A quantitative approach to analysing cortisol response in the horse.

The cortisol response to glucocorticoid intervention has, in spite of several studies in horses, not been fully characterized with regard to the determinants of onset, intensity and duration of response. Therefore, dexamethasone and cortisol response data were collected in a study applying a constant rate infusion regimen of dexamethasone (0.17, 1.7 and 17 µg/kg) to six Standardbreds. Plasma was analysed for dexamethasone and cortisol concentrations using UHPLC-MS/MS. Dexamethasone displayed linear kinetics within the concentration range studied. A turnover model of oscillatory behaviour accurately mimicked cortisol data. The mean baseline concentration range was 34-57 µg/L, the fractional turnover rate 0.47-1.5 1/h, the amplitude parameter 6.8-24 µg/L, the maximum inhibitory capacity 0.77-0.97, the drug potency 6-65 ng/L and the sigmoidicity factor 0.7-30. This analysis provided a better understanding of the time course of the cortisol response in horses. This includes baseline variability within and between horses and determinants of the equilibrium concentration-response relationship. The analysis also challenged a protocol for a dexamethasone suppression test design and indicated future improvement to increase the predictability of the test.

Mathematical modeling and simulation in animal health. Part I: Moving beyond pharmacokinetics.

The application of mathematical modeling to problems in animal health has a rich history in the form of pharmacokinetic modeling applied to problems in veterinary medicine. Advances in modeling and simulation beyond pharmacokinetics have the potential to streamline and speed-up drug research and development programs. To foster these goals, a series of manuscripts will be published with the following goals: (i) expand the application of modeling and simulation to issues in veterinary pharmacology; (ii) bridge the gap between the level of modeling and simulation practiced in human and veterinary pharmacology; (iii) explore how modeling and simulation concepts can be used to improve our understanding of common issues not readily addressed in human pharmacology (e.g. breed differences, tissue residue depletion, vast weight ranges among adults within a single species, interspecies differences, small animal species research where data collection is limited to sparse sampling, availability of different sampling matrices); and (iv) describe how quantitative pharmacology approaches could help understanding key pharmacokinetic and pharmacodynamic characteristics of a drug candidate, with the goal of providing explicit, reproducible, and predictive evidence for optimizing drug development plans, enabling critical decision making, and eventually bringing safe and effective medicines to patients. This study introduces these concepts and introduces new approaches to modeling and simulation as well as clearly articulate basic assumptions and good practices. The driving force behind these activities is to create predictive models that are based on solid physiological and pharmacological principles as well as adhering to the limitations that are fundamental to applying mathematical and statistical models to biological systems.

Plasma concentration-dependent suppression of endogenous hydrocortisone in the horse after intramuscular administration of dexamethasone-21-isonicotinate.

Detection times and screening limits (SL) are methods used to ensure that the performance of horses in equestrian sports is not altered by drugs. Drug concentration-response relationship and knowledge of concentration-time profiles in both plasma and urine are required. In this study, dexamethasone plasma and urine concentration-time profiles were investigated. Endogenous hydrocortisone plasma concentrations and their relationship to dexamethasone plasma concentrations were also explored. A single dose of dexamethasone-21-isonicotinate suspension (0.03 mg/kg) was administered intramuscularly to six horses. Plasma was analysed for dexamethasone and hydrocortisone and urine for dexamethasone, using UPLC-MS/MS. Dexamethasone was quantifiable in plasma for 8.3 ± 2.9 days (LLOQ: 0.025 µg/L) and in urine for 9.8 ± 3.1 days (LLOQ: 0.15 µg/L). Maximum observed dexamethasone concentration in plasma was 0.61 ± 0.12 µg/L and in urine 4.2 ± 0.9 µg/L. Terminal plasma half-life was 38.7 ± 19 h. Hydrocortisone was significantly suppressed for 140 h. The plasma half-life of hydrocortisone was 2.7 ± 1.3 h. Dexamethasone potency, efficacy and sigmoidicity factor for hydrocortisone suppression were 0.06 ± 0.04 µg/L, 0.95 ± 0.04 and 6.2 ± 4.6, respectively. Hydrocortisone suppression relates to the plasma concentration of dexamethasone. Thus, determination of irrelevant plasma concentrations and SL is possible. Future research will determine whether hydrocortisone suppression can be used as a biomarker of the clinical effect of dexamethasone.

