Errors in estimating the unbound fraction of drugs due to the volume shift in equilibrium dialysis

Non-specific binding of compounds in in vitro metabolism assays: a comparison of microsomal and hepatocyte binding in different species and an assessment of the accuracy of prediction models

Non-specific binding in in vitro metabolism systems leads to an underestimation of the true intrinsic metabolic clearance of compounds being studied. Therefore in vitro binding needs to be accounted for when extrapolating in vitro data to predict the in vivo metabolic clearance of a compound. While techniques exist for experimentally determining the fraction of a compound unbound in in vitro metabolism systems, early in drug discovery programmes computational approaches are often used to estimate the binding in the in vitro system.Experimental fraction unbound data (n = 60) were generated in liver microsomes (fu_mic) from five commonly used pre-clinical species (rat, mouse, dog, minipig, monkey) and humans. Unbound fraction in incubations with mouse, rat or human hepatocytes was determined for the same 60 compounds. These data were analysed to determine the relationship between experimentally determined binding in the different matrices and across different species. In hepatocytes there was a good correlation between fraction unbound in human and rat (r²=0.86) or mouse (r²=0.82) hepatocytes. Similar correlations were observed between binding in human liver microsomes and microsomes from rat, mouse, dog, Göttingen minipig or monkey liver microsomes (r² of >0.89, n = 51 - 52 measurements in different species). Physicochemical parameters (logP, pKa and logD) were predicted for all evaluated compounds. In addition, logP and/or logD were measured for a subset of compounds.Binding to human hepatocytes predicted using 5 different methods was compared to the measured data for a set of 59 compounds. The best methods evaluated used measured microsomal binding in human liver microsomes to predict hepatocyte binding. The collated physicochemical data were used to predict the human fu_mic using four different in silico models for a set of 53-60 compounds. The correlation (r²) and root mean square error between predicted and observed microsomal binding was 0.69 & 0.20, 0.47 & 0.23, 0.56 & 0.21 and 0.54 & 0.26 for the Turner-Simcyp, Austin, Hallifax-Houston and Poulin models, respectively. These analyses were extended to include measured literature values for binding in human liver microsomes for a larger set of compounds (n=697). For the larger dataset of compounds, microsomal binding was well predicted for neutral compounds (r²=0.67 - 0.70) using the Poulin, Austin, or Turner-Simcyp methods but not for acidic or basic compounds (r²<0.5) using any of the models. While the lipophilicity-based models can be used, the in vitro binding should be measured for compounds where more certainty is needed, using appropriately calibrated assays and possibly established weak, moderate, and strong binders as reference compounds to allow comparison across databases.

Optimizing a High-Throughput Solid-Phase Microextraction System to Determine the Plasma Protein Binding of Drugs in Human Plasma

Plasma protein binding refers to the binding of a drug to plasma proteins after entering the body. The measurement of plasma protein binding is essential during drug development and in clinical practice, as it provides a more detailed understanding of the available free concentration of a drug in the blood, which is in turn critical for pharmacokinetics and pharmacodynamics studies. In addition, the accurate determination of the free concentration of a drug in the blood is also highly important for therapeutic drug monitoring and in personalized medicine. The present study uses C₁₈-coated solid-phase microextraction 96-pin devices to determine the free concentrations of a set of drugs in plasma, as well as the plasma protein binding of drugs with a wide range of physicochemical properties. It should be noted that the extracted amounts used to calculate the binding constants and plasma protein bindings should be measured at respective equilibrium for plasma and phosphate buffer. Therefore, special attention is placed on properly determining the equilibration times required to correctly estimate the free concentrations of drugs in the investigated systems. The plasma protein binding values obtained with the 96-pin devices are consistent with those reported in the literature. The 96-pin device used in this research can be easily coupled with a Concept96 or other automated robotic systems to create an automated plasma protein binding determination protocol that is both more time and labor efficient compared to conventional equilibrium dialysis and ultrafiltration methods.

Electromembrane extraction as a new approach for determination of free concentration of phenytoin in plasma using capillary electrophoresis

PURPOSE

Electromembrane extraction is a new membrane-based extraction method in which charged compounds are extracted by an electric field. So far, this method has been used to extract and isolate a variety of acidic and basic drugs from various samples, including blood and plasma. However, in this procedure, it is not yet clear whether only unbound fraction of a drug is extracted or the total drug. The aim of this study is to reveal the nature of drug extraction in the presence of plasma proteins.

METHODS

To determine the nature of the extraction, the electromembrane extraction was performed from plasma solutions of phenytoin with concentrations 0.03 and 1.0 μg/mL, then the result was compared with the values obtained from the electromembrane extraction of ultrafiltrate of the same solutions (free concentration) and protein-free ultrafiltrate of plasma with final concentration of 0.03 and 1.0 μg/mL (total concentration). For this purpose, EME followed by capillary electrophoresis coupled with diode array detection was optimized and validated.

RESULTS

The results showed that the electromembrane extraction method was only able to extract the unbound fraction of phenytoin from plasma samples. The method was validated over a concentration range of 0.03-4 μg/mL. The inter and intra-assay precisions were less than 6.7%. The phenytoin protein binding was also determined to be in agreement with the literature data and confirms the validity of this method.

CONCLUSION

This sensitive and quick EME approach for determining the free concentration of a phenytoin, can be a good alternative to classic methods for therapeutic drug monitoring and pharmacokinetic studies.

Assay validation and determination of in vitro binding of mefloquine to plasma proteins from clinically normal and FIP-affected cats

The antimalarial agent mefloquine is currently being investigated for its potential to inhibit feline coronavirus and feline calicivirus infections. A simple, high pressure liquid chromatography assay was developed to detect mefloquine plasma concentrations in feline plasma. The assay’s lower limit of quantification was 250 ng/mL. The mean ± standard deviation intra- and inter-day precision expressed as coefficients of variation were 6.83 ± 1.75 and 5.33 ± 1.37%, respectively, whereas intra- and inter-day accuracy expressed as a percentage of the bias were 11.40 ± 3.73 and 10.59 ± 3.88%, respectively. Accordingly, this validated assay should prove valuable for future in vivo clinical trials of mefloquine as an antiviral agent against feline coronavirus and feline calicivirus. However, the proportion of mefloquine binding to feline plasma proteins has not been reported. The proportion of drug bound to plasma protein binding is an important concept when developing drug dosing regimens. As cats with feline infectious peritonitis (FIP) demonstrate altered concentrations of plasma proteins, the proportion of mefloquine binding to plasma proteins in both clinically normal cats and FIP-affected cats was also investigated. An in vitro method using rapid equilibrium dialysis demonstrated that mefloquine was highly plasma protein bound in both populations (on average > 99%).

