W. D. Hooper

Plasma protein binding of diphenylhydantoin. Effects of sex hormones renal and hepatic disease.

The in vitro binding of diphenylhydantoin (DPH) to the protein in plasma from 97 volunteers has been studied using ultrafiltration at 37° C. The capacity of plasma protein to bind DPH did not differ significantly between pregnant women (11.6 ± 1.7% of total drug unbound), women taking oral contraceptives (9.9 ± 1.7% unbound), healthy males (10.6 ± 1.3% unbound), and healthy females (11.0 ± 3.2% unbound). However, in plasma trom patients with renal disease (15.8 ± 3.9% unbound), hepatic disease (15.9 ± 6.0%) or hepatorenal disease (15.6 ± 5.4%), the protein binding of DPH was significantly decreased. These changes in protein binding were found to correlate better with changes in albumin and bilirubin levels in plasma than with any of 13 other biochemical parameters examined.

Valproate-Associated Hepatotoxicity and its Biochemical Mechanisms

SummaryIntake of the anticonvulsant drug valproic acid, or its sodium salt, has been associated with occasional instances of severe and sometimes fatal hepatotoxicity. Probably at least 80 cases have occurred worldwide. The syndrome affects perhaps 1 in 10,000 persons taking the drug, and usually develops in the early weeks or months of therapy. Most instances have involved children, usually those receiving more than 1 anticonvulsant. Multiple cases have occurred in 2 families. The typical presentation is of worsening epilepsy, increasing depression of consciousness, and progressive clinical and biochemical evidence of liver failure. The liver has sometimes shown hepatocyte necrosis, and on other occasions widespread microvesicular steatosis, while cholestatic changes have also occurred. The appearances are interpreted as consistent with a drug toxicity reaction.During the hepatotoxicity increased amounts of unsaturated metabolites of valproate, notably 4-en-valproate, have been found in blood and urine. In 4 cases there has been evidence of impaired β-oxidation of valproate with, in 1 case, accumulation of isomers of valproate glucuronide caused by intramolecular rearrangement of the conjugate. There are molecular structural similarities between 4-en-valproate and 2 known hepatotoxins (4-en-pentanoate and methylenecyclopropylacetic acid, the latter being responsible for hypoglycin poisoning). There are also clinical and histopathological similarities between valproate hepatotoxicity and both hypoglycin poisoning and certain spontaneous disorders of isoleucine metabolism (one pathway ofvalproate metabolism is analogous to oxidative degradation of isoleucine). Unsaturated metabolites of valproate, in particular 4-en-valproate, may contribute to the hepatotoxicity of the drug. However, since the hepatotoxicity appears to involve an element of idiosyncrasy, the primary defect in some cases may be an inherited or acquired deficiency in the drug’s β-oxidation. This defect may divert valproate metabolism towards ω-oxidation, with increased formation of the toxin 4-en-valproate, but may also allow increased formation of a toxic metabolite derived from isoleucine, since β-oxidation of isoleucine derivatives will also be impaired.

Plasma protein binding of carbamazepine.

The binding of carbamazepine to the pro teins of human plasma has been studied using ultrafiltration techniques. In vitro studies at 37° C showed the relation between concentration of unbound drug and total drug to be linear thraugh the range of total concentration of 5 to 50 µg/ml. The per cent unbound drug increased slightly as concentration increased. There was tittle difference between the extent 0/ binding at 4° C and 20° C, but more carbamazepine was unbound at 37° C. Under in vitro conditions, 6 other anticonvulsants, and aspirin, were tested individually, each at high therapeutic or toxic concentration, and shown not to displace carbamazepine from plasma proteins to a significant degree. The extent of binding of carbamazepine in vivo was determined in a total of 54 plasma sampies collected from treated patients; 26.9 ± SD 9.4% of the drug was unbound. In blood sampies from 23 of these patients, the red cell concentration of carbamazepine averaged 38.3 ± SD 17.9% of the plasma concentration. The effects of hepatic and renal diseases on the carbamazepine binding capacity of plasma proteins were assessed by comparing the binding capacity of plasma from diseased persons with that from normal subjects. There was no significant difference in binding capacity between plasma from patients with renal disease and that from normal subjects. However, the plasma from patients with hepatic disease bound a stightly lower percentage of carbamazepine than did normal plasma (p < 0.05). This alteration did not correlate with changes in any of 15 biochemical parameters measured in these patients. The clinical significance of these results is discussed.

THE INFLUENCE OF AGE AND GENDER ON THE STEREOSELECTIVE METABOLISM AND PHARMACOKINETICS OF MEPHOBARBITAL IN HUMANS

In this clinical investigation, four groups of subjects (eight young women and eight young men [age range, 18 to 25 years], and eight elderly women and eight elderly men [>60 years of age]) received single oral doses (400 mg) of racemic mephobarbital. The apparent total body clearance of R‐mephobarbital was much greater and the elimination half‐life was much shorter in the young men compared with the other three groups. This enantiomer displayed an age‐dependent gender effect and a gender‐dependent age effect in its metaoblism. The apparent total body clearance of the S‐enantiomer was much lower than that of the R‐enantiomer in all subjects and did not differ between subject groups, although the elimination half‐life was slightly but signifiantly shorter in young males. A consequence of these enantiomeric differences was an apparently enhanced stereoselectivity in the metabolism of mephobarbital in young men. These substantial influences of age and gender on the stereoselective disposition of mephobarbital are consistent with recent findings concerning the expression and regulation of cytochrome P450 enzymes.

