Carol A. Shively

Altered plasma half‐lives of antipyrine, propylthiouracil, and methimazole in thyroid dysfunction

In normal, nonmedicated volunteers and in patients with thyroid disorders the plasma half‐lives of antipyrine, propylthiouracil, and methimazole were determined after single oral doses. The plasma half‐lives ± S.D. of antipyrine, propylthiouracil, and methimazole were 11.9 ± 1.4 hr, 6.7 ± 1.0 hr, and 9.3 ± 1.4 hr, respectively, in normal volunteers, but were shortened to 7.7 ± 1.2 hr, 4.3 ± 0.7 hr, and 6.9 ± 0.6 hr, respectively, in hyperthyroid patients. In hypothyroid patients the plasma half‐lives of these drugs were prolonged to 26.4 ± 4.0 hr, 24.7 ± 34.5 hr, and 13.6 ± 4.8 hr, respectively. Return to the euthyroid state restored plasma half‐lives to or toward normal. Alterations in plasma drug half‐lives during thyroid dysfunction appear to result mainly from accelerated hepatic microsomal drug metabolism in hyperthyroidism and retarded drug biotransformation during hypothyroidism.

Liquid Chromatographic Assay of Warfarin: Similarity of Warfarin Half-Lives in Human Subjects

A high pressure liquid chromatographic assay was developed to measure warfarin concentrations in biological fluids. Twelve healthy, unrelated volunteers received a single oral dose of warfarin (0.75 mg per kilogram of body weight). The mean plasma warfarin half-life was 36.3 � 3.5 hours by liquid chromatography but 55.9 � 8.4 hours by a currently used fluorimetric assay that fails to separate warfarin from its metabolites. Interindividual variation was greater and each half-life longer by the fluorimetric than by the chromatographic procedure. Warfarin shows less interindividual variation than that observed for other drugs primarily metabolized by hepatic microsomal mixed function oxidases. Advantages of specificity, rapidity, sensitivity, accuracy, and simplicity recommend liquid chromatography in the development of other drug assays.

Sex differences in absorption kinetics of sodium salicylate

Sodium salicylate in aqueous solution (9 mg/kg) was given by oral and intravenous routes to normal male and female subjects. Because the bio availability of salicylate was complete, salicylate was given orally in all subsequent experiments. There were sex differences in time required to attain peak salicylate concentration (tmax), but not in maximum plasma salicylate concentration (Cmax). There were no sex differences in apparent volume of distribution, plasma salicylate clearance, or area under the concentration‐time curve. In female subjects, tmax tended to reach a nadir at the middle of the menstrual cycle, when gastric emptying time is shortest, whereas Cmax remained relatively unchanged throughout the menstrual cycle. Equilibrium dialysis studies on the binding of sodium salicylate and of 14C‐racemic warfarin to plasma from 25 normal male and 25 normal female subjects of similar age disclosed no sex differences either in the extent of binding of these drugs or in serum albumin concentration. The possibility of sex differences in rates of gastrointestinal absorption of other drugs should be investigated.

High levels of methylxanthines in chocolate do not alter theobromine disposition

Theobromine disposition was measured twice in 12 normal men, once after 14 days of abstention from all methylxanthines and once after 1 week of theobromine (6 mg/kg/day) in the form of dark chocolate. Mean theobromine t½, apparent volume of distribution, and clearance after abstinence from all methylxanthines were 10.0 hours, 0.76 L/kg, and 0.88 ml/min/kg. High daily doses of chocolate for 1 week did not change these values. After subjects abstained from methylxanthines, urinary radioactivity over 72 hours after a single, oral dose of [8‐14C]theobromine consisted of 42% 7‐methylxanthine, 20% 3‐methylxanthine, 18% theobromine, 10% 7‐methyluric acid, and 10% 6‐amino‐5[N methyl‐formylamino]‐1‐methyluracil. A week of daily theobromine consumption in the form of dark chocolate also did not alter this urinary profile of theobromine and its metabolites. Although these results might appear to differ from other reports of inhibition of theobromine elimination after five consecutive daily doses of theobromine in aqueous suspensions, both the rate and extent of absorption of theobromine in chocolate were less then that of theobromine in solution. Relative bioavailability of theobromine in chocolate was 80% that of theobromine in solution. This reinforces the fundamental principle that both the metabolic and the therapeutic consequences of a particular chemical can differ when that chemical is given in the pure compared with the dietary form.

