From piecewise to full physiologic pharmacokinetic modeling: applied to thiopental disposition in the rat

Utility of Minimal Physiologically Based Pharmacokinetic Models for Assessing Fractional Distribution, Oral Absorption, and Series-Compartment Models of Hepatic Clearance

Minimal physiologically based pharmacokinetic (mPBPK) models are physiologically relevant, require less information than full PBPK models, and offer flexibility in pharmacokinetics (PK). The well-stirred hepatic model (WSM) is commonly used in PBPK, whereas the more plausible dispersion model (DM) poses computational complexities. The series-compartment model (SCM) mimics the DM but is easier to operate. This work implements the SCM and mPBPK models for assessing fractional tissue distribution, oral absorption, and hepatic clearance using literature-reported blood and liver concentration-time data in rats for compounds mainly cleared by the liver. Further handled were various complexities, including nonlinear hepatic binding and metabolism, differing absorption kinetics, and sites of administration. The SCM containing one to five (n) liver subcompartments yields similar fittings and provides comparable estimates for hepatic extraction ratio (ER), prehepatic availability (Fg ), and first-order absorption rate constants (ka ). However, they produce decreased intrinsic clearances (CLint ) and liver-to-plasma partition coefficients (Kph ) with increasing n as expected. Model simulations demonstrated changes in intravenous and oral PK profiles with alterations in Kph and ka and with hepatic metabolic zonation. The permeability (PAMPA P) of the various compounds well explained the fitted fractional distribution (fd ) parameters. The SCM and mPBPK models offer advantages in distinguishing systemic, extrahepatic, and hepatic clearances. The SCM allows for incorporation of liver zonation and is useful in assessing changes in internal concentration gradients potentially masked by similar blood PK profiles. Improved assessment of intraorgan drug concentrations may offer insights into active moieties driving metabolism, biliary excretion, pharmacodynamics, and hepatic toxicity. SIGNIFICANCE STATEMENT: The minimal physiologically based pharmacokinetic model and the series-compartment model are useful in assessing oral absorption and hepatic clearance. They add flexibility in accounting for various drug- or system-specific complexities, including fractional distribution, nonlinear binding and saturable hepatic metabolism, and hepatic zonation. These models can offer improved insights into the intraorgan concentrations that reflect physiologically active moieties often driving disposition, pharmacodynamics, and toxicity.

Exploring the Pharmacokinetic Mysteries of the Liver: Application of Series Compartment Models of Hepatic Elimination

Among the basic hepatic clearance models, the dispersion model (DM) is the most physiologically sound compared with the well-stirred model and the parallel tube model. However, its application in physiologically-based pharmacokinetic (PBPK) modeling has been limited due to computational complexities. The series compartment models (SCM) of hepatic elimination that treats the liver as a cascade of well-stirred compartments connected by hepatic blood flow exhibits some mathematical similarities to the DM but is easier to operate. This work assesses the quantitative correlation between the SCM and DM and demonstrates the operation of the SCM in PBPK with the published single-dose blood and liver concentration-time data of six flow-limited compounds. The predicted liver concentrations and the estimated intrinsic clearance (CLint ) and PBPK-operative tissue-to-plasma partition coefficient (Kp ) values were shown to depend on the number of liver sub-compartments (n) and hepatic enzyme zonation in the SCM. The CLint and Kp decreased with increasing n, with more remarkable differences for drugs with higher hepatic extraction ratios. Given the same total CLint , the SCM yields a higher Kp when the liver perivenous region exhibits a lower CLint as compared with a high CLint at this region. Overall, the SCM nicely approximates the DM in characterizing hepatic elimination and offers an alternative flexible approach as well as providing some insights regarding sequential drug concentrations in the liver. SIGNIFICANCE STATEMENT: The SCM nicely approximates the DM when applied in PBPK for characterizing hepatic elimination. The number of liver sub-compartments and hepatic enzyme zonation are influencing factors for the SCM resulting in model-dependent predictions of total/internal liver concentrations and estimates of CLint and the PBPK-operative Kp . Such model-dependency may have an impact when the SCM is used for in vitro-to-in vivo extrapolation (IVIVE) and may also be relevant for PK/PD/toxicological effects when it is the driving force for such responses.

Development of Physiologically Based Organ Models to Evaluate the Pharmacokinetics of Drugs in the Testes and the Thyroid Gland

We extended a generic whole-body physiologically based pharmacokinetic (PBPK) model for rats and humans for organs of the reproductive and endocrine systems (i.e., the testes and the thyroid gland). An extensive literature search was performed, first, to determine the most generic organ model structures for testes and thyroid across species, and, second, to identify the corresponding anatomic and physiological parameters in rats and humans. The testes and thyroid organ models were implemented in the PBPK modeling software PK-Sim and MoBi. The capability of the PBPK approach to simulate the testes and thyroid tissue concentration data was demonstrated using a series of test compounds. The presented organ model structures and parameterization yielded a close agreement between observed and simulated tissue concentrations over time. The organ models are ready to be used to predict the pharmacokinetics of passively entering drugs in the testes and thyroid tissue in a generic PBPK modeling framework.

Deoxycholic Acid as a Modifier of the Permeation of Gliclazide through the Blood Brain Barrier of a Rat

Major problem for diabetic patients represents damage of blood vessels and the oxidative stress of the brain cells due to increased concentration of free radicals and poor nutrition of brain cells. Gliclazide has antioxidative properties and poor blood brain barrier (BBB) penetration. Bile acids are known for their hypoglycemic effect and as promoters of drug penetration across biological membranes. Accordingly, the aim of this study is to investigate whether the bile acid (deoxycholic acid) can change the permeation of gliclazide, through the blood brain barrier of a rat model type-1 diabetes. Twenty-four male Wistar rats were randomly allocated to four groups, of which, two were given alloxan intraperitoneally (100 mg/kg) to induce diabetes. One diabetic group and one healthy group were given a bolus gliclazide intra-arterially (20 mg/kg), while the other two groups apart from gliclazide got deoxycholic acid (4 mg/kg) subcutaneously. Blood samples were collected 30, 60, 150, and 240 seconds after dose, brain tissues were immediately excised and blood glucose and gliclazide concentrations were measured. Penetration of gliclazide in groups without deoxycholic acid pretreatment was increased in diabetic animals compared to healthy animals. Also in both, the healthy and diabetic animals, deoxycholic acid increased the permeation of gliclazide through that in BBB.

Physiologically based pharmacokinetics of molecular imaging nanoparticles for mRNA detection determined in tumor-bearing mice

Disease detection and management might benefit from external imaging of disease gene mRNAs. Previously we designed molecular imaging nanoparticles (MINs) based on peptide nucleic acids complementary to cancer gene mRNAs. The MINs included contrast agents and analogs of insulin-like growth factor 1 (IGF-1). Analysis of MIN tumor uptake data showed stronger binding in tumors than in surrounding tissues. We hypothesized that MINs with an IGF-1 analog stay in circulation by binding to IGF-binding proteins. To test that hypothesis, we fit the tissue distribution results of several MINs in xenograft-bearing mice to a physiological pharmacokinetics model. Fitting experimental tissue distribution data to model-predicted mass transfer of MINs from blood into organs and tumors converged only when the parameter for MINs bound to circulating IGF-binding proteins was set to 10%-20% of the injected MIN dose. This result suggests that previous mouse imaging trials used more MINs than necessary. This prediction can be tested by a ramp of decreasing doses.

