Concept of a volume of distribution and possible errors in evaluation of this parameter

On Dr Dick Climie and Dr Jack Thomas, and the genesis of chemical-clinical pharmacology in Australian anaesthesia research

Pharmacokinetic and pharmacodynamic research is regularly reported in most contemporary anaesthesia-oriented journals. This sub-specialty area of pharmacology grew rapidly from the 1960s as various essential concepts and tools-laboratory analysis of drug/metabolite concentrations in biofluids, physiological signal collection, and methods for analysing/presenting relevant pharmacokinetic and pharmacodynamic data-started coming together. For Australia, such research began in Sydney in the mid-1960s with collaboration between anaesthetist Dr C.R. Climie (1923-2013) at the Royal Hospital for Women and medicinal chemist Dr J. Thomas OAM (1928-2017), and was achieved through a succession of postgraduate research student projects in the Department of Pharmacy of The University of Sydney, initially supervised by Dr Thomas. These consisted of studies concerned with the systemic absorption and placental transmission of drugs being used in parturients. By the late 1960s, Sydney anaesthetists Drs G.J. Long and C.A. Shanks (1936-1998) were also participating, and the projects were becoming more complex, including studies of the metabolism of local anaesthetics and other drugs by mothers and neonates. Between the mid-1970s and early-1980s, with additional anaesthetists, postgraduate research students and their academic supervisors participating, the projects focussed mainly on the pharmacokinetics and pharmacodynamics of neuromuscular blocking agents. This form of chemical-clinical-pharmacologically-based anaesthesia-oriented research that started in Sydney with the collaboration of Drs Climie and Thomas led to many challenging higher degree projects for pharmaceutical scientists, and access to unprecedented research capabilities for anaesthetists. Most significantly, it established a permanent place for multidisciplinary pharmacokinetic- and pharmacodynamic-based research within Australian academic departments of anaesthesia.

Comparing Mechanistic and Preclinical Predictions of Volume of Distribution on a Large Set of Drugs

PURPOSE

Volume of distribution at steady state (Vd_ss) is a fundamental pharmacokinetic (PK) parameter driven predominantly by passive processes and physicochemical properties of the compound. Human Vd_ss can be estimated using in silico mechanistic methods or empirically scaled from Vd_ss values obtained from preclinical species. In this study the accuracy and the complementarity of these two approaches are analyzed leveraging a large data set (over 150 marketed drugs).

METHODS

For all the drugs analyzed in this study experimental in vitro measurements of LogP, plasma protein binding and pKa are used as input for the mechanistic in silico model to predict human Vd_ss. The software used for predicting human tissue partition coefficients and Vd_ss based on the method described by Rodgers and Rowland is made available as supporting information.

RESULTS

This assessment indicates that overall the in silico mechanistic model presented by Rodgers and Rowland is comparably accurate or superior to empirical approaches based on the extrapolation of in vivo data from preclinical species.

CONCLUSIONS

These results illustrate the great potential of mechanistic in silico models to accurately predict Vd_ss in humans. This in silico method does not rely on in vivo data and is, consequently, significantly time and resource sparing. The success of this in silico model further suggests that reasonable predictability of Vd_ss in preclinical species could be obtained by a similar process.

ADAM, a hands-on patient simulator for teaching principles of drug disposition and compartmental pharmacokinetics

AIMS

To design, construct and validate a pharmacokinetics simulator that offers students hands-on opportunities to participate in the design, administration and analysis of oral and intravenous dosing regimens.

METHODS

The Alberta Drug Administration Modeller (ADAM) is a mechanical patient in which peristaltic circulation of water through a network of silicone tubing and glass bottles creates a representation of the outcomes of drug absorption, distribution, metabolism and elimination. Changing peristaltic pump rates and volumes in bottles allows values for pharmacokinetic constants to be varied, thereby simulating differences in drug properties and in patient physiologies and pathologies. Following administration of methylene blue dye by oral or intravenous routes, plasma and/or urine samples are collected and drug concentrations are determined spectrophotometrically. The effectiveness of the simulator in enhancing student competence and confidence was assessed in two undergraduate laboratory classes.

RESULTS

The simulator effectively models one- and two-compartment drug behaviour in a mathematically-robust and realistic manner. Data allow calculation of numerous pharmacokinetic constants, by traditional graphing methods or with curve-fitting software. Students' competence in solving pharmacokinetic problems involving calculations and graphing improved significantly, while an increase in confidence and understanding was reported.

CONCLUSIONS

The ADAM is relatively inexpensive and straightforward to construct, and offers a realistic, hands-on pharmacokinetics learning opportunity for students that effectively complements didactic lectures.

