Age and propranolol stereoselective disposition in humans

Potential implications of DMET ontogeny on the disposition of commonly prescribed drugs in neonatal and pediatric intensive care units

Introduction: Pediatric patients, especially neonates and infants, are more susceptible to adverse drug events as compared to adults. In particular, immature small molecule drug metabolism and excretion can result in higher incidences of pediatric toxicity than adults if the pediatric dose is not adjusted.Area covered: We reviewed the top 29 small molecule drugs prescribed in neonatal and pediatric intensive care units and compiled the mechanisms of their metabolism and excretion. The ontogeny of Phase I and II drug metabolizing enzymes and transporters (DMETs), particularly relevant to these drugs, are summarized. The potential effects of DMET ontogeny on the metabolism and excretion of the top pediatric drugs were predicted. The current regulatory requirements and recommendations regarding safe and effective use of drugs in children are discussed. A few representative examples of the use of ontogeny-informed physiologically based pharmacokinetic (PBPK) models are highlighted.Expert opinion: Empirical prediction of pediatric drug dosing based on body weight or body-surface area from the adult parameters can be inaccurate because DMETs are not mature in children and the age-dependent maturation of these proteins is different. Ontogeny-informed-PBPK modeling provides a better alternative to predict the pharmacokinetics of drugs in children.

Pharmacokinetic and Pharmacodynamic Considerations for Drugs Binding to Alpha-1-Acid Glycoprotein

According to the free drug hypothesis only the unbound drug is available to act at physiological sites of action, and as such the importance of plasma protein binding primarily resides in its impact on pharmacokinetics and pharmacodynamics. Of the major plasma proteins, alpha-1-acid glycoprotein (AAG) represents an intriguing one primarily due to the high affinity, low capacity properties of this protein. In addition, there are marked species and age differences in protein expression, homology and drug binding affinity. As such, a thorough understanding of drug binding to AAG can help aid and improve the translation of pharmacokinetic/pharmacodynamic (PK/PD) relationships from preclinical species to human as well as adults to neonates. This review provides a comprehensive overview of our current understanding of the biochemistry of AAG; endogenous function, impact of disease, utility as a biomarker, and impact on PK/PD. Experimental considerations are discussed as well as recommendations for understanding the potential impact of AAG on PK through drug discovery and early development.

Pharmacokinetic Parameters and Over-Responsiveness of Iranian Population to Propranolol

Purpose: Propranolol is the most widely used treatment for cardiovascular diseases. Dosage range in our patients is usually less than the amount mentioned in references. The aim of the present study was to clarify whether pharmacokinetic differences are able to justify the need for the fewer doses in our patients or not.

Methods: Twenty healthy volunteers (10 male) at heart center of Mazandaran University of Medical Sciences were studied. Samples of blood were collected before a single oral dose (40 mg) of Propranolol. Blood samples were taken up to 9 hours after dose. Total plasma concentration of Propranolol was measured by HPLC. Population Pharmacokinetic analysis was performed using population pharmacokinetics modeling software P-Pharm.

Results: The mean value for oral plasma clearance (CL/F) was 126.59 ml/hr. The corresponding values for apparent volume of distribution (V/F), t1/2 beta, maximum blood concentration (C max), and time to reach the maximum blood concentration (T max) were 334.12 Lit, 1.98 hr, 40.25 ng/ml, and 1.68 hr, respectively. The observed mean values of V/F of propranolol in the present study were comparable with those reported in the literature. However, the mean values of CL/F of propranolol in current study was significantly higher than those reported in other population (P-value<0.001).

Conclusion: This study has confirmed that the pharmacokinetic differences are not able to justify over-responsiveness of Iranian population to propranolol. Pharmacodynamic differences in responding to beta blocker drugs by Renin secretion or having a different sensibility to beta receptors might play a role in making our population have a different response to propranolol.