Quantitative relationship between the octanol/water partition coefficient and the diffusion limitation of the exchange between adipose and blood

Background

The goal of physiologically based pharmacokinetics (PBPK) is to predict drug kinetics from an understanding of the organ/blood exchange. The standard approach is to assume that the organ is "flow limited" which means that the venous blood leaving the organ equilibrates with the well-stirred tissue compartment. Although this assumption is valid for most solutes, it has been shown to be incorrect for several very highly fat soluble compounds which appear to be "diffusion limited". This paper describes the physical basis of this adipose diffusion limitation and its quantitative dependence on the blood/water (K_bld-wat) and octanol/water (K_ow) partition coefficient.

Methods

Experimental measurements of the time dependent rat blood and adipose concentration following either intravenous or oral input were used to estimate the "apparent" adipose perfusion rate (F_A) assuming that the tissue is flow limited. It is shown that the ratio of F_A to the anatomic perfusion rate (F) provides a measure of the diffusion limitation. A quantitative relationship between this diffusion limitation and K_bld-wat and K_ow is derived. This analysis was applied to previously published data, including the Oberg et. al. measurements of the rat plasma and adipose tissue concentration following an oral dose of a mixture of 13 different polychlorinated biphenyls.

Results

Solutes become diffusion limited at values of log K_ow greater than about 5.6, with the adipose-blood exchange rate reduced by a factor of about 30 for a solute with a log K_ow of 7.36. Quantitatively, a plot of F_A/F versus K_ow is well described assuming an adipose permeability-surface area product (PS) of 750/min. This PS corresponds to a 0.14 micron aqueous layer separating the well-stirred blood from the adipose lipid. This is approximately equal to the thickness of the rat adipose capillary endothelium.

Conclusions

These results can be used to quantitate the adipose-blood diffusion limitation as a function of K_ow. This is especially important for the highly fat soluble persistent organic chemicals (e.g. polychlorinated biphenyls, dioxins) whose pharmacokinetics are primarily determined by the adipose-blood exchange kinetics.

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.

A minimal physiological model of thiopental distribution kinetics based on a multiple indicator approach

Currently available models of thiopental disposition kinetics using only plasma concentration-time data neglect the influence of intratissue diffusion and provide no direct information on tissue partitioning in individual subjects. Our approach was based on a lumped-organ recirculatory model that has recently been applied to unbound compounds. The goal was to find the simplest model that accounts for the heterogeneity in tissue partition coefficients and accurately describes initial distribution kinetics of thiopental in dogs. To ensure identifiability of the underlying axially distributed capillary-tissue exchange model, simultaneously measured disposition data of the vascular indicator, indocyanine green, and the marker of whole body water, antipyrine, were analyzed together with those of thiopental. A model obtained by grouping the systemic organs in two subsystems containing fat and nonfat tissues, successfully described all data and allowed an accurate estimation of model parameters. The estimated tissue partition coefficients were in accordance with those measured in rats. Because of the effect of tissue binding, the diffusional equilibration time characterizing intratissue distribution of thiopental is longer than that of antipyrine. The approach could potentially be used in clinical pharmacokinetics and could increase our understanding of the effect of obesity on the disposition kinetics of lipid-soluble drugs.

Blood-brain barrier penetration of zolmitriptan--modelling of positron emission tomography data

Positron emission tomography (PET) with the drug radiolabelled allows a direct measurement of brain or other organ kinetics, information which can be essential in drug development. Usually, however, a PET-tracer is administered intravenously (i.v.), whereas the therapeutic drug is mostly given orally or by a different route to the PET-tracer. In such cases, a recalculation is needed to make the PET data representative for the alternative administration route. To investigate the blood-brain barrier penetration of a drug (zolmitriptan) using dynamic PET and by PK modelling quantify the brain concentration of the drug after the nasal administration of a therapeutic dose. [11C]Zolmitriptan at tracer dose was administered as a short i.v. infusion and the brain tissue and venous blood kinetics of [11C]zolmitriptan was measured by PET in 7 healthy volunteers. One PET study was performed before and one 30 min after the administration of 5 mg zolmitriptan as nasal spray. At each of the instances, the brain radioactivity concentration after subtraction of the vascular component was determined up to 90 min after administration and compared to venous plasma radioactivity concentration after correction for radiolabelled metabolites. Convolution methods were used to describe the relationship between arterial and venous tracer concentrations, respectively between brain and arterial tracer concentration. Finally, the impulse response functions derived from the PET studies were applied on plasma PK data to estimate the brain zolmitriptan concentration after a nasal administration of a therapeutic dose. The studies shows that the PET data on brain kinetics could well be described as the convolution of venous tracer kinetics with an impulse response including terms for arterial-to-venous plasma and arterial-to-brain impulse responses. Application of the PET derived impulse responses on the plasma PK from nasal administration demonstrated that brain PK of zolmitriptan increased with time, achieving about 0.5 mg/ml at 30 min and close to a maximum of 1.5 mg/ml after 2 hr. A significant brain concentration was observed already after 5 min. The data support the notation of a rapid brain availability of zolmitriptan after nasal administration.

Physiologically based pharmacokinetic modelling 2: Predicting the tissue distribution of acids, very weak bases, neutrals and zwitterions

A key component of whole body physiologically based pharmacokinetic (WBPBPK) models is the tissue-to-plasma water partition coefficients (Kpu's). The predictability of Kpu values using mechanistically derived equations has been investigated for 7 very weak bases, 20 acids, 4 neutral drugs and 8 zwitterions in rat adipose, bone, brain, gut, heart, kidney, liver, lung, muscle, pancreas, skin, spleen and thymus. These equations incorporate expressions for dissolution in tissue water and, partitioning into neutral lipids and neutral phospholipids. Additionally, associations with acidic phospholipids were incorporated for zwitterions with a highly basic functionality, or extracellular proteins for the other compound classes. The affinity for these cellular constituents was determined from blood cell data or plasma protein binding, respectively. These equations assume drugs are passively distributed and that processes are nonsaturating. Resultant Kpu predictions were more accurate when compared to published equations, with 84% as opposed to 61% of the predicted values agreeing with experimental values to within a factor of 3. This improvement was largely due to the incorporation of distribution processes related to drug ionisation, an issue that is not addressed in earlier equations. Such advancements in parameter prediction will assist WBPBPK modelling, where time, cost and labour requirements greatly deter its application.

An evaluation of the utility of physiologically based models of pharmacokinetics in early drug discovery

Generic physiologically-based models of pharmacokinetics were evaluated for early drug discovery. Plasma profiles after intravenous and oral dosing were simulated in rat for 68 compounds from six chemical classes. Input data consisted of structure based predictions of lipophilicity, ionization, and protein binding plus intrinsic clearance measured in rat hepatocytes, single measured values of aqueous solubility, and artificial membrane permeability. LogP of compounds was high with a mean of 3.9 while free fraction in plasma (mean 9%) and solubility (mean 37 microg/mL) were low. Predicted and observed clearance and volume showed mean fold-error and R2 of 1.8, 0.56, and 1.9, 0.25 respectively. Predicted bioavailability showed strong bias to under prediction correlated to very low aqueous solubility and a theoretical correction for bile salt solubilization in vivo brought some improvement in average prediction error (to 31%). Overall, this evaluation shows that generic simulation may be applicable for typical drug-like compounds to predict differences in pharmacokinetic parameters of more than twofold based upon minimal measured input data. However verification of the simulations with in vivo data for a few compounds of each compound class is recommended since recent discovery compounds may have properties beyond the scope of the current generic models.

