Plasma protein binding and placental transfer of bupivacaine

Bupivacaine Pharmacokinetics and Breast Milk Excretion of Liposomal Bupivacaine Administered After Cesarean Birth

Very little plasma bupivacaine was transferred into breast milk after liposomal bupivacaine wound infiltration in patients who had undergone cesarean delivery.

To evaluate bupivacaine concentrations in maternal plasma and transfer into breast milk in women undergoing liposomal bupivacaine infiltration in the transversus abdominis plane after cesarean birth.

Lopinavir/ritonavir treatment increases the placental transfer of bupivacaine enantiomers in human immunodeficiency virus-infected pregnant women

AIMS

The present study evaluated the placental transfer and amniotic fluid distribution of bupivacaine enantiomers in health pregnant women and in human immunodeficiency virus (HIV)-infected pregnant women receiving epidural anaesthesia for caesarean section.

METHODS

Twelve HIV-infected pregnant women (HIV group) were treated long-term (at least 8 weeks) with lopinavir/ritonavir (400/100 mg twice daily), and 12 healthy pregnant women (Control group) who submitted to epidural anaesthesia with racemic bupivacaine (75 mg) during caesarean section were investigated. At delivery, samples of maternal and fetal blood and amniotic fluid were collected (10-20 min after drug administration).

RESULTS

The placental transfer ratio of bupivacaine enantiomers was significantly higher among the pregnant women from the HIV group when compared with those from the Control group (Mann-Whitney test, P ≤ 0.05). Placental transfer ratios (median and 25th - 75th percentiles) for (+)-(R)-bupivacaine were 0.58 (0.38-0.82) in the HIV group vs. 0.25 (0.18-0.33) in the Control group, and for (-)-(S)-bupivacaine, they were 0.54 (0.34-0.69) in the HIV group vs. 0.25 (0.19-0.29) in the Control group. The transplacental distribution of bupivacaine was stereoselective only in the HIV group. The umbilical artery/umbilical vein ratio and amniotic fluid/maternal vein ratio were low and nonstereoselective, and no statistically significant differences were observed between the groups.

CONCLUSIONS

This study supports that the placental transfer of both bupivacaine enantiomers was 100% higher in HIV-pregnant women treated with lopinavir/ritonavir when compared with that in healthy pregnant women receiving epidural anaesthesia for caesarean section.

The human placental perfusion model: a systematic review and development of a model to predict in vivo transfer of therapeutic drugs

Dual perfusion of a single placental lobule is the only experimental model to study human placental transfer of substances in organized placental tissue. To date, there has not been any attempt at a systematic evaluation of this model. The aim of this study was to systematically evaluate the perfusion model in predicting placental drug transfer and to develop a pharmacokinetic model to account for nonplacental pharmacokinetic parameters in the perfusion results. In general, the fetal-to-maternal drug concentration ratios matched well between placental perfusion experiments and in vivo samples taken at the time of delivery of the infant. After modeling for differences in maternal and fetal/neonatal protein binding and blood pH, the perfusion results were able to accurately predict in vivo transfer at steady state (R² = 0.85, P < 0.0001). Placental perfusion experiments can be used to predict placental drug transfer when adjusting for extra parameters and can be useful for assessing drug therapy risks and benefits in pregnancy.

Feto-maternal distribution of ropivacaine and bupivacaine after epidural administration for cesarean section