Proposal for defining the relevance of drug accumulation derived from single dose study data for modified release dosage forms

Recently, the European Medicines Agency (EMA) published the new draft guideline on the pharmacokinetic and clinical evaluation of modified release (MR) formulations. The draft guideline contains the new requirement of performing multiple dose (MD) bioequivalence studies, in the case when the MR formulation is expected to show 'relevant' drug accumulation at steady state (SS). This new requirement reveals three fundamental issues, which are discussed in the current work: first, measurement for the extent of drug accumulation (MEDA) predicted from single dose (SD) study data; second, its relationship with the percentage residual area under the plasma concentration-time curve (AUC) outside the dosing interval (τ) after SD administration, %AUC(τ-∞)SD ; and third, the rationale for a threshold of %AUC(τ-∞)SD that predicts 'relevant' drug accumulation at SS. This work revealed that the accumulation ratio RA,AUC , derived from the ratio of the time-averaged plasma concentrations during τ at SS and after SD administration, respectively, is the 'preferred' MEDA for MR formulations. A causal relationship was derived between %AUC(τ-∞)SD and RA,AUC , which is valid for any drug (product) that shows (dose- and time-) linear pharmacokinetics regardless of the shape of the plasma concentration-time curve. Considering AUC thresholds from other guidelines together with the causal relationship between %AUC(τ-∞)SD and RA,AUC indicates that values of %AUC(τ-∞)SD ≤ 20%, resulting in RA,AUC ≤ 1.25, can be considered as leading to non-relevant drug accumulation. Hence, the authors suggest that 20% for %AUC(τ-∞)SD is a reasonable threshold and selection criterion between SD or MD study designs for bioequivalence studies of new MR formulations.

The Connection Between the Steady State (Vss) and Terminal (Vβ) Volumes of Distribution in Linear Pharmacokinetics and The General Proof That Vβ ≥ Vss

The steady state and terminal (area) volumes of distribution are important pharmacokinetic parameters defined as the ratio of the total quantity of drug in the body, A(b)(t), to drug plasma concentration C(p)(t) at steady state and the terminal phase of drug elimination, respectively. The general equations for the approach of C(p)(t), A(b)(t) and the distribution volume A(b)(t)/C(p)(t) to the steady state values (for a continuous constant rate drug infusion) are derived. It is shown that the time course of A(b)(t) near the asymptotic steady state value depends on both the terminal and steady state volumes of distribution, and an accurate equation to determine the time required to reach the steady state is obtained. For a general linear pharmacokinetic system (i.e., with possible drug elimination at any state from any compartment and drug exchange between compartments) it is proven that V(beta) >/= V(ss). A physiologically determined feature, which is the drug input into plasma for reaching the steady state or terminal phase, underlies the proof. If the steady state is reached by a continuous input of drug into some compartment other than plasma, and the terminal volume of distribution is considered after dosing of a drug in the same compartment, then both cases V(ss) <> V(beta) are possible. It is shown that the general exponential series for C(p)(t) after intravenous bolus dose may have negative pre-exponents, unlike a common assumption that all pre-exponents should be positive. Its is figured out that the commonly used equations for the estimation of V(ss) and V(beta) (V(ss) = D x AUMC/AUC(2) and V(beta) = D/(AUC x beta) may yield V(ss) > V(beta) for a linear pharmacokinetic system, contrary to the usual statement (V(ss) < V(beta)) and its seemingly simple proof, which has a flaw. It is shown that the time required to reach the steady state concentration of drug in plasma could be much shorter than a commonly used estimation of 5t(1/2), where t(1/2) is the terminal half-life obtained from the intravenous bolus drug plasma concentration time course.

Critical analysis of the discrepancy betweenV? andVss for drugs exhibiting different two-compartment disposition profiles

It is well known that in the two-compartment open body model the values of apparent volume of distribution (V(beta)) and volume of distribution at steady state (V(ss)) are never identical. There are at least two conditions when V(beta) significantly overestimates V(ss). The first is when most of a drug is eliminated relatively rapidly but a small fraction of the dose persists and gives rise to an extremely long half-life. The second is when a drug is rapidly cleared from the central compartment with a short half-life. The primary purpose of the current paper was to investigate how different two-compartment disposition profiles affect the magnitude of difference between V(beta) and V(ss). Novel equations have been developed that relate the V(beta)/V(ss) ratio to f1 (fraction of drug elimination associated with the distributive phase) and to beta/alpha (ratio of the exponential coefficients). This paper demonstrates mathematically that an increasing value of f1 is associated with a greater divergence between V(beta) and V(ss). A similar relationship was also found for the divergence between the terminal half-life (t(1/2beta)) and the mean residence time (MRT). An increase in the beta/alpha ratio results in a substantial decrease of this discrepancy and provides a maximal possible value, or an upper limit to the V(beta)/V(ss) ratio. The newly derived equations along with their graphical presentation may serve as an excellent predictive tool for checking the accuracy of the experimentally obtained values of V(beta) and V(ss).

Assuming peripheral elimination: its impact on the estimation of pharmacokinetic parameters of muscle relaxants

For anesthetic drugs undergoing nonorgan-based elimination, there is a definite trend towards using pharmacokinetic (PK) models in which elimination can occur from both central (k10) and peripheral compartments (k20). As the latter cannot be assessed directly, assumptions have to be made regarding its value. The primary purpose of this paper is to evaluate the impact of assuming various degrees of peripheral elimination on the estimation of PK parameters. For doing so, an explanatory model is presented where previously published data from our laboratory on three muscle relaxants, i.e., atracurium, doxacurium, and mivacurium, are used for simulations. The mathematical aspects for this explanatory model as well as for two specific applications are detailed. Our simulations show that muscle relaxants having a short elimination half-life are more affected by the presence of peripheral elimination as their distribution phase occupies the major proportion of their total area under the curve. Changes in the exit site dependent PK parameters (Vdss) are also mostly significant when k20 is smaller than k10. Although the physiological processes that determine drug distribution and those affecting peripheral elimination are independent, the two are mathematically tied together in the two-compartment model with both central and peripheral elimination. It follows that, as greater importance is given to k20, the rate of transfer from the central compartment (k12) increases. However, as a result of a proportional increase in the volume of the peripheral compartment, peripheral concentrations remain unchanged whether or not peripheral elimination is assumed. These findings point out the limitations of compartmental analysis when peripheral elimination cannot be measured directly.

