Georges Olive

Vigabatrin : clinical pharmacokinetics

Vigabatrin is a structural analogue of the inhibitory neurotransmitter gamma-aminobutyric acid (GABA). It is supplied as a racemic mixture, with the S(+) enantiomer possessing pharmacological activity. [R,S]-Vigabatrin plasma concentrations can be estimated using high-performance liquid chromatographic methods. Only gas chromatography-mass spectrometry methods allow quantification of the S(+) and R(-) enantiomers. Vigabatrin was rapidly absorbed reaching peak concentrations within 1 to 2h. Area under plasma concentration-time curves indicated dose-linear pharmacokinetics. There was no effect of food on the absorption of vigabatrin. The absorption characteristics of the enantiomers were similar to those of the [R,S]-vigabatrin. No chiral inversion was detected after administration of the pure S(+) enantiomer. Vigabatrin is not protein bound. The apparent volume of distribution of [R,S]-vigabatrin was approximately 0.8 L/kg. Despite the lack of protein binding, cerebrospinal concentrations of the [R,S]-vigabatrin were only 10% of the plasma concentration 6h after a single oral dose. The half-life of [R,S]-vigabatrin was between 5.3 and 7.4h, the half-life of the enantiomers were 7.5 and 8.1h for the S(+) and the R(-) forms, respectively. The major route of elimination was renal excretion; urinary recovery of the [R,S]-vigabatrin was close to 70%. Pharmacokinetic studies in epileptic children did not show any significant effect of maturation on the disposition of the S(+) enantiomer: the half-life and the renal clearance were similar to adult values. Data suggest a lower bioavailability in children. In adults with epilepsy, the half-life of the [R,S]-vigabatrin ranged from 4.2 and 5.6h, similar to that measured in healthy adults. In elderly nonepileptic volunteers the pharmacokinetics of the enantiomers of vigabatrin showed delayed absorption, a major increase in peak concentration and a prolonged half-life. These changes were attributed to decreased renal clearance of vigabatrin. A nonlinear relationship between renal clearance and creatinine clearance was suggested. Vigabatrin caused a 20% fall in plasma phenytoin concentrations, the mechanism of which has not been elucidated. There were no other interactions with most concurrently administered anticonvulsants. The usual dosage of vigabatrin as add-on treatment in adults is 2 to 4g daily. Higher dosages up to 80 mg/kg daily were required in children. A dosage adjustment was recommended in any patient with decreased renal clearance. Although anticonvulsant effects were clearly related to dosage, monitoring of plasma concentrations of vigabatrin as a guide to dosage is unlikely to be of as much value as with other antiepileptic drugs. The action of the drug long outlasts its presence in plasma.

Influence of stiripentol on cytochrome P450-mediated metabolic pathways in humans: In vitro and in vivo comparison and calculation of in vivo inhibition constants

The spectrum of cytochrome P450 inhibition of stiripentol, a new anticonvulsant, was characterized in vitro and in vivo.

Maturation of caffeine metabolic pathways in infancy.

The maturation of the different pathways of caffeine metabolism was studied during infancy. The group of children (n = 14) consisted of four premature newborn infants and 10 older infants who received caffeine citrate solution. Caffeine and 11 of its metabolites were measured by HPLC. Total demethylation and N3‐ and N7‐demethylation increase exponentially with postnatal age; the plateau is reached by 120 days and accounts for 58.6%, 90.5%, and 79.3%, respectively. N1‐demethylation shows no variation with postnatal age. It is suggested that N3‐demethylation is more important in young infants than in adults and that maturation of N1‐demethylation occurs later than 19 months of age. 8‐Hydroxylation is mature as early as 1 month of age and may be higher in infants than in adults. Acetylation is not mature before at least 1 year of age. Differences in maturation rate of acetylation may be related in part to genetic acetylator status.

