Local application of engineered insulin-like growth factor I mRNA demonstrates regenerative therapeutic potential in vivo.

Insulin-like growth factor I (IGF-I) is a growth-promoting anabolic hormone that fosters cell growth and tissue homeostasis. IGF-I deficiency is associated with several diseases, including growth disorders and neurological and musculoskeletal diseases due to impaired regeneration. Despite the vast regenerative potential of IGF-I, its unfavorable pharmacokinetic profile has prevented it from being used therapeutically. In this study, we resolved these challenges by the local administration of IGF-I mRNA, which ensures desirable homeostatic kinetics and non-systemic, local dose-dependent expression of IGF-I protein. Furthermore, IGF-I mRNA constructs were sequence engineered with heterologous signal peptides, which improved in vitro protein secretion (2- to 6-fold) and accelerated in vivo functional regeneration (16-fold) over endogenous IGF-I mRNA. The regenerative potential of engineered IGF-I mRNA was validated in a mouse myotoxic muscle injury and rabbit spinal disc herniation models. Engineered IGF-I mRNA had a half-life of 17-25 h in muscle tissue and showed dose-dependent expression of IGF-I over 2-3 days. Animal models confirm that locally administered IGF-I mRNA remained at the site of injection, contributing to the safety profile of mRNA-based treatment in regenerative medicine. In summary, we demonstrate that engineered IGF-I mRNA holds therapeutic potential with high clinical translatability in different diseases.

Model-based prediction of phase III overall survival in colorectal cancer on the basis of phase II tumor dynamics.

PURPOSE: We developed a drug-disease simulation model to predict antitumor response and overall survival in phase III studies from longitudinal tumor size data in phase II trials. METHODS: We developed a longitudinal exposure-response tumor-growth inhibition (TGI) model of drug effect (and resistance) using phase II data of capecitabine (n = 34) and historical phase III data of fluorouracil (FU; n = 252) in colorectal cancer (CRC); and we developed a parametric survival model that related change in tumor size and patient characteristics to survival time using historical phase III data (n = 245). The models were validated in simulation of antitumor response and survival in an independent phase III study (n = 1,000 replicates) of capecitabine versus FU in CRC. RESULTS: The TGI model provided a good fit of longitudinal tumor size data. A lognormal distribution best described the survival time, and baseline tumor size and change in tumor size from baseline at week 7 were predictors (P < .00001). Predicted change of tumor size and survival time distributions in the phase III study for both capecitabine and FU were consistent with observed values, for example, 431 days (90% prediction interval, 362 to 514 days) versus 401 days observed for survival in the capecitabine arm. A modest survival improvement of 39 days (90% prediction interval, -21 to 110 days) versus 35 days observed was predicted for capecitabine. CONCLUSION: The modeling framework successfully predicted survival in a phase III trial on the basis of capecitabine phase II data in CRC. It is a useful tool to support end-of-phase II decisions and design of phase III studies.

Allometric scaling of pharmacodynamic responses: application to 5-Ht1A receptor mediated responses from rat to man.

PURPOSE: The aim of the present study was to assess whether two widely used biomarkers for 5-HT(1A)-receptor mediated responses in the rat (hypothermia and corticosterone increase) could be scaled to man using allometric principles. MATERIALS AND METHODS: Mechanism-based pharmacokinetic-pharmacodynamic (PK-PD) models were developed and characterized in rats for the standard 5-HT(1A)-receptor agonists, buspirone and flesinoxan. Allometric scaling was investigated on the basis of simulation taking into account the inter-individual variability and clinical study design. The model-predicted effects of both flesinoxan and buspirone were compared to those published in the literature. RESULTS: The main finding of this analysis was that for both hypothermia and cortisol increase, the model could predict the extent of the pharmacological response in man adequately. For the hypothermic response, the time course of the response was also predicted with a high degree of accuracy. In contrast, in the case of the cortisol response, the observed time lag was, despite the fact that it fell within the model uncertainty, not predicted. CONCLUSIONS: Based on these analyses, it is concluded that allometrically scaled mechanism based PK-PD models are promising as a means of predicting the pharmacodynamic responses in man. This approach provides for a novel way of interpreting and scaling pre-clinical pharmacological responses and ultimately facilitates the understanding and prediction of pharmacological responses in man.

