Human pharmacokinetics of tolfenamic acid,a new anti-inflammatory agent

Summary The pharmacokinetics of tolfenamic acid, a new anti-inflammatory agent was studied in six healthy volunteers after an intravenous dose of 100 mg and oral doses of 100, 200, 400 and 800 mg. The disposition of intravenous tolfenamic acid could be described by two-compartment open model, with a central compartment volume (V_dc) of 5.6±0.31 (mean±SE), volume during -phase (V_d) of 31±21, and a total elimination rate constant (k₁₀) 1.6±0.1 h^–1. The terminal elimination half-life was 2.5±0.6 h and the total plasma clearance 155±15 ml/min. The elimination occured principally by extrarenal mechanisms, the recovery of unchanged drug together with is glucuronide in urine averaging only 8.8% of the intravenous dose. The binding of tolfenamic acid to plasma proteins averaged 99.7%. The gastrointestinal absorption had a mean half-life of 1.7±0.1 h. Based on comparison of areas under the plasma concentration time-curves after intravenous and oral administration, the biovailability of tolfenamic acid capsules averaged 60%. The rate and extent of absorption and the rate of elimination of tolfenamic acid were independent of dose.     

Dosage adjustment for ceftazidime in patients with impaired renal function

Summary Ceftazidime has good antibacterial activity against many Gram-negative micro-organisms including Ps. aeruginosa.The aim of the present study was to calculate a dosage adjustment regimen for renal failure patients and to test it in a second group of patients. A study was made of the pharmacokinetics of ceftazidime 1 g given as a single bolus i.v. injection in 20 patients in an intensive care unit with varying degrees of renal function, including patients on regular haemodialysis. The serum half-life of elimination (t_1/2) varied from 1.6 to 45 h depending on renal function. During haemodialysis the mean t_1/2 was 4.7 h. A good correlation between the renal clearance of creatinine and ceftazidime was observed. In most patients protein binding was lower than previously observed. From the pharmacokinetic data, a dosage adjustment regimen for patients with renal insufficiency was calculated, which studies in 7 further patients showed to be effective.Abbreviations C_t serum concentration at time t - and slopes of the bi-exponential curve of the fast and the slow disposition (overall elimination) processes - A and B intercepts of the distribution slope and elimination slope extrapolated to the ordinate - t_1/2 serum half-life of elimination - k₁₂ distribution rate constant for transfer from the central to the peripheral compartment - k₂₁ distribution rate constant for transfer from the peripheral to the central compartment - k₁₃ elimination rate constant from the central compartment - k_N and elimination rate constants from the central compartment in normal and impaired renal function, respectively - AUC area under the serum concentration-time curve from zero to infinity - V_t total apparent volume of distribution (area method) - V₁ and V₂ distribution volumes in the central and the peripheral compartments, respectively - CL total body clearance of ceftazidime - CL_R renal clearance of ceftazidime - CL_CR renal clearance of creatinine - T and dosing intervals in normal and impaired renal function - (m) pe (mean) prediction error - rmspe root mean squared prediction error     

Pharmacokinetics of bolus intravenous and oral doses of L-carnitine in healthy subjects

Summary The pharmacokinetics of single intravenous and oral doses of L-carnitine 2 and 6 g was studied in 6 healthy subjects on a low-carnitine diet. Carnitine was more rapidly eliminated from plasma after the 6 g dose. Comparing the doses, the t_1/2 of the elimination phase () was 6.5 h vs 3.9 h, the elimination constant 0.40 vs 0.50 h^–1 and the plasma carnitine clearance was 5.4 vs 6.11 · h^–1 for the 2 g and 6 g doses, respectively, showing dose-related elimination. Saturable kinetics were not found. The apparent volumes of distribution after the two doses were not significantly different and were of the same order as the total body water. Urinary recoveries of the 2 g and 6 g doses were 70% and 82%, respectively, during the first 24 h. Following the oral doses, there was no significant difference between the areas under the plasma carnitine concentration-time curves. Urinary recovery was 8% and 4% for the 2 g and 6 g doses during the first 24 h. Oral bioavailability was 16% for the 2 g dose and 5% for the 6 g dose. The results suggest that the mucosal absorption of carnitine was already saturated by the 2 g dose.     