Incorporation of the Time-Varying Postprandial Increase in Splanchnic Blood Flow into a PBPK Model to Predict the Effect of Food on the Pharmacokinetics of Orally Administered High-Extraction Drugs

Following a meal, a transient increase in splanchnic blood flow occurs that can result in increased exposure to orally administered high-extraction drugs. Typically, physiologically based pharmacokinetic (PBPK) models have incorporated this increase in blood flow as a time-invariant fed/fasted ratio, but this approach is unable to explain the extent of increased drug exposure. A model for the time-varying increase in splanchnic blood flow following a moderate- to high-calorie meal (TV-Q _Splanch) was developed to describe the observed data for healthy individuals. This was integrated within a PBPK model and used to predict the contribution of increased splanchnic blood flow to the observed food effect for two orally administered high-extraction drugs, propranolol and ibrutinib. The model predicted geometric mean fed/fasted AUC and C _max ratios of 1.24 and 1.29 for propranolol, which were within the range of published values (within 1.0-1.8-fold of values from eight clinical studies). For ibrutinib, the predicted geometric mean fed/fasted AUC and C _max ratios were 2.0 and 1.84, respectively, which was within 1.1-fold of the reported fed/fasted AUC ratio but underestimated the reported C _max ratio by up to 1.9-fold. For both drugs, the interindividual variability in fed/fasted AUC and C _max ratios was underpredicted. This suggests that the postprandial change in splanchnic blood flow is a major mechanism of the food effect for propranolol and ibrutinib but is insufficient to fully explain the observations. The proposed model is anticipated to improve the prediction of food effect for high-extraction drugs, but should be considered with other mechanisms.

Development of In Vitro Pharmacokinetic Screens Using Caco-2, Human Hepatocyte, and Caco-2/Human Hepatocyte Hybrid Systems for the Prediction of Oral Bioavailability in Humans

In this study, in vitro systems were used to build 2 pharmacokinetic models that predict human oral bioavailability: the Caco-2/hepatocyte combination model and the Caco-2/hepatocyte hybrid model. Data obtained in vitro on Caco-2 cell permeability and hepatocyte clearance are routinely used to predict the fraction of absorption after oral administration and the extent of first-pass metabolism, respectively. In the Caco-2/hepatocyte combination model, results from a Caco-2 cell permeability assay and a hepatocyte clearance assay were combined to project oral bioavailability. Comparison of oral bioavailabilities predicted by the combination model and reported oral bioavailabilities in humans for 30 marketed compounds resulted in a modest correlation ( r2 = 0.66). The Caco-2/hepatocyte hybrid model, as previously reported, joins the Caco-2 and hepatocyte clearance systems into 1 assay. Improvements to the previous model were made by incorporating an elimination phase into the Caco-2/hepatocyte hybrid model. In the new hybrid model, the compound was added to a Caco-2-containing donor compartment and allowed to permeate for 2 h to a hepatocyte-containing receiver compartment. Subsequently, to mimic an elimination phase, the donor compartment was removed, and permeated compound was incubated with hepatocytes alone for an additional 3 h. The area under the concentration versus time curve (AUC) was determined for each of the same 30 marketed compounds assessed by the combination model. A linear regression analysis comparing the in vitro AUCs and reported oral bioavailabilities in humans showed a reasonable correlation ( r 2 = 0.73). This study demonstrates that the Caco-2/hepatocyte hybrid model is more favorable and further proves the potential and feasibility of using in vitro screenings for the prediction of in vivo pharmacokinetics in humans. ( Journal of Biomolecular Screening 2007:1084-1091)

Population pharmacokinetics of the selective serotonin 5-HT1A receptor partial agonist piclozotan

BACKGROUND/AIMS

Serotonin (5-HT) and its receptors are known to play important roles in various physiological and pathophysiological processes. The 5-HT1A receptor subtype is thought to be involved in psychiatric disorders, immunomodulation, and in cerebral ischemic conditions. Piclozotan, a selective and potent partial agonist of 5-HT1A, has been shown to be neuroprotective against ischemic neuronal damage in animal models. Its pharmacokinetics (PK), tolerability, and safety have been evaluated in patients with acute ischemic stroke. The aim of the study was to describe piclozotan PK, using population PK modeling.