Analyzing multi-response data using forcing functions

INTRODUCTION

Two analytic strategies can be taken to the analysis of multi-response data: a multivariate output model can be fit to all the response components simultaneously (SIM), or each response component can be fit separately to a univariate output model, conditioning in some way on the non-modeled components, the so-called forcing function approach (FFA). Focusing on a special case of multi-response model corresponding to a (pharmacokinetic) physiological f low model (PFM), the aims of this study are to (i) provide an algorithm for applying FFA to multi-response data from a PFM; (ii) examine the performance of FFA vs. SIM under optimal conditions for both, and in the presence of model misspecification; (iii) make recommendations regarding the use of FFA for multi-response data analysis.

METHODS

The basic PFM we use (variants of the basic model are used for simulation) has four homogenous compartments among which drug distributes. All are sampled arterial blood (A), non-eliminating tissue (N), eliminating tissue (E), and venous blood (V), which is also the drug dosing site. Parameters are blood f low rates to E and N, volumes of distribution of A, E, N, and V, elimination rate constant from E, and observation error variances. Observations from a generic individual under various study designs and parameter values are simulated. Using data-analytic models (DAM) both the same as, and different than the data simulation model (DSM), SIM fits the PFM to all data simultaneously; FFA first fits each type of response (one per tissue) separately, approximating the tissue's input by linearly interpolating the observed concentrations from the donor tissue(s), estimates the identifiable parameter combinations for the response type, and then solves the simultaneous equations linking these across tissues, to obtain the primary model parameters of interest. This simulation and analysis steps are repeated to generate reliable performance statistics. Performances are compared with respect to parameter estimation error (when DAM and DSM are identical), and interpolated prediction error (when DAM and DSM are/are-not identical). The ability of SIM and FFA to identify the correct analytic model is also examined by comparing their failure rates in rejecting the wrong DAM.

RESULTS

The parameter estimation errors with FFA are generally about two times greater than those with SIM when the DAM is identical to the DSM. The prediction errors of FFA are about ten times greater than those of SIM when the DAM is identical to the DSM, and are about three times greater when the two are different. However, SIM fails to identify the correct model twice as often as FFA.

CONCLUSIONS

Despite its greater convenience for model building, and its clear advantages for model identification, FFA's final parameter estimates cannot be trusted when the multi-response system being modeled involves feedback. The size of the ratio of the two FFA residuals (obtained from the response-specific fits and from predictions made with the final FFA parameters) can, however, be used to indicate when FFA's final estimates may be trustworthy.

A physiologically-based pharmacokinetic model of drug detoxification by nanoparticles

Nanoparticles (NPs) may be capable of reversing the toxic effects of drug overdoses in humans by adsorbing/absorbing drug molecules. This paper develops a model to include the kinetic effects of treating drug overdoses by NPs. Depending on the size and the nature of the NPs, they may either pass through the capillary walls and enter the tissue space or remain only inside the capillaries and other blood vessels: models are developed for each case. Furthermore, the time scale for equilibration between the NP and the blood will vary with the specific type of NP. The NPs may sequester drug from within the capillaries depending on whether this time scale is larger or smaller than the residence time of blood within the capillary. Models are developed for each scenario. The results suggest that NPs are more effective at detoxification if they are confined to the blood vessels and do not enter the tissues. The results also show that the detoxification process is faster if drug uptake occurs within the capillaries. The trends shown by the model predictions can serve as useful guides in the design of the optimal NP for detoxification.

Physiologically based pharmacokinetic modeling of arterial - antecubital vein concentration difference

BACKGROUND

Modeling of pharmacokinetic parameters and pharmacodynamic actions requires knowledge of the arterial blood concentration. In most cases, experimental measurements are only available for a peripheral vein (usually antecubital) whose concentration may differ significantly from both arterial and central vein concentration.

METHODS

A physiologically based pharmacokinetic (PBPK) model for the tissues drained by the antecubital vein (referred to as "arm") is developed. It is assumed that the "arm" is composed of tissues with identical properties (partition coefficient, blood flow/gm) as the whole body tissues plus a new "tissue" representing skin arteriovenous shunts. The antecubital vein concentration depends on the following parameters: the fraction of "arm" blood flow contributed by muscle, skin, adipose, connective tissue and arteriovenous shunts, and the flow per gram of the arteriovenous shunt. The value of these parameters was investigated using simultaneous experimental measurements of arterial and antecubital concentrations for eight solutes: ethanol, thiopental, 99Tcm-diethylene triamine pentaacetate (DTPA), ketamine, D2O, acetone, methylene chloride and toluene. A new procedure is described that can be used to determine the arterial concentration for an arbitrary solute by deconvolution of the antecubital concentration. These procedures are implemented in PKQuest, a general PBPK program that is freely distributed http://www.pkquest.com.

RESULTS

One set of "standard arm" parameters provides an adequate description of the arterial/antecubital vein concentration for ethanol, DTPA, thiopental and ketamine. A significantly different set of "arm" parameters was required to describe the data for D2O, acetone, methylene chloride and toluene - probably because the "arm" is in a different physiological state.

CONCLUSIONS

Using the set of "standard arm" parameters, the antecubital vein concentration can be used to determine the whole body PBPK model parameters for an arbitrary solute without any additional adjustable parameters. Also, the antecubital vein concentration can be used to estimate the arterial concentration for an arbitrary input for solutes for which no arterial concentration data is available.

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.

Factors determining the duration of tracheal intubation in cardiac surgery: a single-centre sequential patient audit

BACKGROUND AND OBJECTIVE

The study was designed to identify those factors associated with early tracheal extubation following cardiac surgery. Previous studies have tended to concentrate on surgery for coronary artery bypass or on other selected cohorts.

METHODS

Sequential cohort analysis of 296 unselected adult cardiac surgery patients was performed over 3 months.

RESULTS

In total, 39% of all patients were extubated within 6 h, 89% within 24 h and 95% within 48 h. Delayed extubation (>6 h after surgery) appeared unrelated to age, gender, body mass index, a previous pattern of angina or myocardial infarction, diabetes, preoperative atrial fibrillation, and preoperative cardiovascular assessment, as well as other factors. Delayed tracheal extubation was associated with poor left ventricular, renal and pulmonary function, a high Euroscore, as well as the type, duration and urgency of surgery. Early extubation (<6 h) was not associated with a reduced length of stay in either the intensive care unit or in hospital compared with patients who were extubated between 6 and 24 h. In these groups, it is presumed that organizational and not clinical factors appear to be responsible for a delay in discharge from intensive care. Patients who were extubated after 24 h had a longer duration of hospital stay and a greater incidence of postoperative complications. Postoperative complications were not adversely affected by early tracheal extubation.

CONCLUSIONS

In an unselected sequential cohort, both patient- and surgery-specific factors may be influential in determining the duration of postoperative ventilation of the lungs following cardiac surgery. In view of the changing nature of the surgical population, regular re-evaluation is useful in reassessing performance.

Injection pain of rocuronium and vecuronium is evoked by direct activation of nociceptive nerve endings

BACKGROUND AND OBJECTIVE

Rocuronium and, to a lesser extent, vecuronium can induce burning sensations associated with withdrawal reactions during administration. Dermal microdialysis in human and electrophysiological recordings of nociceptors in mouse skin were used to elucidate the underlying mechanisms of pain induction.

METHODS

Microdialysis catheters were inserted intradermally into the forearm of 10 volunteers and were perfused with two different concentrations of rocuronium and vecuronium (1 and 10 mg mL(-1)) or a control. Dialysis samples were taken every 15 min and analysed for protein, histamine, tryptase and bradykinin content. Pain intensity was rated on a numerical scale of 0-10. In a parallel design, activation of cutaneous nociceptors was assessed directly in a skin-nerve in vitro preparation of the mouse hind paw. The receptive fields of identified single C-nociceptors (n = 12) were superfused with rocuronium or vecuronium solutions (10 mg mL(-1)) at physiological pH.