Population pharmacodynamic modelling of lorazepam- and midazolam-induced sedation upon long-term continuous infusion in critically ill patients.

OBJECTIVE: The objective of the present investigation was to develop a population pharmacodynamic model for midazolam- and lorazepam-induced sedation upon long-term continuous infusion in critically ill patients. METHODS: The study was conducted in 59 patients receiving lorazepam and 54 patients receiving midazolam by continuous infusion for at least 24 h. Repeated blood samples were obtained for determination of the concentrations of lorazepam and midazolam. The level of sedation was assessed using a 5-point sedation scale. RESULTS: The pharmacokinetics of lorazepam and midazolam was described with previously proposed pharmacokinetic models. For the pharmacodynamics, the probability that the sedation was equal to or more than a specific score was described using a sigmoid E(max) model. The EC(50) values of lorazepam for the sedation scores equal or larger than 2-5 were 6.1, 57, 184 and 529 ng/ml, respectively. The corresponding values for midazolam were 216, 483, 1,100 and 2,200 ng/ml. Inter-individual variability in the EC(50) values was relatively high with a CV of 68% for lorazepam and 86% for midazolam (p=0.035). No covariates explaining part of the observed inter-individual variability were identified. CONCLUSION: The population pharmacodynamic model shows a similarly wide intra- and inter-individual variability in the pharmacodynamics of both lorazepam and midazolam. Thus, the previously observed differences in "ease of titration" between lorazepam and midazolam are unrelated to pharmacodynamic factors.

Allometric relationships between the pharmacokinetics of propofol in rats, children and adults.

AIMS: Allometric equations have proven useful for the extrapolation of animal data to determine pharmacokinetic parameters in man. It has been proposed that these equations are also applicable over the human size range including the paediatric population. The aim of this work was to study the relationship between various pharmacokinetic parameters for propofol and body weight using data from rats, children and adults. Furthermore, the utility of allometric scaling is evaluated by the prediction of propofol concentrations in humans based on data obtained in the rat. METHODS: The relationship between the pharmacokinetic parameters of propofol obtained in rats, children and adults was analyzed by plotting the logarithmically transformed parameters against the corresponding logarithmically transformed body weights. In addition, based on allometric equations, pharmacokinetic parameters obtained in rats were scaled to humans. These parameters were used to simulate propofol concentrations in long-term sedated critically ill patients using NONMEM. Simulated concentrations were then compared with actually observed concentrations in humans. RESULTS: The relationship between pharmacokinetic parameters of propofol from rats, children and adults was in good agreement with those from the literature on allometric modelling. For clearance, intercompartmental clearance, central volume of distribution and peripheral volume of distribution, the power parameters were 0.78, 0.73, 0.98 and 1.1, respectively, and r2 values for the linear correlations were 0.990, 0.983, 0.977 and 0.994, respectively. On the basis of data obtained after a single bolus injection in the rat, adequate predictions of propofol concentrations in critically ill patients can be made using allometric equations, despite the long-term nature of the use of the drug, the large number of infusion changes per day and/or differences in state of health and age. CONCLUSIONS: For propofol, allometric scaling has proved to be valuable for cross species extrapolation. Furthermore, the use of the allometric equation between adults and children seems to be an adequate tool for the development of rational dosing schemes for children of varying body weights, and requires further study.

Pharmacodynamics of oral ganciclovir and valganciclovir in solid organ transplant recipients.