Animal Health Modeling & Simulation Society: a new society promoting model-based approaches in veterinary pharmacology.

The Animal Health Modeling & Simulation Society (AHM&S) is a newly founded association (2012) that aims to promote the development, application, and dissemination of modeling and simulation techniques in the field of Veterinary Pharmacology and Toxicology. The association is co-chaired by Pr. Johan Gabrielsson (Europe), Pr. Jim Riviere (USA), and secretary Dr. Jonathan Mochel (Switzerland). This short communication aims at presenting the membership, rationale and objectives of this group.

Translational neuropharmacology and the appropriate and effective use of animal models.

This issue of the British Journal of Pharmacology is dedicated to reviews of the major animal models used in neuropharmacology to examine drugs for both neurological and psychiatric conditions. Almost all major conditions are reviewed. In general, regulatory authorities require evidence for the efficacy of novel compounds in appropriate animal models. However, the failure of many compounds in clinical trials following clear demonstration of efficacy in animal models has called into question both the value of the models and the discovery process in general. These matters are expertly reviewed in this issue and proposals for better models outlined. In this editorial, we further suggest that more attention be made to incorporate pharmacokinetic knowledge into the studies (quantitative pharmacology). We also suggest that more attention be made to ensure that full methodological details are published and recommend that journals should be more amenable to publishing negative data. Finally, we propose that new approaches must be used in drug discovery so that preclinical studies become more reflective of the clinical situation, and studies using animal models mimic the anticipated design of studies to be performed in humans, as closely as possible.

Quantitative pharmacology or pharmacokinetic pharmacodynamic integration should be a vital component in integrative pharmacology.

Pharmacodynamics (PD) examines the relationship between drug concentration and onset, intensity, and duration of the pharmacological effect. Pharmacokinetics (PK) is the science of the time course of drugs in the organism. The quantitative pharmacological approach focuses on concentration-response and response-time relationships, with special emphasis on the proposed impact of the drug on the disease. The review aims to raise awareness among pharmacologists with regard to why pharmacokinetic-pharmacodynamic (PKPD) integration is essential in basic pharmacology research to improve interpretation of data. Quantitative pharmacology is vital in drug discovery for target validation, optimizing the development of lead compounds, and scaling compounds to humans and has become mandatory for regulatory bodies. However, its use is still comparatively rare in experimental pharmacology, and its absence diminishes the interpretative value of published experimental data and can allow the presentation of misleading information. A primary requirement for PKPD integration is establishing the inter-relationships between in vitro and in vivo PK and PD properties and extrapolation to the known or possible future clinical use of a compound. This review examines the use of PKPD in experimental pharmacology by reviewing drug exposure measurements, plasma protein binding, exposure-effect relationships, and the measurement of active metabolites. It examines the significance of dosing schedules, the importance of target engagement, and problems in examining time-response relationships. It shows how quantitative pharmacology adds significant value to study design and examines why ignoring pharmacokinetics can lead to misleading results and conclusions. Finally, a guide list of points to be considered when performing studies is provided.

Do ethanol and deuterium oxide distribute into the same water space in healthy volunteers?

