Principles and Practice of Pharmaceutical Medicine

microsomes will be sufficient. However, if the
potential for non-microsomal biotransformation
exists, then a differentin vitrosystem, such as
hepatocyte suspensions, should be used. In the
illustration above, it turned out, as far as clearance
of compound X is concerned, human is specifically
like a rat and unlike a dog.
The third caveat is that one must consider the
variability in the expression of metabolizing
enzymes between individuals. Oxidative metabo-
lism (seenin vivoand in microsomal enzymes), and
especially cytochrome P 450 s, vary tremendously
between human individuals (Meyer, 1994; Shimada
et al., 1994). Had we used a single donor micro-
somal sample, rather than pooled liver microsomes
(a pool consisting of at least eight individual
donors), to scalein vitrodata toin vivohepatic
clearance, we might have made greatly misleading
predictions (note that oxidative, initial drug meta-
bolism is sometimes called ‘phase I metabolism’ in
the literature, causing ambiguity with the stage of
drug development or type of clinical trial).

Volumes of distribution

Review of elementary concepts

Volume of distribution is a theoretical concept that
may or may not correspond to the anatomical
compartment(s) which drugs or metabolites may
access after dosing. When size of the dose (D)is
known, and when drug concentration (C) may be
found by sampling biological fluids, then, in the
simplest case, the volume of distribution (VD) is:

VD¼D=C

Clinical protocols can usually only prescribe the
sampling of a subset of compartments when a drug
is known to distribute widely in the body. For
example, a lipophilic drug may penetrate lipophilic
organs such as brain, and, obviously, brain sam-
pling simply for pharmacokinetic purposes is
usually possible only in animals. In such cases,
blood concentrations fall far lower than if the
dose had distributed solely into the circulating
compartment; Cbecomes very small, and VD

becomes correspondingly very large. The opposite

effect would require the drug to be restricted to a

fraction of the compartment that is sampled, essen-

tially suggesting that too few compartments have

been postulated, and the effect is almost never

encountered. Again, see Curry (1980) or Benet

et al. (1996) for expansion of these elementary

aspects of volume of distribution.

Prediction of human holumes of distribution

The free (not plasma protein bound) volume of

distribution of experimental drugs is generally con-

sidered to be constant for all species. Thus, the

volume of distribution in humans can easily be

predicted through a simple proportionality

betweenin vitroplasma protein binding data in

humans and in a preclinical species, andin vivo

volume of distribution in that same preclinical

species:

VDhuman¼

VDpre-clinical speciesfuhuman

fupre-clinical species

where fu is fraction unboundV 0 plasma proteins.

Table 8.3 shows the predicted volume of distribu-

tion of a single intravenous bolus dose of com-

pound X in humans; this is found by using the

above equation, anin vitro estimate of protein

binding data for rat and dog plasmas and the

observed volumes of distribution for these two

speciesin vivo. For humans, VDhumanwas pre-

dicted to be 3.48–4.591 kg
^1 using the rat data

and 3.01–5.061 kg
^1 using the dog data.

Table 8.3 In vitroplasma protein binding,in vivo

volume of distribution and predicted volume of

distribution in humans

Fraction of Predicted

compound X In vivo volume of

unbound in volume of distribution

the plasma distribution in humans

(fu) (l kg) (l kg)

Rat 0.45 3.02–3.97 3.48–4.59

Human 0.52 – –

Dog 0.66 3.82–6.43 3.01–5.06

84 CH8 PHASE I: THE FIRST OPPORTUNITY FOR EXTRAPOLATION FROM ANIMAL DATA