G. M. Pacifici

Placental Transfer of Drugs Administered to the Mother

SummaryDrugs administered to mothers have the potential to cross the placenta and reach the fetus. Under particular circumstances, the comparison of the drug concentration in the maternal and fetal plasma may give an idea of the exposure of the fetus to the maternally administered drugs. In this review drugs are classified according to their type of transfer across the placenta.Several drugs rapidly cross the placenta and pharmacologically significant concentrations equilibrate in maternal and fetal plasma. Their transfer is termed ‘complete’. Other drugs cross the placenta incompletely, and their concentrations are lower in the fetal than in maternal plasma. The majority of drugs fit into 1 of these 2 groups. A limited number of drugs reach greater concentrations in fetal than maternal plasma. It is said that these drugs have an ‘exceeding’ transfer. The impression prevails that suxamethonium chloride (succinylcholine chloride) and doxorubicin do not cross the placenta. However, a careful analysis of the literature suggests that this impression is wrong and that all drugs cross the placenta, although the extent transfer varies considerably.The following parameters were considered as possible factors determining the extent of placental transfer: (i) the molecular weight of the drug; (ii) the pKa (pH at which the drug is 50% ionised); and (iii) the extent of drug binding to the plasma protein. Drugs with molecular weights greater than 500D have an incomplete transfer across the human placenta. Strongly dissociated acid drug molecules should have an incomplete transfer, but this does not seem to be an absolute rule. For example, ampicillin and methicillin transfer completely and they are strongly dissociated at physiological pH. The extent of drug binding to plasma protein does not influence the type of drug transfer across the human placenta.

Methods of determining plasma and tissue binding of drugs. Pharmacokinetic consequences.

SummaryThe available techniques for the investigation of drug binding to plasma and tissues protein are reviewed and the advantages and disadvantages of the various techniques stated. A comparison of different plasma protein binding techniques is made which shows that the size of the unbound fraction of drug may be influenced by the method used. Protein binding may be assayed by methods including equilibrium dialysis, ultrafiltration, ultracentrifugation, gel filtration, binding to albumin microspheres and circular dichroism. Tissue binding techniques can involve testing binding to isolated organs, tissue slices, homogenates and isolated subcellular particles. Details of the available methods to compute pharmacokinetic constants are given. Stereoselective binding has been investigated for a limited number of drugs and the difference in the binding of 2 enantiomers is usually modest. The measurement of the binding constants is often required to characterise the drug-protein interaction. Mathematical and graphical methods to compute the pharmacokinetic parameters are discussed. The implications of binding on the volume of distribution and clearance of drugs are examined.

Sulphation of resveratrol, a natural compound present in wine, and its inhibition by natural flavonoids

1. Resveratrol, a polyphenolic compound present in grape and wine, has beneficial effects against cancer and protective effects on the cardiovascular system. Resveratrol is sulphated, and the hepatic and duodenal sulphation might limit the bioavailability of this compound. The aim of this study was to see whether natural flavonoids present in wine, fruits and vegetables inhibit the sulphation of resveratrol in the human liver and duodenum. 2. In the liver, IC50 for the inhibition of resveratrol sulphation was 12 ± 2 pM (quercetin), 1.0 ± 0.04 μM (fisetin), 1.4 ± 0.1 μM (myricetin), 2.2 ± 0.1 μM (kaempferol) and 2.8 ± 0.2 μM (apigenin). Similarly, in the duodenum, IC50 was 15 ± 2 pM (quercetin), 1.3 ± 0.1 μM (apigenin), 1.3 ± 0.5 μM (fisetin), 2.3 ± 0.1 μM (kaempferol) and 2.5 ± 0.3 μM (myricetin). 3. The type of inhibition of quercetin on resveratrol sulphation was studied in three liver samples and was determined to be non-competitive and mixed in nature. Km (mean ± SD; μM) was 0.23 ± 0.07 (control), 0.40 ± 0.08 (5 pM quercetin) and 0.56 ± 0.09 (10 pM quercetin). Vmax (mean ± SD; pmol·min−1·mg−1) was 99 ± 11 (control), 73 ± 15 (5 pM quercetin) and 57 ± 10 (10 pM quercetin). K1 and K1es estimates (mean ± SD) were 3.7 ± 1.8 pM and 12.1 ± 1.7 pM respectively (p = 0.010). 4. Chrysin was a substrate for the sulphotransferase(s) and an assay was developed for measuring the chrysin sulphation rate in human liver. The enzyme followed Michaelis‐Menten kinetics and Km and Vmax (mean ± SD) measured in four livers were 0.29 ± 0.07 μM and 43.1 ± 1.9 pmol·min−1·mg−1 respectively. 5. Catechin was neither an inhibitor of resveratrol sulphation nor a substrate of sulphotransferase. 6. These results are consistent with the view that many, but not all, flavonoids inhibit the hepatic and duodenal sulphation of resveratrol, and such inhibition might improve the bioavailability of this compound.