Brain Distribution of Drugs: Pharmacokinetic Considerations

It is crucial to understand the basic principles of drug transport, from the site of delivery to the site of action within the CNS, in order to evaluate the possible utility of a new drug candidate for CNS action, or possible CNS side effects of non-CNS targeting drugs. This includes pharmacokinetic aspects of drug concentration-time profiles in plasma and brain, blood-brain barrier transport and drug distribution within the brain parenchyma as well as elimination processes from the brain. Knowledge of anatomical and physiological aspects connected with drug delivery is crucial in this context. The chapter is intended for professionals working in the field of CNS drug development and summarizes key pharmacokinetic principles and state-of-the-art experimental methodologies to assess brain drug disposition. Key parameters, describing the extent of unbound (free) drug across brain barriers, in particular blood-brain and blood-cerebrospinal fluid barriers, are presented along with their application in drug development. Special emphasis is given to brain intracellular pharmacokinetics and its role in evaluating target engagement. Fundamental neuropharmacokinetic differences between small molecular drugs and biologicals are discussed and critical knowledge gaps are outlined.

Drug-carrier binding and enzymatic carrier digestion in amorphous solid dispersions containing proteins as carrier

In order to further explain the ability of gelatin 50PS and bovine serum albumin (BSA) to generate supersaturation of a series of poorly soluble drugs (carbamazepine, cinnarizine, diazepam, itraconazole, nifedipine, indomethacin, darunavir (ethanolate), ritonavir, fenofibrate, griseofulvin, ketoconazole, naproxen, phenylbutazone and phenytoin), drug-polymer binding was investigated using solution NMR and equilibrium dialysis experiments. Binding characteristics of the biopolymers were compared to those of PVP, PVPVA and HPMC. Since both biopolymers are prone to enzymatic digestion, we evaluated the influence of proteolytic enzymes like pepsin and pancreatin on the dissolution properties of poorly soluble compounds when formulated as amorphous solid dispersions with gelatin 50PS and BSA. Evidence is being presented that supports the importance of drug-polymer binding in inducing and stabilizing supersaturation of poorly soluble drugs and enhancing dissolution from ASDs. In fact, BSA displayed drug binding with nearly all tested model drugs while in case of gelatin 50PS binding was observed for 5 out of 12 drugs. Addition of pepsin or pancreatin during dissolution of the biopolymer-containing ASDs leads to a drop in the concentration of the drug pointing to enzymatic digestion of the gelatin and BSA. However, after digestion, these formulations still outperformed their crystalline counterparts.

Determination of binding properties of ampicillin in drug-human serum albumin standard solution using N-vinylpyrrolidone copolymer combined with the micellar systems

It is well-known that only the unbound (free) drug fraction can achieve a pharmacological effect. Therefore the determination of free drug concentration is a very important issue in the field of pharmacology. In this study poly-1-vinyl-2-pyrrolidone (VP) crosslinked with divinylbenzene (DVB) compared with the micellar liquid chromatography (MLC) with and without pre-made drug adsorption was used for quantitative analysis of free ampicillin concentration in the standard solution of drug-human serum albumin owing to its ability to block protein adsorption. The commonly recognized adsorption method based on drug adsorption on VP-DVB has been compared to the entirely new application of MLC with direct sample injection (DSI) not requiring pre-made adsorption. Micellar aggregates are able to solubilize various compounds therefore micellar environment can be used for direct determination of free drug concentration. The obtained results show that the free drug concentration values obtained in the micellar systems based on cetyltrimethylammonium bromide (CTAB) (93.98μgL^-1, 78.3%) as well as on polyoxyethylene (23) lauryl ether (Brij35) (91.15μgL^-1, 75.9%) are similar to those obtained after the drug adsorption on VP-DVB using both RP-HPLC (95.85μgmL^-1, 79.9%) and spectrophotometry (96.47μgmL^-1, 80.4%). However, only %PPB (% plasma protein binding) value calculated on the basis of Brij35 retention factor is similar to the literature data. The obtained results are within the analytical range of % of free drug concentration. Therefore N-vinylpyrrolidone copolymer as well as micellar system based on the non-ionic surfactant can be successfully applied for determination of free drug concentration. Moreover, the new application of MLC with DSI can be recognized as a promising, fast and simple method for quantitative determination of free drug concentration.

Interpreting physicochemical experimental data sets

With the wealth of experimental physicochemical data available to chemoinformaticians from the literature, commercial, and company databases an increasing challenge is the interpretation of such datasets. Subtle differences in experimental methodology used to generate these datasets can give rise to variations in physicochemical property values. Such methodology nuances will be apparent to an expert experimentalist but not necessarily to the data analyst and modeller. This paper describes the differences between common methodologies for measuring the four most important physicochemical properties namely aqueous solubility, octan-1-ol/water distribution coefficient, pK(a) and plasma protein binding highlighting key factors that can lead to systematic differences. Insight is given into how to identify datasets suitable for combining.

A general chemiluminescence strategy for measuring aptamer-target binding and target concentration

Although much effort has been made for studies on aptamer-target interactions due to promising applications of aptamers in biomedical and analytical fields, measurement of the aptamer-target binding constant and binding site still remains challenging. Herein, we report a sensitive label-free chemiluminescence (CL) strategy to determine the target concentration and, more importantly, to measure the target-aptamer binding constant and binding site. This approach is suitable for multiple types of targets, including small molecules, peptides, and proteins that can enhance the CL initiated by N-(aminobutyl)-N-ethylisoluminol functionalized gold colloids, making the present method a general platform to investigate aptamer-target interactions. This approach can achieve extremely high sensitivity with nanogram samples for measuring the target-aptamer binding constant. And the measurement could be rapidly performed using a simple and low-cost CL system. It provides an effective tool for studying the binding of biologically important molecules to nucleic acids and the selection of aptamers. Besides, we have also discovered that the 14-mer aptamer fragment itself split from the ATP-binding aptamer could selectively capture ATP. The binding constant, site, and conformation between ATP and the 14-mer aptamer fragment were obtained using such a novel CL strategy and molecular dynamic simulation.