Prospective use of population pharmacokinetics/pharmacodynamics in the development of cisatracurium

PURPOSE

The population PK/PD approach was prospectively used to determine the PK/PD of cisatracurium in various subgroups of healthy surgical patients.

METHODS

Plasma concentration (Cp) and neuromuscular block data from 241 patients in 8 prospectively-designed Phase I-III trials were pooled and analyzed using NONMEM. The analyses included limited Cp-time data randomly collected from 186 patients in efficacy/safety studies and full Cp-time data from 55 patients in pharmacokinetic studies. The effects of covariates on the PK/PD parameters of cisatracurium were evaluated. The time course of neuromuscular block was predicted for various patient subgroups.

RESULTS

The population PK/PD model for cisatracurium revealed that anesthesia type, gender, age, creatinine clearance, and presence of obesity were associated with statistically significant (p < 0.01) effects on the PK/PD parameters of cisatracurium. These covariates were not associated with any clinically significant changes in the predicted recovery profile of cisatracurium. Slight differences in onset were predicted in patients with renal impairment and patients receiving inhalation anesthesia. Based on the validation procedure, the model appears to be accurate and precise.

CONCLUSIONS

The prospective incorporation of a population PK/PD strategy into the clinical development of cisatracurium generated information which influenced product labeling and reduced the number of studies needed during development.

Alternative continuous infusion method for analysis of enterohepatic circulation and biliary excretion of cefixime in the rat

The enterohepatic circulation and biliary excretion of cefixime during continuous infusion were evaluated in rats based on the recirculatory concept. The Laplace-transformed equations for the enterohepatic circulation according to this concept were derived by means of the combination of transfer function. The transformed equations were simultaneously fitted to the time courses of plasma concentration in rats with laparotomy and with bile duct cannula by means of a nonlinear regression program, MULTI(FILT), into which the fast inverse Laplace transform was incorporated. The optimum model was selected on the basis of Akaike's information criterion (AIC). The time course of drug accumulation in the bile during infusion starts with a relatively gentle slope and finally approaches the asymptote with a constant slope. The kinetic significance of this asymptote was explained using the time courses of the cumulative amount excreted into the bile of rats with bile duct cannulation. The local moment characteristics for a single pass through enterohepatic circulation were further calculated from the time courses of both the plasma concentration and the excreted amount into the bile. The recovery ratio (Fc) and the mean circulatory time (tc) through a single pass of enterohepatic circulation were estimated to be 31.1% and 0.925 h, respectively. The recovery ratio (Fa) and the mean transit time (ta) for the complicated process from the access to the bile duct into the systemic circulation such as transport through the bile duct, absorption from the intestinal tract, and transit through the portal system were 76.4% and 0.0231 h, respectively. The recovery ratio (Fb) and the mean transit time (tb) for the disposition process through the systemic circulation into the bile were 40.7% and 0.902 h, respectively.

Absorption and disposition of a new antiarrhythmic agent bidisomide in man

Absorption and disposition of bidisomide were studied in 12 healthy male subjects after a 20-min iv (1 mg/kg; N = 6) infusion and oral (2 mg/kg; N = 6) administration of the 14C-labeled drug. The oral absorption profile of unlabeled bidisomide was also studied after administration of a solution by a nasoenteric tube to different sites of the gastrointestinal tract (stomach, duodenum, jejunum, and ileum). The systemic availability was 61%. Absorption was slow initially and then rapid, achieving peak plasma concentrations between 2 and 4 hr. Less than complete systemic availability was attributed to incomplete absorption rather than first-pass metabolism. When the drug solution was delivered directly to the stomach, two distinct peak plasma levels were found. This was attributed to the more rapid absorption of bidisomide in the duodenum and ileum (and/or possibly colon). Following an iv dose, plasma levels of the drug declined with mean half-lives of 0.11, 2.0, and 12 hr for alpha, beta, and gamma phases, respectively, and a plasma clearance of 380 mL/min. The percentages of the dose recovered as bidisomide in urine and feces were 19 +/- 1 and 29 +/- 4 for the iv dose and 9.1 +/- 0.9 and 48 +/- 5 for the oral dose. Bidisomide did not exhibit substantial enantioselective pharmacokinetics in plasma regardless of the route of administration. The mean urinary excretion of the (-) enantiomer was, however, slightly higher than that of the (+) enantiomer, with (-)/(+) enantiomeric ratios of 1.2 and 1.3 after iv and oral administration, respectively. The enantiomeric ratio of bidisomide recovered in the feces was approximately 1.