Antipyrine and warfarin disposition in a patient with idiopathic hypoalbuminemia

In a subject with the rare condition of idiopathic hypoalbuminemia, antipyrine and warfarin disposition was investigated to determine whether, in an otherwise healthy subject, low albumin concentrations affect the kinetics of these prototypic drugs. Because antipyrine is negligibly bound to albumin its disposition would be expected to be normal, but because at usual therapeutic doses warfarin is 99% bound to albumin, warfarin elimination would be expected to be accelerated in idiopathic hypoalbuminemia. In our patient antipyrine disposition was normal, whereas warfarin clearance was increased and plasma warfarin half‐life reduced. The apparent volume of distribution of warfarin was within normal range. Warfarin binding to the patients plasma was decreased; the free fraction of warfarin in plasma was correspondingly elevated. Albumin isolated from the patient and purified exhibited normal kinetic values for warfarin binding. Caution is necessary in extending these results to other drugs and other patients with idiopathic hypoalbuminemia since the validity of such extrapolations depends on the extent to which the drug investigated is normally bound to albumin and also on the magnitude of the particular patients hypoalbuminemia at the precise time of study.

Temporal variations of antipyrine half-life in man.

Temporal variations in antipyrine disposition were studied in 19 normal male volunteers using salivary antipyrine half‐lives determined after a single oral dose of antipyrine (18 mg Ikg) given at 7:00 A.M. Specimens of saliva obtained at half‐hour intervals from 10:30 A.M. to 2:00 P.M. and also from 10:30 P.M. to 2:00 A.M. were designated the noon and midnight periods, respectively. In these 19 volunteers there were no significant differences between the mean antipyrine half‐lives calculated using only the noon period (12.8 ± 3.0 hr SD), those calculated using only the midnight period (13.2 ± 3.6 hr SD), and those determined using the whole 24‐hr period (13.0 ± 2.5 hr SD); neither were there significant differences among these periods for mean antipyrine metabolic clearance rates and mean apparent volumes of distribution. However, between the noon and midnight periods, 12 of the 19 subjects changed antipyrine half‐life by more than 10%. In the 12 subjects in whom the change exceeded 10%, there were 7 increases and 5 decreases in antipyrine half‐life. The range of extremes was more than 2‐fold, from an increase in half‐life of 173% in one individual to a decrease of 42% in another. That these temporal changes were not random fluctuation is suggested by repeated studies performed in the 5 subjects who exhibited the largest alterations; when the temporal change exceeded 10%, values were reproducible with respect to direction as well as magnitude. The observation that neither the direction nor the magnitude of temporal variation was altered by antipyrine administration at 7:00 P.M., rather than 7:00 A.M., also supports the concept that an endogenous circadian rhythm affects drug metabolism. Another rhythm, noted over a much shorter period of time, was a waver in the curves for both saliva and plasma antipyrine concentrations of all subjects. Since the same rhythm was also observed when antipyrine was measured in numerous aliquots of a single sample, the waver appeared to be artifactual in nature arising from random variability in the assay.

Theobromine metabolism and pharmacokinetics in pregnant and nonpregnant Sprague-Dawley rats.

The plasma kinetics of po administered theobromine (TBR) were determined in timed-pregnant (P) and nonpregnant (NP) Sprague-Dawley rats at doses of 5, 10, 50, and 100 mg/kg using TBR sodium acetate with 10 microCi [8-14C]TBR as a radioactive tracer. Since plasma radioactivity consisted of greater than 99% TBR and less than 1% metabolites as shown by high-performance liquid chromatographic (HPLC) methods, liquid scintillation counting was used to quantify plasma TBR. No dose-dependent kinetics were observed in mean TBR plasma half-life, volume of distribution, systemic clearance, area under the curve-dose normalized, or time to reach maximum plasma concentration in either P or NP rats. The kinetic parameters of P rats were strikingly similar to NP rats at all TBR dosage levels employed. Analysis of urinary metabolites by HPLC and a radioactivity monitoring system after a single po TBR dose of 5 and 100 mg/kg with 10 microCi [8-14C] TBR revealed similar qualitative metabolic patterns in P and NP rats. Compounds identified in the urine were TBR (39 to 62%), 6-amino-5-[N-methylformylamino]-1-methyluracil (20 to 32%), 3-methylxanthine and 7-methylxanthine (8 to 15%), 3,7-dimethyluric acid (5 to 10%), and 7-methyluric acid (5 to 7%).

Diet-induced alterations in theobromine disposition and toxicity in the rat.

