Ronald D. Schoenwald

Furosemide (frusemide). A pharmacokinetic/pharmacodynamic review (Part I).

SummaryFurosemide (frusemide) is a potent loop diuretic used in the treatment of oedematous states associated with cardiac, renal and hepatic failure, and for the treatment of hypertension. Therapy is frequently complicated by apparently erratic systemic availability from the oral route and from unpredictable responses to a given dosage. The exact mechanism of action is not fully understood, but furosemide is believed to act at the luminal surface of the ascending limb of the loop of Henle by inhibiting the active reabsorption of chloride. The response to a given dosage is modulated by the fluid and electrolyte balance of the individual. Acute and delayed tolerance has been demonstrated both in animals and in man, and is postulated to be due to the intervention of homeostatic mechanisms influencing fluid and electrolyte balances. Furosemide is delivered to its site of action by active secretion via the nonspecific organic acid pump. Comparisons between the observed diuresis/saluresis and plasma furosemide concentrations, urinary excretion rates and renal clearance found either negative or no correlations with plasma drug concentration but significant correlations with urine measurements. Response is related to the concentration of the drug in urine rather than in plasma. The most common adverse reactions attributable to furosemide therapy are essentially extensions of the therapeutic effects (i.e. fluid and electrolyte disturbances).The pharmacokinetic behaviour of furosemide is marked by a large degree of variability, derived from differences within and between both subjects and study protocols. Part of this variability can be attributed to differences in organ function, which is important in view of the types of patients treated with furosemide. On the other hand, a large proportion remains as inter- and intrasubject variation.The bioavailability of furosemide from oral dosage forms is highly variable. The poor bioavailability has been hypothesised to be due to the poor solubility of the compound, site-specific absorption, presystemic metabolism and/or other unknown mechanisms. Furosemide is highly bound to plasma proteins, almost exclusively to albumin. Although the drug is insoluble in water and favours partitioning into fatty tissue, the high degree of plasma protein binding restricts the apparent volume of distribution at steady-state to values within a multiple of 2 to 5 times the plasma volume. Furosemide has two documented metabolites — furosemide glucuronide and saluamine (CSA). The first is an accepted metabolic product, whereas the status of CSA as a metabolite is highly controversial. The half-life reported for furosemide in normal subjects generally falls in the range of 30 to 120 minutes, but is influenced by underlying disease processes: for example, in patients with end-stage renal disease without other organ impairment it averages 9.7 hours.Since the site of action of furosemide is the luminal surface of the ascending limb of the loop of Henle, the fraction of the dosage excreted unchanged in the urine represents the fraction which is potentially available for pharmacological action. Approximately one-half to two-thirds of an intravenous dosage or a quarter to one-third of an oral dosage will actually be available at the site of action. This general finding is altered by factors which alter the bioavailability and/or urinary delivery of the drug. Clinical nonresponders tend to have decreased excretion percentages.The dose-response relationship of furosemide entails a linear pharmacokinetic relationship superimposed on a nonlinear pharmacodynamic relationship, and the mathematical model deemed most appropriate for the characterisation of the observed pharmacodynamic behaviour is a 4-parameter logistic function.Clinically, furosemide is used by large numbers of diseased patients on a long term basis. The majority of the knowledge that is currently available on its pharmacodynamics is based on the investigation of healthy, drug-free subjects receiving single doses and undergoing concurrent rehydration. This information is useful in delineating the impact of a variety of factors which influence the dose-response relationship, but does not provide the clinician with the answers to important questions regarding the specifics of the therapeutic application of furosemide in diseased populations.