RESULTS

In accordance with clinical observations, microdialysis of rocuronium (10 mg mL(-1)) induced sharp burning pain (NRS 4.1 +/- 1.8), whereas vecuronium given in the usual clinical concentration (1 mg mL(-1)) induced only minor pain sensations (NRS 0.6 +/- 1.3). At equimolar concentrations, pain sensation and concomitant mediator release evoked by both drugs were similar. No correlations were found between pain rating and mediator release. In the in vitro preparation, C-fibres showed a consistent excitatory response with rapid onset after stimulation with vecuronium as well as rocuronium (differences not significant).

CONCLUSIONS

The algogenic effect of aminosteroidal neuromuscular blocking drugs can be attributed to a direct activation of C-nociceptors.

Anaesthetic strategies to reduce perioperative blood loss in paediatric surgery

In adults, a number of measures to reduce perioperative blood loss have been established. These techniques serve to reduce patients' exposure to homologous blood. Most adults are concerned with this issue especially since many patients became infected with human immunodeficiency virus (HIV) during the 1980s through exposure to blood components. While blood-saving strategies are widely used in adults, they are mostly neglected in infants. However, it is these young patients with their whole life in front of them who, it could be argued, would benefit especially from any potentially avoidable infection (HIV, hepatitis, etc.) or immunological complications. In infants and small children, these blood-sparing techniques may not be as effective as in adults and technical limitations may prevent their application. However, some of these measures can be used and may serve to prevent or reduce exposure to homologous blood. In the following review, blood-saving techniques established in adults are described and their applicability for paediatric patients discussed.

Effects of propofol on the systolic and diastolic performance of the postischaemic, reperfused myocardium in rabbits

BACKGROUND AND OBJECTIVE

The effect of propofol on myocardial dysfunction during ischaemia and reperfusion is controversial yet important because of its frequent use in cardiac anaesthesia. Although animal studies suggest a free radical-scavenging potential, the cardioprotective properties of propofol have not been demonstrated consistently in vivo. Previous studies focused on systolic function while diastolic function may be a more sensitive marker of ischaemic injury. The main aim was to document the effect of propofol on diastolic function in isolated, blood perfused rabbit hearts subjected to moderate global ischaemia and reperfusion.

METHODS

Propofol 168 micromol L(-1), or the equivalent of its vehicle, Intralipid, was administered to 34 paced parabiotic Langendorff blood-perfused isolated rabbit hearts before and after 30 min of global normothermic ischaemia. Recovery of systolic function was quantified with the maximum rate of rise of left ventricular pressure. Diastolic performance was assessed using the time constant of the decline in left ventricular pressure (tau) and chamber stiffness (VdP/dV at 12 mmHg).

RESULTS

Recovery of systolic function during reperfusion was comparable in the two groups. There was no difference in left ventricular pressure between the two groups at any time during the experiments. Chamber stiffness increased significantly during ischaemia and reperfusion in the control group (from 34 +/- 9 to 54 +/- 8 mmHg during ischaemia, and 43 +/- 5 mmHg after 30 min reperfusion; mean +/-95% confidence interval) but not in the propofol-treated group (29 +/- 5, 36 +/- 8 and 30 +/- 8 at baseline, ischaemia and 30 min reperfusion, respectively).

CONCLUSIONS

Propofol has no protective effect on active relaxation or on systolic function in the present model, but it reduces ischaemic and postischaemic chamber stiffness.

Effects of bupivacaine used with sevoflurane on the rhythm and contractility in the isolated rat heart

BACKGROUND AND OBJECTIVE

The effects of sevoflurane on bupivacaine cardiotoxicity are mainly attributed to systemic effects. The purpose of this study was to investigate the direct myocardial effects of sevoflurane on bupivacaine toxicity.

METHODS

Hearts of 30 Wistar albino rats were isolated and mounted on a Langendorff apparatus perfused by modified Tyrode solution. Experimental groups were: a sevoflurane group (Group S, n = 10)--following baseline and 20 min (Stage 1) recordings, sevoflurane was added in doses of 1.4% (1 MAC) and 2.8% (2 MAC). In the two bupivacaine groups, bupivacaine 5 micromol (Group B5, n = 10) and bupivacaine 10 micromol (Group B10, n = 10) was added to the solution at Stage 1, and sevoflurane was added to the system as in Group S. Haemodynamic variables, i.e. heart rate, PR interval, QRS duration, left ventricular systolic pressure, contractility (+dp/dtmax), relaxation, time to reach peak systolic pressure, change in left ventricular diastolic pressure from baseline, and rate-pressure product were recorded.

RESULTS

In Group S, there was no change in cardiac rhythm. In bupivacaine groups, severe rhythm disturbances occurred and both the PR intervals and QRS complexes were prolonged significantly. All contractility variables deteriorated and the rate-pressure product decreased by 67-90% with the addition of bupivacaine. In all groups, 2 MAC sevoflurane lowered +dp/dtmax further.

CONCLUSIONS

Sevoflurane does not have any untoward effect on bupivacaine-induced cardiotoxicity in clinically relevant doses in the isolated rat heart.

Effect of the neuronal nitric oxide synthase inhibitor 7-nitroindazole on the righting reflex ED50 and minimum alveolar concentration during sevoflurane anaesthesia in rats

BACKGROUND AND OBJECTIVE

The aim was to determine the effect of acute and chronic administration of 7-nitroindazole, a selective neuronal nitric oxide synthase inhibitor, on the righting reflex ED50 and the minimum alveolar concentration during sevoflurane anaesthesia in rats.

METHODS

7-Nitroindazole was acutely (0, 50 and 100 mg kg(-1)) and chronically (0 and 150 mg kg(-1) day(-1), 4 days) administered to rats. After the preparation, the minimum alveolar concentration and the righting reflex ED50 were measured. The concentration of cGMP in the brain, cerebellum and spinal cord was also measured.

RESULTS

Acute administration reduced the minimum alveolar concentration (50 mg kg(-1), 58.8% (95% CI: 50.3-67.3%) of the baseline value, P < 0.01; 100 mg kg(-1), 55.8 (46.9-64.7), P < 0.01) and the righting reflex ED50 (50 mg kg(-1), 27.2 (17.2-37.2), P < 0.01; 100 mg kg(-1), 14.3 (6.6-22.0), P < 0.01). Chronic administration did not reduce the minimum alveolar concentration; however, it reduced the righting reflex ED50 (65.3 (52.9-77.7), P < 0.01). Overall, the reduction in minimum alveolar concentration in the acute and chronic protocol did not correlate with that of the righting reflex ED50. 7-Nitroindazole (100 mg kg(-1), acute) reduced the cGMP concentration within the cerebellum by 55.4%; however, it did not decrease concentrations in the brain or spinal cord.

CONCLUSIONS

Different mechanisms are responsible for the observed alterations to the minimum alveolar concentration and the righting reflex ED50 following treatment with 7-nitroindazole. The nitric oxide-cGMP pathway might play a less important role in the determination of minimum alveolar concentration than the righting reflex ED50.

Suprascapular nerve block or a piroxicam patch for shoulder tip pain after day case laparoscopic surgery

BACKGROUND AND OBJECTIVE

The reported incidence of shoulder tip pain following laparoscopic surgery varies from 35 to 63%. This study evaluated the analgesic efficacy of either performing a prophylactic suprascapular nerve block with bupivacaine or applying a piroxicam patch to the skin over both shoulders for the relief of shoulder tip pain after laparoscopy.

