Total Plasma Drug Concentrations Research Articles

Biased computation of probability of target attainment for antimicrobial drugs.

The medical literature is replete with articles in which there is confusion between "free concentration" and "unbound fraction" (fu ), which is the ratio of free to total plasma concentration. The lack of clarity in distinguishing between these two terms has led to biased computations, erroneous interpretations, and misleading recommendations. The problems are highlighted in this paper, taking the example of calculation of Probability of Target Attainment (PTA). This metric is used to propose pharmacokinetic/pharmacodynamic (PK/PD) breakpoints required for the interpretation of Antimicrobial Susceptibility Testing. Based on Monte Carlo simulations of the PK/PD index, area under the unbound concentration time curve/minimum inhibitory concentration (fAUC/MIC), computation of PTA from total plasma concentrations scaled by fu ineluctably leads to biased estimates. The bias is greater if the variability associated with fu is added, instead of removing it during this scaling. The explanation for the bias is that total plasma drug concentrations are intrinsically more variable than the corresponding free concentrations. This is due to the variability of antimicrobial binding for total, but not for free plasma concentrations. In consequence, the greater variability always leads to underestimation of the PK/PD cutoff (i.e., the critical MIC that is guaranteed for a given percentile of the population). A further consequence is an increase in calculated dosage required to attain the targeted quantile. This erroneous approach, of using free antimicrobial drug fraction, is not limited to the derivation of PK/PD cutoff, but may also have consequences for antimicrobials drug safety in clinical patients.

Therapeutic drug monitoring of phenytoin and valproic acid in critically ill patients at Windhoek Central Hospital, Namibia.

BackgroundPhenytoin and valproic acid, anticonvulsants, have a low therapeutic index and are highly plasma protein bound, mainly to albumin. Hypoalbuminaemia is common in critically ill patients and increases the unbound drug concentration. Thus, monitoring unbound rather than total plasma drug concentrations is recommended to optimise the dosing of these drugs.ObjectiveThis retrospective study determined unbound plasma concentrations of phenytoin and valproic as a more accurate value of drug levels than total plasma drug concentrations.MethodsTotal plasma concentrations were retrieved for 56 Intensive Care Unit patients for phenytoin and 93 for valproic acid. Total drug concentrations were converted to unbound concentrations using a serum albumin-based normalising equation.ResultsTotal phenytoin plasma concentration was below (41.1% of patients), within (46.4%) or above (12.5%) the therapeutic range (10 μg/mL – 20 μg/mL). However, the predicted unbound plasma concentration of phenytoin was above the therapeutic range (1 μg/mL – 2 μg/mL) in the majority of patients (57.1%). For valproic acid, the total plasma concentration of most patients (87.1%) was below the therapeutic range (50 μg/mL – 100 μg/mL); among remaining patients (12.9%), it was within the therapeutic range. In the majority of patients (91.4%), the predicted unbound plasma concentration of valproic acid was between 2.5 μg/mL and 20 μg/mL.ConclusionThe usefulness of monitoring the total phenytoin or valproic acid levels for dose optimisation is limited as it is an inaccurate indicator of a patient’s drug therapeutic state. Thus, the unbound plasma drug concentrations should be quantified experimentally or predicted in resource-limited settings.

Determination of free clindamycin, flucloxacillin or tedizolid in plasma: Pay attention to physiological conditions when using ultrafiltration.

Pharmacokinetic/pharmacodynamic indices of anti-infective drugs should be referenced to free drug concentrations. In the present study, clindamycin, flucloxacillin and tedizolid have been determined in human plasma by HPLC-UV. The drugs were separated isocratically within 3-6 min on a C18 column using mixtures of phosphate buffer-acetonitrile of pH 7.1-7.2. Sample treatment for the determination of total drug concentrations in plasma included extraction/back-extraction (clindamycin) or protein precipitation (flucloxacillin, tedizolid). The free drug concentrations were determined after ultrafiltration. An ultrafiltration device with a membrane consisting of regenerated cellulose proved to be suitable for all drugs. Maintaining a physiological pH was crucial for clindamycin, whereas maintaining body temperature was essential for tedizolid. The methods were applied to the analysis of total and free drug concentrations in clinical samples and were sufficiently sensitive for pharmacokinetic studies and therapeutic drug monitoring.

