Prediction of changes in the clinical pharmacokinetics of basic drugs on the basis of octanol-water partition coefficients

Development and Evaluation of a Physiologically Based Pharmacokinetic Model for Predicting Haloperidol Exposure in Healthy and Disease Populations

The physiologically based pharmacokinetic (PBPK) approach can be used to develop mathematical models for predicting the absorption, distribution, metabolism, and elimination (ADME) of administered drugs in virtual human populations. Haloperidol is a typical antipsychotic drug with a narrow therapeutic index and is commonly used in the management of several medical conditions, including psychotic disorders. Due to the large interindividual variability among patients taking haloperidol, it is very likely for them to experience either toxic or subtherapeutic effects. We intend to develop a haloperidol PBPK model for identifying the potential sources of pharmacokinetic (PK) variability after intravenous and oral administration by using the population-based simulator, PK-Sim. The model was initially developed and evaluated to predict the PK of haloperidol and its reduced metabolite in adult healthy population after intravenous and oral administration. After evaluating the developed PBPK model in healthy adults, it was used to predict haloperidol–rifampicin drug–drug interaction and was extended to tuberculosis patients. The model evaluation was performed using visual assessments, prediction error, and mean fold error of the ratio of the observed-to-predicted values of the PK parameters. The predicted PK values were in good agreement with the corresponding reported values. The effects of the pathophysiological changes and enzyme induction associated with tuberculosis and its treatment, respectively, on haloperidol PK, have been predicted precisely. For all clinical scenarios that were evaluated, the predicted values were within the acceptable two-fold error range.

Whole blood or plasma: what is the ideal matrix for pharmacokinetic-driven drug candidate selection?

In the present era of drug development, quantification of drug concentrations following pharmacokinetic studies has preferentially been performed using plasma as a matrix rather than whole blood. However, it is critical to realize the difference between measuring drug concentrations in blood versus plasma and the consequences thereof. Pharmacokinetics using plasma data may be misleading if concentrations differ between plasma and red blood cells (RBCs) because of differential binding in blood. In this review, factors modulating the partitioning of drugs into RBCs are discussed and the importance of determining RBC uptake of drugs for drug candidate selection is explored. In summary, the choice of matrix (plasma vs whole blood) is an important consideration to be factored in during drug discovery.

Intravenous lipid emulsion as antidote beyond local anesthetic toxicity: a systematic review

OBJECTIVES

The objective was to asses the efficacy of lipid emulsion as antidotal therapy outside the accepted setting of local anesthetic toxicity.

METHODS

Literature was accessed through PubMed, OVID (1966-February 2009), and EMBASE (1947-February 2009) using the search terms "intravenous" AND ["fat emulsion" OR "lipid emulsion" OR "Intralipid"] AND ["toxicity" OR "resuscitation" OR "rescue" OR "arrest" OR "antidote"]. Additional author and conference publication searches were undertaken. Publications describing the use of lipid emulsion as antidotal treatment in animals or humans were included.

RESULTS

Fourteen animal studies, one human study, and four case reports were identified. In animal models, intravenous lipid emulsion (ILE) has resulted in amelioration of toxicity associated with cyclic antidepressants, verapamil, propranolol, and thiopentone. Administration in human cases has resulted in successful resuscitation from combined bupropion/lamotrigine-induced cardiac arrest, reversal of sertraline/quetiapine-induced coma, and amelioration of verapamil- and beta blocker-induced shock.

CONCLUSIONS

Management of overdose with highly lipophilic cardiotoxic medications should proceed in accord with established antidotal guidelines and early poisons center consultation. Data from animal experiments and human cases are limited, but suggestive that ILE may be helpful in potentially lethal cardiotoxicity or developed cardiac arrest attributable to such agents. Use of lipid emulsion as antidote remains a nascent field warranting further preclinical study and systematic reporting of human cases of use.

Intralipid infusion ameliorates propranolol-induced hypotension in rabbits

OBJECTIVE

Recent case reports of successful amelioration of lipid-soluble drug toxidromes with Intralipid infusion have prompted interest in the scope of lipid emulsions as antidotal therapy. Propranolol is a highly lipid-soluble, nonselective beta-blocker with additional local-anaesthetic properties. We explored the hypothesis that propranolol toxicity may be similarly attenuated by Intralipid infusion in a rabbit model.

METHODS

Twenty sedated, invasively monitored, and mechanically ventilated New Zealand White rabbits underwent propranolol infusion at 4.2 mg/min to a target mean arterial pressure (MAP) of 60% baseline MAP. Animals subsequently received 6 ml/kg 20% Intralipid, or 6 ml/kg 0.9% saline solution over a 4-minute period. Pulse rate and MAP were recorded at 2.5-minute intervals to 15 minutes.