High performance affinity chromatography (HPAC) as a high-throughput screening tool in drug discovery to study drug-plasma protein interactions

Drug-plasma protein binding is an important parameter that, together with other physicochemical properties such as lipophilicity and pK(a), greatly influences drug absorption, distribution, metabolism, and excretion (ADME). Therefore, it is important for pharmaceutical companies to develop a rapid screening assay to examine plasma protein binding during the early stages of the drug discovery process. Human serum albumin (HSA) and α(1)-acid glycoprotein (AGP) are the most important plasma proteins that are capable of binding drugs. In this work, an automated and high-throughput (<3 min/compound) strategy was developed using high performance affinity chromatography (HPAC) with commercial HSA and AGP columns to evaluate drug-plasma protein interactions for drug screening. A generic gradient was used throughout the study to separate drugs that were weakly and tightly bound to HSA and AGP. To accelerate the analysis time, the system was calibrated in a single run by pooling reference compounds without overloading the column. For both HSA and AGP studies, the developed methods were successfully transferred from HPAC-UV to HPAC-MS with single quadrupole MS detection and ammonium acetate, pH 7.0 as a volatile mobile phase. The MS detection enhanced the sensitivity, selectivity, and throughput of the method by pooling unknown compounds. For HSA analyses, the binding percentages obtained using HPAC were well correlated with the binding percentages from the literature. This method was also able to rank compounds based on their affinity for HSA. Concerning the AGP analyses, the quality of the correlation between the binding percentages obtained in HPAC and those from the literature was weaker. However, the method was able to classify compounds into weak, medium, and strong binders and rank compounds based on their affinity for AGP.

Effect of Theophylline on the Intestinal Clearance of Drugs in Rats

Intestinal eχsorption of salicylic acid, urea and quinidine was measured during the perfusion of the rat intestinal lumen with Tyrode solution. The intestinal clearance (CLi) of the three compounds was measured by dividing the rate of appearance in the intestinal luminal perfusate by the plasma concentration of the compound. Co-administration of theophylline (0 −2 mg h−1) with the test agents increased the CLi of salicylic acid, did not alter the CLi of urea, but decreased the CLi of quinidine. The effect of theophylline on the CLi of quinidine was enhanced with increasing dose. Theophylline was found to increase microclimate-pH at the intestinal surface, but the magnitude of ΔpH alone could not explain the effect of theophylline on the CLi of quinidine. The data, together with previous observations, suggest that the intestinal eχsorption of drugs was affected by the microclimate pH and by the unstirred water layer. Theophylline affects CLi of salicylic acid and quinidine partly by increasing the microclimate pH of the intestine. Theophylline may also affect quinidine CLi by inhibiting the carrier-mediated pathway.

Concentration-dependent Exsorption of Quinidine in the Rat Intestine

During intravenous infusion, the luminal concentration of quinidine was higher than the plasma concentration. The intestinal clearance (CLi) of the drug was measured by dividing the rate of appearance of the drug in the intestinal luminal perfusate by the plasma concentration. The CLi of quinidine was therefore much higher than the rate of luminal perfusion. Over the infusion dose range of 0·1–2 mg h−1, the CLi of quinidine decreased with increasing plasma concentration of quinidine. Adding quinidine into the luminal perfusate had little effect on the CLi of quinidine. Co-administration of quinidine with other agents intravenously did not alter the CLi of salicylic acid and urea, while the same treatment decreased the CLi of theophylline and 5-disopyramide. In-vitro experiments on brush-border membrane vesicles showed that quinidine decreased the rate of Na+ uptake and H+ efflux. The inhibition was significant at quinidine concentrations above 20 μm. Quinidine was a more potent inhibitor than amiloride. At quinidine infusion rates less than 2 mg h−1, quinidine concentration in plasma or in the luminal perfusate was at the lower limit of the inhibitory concentration. Microclimate pH at the intestinal surface was also measured. At mid-jejunum, the microclimate pH increased 0·3 pH units by infusing 2 mg h−1 of quinidine, while the microclimate pH at most other measuring sites was not significantly altered by quinidine infusion. It was concluded that quinidine is exsorbed from blood into the intestinal lumen by a carrier-mediated pathway in addition to the passive diffusion. At high plasma concentration, quinidine exsorption becomes saturated. Quinidine inhibited the intestinal exsorption of theophylline and S-disopyramide possibly by competition on the carrier.

Semi-automated protein binding methodology using equilibrium dialysis and a novel mixed-matrix cassette approach

A semi-automated protein binding assay using equilibrium dialysis (ED) and a novel mixed-matrix methodology has been developed. This method decreases mass spectrometer run time and reduces the likelihood of experimental artifacts. In this cassette-based approach, a single matrix is prepared following dialysis by mixing dialyzed plasma and buffer containing different test compounds from the same dialysis plate. This approach differs from the traditional mixed-matrix method where fresh plasma and fresh buffer are mixed with opposing dialyzed samples. This new mixed-matrix methodology is compatible with various high-throughput ED and ultrafiltration devices, many liquid handling systems, and can be used for plasma, serum, albumin, alpha-1 acid glycoprotein, microsomal, and fine tissue homogenate binding studies. The utility of the method can be further enhanced by varying the number of replicates, concentrations, and matrices with simple modifications. Using 29 structurally diverse marketed drugs with a wide range of protein binding values reported in the literature, we have shown the new procedure reduces the total number of samples by nearly half compared to traditional methods, eliminates the need for standard curves, and increases the uniformity of the sample matrix for LC/MS/MS analysis.

Methods for measuring aptamer-protein equilibria: a review

Aptamers are single stranded DNA or RNA molecules that have been selected using in vitro techniques to bind target molecules with high affinity and selectivity, rivaling antibodies in many ways. In order to use aptamers in research and clinical applications, a thorough understanding of aptamer-target binding is necessary. In this article, we review methods for assessing aptamer-protein binding using separation based techniques such as dialysis, ultrafiltration, gel and capillary electrophoresis, and HPLC; as well as mixture based techniques such as fluorescence intensity and anisotropy, UV-vis absorption and circular dichroism, surface plasmon resonance, and isothermal titration calorimetry. For each method the principle, range of application and important features, such as sample consumption, experimental time and complexity, are summarized and compared.