Analysis of Antiplatelet Effect of Ticlopidine In Humans: Modeling Based on Irreversible Inhibition of Platelet Precursors in Bone Marrow

The relationship between plasma concentration of ticlopidine and its inhibitory effect on platelet aggregation in human was analyzed using a pharmacokinetic/pharmacodynamic (PK/PD) model. The data of plasma concentration and inhibitory effect on platelet aggregation were taken from the literature. A two-compartment open model was fitted to plasma ticlopidine concentrations. Assuming that ticlopidine acts on platelet precursors in the bone marrow, the apparent reaction rate constant of ticlopidine and platelet precursors (K), apparent transformation rate constant of platelet precursors (k_r) and apparent elimination rate constant of platelets (k _e ) were estimated. The estimated values ± S.D. were 1.01 ± 1.08 ml g ^–1 hr^–1 for K, 0.265 ± 0.259 hr^–1 for k_r and 0.0747 ± 0.0112 hr ^–1 for k _e . The antiaggregation effects of ticlopidine on platelets after administration of 100, 200, and 300 mg (bid for 8 days) were simulated using the PD parameters of K, k_r, and k_e. While the antiaggregation effect reached steady state within 3–4 days without dose dependency of the interval, the maximum effect increased with dose. Furthermore, changing the elimination rate constant of ticlopidine from the central compartment in the model significantly changed the duration of inhibitory effect of ticlopidine on platelet aggregation. Therefore, the reported long duration of antiplatelet effect after discontinuation of ticlopidine, which is believed to be irreversible binding to the platelet, might have been partially caused by the delayed plasma elimination after a long therapy of ticlopidine. On the other hand, the mean life-span of platelets in the blood estimated by 1/k_e after administration of ticlopidine was 14 hr, far below the life-span of platelets in the blood. For a more detailed analysis of the antiplatelet effect of ticlopidine, the possible contribution of reversible binding of the drug to glycoprotein IIb/IIIa should be considered in future PK/PD models.     

Pharmacokinetics of intravenous and oral bolus doses of L-carnitine in healthy subjects

Summary The pharmacokinetics of single intravenous and oral doses of L-carnitine 2 g and 6 g has been investigated in 6 healthy subjects on a low carnitine diet.Carnitine was more rapidly eliminated from plasma after the higher dose. Comparing the 2-g and 6-g doses, the t_1/2 of the elimination phase () was 6.5 h vs 3.9 h, the elimination constant was 0.40 vs 0.50 h^–1 and the plasma carnitine clearance was 5.4 vs 6.11 × h^–1 (p<0.025), thus showing dose-related elimination.Saturable kinetics was not found in the range of doses given. The apparent volumes of distribution after the two doses were not significantly different and they were of the same order as the total body water. Urinary recoveries after the 2-g and 6-g doses were 70% and 82% during the first 24 h, respectively.Following the two oral dosing, there was no significant difference in AUCs of plasma carnitine. Urinary recoveries were 8% and 4% for the 2-g and 6-g doses during the first 24 h. The oral bioavailability of the 2-g dose was 16% and of the 6 h dose 5%. The results suggest that the mucosal absorption of carnitine is already saturated at the 2-g dose.     