Ropivacaine is a new amino amide local anesthetic less lipophilic and with a lower affinity for plasma proteins than bupivacaine. The purpose of this study was to examine the feto-maternal distribution of ropivacaine and bupivacaine after epidural administration for cesarean section. Healthy parturients were randomly allocated in a double-blinded manner to receive either 0.5% ropivacaine or 0.5% bupivacaine through a lumbar epidural catheter. The total and free concentrations of ropivacaine and bupivacaine were determined in the maternal and umbilical plasma. The total dose (mg/kg) required for adequate surgical anesthesia and the resulting maximum total concentrations did not differ significantly between ropivacaine and bupivacaine. The free plasma clearance of bupivacaine was higher than that of ropivacaine (7382 vs 3344 ml/min, P = 0.0001). The apparent terminal elimination half-life of ropivacaine was shorter than that of bupivacaine (6.0 vs 8.8 h, P = 0.007). At delivery, the maternal free plasma concentration of ropivacaine was more than twice that of the free concentration of bupivacaine (0.072 vs 0.032 microg/ml, P = 0.002). The free concentration of ropivacaine was about twice that of bupivacaine in the umbilical venous (0.06 vs 0.03 microg/ml, P = 0.001) and umbilical arterial (0.05 vs 0.02 microg/ml, P = 0.007) plasma. The more rapid plasma clearance of bupivacaine compared to ropivacaine, leading to lower maternal plasma concentrations and hence to lower umbilical concentrations at delivery, could be explained by the higher lipid solubility, hence greater distribution volume, of bupivacaine.

Stereoselectivity in the placental transfer and kinetic disposition of racemic bupivacaine administered to parturients with or without a vasoconstrictor

The aim of the present study was to investigate the stereoselectivity in the kinetic disposition and the transplacental distribution of bupivacaine in term parturients during labor. Maternal age ranged from 18-37 years and fetal gestational age from 37.6-41.5 weeks. Healthy parturients (n = 23) received epidural 0.5% racemic bupivacaine alone (group A) or combined with epinephrine (group B). Maternal venous blood was sampled at regular intervals until 8 h after drug administration and umbilical venous blood was obtained at delivery. Bupivacaine enantiomers were determined in plasma samples by HPLC using a Chiralcel(R) OD-R column and a UV detector. One- or two-compartment models were fitted to data and differences between the (+)-(R) and (-)-(S) enantiomers were compared with the paired Wilcoxon test (P< 0.05). The influence of epinephrine was evaluated using the unpaired Mann-Whitney test (P< 0.05). The disposition of bupivacaine in maternal plasma was stereoselective, with higher V(d/f) (140.60 vs. 132.81 L for group A and 197.86 vs. 169.46 L for group B) and C(l/f) (29.00 vs. 25.43 L/h for group A and 33.15 vs. 26.39 L/h for group B) and lower t(1/2)beta (3.24 vs. 3.30 h for group A and 4.36 vs. 4.45 h for group B) being observed for (+)-(R)-bupivacaine. The combined administration of epinephrine resulted in higher V(d/f) (197.86 vs. 140.60 L for (+)-(R) and 169.46 vs. 132.81 L for (-)-(S)) and t(1/2)beta values (4.36 vs. 3.24 h for (+)-(R) and 4.45 vs. 3.30 h for (-)-(S)). The transplacental distribution of bupivacaine was stereoselective only when bupivacaine was administered without epinephrine (group B), with a higher cord blood/maternal blood ratio being observed for (-)-(S)-bupivacaine (0.40 vs. 0.35). Chirality 16:65-71, 2004.

Drug Transfer and Metabolism by the Human Placenta

The major function of the placenta is to transfer nutrients and oxygen from the mother to the foetus and to assist in the removal of waste products from the foetus to the mother. In addition, it plays an important role in the synthesis of hormones, peptides and steroids that are vital for a successful pregnancy. The placenta provides a link between the circulations of two distinct individuals but also acts as a barrier to protect the foetus from xenobiotics in the maternal blood. However, the impression that the placenta forms an impenetrable obstacle against most drugs is now widely regarded as false. It has been shown that that nearly all drugs that are administered during pregnancy will enter, to some degree, the circulation of the foetus via passive diffusion. In addition, some drugs are pumped across the placenta by various active transporters located on both the fetal and maternal side of the trophoblast layer. It is only in recent years that the impact of active transporters such as P-glycoprotein on the disposition of drugs has been demonstrated. Facilitated diffusion appears to be a minor transfer mechanism for some drugs, and pinocytosis and phagocytosis are considered too slow to have any significant effect on fetal drug concentrations. The extent to which drugs cross the placenta is also modulated by the actions of placental phase I and II drug-metabolising enzymes, which are present at levels that fluctuate throughout gestation. Cytochrome P450 (CYP) enzymes in particular have been well characterised in the placenta at the level of mRNA, protein, and enzyme activity. CYP1A1, 2E1, 3A4, 3A5, 3A7 and 4B1 have been detected in the term placenta. While much less is known about phase II enzymes in the placenta, some enzymes, in particular uridine diphosphate glucuronosyltransferases, have been detected and shown to have specific activity towards marker substrates, suggesting a significant role of this enzyme in placental drug detoxification. The increasing experimental data on placental drug transfer has enabled clinicians to make better informed decisions about which drugs significantly cross the placenta and develop dosage regimens that minimise fetal exposure to potentially toxic concentrations. Indeed, the foetus has now become the object of intended drug treatment. Extensive research on the placental transfer of drugs such as digoxin and zidovudine has assisted with the safe treatment of the foetus with these drugs in utero. Improved knowledge regarding transplacental drug transfer and metabolism will result in further expansion of pharmacological treatment of fetal conditions.