BACKGROUND: A randomized, double-blind study was conducted to evaluate the pharmacokinetics of ganciclovir following oral administration of ganciclovir or valganciclovir for prophylaxis of cytomegalovirus (CMV) disease in solid organ transplant recipients (n = 240/372). METHODS: The correlations between individual exposure to ganciclovir during prophylaxis, with CMV viremia incidence during and after treatment, CMV disease up to 12 months posttransplant, and hematological toxicity were assessed. RESULTS: Mean daily areas under the curve (AUCs) of ganciclovir from valganciclovir and oral ganciclovir were 46.3 +/- 15.2 and 28.0 +/- 10.9 microg.h/ml (mean +/- SD), respectively. Viremia was suppressed during prophylaxis when exposure to ganciclovir was 40-50 microg.h/ml, AUCs typical of those achieved in valganciclovir-treated patients. The development of viremia 1 month after ending prophylaxis was also reduced with higher ganciclovir AUC (median predicted incidence, 20% and 10% at AUCs of 33 and 50 microg h/ml, respectively). The development of CMV disease within 1 year of transplant was 17.6% and independent of prophylactic exposure to ganciclovir. There was only a weak tendency to increased neutropenia and leukopenia with higher ganciclovir exposure. CONCLUSIONS: The greater systemic exposure to ganciclovir delivered by valganciclovir was associated with delayed development of viremia. There was only a weak association between AUC and hematological toxicity.

Population pharmacokinetics of lorazepam and midazolam and their metabolites in intensive care patients on continuous venovenous hemofiltration.

BACKGROUND: The objective is to study the population pharmacokinetics of lorazepam and midazolam in critically ill patients with acute renal failure who are treated with continuous venovenous hemofiltration (CVVH). METHODS: Twenty critically ill patients with acute renal failure on CVVH therapy were administered either lorazepam (n = 10) or midazolam (n = 10) by continuous infusion. CVVH was performed with an ultrafiltrate flow of 2 L/h with filtrate substitution in the predilution or postdilution mode. Blood flow through the 1.9-m 2 cellulose triacetate membrane filter was 180 mL/min. For 48 hours, multiple blood and ultrafiltrate samples were obtained for determination of concentrations of the drug and its metabolites. RESULTS: The pharmacokinetics of lorazepam is described best by a 1-compartment model. No significant covariates were identified. Total-body clearance was 6.4 L/h, and volume of distribution was 376 L. Ultrafiltration clearance was 0.31 L/h, equivalent to approximately 5% of total clearance. Average degree of plasma protein binding was 82.9% for lorazepam, with a sieving coefficient of 0.16 +/- 0.03. For lorazepamglucuronide, degree of plasma protein binding was 39.5%, and sieving coefficient was 0.48 +/- 0.07. The pharmacokinetics of midazolam is described best by a 1-compartment model. No significant covariates were identified. Total-body clearance was 8.5 L/h, and volume of distribution was 157 L. Clearance by ultrafiltration was 0.055 L/h, equivalent to approximately 0.7% of total clearance. Average degree of plasma protein binding was 95.8%, with a sieving coefficient of 0.04 +/- 0.03. For the metabolite 1-hydroxymidazolamglucuronide, average degree of plasma protein binding was 43.4%, with a sieving coefficient of 0.45 +/- 0.06. CONCLUSION: Neither lorazepam nor midazolam is removed efficiently by CVVH. CVVH contributes significantly to the removal of the glucuronide metabolites lorazepamglucuronide and 1-hydroxymidazolamglucuronide.

5-HT7 receptor efficacy distribution throughout the canine stomach.