BACKGROUND: The volume of distribution at steady state for ethanol (Vss) is thought to be identical to the total body water (TBW). We compared a two-compartment pharmacokinetic model with parallel Michaelis-Menten and first-order renal elimination with the classical one-compartment zero-order elimination model. Ethanol concentration-time profiles were established for breath, venous blood, and urine. The values of Vss obtained for ethanol were compared with TBW determined by deuterium oxide dilution. METHODS: Sixteen healthy volunteers each received a 30-min intravenous infusion of ethanol on two occasions. Ethanol was measured in breath by a quantitative infrared analyzer and in blood and urine by headspace gas chromatography. Deuterium oxide was given as an intravenous injection and measured in serum by isotope-ratio mass spectrometry. Components of variation were calculated by ANOVA to determine the precision of the estimates of Vss and TBW. RESULTS: Mean TBW, determined by deuterium oxide dilution, was 44.1 +/- 3.9 liters (+/-SD) for men, corresponding to 0.61 liters/kg, and 37.4 +/- 3.2 liters for women, or 0.54 liters/kg. Estimates of Vss from blood-ethanol pharmacokinetics were 87.6% of TBW according to isotope dilution and 84.4% for breath analysis with the two-compartment model. This compares with 95.1% and 95.4% for blood and breath alcohol, respectively, when the classical zero-order kinetic analysis is used. The precision of the estimates of Vss and TBW was between +/-1.56 and +/-2.19 liters (95% confidence interval). CONCLUSIONS: Ethanol does not distribute uniformly into the TBW. The precision of measuring Vss by ethanol dilution was comparable to estimates of TBW by isotope dilution. Results of noninvasive breath ethanol analysis compared well with use of venous blood for estimating Vss. The sophisticated two-compartment model was much superior to the classical one-compartment model in explaining the total concentration-time course of intravenously given ethanol.

Pharmacodynamic modelling of reversible gastric acid pump inhibition in dog and man.

H 335/25, a 4-amino quinoline, belongs to a new class of reversible gastric acid pump inhibitors. A potential advantage of such drugs over the irreversible proton pump inhibitors (PPIs) is better control over the effect-time profile. Dose escalation studies were performed to characterize the effect on acid secretion in dogs (n=24) and healthy male subjects (n=12). The effect-time profile was delayed compared to the concentration-time profile. A model-based approach, using non-linear mixed effects modelling, was applied to quantify and elucidate the mechanism for the delayed effect. Three different models were investigated: (1) a slow equilibration preceding the formation of drug-enzyme complex, modelled by an effect-compartment, (2) a slow equilibration between free drug, free enzyme and drug-enzyme complex, described by a kinetic binding model, and (3) a delay between enzyme inhibition and the measured response, described by an indirect response model. Model 2 was shown to be superior to models 1 and 3, for both dog and human data. The dissociation rate constant, k(off), was estimated to be 0.85 and 0.88 h and the calculated equilibration constant, K(d), was 160 and 250 nM in dog and man, respectively. Simulations of the predicted time-course of the effect beyond the 4-5-h observation period was similar for the three models.

Neuroprotective efficacy of AR-A008055, a clomethiazole analogue, in a global model of acute ischaemic stroke and its effect on ischaemia-induced glutamate and GABA efflux in vitro.

We have investigated the neuroprotective properties of AR-A008055 [(+/-)-1-(4-methyl-5-thiazolyl-1-phenyl-methylamine], a novel compound structurally related to clomethiazole. Administration (i.p.) of (+/-)-AR-A008055 60 min after 5 min of global cerebral ischaemia in gerbils produced a dose-dependent protection of the hippocampus from damage. Both enantiomers [(R)-(+)-AR-A008055 and (S)-(-)- AR-A008055] at 600 micromol/kg produced similar protection to that following clomethiazole (600& micromol/kg) and both produced similar and sustained neuroprotection, at 4, 7 and 21 days post-insult. When infused intravenously over a 2-h period, both enantiomers produced concentration-dependent neuroprotection, with the enantiomers providing similar protection at every plasma concentration (50-200 nmol/ml). The efficacy of (S)-(-)-AR-A008055 was similar to clomethiazole, but it was slightly less potent. Ischaemia-induced glutamate efflux from rat brain cortical prisms in vitro was inhibited by both isomers (100 microM). The inhibitory effect of (R)-(+)-AR-A008055 was blocked by bicuculline (10 microM) and picrotoxin (100 microM), while the effect of (S)-(-)-AR-A008055 was only antagonised by picrotoxin. This indicated that (S)-(-)-AR-A008055, like clomethiazole, is able to open the GABA(A)-chloride channel in the absence of endogenous GABA. (R)-(+)-AR-A008055 was more potent than (S)-(-)-AR-A008055 in enhancing the concentration of GABA in the medium following 30 min exposure of tissue to the ischaemic conditions, suggesting that it is an effective GABA uptake inhibitor. This action may explain both its effect on glutamate efflux in vitro and its neuroprotective effect in vivo.