Paediatric regional anaesthesia--what makes a difference?

Regional anaesthetic techniques are useful for providing post-operative pain control for ambulatory surgery in children. This chapter focuses on unique features of paediatric anatomy and physiology that allow successful performance of these techniques. An overview of the effective use of long-acting local anaesthetics in children is presented. Combinations of local anaesthetics and adjunct medications that prolong caudal blockade, the most commonly performed paediatric block, are reviewed, with a goal of providing prolonged analgesia with minimal side-effects. An overview of the use of peripheral nerve blockade and specific recommendations for the use of these blocks in children is also presented.

Transfer of lidocaine and bupivacaine across the isolated perfused human placenta

Drug permeability and pharmacokinetics through the placenta are important factors determining foetal drug exposure. The purpose of the present study was to establish a perfused human placental cotyledon system to assess the placental transfer of lidocaine and bupivacaine, widely used local anaesthetics in obstetric anaesthesia. Term placentas were obtained immediately after delivery with maternal consent and a two-hour recycling perfusion of a single placental cotyledon was performed. Bupivacaine or lidocaine with antipyrine as a reference compound were added to the maternal reservoir and their disappearance from the maternal circulation and appearance to the foetal circulation were followed in five experiments for each drug. Drug concentrations were measured by gas chromatography. Bupivacaine disappeared more rapidly from the maternal circulation than lidocaine. At 2 hr, bupivacaine foetal:maternal concentration ratio was 0.56 +/- 0.12 and 14.6% +/- 2.99 of the total circulating amount was found in the foetal circulation. Lidocaine concentration increased more in the foetal circulation and the foetal:maternal concentration ratio at 2 hr was 0.90 +/- 0.09 (P < 0.01), and 22.1% +/- 2.21 (P < 0.01) was found in the foetal circulation. The maternal to foetal transfer of bupivacaine and lidocaine were 67.2% +/- 0.153 and 98.9% +/- 0.07 (P < 0.05) of that of freely diffusable antipyrine, respectively. Both amide local anaesthetics crossed the dually perfused human placenta rapidly. Bupivacaine disappeared faster than lidocaine from the maternal circulation but less was transferred to foetal circulation. This difference is probably explained by the greater lipophilicity of bupivacaine and hence higher placental binding. These results suggest less foetal drug exposure with bupivacaine than lidocaine.