This study aimed to determine, quantify and explain regional differences in the relaxant response to the selective 5-HT(1) and 5-HT(7) receptor agonist 5-carboxamidotryptamine (5-CT) throughout the canine stomach. Longitudinal muscle strips from eight gastric corpus regions and six antrum regions were mounted for isotonic measurement. The 5-CT-induced relaxation was examined on a prostaglandin F(2alpha)-induced submaximal response, expressed as percentage of this response and fitted to the operational model of agonism (OMOA). 5-HT(7) receptor messenger RNA (mRNA) expression was compared by means of quantitative PCR. 5-CT inhibited PGF(2alpha)-induced tonic contraction (corpus) and increase of phasic contraction amplitude (antrum). The consistent antagonism produced by the selective 5-HT(7) receptor antagonist SB-269970 (10 nm, pA(2) estimates 8.2-8.9) confirmed that in every region, the inhibition by 5-CT was 5-HT(7) receptor mediated. However, variation in the maximum effect (61-108%) and pEC(50) (6.4-8.6) was observed throughout the different regions. The OMOA explained these differences as differences in the efficacy parameter tau (ratio of receptor density and coupling efficiency; log tau estimates ranging from 0.1 to 2.1). The log tau gradient decreases going from the lesser to the greater curvature. A proportional difference (68%) in the relative expression of 5-HT(7) receptor mRNA between the lesser and the greater curvature indicates that differences in receptor density contribute to the observed functional differences. This study illustrates that 5-HT(7) receptors are present throughout the ventral wall of the canine stomach, but the efficacy (expressed as log tau) is clearly greater close to the lesser curvature. Differences in 5-HT(7) receptor expression at least partially explain the functional differences.

Mechanism-based pharmacokinetic-pharmacodynamic modeling of 5-HT1A receptor agonists: estimation of in vivo affinity and intrinsic efficacy on body temperature in rats.

The pharmacokinetic-pharmacodynamic (PK-PD) correlations of seven prototypical 5-HT(1A) agonists were analyzed on the basis of a recently proposed semi-mechanistic PK-PD model for the effect on body temperature. The resulting concentration-effect relationships were subsequently analyzed on the basis of the operational model of agonism to estimate the operational affinity (pK(A)) and efficacy (log tau) at the 5-HT(1A) receptor in vivo. The values obtained in this manner were compared with estimates of the affinity (pK(i)) and intrinsic efficacy (log[agonist ratio]) in a receptor-binding assay. Between 5-HT(1A) agonists wide differences in in vivo affinity and efficacy were observed, with values of the pK(A) ranging from 5.67 for flesinoxan to 8.63 for WAY-100,635 [N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-2-pyridinyl-cyclohexanecarboxamide] and of the log tau ranging from -1.27 for WAY-100,135 [N-(1,1-dimethylethyl)-4-(2-methoxyphenyl)-alpha-phenyl-1-piperazine-propanamide] to 0.62 for R-(+)-8-hydroxy-2-[di-n-propylamino)tetralin. Poor correlations were observed between the in vivo receptor affinity (pK(A)) and the affinity estimates in the in vitro receptor binding assay (pK(i); r(2) = 0.55, P > 0.05), which could in part be explained by differences in blood-brain distribution. In contrast, a highly significant correlation was observed between the efficacy parameters in vivo (log tau) and in vitro (log [agonist ratio]; r(2) = 0.76, P < 0.05). Thus by combining the previously proposed semi-mechanistic PK-PD model for the effect on body temperature with the operational model of agonism, a full mechanistic PK-PD model for 5-HT(1A) receptor agonists has been obtained, which is highly predictive of the in vivo intrinsic efficacy.

Comparative population pharmacokinetics of lorazepam and midazolam during long-term continuous infusion in critically ill patients.