Drug-protein binding: a critical review of analytical tools

The extent of drug binding to plasma proteins, determined by measuring the free active fraction, has a significant effect on the pharmacokinetics and pharmacodynamics of a drug. It is therefore highly important to estimate drug-binding ability to these macromolecules in the early stages of drug discovery and in clinical practice. Traditionally, equilibrium dialysis is used, and is presented as the reference method, but it suffers from many drawbacks. In an attempt to circumvent these, a vast array of different methods has been developed. This review focuses on the most important approaches used to characterize drug-protein binding. A description of the principle of each method with its inherent strengths and weaknesses is outlined. The binding affinity ranges, information accessibility, material consumption, and throughput are compared for each method. Finally, a discussion is included to help users choose the most suitable approach from among the wealth of methods presented.

Use of a surface plasmon resonance method to investigate antibiotic and plasma protein interactions

The pharmacologic effect of an antibiotic is directly related to its unbound concentration at the site of infection. Most commercial antibiotics have been selected in part for their low propensity to interact with serum proteins. These nonspecific interactions are classically evaluated by measuring the MIC in the presence of serum. As higher-throughput technologies tend to lose information, surface plasmon resonance (SPR) is emerging as an informative medium-throughput technology for hit validation. Here we show that SPR is a useful automatic tool for quantification of the interaction of model antibiotics with serum proteins and that it delivers precise real-time kinetic data on this critical parameter.

Development and validation of a 96-well equilibrium dialysis apparatus for measuring plasma protein binding

A 96-well equilibrium dialysis block was designed and constructed that is compatible with most standard 96-well format laboratory supplies and instruments. The unique design of the dialysis apparatus allows one to dispense and aspirate from either or both the sample and dialysate sides from the top of the apparatus, which is not possible with systems currently on the market. This feature permits the investigator to analyze a large number of samples, time points, or replicates in the same experiment. The novel alignment of the dialysis membrane vertically in the well maximizes the surface-to-volume ratio, eliminates problems associated with trapped air pockets, and allows one to add or remove samples independently or all at once. Furthermore, the design of the apparatus allows both the sample and dialysate sides of the dialysis well to be accessible by robotic systems, so assays can be readily automated. Teflon construction is used to minimize nonspecific binding of test samples to the apparatus. The device is reusable, easily assembled, and can be shaken in controlled temperature environments to decrease the time required to reach equilibrium as well as facilitate dissolution of test compounds. Plasma protein binding values obtained for 10 diverse compounds using standard dialysis equipment and the 96-well dialysis block validates this method.

Stereoselectivity of ibuprofen metabolism and pharmacokinetics following the administration of the racemate to healthy volunteers

1. The stereoselective metabolism and pharmacokinetics of the enantiomers of ibuprofen have been investigated following the oral administration of the racemic drug (400 mg) to 12 healthy volunteers.2. The stereochemical composition of the drug in serum, both total and unbound, and drug and metabolites, both free and conjugated, in urine were determined by a combination of the direct and indirect chromatographic procedures to enantiomeric analysis. 3. The oral clearance of (S)-ibuprofen was significantly greater than that of the R-enantiomer (74.5 +/- 18.1 versus 57.1 +/- 11.7 ml min(-1); p < 0.05) and the clearance of (R)-ibuprofen via inversion was ca two fold that via alternative pathways. 4. Some 74.0 +/- 9.6% of the dose was recovered in urine over 24 h as ibuprofen, 2-hydroxyibuprofen and carboxyibuprofen, both free and conjugated with glucuronic acid. Analysis of the stereochemical composition of the urinary excretion products indicated that 68% of the dose of (R)-ibuprofen had undergone chiral inversion. 5. Metabolism via glucuronidation and both routes of oxidation, showed enantio-selectivity for (S)-ibuprofen, the enantiomeric ratios (S/R) in partial metabolic clearance being 7.1, 4.8 and 3.4 for formation of ibuprofen glucuronide, 2-hydroxyibuprofen and carboxyibuprofen respectively.6. Modest stereoselectivity was observed in the formation of (2'R, 2R)- and (2'S, 2S)-carboxyibuprofen in comparison to the alternative diastereoisomers, the ratios in formation clearance being 1.6 and 1.2 respectively.

Development and validation of a liquid chromatographic method for Casiopeina IIIi in rat plasma

A sensitive and specific liquid chromatographic method using extraction with zinc sulfate has been developed for the determination of Casiopeina IIIi and validated over the linear range 5-100 microg/ml in 1 ml of rat plasma. The analysis was performed on a Symmetry C(18) (5 microm) column. The mobile phase was methanol: 0.01 M phosphate buffer pH 6.5 (40:60, v/v). The column effluent was monitored at 262 nm. The results showed that the assay is sensitive at 5 microg/ml. Maximum intra-day coefficient of variation was 10.6%. The recovery obtained in plasma was 87.2%. The method was used to perform protein binding studies by equilibrium dialysis in rat plasma and was found to be satisfactory.

A rapid spectrofluorimetric technique for determining drug-serum protein binding suitable for high-throughput screening

PURPOSE

To develop and validate a rapid method for determining the dissociation constants with which pharmaceutical candidates and drugs bind to serum albumin and to alpha1-acid glycoprotein with the goal of deducing the extent of binding.

METHODS

The quenching of the intrinsic tryptophan fluorescence of serum albumin and alpha1-acid glycoprotein was monitored by spectrofluorimetry and the data were used to calculate the apparent dissociation constant. Sodium warfarin was used to probe the warfarin-binding site of serum albumin and diazepam was used to probe the benzodiazepine binding site. Additionally, the binding of sodium salicylate, phenylbutazone, sulfinpyrazone, iophenoxic acid, theophylline, chloramphenicol, acetaminophen, lithium chloride and ampicillin were also investigated. Chlorpromazine hydrochloride and imipramine hydrochloride were used as probes for alpha1-acid glycoprotein. The assays were also extended to the multiwell format. The quenching curves were fitted to the quadratic binding equation to determine the dissociation constants.

RESULTS

Intrinsic fluorescence measurements are an excellent predictor of the drug binding to human serum albumin and to alpha1-acid glycoprotein. These measurements detect binding to the warfarin and benzodiazepine binding sites of human serum albumin. The dissociation constants estimated using the method compare favorably to the dissociation constants previously reported by Epps et al. using extrinsic fluorescence methodology, and the results correlate well with equilibrium dialysis using drug displacement endpoints.

