Can In Vitro-In Vivo Extrapolation Be Successful? Recognizing the Incorrect Clearance Assumptions

Abstract

For a number of years, our laboratory has been investigating the underlying reasons for the published poor in vitro-in vivo extrapolation (IVIVE) predictability of human clearance both from a theoretical and from an experimental perspective. Here, we critically examine clearance concepts and commonly employed IVIVE approaches, concluding that there is no theoretical reason that IVIVE should work, just as it does not. Our analysis, however, has identified 10 misconceptions and/or poorly understood aspects of clearance that are listed in the Conclusion section of this manuscript. Chief among these are that all published human drug clearance values are arterial clearances-clearance calculated as organ blood flow multiplied by the extraction ratio is the arterial clearance of the organ of elimination (and not the published drug clearance value)-and that the well-stirred model equation taught in all pharmacokinetic courses that relates organ blood flow, fraction unbound in blood, and intrinsic clearance has no validity. We further list 10 conclusions relating to the IVIVE process. The primary IVIVE-related conclusions are that the intrinsic clearance value determined from an in vitro incubation is an arterial intrinsic clearance, there is no theoretical basis upon which an arterial intrinsic clearance can be related to a whole-body arterial clearance to accomplish IVIVE, there are no published data demonstrating that in vitro intrinsic metabolic clearance can predict in vivo organ clearance as IVIVE assumes, and the scientific basis for the hypothesized albumin-mediated hepatic uptake phenomenon is invalid. We further propose three IVIVE process recommendations.

Figure 1.

In vitro-in vivo extrapolation (IVIVE) scheme where an in vitro incubation of hepatocytes or microsomes allows determination of the half-life of drug elimination and an estimate of the in vitro drug intrinsic clearance (CL_int), which is then scaled up to whole organism liver intrinsic clearance. This value is then inserted into an equation representing a model of hepatic elimination, here the purported well-stirred model incorporating whole organism liver blood flow (Q_H) and fraction unbound in blood (f_u,B), with correction for fraction unbound in the incubation mixture (f_u,inc), to predict an in vivo liver clearance (CL_H).

Figure 2.

Physiologic based pharmacokinetic (PBPK) model depicting the exit concentration from the liver (C_{H, out}) and the whole-body venous concentration (C_{venous, whole-body}).

Figure 3.

The effect of dl-propranolol on the disposition of lidocaine under steady-state conditions. Each point represents the mean ± S.E. for 6 dogs.

Figure 4.

Plasma concentrations of lidocaine (30 mg/kg i.v.) before (solid circles) and after (open circles) 0.5 mg/kg of propranolol i.v. Each point represents the mean ± S.E. for 5 dogs.

Figure 5.

Chemical engineering reaction (pharmacokinetic) models at steady-state (logarithmic concentration on Y axis)

Figure 6.

Plot of the predicted and the observed lidocaine concentrations in the effluent blood for models I and II when hepatic blood flow was changed from the control flow rate of 10 to 12, 14, and 16 ml/min per liver. The lines represent predicted data from models I and II, and the points represent observations.

Figure 7.

IPRL steady-state cold taurocholate studies depicting radioactive dose taurocholate disappearance curves obtained when perfusate albumin concentration was 0.5 g/dl (open circles) and 5.0 g/dl (solid circles). The broken line represents the predicted result for an albumin concentration of 5.0 g/dl based on the assumption that uptake is determined by the perfusate concentration of free bile salt utilizing a dispersion model of hepatic elimination.

Figure 8.

Schematic representation of blood and liver concentrations (C_B & C_H) in the extended clearance model where unbound concentrations (C_u) drive hepatic intrinsic metabolic (met), biliary (bil) and transporter efflux (eff) intrinsic clearances and blood transporter influx (inf) intrinsic clearance, as presented by Sodhi et al.

Figure 9.

Prediction by Wood et al. of CL_int,u in hepatocytes (A and C) and microsomes (B and D) in human (A and B) and rat (C and D). Dashed lines represent unity and dotted lines a 2-fold margin of error.