AIMS: It is well established that there is a wide intra- and interindividual variability in dose requirements for lorazepam and midazolam in intensive care patients. The objective of this study was to compare the population pharmacokinetics of lorazepam and midazolam after long-term continuous infusion in mechanically ventilated critically ill patients. METHODS: Forty-nine critically ill patients randomly received either lorazepam (n = 28) or midazolam (n = 21) by continuous infusion for at least 24 h. Multiple blood samples were obtained for determination of the drug and metabolite concentrations by HPLC. Population pharmacokinetic models were developed using the Non-Linear Mixed Effect Modelling (NONMEM) program. The influence of selected covariates was investigated. The prospective performance of the models was evaluated on the basis of results in separate groups of patients for lorazepam (n = 31) and midazolam (n = 33). RESULTS: The pharmacokinetics of lorazepam were best described by a two-compartment model. Alcohol abuse, positive end expiratory pressure (PEEP) and age were identified as significant covariates. Total body clearance for patients without alcohol abuse was 4.13 - (PEEP - 5) x 0.42 l h-1, and 0.74 l h-1 for patients with alcohol abuse. The volume of distribution was 0.74 l, the steady state volume of distribution was 56 - (age - 58) x 2.1 l and the intercompartmental clearance was 10 l h-1. The proportional residual error was 15% and the median absolute prediction error was 13.6% with a bias of 1.5%. The pharmacokinetics of midazolam were best described by a two-compartment model with alcohol abuse, APACHE score and age as significant covariates. Total body clearance for patients without alcohol abuse was 11.3 - (age - 57) x 0.14 l h-1, and 7.27 - (age -57) x 0.14 l h-1 for patients with alcohol abuse. The volume of distribution was 7.15 l, the steady state volume of distribution was 431 l, and the intercompartmental clearance was 40.8 - (APACHE score - 26) x 2.75 l h-1. The proportional residual error was 31% with an additive residual error of 32 ng ml-1. The median absolute prediction error was 12.9% with a bias of 1.2%. The prospective performance in the lorazepam evaluation group was better with the covariate adjusted model, but in the midazolam evaluation group it was not better than with the simple model. In all models a tendency to overestimate the lower plasma concentrations was observed. CONCLUSIONS: The pharmacokinetics of both lorazepam and midazolam were well described by a two-compartment model. Inclusion of alcohol abuse and age as covariates improved both models. PEEP was identified as an additional covariate for lorazepam, and the APACHE score for midazolam. For both drugs there is a large interindividual variability in their pharmacokinetics when used for long-term sedation in critically ill patients. However, the intra-individual variability is much lower for lorazepam.

Population pharmacokinetic and pharmacodynamic modeling of propofol for long-term sedation in critically ill patients: a comparison between propofol 6% and propofol 1%.

OBJECTIVES: A population pharmacokinetic and pharmacodynamic model of propofol for long-term sedation in critically ill patients is described, because limited information is available in these patients. In the models the influence of time-independent covariates, in particular, the propofol formulation (propofol 6% versus propofol 1%), and of time-dependent covariates was investigated. METHODS: Twenty critically ill, mechanically ventilated patients received propofol formulated as propofol 6% (n = 10) or propofol 1% (n = 10) during a 2- to 5-day period. The level of sedation was assessed with the Ramsay 6-point scale. The data from a short-term sedation study in 24 patients after cardiac surgery were included. Population pharmacokinetic and pharmacodynamic modeling was performed with NONMEM. RESULTS: The pharmacokinetics was adequately described by a 2-compartment model. The propofol formulation was not a significant covariate for the pharmacokinetics, whereas serum triglyceride concentration (TG) and relative body temperature (T(c)) were significant covariates for elimination clearance (CL). The population pharmacokinetic parameters were as follows: CL = 2.2 + 0.27 x T(c) - 0.049 x TG (mean, 2.1 L/min); volume of central compartment, 22.2 L; CL (distribution), 1.5 L/min; and volume of peripheral compartment, 168 L. The addition of other time-independent covariates (long-term versus short-term sedation study, as well as physiologic characteristics) or time-dependent covariates (duration of propofol infusion, additional midazolam rates, and hemodynamic parameters) to the model did not improve the quality of fit. For the pharmacodynamics, the probability that the sedation level was equal to, or more than, a specific score was described with the use of a sigmoid inverse logit of the maximal achievable probability model. The values for the inverse logit of the concentration causing half of the maximal effect for Ramsay sedation scores of 2 through 6 were 0.13 +/- 0.09, 0.31 +/- 0.17, 0.56 +/- 0.24, 0.79 +/- 0.31, and 1.78 +/- 0.65 mg/L, respectively (population mean +/- SE). Interindividual variability was high, with a coefficient of variation of 119% in the 50% effective concentration values. No covariates were identified. CONCLUSIONS: The population models in critically ill patients showed no differences in pharmacokinetics or pharmacodynamics between propofol 6% and propofol 1%. TG and T(c) appeared to be significant covariates for elimination clearance. For the pharmacodynamics, when propofol concentrations were between 0.75 and 1.5 mg/L, Ramsay sedation score 6 was most probable (40%-75%) and the probability for Ramsay sedation score 5 was 20% to 40%. Large pharmacodynamic variabilities were observed.