METHODS

A total of 1308 plasma piclozotan concentration measurements from 84 healthy subjects and 412 plasma piclozotan measurements from 74 stroke patients were included in the analysis. Covariates considered during the model building process included disease status, age, weight, sex, smoking status, and alcohol consumption. Data were analyzed using nonlinear mixed-effects modeling with the NONMEM software system. The final model was qualified via predictive check and nonparametric bootstrap procedures.

RESULTS

Piclozotan PK was well described using a 3-compartment model with first-order elimination. Parameter estimates (intersubject variability) were V1, central volume: 64.0 L (66.5%) and CL, systemic clearance: 18.0 L/h (31.4%). Peripheral volumes (V2, V3) were related to total body weight, whereas CL was related to ideal body weight. Clearance decreased with advancing age, yielding a decrease of 35%-65% in patients aged 70-90. There was no discernable difference in PK between healthy subjects and stroke patients.

CONCLUSIONS

Piclozotan disposition was well described by the population model, and the intersubject variability around the estimated parameters was moderate in magnitude (<60%). The population PK analysis of piclozotan allows for characterization of piclozotan exposure in individual subjects based on their age and body weight. The availability of a population PK model will facilitate dose optimization and further clinical development of piclozotan.

Pharmacokinetic interactions of topiramate

Topiramate is a new antiepileptic drug (AED) that has been approved worldwide (in more than 80 countries) for the treatment of various kinds of epilepsy. It is currently being evaluated for its effect in various neurological and psychiatric disorders. The pharmacokinetics of topiramate are characterised by linear pharmacokinetics over the dose range 100-800 mg, low oral clearance (22-36 mL/min), which, in monotherapy, is predominantly through renal excretion (renal clearance 10-20 mL/min), and a long half-life (19-25 hours), which is reduced when coadministered with inducing AEDs such as phenytoin, phenobarbital and carbamazepine. The absolute bioavailability, or oral availability, of topiramate is 81-95% and is not affected by food. Although topiramate is not extensively metabolised when administered in monotherapy (fraction metabolised approximately 20%), its metabolism is induced during polytherapy with carbamazepine and phenytoin, and, consequently, its fraction metabolised increases. During concomitant treatment with topiramate and carbamazepine or phenytoin, the (oral) clearance of topiramate increases 2-fold and its half-life becomes shorter by approximately 50%, which may require topiramate dosage adjustment when phenytoin or carbamazepine therapy is added or discontinued. From a pharmacokinetic standpoint, topiramate is a unique example of a drug that, because of its major renal elimination component, is not subject to drug interaction due to enzyme inhibition, but nevertheless is susceptible to clinically relevant drug interactions due to induction of its metabolism. Unlike old AEDs such as phenytoin and carbamazepine, topiramate is a mild inducer and, currently, the only interaction observed as a result of induction by topiramate is that with ethinylestradiol. Topiramate only increases the oral clearance of ethinylestradiol in an oral contraceptive at high dosages (>200 mg/day). Because of this dose-dependency, possible interactions between topiramate and oral contraceptives should be assessed according to the topiramate dosage utilised. This paper provides a critical review of the pharmacokinetic interactions of topiramate with old and new AEDs, an oral contraceptive, and the CNS-active drugs lithium, haloperidol, amitriptyline, risperidone, sumatriptan, propranolol and dihydroergotamine. At a daily dosage of 200 mg, topiramate exhibited no or little (with lithium, propranolol and the amitriptyline metabolite nortriptyline) pharmacokinetic interactions with these drugs. The results of many of these drug interaction studies with topiramate have not been published before, and are presented and discussed for the first time in this article.