Influence of epidural analgesia on fetal and neonatal well-being

Epidural analgesia is a frequently used method to reduce the pain of child-bearing. Concerns regarding the safety and potential hazards still persist in the medical community. This review intends to examine how epidural analgesia determines the various factors of fetal and neonatal well-being. Placental drug transfer of opiates like morphine, pethidine and fentanyl is rapid and can lead to neonatal depression. Sufentanil seems to be the safest opiate to administer epidurally. Local anaesthetics are transferred to the fetus in substantial amounts, but the reported effects are subtle and are probably inconsequential. Utero- and fetoplacental blood flow seems to be improved by epidural analgesia with local anaesthetics. Even when using stronger solutions for more extensive blockade in patients for caesarean section, no adverse effects could be demonstrated using pulsed Doppler technique as long as prolonged hypotension (> 2 min) is avoided. Hypotension is best prevented with 20-25 ml/kg crystalloid preload and prompt treatment with ephedrine or etilephrine. Addition of adrenaline to local anaesthetics is considered to be safe for the healthy mother and fetus but it should best be avoided in mothers with pregnancy induced hypertension. Fetal and neonatal acid-base balance and gas-exchange are not adversely affected by epidural analgesia. Many studies show that epidural analgesia can indeed protect the fetus if hypotension is prevented. Neonatal well-being evaluated by APGAR, BRAZELTON, SCANLON and NACS scores is not significantly influenced by local anaesthetics. Neonatal depression can occur however with epidural use of morphine, fentanyl and alfentanil. Sufentanil, again in doses up to 30 micrograms in association with bupivacaine seems to be devoid of depressive effects on the neonate. In summary, the anaesthetist has good arguments to reassure his obstetrical colleagues that providing epidural analgesia for pregnant women in labour is a justifiable intervention to support the natural process of child-bearing.

Placental transfer of drugs administered to the mother

Drugs 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.

Factors affecting the maternal-fetal distribution of pethidine and bupivacaine in the rabbit

Twenty pregnant New Zealand white rabbits (mean body weight 4.6 kg) within 3 days of term were anaesthetized and given an intravenous infusion of bupivacaine 1.25 mg/ml with pethidine 1.25 mg/ml at a rate of 12 ml/h for 20 min, 6 ml/h for 60 min and 3 ml/h thereafter. In 10 of the does the solution also contained adrenaline 1.25 microg/ml. Up to 8 fetuses were removed at 15 min intervals from the start of the infusion and umbilical vein pH was measured, together with bupivacaine and pethidine concentrations, in fetal plasma, fetal brain and maternal plasma sampled synchronously. Mean umbilical vein pH fell with time with no significant difference between the groups. Maternal plasma concentrations of both drugs did not alter significantly during the experiment. Maternal clearance of bupivacaine was 85.6 ml/min and of pethidine was 249 ml/min. Despite the three-fold higher maternal plasma concentrations of bupivacaine, concentrations of pethidine in fetal plasma and brain were consistently higher than those of bupivacaine. Fetal plasma pethidine concentrations rose 0.276 microg x ml(-1)h(-1) and bupivacaine concentrations rose 0.184 microg x ml(-1)h(-1). The mean (+/-SD) maximum fetal: maternal plasma ratio for bupivacaine was 0.361+/-0.127 and for pethidine 1.78+/-0.81. The fetal brain:plasma ratio of pethidine was consistently higher than that of bupivacaine and did not change significantly with time, whereas that of bupivacaine fell significantly (P<0.05). Concentrations of bupivacaine and pethidine in fetal and maternal brain were consistently higher with adrenaline, although adrenaline had no significant effect on the concentrations in this or any compartment.

Plasma total and free concentrations of bupivacaine and lignocaine in mother and fetus following epidural administration, singly or together

Forty six women undergoing elective caesarean section under epidural blockade were given 20 ml of either bupivacaine 0.5% plain (B) or with adrenaline 5 microg/ml (B + A), lignocaine 2% with adrenaline 5 microg/ml (L + A) or a mixture of equal parts of L + A and B + A (BLA). Further doses of the same solution were given at 20 min if necessary. Free and total plasma concentrations of the local anaesthetics were measured in maternal venous plasma 10, 20, 40, 60 and 120 min after the main dose and in umbilical venous plasma at delivery. Protein binding of both drugs was significantly higher in mother than baby. Lignocaine did not affect bupivacaine binding but lignocaine binding was higher in group BLA than L + A. Otherwise the presence of one drug had little effect on the disposition of the other drug in mother or baby. No plasma concentrations were considered toxic. Adrenaline did not reduce maternal C(max) of free bupivacaine and appeared to be associated with increased fetal uptake of bupivacaine.