Population pharmacokinetics and Monte Carlo simulations of sulbactam to optimize dosage regimens in patients with ventilator-associated pneumonia caused by Acinetobacter baumannii

BackgroundVentilator-associated pneumonia (VAP) caused by multidrug-resistant (MDR) Acinetobacter baumannii remains one of the leading causes of the high mortality rate in critically ill patients. Sulbactam has been considered as an alternative concomitant medication with other effective antimicrobial agents for the treatment of these MDR microorganisms. The aims of this study were (i) to characterize the population pharmacokinetics (PK) and (ii) to assess the efficacy of various dosage regimens of sulbactam in terms of probability of target attainment (PTA). MethodsThe PK studies were carried out following administration of 2 g of sulbactam every 12 h on the 7th dose of drug administration in 16 patients with VAP, and a Monte Carlo simulation was performed to determine the PTA of achieving 40% and 60% the exposure time during which the total plasma drug concentration remained above the MIC (T>MIC). ResultsThe volume of distribution and total clearance of sulbactam were 22.17 ± 1.60 L and 6.76 ± 2.37 L/h, respectively. For pathogens with a MIC of 8 μg/mL, the high PTAs of achieving (≥90%) 60% T>MIC in patients with serum albumin 1.7–2.4 g/dL and CLCR 90–120 mL/min following administration of sulbactam as a 4-h infusion of 1 g every 6 h, 2 g every 12 h, and 2 g every 8 h were 98.65%, 78.07% and 98.23%, respectively. For pathogens with a MIC of 16 μg/mL, the high PTAs of achieving (≥90%) 60% T>MIC in patients with serum albumin 1.7–2.4 g/dL and CLCR 90–120 mL/min following administration of sulbactam as a 4-h infusion of 2 g every 6 h, and 3 g every 8 h were 98.83% and 95.59%, respectively. ConclusionThese findings indicate that high dosage combination regimens are required for the treatment of life-threatening infections in critically ill patients with VAP.

Identification differential behavior of Gd@C82(OH)22 upon interaction with serum albumin using spectroscopic analysis

The interaction between Gd@C82(OH)22 and serum albumin (HSA and BSA) were investigated by spectroscopic analysis. From the characteristic feature of fluorescence quenching spectra at different temperatures, the inherent binding information including quenching mechanism, association constants, number of binding site, fraction of initial fluorescence and basic thermodynamic parameters were calculated. The binding of Gd@C82(OH)22 to serum albumin caused strong quenching of protein intrinsic fluorescence and the structural changes of serum albumin. At lower concentrations, Gd@C82(OH)22 was likely to rise fluorescence quenching of serum albumin through individual static quenching process by forming a ground-state complex, while dynamic and static coexisting quenching mechanism occurred in high concentration. Bimolecular quenching (Kq) value is twice the diffusion-controlled quenching constant (2.0 × 1010 L mol−1 s−1); binding sites of BSA were slightly more than those of HAS, and all of them reached to 1; the distance r between donor and acceptor was found to be 3.1494 nm and 3.6479 nm for HSA and BSA, respectively, both of which were fewer than 7 nm. It is confirmed that binding interaction for proteins in the presence of drugs was strong, the binding ratio was 1:1, and non-radiative energy transfer from protein to drug was extremely high probability in lower density. Binding process of Gd@C82(OH)22-HSA was driven mainly through van der Waals forces and hydrogen bonding formation, however more likely to be electrostatic interaction involved in the Gd@C82(OH)22-BSA binding process; Binding sites of Gd@C82(OH)22 to serum albumin were near tryprophan (HSA) and tyrosine residues (BSA), respectively. Moreover, a theoretical model of predicting the binding rate of drug to serum albumin was estimated, further analyzed that the binding rate was dynamically altered in various dose of protein and drug. Overall, these results provide potentially significant information for elucidating the distribution, transportation, the apparent relationship between pharmacologic activity and total plasma drug concentration as well as anti-carcinogenic activity and mechanisms in vivo.