Caffeine acetylator phenotyping during maturation in infants

ABSTRACT: Caffeine acetylator phenotype was studied during maturation in 54 8− to 447-d-old children hospitalized for minor disease (group A) and in five 3− to 630-d-old children with Pierre Robin syndrome (group B). In group A, the children received 2.5 mg/kg caffeine orally once between birth and 15 mo. Group B patients were chronically treated with caffeine (2.3 to 15.8 mg/kg/d) for prevention of apneas, and the acetylator phenotype was serially determined. Phenotyping was performed on a spot urine sample collected 2–6 h after drug administration. Caffeine metabolites [5-acetylamino-6-formylamino-3-methyl uracil (AFMU), 1-methylxanthine, 1-methyluric acid, 1,7-methyluric acid, and 1,7-methylxanthine) were measured using HPLC. Acetylator phenotype was determined on the basis of AFMU/1-methylxanthine (ratio 1) and AFMU/AFMU + 1-methyluric acid + 1-methylxanthine + 1,7-methylxanthine + 1,7-methyluric acid (ratio 2) molar ratios. In group A, all children were slow acetylators before 83 d of age (ratio 1 < 0.4; ratio 2 < 0.08), whereas older children included slow and fast acetylators. The acetylation molar ratios differed significantly between age groups and increased with age. The cumulative percentage of fast acetylators increased with age but the plateau was not yet reached at 15 mo. In three children, the phenotyping was repeated after 15 mo: the second determination was consistent with the first one. In group B, all children appeared as slow acetylators on the first phenotyping. Four of them appeared subsequently as fast acetylators; one remained a slow acetylator until 11 mo. These results suggest that maturation of caffeine acetylation occurs during at least the first 15 mo of life for fast acetylators but is not detectable in slow acetylators. Acetylator status cannot reliably be determined before at least 15 mo.

Maturation of caffeine N-demethylation in infancy: a study using the 13CO2 breath test.

ABSTRACT: Four premature neonates and eight infants 1–19 months old received caffeine for apnea. The usual morning oral dose was substituted by 1,3,7 13C-trimethylxanthine (13C-tri CAF) as the citrate salt. Five breath samples were collected the day before (day 1) and the day of 13C-tri CAF administration (day 2). Plasma (after each breath collection) and urine were collected on day 2.13C-CO2 exhalation was determined by isotope ratio mass spectrometry. Caffeine and its metabolites were measured using high-pressure liquid chromatography. Assessment of the labeled CO2 in the breath revealed no detectable 13C-tri CAF N-demethylation activity in infants before 45 wk postconceptional age. However, demethylation (as urinary metabolites) has been detected before that age. Two-, 4-, and 6-h cumulative excretion of 13C-tri CAF as 13C-CO2 increased with postnatal age and correlated with caffeine plasma clearance (r = 0.840, p < 0.01). These results were consistent with those obtained for urinary metabolites. In one infant (19 months old) the cumulative excretion of 13C-CO2 while crying was 65% of the value observed during quiet breathing. The measurement of caffeine demethylation using the caffeine CO2 breath test is feasible in infants and is a safe and noninvasive method to determine age related changes in P4501-dependent N-demethylase activity.

Isoniazid acetylation metabolic ratio during maturation in children

Isoniazid acetylation metabolic ratio (MR) was studied in 61 children with tuberculosis after administration of isoniazid. MR was calculated as the molar acetylisoniazid to isoniazid concentration ratio. MR was used as a probe for N‐acetyltransferase activity and to determine the acetylation phenotype. MR had a bimodal distribution with an antimode between 0.48 and 0.77. MR and the percentage of fast acetylators increased significantly with age. The cumulative frequency of fast acetylators increased with age, with a plateau reached around 4 years. MR value was checked during treatment in 44 children. All children but one who initially appeared as fast acetylators remained in this group after repeated testing. Among the 30 slow acetylators, 12 became fast acetylators, and 10 showed a variable phenotyping at different ages. A bimodal distribution of the isoniazid acetylation MR was shown in children, with an antimode close to that described in the literature and a maturation of isoniazid acetylation during the first 4 years.

Developmental changes of caffeine elimination in infancy

Five neonates (4 premature) and 16 infants (6 prematurely born), 15-588 days old, received caffeine as citrate salt for apnea. Plasma samples were collected 0, 2, 4, 6 h after a dose and before the next scheduled one. Patients 8 and 9 were serially studied. Caffeine plasma concentrations were determined using HPLC. The caffeine elimination half-life and clearance varied linearly with gestational age and exponentially with postnatal age, the plateau being reached during the second trimester of life. Dose regimen guidelines as a function of postnatal age were derived from individually calculated doses and dosing intervals in order to achieve, at steady state, a caffeine mean plasma concentration of 11 mg/l with a minimum of 7.5 mg/l and a maximum of 14.5 mg/l. We suggest dosing intervals for infants before 1 month, 1-2 months, 2-4 months and after 4 months to be equal to 24, 12, 8 and 6 h, respectively. The individual recommended dose varies from 2 to 10 mg/kg (as caffeine base) making caffeine monitoring mandatory in infants.