Pharmacokinetics of sulphadimethoxine in channel catfish (Ictalurus punctatus)

1. Plasma clearance, bioavailability, tissue disposition and elimination of 14C-sulphadimethoxine (SDM) were studied in channel catfish (Ictalurus punctatus) after intravenous (i.v.) and oral dosing (per os; p.o.) at 40 mg/kg body weight. 2. Analysis of blood SDM concentrations over time for intravascularly administered SDM showed that disposition and elimination were best described by a two-compartment pharmacokinetic model; estimated half-lives for SDM in blood were 0.09 and 12.6 h for the distribution and elimination phases, respectively. 3. SDM was found primarily in muscle tissue immediately after oral administration; however, clearance from muscle was rapid, with a half-life of 13.1 h. 4. With time, SDM-derived radioactivity became concentrated in the bile and was eliminated slowly (t 1/2 = 115.5 h). 5. Binding of SDM in channel catfish plasma was low (18%) and was non-specific and dose-independent. 6. With the exception of the initial, rapid clearance of SDM from blood, the pharmacokinetic parameters describing SDM distribution and elimination in channel catfish were similar to values reported for other vertebrate species; the rapid distribution of SDM from blood to the tissues in the catfish may be related to species differences in the plasma binding of SDM.

Time-dependent effects of theophylline on myocardial reactive hyperaemias in the anaesthetized dog

1. The effects of a loading dose of theophylline (5 mg kg-1 i.v.) on the hyperaemias resulting from short-term (15 and 30 s) interruptions in coronary blood flow and intracoronary adenosine were studied at given intervals over a 2 h period in the anaesthetized dog. 2. These hyperaemic responses were affected differently by theophylline and each effect was time-dependent. The reactive hyperaemic response progressively decreased after drug delivery, reaching 46% of control at 2 h. In contrast, after a maximal attenuation to 23% of control 5 min after theophylline, the hyperaemia resulting from intracoronary adenosine progressively increased over the same period, reaching 64% of control 2 h after the loading dose. 3. Two-compartment model results based on plasma theophylline measurements and the time course of theophylline accumulation in pericardial infusates, suggested that complete drug distribution throughout the heart may require at least 20 min following a single intravenous dose. 4. If it is assumed that theophylline blocks coronary vascular adenosine receptors, these pharmacokinetics are consistent with the time-dependent pattern of response attenuation we observed for the adenosine-induced hyperaemias, but they cannot entirely explain the pattern of response attenuation observed for the occlusion-induced hyperaemias. The continued increase in attenuation of this response after complete drug distribution suggests an additional pharmacodynamic action of theophylline. 5. We conclude that a single therapeutic dose of theophylline results in distinct time-dependent pharmacological effects with respect to the ability of the coronary vasculature to dilate in response to temporary interruptions in oxygen supply and in response to exogenously administered adenosine. These effects deserve consideration in both experimental studies in which adenosine antagonists are used to assess adenosine action in vivo, and in clinical practice where theophylline pharmacotherapy for pulmonary disorders is commonplace.

A new potential prodrug to improve the duration of L-dopa: L-3-(3-hydroxy-4-pivaloyloxyphenyl)alanine

L-3-(3-Hydroxy-4-pivaloyloxyphenyl)alanine (1, NB-355) is a novel L-dopa prodrug. After oral administration with carbidopa in rats, 1 demonstrated 2.3 times longer duration (MRT) and 1.4 times larger bioavailability (AUC) on plasma L-dopa concentrations than those of L-dopa itself. Similar results were obtained in dogs. The prolonged profile of L-dopa was parallel to that of carbidopa, and the intact ester was undetectable in rat plasma. After intravenous administration in rats, 1 was converted quickly and completely to L-dopa in the systemic circulation. It was also noted that the oral LD50 value of 1 was greater than 6 g/kg in mice. These data suggest that 1 will offer long-lasting L-dopa therapy for the treatment of Parkinson's disease with little concern about toxicity.

Erythromycin lactobionate: pharmacokinetics and uterine tissue levels

Erythromycin is well-known for its properties of tissular diffusion. An estimation of its concentration in uterine tissue seemed worthwhile, in order to justify the use of erythromycin lactobionate in uterine infections by sensitive micro-organisms. The authors studied the uterine levels of this macrolid, after perfusion of 1g of erythromycin lactobionate, over a period of one hour. The protocol involved a group of 15 women, who were to undergo a hysterectomy for benign non-inflammatory pathology. The uterine specimens and plasmatic pharmacokinetics of the antibiotic constitute the data. The concentrations of antibiotic were determined by microbiological methods. 2 sets of results emerge: during perfusion, the uterine erythromycin levels are equal or inferior to simultaneous serum levels. after perfusion, the uterine levels are superior to serum levels. A bicompartmental linear model was used, which simulated the tissular levels in standard treatment with erythromycin lactobionate. If the erythromycin 4 micrograms/ml critical concentration is compared with uterine tissue concentration, the antibiotic may be expected to be active against a uterine affected by sensitive micro-organisms during a period of approximately 5 hours. This result must be confirmed by a clinical study.