Correlation of plasma anti-tuberculosis drug levels with subsequent development of hepatotoxicity

To compare the free and total plasma drug concentrations of rifampicin (RMP), isoniazid and pyrazinamide in subjects with or without anti-tuberculosis drug-induced hepatotoxicity (DIH). A total of 110 tuberculosis (TB) patients were administered daily anti-tuberculosis treatment and were prospectively followed for the development of DIH. Plasma drug levels were measured at 0, 1, 2 and 4 h on days 1, 7 and 14 of treatment. Plasma drug levels in 15 patients who developed DIH (cases) were compared with 95 patients who did not (controls). Female sex, body mass index < 17 kg/m(2) and baseline serum albumin < 4 g/dl predicted risk of DIH on univariate analyses. Free and total plasma RMP levels (Cmax and AUC0-4) on days 1, 7 and 14 were significantly higher in cases compared to controls and predicted development of DIH. Day 7 total RMP Cmax and AUC0-4 were higher in cases (mean 26.73, standard deviation [SD] 5.72 and 47.58, SD 33.10) than in controls (7.87, SD 10.95 and 14.01, SD 10.69, respectively). Plasma RMP levels were higher in cases than in controls and independently predicted subsequent development of DIH. The Cmax of Day 7 total RMP level (cut-off 12.50 mg/l) predicted subsequent development of DIH in 93.3% of the patients.

Oral absorption and excretion of icaritin, an aglycone and also active metabolite of prenylflavonoids from the Chinese medicine Herba Epimedii in rats

Icaritin (ICT) is a main aglycone and also active intestinal metabolite of prenylflavonoids from the Chinese medicine Herba Epimedii. In the present study, the oral absorption and excretion of this compound was investigated using rats for exploring its fate in the body, so as to better understanding its in vivo pharmacological activities. The free (parent) and total (parent plus conjugated metabolites) ICT concentrations in rat plasma, urine and bile, after intravenous (i.v.) and oral administration both at 5mg/kg, were determined before and after enzymatic hydrolysis with β-glucuronidase/sulphatase, respectively, by a HPLC–UV method. The results showed that free ICT plasma concentration after i.v. dose was rapidly decreased with average t1/2, λ of 0.43h, while the total ICT concentration was decreased slowly with t1/2, λ of 6.86h. The area under the curve of ICT conjugated metabolites was about 11-fold higher than that of free ICT. The majority of ICT in the body was excreted from the bile with 68.05% of dose over 8h after i.v. dosing, in which only 0.15% was in parent form. While very little amount of ICT was excreted from the urine with 3.01% of dose over 24h, in which the parent form was 0.62%. After oral administration, very little amount of parent ICT was detected only in 0.5, 1 or 2h plasma samples with the concentration less than LOQ, however, its total plasma concentration after enzymatic hydrolysis treatment was at relative high level with average maximum concentration of 0.49μg/ml achieved at 1h post dose. The oral bioavailability of ICT was 35% of dose, estimated by its total plasma drug concentrations. It is concluded that ICT can be easily absorbed into the body, and then rapidly conversed to its conjugated metabolites, and finally removed from the body mainly by biliary excretion.

