The
in vitro–in vivo extrapolation (IVIVE) approach for
predicting total plasma clearance (CLtot) has been widely
used to rank order compounds early in discovery. More recently, a
computational machine learning approach utilizing physicochemical
descriptors and fingerprints calculated from chemical structure information
has emerged, enabling virtual predictions even earlier in discovery.
Previously, this approach focused more on in vitro intrinsic clearance
(CLint) prediction. Herein, we directly compare these two
approaches for predicting CLtot in rats. A structurally
diverse set of 1114 compounds with known in vivo CLtot,
in vitro CLint, and plasma protein binding was used as
the basis for this evaluation. The machine learning models were assessed
by validation approaches using the time- and cluster-split training
and test sets, and five-fold cross validation. Assessed by five-fold
validation, the random forest regression (RF) and radial basis function
(RBF) models demonstrated better prediction performance in eight attempted
machine learning models. The CLtot values predicted by
the RF and RBF models were within two-fold of the observed values
for 67.7 and 71.9% of cluster-split test set compounds, respectively,
while the predictivity was worse in the time-split dataset. The predictivity
of both models tended to be improved by incorporating in vitro parameters,
unbound fraction in plasma (f
u,p), and
CLint. CLtot prediction utilizing in vitro CLint and the well-stirred model, correcting for the fraction
unbound in blood, was substantially worse compared to machine learning
approaches for the same cluster-split test set. The reason that CLtot is underestimated by IVIVE is not fully explained by considering
the calculated microsomal unbound fraction (cfu,mic), extended
clearance classification system (ECCS), and omitting high clearance
compounds in excess of hepatic blood flow. The analysis suggests that
in silico machine learning models may have the power to reduce reliance
on or replace in vitro and in vivo studies for chemical structure
optimization in early drug discovery.