RESULTS

MAP was greatest in the Intralipid group (median 69 mmHg, interquartile range [IQR] 17.5 mmHg Intralipid vs. median 53 mmHg, IQR 12.75 mmHg saline; p=0.029) at 15 minutes. No difference was observed in first derivative of MAP, or pulse rate between groups.

CONCLUSIONS

Propranolol-induced hypotension is ameliorated by Intralipid infusion in this intact rabbit model. The mechanism of action remains to be elucidated.

Prediction of Drug Tissue to Plasma Concentration Ratios Using a Measured Volume of Distribution in Combination With Lipophilicity

One of the drug specific parameters needed in physiologically based pharmacokinetic (PBPK) models is the tissue to plasma drug concentration ratios (K(p) values). The aim of this study was to develop an empirical method for predicting K(p) values using a preclinically determined in vivo volume of distribution, in combination with descriptors for drug lipophilicity. Pharmacokinetic data in laboratory animals for a wide range of drug compounds were collected. Obtained correlations between K(p) values for muscle and other tissues, in a training set of 49 compounds, were used to predict K(p) values for a test set of 22 compounds, based on their volume of distribution and lipophilicity. Predicted K(p) values agreed well with experimentally determined values (n = 118), especially for noneliminating tissues (r(2) = 0.81) with 72% and 87% being within a factor +/-2 and +/-3, respectively. In conclusion, we present an empirical method based on a measured volume of distribution and a drug lipophilicity descriptor, which can be used to predict tissue K(p) values with reasonable accuracy.

Human physiologically based pharmacokinetic model for propofol

Background

Propofol is widely used for both short-term anesthesia and long-term sedation. It has unusual pharmacokinetics because of its high lipid solubility. The standard approach to describing the pharmacokinetics is by a multi-compartmental model. This paper presents the first detailed human physiologically based pharmacokinetic (PBPK) model for propofol.

Methods

PKQuest, a freely distributed software routine , was used for all the calculations. The "standard human" PBPK parameters developed in previous applications is used. It is assumed that the blood and tissue binding is determined by simple partition into the tissue lipid, which is characterized by two previously determined set of parameters: 1) the value of the propofol oil/water partition coefficient; 2) the lipid fraction in the blood and tissues. The model was fit to the individual experimental data of Schnider et. al., Anesthesiology, 1998; 88:1170 in which an initial bolus dose was followed 60 minutes later by a one hour constant infusion.

Results

The PBPK model provides a good description of the experimental data over a large range of input dosage, subject age and fat fraction. Only one adjustable parameter (the liver clearance) is required to describe the constant infusion phase for each individual subject. In order to fit the bolus injection phase, for 10 or the 24 subjects it was necessary to assume that a fraction of the bolus dose was sequestered and then slowly released from the lungs (characterized by two additional parameters). The average weighted residual error (WRE) of the PBPK model fit to the both the bolus and infusion phases was 15%; similar to the WRE for just the constant infusion phase obtained by Schnider et. al. using a 6-parameter NONMEM compartmental model.

Conclusion

A PBPK model using standard human parameters and a simple description of tissue binding provides a good description of human propofol kinetics. The major advantage of a PBPK model is that it can be used to predict the changes in kinetics produced by variations in physiological parameters. As one example, the model simulation of the changes in pharmacokinetics for morbidly obese subjects is discussed.

Whole body pharmacokinetic models

The aim of the current review is to summarise the present status of physiologically based pharmacokinetic (PBPK) modelling and its applications in drug research, and thus serve as a reference point to people interested in the methodology. The review is structured into three major sections. The first discusses the existing methodologies and techniques of PBPK model development. The second describes some of the most interesting PBPK model implementations published. The final section is devoted to a discussion of the current limitations and the possible future developments of the PBPK modelling approach. The current review is focused on papers dealing with the pharmacokinetics and/or toxicokinetics of medicinal compounds; references discussing PBPK models of environmental compounds are mentioned only if they represent considerable methodological developments or reveal interesting interpretations and/or applications.The major conclusion of the review is that, despite its significant potential, PBPK modelling has not seen the development and implementation it deserves, especially in the drug discovery, research and development processes. The main reason for this is that the successful development and implementation of a PBPK model is seen to require the investment of significant experience, effort, time and resources. Yet, a substantial body of PBPK-related research has been accumulated that can facilitate the PBPK modelling and implementation process. What is probably lagging behind is the expertise component, where the demand for appropriately qualified staff far outreaches availability.