Urinary excretion of valproate and some metabolites in chronically treated patients

Urinary excretion of the antiepileptic agent valproic acid (VPA) and major metabolites from its glucuronidation, β-oxidation, and ω- and ω1-hydroxylation pathways were studied under steady state conditions in 24 epileptic patients. Some 55 \pm 18% (SD) of the daily dose was recovered in urine, 33 \pm 14% in the form of VPA-glucuronide, 15 \pm 8% as β-oxidation products, and 4 \pm 2% and 2 \pm 1% as products of the ω- and ω1-hydroxylation pathways, respectively. Only 1 \pm 2% of the dose was excreted unchanged. The proportion metabolized by direct glucuronidation tended to increase with dose at the expense of the oxidative pathways, particularly β-oxidation. However, the wide variation in the patterns of urinary metabolite excretion precludes use of routinely collected urinary excretion data as a basis for detecting any but severe noncompliance with VPA therapy or abnormalities of VPA metabolism.

First dose and steady-state pharmacokinetics of oxcarbazepine and its 10-hydroxy metabolite

SummaryThe pharmacokinetics of oxcarbazepine (a new anticonvulsant which is a congener of carbamazepine) and of its 10-hydroxy metabolite were studied at the outset of therapy in 8 adult epileptics comedicated with other anticonvulsants. The pharmacokinetic study was repeated under steady-state conditions after 3 months of drug intake in 6 of these subjects.The plasma elimination half-life of oxcarbazepine appeared to lie in the range 1.0–2.5 h, and that of its 10-hydroxy metabolite averaged 8.4 h. The apparent oral clearance of the parent drug (averaging 2.51·kg−1·h−1) was high enough to suggest substantial presystemic elimination. The oral clearance fell after 3 months of drug intake, but the half-lives of the drug and metabolite showed no statistically significant change over this time. Steady-state plasma levels of both drug and metabolite were linearly related to drug dose, metabolite levels averaging 9 times those of the parent substance.

Aspects of tetrazolium salt reduction relevant to quantitative histochemistry

SummaryUncertainty about the nature of the reduction products of ditetrazolium salts may have limited their use in quantitative histochemistry. Our studies have shown that under appropriate conditions pure Nitro-BT reduces through a red intermediate substance to a stable blue diformazan. Nitrobenzene was found to be a satisfactory solvent for this diformazan. The monotetrazolium INT may also be reduced to a formazan through an intermediate phase. The amounts of definitive formazan produced from both monotetrazolium and ditetrazolium salts may be influenced by the solubility of their intermediate reduction compounds in the systems in which reduction is occurring. The yield of definitive diformazan from Nitro-BT after chemical reduction, and after enzymatic reduction in liver homogenate and sections of a “mock” tissue, was not in linear proportion to the strength of reducing conditions; however, the yields of formazan from the monotetrazoliums INT and MTT were linear. This finding suggests that in quantitative histochemistry it is essential to calibrate reactions involving ditetrazolium reduction.

Effect of felbamate on valproic acid disposition in healthy volunteers : inhibition of β-oxidation

Summary: We assessed the effects of felbamate (FBM) on the disposition of valproic acid (VPA) in healthy volunteer men. Eighteen subjects received sodium VPA, 400 mg/day for 21 days. Plasma and urine samples were taken on day 7 to document the steady‐state disposition of VPA alone. From day 8 to day 21, subjects received placebo or FBM at the following doses (mg/day): 1,200, 2,400, 3,000, or 3,600 (n = 2–4 per group). Many adverse events (AE) occurred from about day 10; 2 subjects dropped out and 1 continued on a reduced FBM dose. Pharmacokinetic studies were repeated on day 21 for the 16 subjects who completed the study. FBM was measured in plasma and urine by high‐performance liquid chromatography (HPLC). VPA and its 2–en, 4–en, and 3–oxo metabolites in plasma, and VPA (nonconjugated and total), and its 3–oxo and 4–hydroxy metabolites in urine, were measured by gas chromatography/mass spectrometry (GUMS). Mean plasma FBM trough concentrations on day 21 ranged from 26.9 μg/ml (1,200 mg dose) to 76.8 μg/ml (3,600–mg dose). Mean plasma VPA Cmax values were 32–42 μg/ml in the various subgroups when VPA only was administered. Higher plasma VPA levels were observed when FBM was administered concurrently (55.4–63.8 μg/ml). The excretion of 3–oxo–VPA in urine was significantly lower on day 21 than on day 7, whereas VPA‐glucuronide was significantly increased. The effects of FBM on VPA disposition were dose dependent and were maximal at ∼2400 mg/day. FBM had caused significant inhibition of the β‐oxidation pathway for VPA metabolic clearance, and this had been largely compensated by increased VPA glucuronidation.

Markers of physical integrity and metabolic viability of the perfused human placental lobule

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Single-dose pharmacokinetics of metoclopramide

SummaryThe time courses of plasma metoclopramide concentrations were followed in six subjects after oral and intravenous single dose administration. Plasma concentration-time data following i.v. administration in each subject were found to fit a two compartment model with a mean terminal half-life of 4.55 h±0.80 h and a mean distribution half-time of 0.35 h±0.09 h. Volumes of distribution were high (3.43±1.181 · kg−1), and clearances (0.53±0.191 · kg−1h−1) approached liver plasma flow. This suggests that metoclopramide occurs at higher concentrations in tissues than in plasma, and that its clearance is probably limited by liver blood flow rather than liver metabolic capacity. The post-absorption decline in metoclopramide plasma levels after oral administration was also biexponential in each subject. The terminal half-life was 5.17 h±0.98 h. Mean volume of distribution and mean clearance were similar to intravenous values (after adjustment for bioavailability). Oral absorption was rapid with peak plasma concentrations being reached at a mean time of 0.93 h. A mean bioavailability of 0.77 was calculated for the six subjects, and it was postulated that this incomplete availability is due to a first-pass effect. The inter-individual variation in the degree of ‘first-pass’ was considerable (0.47–1.14).