Abstract
Extensive studies have been conducted to predict in vivo metabolic clearance from in vitro human liver metabolism parameters (i.e., in vitro–in vivo extrapolation (IVIVE)) with little success. Here, deriving IVIVE from first principles, we show that the product of fraction unbound in the blood and the predicted in vivo intrinsic clearance determined from hepatocyte or microsomal incubations is the lower boundary condition for in vivo hepatic clearance and the prerequisite for IVIVE predictions to be valid, regardless of extraction ratio. For 60–80% of drugs evaluated here, this product is markedly less than the in vivo measured clearance, a result that violates the lower boundary of the predictive relationship. This can only be explained by (a) suboptimal in vitro metabolic stability assay conditions, (b) significant error in the assumption that in vitro intrinsic clearance determinations will predict in vivo intrinsic clearance simply by scaling-up the amount of enzyme (in vitro incubation to in vivo liver), and/or (c) the methods of determining fraction unbound are incorrect. We further suggest that widely employed organ blood flow values underpredict the effective blood flow within the organ by approximately 2.5-fold, thus impacting IVIVE of high clearance compounds. We propose future pathways that should be investigated in terms of the relationship to experimentally measured clearance values, rather than model-dependent intrinsic clearance. IVIVE outcome can be improved by estimating the ratio of unbound drug concentration in the liver tissue to the liver plasma, examining the assumption of the free drug theory (i.e., there are no transporter effects at the blood cell membrane) and the finding that the upper limit of organ clearance may be greater than blood flow entering the organ.
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Abbreviations
- ADME:
-
Absorption, distribution, metabolism, excretion
- AUC:
-
Area under the concentration–time curve
- BDDCS:
-
Biopharmaceutics Drug Disposition Classification System
- \( \frac{B}{P} \) :
-
Blood to plasma partitioning ratio
- CL:
-
Clearance
- CLint :
-
Intrinsic clearance
- CYP3A:
-
Cytochrome P450 3A
- D :
-
Dose
- ECCS:
-
Extended Clearance Classification System
- F :
-
Bioavailability
- f u,B :
-
Fraction of unbound drug in the blood
- f u, inc :
-
Fraction of unbound drug in an in vitro incubation
- f u,P :
-
Fraction of unbound drug in the plasma
- IVIVE:
-
In vitro–in vivo extrapolation
- NME:
-
New molecular entity
- Q H :
-
Hepatic blood flow
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Acknowledgments
The authors thank members of our laboratory Dr. Jin Dong (present address: Simulations Plus), Alan R. Wolfe, Shufang Liu (present address: SUNY Buffalo), and Dr. Giovani Bocci (present address: University of New Mexico) for the very helpful discussions.
Funding
This work was supported in part by a Mary Anne Koda-Kimble Seed Award for Innovation. Dr. Benet is a member of the UCSF Liver Center supported by NIH Grant P30 DK026743. Ms. Sodhi was supported in part by an American Foundation for Pharmaceutical Education Pre-Doctoral Fellowship, NIGMS Grant R25 GM56847 and a Louis Zeh Fellowship.
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Benet, L.Z., Sodhi, J.K. Investigating the Theoretical Basis for In Vitro–In Vivo Extrapolation (IVIVE) in Predicting Drug Metabolic Clearance and Proposing Future Experimental Pathways. AAPS J 22, 120 (2020). https://doi.org/10.1208/s12248-020-00501-9
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DOI: https://doi.org/10.1208/s12248-020-00501-9