Plasma protein binding of disopyramide in pregnant and postpartum women, and in neonates and their mothers

1. The protein binding of disopyramide was measured in plasma obtained from nonpregnant women, pregnant women in the first, second, and third trimesters, matched pairs of mothers and neonates (cord plasma), and 1 month postpartum women (n = 6 or 8 of each). 2. Plasma samples spiked with 0.2-12.0 micrograms ml-1 of the drug were ultrafiltered and the free fractions were measured with a fluorescent polarization immunoassay. 3. The mean (+/- s.d.) percentages of free drug at a total concentration of 3.0 micrograms ml-1 observed in the third trimester (46 +/- 9%) and neonate (79 +/- 5%) groups were greater (P less than 0.05 or 0.01) than that in the non-pregnant group (34 +/- 7%). In contrast, the corresponding value observed in the postpartum group (23 +/- 8%) was less (P less than 0.05) than that in the non-pregnant group. In addition, there was a significant (P less than 0.01) difference in the mean percentage of free drug at 3.0 micrograms ml-1 in plasma from mothers (43 +/- 9%) and neonates (79 +/- 5%). 4. A multiple regression analysis indicated that alpha 1-acid glycoprotein (r = -0.88, P less than 0.01), rather than albumin (r = -0.008), dominated the binding of disopyramide within the therapeutic range of drug concentration. An analysis of the binding parameters of disopyramide suggested that alterations in binding were attributable to changes in the capacity rather than the affinity of binding.(ABSTRACT TRUNCATED AT 250 WORDS)

Influence of maternal plasma protein binding on fetal unbound plasma concentration of propranolol in the pregnant ewe

According to theory, for a drug of nonrestrictive, flow-limited clearance, a change during pregnancy of the unbound fraction (fu) of drug in maternal plasma should cause a change in steady-state unbound plasma drug concentration (Cu) in maternal plasma, which should also cause a change in fetal Cu. This theory was examined in 14 chronically cannulated, unanesthetized pregnant ewes in which 28 separate experiments were performed during the latter part of gestation. An initial bolus dose and 3-h constant rate infusion of propranolol were administered via the maternal jugular vein and steady-state maternal and fetal carotid arterial plasma total and unbound propranolol concentrations were measured. Fetal Cu (32 +/- 21 ng/mL) was significantly less than maternal Cu (78 +/- 52 ng/mL), due to previously demonstrated fetal hepatic extraction of propranolol. Notwithstanding fetal elimination, there was a significant correlation between fetal Cu and maternal Cu (r = 0.41, p less than 0.025). There was also a strong correlation between fetal Cu and the maternal unbound fraction of drug (fu; r = 0.75, p less than 0.001). We conclude that for propranolol, a drug of nonrestrictive, flow-limited clearance, changes in maternal fu can have a significant influence on fetal Cu, and therefore would be expected to influence the pharmacological effect of the drug in the fetus.

[Acute toxicity of local anesthetics as a function of the patient's condition]

A patient's condition may alter the pharmacokinetic and pharmacodynamic characteristics of local anaesthetics and so increase the risk of toxicity. In the elderly patient, the elimination half-life is increased for both lidocaine and bupivacaine; the risk of overdose is therefore increased when the local anaesthetic agent is given in repeated doses and as a continuous infusion. Cardiotoxicity due to bupivacaine seems to be worsened by pregnancy. In the foetus and newborn, local anaesthetic toxicity gives the same clinical picture as in the adult and is increased in the presence of acidosis and anoxia. Bupivacaine depressive effects are increased by tachycardias, intraventricular blocks and all the conditions which are known to depolarize the cardiac cell membrane (e.g. hyperkaliemia, acidosis, severe hypoxia, myocardial ischaemia). Drug interactions may also potentiate the toxicity of lidocaine and bupivacaine, such as calcium blockers and diazepam. The effects of other conditions (cirrhosis, renal failure, epilepsy) and other drug interactions, specially those modifying free fraction and elimination of local anaesthetics, are also discussed.