CONCLUSIONS

These measurements can be carried out with small samples and do not require separation of the bound and unbound species. Additionally, the proposed methods eliminate membrane separations, are not compound specific and do not require analytical chromatography or mass spectrometry for quantitation. Spectrofluorimetry may prove to be a useful method for rapidly determining the protein binding of combinatorial libraries.

Stereoselective binding of zopiclone to human plasma proteins

The binding of racemic zopiclone (ZOP) and of its two enantiomers to plasma proteins, albumin and alpha 1-acid glycoprotein were compared. Our work shows that the binding of ZOP to human plasma proteins is stereoselective. The total plasma protein binding percentages were 79.3 +/- 5.5%, 83.8 +/- 5.2%, and 75.1 +/- 2.1%, for racemic zopiclone, (-)zopiclone and (+)zopiclone, respectively. These results were confirmed by the analysis of samples obtained from healthy volunteers after the oral administration of ZOP. The anticoagulant used for sampling was also shown to have an influence on the percentage binding and on its stereoselectivity. Considering albumin and alpha 1-acid glycoprotein separately, stereoselectivity was also observed.

Disposition of metronidazole in hens (Gallus gallus) and quails (Coturnix coturnix japonica): pharmacokinetics and whole-body autoradiography

Hens were given single intravenous or oral doses (30 mg/kg body weight) of metronidazole and the plasma concentrations of the drug were determined by high-performance liquid chromatography (HPLC) at intervals from 10 min to 24 h after drug administration. Pharmacokinetic variables were calculated by the Lagrange algorithm technique. The elimination half-life (t1/2 beta) after the intravenous injection was 4.2 +/- 0.5 h, the volume of distribution (Vd(ss) 1.1 +/- 0.2 L/kg and the total body clearance (ClB) 131.2 +/- 20 mL/h.kg. Oral bioavailability of the metronidazole was 78 +/- 16%. The plasma maximum concentration (Cmax) 31.9 +/- 2.3 micrograms/mL was reached 2 h after the oral administration and the oral elimination half-life (t1/2 beta) was 4.7 +/- 0.2 h. The binding of metronidazole to proteins in hen plasma was very low (less than 3%). Whole body autoradiography of [3H] metronidazole in hens and quails showed an even distribution of labelled material in various tissues at short survival intervals (1-4 h) after oral or intravenous administration. A high labelling was seen in the contents of the small and large intestines. In the laying quails a labelling was also seen in the albumen and in a ring in the periphery of the yolk at long survival intervals. Our results show that a concentration twofold above the MIC is maintained in the plasma of hens for at least 12 h at an oral dose of 30 mg/kg metronidazole.

Drug-protein binding sites. New trends in analytical and experimental methodology

In the last few years, continuous progress in instrumental analytical methodology has been achieved with a substantial increase in the number of new, more specific and more flexible methods for ligand-protein assays. In general, the methods used for drug-protein binding studies can be divided into two main groups: separation methods (enabling the calculation of binding parameters, i.e. the number of binding sites and their respective affinity constants) and non-separation methods (describing predominantly qualitative parameters of the ligand-protein complex). This review will be focussed particularly on recent trends in the development of drug-protein binding methods including stereoselective and non-stereoselective aspects using chromatography, capillary electrophoresis and microdialysis as compared to the "conventional approach" using equilibrium dialysis, ultrafiltration or size exclusion chromatography. The advantages and limitations of various methods will be discussed including a focus on "optimal" experimental strategies taking into account in vitro, ex vivo and/or in vivo studies. Furthermore, the importance of some particular aspects concerning the drug binding to proteins (covalent binding of drugs and metabolites, stereoselective interactions and evaluation of binding data) will be outlined in more detail.

Influence of metabolites on protein binding of verapamil enantiomers

This study investigated the effect of verapamil metabolites on R- and S-verapamil protein binding in plasma samples collected from subjects prior to rac-verapamil dosing and following single dose and steady state rac-verapamil dosing. In vitro studies of the effects of norverapamil, D617 and D620 on R- and S-verapamil protein binding were also performed. Protein binding of R- and S-verapamil was unchanged following single and multiple doses of rac-verapamil as compared with protein binding in pre-dose samples. In vitro, norverapamil had no effect on R- and S-verapamil protein binding up to 1000 ng ml-1. Norverapamil 5000 ng ml-1 caused a 30% increase in free fraction of both R- and S-verapamil. D617 and D620 concentrations up to 5000 ng ml-1 had no effect on R- and S-verapamil protein binding. We conclude the metabolites of verapamil have no clinically significant effect on R- and S-verapamil protein binding.

Evaluation of ultrafiltration for the free-fraction determination of single photon emission computed tomography (SPECT) radiotracers: beta-CIT, IBF, and iomazenil

An ultrafiltration system was evaluated for the free-fraction measurement of SPECT radiotracers (beta-CIT, IBF, and iomazenil) used in functional brain imaging. The effect of temperature, storage, centrifugal force, tracer concentration, and percentage filtered demonstrated a relative error of < 9%. As a result of the minimal temperature effect, 25 degrees C was employed for all measurements. A comparison of the ultrafiltration system with equilibrium dialysis revealed < 5% difference for beta-CIT and iomazenil, but 16% for IBF. Additionally, the time and ease of operation considerably favored the ultrafiltration system. The precision quantitated by repetition was < 6% for between-run and within-run variability. In conclusion, ultrafiltration provided rapid results, demonstrated minor analytical errors, revealed generally good correlation with equilibrium dialysis, and allowed excellent precision.