Pharmacokinetics in drug discovery and development

BASIC PRINCIPLES Basic Principles INDUSTRIAL AND REGULATORY APPLICATIONS PK/PD Approach Pharmacokinetics and Metabolism in Drug Discovery and Preclinical Development Phase I, II, and III FDA Submissions Bioavailability and Bioequivalence CLINICAL APPLICATIONS General Approaches to Clinical Pharmacokinetic Monitoring Aminoglycosides and Other Antibiotics Cardiovascular Agents Psychotropic Agents Theophylline Anticonvulsant Agents RESEARCH APPLICATIONS Classical Modeling Noncompartmental Modeling, An Overview Physiologically-Based Pharmacokinetic Models Population Pharmacokinetics The Linear Systems Approach

Ocular pharmacokinetic models of clonidine-3H hydrochloride

A single topical instillation of clonidine-3H HCl solution (0.2%) was administered to the rabbit eye (30 μl) in order to study the drugs ocular pharmacokinetics. Seven different tissues and plasma were excised and assayed for drug over 180min. By 45–60 min pseudoequilibrium is reached for the cornea, iris/ciliary body, and aqueous humor. Thereafter, drug levels in these tissues decline in parallel. The data are fit separately to a physiological model and a classical diffusion model for which seven ocular tissue compartments and a plasma reservoir are constructed for each model. Clearance terms and distribution equilibrium coefficients are determined from the tissue level data and used as parameters in fitting the mass balance differential equations representing the physiological model. The model parameters can also be fit to a 0.4% single dose. In a separate experiment, a topical infusion technique was designed to provide a constant rate input to the cornea until an apparent steady state was reached in aqueous humor at 55 min. Aqueous humor levels were assayed for clonidine over the infusion and postinfusion periods. The physiological model parameters are fit to the topical infusion data and show good agreement between the predicted and experimental data. The classical model is too complex to fit the data to integrated exponential equations primarily because the method of residuals is inadequate in determining a sufficient set of initial estimates. This is overcome by dividing the eight-compartment model into seven fragmental models, each representing one to five compartments. A stepwise procedure is developed in which initial estimates are obtained for each separate fragmental model and refined. The refined parameter values can then be used as initial estimates for the complex model. Differential equations for the complex model are fit simultaneously to tissue levels representing each compartment. By observation, the classical model fit the data more closely than the physiological model. Statistical moment theory is also applied to the topical infusion data to determine ocular pharmacokinetic parameters for clonidine. The calculated values are: corneal absorption rate constantka, 0.00139 min−1; aqueous humor elimination rate constantk10, 0.0658min−1; mean residence timeMRTd, 35.6 min; apparent steadystate volume of distributionVss, 0.530 ml; and ocular clearanceQe, 14.9 =μl/min. The fraction absorbed from the single instillation is estimated as 0.0163.

Ocular pharmacokinetics and pharmacodynamics of phenylephrine and phenylephrine oxazolidine in rabbit eyes.

The aqueous humor concentration of phenylephrine and its corresponding mydriatic response were measured over time in New Zealand albino rabbit eyes following a 10-µl topical instillation of a phenylephrine HC1 viscous solution (10%) or a phenylephrine oxazolidine (prodrug) suspension in sesame oil (1 and 10%). The bioavailability of a 1% prodrug suspension in the rabbit eye (AUC of aqueous humor concentration vs time) was 30% lower than that of a 10% phenylephrine solution (P < 0.1) with the exception that the peak time occurred 34 min earlier with the prodrug. A 10% prodrug suspension increased the aqueous humor bioavailability approximately eightfold but improved the mydriatic activity (AUC of mydriasis vs time) only fourfold. The pharmacokinetic parameters, apparent absorption, and elimination rate constants, of phenylephrine and the prodrug were determined from aqueous humor concentration–time and mydriasis–time profiles. The study showed that the kinetic parameters of phenylephrine estimated from its mydriasis profile do not accurately reflect the kinetics of drug distribution in the iris. These parameters also varied with the instillation of phenylephrine solution or prodrug suspensions. A mydriatic tolerance of the pupil response was apparent after the topical instillation of phenylephrine solution. The mydriatic tolerance may be due to the decrease in receptor number in the iris dilator muscle.

