increased. When this effect was taken into account similar P(app) estimates were obtained by the two approaches. Thus the convention of using a constant value of intestinal radius in the steady-state equation leads to an over estimate of the P(app) when high flow rates are employed. The different trends observed, between P(app) and perfusate flow rate, for the two compounds, macrogol 4000 and diclofenac, may be linked to increased surface area and exposure to membrane pores of larger size. The longitudinal spreading coefficient, De, increased with flow rate and was approximately 1000 times greater than that estimated for molecular diffusion. The high values obtained were consistent with the non-smooth biological surface and peristaltic movement present in-vivo.