Pharmacokinetics of ethynyl estradiol: a current view

There are large discrepancies in the values reported for the parameters of ethynyl estradiol pharmacokinetics. Economies in sampling frequency (in the hope that computer simulation will serve the purpose), difficulties in measurement of plasma levels resulting from the small doses used, extensive oxidative metabolism, distribution, and enterohepatic circulation all complicate the effort to obtain reliable data. The problems in quantitating various pharmacokinetic parameters and the techniques for their evaluation, as well as methods for the determination of the requisite sampling times and frequencies are discussed, and best estimates for the various parameters, as derived from the literature, are provided.

Comparative pharmacokinetics of YM-13115, ceftriaxone, and ceftazidime in rats, dogs, and rhesus monkeys

The pharmacokinetics of YM-13115, ceftriaxone, and ceftazidime were studied in rats, dogs, and rhesus monkeys (only YM-13115 and ceftriaxone were studied in rhesus monkeys). The plasma half-lives in rats were 48 min for YM-13115, 34 min for ceftriaxone, and 14 min for ceftazidime. In dogs, they were 21.9 min for YM-13115, 50.7 min for ceftriaxone, and 49.0 min for ceftazidime. In monkeys, they were 5.30 h for YM-13115 and 3.40 h for ceftriaxone. The 24-h urinary recoveries in rats were 26.7% of the dose for YM-13115, 32.0% for ceftriaxone, and 97.1% for ceftazidime. In dogs, they were 13.3% for YM-13115, 62.5% for ceftriaxone, and 86.3% for ceftazidime. In monkeys, they were 22.5% for YM-13115 and 29.3% for ceftriaxone. The 24-h biliary recoveries in rats were 72.2% for YM-13115, 61.8% for ceftriaxone, and 0.63% for ceftazidime.

Theorem on the apparent volume of distribution and the amount of drug in the body at steady state

For an N-compartmental system, with irreversible drug loss from the sampled compartment, the equilibrium concentration, C (infinity), obtained with a zero-order drug input is related to the total amount of drug in the system, T, by C (infinity) V = T. The scalar V is the volume of distribution of the corresponding closed system. The moment functions of the open system define V, and hence T is directly calculable. The derivation is general in the sense that the topology of the system is not specified and no functional form for C (t) is required.

Pharmacokinetics of cefpiramide (SM-1652) in humans

The pharmacokinetics of cefpiramide (SM-1652) were studied after the intravenous administration of single or multiple doses to 21 healthy volunteers. The cefpiramide concentration in plasma at time zero after a bolus intravenous injection of 500 or 1,000 mg was 152 or 303 micrograms/ml, respectively. The maximum cefpiramide level in plasma at the end of a 1-h infusion of 1,000 or 2,000 mg was 166 or 317 micrograms/ml, respectively. The mean plasma half-life of cefpiramide in 15 subjects who received a single dose of 500 or 1,000 mg was 4.44 h. There was no evidence of drug accumulation in plasma when 500 or 1,000 mg of cefpiramide was administered 11 times at 12-h intervals. Urinary excretion of cefpiramide over a 24-h period was ca. 22.5%, regardless of the intravenous administration technique and the dosage. Fecal recoveries of cefpiramide varied from 0 to 36.9% in different subjects.

Interspecies similarities in the disposition of 3H-dihydroergotamine following subcutaneous administration in man and rabbits

The disposition of dihydroergotamine methanesulfonate following single subcutaneous doses was studied in man and the rabbit using radiotracer techniques. 3H-Dihydroergotamine was almost immediately and completely absorbed from the injection site; peak blood radioactivity levels were attained within 1 h of drug administration in both species. The disappearance of radioactivity from blood was biphasic, with t 1/2,alpha and t 1/2,beta values of 2.9 and 16.9 h, respectively, in man and 2.9 and 14.7 h, respectively, in the rabbit. Apparent volumes of distribution were 18.9 Liter/kg in man and 30.4 Liter/kg in the rabbit. The excretion pattern of dihydroergotamine and its metabolites was also similar for the two species, with biliary elimination being the predominant route. At 4-5 days postdosing, 80-85% of the administered radioactivity was recovered in the feces and urine. The rabbit appears to be an adequate animal model for the study of dihydroergotamine pharmacokinetics in man.

Kinetic profile in blood and brain of the cholinesterase reactivating oxime HI-6 after intravenous administration to the rat

The kinetic profile of the oxime HI-6, a potent cholinesterase reactivator, after iv administration of 0.1325 mmol/kg (50 mg/kg) to rats is described. The blood concentrations measured over a period of 300 min can be described by a two-compartment open model. Excretion occurred only by the kidney. Approximately 60% was excreted in unmetabolized form. Brain tissue concentrations were significantly above the detection limit of the HPLC analysis procedure even when a correction was made for the amount of HI-6 present in brain blood. The concentration in brain tissue is built up according to a rapid equilibration mechanism. Disappearance of HI-6 from brain occurred slowly compared to the elimination of HI-6 from blood. These findings are discussed in the light of the existing knowledge on the kinetic behavior of polar solutes in the central nervous system. Whether the concentrations of HI-6 built up in the brain are relevant for the therapy of organophosphate intoxications cannot be determined from the experiments described.

Is volume of distribution at steady state a meaningful kinetic variable?