Modeling of dose-response-time data: four examples of estimating the turnover parameters and generating kinetic functions from response profiles.

The most common approach to in vivo pharmacokinetic and pharmacodynamic modeling involves sequential analysis of the plasma concentration versus time and then response versus time data, such that the plasma kinetic model provides an independent function, driving the dynamics. However, response versus time data, even in the absence of measured drug concentrations, inherently contain useful information about the turnover characteristics of response (turnover rate, half-life of response), the drug's biophase kinetics (F, half-life) as well as the pharmacodynamic characteristics (potency, intrinsic activity). Previous analyses have assumed linear kinetics, linear dynamics, no time lag between kinetics and dynamics (single-valued response), and time constant parameters. However, this report demonstrates that the drug effect can be indirect (antinociception, cortisol/adrenocorticotropin (ACTH), body temperature), display nonlinear kinetics, display feedback mechanisms (nonstationarity, cortiso/ACTH) and exhibit hysteresis with the drug levels in the biophase (antinociception, body temperature). It is also demonstrated that crucial determinants of the success of modeling dose-response-time data are the dose selection, multiple dosing, and to some extent different input rates and routes. This report exemplifies the possibility of assigning kinetic forcing functions in pharmacodynamic modeling in both preclinical and clinical studies for the purpose of characterizing (discrimination between turnover and drug-specific parameters) response data and optimizing subsequent clinical protocols, and for identification of inter-individual differences.

A turnover model of irreversible inhibition of gastric acid secretion by omeprazole in the dog.

A turnover model for irreversible inhibition of gastric acid secretion by omeprazole in gastric fistula dogs was developed using data from studies with both short- and long-term measurement periods. In the short-term experiments, after stimulation of acid secretion with histamine, the dogs were infused i.v. with omeprazole and acid secretion was measured for 5 h. Dose and infusion times were varied to produce different concentration-time profiles and schedule dependence in the inhibitory effect of omeprazole was observed. In the long-term experiments, dogs were given single intraduodenal doses, which inhibited the acid secretion for several days. Combining the short-term and long-term data allowed the observation of a biphasic recovery of acid secretion that was described by the turnover model. Second order association rate constants (k(ome)) for the covalent binding of omeprazole to H(+),K(+)-ATPase were estimated to 11 and 3.0 l/micromol/h for the i.v. and intraduodenal experiments, respectively. The apparent turnover rate constant of the enzyme (k(out)) was 0.013 h(-1) and the corresponding half-life of inhibition of acid secretory capacity was 54 h. The potency, calculated as k(out) over k(ome), was 4.3 and 1.2 nM for the intraduodenal and i.v. doses, respectively. Allometric scaling of the model resulted in trustworthy predictions for observations previously done in humans. The model predicted a good correlation between maximal inhibitory effect and exposure (area under the plasma concentration curve). The time dependence in this relation was also predicted by the model.

Within- and between-subject variations in pharmacokinetic parameters of ethanol by analysis of breath, venous blood and urine.

AIMS: To evaluate the prerequisites for using ethanol dilution to estimate total body water, we studied the within- and between-subject variation in the parameter estimates of a two-compartment model for ethanol pharmacokinetics with parallel Michaelis-Menten and first-order renal elimination. Because sampling of breath might be preferable in some clinical situations the parameter estimates derived from breath and venous blood were compared. METHODS: On two occasions, ethanol 0.4 g kg-1 was given by intravenous infusion to 16 volunteers after they had fasted overnight. The proposed model was fitted by means of nonlinear regression to concentration-time data measured in the breath, venous blood and urine during 360 min. The model contained six parameters: Vmax and Km (Michaelis-Menten elimination constants), CLd (intercompartmental distribution parameter), VC and VT (volumes of the central and tissue compartment, respectively) and CLR (renal clearance). The volume of distribution, Vss, was calculated as the sum of VC and VT. RESULTS: The mean +/- total s.d. of the parameter estimates derived from blood data were Vmax 95 +/- 25 mg min-1, Km 27 +/- 19 mg l-1, CLd 809 +/- 232 ml min-1, VC 14.5 +/- 4.3 l, VT 21. 2 +/- 4.4 l, CLR 3.6 +/- 2.0 ml min-1 and Vss 35.8 +/- 4.3 l. The variation within subjects amounted to 3%, 9%, 21%, 21%, 17%, 26% and 2%, respectively, of the total variation. Breath samples were associated with a similar or lower variation than blood, both within and between subjects. About 1.5% of the infused ethanol was recovered in the urine. CONCLUSIONS: The low within-subject variation of the key parameter Vss (only 2%) suggests that ethanol dilution analysed by the pharmacokinetic model applied here may be used as an index of the total body water. Breath samples yielded at least as good reproducibility in the model parameters as venous blood.