Glucuronidation of resveratrol, a natural product present in grape and wine, in the human liver

1. Resveratrol, a polyphenolic compound present in grape and wine, has beneficial effects against cancer and protective effects on the cardiovascular system. It has been shown that the compound is sulphated in human liver and the aims of the present investigation were to study resveratrol glucuronidation in human liver microsomes and to determine whether flavonoids inhibit resveratrol glucuronidation. 2. A simple and reproducible radiometric assay for resveratrol glucuronidation was developed. The assay employed uridine-5′-diphosphoglucuronic acid-[14C] and unlabelled resveratrol. Resveratrol-glucuronide was isolated by TLC. The intra- and interassays variabilities were 1 and 1.5%, respectively. 3. The rate of resveratrol glucuronidation was measured in 10 liver samples. The mean ± SD and median of resveratrol glucuronidation rate were 0.69 ± 0.34 and 0.80 nmol/min/mg, respectively. Resveratrol glucuronosyl transferase followed Michaelis-Menten kinetics and the Km and Vmax (mean ± SD; n = 5) were 0.15 ± 0.09 mm and 1.3 ± 0.3 nmol/min/mg, respectively. The intrinsic clearance was 11 ± 4 × 10−3 ml/min.mg. 4. The flavonoid quercetin inhibited resveratrol glucuronidation and its IC50 (mean ± SD; n = 3) was 10 ± 1 μM. Myricetin, catechin, kaempferol, fisetin and apigenin (all at 20 μM) inhibited resveratrol glucuronidation and the percent of control ranged between 46% (catechin) to 72% (apigenin). 5. The present results show that resveratrol is glucuronated in the human liver. Glucuronidation may reduce the bioavailability of this compound however, flavonoids inhibit resveratrol glucuronidation and such an inhibition might improve the bioavailability of resveratrol.