Plasma protein binding of the experimental antitumour agent acridine-4-carboxamide in man, dog, rat and rabbit

The plasma binding of N-[2-(dimethylamino)ethyl]acridine-4-carboxamide (AC) was investigated in-vitro by equilibrium dialysis for 3 h at 37 degrees C against isotonic phosphate buffer (pH 7.35) using [3H]AC. There were significant species differences with the smallest % free fraction (mean +/- s.d.) occurring in human plasma (3.4 +/- 0.2), followed by dog (8.1 +/- 0.4), mouse (14.8 +/- 0.8), rat (16.3 +/- 0.9) and rabbit (20.2 +/- 0.7). In plasma from healthy individuals (n = 5), the % free fraction ranged from 2.7 to 3.8. In physiological solutions of human proteins, the greatest binding was observed for alpha 1-acid glycoprotein (AAG) (0.75 g L-1) with a mean free fraction of 24.1 +/- 2.2%, followed by albumin (40 g L-1) with 31.6 +/- 0.7 and 39.8 +/- 2.5% for fatty-acid-free and globulin-free, respectively. There was also some binding to globulins (5 g L-1) with a mean % free fraction of 70.3 +/- 1.6 and 84.8 +/- 2.2 for Cohn's fraction I and IV, respectively. Binding data from the displacement of [3H]AC by increasing concentrations of AC in human AAG (0.75 g L-1) or albumin solution (40 g L-1) indicated that AAG had 10-fold greater binding affinity for AC (Ka, 7.8 x 10(4) M-1) compared with albumin (Ka, 6.8 x 10(3) M-1). In human plasma enriched with AAG there was a significant negative linear correlation (r = 0.932; P < 0.001) between % AC free fraction and increasing AAG concentration over the range 0.6-4.5 g L-1.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics and electrophysiological effects of intravenous ajmaline

The pharmacokinetics of ajmaline were studied in 10 patients with suspected paroxysmal atrioventricular block who received a 1 mg/kg intravenous dose over 2 minutes for diagnostic purposes (ajmaline test). Plasma concentration decay followed a triexponential time course with a final half-life much longer (7.3 +/- 3.6 hours) than that previously found by other investigators (about 15 minutes). Mean total plasma clearance and renal clearance were 9.76 ml/min/kg and 0.028 ml/min/kg, respectively. Although most of the dose was eliminated through the extrarenal route (only 3.5% of the intravenous dose was recovered in urine), no fluorescent metabolites could be detected either in plasma or urine. The steady-state volume of distribution averaged 6.17 L/kg, and plasma protein binding ranged between 29 and 46%. Three patients developed a transient atrioventricular block after ajmaline administration. In the remainder, the drug prolonged atrio-His bundle (AH interval), His bundle-ventricular (HV interval) and intraventricular (QRS interval) conduction times. Corrected ventricular repolarisation time (QTc interval) showed less marked changes, which were biphasic at times. The mean maximum ajmaline-induced increase in HV interval was 98%, in QRS was 58%, in AH was 30%, and in QTc was 17%. In most cases the time course of electrocardiographic changes lagged behind that of plasma concentrations, suggesting a delayed equilibrium of plasma concentrations with the site of action (hysteresis). Despite that, the pharmacokinetic-pharmacodynamic model, which accounted for hysteresis, failed to fit the experimental data adequately.(ABSTRACT TRUNCATED AT 250 WORDS)

Dual effect of valproic acid on the pharmacokinetics of phenytoin

To evaluate the effects of valproic acid on the disposition of phenytoin, a single dose of 600 mg valproic acid and multiple doses of valproic acid (200 mg four times a day for 5 days) were administered together with a single oral dose of 600 mg phenytoin to 12 young male volunteers. Fraction of unbound phenytoin and the area under curve (AUC) of the total and unbound phenytoin in plasma were compared with the control phase in which only 600 mg phenytoin was given. Valproic acid increased the unbound fraction of phenytoin in both single- and multiple-dose studies by 15 per cent and 41 per cent, respectively. Single-dose valproic acid increased the total AUC of phenytoin by 11 per cent. Multiple-dose valproic acid decreased the total AUC by 7 per cent. Single- and multiple-dose valproic acid increased the unbound AUC by 25 per cent and 18 per cent, respectively, probably due to the inhibition on the metabolizing enzymes. We concluded that there are at least two mechanisms involved in valproic acid-phenytoin interaction. Whereas valproic acid displacing phenytoin on the plasma protein decreased the total drug concentration of phenytoin, the enzyme inhibition by valproic acid increased both the total and unbound concentration of phenytoin. The two conflicting mechanisms may result in different effects on the total plasma concentration of phenytoin. Therapeutic drug monitoring based on the total concentration of phenytoin may be misleading when valproic acid is co-administered.

Disposition of total and unbound etoposide following high-dose therapy

Total and unbound etoposide pharmacokinetics were studied in 16 adult patients (median age, 34 years; range, 18-61 years) undergoing autologous bone marrow transplantation for advanced lymphoma after receiving high-dose etoposide (35-60 mg/kg) as a single intravenous infusion. Pretreatment values for mean serum albumin and total bilirubin were 3.0 +/- 0.4 g/dl and 0.5 +/- 0.4 mg/dl, respectively. Etoposide plasma concentrations and protein binding (%unbound) were determined by high-performance liquid chromatography (HPLC) and equilibrium dialysis, respectively. Pharmacokinetic parameters for unbound and total etoposide were calculated by nonlinear regression analysis using a two-compartment model. The mean (+/- SD) parameters for total etoposide included: clearance (CL), 31.8 +/- 17.7 ml min-1 m-2; volume of distribution (Vss), 11.5 +/- 5.9 l/m2, and terminal half-life (t1/2 beta), 7.2 +/- 3.7 h. Mean unbound CL was 209.6 +/- 62.7 ml min-1 m-2 and %unbound was 16% +/- 5%. The mean etoposide %unbound was inversely related to serum albumin (r2 = 0.45, P = 0.0043). The mean %unbound at the end of the etoposide infusion was higher than that at the lowest measured concentration (21% vs 13%, respectively; P = 0.017), suggesting that concentration-dependent binding may occur after high etoposide doses. The median total CL was higher in patients with serum albumin concentrations of < or = 3.0 g/dl than in those with levels of > 3.0 g/dl (34.6 vs 23.5 ml min-1 m-2, P = 0.05). Total CL was directly related to %unbound (r2 = 0.61, P = 0.0004). Unbound CL was unrelated to either serum albumin or %unbound. These results demonstrate that hypoalbuminemia is independently associated with an increased etoposide %unbound and rapid total CL after the administration of high-dose etoposide. Unbound CL in hypoalbuminemic patients is unchanged in the presence of normal total bilirubin values.