The pharmacokinetics of alcohol in human breath,venous and arterial blood after oral ingestion

Summary The concentration-time profile of ethanol in breath air (AAC), arterial (ABAC) and venous blood (VBAC) of human volunteers was studied after four different oral doses of absolute alcohol — 0.5, 0.75, 1.0, and 1.25 g/kg body weight. Seventy-eight single dose experiments were carried out in 42 subjects. In all 78 studies AAC was measured and VBAC was estimated simultaneously in blood collected from a cubital vein of 36 volunteers. Arterial blood, too, was collected from 8 subjects from a catheter in a brachial artery. All blood alcohol concentrations were analysed independently by gas chromatography (GLC) and an enzymatic (ADH) method. A one-compartment open model with first order absorption and pseudo-zero-order elimination was employed to calculate the pharmacokinetic parameters. The average values for the first order absorption rate constant (k_a) ranged from 2.2 to 3.1, from 2.4 to 2.6 and from 1.0 to 1.7 h^–1 for AAC, ABAC and VBAC, respectively. The pseudo-zero-order elimination rate constant () was 0.17 to 0.18, 0.21 to 0.22 and 0.26 to 0.27 g × 1^–1 × h^–1, respectively. During absorption ABAC tended to be higher than VBAC, peaking at a higher level (C_max) and with a shorter time to peak (t_max) until an arterio-venous concentration equilibrium was reached, whereafter VBAC remained above ABAC. Although there was a close relationship between AAC, ABAC and VBAC during elimination, AAC closely followed the pattern of ABAC during absorption and tended to deviate from VBAC. AAC, therefore, is a much better predictor of ABAC during absorption than VBAC.     

A dose-dependent pharmacokinetic study on caffeic acid in rabbits after intravenous administration

The dose-dependent pharmacokinetics of caffeic acid (CA) were studied in rabbits. Three different doses (5, 10, and 25 mg kg⁻¹) were administered intravenously to six rabbits each. The concentration–time profiles for CA could be fitted by a two-compartment model for each dose. The results showed that total-body clearance and elimination rate constant from the central compartment (k₁₀) after a 5 mg kg⁻¹ dose were greater than those after the other two doses. Furthermore, the terminal elimination half-life (β half-life) and mean residence time (MRT) after a 5 mg kg⁻¹ dose were less than after the other doses. The AUC value increased linearly with dose within the range of 10–25 mg kg⁻¹. Most of the unchanged caffeic acid was excreted in the urine within 2 h. The percentage of unchanged caffeic acid excreted in the urine was 63·4, 60·0, and 55·4% after doses of 5, 10, and 25 mg kg⁻¹, respectively, which was not significantly different. However, significant differences in the renal clearances and renal excretion rate constants were observed with a 5 mg kg⁻¹ dose compared to the other doses. On the other hand, nonrenal clearances and nonrenal excretion rate constants showed no dose-related differences. The differences observed in total-body clearance, k₁₀, β half-life, and MRT between a 5 mg kg⁻¹ dose and the other doses can be explained on the basis of the differences in renal clearance and renal excretion rate constants. ©1997 John Wiley & Sons, Ltd.     

Pharmacokinetics of tolfenamic acid in patients with cirrhosis of the liver

Summary Six patients with alcoholic cirrhosis of the liver received 100 mg tolfenamic acid p.o. and i.v. The disposition of tolfenamic acid could be described by a two-compartment open body model, with a mean central compartment volume of 8.71, and a -phase volume of 251. The elimination rate constant k_e averaged 1.13 h^–1 and the half-life of the -phase was 1.73 h; the mean total plasma clearance was 159 ml/min. These pharmacokinetic parameters differed only slightly from those in two groups of healthy volunteers studied previously; k_e was significantly reduced by about 30% in the patients but none of the other parameters differed significantly. There was good correlation between individual elimination rate constants or plasma clearances with the liver function tests, serum albumin and P-coagulation factors. Oral absorption was good and bioavailability of about 100% was shown by comparison of the areas under the plasma concentration — time curves after i.v. and p.o. administration. Metabolism was qualitatively and quantitatively very similar to previous observations in healthy volunteers. There seems no reason to reduce the dose of tolfenamic acid in patients with compensated alcoholic cirrhosis.     