Abstract 5464: Determination of total and encapsulated drug pharmacokinetics for CPX-351, a nanoscale liposomal fixed molar ratio of cytarabine-daunorubicin (Cyt:Daun)

Abstract Background: CPX-351 is a liposomal formulation that delivers elevated concentrations of Cyt and Daun after administration in vivo and maintains the two drugs at a 5:1 molar ratio shown to be synergistic in preclinical studies. Pharmacokinetic analysis of CPX-351 in advanced leukemia patients demonstrated prolonged plasma exposure of the drugs with quantifiable concentrations of both agents being observed as long as 7 days after the final CPX-351 dose. Here we developed a method to separate liposome-bound from unencapsulated drugs, thereby allowing the pharmacokinetics of encapsulated agents to be determined in comparison to total plasma drug concentrations. Methods: CPX-351 was administered to female Sprague-Dawley rats via IV bolus injection. Plasma samples were harvested at five timepoints from 1 – 24 hours. Unencapsulated drug analysis was conducted by addition of internal standards (gemcitabine (Cyt) and doxorubicin (Daun)) to the plasma, followed by a 5-fold dilution in buffer and ultrafiltration using Microcon YM-30. Unencapsulated drug levels were determined by reversed-phase HPLC-MS (SIM, 244.2 m/z (Cyt) and 528.3 m/z (Daun)). Total analysis was conducted by protein precipitation and analysis was determined by reversed-phase HPLC-UV (Cyt) and reversed-phase HPLC-Fluorescence (Daun). Results: A separation method was developed to separate 0.22 – 4.33% unencapsulated Cyt and 0.13 – 2.50% unencapsulated Daun from liposome in plasma matrix. Encapsulated drug was calculated by subtracting the amount of unencapsulated analyte from the corresponding amount of total analyte. The total drug curves and the calculated encapsulated drug curves from 1 – 24 hours were superimposable for Cyt and Daun. A maximum of 1.9% unencapsulated Cyt and 0.4% unencapsulated Daun was measured in the plasma at any time point. Conclusions: Unencapsulated drug levels in rat plasma are insignificant compared to the total drug levels such that levels of total and encapsulated are within the spread of the data and therefore indistinguishable. These results suggest that CPX-351 remains in circulation in rats as intact liposomes at least 24 hours post administration. Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 102nd Annual Meeting of the American Association for Cancer Research; 2011 Apr 2-6; Orlando, FL. Philadelphia (PA): AACR; Cancer Res 2011;71(8 Suppl):Abstract nr 5464. doi:10.1158/1538-7445.AM2011-5464

Factors contributing to variability in drug pharmacokinetics. II. Drug distribution.

Many factors determine the distribution of a drug within the body. All factors may be altered in physiological and pathological states, and this may result in a need to change either the dose of the drug administered or the dosage interval. Changes in plasma protein binding may also alter the interpretation of the relationship between total plasma drug concentration and drug effect, and in such circumstances the free drug concentration may provide a better guide to optimal therapy.

Impact of pH on Plasma Protein Binding in Equilibrium Dialysis

Many pharmacokinetic analyses require unbound plasma concentrations, including prediction of clearance, volume of distribution, drug-drug interactions, brain uptake analysis, etc. It is most often more convenient to measure the total drug concentration in plasma rather than the unbound drug concentration. To arrive at unbound plasma concentrations, separate in vitro determinations of the plasma protein binding of a drug are usually carried out in serum or in plasma, and the plasma pharmacokinetic results are then mathematically adjusted by this fraction unbound ( f u,p). Plasma protein binding or the drug fraction unbound in plasma ( f u,p) is known to be affected by protein, drug, free fatty acid concentrations, lipoprotein partitioning, temperature, pH, and the presence or absence of other drugs/displacing agents within plasma samples. Errors in f u,p determination caused by lack of adequate pH control in newer assay formats for plasma protein binding (e.g., 96-well equilibrium thin walled polypropylene dialysis plates) will have significant drug-specific impact on these pharmacokinetic calculations. Using a diverse set of 55 drugs and a 96-well equilibrium dialysis plate format, the effect of variable pH during equilibrium dialysis experiments on measured values of f u,p was examined. Equilibrium dialysis of human plasma against Dulbecco's phosphate buffered saline at 37 degrees C under an air or 10% CO 2 atmosphere for 22 h resulted in a final pH of approximately 8.7 and 7.4, respectively. The ratio of f u,p at pH 7.4 (10% CO 2) vs pH 8.7 (air) was >or=2.0 for 40% of the 55 compounds tested. Only one of the 55 compounds tested had a ratio <0.9. Select compounds were further examined in rat and dog plasma. In addition, physicochemical properties were calculated for all compounds using ACD/Labs software or Merck in-house software and compared to plasma protein binding results. Changes in plasma protein binding due to pH increases which occurred during the equilibrium dialysis experiment were not species specific but were drug-specific, though nonpolar, cationic compounds had a higher likely hood of displaying pH-dependent binding. These studies underscore the importance of effectively controlling pH in plasma protein binding studies.