When drug therapy gets old: pharmacokinetics and pharmacodynamics in the elderly

The age-related changes in the functions and composition of the human body require adjustments of drug selection and dosage for old individuals. Drug excretion via the kidneys declines with age, the elderly should therefore be treated as renally insufficient patients. The metabolic clearance is primarily reduced with drugs that display high hepatic extraction ('blood flow-limited metabolism'), whereas the metabolism of drugs with low hepatic extraction ('capacity-limited metabolism') usually is not diminished. Reduction of metabolic drug elimination is more pronounced in malnourished or frail subjects. The water content of the aging body decreases, the fat content rises, hence the distribution volume of hydrophilic compounds is reduced in the elderly, whereas that of lipophilic drugs is increased. Intestinal absorption of most drugs is not altered in the elderly. Aside of these pharmacokinetic changes, one of the characteristics of old age is a progressive decline in counterregulatory (homeostatic) mechanisms. Therefore drug effects are mitigated less, the reactions are usually stronger than in younger subjects, the rate and intensity of adverse effects are higher. Examples of drug effects augmented is this manner are postural hypotension with agents that lower blood pressure, dehydration, hypovolemia, and electrolyte disturbances in response to diuretics, bleeding complications with oral anticoagulants, hypoglycemia with antidiabetics, and gastrointestinal irritation with non-steroidal anti-inflammatory drugs. The brain is an especially sensitive drug target in old age. Psychotropic drugs but also anticonvulsants and centrally acting antihypertensives may impede intellectual functions and motor coordination. The antimuscarinic effects of some antidepressants and neuroleptic drugs may be responsible for agitation, confusion, and delirium in elderly. Hence drugs should be used very restrictively in geriatric patients. If drug therapy is absolutely necessary, the dosage should be titrated to a clearly defined clinical or biochemical therapeutic goal starting from a low initial dose.

Development and validation of a 96-well equilibrium dialysis apparatus for measuring plasma protein binding

A 96-well equilibrium dialysis block was designed and constructed that is compatible with most standard 96-well format laboratory supplies and instruments. The unique design of the dialysis apparatus allows one to dispense and aspirate from either or both the sample and dialysate sides from the top of the apparatus, which is not possible with systems currently on the market. This feature permits the investigator to analyze a large number of samples, time points, or replicates in the same experiment. The novel alignment of the dialysis membrane vertically in the well maximizes the surface-to-volume ratio, eliminates problems associated with trapped air pockets, and allows one to add or remove samples independently or all at once. Furthermore, the design of the apparatus allows both the sample and dialysate sides of the dialysis well to be accessible by robotic systems, so assays can be readily automated. Teflon construction is used to minimize nonspecific binding of test samples to the apparatus. The device is reusable, easily assembled, and can be shaken in controlled temperature environments to decrease the time required to reach equilibrium as well as facilitate dissolution of test compounds. Plasma protein binding values obtained for 10 diverse compounds using standard dialysis equipment and the 96-well dialysis block validates this method.

Protein binding predictions in infants

Plasma binding protein levels are lower in the newborn than in the adult and gradually increase with age. At birth, human serum albumin (HSA) concentrations are close to adult levels (75%-80%), while alpha 1-acid glycoprotein (AAG) is initially half the adult concentration. As a result, the extent of drug binding to HSA is closer to that of the adult than are those drugs bound largely to AAG. A model that incorporates the fraction unbound in adults and the ratio of the binding protein concentration between infants and adults successfully predicted the fraction unbound in infants and children.

Synthesis, stereoselective enzymatic hydrolysis, and skin permeation of diastereomeric propranolol ester prodrugs