METHODS

Sixty healthy informed female patients were randomly assigned to one of three groups: (a) a control group (n = 20), no treatment; (b) a suprascapular nerve block group (n = 20) in which a bilateral suprascapular nerve block was performed before induction of anaesthesia with 5 mL 0.5% bupivacaine with epinephrine; and (c) a piroxicam patch group (n = 20) in which a 48 mg piroxicam patch on the skin of each shoulder was applied before induction of anaesthesia. All patients received a total intravenous anaesthesia technique with propofol, fentanyl and vecuronium. Shoulder tip and wound pain were recorded on a visual analogue pain scale at five time intervals for 24 h after surgery.

RESULTS

A total of 80% of patients in the control group, 75% in the suprascapular nerve block group and 45% in the piroxicam patch group complained of shoulder tip pain during the recording period (P < 0.05). The scores for shoulder tip pain in the piroxicam patch group were significantly lower compared with the control group at 3, 6 and 12 h, and compared with the suprascapular nerve block group at 6 and 12 h. The need for analgesics was also significantly lower in the piroxicam patch group compared with the other two groups.

CONCLUSIONS

Prophylactic piroxicam patches are effective and safe for the relief of shoulder tip pain after laparoscopy. Bilateral suprascapular nerve block is not effective in this setting.

Treatment of hiccup during general anaesthesia or sedation: a qualitative systematic review

BACKGROUND AND OBJECTIVE

Acute hiccup is a minor complication that can occur during sedation or general anaesthesia. The disorder can disturb the surgical field, might interfere with lung ventilation or could hamper diagnostic procedures. The objective was to perform a systematic search for interventions aimed at treating hiccup occurring during anaesthesia or sedation.

METHODS

A systematic search for reports describing interventions to treat hiccup in conjunction with anaesthesia was carried out (MEDLINE, EMBASE, Cochrane-Library, manual screening of reference lists and review articles, up to December 2001). Search terms were 'hiccup', 'singultus' or 'hiccough'.

RESULTS

Twenty-six reports involving approximately 581 patients focused on hiccup remedies in the anaesthesia setting. Only one report was substantiated by a randomized controlled trial. This investigated methylphenidate 10 mg intravenously in 51 patients, which did not show a beneficial effect compared with placebo. Hiccup was a self-limiting phenomenon. Case series and case reports focused on various systemically applied drugs in 12 reports, stimulating techniques (e.g. pharyngeal stimulation) in seven, topical applied remedies (e.g. intranasal ice-cold water) in four, and ventilation techniques (e.g. continuous positive pressure ventilation) in two.

CONCLUSIONS

A large variety of interventions have been proposed for the treatment of hiccup during anaesthesia and sedation. However, perioperative treatment is still based on empirical findings and no treatment is 'evidence-based'. Thus, no valid recommendations for the treatment of hiccup can be derived. Uncontrolled observations are inadequate to establish treatment efficacy.

Pharmacokinetics and pharmacodynamics of the new propofol prodrug GPI 15715 in rats

BACKGROUND AND OBJECTIVE

We studied the pharmacokinetics and pharmacodynamics of GPI 15715 (Aquavan injection), a new water-soluble prodrug metabolized to propofol by hydrolysis.

METHODS

Nine adult male Sprague-Dawley rats (398 +/- 31 g) received a bolus dose of 40 mg GPI 15715. The plasma concentrations of GPI 15715 and propofol were determined from arterial blood samples, and the pharmacokinetics of both compounds were investigated using compartment models whereby the elimination from the central compartment of GPI 15715 was used as drug input for the central compartment of propofol. Pharmacodynamics were assessed using the median frequency of the EEG power spectrum.

RESULTS

A maximum propofol concentration of 7.1 +/- 1.7 microg mL(-1) was reached 3.7 +/- 0.2 min after bolus administration. Pharmacokinetics were best described by two-compartment models. GPI 15715 showed a short half-life (2.9 +/- 0.2 and 23.9 +/- 9.9 min), an elimination rate constant of 0.18 +/- 0.01 min(-1) and a central volume of distribution of 0.25 +/- 0.02 L kg(-1). For propofol, the half-life was 1.9 +/- 0.1 and 45 +/- 7 min, the elimination rate constant was 0.15 +/- 0.02 min(-1) and the central volume of distribution was 2.3 +/- 0.6 L kg(-1). The maximum effect on the electroencephalogram (EEG)--EEG suppression for >4 s--occurred 6.5 +/- 1.2 min after bolus administration and baseline values of the EEG median frequency were regained 30 min later. The EEG effect could be described by a sigmoid Emax model including an effect compartment (E0 = 16.9 +/- 7.9 Hz, EC50 = 2.6 +/- 0.8 microg mL(-1), ke0 = 0.35 +/- 0.04 min(-1)).

CONCLUSIONS

Compared with known propofol formulations, propofol from GPI 15715 showed a longer half-life, an increased volume of distribution, a delayed onset, a sustained duration of action and a greater potency with respect to concentration.

High-affinity block of voltage-operated rat IIA neuronal sodium channels by 2,6 di-tert-butylphenol, a propofol analogue

BACKGROUND AND OBJECTIVE

Propofol is a phenol derivative (2,6 di-isopropylphenol) with a unique effect profile including activating effects on GABA(A) and blocking effects on voltage-operated sodium channels. If the substituents in the 2- and the 6-positions are replaced by tert-butyl groups, the resulting phenol derivative, 2,6 di-tert-butylphenol, despite being a close structural propofol analogue, completely lacks GABA(A) receptor effects. The aim of this in vitro study was to investigate the effects of 2,6 di-tert-butylphenol on voltage-operated neuronal sodium channels in order to determine whether and, if so, how these structural changes alter the sodium channel-blocking effect seen with propofol.

METHODS

Whole-cell sodium inward currents through heterologously expressed rat type IIA sodium channels were recorded in the absence and presence of definite concentrations of 2,6 di-tert-butylphenol and propofol.

RESULTS

When applied at concentrations > or = 30 micromol, 2,6 di-tert-butylphenol completely and irreversibly blocked sodium inward currents. The blockade equilibrium time was about 2 min. A partial washout was possible only if the application was stopped before the equilibrium of the blockade was achieved.

CONCLUSIONS

2,6 Di-tert-butylphenol exerts a high-affinity block of neuronal sodium channels. Apparently, the slight structural differences of 2,6 di-tert-butylphenol in comparison with propofol--which account for the lack of GABA(A) receptor effects--enhance its voltage-operated sodium channel-blocking effects. As 2,6 di-tert-butylphenol is much more potent than most sodium channel blockers in clinical use, it might be of interest in the development of local anaesthetics.

Effects of rolipram, pimobendan and zaprinast on ischaemia-induced dysrhythmias and on ventricular cyclic nucleotide content in the anaesthetized rat

BACKGROUND AND OBJECTIVE

This study was designed to compare the haemodynamic, electrophysiological and pharmacodynamic effects of three selective inhibitors of the different isoenzyme forms of phosphodiesterase (PDE) on ischaemia-induced dysrhythmias in the anaesthetized rat. The drugs used were pimobendan, a selective PDE III inhibitor, rolipram, a selective PDE IV inhibitor, and zaprinast, a selective PDE V inhibitor.

METHODS

The coronary artery was occluded 15 min after commencing drug administration, and myocardial ischaemia was maintained for 30 min during which the heart rate and mean arterial pressure were recorded. cAMP and cGMP were determined by radioimmunoassay.