Pharmacokinetics and Pharmacodynamic Assessment of Imipenem in the Intraperitoneal Fluid of Abdominal Surgery Patients

Background: Imipenem is used to treat intra-abdominal infections, however, this treatment would benefit from a better understanding of imipenem penetration into the abdominal cavity. Methods: Imipenem 500 mg was infused to 10 laparotomy patients. The total drug concentrations in plasma and intraperitoneal fluid were analyzed noncompartmentally and 3-compartmentally (population pharmacokinetic analysis), and used for a Monte Carlo simulation with minimum inhibitory concentration data. Results: The AUC_0–∞ ratio of the intraperitoneal fluid to the plasma was 0.82 ± 0.06 (mean ± SD, n = 6). The pharmacodynamic exposures in the intraperitoneal fluid were higher than in the plasma. The probabilities of achieving a bacteriostatic/bactericidal target (20/40% of the time above minimum inhibitory concentration for 24 h) in the intraperitoneal fluid using a regimen of 500 mg every 8 h against the Bacteroidesfragilis group, Escherichia coli and Klebsiella species were 97.3/89.1, 98.8/97.2 and 100/98.2%, respectively. However, a regimen of 1,000 mg every 8 h was needed to achieve 96.7/89.3% against Pseudomonas aeruginosa. Conclusion: These results may help us to understand the intraperitoneal penetration of imipenem and to determine the dosing regimen for intra-abdominal infections.

Toxicokintic and toxicodynamic analysis of clofibrate based on free drug concentrations in nagase analbuminemia rats (NAR)

Toxicokinetics (TK) is usually performed by measurement of the total drug concentrations in plasma. However, free drug concentrations in plasma are considered to correlate directly with toxicodynamics (TD). In the present study, to evaluate the applicability of TK/TD analysis based on free drug concentrations, we investigated the TK/TD of clofibrate, which binds to albumin with a higher ratio, using an albumin-deficient mutant strain, Nagase analbuminemia rats (NAR). TK, blood chemistry, histopathology, drug and fatty acid metabolizing enzymes and microarray analysis in the liver were examined after a 4-day oral administration of clofibrate. Compared to Sprague-Dawley (SD) rats, the parent strain of NAR, 4.1-fold higher AUC(0-24hr) based on free drug concentrations (3445 versus 844 microg.hr/ml) was observed in NAR when both rats showed the same level of AUC(0-24hr) based on the total drug concentrations (4436 versus 4237microg.hr/ml). Additionally, more severe hepatocellular hypertrophy, increase in aspartate transaminase (AST), alanine aminotransferase (ALT) and lactate dehydrogenase (LDH), decrease in total cholesterol (T.CHO), phospholipid (PL), triglyceride (TG), and non-esterified fatty acid (NEFA), and increase in the mRNA levels of fatty acid metabolizing enzymes (FAOS, CAT, and CPT) were observed in NAR at the same dose. These results demonstrated that NAR developed more severe toxicities and pharmacological effects than SD rats correlating with the higher AUC of the free drug concentrations. The results also suggested that TK/TD analysis based on the free drug concentration is appropriate to interpret the relationship between exposure and toxicity in cases of protein binding saturation including protein decrease or species differences on protein binding, especially when drugs showing a higher protein binding ratio are dosed.