To assess the potential influence of diet on theobromine (TBR) disposition and the development of TBR-induced thymic and testicular toxicity, male Sprague-Dawley rats were given the following diets ad libitum for 28 days: (1) semipurified (S); (2) commercial chow (Ch); (3) semipurified + 0.6% TBR (S + TBR); or (4) commercial chow + 0.6% TBR (Ch + TBR). Toxicity endpoints determined in each TBR group indicated that Ch + TBR-treated animals did not exhibit the marked reduction in body weight or testicular atrophy induced by the S + TBR diet, although thymic weight was lower regardless of diet. Metabolic studies performed after the 28-day feeding period using 5 mg/kg TBR + 10 microCi [8-14C]TBR revealed an overall inductive effect of Ch on TBR metabolism as shown by increased urinary excretion (0-24 hr) of the major TBR metabolite, 6-amino-5[N-methylformylamino]-1-methyluracil (6-AMMU), as well as 7-methylxanthine + 3-methylxanthine (7-MX + 3-MX) and 3,7-dimethyluric acid (3,7-DMU). Consumption of 0.6% TBR for 28 days in either S or Ch diets also induced its own metabolism, as shown by decreased urinary excretion of unchanged TBR and increased conversion primarily to 3,7-DMU. Fecal 14C elimination (0-24 hr) was similar between animals fed S and Ch diets, indicating no effect of control diet on TBR bioavailability. Since serum TBR concentrations and overall toxicity were lower in Ch + TBR-treated animals than in S + TBR treated animals, yet TBR bioavailability was similar, this effect was attributed to the inducing potential of the Ch diet on TBR metabolism and clearance. Investigators are cautioned to consider the potential effect of diet on metabolism when performing and evaluating toxicological studies.

Dietary patterns and diurnal variations in aminopyrine disposition

In healthy, nonmedicated male subjects, patterns of eating and fasting determined diurnal variations in aminopyrine disposition. Under dietary conditions used in our laboratory to study temporal variation in the disposition of aminopyrine (a 12‐hr fast preceding 8 A.M. dose and a 5 P.M. meal before 8 P.M. drug dosing) there was a mean diurnal variation of 33% in saliva aminopyrine half‐life (t½) and of 46% in apparent volume of distribution (aVd) with shorter t½ and lower aVd at 8 A.M. Mean metabolic clearance rate (Cl) was unchanged. Fasting for 32 and 44 hr before aminopyrine at 8 A.M. and 8 P.M. ablated this temporal variation in aminopyrine t½ and aVd. Reversal of eating times (a 12‐hr fast before the 8 P.M. aminopyrine dose and a 5 A.M. meal before the 8 A.M. dose) reversed the direction of the diurnal variation of aminopyrine t½ and aVd so that aminopyrine t½ and aVd were larger at 8 A.M. but fasting for 32 or 44 hr did not alter the normal circadian rhythms of plasma 11‐hydroxycorticoids identified in our subjects at 8 A.M. and 8 P.M. Plasma protein binding of aminopyrine, determined at 8 A.M. after a 12‐hr fast (29.8%), was higher than at 8 P.M. (23.3%, p < 0.05). Aminopyrine protein binding rose to 38.1% and 32.9% after a fast of 32 and 44 hr. Mean bioavailability and area under the curve of aminopyrine determined in plasma at 8 A.M. and 8 P.M. showed no diurnal change. As in saliva, aminopyrine concentrations in plasma revealed that mean aminopyrine t½ was 34% longer, mean aminopyrine aVd 33% larger, but mean aminopyrine Cl the same at 8 P.M. as at 8 A.M. These results suggest that meals decrease protein binding of aminopyrine, leading to increased aminopyrine aVd and t½, possibly due to increased tissue uptake of aminopyrine. Aminopyrine plasma concentrations after oral doses of 2, 4, and 8 mg/kg at 1‐wk intervals to each of six normal male subjects revealed dose dependence of aminopyrine t½ and hepatic first‐pass effect.

A Sensitive Gas-Chromatographic Assay Using a Nitrogen-Phosphorus Detector for Determination of Antipyrine and Aminopyrine in Biological Fluids

A method using gas chromatography with organic nitrogen-sensitive detection is described for measurement of antipyrine and aminopyrine concentrations in biological fluids. The analysis was performed isothermally on 3% SP-2250 DB after alkalinized saliva was extracted into chloroform. Phenacetin served as internal standard. Low oral doses of antipyrine (1.0-1.8 mg/kg) and/or aminopyrine (2 mg/kg) were measured accurately in saliva of normal human subjects. The standard curves for antipyrine and aminopyrine were linear from 0 to 10 microgram/ml. The coefficient of variation, determined at a salivary concentration of 2 microgram/ml, was 1.7% for antipyrine and 2.4% for aminopyrine. Saliva concentrations obtained by this method in normal human subjects after either an oral dose of antipyrine (18 mg/kg) or aminopyrine (9 mg/kg) agreed closely with those determined by the flame ionization gas-chromatographic method used to measure higher concentrations of antipyrine and aminopyrine. Antipyrine (1.8 mg/kg) administered concomitantly with aminopyrine (1 mg/kg or 2 mg/kg) to normal male volunteers prolonged mean saliva antipyrine half-life by about 25-33% compared to values obtained when these same subjects received the same dose of antipyrine alone.