Toxicology of Industrial Compounds

Table 23.4 Kinetic parameters for the in vitro hydrolysis of Compound D and E by
rat serum and organ homogenates
Notes:
a The apparent Vmax value for serum is given in µmol min −1 ml −1.
H.THOMAS ET AL. 325
b Initial velocity of hydrolysis at 0.2 mM ester concentration.
Hydrolysis of Compound D was determined in 50 mM Tris/phosphate buffer, pH 7.
5, containing either 1 per cent (v/v) rat serum, or 1.25 per cent (w/v) rat liver
homogenate or 10 per cent (w/v) small intestine homogenate. Similarly, hydrolysis
of Compound E was assessed using 98 per cent (v/v) rat serum or in the presence of
10 mM Tris/HCl buffer, pH 7.5, containing 250 mM sucrose and either 24.5 per
cent (w/v) rat liver homogenate or 19.6 per cent (w/v) small intestine homogenate.
effects. This question was addressed in a pharmacokinetic study under
conditions of single oral administration of Compounds D and E at a dose
level of 10 mg kg −1 each (Table 23.5).
14 C-Phenyl-labelled Compound D was readily absorbed from the
gastrointestinal tract. Maximal blood radioactivity was reached between 1
and 2 h and subsequently eliminated with an apparent half-life of 10.0–11.
8 h. After 48 h only minute amounts of radioactivity equalling about 3 per
cent of the blood levels in T max were detectable. Analysis of the resulting
blood metabolite pattern largely confirmed the findings of the preceding in
vitro investigation of enzymatic Compound D hydrolysis: particularly
during periods of high parent equivalent concentrations in blood as
recorded between 30 min and 6 h after dosing, hydrolysis appeared to be
the major metabolic pathway as evidenced by the high concentrations of
the carboxylic acid, Compound C (34–77 per cent), and an unidentified
metabolite (17–36 per cent) which was regarded to have evolved from
Compound C by an additional metabolic step (Table 23.5).
Quite surprisingly, Compound E was found to be absorbed to a much
lower extent than Compound D, with a C max after 1 h of less than one
tenth of the value seen with the latter. Elimination with an apparent halflife
of 12.0 h and slightly higher residual radioactivity of approximately 4.8
per cent of the blood levels recorded at T max after 48 h indicated only a
slightly reduced elimination rate as compared to Compound D. Also, quite
different from the initially anticipated result, hydrolysis contributed
substantially to the rapid metabolism of Compound E. The 24 h AUC
values revealed a 23 per cent and 36 per cent contribution of the carboxylic