Factors affecting in vitro protein binding of etoposide in humans

Clinical studies have demonstrated that the plasma protein binding of etoposide, a widely used anticancer drug, is extensive (approximately 94%), highly variable among patients (10-fold range), and significantly related to serum albumin and total bilirubin concentration. The present study was designed to more thoroughly evaluate factors likely to affect etoposide protein binding under controlled in vitro conditions where single variables could be changed. Protein binding was determined using an equilibrium dialysis method with tritiated etoposide. The binding of etoposide was similar in serum or plasma, and heparin had no effect on binding. Etoposide binding decreased with increased pH, but no clinically significant difference was noted within the range of physiologic pH. Etoposide binding evaluated in single-source donor plasma was concentration-dependent over a concentration range of 1 to 250 micrograms/mL. Etoposide binding parameters determined in normal human plasma were characterized by a single class of binding sites of moderate affinity (K = 2.88 +/- 0.47 x 10(4)) and high capacity (nP = 5.07 +/- 0.5 x 10(-4); where n is the number of binding sites). The etoposide binding ratio was significantly correlated with albumin concentration (r2 = 99%, p less than 0.05). The characteristics of etoposide binding in a 4.0-g/dL solution of human serum albumin (K = 3.56 +/- 1.22 x 10(4) and nP = 5.58 +/- 0.16 x 10(-4)) suggest that the single class of binding sites is on albumin. Bilirubin caused a significant decrease in K, consistent with competitive binding, but only at higher bilirubin concentrations.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics and toxicity of the antitumour agent N-[2-(dimethylamino)ethyl]acridine-4-carboxamide after i.v. administration in the mouse

The pharmacokinetics, tissue distribution and toxicity of the antitumour agent N-[2-(dimethylamino)ethyl]acridine-4-carboxamide(AC) were studied after i.v. administration to mice. Over the dose range of 9-121 mumol/kg (3-40 mg/kg), AC displayed linear kinetics with the following model-independent parameters: clearance (C), 21.0 +/- 1.9 1 h-1 kg-1; steady-state volume of distribution (Vss), 11.8 +/- 1.4 l/kg; and mean residence time (MRT), 0.56 +/- 0.02 h. The plasma concentration-time profiles for AC fitted a two-compartment model with the following parameters: Cc, 19.4 +/- 2.3 1 h-1 kg-1; Vc, 7.08 +/- 1.06 l/kg; t1/2 alpha 13.1 +/- 3.5 min; and t1/2Z, 1.60 +/- 0.65 h. AC displayed moderately high binding in healthy mouse plasma, giving a free fraction of 15.9%-25.3% over the drug concentration range of 1-561 microM. After the i.v. administration of 30 mumol/kg [3H]-AC, high radioactivity concentrations were observed in all tissues (especially the brain and kidney), showing a high t1/2c value (37-59 h). At 2 min (first blood collection), the AC concentration as measured by high-performance liquid chromatography (HPLC) comprised 61% of the plasma radioactivity concentration (expressed as AC equivalents/l). By 48 h, 73% of the dose had been eliminated, with 26% and 47% of the delivered drug being excreted by the urinary and faecal routes, respectively; less than 1% of the total dose was excreted as unchanged AC in the urine. At least five distinct radiochemical peaks were distinguishable by HPLC analysis of plasma extracts, with some similar peaks appearing in urine. The 121-mumol/kg dose was well tolerated by mice, with sedation being the only obvious side effect and no significant alterations in blood biochemistry or haematological parameters being recorded. After receiving a dose of 152 mumol/kg, all mice experienced clonic seizures for 2 min (with one death occurring) followed by a period of sedation that lasted for up to 2 h. No leucopenia occurred, but some mild anaemia was noted. There was no significant change in blood biochemistry. A further 20% increase in the i.v. dose (to 182 mumol/kg) resulted in mortality, with death occurring within 2 min of AC administration.

Aspirin alters methotrexate disposition in rheumatoid arthritis patients

Intravenous methotrexate (MTX) (10 mg), either alone or with oral aspirin (ASA) (3,900 mg/day), was administered to 15 patients with rheumatoid arthritis. Systemic and renal clearance of MTX were lower, and the unbound fraction of MTX was higher when patients were also receiving ASA than when taking MTX alone. No acute hematologic, renal, or hepatic toxicity was observed with either treatment. The findings of this study therefore indicate that concomitant aspirin therapy acutely alters the clearance of low-dose MTX in patients with rheumatoid arthritis.

Interspecies differences in the effect of pH on gallopamil protein binding to albumin and alpha 1-acid glycoprotein

There is little information about the factors which influence drug protein binding between species. We have therefore investigated the role of pH on the binding of gallopamil, a calcium channel antagonist known to exhibit pH-sensitive binding, among four species, human, baboon, bovine, and canine. We used pure protein solutions of alpha 1 acid glycoprotein (AAG) (60 mg.l-1), albumin (45 gm.l-1), and their combination and three values of pH, 7.0, 7.4, and 8.0. Gallopamil protein binding was determined over a concentration range of 2.0 x 10(-7) mol.l-1 to 2.1 x 10(-3) mol.l-1 using equilibrium dialysis. Gallopamil binding in all solutions was best described using a two binding site model in the combination solution and a one binding site model in the pure solutions. pH did not affect the number of identical binding sites. However, the influence of pH on gallopamil binding was species specific. Increasing the pH from 7.0 to 8.0 influenced binding affinity differently between species. There were directionally similar changes in unbound fraction at a gallopamil concentration of 2 x 10(-7) mol.l-1 as pH increased, although there were species differences in the degree of change. In protein solutions containing both AAG and albumin a reduction in pH from 7.4 to 7.0 resulted in species-specific increases in the unbound fraction. Increasing the pH from 7.4 to 8.0 again resulted in species-specific reductions in the unbound fraction of gallopamil. Similar changes were seen when pure AAG or albumin solutions were used, indicating species variance in both gallopamil protein binding and the effect of pH on binding.

Racial differences in drug response: isoproterenol effects before and after propranolol

The aim of this study was to determine in young, healthy men the relative contribution of pharmacodynamic factors inherent between two groups known to respond differently to hypertensive therapy. Black (n = 10) and white (n = 10) men received an isoproterenol sensitivity test before and after propranolol (0.1 mg/kg, then 50 micrograms/min). There were greater increases (twofold) in systolic BP following the 1.0- and 1.5-microgram isoproterenol dose (P less than 0.05) in the black group. During propranolol there were no differences in free (1)-propranolol concentrations between the groups; however, propranolol decreased resting heart rate in the white group more than in the black group (P less than 0.05). Cardiac index decreased less in the black group compared to the white group (P less than 0.05). Following the second isoproterenol challenge, there again were greater increases in systolic BP in the black group at both the 10- and the 20-micrograms isoproterenol dose (P less than 0.05). Our study has highlighted the importance of cross-racial studies in evaluating drug effects.