A MODEL BASED ASSESSMENT OF REDISTRIBUTION DEPENDENT ELIMINATION AND BIOAVAILABILITY OF RIFABUTIN

The autoinduction characteristic of rifabutin (RIF) following multiple oral dosing was investigated via pharmacokinetic modeling. A two-compartment model with first-order absorption was fit to plasma RIF data obtained from a study conducted in healthy normal volunteers following both a single and multiple oral doses. Parameter estimates showed an elimination rate constant (k₁₀) of about 0·12--0·14 h⁻¹ which was independent of the single or multiple-dosing condition. The lower-than-expected drug accumulation following multiple dosing seems to suggest that prolonged dosing perturbs the linear kinetic system. However, this analysis has shown no significant changes (p>0·05) in the rate constants describing RIF absorption, tissue distribution/redistribution, and elimination. The mean rate of drug redistribution from the tissue compartment (k₂₁; 0·04--0·06 h⁻¹) was twofold to threefold lower than k₁₀, and, with a large steady-state distribution volume (V_ss/F after a single dose, 1630 L), RIF elimination appears to be dependent on drug redistribution. This hypothesis was further supported by a significant correlation (p<0·01) BETWEEN RIF TISSUE REDISTRIBUTION (k₂₁) and terminal disposition phase rate (λ_z) constants. The redistribution dependent elimination of RIF also helps explain the stability of the terminal half-life under both single and multiple-dosing paradigms. Urinary excretion of RIF and its 25-O-deacetyl metabolite totalled less than 7% of the oral dose following single dosing, and decreased to about 4% after multiple dosing. For individual patients, the decrease in urinary recovery of the 25-O-deacetyl metabolite was directly proportional to the decrease in urinary RIF recovery. In addition, both estimates of the model intercepts (A and B) were lower following multiple dosing. Further analyses revealed a linear relationship between A and B intercepts, and also between the urinary RIF recovery and the B intercept. These relationships, in conjunction with the lack of significant increase in the rate of elimination, indicate that induction of presystemic extrahepatic metabolism and/or decrease in the extent of oral absorption may be the primary causes for the lower-than-expected systemic RIF plasma levels after multiple oral dosing.     

Application of the Wagner-Nelson absorption method to the two-compartment open model

This report considers the application of the Wagner- Nelson method to both one- and two- compartment open model data when there is no competing reaction at the absorption site. Equations are derived which show that application of the Wagner- Nelson method to data which obey the twocompartment open model with first-order absorption allows accurate estimation of not only the rate constant k_a but also the parameters of the two- compartment open model, namely k₁₂, k₂₁, and k_el. In the example given, this new method was more accurate than the classical feathering or back- projection method. The appropriate criterion for collapsing the two- to the onecompartment open model is given. In cases where the one-compartment open model applies, and absorption is first order but abruptly ceases after some time, it is shown that k_a may be accurately estimated by application of the Guggenheim method to the A_T/V values calculated for the absorption phase.Supported in part by Public Health Service Grant 5-P11-GM15559.     

Pharmacokinetics of sulfametopyrazine (Kelfizine W) effects of a single oral dose