The Relevance of Microdialysis for Clinical Oncology

Conventional pharmacokinetic trials in oncology measure the total amount of drug and its metabolites in the plasma of patients. These total drug concentrations are then correlated with clinical parameters such as toxicity and/or drug efficacy. However, total drug concentrations in plasma are often not directly related to tumor or tissue concentrations and the kinetics in (tumor) tissue may be very different from the kinetics of the drug in plasma. As only the unbound fraction of drugs can passively diffuse or can be actively transported across the cell membrane of endothelial cells and penetrate the tissues and tumor cells, binding of drugs to plasma components may, in part, be responsible for the variability in clinical outcomes. Also, variation in binding to cellular components may be important. As an overall consequence, in in vivo models high inter-individual variabilities of drug concentrations in the tumor have been observed, which affected drug activity. Many current cancer drug-therapies aim for a clinical benefit at the edge of acceptable toxicity, without paying attention to (variability in) plasma and tumor or tissue pharmacokinetics. This review discusses the potential applicability of microdialysis, a minimally invasive method used to determine the unbound fraction of drugs in blood or in the interstitial fluid of tissues, in clinical oncology. Although faced with some methodological challenges, the technique offers many advantages over others, and enables direct tumor assessment for pharmacokinetics and even pharmacodynamics.

Myocardium distribution of sertindole and its metabolite dehydrosertindole in guinea‐pigs

Sertindole, like other atypical antipsychotics, has been shown to increase the action potential duration and QT interval in a concentration dependent manner, in in vitro electrophysiological studies. However, this does not always translate into increased duration of the QT interval, increased risk of torsade de pointes or sudden death in clinical practice. The reasons for these apparent discrepancies are unclear and many studies have underscored the importance of the interpretation of in vitro electrophysiological data in the context of other pharmacodynamic (e.g. cardiac ion channels target, receptor affinity) and pharmacokinetic parameters (total plasma drug concentration and drug distribution). To address the possible relevance of the concentrations used in experimental studies, the myocardium distribution of sertindole and its metabolite was determined after single and repeated intraperitoneal administration to guinea-pigs. The data suggest that the plasma concentration appears to predict the concentration in the myocardium and that the myocardium concentrations of sertindole are 3.1 times higher than plasma concentrations. Using these data, the relevance of in vitro electrophysiological studies to clinical plasma concentrations has been appraised.

Prediction of In Vivo Drug-Drug Interactions from In Vitro Data

Quantitative predictions of in vivo drug-drug interactions (DDIs) resulting from metabolic inhibition are commonly made based upon the inhibitor concentration at the enzyme active site [I] and the in vitro inhibition constant (K(i)). Previous studies have involved the use of various plasma inhibitor concentrations as surrogates for [I] along with K(i) values obtained from published literature. Although this approach has resulted in a high proportion of successful predictions, a number of falsely predicted interactions are also observed. To focus on three issues that may influence the predictive value of the [I]/K(i) ratio approach: (i) the use of unbound K(i) (K(i,u)) values generated from standardised in vitro experiments compared with literature values; (ii) the selection of an appropriate [I]; and (iii) incorporation of the impact of intestinal metabolic inhibition for cytochrome P450 (CYP) 3A4 predictions. To this end we have selected eight inhibitors of CYP2C9, CYP2D6 and CYP3A4 and 18 victim drugs from a previous database analysis to allow prediction of 45 clinical DDI studies. In vitro kinetic and inhibition studies were performed in human liver microsomes using prototypic probe substrates of CYP2C9 and CYP2D6, with various inhibitors (miconazole, sulfaphenazole, fluconazole, ketoconazole, quinidine, fluoxetine, fluvoxamine). The K(i) estimates obtained were corrected for non-specific microsomal binding, and the K(i,u) was incorporated into in vivo predictions using various [I] values. Predictions for CYP3A4 were based upon in vitro data obtained from a previous publication within our laboratory, and an assessment of the impact of the interaction in the gut wall is included. Predictions were validated against 45 in vivo studies and those within 2-fold of the in vivo ratio of area under the plasma concentration-time curve of the substrate, in the presence and absence of the inhibitor (AUC(i)/AUC) were considered successful. Predictions based upon the average systemic total plasma drug concentration ([I](av)) [incorporating the effects of parallel drug elimination pathways] and the K(i,u) value resulted in 91% of studies predicted to within 2-fold of the in vivo AUC(i)/AUC. This represents a 35% improvement in prediction accuracy compared with predictions based upon total K(i) values obtained from various published literature sources. A corresponding reduction in bias and an increase in precision were also observed compared with the use of other [I] surrogates (e.g. the total and new unbound maximum hepatic input plasma concentrations). No significant improvement in prediction accuracy was observed by incorporating consideration of gut wall inhibition for CYP3A4. DDI predictions based upon the use of K(i,u) data obtained under a set of optimal standardised conditions were significantly improved compared with predictions using in vitro data collated from various sources. The use of [I](av) as the [I] surrogate generated the most successful predictions as judged by several criteria. Incorporation of either plasma protein binding of inhibitor or gut wall CYP3A4 inhibition did not result in a general improvement of DDI predictions.