The Role of Tear Proteins in Tear Film Stability in the Dry Eye Patient and in the Rabbit

Sigma receptors have been identified in the lacrimal glands of rabbits; stimulation of these receptors results in the modulation of protein secretion from acinar cells. 1–4 When acinar cells were incubated with various sigma ligands, it was established that increases and decreases in protein release could be used to identify agonists and antagonists.4 Following the application of a newly designed agonist, N,N-dimethyl-2-phenylethylamine HC1 (AF2975), to the rabbit eye, a statistically significant increase in total protein was observed, when compared to either baseline values or the fellow eye.3 Tears were collected, and protein fractions were separated into various fractions with the use of size-exclusion high-pressure liquid chromatography (SE-HPLC).4 In particular, a 23 min protein fraction (about 16–18 kDa) was found to increase by 150% and 90% at 10 and 60 min, respectively following the topical application of AF2975 (50 μl of 0.15%) to the rabbit eye. Other protein peaks also showed an increase, but much less than the 23 min peak. Following desalting and concentrating, it was possible to separate the 23 min protein peak from rabbit tears into five isoforms using isoelectric chromatofocusing and a protein standard with a pI of 4.6. The isoforms were acidic with pIs higher than 4.6; however, the 23 min protein peak has not been specifically identified, which would require determination of its amino acid sequence. The function of the small-molecular-weight 23 min protein fraction is not known, but has been contrasted to the human tear fraction commonly referred to as tear-specific prealbumin (TSP) or as proteins migrating faster than albumin (PMFA).5–7

The Effect of Reduced Eyedrop Size and Eyelid Closure on the Therapeutic Index of Phenylephrine

In this study we examined the relative effects of reducing eyedrop size (from 30 microliters to 10 microliters) and eyelid closure on the ocular efficacy and systemic absorption of 10% phenylephrine. Thirteen subjects participated in a quadruple crossover study that involved dilation with a 10-microliters and a 30-microliters drop of phenylephrine with and without eyelid closure. The 10-microliters drop was just as effective for pupillary dilation as the 30-microliters drop. Eyelid closure improved dilation for both drop sizes. Both eyelid closure and reducing the drug volume decreased systemic absorption of phenylephrine as measured by plasma concentration. When used together, eyelid closure and the smaller drop size reduced plasma concentration by 45%. The therapeutic index for 10% phenylephrine appears to be improved by using a 10-microliters drop followed by eyelid closure.

Biopharmaceutical evaluation of ibufenac, ibuprofen, and their hydroxyethoxy analogs in the rabbit eye

Two new structural analogs, 2-(4-hydroxyethoxyphenyl)acetic acid [R3] and 2-(4-hydroxyethoxyphenyl)propionic acid [R4], along with their parent compounds, ibufenac and ibuprofen, were evaluated for their biopharmaceutical properties. The analogs represented substitution of the lipophilic isobutyl side chains of ibufenac and ibuprofen with hydrophilic hydroxyethoxy side chains. Anti-inflammatory activity was evaluated by administering drugs topically to inhibit inflammation induced by using either clove oil or arachidonic acid. The rank order of activity was ibufenac ≅ ibuprofen > R3 ≅ R4. The new compounds, R3 and R4, were highly water soluble (>60-fold) and partitioned less (<1/1500-fold) into the lipid phase when compared to ibufenac and ibuprofen. R3 and R4 each had apparent corneal permeability coefficients of 6×10−6cm/sec, whereas ibufenac and ibuprofen yielded values of about 22×10−6cm/sec. In an ocular pharmacokinetic study in the rabbit eye, constant concentrations of each compound were maintained on the cornea in a cylinder or welt fixed to the cornea, resulting in a constant input rate. This method circumvented parallel loss routes at the absorption site including nasolacrimal drainage. From area calculations the dispositions of the compounds within the eye were described by mean residence times, steady state volumes of distributions, and clearance rates. R3 and R4 were more slowly absorbed, retained within eye tissues longer, and were cleared more slowly from the eye than ibufenac and ibuprofen. The aqueous humor concentration-time profiles were also computer-fitted to equations representing classical pharmacokinetic models. For ibufenac and ibuprofen, the entire cornea was assumed to be the net barrier for entry into the anterior chamber. Whereas, for R3 and R4, the corneal epithelium and endothelium were presumed to be the diffusional barriers into and out of the stroma, the latter treated as a compartment. Aqueous humor concentrations of each drug fit the models reasonable well and agreed with conclusions made from the use of area calculations. The drop volume method was used to measure the surface tension of each compound. Both ibufenac and ibuprofen were considerably more surface active than R3 or R4. The greater surface tension measured for ibufenac and ibuprofen correlated to the subjective observations of ocular discomfort for these drugs.