Clinical pharmacokinetics of aminoglycosides in the neonate: a review

BackgroundSepsis is common in neonates and is a major cause of morbidity and mortality. Sixty percent of preterm neonates receive at least one antibiotic, and 43% of the antibiotics administered to these neonates are aminoglycosides. The clearance (Cl), serum half-life (t1/2), and volume of distribution (Vd) of aminoglycosides change during the neonatal life, and the pharmacokinetics of aminoglycosides need to be studied in neonates in order to optimise therapy with these drugs.ObjectiveThe aim of this work is to review the published data on the pharmacokinetics of aminoglycosides in order to provide a critical analysis of the literature that can be a useful tool in the hands of physicians.MethodsThe bibliographic search was performed electronically using PubMed, as the search engine, through July 11th, 2008. Firstly, a Medline search was performed with the keywords “pharmacokinetics of aminoglycosides in neonates” with the limit of “human”. Other Medline searches were performed with the keywords “pharmacokinetics of … in neonates” followed by the name of the aminoglycosides: amikacin, gentamicin, netilmicin and tobramycin. In addition, the book Neofax: A Manual of Drugs Used in Neonatal Care by Young and Mangum (Thomson Healthcare, 2007) was consulted.ResultsThe aminoglycosides are mainly eliminated by the kidney, and their elimination rates are reduced at birth. As a consequence Cl is reduced and t1/2 is prolonged in the neonate as compared to more mature infants. The high body-water content of the neonate results in a large Vd of aminoglycosides as these drugs are fairly water soluble. Postnatal development is an important factor in the maturation of the neonate, and as postnatal age proceeds, Cl of aminoglycosides increases.ConclusionThe maturation of the kidney governs the pharmacokinetics of aminoglycosides in the infant. Cl and t1/2 are influenced by development, and this must be taken into consideration when planning a dosage regimen with aminoglycosides in the neonate. Aminoglycosides are fairly water soluble, and the larger water content of neonates yields a larger Vd in these patients.

Distribution of UDP-glucuronosyltransferase and its endogenous substrate uridine 5′-diphosphoglucuronic acid in human tissues

SummaryThe activity of UDP-glucuronosyltransferase (UDPGT) and the concentration of its endogenous substrate, 5′-diphosphoglucuronic acid (UDPGA), have been measured in human liver, kidney, lung and intestinal mucosa.The activity of UDPGT was tissue- and substrate-dependent. The liver/kidney and liver/intestine ratios for UDPGT varied over one order of magnitude with three substrates. The highest activity of UDGPT in extrahepatic tissues was in the kidney, with 1-naphthol as substrate; it was about half of the hepatic activity.The concentration (μmol · kg−1) of UDPGA was 279 (liver), 17.4 (kidney), 19.3 (intestinal mucosa) and 17.2 (lung), it was at least 15-fold higher in liver than the other tissues, and the concentration in kidney, lung and intestinal mucosa was similar.The kinetics of UDPGT in a liver homogenate at varying concentrations of UDPGA and fixed concentration of 1-naphthol, ethinyloestradiol, and morphine was also measured. The apparent kM for UDPGT depended upon the chemical nature of the UDPGA-acceptor substrate; average values of kM were 63, 300, and 700 μmol · 1−1 for 1-naphthol, ethinyloestradiol and morphine respectively. These values are, respectively, lower, similar to and higher than the hepatic concentration of UDPGA.Under certain circumstances UDPGA may be the limiting factor in the in vivo glucuronidation of drugs by extrahepatic tissues.

Differential Distribution of Phenol and Catechol Sulphotransferases in Human Liver and Intestinal Mucosa

Phenol and catechol sulphotransferases were studied with p-nitrophenol and dopamine as substrates in the mucosa of the ileum and colon obtained from 6 subjects and also in the liver from 6 subjects. The ileum and colon were from the same donor. The kinetics of phenol and catechol sulphotransferases were studied in each tissue specimen. The maximum velocity of reaction (Vmax) for phenol sulphotransferase (in pmol X min-1 X mg-1; mean +/- SD) was 165 +/- 28 (ileum), 79 +/- 42 (colon) and 1,361 +/- 370 (liver), whereas Vmax for catechol sulphotransferase was 489 +/- 75 (ileum), 198 +/- 93 (colon) and 39 +/- 23 (liver). Phenol sulphotransferase is the predominant pathway in the liver, whereas catechol sulphotransferase is the predominant pathway in the intestine. The ileum catalysed the sulphation of p-nitrophenol and dopamine at a higher rate than the colon. The Michaelis-Menten constant (Km) for phenol sulphotransferase (in mumol/l; mean +/- SD) was 0.96 +/- 0.11 (ileum), 1.00 +/- 0.19 (colon) and 0.84 +/- 0.07 (liver), whereas Km for catechol sulphotransferase was 17.8 +/- 2.8 (ileum), 18.2 +/- 3.4 (colon) and 21.4 +/- 1.2 (liver). Km values of hepatic phenol or catechol sulphotransferases are not different from those of intestinal enzymes. Previous work has shown that 2-naphthol sulphotransferase obeys non-Michaelis-Menten kinetics in the human intestinal mucosa [Pharmacology, 1988;43:411]. Here, we show that 2-naphthol is sulphated by at least two enzymes in human intestine.