Pharmacokinetic-pharmacodynamic modeling of buspirone and its metabolite 1-(2-pyrimidinyl)-piperazine in rats.

The objective of this investigation was to compare the in vivo potency and intrinsic activity of buspirone and its metabolite 1-(2-pyrimidinyl)-piperazine (1-PP) in rats by pharmacokinetic-pharmacodynamic modeling. Following intravenous administration of buspirone (5 or 15 mg/kg in 15 min) or 1-PP (10 mg/kg in 15 min), the time course of the concentrations in blood were determined in conjunction with the effect on body temperature. The pharmacokinetics of buspirone and 1-PP were analyzed based on a two-compartment model with metabolite formation. Differences in the pharmacokinetics of buspirone and 1-PP were observed with values for clearance of 13.1 and 8.2 ml/min and for terminal elimination half-life of 25 and 79 min, respectively. At least 26% of the administered dose of buspirone was converted into 1-PP. Complex hypothermic effects versus time profiles were observed, which were successfully analyzed on the basis of a physiological indirect response model with set-point control. Both buspirone and 1-PP behaved as partial agonists relative to R-(+)-8-hydroxy-2-(di-n-propylamino)tetralin (R-8-OH-DPAT) with values of the intrinsic activity of 0.465 and 0.312, respectively. Differences in the potency were observed with values of 17.6 and 304 ng/ml for buspirone and 1-PP, respectively. The results of this analysis show that buspirone and 1-PP behave as partial 5-hydroxytryptamine(1A) agonists in vivo and that following intravenous administration the amount of 1-PP formed is too small to contribute to the hypothermic effect.

Pharmacokinetics and effects of propofol 6% for short-term sedation in paediatric patients following cardiac surgery.

AIMS: This paper describes the pharmacokinetics and effects of propofol in short-term sedated paediatric patients. METHODS: Six mechanically ventilated children aged 1-5 years received a 6 h continuous infusion of propofol 6% at the rate of 2 or 3 mg kg-1 h-1 for sedation following cardiac surgery. A total of seven arterial blood samples was collected at various time points during and after the infusion in each patient. Pharmacokinetic modelling was performed using NONMEM. Effects were assessed on the basis of the Ramsay sedation score as well as a subjective sedation scale. RESULTS: The data were best described by a two-compartment pharmacokinetic model. In the model, body weight was a significant covariate for clearance. Pharmacokinetic parameters in the weight-proportional model were clearance (CL) = 35 ml kg-1 min-1, volume of central compartment (V1) = 12 l, intercompartmental clearance (Q) = 0.35 l min-1 and volume of peripheral compartment (V2) = 24 l. The interindividual variabilities for these parameters were 8%, < 1%, 11% and 35%, respectively. Compared with the population pharmacokinetics in adults following cardiac surgery and when normalized for body weight, statistically significant differences were observed the parameters CL and V1 (35 vs 29 ml kg-1 min-1 and 0.78 vs 0.26 l kg-1P < 0.05), whereas the values for Q and V2 were similar (23 vs 18 ml kg-1 min-1 and 1.6 vs 1.8 l kg-1, P > 0.05). In children, the percentage of adequately sedated patients was similar compared with adults (50% vs 67%) despite considerably higher propofol concentrations (1.3 +/- 0.10 vs 0.51 +/- 0.035 mg l-1, mean +/- s.e. mean), suggesting a lower pharmacodynamic sensitivity to propofol in children. CONCLUSIONS: In children aged 1-5 years, a pharmacokinetic model for propofol was described using sparse data. In contrast to adults, body weight was a significant covariate for clearance in children. The model may serve as a useful basis to study the role of covariates in the pharmacokinetics and pharmacodynamics of propofol in paediatric patients of different ages.