Transfer of Salbutamol in the Human Placenta in vitro

The maternal-fetal transfer of Salbutamol was studied in vitro, in dual perfusion of isolated human placental lobules. The fraction of drug transferred to the fetal side was compared to reference substances:((14)C)-antipyrine and ((3H)-inulin. Salbutamol transfer (12%) represents about 39% of antipyrine and five times of inulin transfer. Studies with varying Salbutamol concentrations in the maternal arterial perfusate revealed that the drug transfer to the fetal side was effected by a flow-dependent diffusion process. We conclude that the reported effects on the fetus following a maternal drug infusion are mediated by the drug transferred across the placenta.

Culture of endothelial cells from human placental microvessels

Abstract.Endothelial cells are known to participate in angiogenesis, adaptation of vascular tonus and maintenance of blood fluidity in the microcirculation. To investigate these functions in the placenta, we devised a method of isolation and culture of endothelial cells from villous microvessels. In primary culture, these intraplacental endothelial cells exhibited many features observed in microvascular endothelium from other organs: spindle-shape, rosette associations, circular arrangements and confluence. In contrast to the confluent endothelial cells derived from the umbilical vein, cells from microvessels did not form a cobblestone network. After trypsin digestion of microvessels, magnetic microbeads coated with S-Endol immunoglobulin, antithrombomodulin and Ulex europaeus-I lectins were tested for sorting endothelial cells. Only the microbeads coated with antithrombomodulin allowed a suitable magnetic cell separation after trypsinization. By contrast, the microbeads coated with each of these antibodies or with lectins attached to confluent cells from the second passage. The microbeads detached from the cells at different rates. Their examination by scanning microscopy indicates that a portion of these microbeads was phagocytosed. Microvascular endothelial cells from the second passage were intensively stained by the anti-von Willebrand reaction and only weakly by the anti-smooth muscle α-actin reaction. They incorporated acetylated-low density lipoproteins coupled to a fluorescent probe. The positive reactions against the anti-von Willebrand factor and the uptake of the fluorescent acetylated-low density lipoproteins were modified after eight passages.

Isoniazid dose adjustment in a pediatric population.

This retrospective analysis was designed to evaluate the inactivation index (I3) method used to adjust the isoniazid dose during long-term administration in a pediatric population. Before starting on antituberculosis therapy, sixty-one children received one 10 mg.kg-1 isoniazid test-dose (D). The isoniazid and acetyl isoniazid concentrations were measured by high-performance liquid chromatography on a plasma sample collected 3 hours (C3h) after administration. The patients were separated into slow and fast acetylator groups according to the metabolic ratio. The dose adjustment method using the I3 is based on the assumption that there is a linear correlation between C3h and D [C3h = (I3 x D) - 0.6] in which the slope is I3 and the Y intercept is equal to -0.6 mg.l-1. I3 was determined from a single plasma concentration determination and used to calculate the dose recommended to obtain a desired C3h equal to 1.5 micrograms.ml-1: recommended dose (mg.kg-1) = (1.5 + 0.6)/I3.I3 was significantly higher in the slow acetylator group (0.55 +/- 0.16) than in the fast one (0.26 +/- 0.13), which leads us to recommend a significantly lower dose in the slow acetylator group (4.2 +/- 1.5 mg.kg-1) than in the fast one (10.3 +/- 4.6 mg.kg-1). The data obtained in a subgroup of 21 patients who had at least three consecutive determinations of C3h after different dosages allowed us to verify that there was a linear correlation between C3h and the dose. The mean slope of the correlation lines in that subgroup was 0.61 +/- 0.25 and the 95% confidence interval of the estimated Y-intercept include the theoretical value of -0.60, which shows that our data are consistent with those previously reported in adults. The percentage of patients with a C3h plasma concentration within the expected range (1.5 +/- 0.5 micrograms.ml-1) was significantly higher (69%) in those whose dose was derived from the calculation than in the others (25%). Within each acetylator group, the range of the recommended dose varied widely, and these results emphasize the usefulness of individual dose adjustment based on the inactivation index method.