Multivariate methods in developing an evolutionary strategy for tablet formulation.

The aim of this study was to develop a new strategy for choosing excipients in tablet formulation. Multivariate techniques such as principal component analysis (PCA) and experimental design were combined in a multivariate design for screening experiments. Of a total 87 investigated excipients, the initial screening experiments contained 5 lubricants, 9 binders, and 5 disintegrants, and 35 experiments were carried out. Considering a reduced factorial design was used, the resulting PCA and partial least squares (PLS) models offered good insight into the possibilities of tablet formulation. It also offered solutions to the problems and clearly gave directions for optimum formulations. Further, it offered several alternatives for achieving quality formulations. Additional experiments conducted to validate and verify the usefulness of the model were successful, resulting in several tablets of good quality. The conclusion is that a multivariate strategy in tablet of formulation is efficient and can be used to reduce the number of experiments drastically. Combining multivariate characterization, physicochemical properties, experimental design, multivariate design, and PLS would lead to an evolutionary strategy for tablet formulation. Since it includes a learning strategy that continuously incorporates data for new compounds and from conducted experiments, this would be an even more powerful tool than expert systems.

The influence of drug input rate on the development of tolerance to frusemide.

AIMS: Understanding the impact of drug input rate on its pharmacokinetic-pharmacodynamic relationship may lead to a more optimal drug therapy. The aim of the present study was to investigate the influence of the rate of administration on tolerance development to frusemide, by giving the drug at four different infusion rates. METHODS: Eight healthy volunteers were given 10 mg of frusemide on four different occasions, as a constant-rate intravenous infusion during 10, 30, 100 and 300 min, respectively. Urinary volume and contents of frusemide and sodium were measured in samples collected over 8 h. RESULTS: The four different infusion rates systematically influenced the frusemide excretion rate versus diuretic and natriuretic response relationship. Counter-clockwise hysteresis occurred for the most rapid infusion rate, whereas a progressive clockwise hysteresis was observed for the slower infusions, indicating development of tolerance. For each subject, diuresis and natriuresis were modeled for all four treatments simultaneously, using a feedback tolerance model. It was not possible to describe the data using a model without tolerance. The time course of tolerance development showed remarkable differences between the infusion rates. The intensity of maximum tolerance development was significantly less for the slowest infusion rate compared with the more rapid infusions and it appeared significantly later. However, no differences in diuretic or natriuretic response were found between the treatments. CONCLUSIONS: The direction of the hysteresis loop is dependent on the input rate of frusemide. After the administration of a single low dose of frusemide, the time course of tolerance, rather than the integrated time course of tolerance, is influenced by the drug input rate.

Time course of enzyme induction in humans: effect of pentobarbital on nortriptyline metabolism.

To study the effect of induction we gave six male volunteers 10 mg nortriptyline three times a day for 4 weeks and 0.2 gm pentobarbital on days 8 to 21. Plasma and urinary levels of nortriptyline and metabolites were measured. The rate and extent of induction of the enzyme(s) were estimated by a model with use of nortriptyline concentrations. There was a marked decrease of nortriptyline levels after 2 days of pentobarbital treatment. Total clearance of nortriptyline increased more than twofold (range, 1.6-fold to 4.1-fold). Apparent metabolic clearance by 10-hydroxylation increased markedly. The decrease in nortriptyline levels was more rapid than the increase after pentobarbital cessation, fitting with the theory of the model. The induction of nortriptyline metabolism is probably mainly the result of an increase in a non-CYP 2D6 P450 isozyme, possibly CYP 3A4 or a CYP 2C form. More knowledge of induction characteristics of drugs should lead to better predictions of decreased effects and appearance of adverse effects. The kinetic model used for analysis of our data could then be useful.