Summary The pharmacokinetics of sulfametopyrazine were studied for seven days after a single oral dose of 2 g. in healthy volunteers in order to establish its chemotherapeutic value. — The appearance and disappearance of the drug in the plasma were evaluated both for compounds with a free amino group and for total sulphonamides. The half-life and absorption, distribution, elimination and excretion coefficients were calculated, as well as the concentrations in plasma water and interstitial fluid. The estimated drug concentrations in the urine agreed with those calculated from the excretion coefficients. — In all subjects at the end of the seventh day the concentrations in all body compartments of active compounds exceeded the minimum required for a therapeutic effect. The highest concentrations found in the urine were always significantly lower than the drug's basal solubility at pH 5, thus excluding any risk of crystalluria.Glossary of symbols total binding capacity of plasma proteins for SMP - Specific gravity of blood ( Bl), and interstitial fluid (IF) - minimum inhibitory concentration for bacterial growth. Evaluation of against E. coli or other pathogenic bacteria in a medium free of antagonists [29] - ratio of dose interval to half-life - dose interval - safety factor. Proportionality constant between andc _min for a therapeutic efficacy of 95 per cent - fraction of the administered drug absorbed from the depot (gastrointestinal tract etc.) - distribution coefficient with respect to the drug concentration in blood plasma (ml/g) - D* initial dose of the drug - D maintenance dose - M molecular weight of the drug (280) - G weight of subject (kg) - F area between time axis and concentration curve (plotted c' values) - t _50% apparent biological half-life - w ¹ water content of plasma (ml/ml) - p protein concentration of plasma (pBl), or interstitial fluid (pIF) (g/l) - c _IF concentration in the interstitial fluid - C ₀ concentration in plasma at zero time after i.v. administration - c ₁ ⁰ concentration in plasma after oral absorption extrapolated to zero time - c ₁ concentration in plasma water of the drug with free amino function - c _min minimum inhibitory concentration needed in plasma water (minimum therapeutic concentration) - k ₀₁ rate constant for absorption - k ₁ rate constant for absorption determined at timet; (similarlyk₂) - K total elimination coefficient - k _el rate constant for elimination - k _F rate constant for formation of metabolites - k _D excretion coefficient of SMP with free amino function - k _U coefficient of metabolite excretion - D ₀ quantity of SMP in the body at time zero - D _B quantity of SMP in the body at timet - D _U quantity of SMP excreted in the urine at timet - M _F quantity of metabolites formed at timet - M _B quantity of metabolites present in the body at timet - M _U quantity of metabolites excreted in the urine at timet - K dissociation constant for the sulphonamide-protein complex - notation for quantities related to drug concentrations in plasma, e.g. c (corresponding term without refer to plasma water)     

A pharmacokinetic slide rule for more accurate drug treatment

Summary A slide rule has been devised which is based on the general mathematical models of pharmacokinetics. It permits calculation of exact dosage regimens for individual patients from certain basic parameters. First, from the patient's renal clearance, the proportionality constant characterizing renal excretion of a certain drug (a) and its non-renal rate constant of elimination (k _nr), the rate constant of total elimination (k _e) can be calculated. Second, fromk _e, the apparent volume of distribution (V _d) and the desired final mean concentration of a drug (c), exact values can readily be obtained for the loading dose (D*) and the dosage schedule, which consists of the maintenance dose (D), the dosing intervals () and the infusion rate for intravenous administration. In addition the slide rule provides information about the rate at which c is reached ifD alone is administered at , and the fluctuation in the concentration around c to be anticipated during . By use of this calculation, the slide rule facilitates the decision whether a loading dose should be given, and what dosage schedule is best suited to the therapeutic problem. It is possible, therefore, to calculate exact dosage regimens for individual patients, even for those with excretory dysfunction. The slide rule should also help physicians to comprehend the nature and significance of pharmacokinetic mechanisms.     

Pharmacokinetics of ibuprofen in man IV: Absorption and disposition

Fifteen normal male volunters received 400, 800, and 1200 mg doses of ibuprofen as 1, 2, or 3 tablets, respectively, in crossover fashion, then 420 mg in solution form during the fourth week. Plasma concentration of ibuprofen was measured by an HPLC method. Individual subject concentration-time (C,t)data following the solution were analyzed by two different methods, and results unequivocally indicated the open two compartment model with first order absorption. However, the computer fitting of both arithmetic and geometric mean concentrations led to a different model. A method was developed to obtain absorption data (fraction of drug absorbed, F_a,versus time) for a multicompartmental system from oral data alone, without intravenous data. The method assumes that V_p is constant intrasubject and that absorption is complete following administration of both the solution and tablets. The method was successfully applied to the ibuprofen tablet data. It was shown also that such a method is necessary to obtain ibuprofen absorption data since intrasubject variation of the microscopic rate constants k₁₂, k_a21,and k_el (as reflected by the intrasubject variation of the hybrid rate parameters ₁ and ₂ or and a) is of the same order of magnitude as intersubject variation. Absorption of ibuprofen from tablets was shown not to be simple first order as for the solution. The absorption profiles following one tablet were S- shaped, while those following 2 or 3 tablets had partial linear segments indicating zero order absorption.This investigation was supported in part by a contract from The Upjohn Company. Gregory J. Szpunar was partly supported by the American Foundation for Pharmaceutical Education.     