Plasma concentrations of bupivacaine during extradural anaesthesia for caesarean section. The effect of adrenaline

The clinical effects and plasma levels associated with the use of 0.5% bupivacaine with and without the addition of 1:200,000 adrenaline (5 micrograms/ml) were studied in 30 patients who underwent extradural anaesthesia for elective Caesarean section. The addition of adrenaline to bupivacaine prolongs analgesia, reduces the degree of hypotension and delays its onset. Plasma bupivacaine levels were consistently lower when adrenaline was added, but this difference was significant only at 10 minutes after the initial dose. Prolonging the interval between increments seems to be a more reliable way to reduce plasma concentration than the addition of the catecholamine.

Plasma bupivacaine concentrations during postoperative continuous epidural analgesia

Postoperative analgesia and plasma concentrations of bupivacaine were evaluated in six patients receiving intermittent epidural bupivacaine for a period of 72 hours following large bowel surgery. Repeated doses were administered every hour by a new automatic, specially designed pump. Venous plasma bupivacaine concentrations showed accumulation for up to 48-60 hours during administration and were mostly less than 2 micrograms/ml except in an elderly, frail patient in whom a peak of 3.9 micrograms/ml was observed. This convenient, low-dose, pulsed technique provided excellent analgesia although special care may be necessary in the elderly and frail patient if potentially toxic levels are to be avoided.

Placental transfer of bupivacaine, pethidine and lignocaine in the rabbit. Effect of umbilical flow rate and protein content

The factors determining the placental transfer of drugs used in labour were studied in the rabbit placenta perfused in situ with Krebs bicarbonate buffer. During concurrent maternal intravenous infusion of bupivacaine, lignocaine, pethidine and antipyrine, drug concentrations were measured in maternal arterial plasma and placental effluent perfusate, the flow rate and protein content of which were varied. Protein binding and content were also measured. Placental clearance of antipyrine, which is unbound, was unaltered by perfusate protein content, and increased with umbilical perfusate flow up to 2 ml/min. Clearance of lignocaine and pethidine, which were 20-30% protein bound, increased to a small extent with perfusate protein, and were flow-dependent up to the maximum perfusate flow of 4 ml/min. Clearances of bupivacaine, which was greater than 80% bound, increased markedly with perfusate protein but, though flow-dependent, was one-tenth to one-fifth that of the other drugs. Fetal binding and glycoprotein content were less than maternal, hence the equilibrium fetal: maternal ratio is predictably lower for the highly bound bupivacaine than for lignocaine or pethidine. Measured fetal: maternal ratios of bupivacaine were, however, only one-half to one-third of the predicted equilibrium values, suggesting that bupivacaine does not unbind readily in a single transit through the rabbit placenta. Thus, though bupivacaine crosses the placenta more slowly than the other drugs, the fetal dose of all these drugs will be greatest in healthy babies with good placental blood flows and high plasma proteins.

Transfer of bupivacaine, lignocaine and pethidine across the rabbit placenta: influence of maternal protein binding and fetal flow

The rabbit placenta perfused in itu was used to investigate the factors determining the placental transfer of drugs used in labour. Each doe was given an intravenous infusion of pethidine, lignocaine, bupivacaine and antipyrine concurrently, and the umbilical circulation was artificially perfused with Mammalian Krebs' bicarbonate buffer. The umbilical flow rate was varied between 0.25 and 4.0 ml/min. Drugs were analysed in maternal plasma and umbilical effluent by gas liquid chromatography . Maternal protein binding and lipid solubilities were also determined, and were high for bupivacaine, low for antipyrine and intermediate for pethidine and lignocaine. The Cuv /Cma (mean +/- s.e.) at 1.0 ml/min for antipyrine was 0.74 +/- 0.036; for pethidine, 0.64 +/- 0.04; for lignocaine, 0.5 +/- 0.026; and for bupivacaine 0.072 +/- 0.006. This is the same rank order as is observed for the drugs in humans. The placental clearance increased with flow rates up to 2.0 ml/min for antipyrine and up to 4.0 ml/min and probably more for pethidine, lignocaine and bupivacaine. Transfer rate is therefore reduced by maternal protein binding, is flow-dependent at low flows, and permeability-dependent at high flows for the less lipid-soluble compounds.