Influence of CYP2D6 polymorphism on the pharmacokinetics and pharmacodynamic of tolterodine.

OBJECTIVE: To determine whether cytochrome P450 2D6 (CYP2D6) is involved in the metabolism of tolterodine by investigating potential differences in pharmacokinetics and pharmacodynamic (heart rate, accommodation, and salivation) of tolterodine and its 5-hydroxymethyl metabolite between poor metabolizers and extensive metabolizers of debrisoquin (INN, debrisoquine). METHODS: Sixteen male subjects (eight extensive metabolizers and eight poor metabolizers) received 4 mg tolterodine by mouth twice a day for 8 days followed by a single intravenous infusion of 1.8 mg tolterodine for 30 minutes after a washout period. Doses were given as the tartrate salt. The pharmacokinetics of tolterodine and 5-hydroxymethyl metabolite were determined, and the pharmacodynamic were measured. RESULTS: The mean systemic clearance of tolterodine was significantly lower (p < 0.001) among poor metabolizers (9.0 +/- 2.1 l/hr) compared with extensive metabolizers (44 +/- 13 L/hr), resulting in a fourfold longer elimination half-life (p < 0.001). The terminal half-life of the 5-hydroxymethyl metabolite (2.9 +/- 0.4 hours) was slightly longer than that of the parent compound (2.3 +/- 0.6 hours) among extensive metabolizers, but the 5-hydroxymethyl metabolite was undetectable in the serum of poor metabolizers. Only minor differences in pharmacodynamic effects after tolterodine dosage were observed between the groups. Tolterodine caused a similar decrease in salivation in both panels. The decrease occurred when the concentration of unbound tolterodine and 5-hydroxymethyl metabolite among extensive metabolizers was comparable with that of tolterodine among poor metabolizers. CONCLUSIONS: Tolterodine is extensively metabolized by CYP2D6 with high specificity. Despite the effect on pharmacokinetics, the CYP2D6 polymorphism does not appear to be of great importance in the antimuscarinic effect, probably because of the additive action of parent drug and active metabolite.

Pharmacokinetics and pharmacodynamics of tolterodine in man: a new drug for the treatment of urinary bladder overactivity.

The aim of this study was to determine the pharmacokinetics, pharmacodynamics, and safety of tolterodine following single oral and intravenous doses in healthy volunteers. A secondary aim was to identify major urinary metabolites and determine mass balance. Single oral doses of 0.2, 0.4, 0.8, 1.6, 3.2, 6.4, and 12.8 mg of tolterodine (as the tartrate salt) were given to 17 healthy male volunteers. Two intravenous doses (0.64 and 1.28 mg) were administered to 8 of the volunteers and mass balance was studied after a single oral dose of 5 mg (14C)-tolterodine in 6 subjects. Tolterodine was rapidly absorbed following oral administration (time to peak serum concentration 0.9 +/- 0.4 h). The absolute bioavailability was highly variable, ranging from 10 to 70%. The volume of distribution at steady-state ranged from 0.9 to 1.6 l/kg and systemic clearance ranged from 0.23 to 0.52 l/h/kg, which resulted in a terminal half-life of 2-3 h. Tolterodine exhibited high first-pass metabolism and 2 hepatic metabolic pathways were identified: oxidation and dealkylation. Independent of route of administration, < 1% of the parent compound was excreted unchanged in urine. Five metabolites were structurally identified in urine. Following oral administration of (14C)-tolterodine, the excretion of radioactivity into urine and feces was 77 +/- 4.0% and 17 +/- 3.5%, respectively. Tolterodine decreased stimulated salivation after 3.2 mg, increased heart rate after 6.4 mg, and nearpoint of vision after 12.8 mg. Six of 8 subjects reported micturition difficulties after a dose of 12.8 mg. The lack of a direct relationship between tolterodine serum concentrations and effects on stimulated salivation suggested the presence of pharmacologically active metabolite(s).

Pharmacodynamic modeling of furosemide tolerance after multiple intravenous administration.