Albendazole kinetics in patients with echinococcosis: Delayed absorption and impaired elimination in cholestasis

Summary The pharmacokinetics of albendazole and its main metabolite, albendazole sulphoxide, have been examined after giving a single oral dose of 200 mg albendazole to 19 patients with either Echinococcus multilocularis or E. granulosus, 5 of whom had significant extrahepatic obstruction due to the underlying disease. The AUC of albendazole sulphoxide was increased in the latter patients (mean 122 mol · h · l^–1 compared to 17 mol · h · l^–1 in the non-obstructed group). Obstructed patients had delayed absorption, k_a averaging 0.39 compared to 1.41 h^–1 in non-obstructed patients. The corresponding elimination rate constant, k_e was also prolonged, averaging 0.041 and 0.13 h^–1 in the two groups, respectively. Four patients were restudied after complete or partial resolution of the cholestasis. The pharmacokinetic parameters in them had returned towards values comparable to those in the non-obstructed patients.     

Two Compartment Analysis of Plasma Elimination of Phenazone in Normals and in Patients with Cirrhosis of the Liver

Abstract The elimination kinetics of phenazone (NFN) after intravenous injection was investigated in seven healthy volunteers and in five patients with chronic liver disease. The plasma concentration/time data of phenazone were analysed according to a one compartment linear model and a two compartment linear open model, respectively. The elimination rate constant, the volume of distribution, and the clearance estimated by the one compartment model analysis correlated significantly with the corresponding parameters of the two compartment model analysis (r = +0.892, r = +0.989 and r = +0.999, respectively). The initial disposition constant (α) and the rates of distribution of phenazone between the central and peripheral compartment, k₁₂ and k₂₁ were unaltered in the patients with liver disease, whereas the clearance was significantly reduced. These results confirm that the plasma clearance of phenazone is reliably investigated by sampling only from the terminal phase of the plasma elimination curve.     

Behaviour of glibenclamide on repeated administration to diabetic patients

Summary Six maturity onset diabetic patients took glibenclamide 5 mg by mouth, every morning 10 min before a standard breakfast. Serum levels of immunoreactive glibenclamide, glucose and immunoreactive insulin were measured repeatedly on the first and 15th days of treatment. Measured glibenclamide blood levels were in close agreement with an analogue computer simulation of data obtained from healthy volunteers: there was no accumulation of drug in the blood, but there was strong evidence for the existence of a slowly equilibrating deep compartment. Considerable insulin release and correction of the breakfast-induced hyperglycaemia were observed immediately after administration of the drug, as well as 5 h later, at lunch time. The clinical significance of blood levels of glibenclamide, as well as the correlation of pharmacokinetics with pharmacodynamics, are discussed in the light of these results.Glossary of symbols IR- immuno-reactive - GLI glibenclamide - IRI immuno-reactive insulin - GLU glucose - AK 1 values obtained with patient AK on the first day of treatment - AK 15 values obtained with patient AK on the 15th day of treatment - b serum level - b_max maximal serum level - t time after dose - t_max time of maximal serum level - G gastro-intestinal system - B central compartment (blood) - T peripheral compartment (tissue) - E excreta - M,N coefficients of the equation of a bi-exponential decay curve - µ, v exponents of the equation of a bi-exponential decay curve - e base of natural logarithms - K_BG K_EB K_TB K_BT first order rate constants (e. g. K_BG means: into B, from G) - K_BG first order rate constants - etc. not corrected for the volume of distribution     

Population pharmacokinetics and pharmacodynamics of escitalopram in overdose and the effect of activated charcoal

AIMS

To describe the pharmacokinetics and pharmacodynamics (PKPD) of escitalopram in overdose and its effect on QT prolongation, including the effectiveness of single dose activated charcoal (SDAC).