Ocular Pharmacokinetics of a Novel Tetrahydroquinoline Analog in Rabbit: Compartmental Analysis and PK–PD Evaluation

The elimination kinetics of the pharmacologically active compound 1-ethyl-6-fluoro-1,2,3,4-tetrahydroquinoline (MC4) were characterized along with pharmacodynamic (PD) measurements. Four compartmental models based on ocular anatomy, physiology, and possible absorption and disposition pathways were proposed to model the pharmacokinetic (PK) data in WinNonlin and the best model was chosen based on statistical and goodness-of-fit criteria. A three-compartment physiologic-based PK model with a bidirectional transfer between cornea and aqueous humor and a unidirectional transfer between aqueous humor and iris-ciliary body best described the data. The ocular PD parameters, maximum effect attributed to drug (E(max)) and drug concentration which produces 50% of maximum effect (EC(50)), were estimated with change in intraocular pressure (ΔIOP) as the effect (PD response) in the effect compartment model (PK-PD link model) using aqueous humor concentration-time and ΔIOP-time profiles. The link model better described the effect compartment concentrations than a simple E(max) model that used iris-ciliary body concentration-time data, indicating that there is an apparent temporal displacement between aqueous humor concentration (plasma/central compartment equivalent) and pharmacological effect. A physiologically plausible value of 0.0159 min(-1) was obtained for the drug elimination rate constant (k(eo)) from the effect site to account for equilibration time in the biophase. Hysteresis was observed for the iris-ciliary body, aqueous humor drug concentrations, and effect data, further confirming the utility of the link model to describe the PD of MC4.

The effectiveness of teaching clinical pharmacokinetics by computer

Clinical Pharmacology and Therapeutics (1993) 53, 617–621; doi:10.1038/clpt.1993.81

The effects of sigma ligands on protein release from lacrimal acinar cells: a potential agonist/antagonist assay.

Sigma receptor antagonists have been proposed as leading clinical candidates for use in various psychotic disorders. Prior to clinical testing, it is imperative that a new agent be correctly identified as an antagonist and not an agonist since the latter may worsen the psychosis. For sigma-ligands many behavioral and pharmacological assays have been developed in an attempt to classify agonist/antagonist activity. These assays evaluate a response or a behavior in an animal model that can be related to clinical efficacy. However, is the action by the presumed antagonist a consequence of sigma-receptor activity? Previously we have identified sigma-receptors in acinar cells of the main lacrimal gland of the New Zealand white rabbit and have measured protein release after the addition of various N,N-disubstituted phenylalkylamine derivatives known to be sigma-ligands by receptor binding studies. Although protein release from acinar cells has been attributed to either muscarinic or alpha-adrenergic stimulation, protein release from sigma-receptor stimulation was also confirmed. In the reported studies here, we isolated and incubated acinar cells with varying concentrations of known sigma-ligands and measured protein concentration. A knowledge of the receptor profile for the disubstituted phenylalkylamines permitted experiments to be designed in which various alpha, muscarinic, serotonergic, and dopaminergic antagonists could be added in equimolar concentrations. Under the conditions of these experiments, statistically significant increases in protein release for sigma-ligands could be attributed to stimulation of sigma-receptors. Haloperidol, an apparent sigma-antagonist, caused a statistically significant decrease in protein release and also inhibited protein release when tested with a known sigma-ligand, AF2975 [N,N-dimethyl-2-phenylethylamine]. In this system, stimulation and inhibition of protein release were defined as agonist and antagonist behavior, respectively. Of particular interest were the results for BMY14802 and +/- pentazocine, both of which were found to be agonists. Various antipsychotic and antidepressant drugs were measured for their agonist/antagonist behavior. Because of multireceptors present in acini, their agonist or antagonist behaviour could not be attributed solely to interaction with the sigma-receptor unless specific antagonists were added.