Influence of synaptic serotonin level on [18F]altanserin binding to 5HT2 receptors in man

The feasibility of in vivo serotonin 5HT(2) receptor binding measurement using [18F]altanserin as a radioligand has been well established. In this study, the postsynaptic receptor binding potential of this ligand was examined as a possible indicator of synaptic serotonin content after pharmacological challenge. Studies were performed in 11 subjects with a history of recurrent major depression. Six of them received serotonergic antidepressive treatment at the time of the experiment, the other five patients were untreated. Two PET measurements were carried out in each subject within 2 or 3 days. Before one of the measurements, 25 mg of the serotonin re-uptake inhibitor clomipramine were given intravenously, the other measurement was done without pharmacological challenge. The data were analyzed using non-linear least-square regression and Logan's graphical method. In the whole group of subjects, binding potential and distribution volume of altanserin decreased following clomipramine challenge. The decrease was between 14 (P=0.03) and 23% (P=0.004). This effect was mainly seen in subjects not on antidepressive medication. Clomipramine challenge probably increased the synaptic serotonin level, which competed with altanserin leading to the lowered binding potential. The paradigm might, thus, be useful to estimate serotonin release in vivo. Pretreatment with serotonergic antidepressants reduces the effect of clomipramine.

Effect of meropenem on disposition kinetics of valproate and its metabolites in rabbits

PURPOSE

We investigated the effect of meropenem (MEPM) on the disposition kinetics of valproate (VPA) and its metabolites in rabbits.

METHODS

Rabbits were given 75 mg/kg VPA intravenously with or without 300 mg/kg MEPM.

RESULTS

The plamsa total clearance of VPA was significantly increased to about 1.5 times the control (6.09 mL/min/kg vs. 4.28 mL/min/kg) by MEPM (P < 0.05). The values of the area under the plasma concentration-time curve (AUC) of 2-en-VPA, a product of beta-oxidation, and VPA-glucuronide (VPA-G) were significantly decreased to about 55% and 78% of the control, respectively (P < 0.05). The cumulative urinary excretions of VPA in the control and MEPM-treated groups were 0.54% and 0.62% of the dose, respectively, whereas those of VPA-G were 45.6% and 62.5%, respectively. The urinary excretion of VPA-G was significantly increased by MEPM (P < 0.05). Further, in the case of 33.8 mg/kg VPA-G administered intravenously the AUC value of VPA-G was unchanged by MEPM, whereas that of the generated VPA was significantly decreased to about half of the control.

CONCLUSIONS

The increase of the total clearance of VPA caused by MEPM appears to be a consequence of increased renal clearance of VPA-G, as well as suppression of VPA-G hydrolysis in the liver.

[Studies on the mechanism of subcellular distribution of basic drugs based on their lipophilicity]

This paper described the studies on the mechanism of subcellular distribution of lipophilic weak bases. Although the tissue distribution of basic drugs appeared to decrease with time simply in parallel with their plasma concentration, their subcellular distribution in various tissues exhibited a variety of patterns. Basic drugs were distributed widely in various tissues, but were concentrated in lung granule fraction, where their accumulation was dependent on their lipophilicity and lysosomal uptake. As the plasma concentration of drugs decreased after maximum level, the contribution of lysosomes to their subcellular distribution increased. The uptake of the basic drugs into lysosomes depended both on their intralysosomal pH and on the drug lipophilicity. As the lipophilicity of the basic drugs increased, they accumulated more than the values predicted from the pH-partition theory and raised the intralysosomal pH more potently, probably owing to their binding with lysosomal membranes with or without additional intralysosomal aggregation. These phenomena should be considered as a basis of drug interaction in clinical treatments.

Quantitative structure-pharmacokinetics relationships: II. A mechanistically based model to evaluate the relationship between tissue distribution parameters and compound lipophilicity

The tissue-to-unbound plasma distribution coefficients (Kpus) of 14 rat tissues after i.v. administration of nine 5-n-alkyl-5-ethyl barbituric acids, determined in a previous study, were used to identify a model of the relationship between tissue distribution and lipophilicity of the homologs, expressed in terms of their octanol to water partition ratio, P. Based on mechanistic considerations and assumptions, the parameter model was expressed as Kpu tau = fw.tau [1 + a tau (nPt.tau)Pb tau], where fw.tau is the tissue water content. (nPt. tau) is the binding capacity of the tissue, n is the number of the binding sites, a tau and b tau are the parameters of the relationship Ka tau = a tau Pb tau; and Ka tau is the binding association constant of each tissue. The parameter model was linearized and fitted to the predetermined Kpu values, yielding correlation coefficients ranging between .940 and .997. The predictive performance of the parameter model was evaluated using a leave-one-out procedure with subsequent computation of the mean prediction error (ME = measurement of the prediction bias) and the square root of the mean squared prediction error (RMSE = measurement of the prediction accuracy). The ME varied between -22.48 and 61.14%, indicating a slight tendency for overpredicting. The RMSE was between 24.73 and 102% for the individual tissues across the different homologs; and between 28.33 and 85.2% for the individual homologs across the different tissues. The apparently high Kpu prediction errors, when translated through the low sensitivity of the barbiturate whole-body physiologically based pharmacokinetic model, established previously, leads to predicted tissue concentration-time profiles within 5 to 20% of the original ones. Therefore, it is concluded, that the identified mechanistically based model is a good predictor of the tissue-to-unbound Kpus in the rat tissues.