Pharmacokinetic-pharmacodynamic modelling of the hypothermic and corticosterone effects of the 5-HT1A receptor agonist flesinoxan.

The current investigation describes the pharmacokinetic-pharmacodynamic correlation of the hypothermic and the corticosterone effect of flesinoxan in the rat simultaneously. A specific objective was to determine the influence of handling the animal. The pharmacokinetic-pharmacodynamic correlation was determined following intravenous administration of 3 and 10 mg/kg flesinoxan in 5 or 15 min. Serial blood samples were obtained for determination of the time course of the flesinoxan and corticosterone concentrations by high performance liquid chromatography. Body temperature was monitored using a telemetric probe. The pharmacokinetics of flesinoxan were described using a three-compartment model. Both the hypothermic and the corticosterone response were successfully described using a physiological indirect response model. It is shown that customizing the animal prior to the experiment has no influence on the pharmacokinetic-pharmacodynamic parameter estimates. Furthermore, the similarity in potency between the hypothermic and corticosterone effects suggests that both are mediated via tissues with a similar receptor-effector coupling efficiency.

A competitive interaction model predicts the effect of WAY-100,635 on the time course of R-(+)-8-hydroxy-2-(di-n-propylamino)tetralin-induced hypothermia.

The objective of this investigation was to characterize quantitatively the pharmacodynamic interaction between N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-cyclohexanecarboxamide (WAY-100,635) and R-(+)-8-hydroxy-2-(di-n-propylamino)tetralin (R-8-OH-DPAT) in vivo. The 8-OH-DPAT-induced change in body temperature was used as a pharmacodynamic endpoint. Four groups of rats each received 1 mg/kg 8-OH-DPAT in 5 min during computer-controlled infusions of physiological saline or WAY-100,635, targeted at steady-state concentrations of 20, 85, and 170 ng/ml. Body temperature was monitored continuously with a telemetric system, and frequent blood samples were obtained to determine the pharmacokinetics of both drugs. Large differences in pharmacokinetics were observed between WAY-100,635 and R-8-OH-DPAT, reflected in values of the terminal elimination half-life of 33 and 143 min, respectively. Infusion of WAY-100,635 had no influence on the pharmacokinetics of R-8-OH-DPAT. With regard to the pharmacodynamics, clear antagonism of the R-8-OH-DPAT-induced hypothermia was observed. The complex pharmacological effect versus time profiles of R-8-OH-DPAT were analyzed on the basis of an indirect physiological response model with set point control coupled to a competitive interaction model for an agonist and antagonist acting at a common receptor. This model converged, yielding precise estimates of the pharmacodynamic parameters of both WAY-100,635 and R-8-OH-DPAT, which were independent of the infusion rate of WAY-100,635. The estimated in vivo binding constant of WAY-100,635 was 0.98 ng/ml (2.3 nM), which is very similar to the reported value from in vitro receptor binding assays. The findings of this investigation show that, in contrast to earlier reports in the literature, WAY 100,635 behaves as a pure competitive antagonist at the 5-hydroxytryptamine(1A) receptor in vivo.