Pharmacokinetic volumes of distribution (Vd) are commonly calculated either by the steady-state method (Vdss) or the area method (Vdarea). Vdss is traditionally perceived as the least biased and most reliable indicator of the extent of distribution, but Vdss in fact has far greater practical and theoretical limitation than does Vdarea. After single doses or multiple discrete doses of a drug, Vdarea correctly relates plasma concentration to amount of drug in the body at all times after distribution equilibrium is attained. Vdss, on the other hand, is a correct proportionality constant only during continuous intravenous infusion or at a single instant in time after discrete dosing. Furthermore, calculated values of Vdss are strongly dependent on the precise configuration of the initial distributional phase of the plasma concentration curve, which may be difficult or impossible to delineate because of variance arising from methodologic artefacts or unexplained causes. Such variance can lead to large nonphysiologic within- and between-individual variability in Vdss. Vdarea, on the other hand, is relatively independent or artefactual changes in the initial distribution profile. Finally, experimental observations indicate that elimination depends physiologically on distribution in the absence of changes in clearance, not the reverse. The relation of distribution and elimination holds whether the steady-state method or the area method is used to calculate Vd. Thus, Vdarea is a more reliable and generally valid descriptor of the extent of drug distribution than is Vdss.

Estimation of absolute bioavailability assuming steady state apparent volume of distribution remains constant

The limitations of using estimates of extent of bioavailability (F) based on the assumption that either clearance (CL) or Varea remain, constant are discussed in relation to the situation where CL changes between doses. When estimates of F assume CL to remain constant, the extent of the error is the same for all drugs where the percentage change in CL is the same. Assuming Varea to remain constant, the error in F will vary between drugs for similar percentage changes in CL and is related to the extent to which the kinetics of the disposition process deviate from a one compartment body model. A noncompartmental method is described where, provided the reference dose is given intravenously, F can be estimated based on the assumption that Vss remains constant between doses. This method is more accurate than those based on the assumption that either CL or Varea remain, constant when CL changes between doses, but is subject to error when the terminal log-linear slope of Cp vs. time better reflects the process of absorption rather than elimination.

Some considerations on the estimation of steady state apparent volume of distribution and the relationships between volume terms

Application of statistical moment theory to four methods which do not involve detailed compartmental analysis for the determination of Vss shows them to be equal. Assuming drug to be eliminated exclusively from the central compartment results in the minimum value of Vss being determined. A method for determining the maximum possible value of Vss is shown which uses AUC, dose (iv), and the exponents which describe the plasma-concentration time curve. The relationships between the volume terms Vextrap, Varea and Vss are discussed in terms of moment theory.

Pharmacokinetic evaluation of the blood-to-lymph transfer of cyclosporin A in rats

The blood-to-lymph transfer kinetics of cyclosporin A (CyA) were investigated in thoracic duct cannulated rats after a 3 mg kg-1 i.v. bolus injection of tritium-labelled drug. Blood CyA concentrations declined bi-exponentially with a terminal half-life of 13.1 +/- 1.8 h. The appearance rate of CyA in the lymph was also bi-phasic, initially rising and then declining in parallel with the concentrations of CyA in the blood. A pharmacokinetic model describing the blood-to-lymph transfer kinetics of CyA is presented. From the slopes of graphs relating the appearance rate of CyA in the lymph to the corresponding midpoint blood drug concentration, the estimated blood-to-lymph clearance rate of CyA (ClL)b was 0.262 +/- 0.132 ml h-1. (ClL)b was positively correlated with the average lymph flow rate (QL) but was significantly less than QL. On the other hand, the plasma-to-lymph clearance rate of CyA (ClL)p of 0.393 +/- 0.198 ml h-1 was positively correlated and similar to QL. Lymph concentrations of CyA were approximately 40-60 per cent of the corresponding blood concentrations. The results of this investigation showed the existence of an intervening compartment between the blood and lymph fluids which would be consistent with the presence of lymph nodes. It was shown that the blood-to-lymph transfer of CyA was dependent on the flow rate of the lymph and that CyA appeared to be cleared from the plasma (or serum) fraction of the blood.

Quinine pharmacokinetics and toxicity in cerebral and uncomplicated Falciparum malaria

Acute pharmacokinetics of intravenously infused quinine were studied in 25 patients with cerebral malaria and 13 with uncomplicated falciparum malaria. In patients with cerebral malaria receiving the standard dose of 10 mg/kg every eight hours, plasma quinine concentrations consistently exceeded 10 mg/liter, reaching a peak 60 +/- 25 hours (mean +/- 1 S.D.) after treatment was begun and then declining. Quinine total clearances (Cl) and total apparent volumes of distribution (Vd) were significantly lower than in uncomplicated malaria (Cl, 0.92 +/- 0.42 compared with 1.35 +/- 0.6 ml/min/kg, p = 0.03; Vd, 1.18 +/- 0.37 compared with 1.67 +/- 0.34 liter/kg, p = 0.0013). There was no significant difference between the two groups in elimination half-times (t/2) or renal clearances (Cu) (t/2, 18.2 +/- 9.7 compared with 16 +/- 7.0 hours; Cu, 0.21 +/- 0.16 compared with 0.21 +/- 0.08 ml/min/kg). In nine patients studied following recovery, Cl (3.09 +/- 1.18 ml/min), Vd (2.74 +/- 0.47 liter/kg), and Cu (0.53 +/- 0.22 ml/min/kg) were significantly greater (p less than or equal to 0.0004), and t/2 was significantly shorter (11.1 +/- 4.1 hours, p = 0.006) than during the acute illness. Cu accounted for approximately 20 percent of Cl in all groups. Renal failure did not alter the disposition kinetics in cerebral malaria. There was no clinical or electrocardiographic evidence of cardiotoxicity and no permanent neurotoxicity. Quinine toxicity in cerebral malaria has probably been overemphasized. The benefits of high plasma concentrations in the acute phase of this life-threatening disease appear to outweigh the risks, particularly in view of the increasing resistance of Plasmodium falciparum to quinine in Southeast Asia.