RESULTS

Pretreatment with rolipram decreased the duration of ventricular tachycardia without any change in the incidences of dysrhythmias or the mortality rate. This drug did not modify ventricular content of adenosine 3',5'-cyclic monophosphate (cAMP) or guanosine 3',5'-cyclic monophosphate (cGMP). Pimobendan (1 mg kg(-1) + 0.1 mg kg(-1) min) decreased the duration of ventricular tachycardia. This dose of pimobendan and zaprinast (1 mg kg(-1) + 0.1 mg kg(-1) min(-1)) increased the incidence rate of ventricular fibrillation following coronary artery ligation and the mortality rate. Moreover, both drugs increased cGMP in the ventricle.

CONCLUSIONS

The results demonstrated that pimobendan and zaprinast increased the incidence of dysrhythmias and the mortality rate, which was accompanied by an increase in the ventricular content of cGMP. Rolipram decreased the duration of ventricular tachycardia without a change in the cyclic nucleotide content or in the mortality rate.

Prediction of the volume of distribution of a drug: which tissue-plasma partition coefficients are needed?

The aim of this study was to identify the tissue-plasma partition coefficients (Kp) needed for an initial prediction of the volume of distribution at steady state (Vd(ss)) of a drug in humans. Values of Kp were collected from the literature. Only Kp values plausibly representing true steady state distribution were accepted, and data had to be available for muscle, fat, skin and at least five other organs. The apparent volume of distribution of a drug in an organ/tissue (Vapp) was calculated as Kp multiplied by the volume of the organ/tissue, and the Vd(ss) as the sum of all available Vapp values. The percentage contribution of each Vapp to the Vd(ss) was estimated. In addition, linear regressions were calculated between Kp values of all drugs in a specific organ/tissue and Kp in muscle or fat. Finally, the Vd(ss) was re-calculated using (for basic drugs) the Kp in fat to calculate Vapp in fat and lungs and the Kp in muscle for the Vapp of all other organs/tissues. The two sets of estimates of Vd(ss) were compared by linear regression. The same calculations were performed for acidic drugs, except that muscle Kp was used also forthe lungs. Distribution to fat and muscle accounted for 84% (61-91%) (median and range) of the total estimated Vd(ss) of the basic drugs (n = 17). The regressions between Kp in organs/tissues and muscle Kp were statistically significant except in the case of liver. For acidic drugs (n = 18), distribution to fat and muscle accounted for 65% (42-92%) of Vd(ss), and the regressions of Kp were significant for all organs/tissues except kidney and bone. For both types of drugs, correlations between organ/tissue Kp values and Kp in fat were generally worse. There were excellent linear correlations between Vd(ss) calculated by means of only two Kp values and the originally calculated Vd(ss) (r2> or = 0.99 for both basic and acidic drugs; slopes were notsignificantly different from unity). Thus, initial estimation of the Vd(ss) of a new drug can normally be based on only two Kp values, those of muscle and fat. The muscle Kp can be used to represent all lean tissues, including the residual "carcass", with the exception that fat Kp can be used for distribution of basic drugs to lungs.

The concordance of early antipyrine and thiopental distribution kinetics

Studies of factors affecting the initial disposition of drugs with a rapid onset of effect following i.v. administration have used antipyrine as a surrogate for lipophilic drugs because it lacks cardiovascular effects. The present study tested the assumption that antipyrine is a useful surrogate for the flow-dependent tissue distribution of the lipophilic drug thiopental by comparing the recirculatory pharmacokinetic models of antipyrine and thiopental disposition after concomitant administration to five dogs anesthetized with 1.5% halothane. The pharmacokinetics of indocyanine green, a marker of the intravascular behavior of antipyrine and thiopental, and antipyrine in these dogs was nearly identical to that described previously in dogs anesthetized with 1.5% halothane but not given thiopental. The total volume of distribution of the highly lipophilic drug thiopental was more than 60% larger than that of antipyrine, 53 versus 33 liters, respectively. Nonetheless, the initial distribution kinetics of the two drugs, including the pulmonary tissue volume and the volume of the nondistributive pathway as well as the clearance to it, were nearly identical. As a result, the fraction of cardiac output involved in distribution of the two drugs to peripheral tissues was similarly identical, although the distribution of cardiac output between clearance to the rapidly equilibrating tissues and clearance to the slowly equilibrating tissues differed slightly. This study validates the assumption that antipyrine is a useful surrogate for lipophilic drugs in pharmacokinetic studies in which physiologic stability is desirable to meet the assumption of system stationarity.

Prediction of the disposition of midazolam in surgical patients by a physiologically based pharmacokinetic model

The aim of this study was to predict the disposition of midazolam in individual surgical patients by physiologically based pharmacokinetic (PBPK) modeling and explore the causes of interindividual variability. Tissue-plasma partition coefficients (k(p)) were scaled from rat to human values by a physiologically realistic four-compartment model for each tissue, incorporating the measured unbound fraction (f(u)) of midazolam in the plasma of each patient. Body composition (lean body mass versus adipose tissue) was then estimated in each patient, and the volume of distribution at steady state (V(dss)) of midazolam was calculated. Total clearance (CL) was calculated from unbound intrinsic CL, f(u), and estimated hepatic blood flow. Curves of midazolam plasma concentration versus time were finally predicted by means of a perfusion-limited PBPK model and compared with measured data. In a first study on 14 young patients undergoing surgery with modest blood loss, V(dss) was predicted with an only 3.4% mean error (range -24-+39%) and a correlation between predicted and measured values of 0.818 (p < 0.001). Scaling of k(p) values by the four-compartment model gave better predictions of V(dss) than scaling using unbound k(p). In the PBPK modeling, the mean +/- standard deviation (SD) prediction error for all data was 9.7 +/- 33%. In a second study with 10 elderly patients undergoing orthopedic surgery, hemodilution and blood loss led to a higher f(u) of midazolam. The PBPK modeling correctly predicted a marked increase in V(dss), a smaller increase in CL, and a prolonged terminal half-life of midazolam, as compared with findings in the first study. Interindividual variation in the disposition of midazolam could thus in part be related to the physiological characteristics of the patients and the f(u) of the drug in their plasma.

Prediction of adipose tissue: plasma partition coefficients for structurally unrelated drugs

Tissue:plasma (P(t:p)) partition coefficients (PCs) are important parameters describing tissue distribution of drugs. The ultimate goal in early drug discovery is to develop and validate in silico methods for predicting a priori the P(t:p) for each new drug candidate. In this context, tissue composition-based equations have recently been developed and validated for predicting a priori the non-adipose and adipose P(t:p) for neutral organic solvents and pollutants. For ionizable drugs that bind to different degrees to common plasma proteins, only their non-adipose P(t:p) values have been predicted with these equations. The only compound-dependent input parameters for these equations are the lipophilicity parameter, such as olive oil-water PC (K(vo:w)) or n-octanol-water PC (P(o:w)), and/or unbound fraction in plasma (fu(p)) determined under in vitro conditions. Tissue composition-based equations could potentially also be used to predict adipose tissue-plasma PCs (P(at:p)) for ionized drugs. The main objective of the present study was to modify these equations for predicting in vivo P(at:p) (white fat) for 14 structurally unrelated ionized drugs that bind substantially to plasma macromolecules in rats, rabbits, or humans. The second objective was to verify whether K(vo:w) or P(o:w) provides more accurate predictions of in vivo P(at:p) (i.e., to verify whether olive oil or n-octanol is the better surrogate for lipids in adipose tissue). The second objective was supported by comparing in vitro data on P(at:p) with those on olive oil-plasma PC (K(vo:p)) for five drugs. Furthermore, in vivo P(at:p) was not only predicted from K(vo:w) and P(o:w) of the non-ionized species, but also from K*(vo:w) and P*(o:w), taking into account the ionized species in addition. The P(at:p) predicted from K*(vo:w), P*(o:w), and P(o:w) differ from the in vivo P(at:p) by an average factor of 1.17 (SD = 0.44, r = 0.95), 15.0 (SD = 15.7, r = 0.59), and 40.7 (SD = 57.2, r = 0.33), respectively. The in vitro values of K(vo:p) differ from those of P(at:p) by an average factor of 0.86 (SD = 0.16, r = 0.99, n = 5). The results demonstrate that (i) the equation using only data on fu(p) as input and olive oil as lipophilicity surrogate is able to provide accurate predictions of in vivo P(at:p), and (ii) olive oil is a better surrogate of the adipose tissue lipids than n-octanol. The present study is an innovative method for predicting in vivo fat partitioning of drugs in mammals.

