Pharmacokinetics of acetaminophen, antipyrine, and salicylic acid in the lactating and nursing rabbit, with model predictions of milk to serum concentration ratios and neonatal dose

N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GF120918) as a chemical ATP-binding cassette transporter family G member 2 (Abcg2) knockout model to study nitrofurantoin transfer into milk

Genetic knockout mice studies suggested ATP-binding cassette transporter family G member 2 (ABCG2)/Abcg2 translocates nitrofurantoin at the mammary-blood barrier, resulting in drug accumulation in milk. The purpose of this study was to establish the role of Abcg2 in nitrofurantoin accumulation in rat milk using N-(4-[2-(1,2,3,4-tetrahydro-6,7-dimethoxy-2-isoquinolinyl)ethyl]-phenyl)-9,10-dihydro-5-methoxy-9-oxo-4-acridine carboxamide (GF120918) as a "chemical knockout" equivalent. The inhibitory effect of GF120918 was verified in Madin-Darby canine kidney II cells stably expressing rat Abcg2 with Hoechst 33342 and nitrofurantoin flux in Transwells. Nitrofurantoin was infused (0.5 mg/h) in the absence and presence of GF120918 (10 mg/kg in dimethyl sulfoxide) to Sprague-Dawley lactating female rats using a balanced crossover design. Administration of GF120918 increased nitrofurantoin concentration in serum (from 443 +/- 51 to 650 +/- 120 ng/ml) and decreased concentration in milk (from 18.1 +/- 0.9 to 1.9 +/- 1.2 microg/ml), resulting in corresponding mean values for milk to serum concentration ratio (M/S) of 41.4 +/- 19.1 versus 3.04 +/- 2.27 in the absence and presence of GF120918 (p < 0.05), respectively. There was a decrease in systemic clearance with GF120918 (2.8 +/- 0.5 l/h/kg) compared with vehicle controls (4.1 +/- 0.5 l/h/kg; p < 0.05). Western blot analysis revealed good expression of Abcg2 and no P-glycoprotein (P-gp) expression in mammary gland, whereas immunohistochemistry confirmed the apical expression of Abcg2 in lactating mammary gland epithelia. Nitrofurantoin active transport into rat milk can be inhibited by GF120918 resulting in a 10-fold lower M/S. Although GF120918 inhibits both Abcg2 and P-gp, the high expression of Abcg2 and the absence of detectable P-gp expression in lactating mammary gland validate an important role for Abcg2 in nitrofurantoin accumulation in rat milk. GF120918 is particularly useful as a rat chemical knockout model to establish ABCG2's role in drug transfer into milk during breastfeeding.

The effect of food or water deprivation on paracetamol pharmacokinetics in calves

This study investigated the effect of food or water deprivation on the pharmacokinetics of paracetamol in 30 Holstein-Friesian preruminant calves (10 controls, 10 food withheld and 10 water-deprived) aged 24-25 days. Control calves were given paracetamol at 24-25 days and again at 28-29 days of age. In the food withheld and water-deprived calves paracetamol studies were performed before and after 4 days of food or water deprivation. In the control group there were no significant differences in pharmacokinetic parameters for paracetamol in 24-25 and 28-29-day-old calves. Witholding food for 4 days was associated with an increase in the mean residence time (MRT) of paracetamol (P < 0.01). When food was withheld total body clearance (ClB) of paracetamol was significantly decreased (P < 0.05). The volume of distribution (Vss) was not significantly altered. Similarly, water deprivation was associated with a significant increase in MRT and significant decrease in ClB of paracetamol (P < 0.01). The Vss was not significantly altered. Food or water deprivation also influenced the formation of major metabolites (glucuronide and sulphate) of paracetamol. It is concluded that food or water deprivation may impair the elimination drugs that undergo metabolism by UDP-glucuronyltransferase and sulphotransferase in cattle.

Models and methods for predicting drug transfer into human milk

Adverse effects in infants due to the ingestion of drugs and other xenobiotics remain an area of concern. A key parameter in assessing infant exposure via breast milk, the milk to plasma concentration ratio (M/P), has not been determined in vivo in humans for most drugs. There are various methods for predicting M/P, which involve in vitro experiments in mammary cell monolayers, assessment of drug binding to plasma and milk protein and lipid, in vivo experiments in animals, and regression models based on a compound's physicochemical characteristics. This article reviews these approaches in terms of their utility, advantages and disadvantages. Some combination of these methods is necessary for reasonably accurate prediction of M/P in humans.