Placental drug transfer: effects of gestational age and species

The available evidence suggests that for most drugs, adverse effects in the fetus may vary with gestational state and among species due to: (i) changes in the fetal exposure to the drug (i.e. due to changes in the pharmacokinetics of the drug in the mother and/or the fetus), or to (ii) changes in the susceptibility of the fetus to the drug. The fetal exposure to a drug during gestation is influenced more by the varying capacity of mother and/or fetus to eliminate the drug than by any intrinsic 'barrier' phenomenon at the placenta. Although differential maternal/fetal protein binding, active transplacental transport processes and 'ion-trapping' effects may influence the fetal exposure of some drugs, the main mechanisms by which fetal exposure may be modulated during pregnancy are via the capacity for irreversible drug elimination--by the fetus or, less often, by the placenta. The susceptibility of a fetus to adverse drug reactions is determined by the ontogeny of vital processes and the nature of the interaction between the drug and the process. Hence 'gestational state' and 'species' dependent differences in adverse drug effects, in the presence of a constant level of exposure of drug, reflect the time dependent appearance of these processes and the differences in ontogeny of the processes among species. At present, no studies have attempted to relate the measured fetal drug exposure to the intensity of a drug response at different stages of gestation or among species. Although there is a dearth of information in this field, it is apparent that in all species the placentas of all species pose little obstruction to the passage of xenobiotics (including drugs), to the fetus. The consequence of this exposure will depend on a myriad of pharmacokinetic and pharmacodynamic considerations for a given substance in a given species. Hence the outcome cannot be predicted, but must be empirically determined. Extrapolation of findings among different drugs, species and gestational states must be undertaken with caution, recognizing the above considerations and limitations.

Placental transfer of ranitidine during steady-state infusions of maternal and fetal sheep

The placental transfer of ranitidine was studied at pharmacokinetic steady state in anesthetized, full-term, pregnant sheep. Ranitidine was administered to the ewe in three preparations and to the fetus in three other sheep. In all experiments, dose size was based on the combined maternal-fetal weight. Steady-state plasma levels were reached within 4 hr by using an initial intravenous bolus dose followed by continuous infusion. Following maternal dosage, the mean maternal (jugular vein) steady-state concentration (CMss) at 4 hr was 842 +/- 66 ng/ml (SEM), the mean fetal (carotid artery) steady-state concentration (CFss) was 26.5 +/- 4.2 ng/ml, and the mean fetal umbilical venous steady-state concentration was 28.9 +/- 3.5 ng/ml. Both the fetal and umbilical plasma concentrations were significantly less than the maternal plasma concentrations (p less than 0.01). With fetal dosage, mean CMss was 414 +/- 42 ng/ml at 4 hr and was significantly less than the mean CFss value at the same time, which was 6890 +/- 360 ng/ml (p less than 0.005). Ranitidine was not bound extensively to plasma proteins in the ewe or the fetus (range 12-55% bound). The reversal of the CMss/CFss gradient with the change from maternal to fetal administration and the low binding of the drug shows that the gradient following maternal dosage cannot be explained by ion-trapping or differential plasma protein binding. As active placental transport is considered unlikely, the low fetal plasma concentrations are probably due to the presence of significant fetal elimination of ranitidine. Furthermore, the substantial gradient between maternal and umbilical venous plasma concentrations suggests that placental elimination of ranitidine should also be considered.

The developing liver: the steady-state disposition of propranolol in pregnant sheep