Comparative pharmacokinetics of benzodiazepines in dog and man

The pharmacokinetic parameters disposition half-life, metabolic clearance, volume of distribution, intrinsic clearance of unbound drug, and (distributive tissue volume/unbound fraction in tissue) were compared for 12 benzodiazepines in dog and man. With the exception of volume of distribution, statistically significant correlations were obtained when parameters were plotted on a double logarithmic grid. In general, benzodiazepines were metabolized more rapidly and exhibited greater tissue distribution in dog than in man. The variability in parameters was such, however, as to make extrapolations from one species to another subject to considerable error.

Glycerol clearance in alcoholic liver disease

Glycerol clearance was studied by a primed dose-constant infusion technique in 14 patients with alcoholic liver disease and six normal control subjects. Fasting blood glycerol concentrations were raised in the alcoholic subjects (0.09 +/- 0.01 vs 0.06 +/- 0.01 mumol/l, p less than 0.05) and glycerol clearance was impaired (24.5 +/- 1.9 vs 37.5 +/- 3.2 ml/kg/min, p less than 0.005). Endogenous production rate of glycerol and distribution space at steady state were similar in alcoholic and control subjects. The metabolic clearance rate of glycerol correlated negatively with basal glycerol concentrations. Thus tissue uptake of glycerol is impaired in liver disease. As glycerol is metabolised primarily in the liver by conversion to glucose, these data suggest a defect of gluconeogenesis in alcoholic liver disease.

Pharmacokinetics of theophylline in patients following short-term intravenous infusion

Serum theophylline concentrations after intravenous administration of a new short-term infusion (Euphyllin Kurzzeitinfusion) were measured in 50 out-patients with chronic obstructive airways disease (COAD). An intravenous infusion of theophylline ethylenediamine 480 mg (corresponding to approximately 350 mg anhydrous theophylline) in 50 ml isotonic solution was given in 20 min. Blood samples were taken beforehand and 25 to 30 min and 1, 3 and 6 h after starting the infusion. 86% of the patients had a one-hour serum level in he therapeutic range of 8.20 mg/l, and 2 h later, this was true of 64% of the patients. The short-term infusion was well tolerated, even in cases with unknown high pre-infusion serum levels. Pertinent pharmacokinetic parameters were determined, such as total body clearance, apparent volume of distribution, and half-life of elimination. Geometric mean an 95%-confidence limits, derived from the log-normal distribution of these parameters, were: Cl = 0.044 (0.018-0.190) l/h/kg ideal body weight, Vd = 0.451 (0.258-0.789) l/kg ideal body weight, and t 1/2(el) = 7.1 (2.6-19.1) h.

Pharmacokinetics of 3H-phenylephrine in man

7-3H-phenylephrine was given to 15 volunteers by a short-infusion n = 4) or p.o. (10 volunteers, 1 patient with porto-caval anastomosis). Analysis of serum for free 3H-phenylephrine and fractionation of urinary radioactivity was performed by ion-exchange and thin-layer chromatography. As almost the same 3H-activity was excreted in urine after i.v. and p.o. administration, 86% and 80% of the dose respectively, complete enteral absorption can be assumed. A considerable difference was seen in the fraction of free phenylephrine, i.v. 16% of the dose versus p.o. 2.6%, which suggested reduced bioavailability. This was confirmed by comparison of the areas under the serum curve, which showed a bioavailability factor of 0.38. The result for the patient with porto-caval anastomosis was comparable to that in the normal volunteers. The biological half-life of 2 to 3h was comparable to that of structurally related amines, as were the total clearance of 2 l/h, and the volume of distribution of 340 l. Metabolism to phenolic conjugates mainly after oral ingestion, and to m-hydroxymandelic acid after i.v. injection, again demonstrated that m-hydroxylated amines are predominantly conjugated during the "first-pass" metabolism.

Plasma propranolol concentrations in rats with adjuvant-induced arthritis

Previous work has shown that after a single oral dose, plasma propranolol concentrations in patients with active inflammatory disease are significantly higher than those in healthy subjects. After oral administration of propranolol (2 mg) to arthritic rats, the area under the mean drug concentration-time curve in plasma was approximately 10 times greater than that in control animals. After intravenous administration (0.25 mg) the area in arthritic rats was approximately doubled compared with that in controls. The mechanisms causing these changes are not known, but it is probable that increased drug binding to an acute phase reactant in plasma, together with decreased first-pass hepatic clearance in arthritic rats after oral dosing, are involved.