METHODS

The data set included 78 escitalopram overdose events (median dose, 140 mg [10–560 mg]). SDAC was administered 1.0 to 2.6 h after 12 overdoses (15%). A fully Bayesian analysis was undertaken in WinBUGS 1.4.3, first for a population pharmacokinetic (PK) analysis followed by a PKPD analysis. The developed PKPD model was used to predict the probability of having an abnormal QT as a surrogate for torsade de pointes.

RESULTS

A one compartment model with first order input and first-order elimination described the PK data, including uncertainty in dose and a baseline concentration for patients taking escitalopram therapeutically. SDAC reduced the fraction absorbed by 31% and reduced the individual predicted area under the curve adjusted for dose (AUC_i/dose). The absolute QT interval was related to the observed heart rate with an estimated individual heart rate correction factor (α = 0.35). The heart rate corrected QT interval (QT_c) was linearly dependent on predicted escitalopram concentration [slope = 87 ms/(mg l^–1)], using a hypothetical effect-compartment (half-life of effect-delay, 1.0h). Administration of SDAC significantly reduced QT prolongation and was shown to reduce the risk of having an abnormal QT by approximately 35% for escitalopram doses above 200 mg.

CONCLUSIONS

There was a dose-related lengthening of the QT interval that lagged the increase in drug concentration. SDAC resulted in a moderate reduction in fraction of escitalopram absorbed and reduced the risk of the QT interval being abnormal.     

Pharmacokinetics of clonidine in the rat and cat

To investigate the pharmacokinetic behavior of clonidine, rats were given clonidine intravenously at 125, 250, and 500g/kg and blood clonidine concentrations were followed for 6 hr. The disposition of clonidine in two brain regions was studied in rats after an i. v. dose of 500 g/kg. The liver clearance in rats was investigated by liver perfusion techniques. The results obtained indicate that the disposition characteristics of clonidine can be described by a two-compartment open model in both rats and cats. The penetration of clonidine into tissues is rapid, and brain levels in rats were about 1.7 times higher than blood levels. Brain tissues were found to be an indistinguisible part of the central (blood) compartment. Dose-dependent pharmacokinetic behavior was found for clonidine in rats at the doses used. This was demonstrated by a decrease of both the rate constant of distribution to the peripheral compartment and the overall elimination rate constant from the body, with increase in dose. As a consequence, the volume of distribution and the clearance both decreased with increasing dose. Possible explanations for the dose-dependent behavior of clonidine are discussed.     

Microbial pharmacodynamics of piperacillin in neutropenic mice of systematic infection due toPseudomonas aeruginosa

Mathematical solutions for two possible pharmacodynamic interactions (linear nonsaturable and nonlinear saturable) between antibiotics and microorganisms derived from the incorporation of clinically relevant antibiotic dosage regimens such as single bolus dosing, multiple doses, and constant infusion at steady state have been obtained. It is concluded that the saturable nonlinear interaction model between the tested antibiotic and microorganism appears appropriate. The model and its derived equations are capable of describing in vivobacterial growth of P. aeruginosaafter single bolus dosing and multiple doses of piperacillin as described by a linear one-compartment pharmacokinetic model. The activity of piperacillin against P. aeruginosain the neutropenic mouse systemic infection model can be described by an equation with three dynamic parameters: the bacterial growth rate constant k _app ,0.02345min ^–1, the bacterial killing rate constant k _kill ,0.02623 min ^–1, and the Michaelis-Menten type saturation constant K_m, 0.05467 g/ml. The concept and derived equations for the optimal dosing interval and minimum critical concentration are of clinical importance for the proper selection of antibiotic dosage regimens.