Abstract
Here we re-examine the hepatic extraction for diazepam, the only drug for which isolated perfused rat liver (IPRL) studies have been reported to not be consistent with the well-stirred model of organ elimination when only entering and exiting liver concentration measurements are available. First we examine the time dependency of diazepam equilibrium fraction unbound measurements from 4 to 24 hours, reporting the continuing increases with time. We conclude that the time dependency of equilibrium protein binding measurements for very highly bound drugs may be an issue that is not readily overcome. Examining Cout/Cin (Fobs) measurements for diazepam when no protein is added to the incubation media, we observe IPRL outcomes consistent with previous reports of Rowland and co-workers. Recent studies have shown the marked under-predictability of in vivo clearance from in vitro measures of elimination in the absence of protein for very highly bound drugs, which is markedly diminished in the presence of albumin. Thus, we measured Fobs for diazepam at additional low concentrations of protein that would allow discrimination of the models of hepatic elimination and our results are not consistent with the dispersion and parallel-tube models. Therefore, although we can confirm similar outcomes to that reported by Rowland and co-workers when no protein is added to perfusion media, our IPRL results for diazepam, as well of that of Rowland and co-workers, cannot be reasonably interpreted as proving that hepatic organ elimination is model independent or supporting the dispersion and parallel-tube models of organ elimination.
SIGNIFICANCE STATEMENT The only experiments for which isolated perfusion rat liver studies do not support hepatic clearance being best described by the well-stirred model were carried out with diazepam at zero protein concentration. We repeat those studies confirming the previous results at zero protein concentration, but the addition of low protein binding conditions capable of differentiating the various models of hepatic elimination are more consistent with the well-stirred model of hepatic elimination. These experimental studies do not support the preference for alternate models of hepatic elimination, nor the proposal that hepatic organ clearance is model independent.
- animal/nonclinical/preclinical
- isolated perfused liver
- liver physiology/models
- liver/hepatic
- protein binding
- The American Society for Pharmacology and Experimental Therapeutics