Preliminary studies of offspring exposure to phenylbutazone and ivermectin during the perinatal period in a Holstein cow-calf model

The pregnant Holstein cow and her newborn calf were evaluated as an animal model to study in utero and for lactational drug transfer and offspring exposure. A nonsteroidal antiinflammatory drug, phenylbutazone, and an antiparasitic drug, ivermectin, were tested in the model. Prior to parturition, pregnant cows were dosed orally to steady state with phenylbutazone at 4 g/day or given a single subcutaneous injection of 200 microg ivermectin/kg body wt. The level of drug transferred to calves exposed in utero, in utero combined with lactational exposure, and via lactational exposure only, was measured from days 1 through 7 postpartum. At birth the plasma level in phenylbutazone-exposed calves was approximately one-half the dam's steady-state level. For ivermectin-exposed calves, plasma levels were at or below the limit of quantitation (0.5 ng/ml) at birth, suggesting that placental transfer of ivermectin is limited in the cow. For both drugs, rapid accumulation of the drug in calf plasma occurred with lactational exposure to a mean daily dose of 2 microg ivermectin/kg body wt or 0.1 mg phenylbutazone/kg body wt/day for the first 7 days of life. The accumulation observed in the newborn calf is attributed to the lipid solubility and long elimination half-lives of these drugs. These results demonstrate that drug transfer and offspring exposure can be studied using the cow-calf model. The data also highlight the importance of considering not only the dose but also physicochemical characteristics and pharmacokinetics of the drug in the offspring when evaluating the safety of a newborn's exposure to a drug in breast milk.

Penetration of diltiazem into breast milk and its pharmacokinetics in the lactating rabbit

1. The aim was to investigate the milk transfer and pharmacokinetics of diltiazem (DTZ) in the lactating rabbit following DTZ intravenous (i.v.) administration. In addition, DTZ metabolism in mammary tissue and milk was also studied. 2. The pharmacokinetic parameters that largely determine drug disposition (AUC, VD, CL) showed no significant differences between the non-lactating and lactating rabbit. 3. When DTZ was administered to the lactating rabbit, the observed DTZ milk-to-blood AUC ratio (M/B) closely correlated with the calculated ratio, as predicted by a diffusional model, suggesting that DTZ passes into milk via non-ionic diffusion and that other factors which may affect the milk transfer seem to have limited relevance. 4. After a single intravenous dose of DTZ to the lactating rabbit, deacetyldiltiazem (M1) and demethyldiltiazem (MA) were observed in blood, but only M1 could be detected in milk. 5. In conclusion, DTZ seems to diffuse freely into milk after its i.v. administration to the lactating rabbit and should not be given to nursing mammals because of the potential risk for their young. This risk may be even higher because of the presence of M1 (a pharmacologically active metabolite) in milk after administration of the parent drug.

Uncertainty factors for chemical risk assessment: interspecies differences in glucuronidation

For the risk assessment of effects other than cancer, a safe daily intake in humans is generally derived from a surrogate threshold dose (e.g. NOAEL) in an animal species to which an uncertainty factor of 100 is usually applied. This 100-fold is to allow for possible interspecies (10-fold) and interindividual (10-fold) differences in response to a toxicant, and incorporates toxicodynamic and toxicokinetic aspects of variability. The current study determined the magnitude of the interspecies differences in the internal dose of compounds for which glucuronidation is the major pathway of metabolism in either humans or in the test species. The results showed that there are major interspecies differences in the nature of the biological processes which influence the internal dose, including the route of metabolism, the extent of presystemic metabolism and enterohepatic recirculation. The work presented does not support the refinement of the interspecies toxicokinetic default to species- and pathway-specific values, but demonstrates the necessity for risk assessments to be carried out using quantitative chemical-specific data which define the fundamental processes which will influence the internal dose of a chemical (toxicokinetics), or the interaction of toxicant with its target site (toxicodynamics).

Bioavailability and mammary excretion of bisphenol a in Sprague-Dawley rats

This study reports the absolute oral bioavailability and mammary excretion of bisphenol A in rats. The oral bioavailability was determined after administration of relatively low iv (0.1 mg/kg) and oral (10 mg/kg) doses of bisphenol A to rats. After iv injection, serum levels of bisphenol A declined biexponentially, with the mean initial distribution and terminal elimination half-lives being 6.1 +/- 1.3 min and 52.5 +/- 2.4 min, respectively. The systemic clearance (Cls) and the steady-state volume of distribution (Vss) averaged 107.9 +/- 28.7 m/min/kg and 5.6 +/- 2.4 L/kg, respectively. Upon oral administration, the maximum serum concentration (Cmax) and the time to reach the maximum concentration (Tmax) were 14.7 +/- 10.9 ng/ml and 0.2 +/- 0.2 h, respectively. The apparent terminal elimination half-life of bisphenol A (21.3 +/- 7.4 h) after oral administration was significantly longer than that after iv injection, indicating the flip-flop of the absorption and elimination rates. The absolute oral bioavailability of bisphenol A was low (5.3 +/- 2.1%). To determine the extent of mammary excretion, bisphenol A was given by simultaneous iv bolus injection plus infusion to steady state at low, medium, and high doses. The steady-state serum levels of bisphenol A were linearly increased with higher dosing rates. The systemic clearance (mean range, 119.2-154.1 ml/min/kg) remained unaltered over the dosing rate studied. The levels of bisphenol A in milk exceeded those in serum, with the steady-state milk to serum concentration ratio being 2.4-2.7.