OBJECTIVE: Physiologic indirect-response models have been proposed to account for the pharmacodynamics of drugs with an indirect mechanism of action, such as furosemide. However, they have not been applied to tolerance development. The aim of this study was to investigate the development of tolerance after multiple intravenous dosing of furosemide in healthy volunteers. METHODS: Three repetitive doses of 30 mg furosemide were given as rapid intravenous infusions at 0, 4, and 8 hours to eight healthy volunteers. Urine samples were collected for a period up to 14 hours after the first dose. Volume and sodium losses were isovolumetrically replaced with an oral rehydration fluid. RESULTS: Tolerance was demonstrated as a significant decrease in diuretic and natriuretic response over time. Total mean diuresis was 35% lower (p < 0.01) and total mean natriuresis was 52% lower (p < 0.0001) after the third dose of furosemide compared with the first dose. However, there were considerable interindividual variations in the rate and extent of tolerance development for both diuresis and natriuresis. Pharmacokinetic-pharmacodynamic modeling of tolerance development was performed with use of an indirect-response model with an additional "modifier" compartment. This model gave an accurate description of the diuretic and natriuretic data after multiple dosing of furosemide and enabled the estimation of a lag-time for tolerance and a rate constant for tolerance development. Physiologic counteraction was demonstrated as a significant increase in plasma active renin levels (p < 0.00001) and a decrease in atrial natriuretic peptide levels (p < 0.005) during the day, concomitantly with the development of a negative sodium balance. This may be viewed as physiologic reflections of the modifier in our model. CONCLUSION: Indirect-response models may be successfully applied for tolerance modeling of drugs after multiple dosing.

Pharmacokinetic data on estradiol in light of the estring concept. Estradiol and estring pharmacokinetics.

The pharmacokinetics and pharmacodynamics of estradiol in humans are briefly reviewed in this paper. The estradiol vaginal ring was designed and developed to obtain controlled local delivery of very low doses of estradiol resulting in a marginal effect on plasma concentrations of estradiol, associated with a margin of safety over a prolonged period of time. Three clinical studies in postmenopausal women with signs and symptoms while plasma levels remain virtually unchanged in the lower part of the normal postmenopausal range. No pharmacodynamic effects of estrogen could be detected, reflecting a very low systemic exposure. The estradiol exposure and the plasma concentration time course during treatment with first and second ring during first month of the intended three month treatment period are compared. The very low systemic exposure is discussed in relation to existing therapies. Ongoing pharmacokinetic work with estradiol vaginal ring is briefly described. It is concluded that the estradiol vaginal ring is a very stable alternative to existing vaginal therapies, resulting in a very low systemic exposure to estradiol, and is associated with a minimal risk of drug interaction and has a high margin of safety.

New kinetic data on estradiol in light of the vaginal ring concept.

The principal estrogen produced by the functioning premenopausal ovary is 17 beta-estradiol. At the point of irreversible ovarian failure, at menopause, the production of estradiol decreases dramatically, which results in circulating serum levels less than 120 pmol/l. It is important to recognise the pharmacokinetic and metabolic outcomes associated with dosage and route of delivery of estrogen. One of the most promising methods of administering estrogen replacement therapy (ERT) for local effects is the estradiol vaginal ring designed for a controlled continuous low release (7.5 micrograms estradiol/24 h) over a period of 90 days. The present study was undertaken to characterise the basal endogenous turnover of estradiol in postmenopausal women. Information on the disposition of estradiol after an intravenous dose formed the base of the kinetic model. The rate of extent of absorption of estradiol was assessed after ring application. Individual serum concentrations of estradiol were analysed without subtraction of the basal estradiol levels. The results indicate a rapidly eliminated compound (plasma clearance 2 l/min) with a distribution of approximately 50 l, resulting in an efficient half-life of about 20 min. The endogenous production was highly variable (< 1-44 micrograms/24 h). The steady-state estradiol levels following ring application did not increase and were well within the normal basal estradiol range seen in untreated women. In light of the present findings, the low daily dose, the low availability of estradiol across the vaginal wall and the controlled local delivery, favour the use of the estradiol vaginal ring.