The effect of probenecid on the pharmacokinetics and distribution of cefoxitin in healthy volunteers

1 Cefoxitin was given by acute intravenous injection to six healthy volunteers, in a crossover study to investigate the effects of concurrent probenecid administration. 2 Serum antibiotic concentrations were determined by microbiological assay. Cefoxitin concentrations were simultaneously determined in the fluid of blisters produced by topical cantharides. All antibiotic was accounted for in the urine. 3 Cefoxitin was administered by intravenous infusion, subsequent to a loading dose, to produce steady state levels in the region of 10 microgram/ml, in one volunteer. The procedure was later repeated after prior administration of probenecid in the same subject. 4 Pharmacokinetic analyses indicated significant changes only in the parameters associated with renal excretion of drugs. Clearance was reduced by half. 5 The absolute and relative amounts of antibiotic in the central and peripheral compartments were calculated for both modes of administration. In the acute study probenecid produced a small change in distribution away from the peripheral or tissue compartment, towards the central compartment. 6 There was no elevation of initial serum concentrations and sustained levels of antibiotic could be accounted for principally by retarded excretion produced by probenecid, with little contribution by alteration in the disposition of antibiotic. 7 The sustained serum levels of cefoxitin that resulted from its decreased excretion were also reflected in blister fluid. It was concluded that the sustained cefoxitin levels produced by probenecid resulted in similar raised levels in the peripheral or "tissue' compartment, since the redistribution away from the peripheral compartment did not contribute materially to other changes in disposition of drug.

Comparative pharmacokinetics of quinidine and its O-desmethyl metabolite in rabbits

The pharmacokinetics of quinidine and the O-desmethyl metabolite 6'-hydroxycinchonine were studied in rabbits. After intravenous bolus injections of equimolar doses, the blood concentration-time curves of each agent declined biexponentially, which was characteristic of a two-compartment open model. A significant difference between quinidine and 6'-hydroxycinchonine was observed for the following parameters: t1/2 alpha, beta, t1/w beta, k12, Vd beta, and the intercompartmental distribution ratio, k12/k21. The results showed that the distribution of the metabolite was slower than quinidine and suggested that it also was less extensive. The Vd beta value for 6'-hydroxycinchonine was approximately one-half of the value observed for quinidine, and its k12/k21 ration was about one-fourth of the quinidine value. The terminal half-lives for quinidine and the metabolite were 132.4 +/- 27.1 and 65.4 +/- 34.4 min, respectively. Total body clearance was similar for both compounds. The findings of this study could be explained by the greater polarity or greater water solubility of 6'-hydroxycinchonine. The limited data in humans coupled with the results of this investigation suggest that, although it is intrinsically active, the O-demethylated metabolite probably contributed little to the antiarrhythmic effects seen after quinidine administration in humans.

Simplified method for estimating volume of distribution at steady state

Current methods for measuring the volume of distribution at steady state (Vdss) require extrapolation of the plasma concentration curve from time zero to infinity (AUC0-infinity). A new method for measuring Vdss is described. This method saves time since it requires following the plasma concentration curve only until a steady state is reached. The Vdss of sulfamethizole was measured simultaneously by the new method (2 hr) and by established methods (7 hr) in six dogs. There was not significant difference between the results obtained by the two methods.

Influence of sulfaethidole on the human pharmacokinetics of dicloxacillin

In a five-patient crossover study, serum levels of dicloxacillin after intravenous administration of dicloxacillin in the absence and presence of sulfaethiodole were measured. Significantly greater serum concentrations of dicloxacillin were noted when dicloxacillin was administered with sulfaethidole. Pharmacokinetic evaluation of the data suggests that the higher serum concentrations were primarily the result of changes in the extravascular distribution for dicloxacillin in the presence of sulfaethidole. Although examination of the distribution rate constants for dicloxacillin in a two-compartment open model would suggest a lowering of serum concentrations, the experimental data clearly indicate that the serum and tissue compartment dicloxacillin concentrations increased in the presence of sulfaethiodle, indicating that protein binding in the central as well as extravascular compartments could be affected by sulfaethidole.

Pharmacologic studies with radiolabeled N-trifluoroacetyladriamycin-14-valerate (AD 32). Comparison of total anthracycline fluorescence and radioactivity in mouse serum and urine

In connection with pharmacologic studies with AD 32, isotopically-labeled drug prepared from 1-[14C]-trifluoroacetic anhydride and adriamycin-14-valerate was used to determine murine serum and urine levels of radioactivity. Other studies, performed in parallel, measured serum and urinary total fluorescence. Serum fluorescence disappeared in a biphasic pattern, with an initial rapid rate of disappearance followed by a somewhat slower phase. For the first hour, serum radioactivity levels were not significantly different than those measured by fluorescence. After this, however, serum radioactivity decayed at a much slower rate than did fluorescence. Furthermore, a large fraction of the injected radioactivity was found excreted in the urine, whereas urine accounted for only a small fraction of the fluorescence. These results suggest the formation, in part, of a hitherto unrecognized nonfluorescent metabolite, most probably N-trifluoracetyldaunosamine.