Volume of Distribution at Steady State for a Linear Pharmacokinetic System with Peripheral Elimination

The problem of finding the steady-state volume of distribution Vss for a linear pharmacokinetic system with peripheral drug elimination is considered. A commonly used equation Vss = (D/AUC)*MRT is applicable only for the systems with central (plasma) drug elimination. The following equation, Vss = (D/AUC)*MRTint, was obtained, where AUC is the commonly calculated area under the time curve of the total drug concentration in plasma after intravenous (iv) administration of bolus drug dose, D, and MRTint is the intrinsic mean residence time, which is the average time the drug spends in the body (system) after entering the systemic circulation (plasma). The value of MRTint cannot be found from a drug plasma concentration profile after an iv bolus drug input if a peripheral drug exit occurs. The obtained equation does not contain the assumption of an immediate equilibrium of protein and tissue binding in plasma and organs, and thus incorporates the rates of all possible reactions. If drug exits the system only through central compartment (plasma) and there is an instant equilibrium between bound and unbound drug fractions in plasma, then MRTint becomes equal to MRT = AUMC/AUC, which is calculated using the time course of the total drug concentration in plasma after an iv bolus injection. Thus, the obtained equation coincides with the traditional one, Vss = (D/AUC)*MRT, if the assumptions for validity of this equation are met. Experimental methods for determining the steady-state volume of distribution and MRTint, as well as the problem of determining whether peripheral drug elimination occurs, are considered. The equation for calculation of the tissue–plasma partition coefficient with the account of peripheral elimination is obtained. The difference between traditionally calculated Vss = (D/AUC)*MRT and the true value given by (D/AUC)*MRTint is discussed. © 2004 Wiley-Liss, Inc. and the American Pharmacists Association J Pharm Sci 93:1628–1640, 2004

A comparison of the receptor binding and HERG channel affinities for a series of antipsychotic drugs

Many antipsychotic drugs produce QT interval prolongation on the electrocardiogram (ECG). Blockade of the human cardiac K + channel known as human ether- a- go- go-related gene (HERG) often underlies such clinical findings. In fact, HERG channel inhibition is now commonly used as a screen to predict the ability of a drug to prolong QT interval. However, the exact relationship between HERG channel blockade, target receptor binding affinity and clinical QT prolongation is not known. Using patch-clamp electrophysiology, we examined a series of seven antipsychotic drugs for their ability to block HERG, and determined their IC 50 values. We then compared these results to their binding affinities ( K i values) for the dopamine D 2 receptor, the 5-HT 2A receptor and, where available, to clinical QT prolongation data. We found that sertindole, pimozide and thioridazine displayed little (<10-fold) or no selectivity for dopamine D 2 or 5-HT 2A receptors relative to their HERG channel affinities. This lack of selectivity likely underlies the significant QT interval prolongation observed with administration of these drugs. Of the other drugs tested (ziprasidone, quetiapine, risperidone and olanzapine), olanzapine displayed the greatest selectivity for dopamine D 2 and 5-HT 2A receptor binding (100–1000-fold) compared to its HERG channel IC 50. We also compared these HERG channel IC 50 values to QT interval prolongation and plasma drug levels obtained in a recent clinical study. We found that the ratio of total plasma drug concentration to HERG IC 50 value was indicative of the degree of QT prolongation observed. Target receptor affinity and expected clinical plasma levels are important parameters to consider for the interpretation of HERG channel data.