Four diastereomeric propranolol ester prodrugs (1S2S, 1S2R, 1R2S, 1R2R) were synthesized by treating pure R- and S-propranolol hydrochloride with pure enantiomers R- and S-phenylbutyryl chloride. A HPLC technique using alpha-1 acid glycoprotein (chiral AGP) column was developed to study the racemization of propranolol enantiomers during synthesis and hydrolysis studies. A reversed phase HPLC method was also developed to simultaneously analyze propranolol and the ester prodrug. Hydrolysis of these esters was studied in different rat tissue homogenates, i.e., liver, intestine, plasma, skin, brain, and pure plasma cholinesterases, i.e., butyryl cholinesterase (EC 3.1.1.8) and acetyl cholinesterase (EC 3.1.1.7). In vitro percutaneous permeation studies across full thickness shaved rat skin were performed using standard side-by-side diffusion cells at 37 degrees C. The disappearance of the diastereomeric ester prodrugs in rat tissue homogenates followed apparent first-order kinetics and was stereoselective. The ratio of brain to plasma hydrolytic rate constants are 27.8, 5.58, 6.07, and 2.97 for 1S2S, 1R2R, 1R2S, and 1S2R esters, respectively. Hydrolysis of all four diastereomeric ester prodrugs was faster by acetyl cholinesterase than butyryl cholinesterase and is stereoselective. The permeability coefficients [Kp x 10(3) (cm h-1)] are 1.40 +/- 0.30, 1.41 +/- 0.27, 42.20 +/- 1.24, 29.26 +/- 3.41, 16.27 +/- 3.12, 12.99 +/- 2.84 for (R)-propranolol, (S)-propranolol, 1S2S, 1R2S, 1S2R, and 1R2R ester prodrugs, respectively. The results indicate that the 1R2S diastereomeric ester prodrug of propranolol shows greatest stability in liver and intestinal tissues while it exhibits fairly rapid conversion in plasma. The results also suggest the configuration on the second chiral carbon atom to be the determinant in the rate of hydrolysis of all the diastereomeric prodrugs in all biological media examined. The Kp of all four prodrugs markedly increased compared to that of the parent drug, with 1S2S showing a 30-fold increase in skin permeability, the highest among all four prodrugs.

(S)-4'-hydroxypropranolol causes product inhibition and dose-dependent bioavailability of propranolol enantiomers in the isolated perfused rat liver and in rat liver microsomes

1. Previous evidence suggests that the dose-dependent bioavailability of racemic propranolol may be partly due to product inhibition. We have examined this further by studying the individual enantiomers of propranolol in the perfused rat liver (IPRL) and in rat liver microsomes. 2. In recirculating IPRL experiments, (R)-propranolol (n = 7) or (S)-propranolol (n = 4) were infused at rates of 75, 150 and 231 nmol/min for three sequential 36-min phases. In single-pass experiments, (R)-propranolol (n = 4) or (S)-propranolol (n = 4) were administered at rates of 80, 136 and 239 nmol/min for three sequential 30-min phases. Steady-state bioavailability increased 10-20-fold over this dose range with both enantiomers in both recirculating and single-pass experiments. At the higher administration rates of (S)-propranolol, bioavailability in recirculating experiments was significantly greater than that in single-pass experiments, whereas there was no significant difference for (R)-propranolol. This suggests product inhibition of (S)- but not (R)-propranolol metabolism. 3. Of the metabolites examined, racemic 4'-hydroxypropranolol (4-OHP) inhibited the formation of 4-OHP, 5'-hydroxypropranolol (5-OHP) and desisopropylpropranolol (DIP) from (S)-propranolol and (R)-propranolol in microsomal studies (IC50 20 microM). Tissue levels of (S)-4-OHP in recirculating experiments (28.0 microM) at the highest dose (239 nmol/ min) of (S)-propranolol were greater than its IC50 of 20 microM, suggesting that 4-OHP is the inhibiting metabolite in the intact liver. The absence of evidence for product inhibition with (R)-propranolol in perfused livers suggests that (S)-4-OHP inhibits 4-hydroxylation of each isomer but (R)-4-OHP does not. 4. We conclude that in the recirculating IPRL, product inhibition of propranolol metabolism is evident with the (S)-isomer, but not he (R)-isomer, and that the inhibiting metabolite is (S)-4-OHP.

The secretion of propranolol enantiomers in human saliva: evidence for active transport?