Pharmacokinetics of verapamil in lactating rabbits. Prediction of verapamil distribution into rabbit milk

In this work, we have studied the pharmacokinetics and milk penetration of verapamil following intravenous administration in lactating rabbits. Milk-to-serum drug concentration ratios (M/B(obs)) have been determined using area under the milk and serum concentration-time profiles, and the resulting values have then been compared with those obtained by theoretical classical diffusion milk transfer models that were described by Fleishaker et al. [J. Pharm. Sci. 76 (1987) 189.], Atkinson and Begg [Br. J. Clin. Pharmacol. 25 (1990) 495.], and Stebler and Guentert [Pharm. Res. 9 (1992) 1299.]. The pharmacokinetic profile of verapamil in lactating rabbits following endovenous administration is described in the form of a two-compartment model. Moreover, we detected an important milk transfer after endovenous administration of verapamil in lactating rabbits. M/B(obs) was near 15. The classical diffusional models mentioned were not able to predict this extensive transfer of verapamil into rabbit milk. However, when the classical Fleishaker equation was modified and a stepwise regression was carried out, we found that the M/B(obs) value could be predicted using the plasma and milk protein binding.

Mammary excretion of cannabidiol in rabbits after intravenous administration

The present study examined the distribution of cannabidiol into milk after an intravenous bolus injection (3 mg kg-1) to lactating rabbits. Drug concentrations in milk and serum were measured by HPLC. Cannabidiol was excreted into milk rapidly and the drug levels in milk increased over a 4-24-h period following the maternal injection. The mean milk to serum concentration ratio was 25.9, indicating a significant accumulation of the drug in milk.

Studies on paracetamol binding to serum proteins

There is a limited amount of data on the binding of paracetamol to plasma proteins. It has been suggested that binding might influence the ability of some analytical methods to quantify the total amount of drug present in the plasma fraction--the basis of clinical experience in risk assessment and antidote usage. We have investigated the binding of paracetamol to plasma proteins using an ultrafiltration technique. In overdose and spiked uraemic plasma samples the mean percentage of paracetamol bound was 24.1 (1SD = 7.0) with no significant correlation with drug levels or degree of uraemia. There is a small but significant increase in binding with increasing serum albumin concentration both in plasma (rs = 0.549, P = 0.014) and in pure serum albumin solutions (rs = 0.848, P < 0.001).

Modelling of the passage of drugs into milk

Pharmacokinetic modelling of plasma to milk transfer of drugs has assisted our understanding of the milk to plasma ratio (M/P) and pitfalls associated with it. The most useful way of measuring M/P ratios is, however, by model independent analysis. Physiological models have been proposed to enable the prediction of M/P ratios. The most accurate in prospective performance is the log-transformed phase distribution model. This model, developed by stepwise multiple linear regression analysis, also assists in the understanding of the relative contribution of the various physiological factors involved in the distribution of drugs into milk.

Antipyrine as a dialyzable reference to correct differences in efficiency among and within sampling devices during in vivo microdialysis

Antipyrine was investigated as a dialyzable substance that could be used to quantitate relative differences in the efficiency of dialysis among multiple microdialysis probes and by a single probe over time. The contribution of effective membrane surface area to recovery variability was tested by the introduction of air into microdialysis probes. Reduction of effective membrane surface area reduced antipyrine recovery. Dialysates from probes implanted in the jugular vein, brain, and liver of rats receiving antipyrine demonstrated differences in antipyrine concentration among probes within the same rat. These results suggest dissimilar efficiencies of the probes to recover antipyrine, which should be uniformly distributed throughout body water. Dialysates from blood, brain, and liver probes in rats that received both antipyrine and tritiated water (3H2O) showed differences in antipyrine and 3H2O concentrations among probes. Variability of antipyrine and 3H2O concentrations over time within a probe were positively correlated, suggesting that the cause(s) of temporal variability affected both of these markers of body water. Correction of antipyrine tissue/blood ratios, using 3H2O blood/tissue ratios from the same sampling period, reduced the variability in antipyrine tissue/blood ratios, producing ratios closer to the expected value of 1. Differences in probe efficiency contributing to the variability of antipyrine and 3H2O recovery would also be expected to influence the recovery of other substances during microdialysis. The administration of antipyrine during microdialysis experiments is suggested to enable reduction of temporal and site-related differences in substance recovery that are due to differences in probe efficiency. Other methods are necessary to determine the actual extracellular concentration of dialyzed substances and the integrity of the blood-brain barrier.