The steady-state disposition of the beta-adrenoreceptor blocking drug, propranolol, and its metabolite, 4-hydroxypropranolol, was studied in the anesthetized pregnant sheep near term. Following infusion of propranolol to the mother, steady-state plasma concentrations were obtained at three dosage levels in each of the eight animals studied. Blood samples were obtained from : (i) maternal facial artery and hepatic vein; (ii) umbilical vein, and (iii) fetal carotid artery, portal vein, and right hepatic vein. Although total propranolol concentrations in maternal plasma were different from those in umbilical venous plasma, the concentrations of unbound drug were similar, suggesting that unbound drug had equilibrated across the placenta. Approximately 10% of propranolol was unbound in maternal plasma, compared with 30% in fetal plasma. Hepatic extraction of propranolol by the maternal liver was high, so that only 3 to 4% of the drug presented to the liver escaped uptake. In contrast, fetal hepatic extraction was low. Negligible amounts were removed from portal vein blood (right lobe), although one third was extracted from umbilical vein blood (left lobe). Mean peripheral arterial concentrations of 4-hydroxypropranolol were 2- to 3-fold higher in the fetus than in the mother. We conclude that the fetus is exposed to significant concentrations of propranolol and that the fetal hepatic extraction of this drug is considerably less efficient than in the adult. In contrast to the adult liver, the fetal liver does not appear to act as a single homogeneous organ, but rather as two distinct subunits.

Relationship between the transplacental gradients of bupivacaine and alpha 1-acid glycoprotein

1 The binding of bupivacaine (400 ng/ml) to isolated alpha 1-acid glycoprotein was studied at two protein concentrations. At 20 mg/100 ml the extent of bupivacaine binding was 31.0 +/- 1.8% (mean +/- s.d., n = 4), and at a protein concentration of 60 mg/100 ml binding of bupivacaine was 85.8 +/- 1.5% (n = 4). 2 Bupivacaine and alpha 1-acid glycoprotein concentrations were measured in plasma samples collected from a maternal peripheral vein and the umbilical vein at delivery (n = 23). The ratio of the foetal:maternal bupivacaine concentrations ranged from 0.17 to 0.52, while the foetal:maternal ration for alpha 1-acid glycoprotein concentrations ranged from 0.20 to 0.96. A positive relationship emerged between the two ratios (P less than 0.01). 3 The alpha 1-acid glycoprotein concentration gradient across the placenta, and interindividual variability in the gradient appear to contribute to the low and variable transplacental bupivacaine concentration ratio observed.

Drugs in obstetric anaesthesia

From the anaesthetist's viewpoint, pharmacokinetics of pregnancy, drugs affecting uterine muscle tension and foetal effects of anaesthetic drugs are reviewed.

Determination of low levels of bupivacaine (Marcaine) in plasma during epidural analgesia

A rapid method is described for the determination of plasma levels of bupivacaine used for epidural analgesia during childbirth. Data obtained in a clinical study are also given.

Brain-to-blood and saliva-to-blood mepivacaine ratios in rats

Mepivacaine hydrochloride, 25 and 50 mg/kg sc (with sacrifice at 15 min) produced higher (p less than 0.005) drug levels in neonate (24--36-hr-old) rat brain and blood than in adult rat brain and blood; however, there was no significant difference in the brain-to-blood ratio of the drug between neonates and adults at either dose level. Intraarterial infusion of mepivacaine hydrochloride (20 micrograms/min) in adult rats resulted in measurable (GLC) mepivacaine base levels in pilocarpine-induced parotid salivary secretions collected throughout 30- and 45-min infusion periods. The saliva-to-blood ratios (+/- SEM) of mepivacaine base were 0.64 +/- 0.13 after a 30-min infusion and 2.13 +/- 0.48 after a 45-min infusion.

Pharmacokinetics and metabolism of the anilide local anaesthetics in neonates. 11 Etidocaine

The urinary elimination of etidocaine and several of its metabolites was investigated in neonates whose mothers had received one or more doses of etidocaine during labour. The urine collection period ranged among the neonates from 21.4 to 47.0 h post-partum. The total amounts of etidocaine and its metabolites recovered in neonatal urine represented a mean of 0.12% of the maternal dose. Some differences in the pattern of urinary metabolites were observed between neonates and adults. Mean half-life of elimination of etidocaine calculated from sigma-minus plots of the neonatal urinary data was 6.42 h. This is greater than that previously reported following intravenous administration of etidocaine to adults (2.6 h). The slower rate of elimination in neonates is probably due to an increased neonatal volume of distribution since there is evidence to show that etidocaine is extensively metabolised by the neonate.