To study the possible transport routes which may lead to the presence of a drug in saliva, the concentration-time curves of the separate enantiomers of propranolol were measured in human saliva and plasma after oral administration of 10 mg of propranolol hydrochloride. Saliva samples were taken with the Salivette device. Plasma and saliva concentrations of the enantiomers of propranolol were determined by HPLC with fluorescence detection. The transport of propranolol from plasma to the salivary gland appears to be not stereospecific and not saturable. Therefore, there is no indication that the transport of propranolol to the salivary gland is active. The concentrations of both enantiomers of propranolol in saliva, however, were higher than those of both enantiomers in venous plasma. In the past this phenomenon was interpreted as an indication of active transport, but it could be explained by the fact that salivary concentration more closely reflects the central compartment than that of peripheral venous blood.

Stereochemical aspects of pharmacotherapy

Over the past 15 years stereoselectivity has become a well-recognized consideration in clinical pharmacology. Drugs that have an asymmetric center or plane of symmetry within their molecular structure are said to be chiral. They are available as pairs of nonsuperimposable mirror images, called enantiomers, that share essentially the same physicochemical properties. These three-dimensional structural differences, however, can translate into enantiospecific pharmacologic or pharmacokinetic properties, which may be important in understanding the clinical pharmacology of chiral drugs. Most chiral drugs are available as the racemate, in which equal proportions of the two enantiomers are administered concurrently. The pharmacologic and disposition properties of many chiral drugs are documented to be stereospecific, and this has influenced the regulatory requirements for the approval of new drug candidates. Due to this influence on new drug development, the possible issues surrounding racemic drugs will undoubtedly affect the types of pharmaceuticals that are used clinically in the next century. Accordingly, considerable advances have been made in producing optically pure drug. It should be emphasized, however, that stereochemically pure drugs are not necessarily superior to the respective racemates.

Enantioselective inhibitory effect of nicardipine on the hepatic clearance of propranolol in man

The influence of a single oral dose of 30 mg nicardipine on the pharmacokinetics of (R)- and (S)-propranolol, given orally as rac-propranolol 80 mg, was studied in 12 healthy volunteers. The plasma concentrations were higher for the (S)-enantiomer than for the (R)-enantiomer. The Cl(o) and the Cl'intr of (S)-propranolol were significantly lower than the Cl(o) and Cl'intr of (R)-propranolol. The unbound fraction of (R)-propranolol was significantly higher than that of (S)-propranolol. Coadministration of nicardipine significantly increased the AUC and Cmax and significantly decreased the Cl(o) and Cl'intr for unbound drug of (R)- and (S)-propranolol. These changes were more important for (R)- than for (S)-propranolol. The protein binding was not altered by nicardipine. The enantioselective effect of nicardipine on the metabolic clearance of propranolol appears to be due to an interaction at the level of the metabolizing enzymes. The effect on blood pressure of rac-propranolol was little affected when nicardipine was coadministered with rac-propranolol, and its bradycardic effect was reduced.

Substrate stereoselectivity and enantiomer/enantiomer interaction in propranolol metabolism in rat liver microsomes

The substrate stereoselectivity and enantiomer/enantiomer interaction of (S)- and (R)- propranolol for the formation of their metabolites were investigated in rat liver microsomal fractions. The enantiomers of primary metabolites of propranolol, 4-, 5-, 7-hydroxy- and N-desisopropyl-propranolol were separated and assayed by an HPLC method employing a chiral ovomucoid column. Regioselective substrate stereoselectivity (R < S for 4- and 5-hydroxylations; R > S for 7-hydroxylation; R = S for N-desisopropylation) was observed in the formation of propranolol metabolites when the individual enantiomers or a racemic mixture of propranolol were used as substrates. Concentration-dependent metabolic inhibition of propranolol enantiomers by their optical isomers was also observed. In addition, the inhibition of propranolol 4-, 5- and 7-hydroxylations between the enantiomers showed a typical competitive nature. These findings suggested that the propranolol enantiomers competed for the same enzyme, probably a cytochrome P450 isozyme in the CYP2D subfamily.