Glutathione S-transferase in humans: development and tissue distribution.

Glutathione S-transferase (GST) was investigated with benzo(a)pyrene-4,5-oxide (BPO) as substrate in tissue specimens from 26 fetal and 27 adult livers and 27 placentas. The average (±SEM) of GST activity in the cytosol was 1.80±0.18 (fetal liver), 3.05 ± 0.30 (adult liver) and 1.18 ±0.07 (placenta) nmol/min/mg. GST was also investigated in human fetal and adult lungs, kidneys and gut. In these tissues the average (±SEM) GST activity ranged between 0.71±0.12 (adult intestine) and 2.11±0.18 (fetal lungs) nmol/min/mg. Whereas in the fetal liver the conjugation of BPO was catalyzed at a rate of about two-thirds of the adult rate, similar or higher GST activities were found in the fetal non-hepatic tissues as compared to the adult organs. No correlation was found between the activity of the GST in fetal liver and placenta and the gestational age (11–25 weeks). GST develops before the 11th week of gestation and it does not undergo changes during the mid-gestation. No correlation was found between GST activity in adult liver and age (32–70 years).

Sulphation and glucuronidation of ritodrine in human foetal and adult tissues

SummaryRitodrine is a β2-adrenoceptor agonist used for the management of preterm labour. It is inactivated by conjugation with sulphate and glucuronic acid. There is more ritodrine sulphate than ritodrine glucuronide in urine from the newborn whereas equal amounts of ritodrine glucoronide and sulphate are excreted in maternal urine [Clin. Pharmacol. Ther 44, 634–641, 1988]. We show that, in the mid-gestational human fetal liver, ritodrine sulphotransferase is well expressed, whereas the glucuronidation of ritodrine is little developed compared to the adult liver. The average sulphotransferase activity was 308 pmol·min−1 per mg protein in fetal (N=48) and 145 pmol·min−1 per mg protein in adult (N=32) liver. The rates of ritodrine sulphation in fetal gut, lung and kidney were higher than in the corresponding adult tissues. The development and tissue distribution patterns of ritodrine sulphotransferase are consistent with those of dopamine sulphotransferase. Ritodrine and dopamine are sulphated by thermolabile enzymes. The activity of glucuronyl transferase was measurable in only 5 of the 48 foetal livers assayed, and in those in which could be assayed, the average activity was 44.6 pmol·min−1 per mg protein, one-tenth of that in adult livers (524 pmol·min−1 per mg protein).

Cytosolic epoxide hydrolase in humans: development and tissue distribution

Cytosolic epoxide hydrolase activity was measured towards trans-stilbene oxide in 41 human adult livers, in 40 fetal livers, in 17 placentas and in fetal and adult lungs, kidneys and gut. The cytosolic epoxide hydrolase activity was measurable in all specimens investigated. The rate of formation of trans-stilbene glycol (pmol/min per mg protein, mean±SD) was 55.2±89.6 (fetal liver). 303.2±73.2 (adult liver) and 18.8±13.1 (placenta) In the fetal extrahepatic tissues, the cytosolic epoxide hydrolase activity was 70.0±9.4 (adrenals), 47.6±7.2 (gut), 69.4±22.5 (kidneys) and 43.2±19.2 (lungs) pmol/min per mg protein, whereas in the adult tissues it was 131.2±63.1 (kidneys), 27.8±20.3 (intestine), 8.5±2.8 (lungs) and 7.2±4.2 (urinary bladder) pmol/min per mg protein.