Correlation between the pharmacokinetics of oxindanac and inhibition of serum thromboxane B2in calves

The pharmacokinetics and pharmacodynamics of the non-steroidal antiinflammatory drug, oxindanac, were assessed simultaneously in calves after intravenous (i.v.) administration at dose rates of 0.5, 1, 2, 4 and 8 mg/kg. Plasma pharmacokinetic data were fitted to either two or three compartment open models. The elimination t1/2 was constant in the dose range 0.5 to 4 mg/kg (20.2-22.8 h) and shorter at 8 mg/kg (14.7 h). The pharmacodynamics of oxindanac were assessed by its inhibition of serum TxB2, an index of platelet cyclo-oxygenase activity. Plots of total plasma oxindanac concentration vs. inhibition of serum TxB2 fitted in all cases a sigmoidal Emax equation. There were no significant differences in the estimates for ED50 (1.6-1.9 micrograms/ml), Hill constant (1.3-2.7) or Emax between the doses used in the in vivo studies or when blood was spiked with oxindanac in vitro. Plots of inhibition of serum TxB2 vs. time were prepared from the pharmacokinetic model equations in each calf in combination with a single sigmoidal Emax plot generated in vitro. These data were not significantly different from the results produced in vivo. It is concluded that oxindanac causes reversible inhibition of platelet cyclo-oxygenase in calves. Its inhibition of serum TxB2 can be predicted from total plasma drug concentration, as described by a multicompartmental model, and sigmoidal Emax enzyme kinetics. It was not necessary to take into account factors such as drug equilibration between plasma and its target site, free vs. total drug concentration or chirality. This simple model may be useful for predicting the pharmacodynamics of oxindanac in other species.

Tenoxicam: Acute Dose-Dependent Disposition Studies in Rats

Single intravenous bolus doses of tenoxicam of 2.5,5, and 10 mg/kg were administered to male Wistar rats to determine the effects of dose on tenoxicam pharmacokinetics. Predicted apparent volume of distribution at steady state (Vdsa) and total plasma clearance (CL) were, respectively, 42 and 45% higher in the animals given 10-mg/kg dose than the animals given 2.5- and 5-mg/kg doses. Binding of tenoxicam to plasma proteins showed saturability, with a 33% higher unbound fraction of tenoxicam in plasma when total drug concentration in plasma was 36 mg/L (high dose group) in comparison with animals given the low doses (12 and 20 mg/L). The blood-to-plasma concentration ratio of tenoxicam was concentration independent and therefore did not account for the observed dose-dependent changes in Vdaa and CL

Stereoselective and isozyme‐selective drug interactions

A surprisingly large number of marketed drugs are racemic mixtures. The pharmacokinetic literature on racemic drugs contains a vast amount of information on drug-drug interactions derived from the measurement of total drug concentrations in plasma and urine. The appreciation of the role of stereochemistry in drug interactions with racemic warfarin resulted in a long-overdue scientific rigor being applied to the study of drug interactions. It also compelled us to recognize that much of the literature was uninterpretable. A better understanding of oxidative metabolism, particularly the complexity of the cytochrome P-450 family of enzymes, has also strengthened the scientific basis of drug interactions. We now recognize that investigators and clinicians must consider both stereoselectivity and isozyme selectivity in the study of drug interactions to understand the nature of the interaction so as to more effectively use new and potent drugs.