Tissue distribution and physiologically based pharmacokinetics of antisense phosphorothioate oligonucleotide ISIS 1082 in rat

The aim of this study was to develop a whole body physiologically based model of the pharmacokinetics (PBPK) of the phosphorothioate oligonucleotide (PS-ODN) ISIS 1082 in vivo. Rats were administered an intravenous (i.v.) bolus dose of ISIS 1082 (10 mg/kg plus 3H tracer), and arterial blood and tissues were taken at specific times up to 72 hours. Radioactivity was measured in all samples. The parent compound was determined specifically in blood and tissues at 90 minutes and in liver and kidney also at 24 hours, using capillary gel electrophoresis (CGE). A whole body PBPK model was fitted to the combined blood and tissue radioactivity data using nonlinear regression analysis. CGE analysis indicated that the predominant species in plasma and all tissues is ISIS 1082, together with some n-1 and n-2 metabolites. Total radioactivity primarily reflects these species. The whole body model successfully described temporal events in all tissues. However, to adequately model the experimental data, all tissues had to be partitioned into vascular and extravascular spaces to accommodate the relatively slow distribution of ISIS 1082 out of blood because of a permeability rate limitation. ISIS 1082 distributes extensively into tissues, but the relative affinity varies enormously, being highest for kidney and liver and lowest for muscle and brain. A whole body PBPK model with a permeability rate limited tissue distribution was developed that adequately described events in both blood and tissue for an oligonucleotide. This model has the potential not only to characterize the events in individual tissues throughout the body for such compounds but also to scale across animal species, including human.

Clearance of fentanyl, alfentanil, methohexitone, thiopentone and ketamine in relation to estimated hepatic blood flow in several animal species: application to prediction of clearance in man

We have used estimated hepatic blood flow (Qhep) as an aid to evaluate clearance (CL) values in animals and to predict clearance in man of five anaesthetic agents: fentanyl, alfentanil, methohexitone, thiopentone and ketamine. The disposition of methohexitone was determined in rats and that of ketamine in rats, rabbits and pigs. Further data were compiled from the literature and supplemented experimentally as needed. Allometric interspecies scaling, according to three different methods, was used to estimate blood clearance and unbound clearance (CLu) in man. The results of scaling according to the three different methods were evaluated in relation to estimated hepatic extraction ratio (CL/Qhep) of the drugs. In most animals the clearance of the drugs were comparable with or lower than estimated Qhep. However, ketamine showed extensive extrahepatic clearance in rabbits. Prediction of clearance in man was successful by at least one method for all five drugs, while prediction of CLu generally failed. Estimates of CL/Qhep gave no indication as to the choice of the best method. Volume of distribution at steady state could be predicted for alfentanil, thiopentone and ketamine. Comparison of clearance with Qhep should be used to evaluate clearance data in animals, however estimation of hepatic extraction ratios appears to be of little use for allometric scaling. The use of ketamine as an anaesthetic agent in rabbits is questionable, while the use of fentanyl in pigs, methohexitone in rats and ketamine in rats and pigs is well supported by the pharmacokinetic data.

A priori prediction of tissue:plasma partition coefficients of drugs to facilitate the use of physiologically-based pharmacokinetic models in drug discovery

The tissue:plasma (P(t:p)) partition coefficients (PCs) are important drug-specific input parameters in physiologically based pharmacokinetic (PBPK) models used to estimate the disposition of drugs in biota. Until now the use of PBPK models in early stages of the drug discovery process was not possible, since the estimation of P(t:p) of new drug candidates by using conventional in vitro and/or in vivo methods is too time and cost intensive. The objectives of the study were (i) to develop and validate two mechanistic equations for predicting a priori the rabbit, rat and mouse P(t:p) of non-adipose and non-excretory tissues (bone, brain, heart, intestine, lung, muscle, skin, spleen) for 65 structurally unrelated drugs and (ii) to evaluate the adequacy of using P(t:p) of muscle as predictors for P(t:p) of other tissues. The first equation predicts P(t:p) at steady state, assuming a homogenous distribution and passive diffusion of drugs in tissues, from a ratio of solubility and macromolecular binding between tissues and plasma. The ratio of solubility was estimated from log vegetable oil:water PCs (K(vo:w)) of drugs and lipid and water levels in tissues and plasma, whereas the ratio of macromolecular binding for drugs was estimated from tissue interstitial fluid-to-plasma concentration ratios of albumin, globulins and lipoproteins. The second equation predicts P(t:p) of drugs residing predominantly in the interstitial space of tissues. Therefore, the fractional volume content of interstitial space in each tissue replaced drug solubilities in the first equation. Following the development of these equations, regression analyses between P(t:p) of muscle and those of the other tissues were examined. The average ratio of predicted-to-experimental P(t:p) values was 1.26 (SD = 1.40, r = 0.90, n = 269), and 85% of the 269 predicted values were within a factor of three of the corresponding literature values obtained under in vivo and in vitro conditions. For predicted and experimental P(t:p), linear relationships (r > 0.9 in most cases) were observed between muscle and other tissues, suggesting that P(t:p) of muscle is a good predictor for the P(t:p) of other tissues. The two previous equations could explain the mechanistic basis of these linear relationships. The practical aim of this study is a worthwhile goal for pharmacokinetic screening of new drug candidates.

Physiologically based pharmacokinetics of cyclosporine A: reevaluation of dose-nonlinear kinetics in rats

The disposition kinetics of Cyclosporine A (CyA) in rat, based on measurement in arterial blood, appeared dose-linear over a wide i.v. dose range (1.2-30 mg/kg). Physiologically based pharmacokinetic (PBPK) analysis, however, demonstrated that this was an apparent observation resulting from counterbalancing nonlinear factors, such as saturable blood and tissue distribution, as well as clearance (CLb). A PBPK model was successfully developed taking into account these multiple nonlinear factors. Tissue distribution was distinctly different among various organs, being best described by either a linear model (muscle, fat; Model 1), one involving instantaneous saturation (lung, heart, bone, skin, thymus; Model 2), noninstantaneous saturation (kidney, spleen, liver, gut; Model 3), or one with saturable efflux (brain; Model 4). Overall, the whole body volume of distribution at steady state for unbound CyA (Vuss) decreased with increasing dose, due at least in part to saturation of tissue-cellular cyclophilin binding. Clearance, essentially hepatic, and described by the well-stirred model, was also adequately characterized by Michaelis-Menten kinetics, Km 0.60 microgram/ml. In model-based simulations, both volume of distribution at steady state (Vss,b) and CLb varied in a similar manner with dose, such that terminal t1/2 remained apparently unchanged; these dose responses were attenuated by saturable blood binding. CyA concentration measured in arterial blood was not always directly proportional to the true exposure, i.e., unbound or target tissue concentrations. The PBPK model not only described comprehensively such complicated PK relationships but also permitted assessment of the sensitivity of individual parameters to variation in local nonlinear kinetics. Using this approach, dose-dependent CyA uptake into brain was shown to be sensitive to both active and passive transport processes, and not merely the affinity of the active (efflux) transporter at the level of the blood-brain barrier.

