J. D. BAGGOT AND L. E. DAVIS 
693 
Nuclear) and liquid scintillation counting 
(Packard Tri Carb Model 3380 liquid scintilla- 
tion spectrometer) was employed to measure 
the extent of plasma protein binding in vitro. 
The extent of protein binding was determined 
at a drug concentration of 1 X 10"^ M (activ- 
ity 0.156 Ci/ml). 
RESULTS AND DISCUSSION 
The disappearance of amphetamine from the 
blood plasma of the various species is shown 
(Figure 1). The distribution of this drug was 
rapid, less than thirty minutes, and elimination 
followed first-order kinetics in all species. Val- 
ues for the kinetic constants are tabulated in 
Table I. The mean biological half-life under 
normal conditions of fluctuating urinary pH in 
500 
1 2 3 4 5 6 
TIME IN HOURS 
Figure 1. — Disappearance of amphetamine from the 
blood plasma of several species following the intra- 
venous injection of amphetamine sulphate (0.66 
mg/kg, calculated as free base). Elimination was 
first-order in all species. 
these randomly selected groups of animals var- 
ied among species (F-test, p<0.01). The short- 
est half -life of this drug was in goats; the 
half-life was of intermediate duration in ponies, 
rabbits, chickens and swine, while dogs and cats 
had longer half-lives. No sex difference in per- 
sistence of the drug was observed in any species 
studied. The value B may be defined as the con- 
centration of drug which would be found in the 
plasma at time zero if the distribution of drug 
to tissue fluids occurred instantaneously. On ex- 
trapolating to zero time (Figure 1), the mean 
values for B varied from 235 ng ml"^ in goats 
to 438 ng ml~^ in chickens. 
The assay procedure measured both free and 
bound amphetamine in plasma samples and 
since only unbound drug was available for dif- 
fusion out of the circulation, the apparent vol- 
ume of distribution had to be corrected for the 
extent of plasma protein binding. The percent 
amphetamine bound to plasma proteins was in- 
dependent of drug concentration within the 
range of concentrations studied (2.5 X 10"''^ M 
to 4.0 X 10-6 M) . Extent of protein binding in- 
dependent of drug concentration was reported 
for desmethylimipramine^6 and for ampheta- 
mine^'^ in human plasma. Preliminary work sug- 
gests that the extents of plasma protein binding 
of chloramphenicol and morphine in an animal 
species are independent of drug concentration 
within the therapeutic range of plasma levels. 
In contrast, the extent of plasma protein binding 
of quinidine was concentration dependent.^^ 
The extent of binding of amphetamine (mean 
± S.E.) at a drug concentration of 1 X 10-« M 
and the total plasma protein concentration for 
Table I. — Pharmacokinetic Constants (Mean ± S.E.) 
Describing the Distribution and Elimination of Am- 
phetamine in Different Species of Domestic Animals 
Species B ti/a jQ— 1 (litre/ksr) 
(number) (ng/ml) (hr ) (hr ) V'd 
Goat (10) 235 ± 15 0.62 ± 0.04 1.118 ± 0.08 3.08 
Swine (9) 330 ± 15 1.05 ± 0.05 0.654 ± 0.03 2.23 
Pony (6) 281 ± 34 1.39 ± 0.08 0.499 ± 0.03 2.61 
Rabbit (4) . 333 ± 8 1.40 ± 0.08 0.498 ± 0.03 2.29 
Chicken (8) .... 438 ± 15 2.27 ± 0.17 0.306 ± 0.03 1.81 
Dog (11) 263 ± 10 4.60 ± 0.24 0.159 ± 0.01 2.67 
Cat (6) 401 ± 5 6.53 ± 0.39 0.108 ± 0.01 1.83 
B = Theoretical drug concentration in plasma at time zero 
ti/2 = Biological half-life 
p = Overall elimination rate constant 
V'd = Apparent specific volume of distribution 