Electroencephalographic effects of thiopentone and its enantiomers in the rat: correlation with drug tissue distribution

1. To better understand the pharmacology of the thiopentone enantiomers, we studied their quantitative electroencephalographic effects and their distribution into vital tissues. 2. Adult Wistar rats were infused with rac-, R- or S-thiopentone at 4 mg kg(-1)min(-1) until death ensued. The EEG signal was acquired continuously; serial arterial plasma and terminal tissue thiopentone concentrations were measured enantiospecifically. Relevant drug tissue : plasma distribution coefficients and plasma concentration-EEG effect relationships were determined. 3. Doses (mg kg(-1)) (mean+/-s.e.mean) for anaesthesia (toe pinch) and lethality (respiratory failure), respectively, decreased in the order R-thiopentone (55.8+/-2.4 and 176.2+/-11.2)> rac-thiopentone (39.3+/-2.1 and 97.5+/-3.9)> S-thiopentone (35.6+/-1.9 and 74.2+/-5.2); plasma drug concentrations (microg ml(-1)) decreased in the order R-thiopentone (66.3+/-4.5 and 89.8+/-5.2)> rac-thiopentone (56.7+/-2.0 and 77. 8+/-2.8)> S-thiopentone (55.0+/-1.9 and 64.1+/-2.8). 4. Initial EEG activation was similar for all thiopentone forms. Plasma drug concentrations for the same extent of EEG deactivation reflected the potency order. 5. After infusion of rac-thiopentone, tissue : plasma distribution coefficients were higher for R- than for S-thiopentone in brain and visceral regions, but not in fat or muscle. After infusion of the separate enantiomers, the relative heart : brain distribution ratio was for S-thiopentone was double that for R-thiopentone. 6. The therapeutic index of R-thiopentone (3.16+/-0. 14) was more advantageous than either rac-thiopentone (2.52+/-0.13) or S-thiopentone (2.10+/-0.14), possibly due to the relatively greater distribution into CNS tissues than heart. The data suggest that R-thiopentone could make a satisfactory single enantiomer substitute for rac-thiopentone.

Distribution of ciprofloxacin in the isolated rat lung in the presence and absence of tissue oedema

A series of experiments with the isolated perfused rat lung was carried out to study the distribution of ciprofloxacin in this tissue under different experimental conditions; the influence of drug administration rate and the presence of oedema were evaluated by comparison of the statistical moment and distribution coefficient values as well as the unit disposition function (UDF) profiles in the tissue. The polyexponential character of the outflow perfusate curves indicates a distribution behaviour which does not correspond to the 'well stirred model' but rather to the existence of some type of diffusion barriers and/or tissue binding. The presence of oedema in the tissue, induced by insufficient oxygenation of perfusate, significantly increases the distribution coefficient of ciprofloxacin and leads to relevant modifications in the unit disposition function (UDF) of the isolated lung, while administration rate does not significantly affect the kinetic behaviour of the drug in this tissue.

Renal handling of biphosphonate alendronate in rats

Alendronate is a bisphosphonate that is secreted via a saturable pathway in rat kidney. This study is designed to discover if the rate-determining step in its net renal secretion is uptake into the renal tubule. The tissue uptake clearance of alendronate by the kidney, estimated from an integration plot analysis and normalized with respect to plasma protein binding, was 4.2 times higher at a tracer dose than that of inulin, indicating uptake of alendronate by the renal tubules. The uptake clearance is comparable with the net secretion clearance obtained from an infusion study, indicating that the rate-determining step in the net secretion is uptake under the tracer conditions. When the dose was increased, however, there was no reduction in uptake clearance while the net secretion clearance fell to almost zero. The urinary excretion clearance defined with respect to the steady state concentration in the kidney also fell to almost zero. This result suggests that saturation of the net secretion of alendronate is caused by saturation of membrane transport through the brush-border membrane. Thus, it would seem that there is a transport mechanism for alendronate on the brush-border membrane of kidney epithelial cells.

Pharmacokinetics and organ distribution of cationized colchicine-specific IgG and Fab fragments in rat

Pharmacokinetics of cationized goat colchicine-specific polyclonal immunoglobulin G (IgG) and antigen binding fragment (Fab) (cIgG and cFab, respectively) were studied in male adult Sprague-Dawley rats and compared with those of the native proteins (nIgG and nFab). All proteins were radioiodinated by the Iodogen method, and kinetics were investigated following trichloroacetic acid (TCA) precipitation or immunoprecipitation. Deiodination and catabolism were more pronounced with the cationized than the native proteins, especially for cFab. Both cIgG and cFab in plasma decreased more rapidly than nIgG and nFab. The elimination half-lives were 52.9 and 81.8 h for cIgG and nIgG, respectively. In addition, there was a 74-fold increase in the volume of distribution and a 114-fold increase in the systemic clearance of cIgG compared with nIgG. For cFab, the volume of distribution and systemic clearance were increased 6.4- and 3.5-fold, respectively. Organ uptake of cIgG and cFab was markedly increased compared with that of nIgG and nFab, especially in kidney, liver, spleen, and lung. Renal clearance of cIgG and cFab was also increased 30- and 10-fold compared with that of nIgG and nFab, respectively. The present data suggest that cationization of colchicine-specific IgG and Fab fragments increased the organ distribution and greatly altered their pharmacokinetics. Nevertheless, the smaller molecular size of Fab versus IgG did not enhance the distribution and clearance of cFab. These data pave the way for evaluating the biological efficacy of these more tissue-organ-interactive antibodies.

Lumping of whole-body physiologically based pharmacokinetic models

Lumping is a common pragmatic approach aimed at the reduction of whole-body physiologically based pharmacokinetic (PBPK) model dimensionality and complexity. Incorrect lumping is equivalent to model misspecification with all the negative consequences to the subsequent model implementation. Proper lumping should guarantee that no useful information about the kinetics of the underlying processes is lost. To enforce this guarantee, formal standard lumping procedures and techniques need to be defined and implemented. This study examines the lumping process from a system theory point of view, which provides a formal basis for the derivation of principles and standard procedures of lumping. The lumping principle in PBPK modeling is defined as follows: Only tissues with identical model specification, and occupying identical positions in the system structure should be lumped together at each lumping iteration. In order to lump together parallel tissues, they should have similar or close time constants. In order to lump together serial tissues, they should equilibrate very rapidly with one another. The lumping procedure should include the following stages: (i) tissue specification conversion (when tissues with different model specifications are to be lumped together); (ii) classification of the tissues into classes with significantly different kinetics, according to the basic principle of lumping above; (iii) calculation of the parameters of the lumped compartments; (iv) simulation of the lumped system; (v) lumping of the experimental data; and (vi) verification of the lumped model. The use of the lumping principles and procedures to be adopted is illustrated with an example of a commonly implemented whole-body physiologically based pharmacokinetic model structure to characterize the pharmacokinetics of a homologous series of barbiturates in the rat.