Stereoselective inhibition of diphenylhydantoin metabolism by p-hydroxyphenyl-phenylhydantoin enantiomers in rats

Different doses of rac-p-HPPH (0.4 and 4 mg/h) were given repeatedly to rats infused with [14C]phenytoin. The serum levels of 14C-labeled and unlabeled p-HPPH, and [14C]phenytoin were measured by an HPLC method and radiometric analysis. The clearance of phenytoin and p-HPPH was determined by rate of dosing divided by the steady-state concentration. The phenytoin clearance was significantly lower in the high dose p-HPPH injection group than in the low dose group (87 versus 262 ml/h), whereas p-HPPH clearance showed no difference. The formation clearance of [14C]p-HPPH was also significantly lower in rats injected with high dose of p-HPPH (35 versus 169 ml/h). The clearance of other elimination pathways was also lower in rats with high dose of p-HPPH (53 versus 89 ml/h). The serum protein binding of phenytoin was lower in rats injected with high dose of p-HPPH. The result indicated that injections of rac-p-HPPH mainly inhibited on the formation of p-HPPH itself. The formation of (R)-p-HPPH and (S)-p-HPPH in microsomal preparation was measured by a ligand-exchange chromatographic method. The formation of (S)-p-HPPH or (R)-p-HPPH was not only inhibited by the enantiomer itself, but also cross-inhibited by the other enantiomer. To the formation of either (S)-p-HPPH or (R)-p-HPPH, (S)-p-HPPH showed a higher inhibitory activity. The use of rac-p-HPPH to inhibit phenytoin metabolism in vivo involved several mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)

Separation procedures used to reveal and follow drug-protein binding

The review gives a critical evaluation of the different separation procedures used to study drug-protein interactions and describes their various fields of application. For pharmacological studies, the most widely used methods are dialysis and ultrafiltration, because they allow measurements with solutions of high protein concentrations, such as those found in therapeutic conditions. Both techniques use membrane devices, which may induce additional binding effects. Another drawback of these techniques is the need for radiolabelled compounds. Chromatographic methods, which now take advantage of the technology of high-performance liquid chromatography, are generally faster and do not use drug labelling because of the higher sensitivities of the detectors. Two different approaches are possible: either all the interacting species (protein and drug) are dissolved in the mobile phase, or one of them (protein or drug) is immobilized on the support. Several chromatographic methods are available for studies in solution that differ according to the sample injection mode (frontal or zonal elution) and the nature of the mobile phase used. They include quantitation of the drug-protein complex by zonal elution, the Hummel and Dreyer method, frontal elution, the vacancy peak method, and retention analysis by zonal elution. Frontal elution is the most rigorous method since all the species at equilibrium are present in the mobile phase with known and constant concentrations. The most promising one is the Hummel and Dreyer method, because of the very small amount of protein injected in the mobile phase containing the drug. Drug-protein interactions may be studied by affinity chromatography by immobilizing one of the interacting species on the support. Comparison of the constants obtained with methods when both the drug and the protein are in solution is questionable, since the immobilized species in affinity separations differ in their physical properties from those in solution. The main advantage with studies on immobilized proteins is the easy comparison of the binding properties of various drugs, especially when they are enantiomeric. The results of the binding constants measured by different separation methods are given for the albumin-phenylbutazone and albumin-warfarin systems. Good agreement is generally obtained, which proves the validity of using chromatography as a tool to study drug-protein interactions.

Role of unstirred water layer in the exsorption of quinidine

Intestinal ++exsorption of salicylic acid, thiopentone, theophylline, and quinidine was measured during perfusion of the intestinal lumen with Tyrode solution. The effect of pectin or bovine serum albumin added to the perfusate on intestinal clearance (CLi) was investigated. Increasing pectin concentration from 0.0 to 0.5, 1.0 and 1.5% gave CLi values for quinidine of 499 +/- 18, 363 +/- 35, 237 +/- 56, and 300 +/- 28 mL h-1 kg-1, respectively. One per cent of pectin in the perfusate also decreased the CLi of thiopentone, but had no effect on the CLi of salicylic acid or theophylline. Pectin may have increased the thickness of the unstirred water layer on the mucous membrane and the resistance of drug exsorption for some drugs. When bovine serum albumin was added, drug binding in the perfusate increased, and the CLi of salicylic acid, thiopentone, and theophylline increased; the CLi of quinidine was unaltered. Co-administration of theophylline with quinidine decreased the CLi of quinidine without affecting quinidine binding in serum or in the perfusate. The CLi theophylline was not affected by quinidine. These observations are consistent with the hypothesis that the exsorption of quinidine is rate-limited by diffusion through the unstirred water layer on the mucous membrane. The CLi of quinidine is affected by the microclimate-pH in the unstirred water layer. An alternative possibility is that quinidine exsorption is mediated by a carrier-transport pathway.

Comparative drug exsorption in the perfused rat intestine

The factors affecting drug exsorption into the gastrointestinal tract are uncertain. In this study, the intestinal clearance (CLi) of compounds which vary in their lipophilicity, serum protein binding, molecular weight and ionic charge at physiological pH, has been measured. Male Sprague-Dawley rats with ligated bile ducts were infused with the test compounds through the jugular vein. The small intestine was intubated and perfused with Tyrode solution at 20 mL h-1. The CLi of the compounds investigated (urea, polyethylene glycol, inulin, albumin, dextran, barbituric acid, salicylic acid, thiobarbital, thiopental, thioseconal, theophylline, S-disopyramide and quinidine) was determined under anaesthesia by dividing the rate of a component's appearance rate in the perfusate by its carotid arterial concentration. Serum protein binding of the compounds was determined by equilibrium dialysis. The n-octanol-water partition coefficients of the compounds were measured as indices of lipophilicity. The CLi values of dextran, albumin, inulin, polyethylene glycol and urea were 0.56, 1.03, 4.5, 4.8 and 12.0 mL h-1, respectively. The larger the molecular weight of a compound, the smaller its CLi. The molecular weight is apparently one of the major determinants of CLi. Thiobarbital, thiopental and thioseconal are compounds of similar structure with increasing lipophilicity and serum protein binding. The CLi of thiobarbital, thiopental and thioseconal was proportional to the unbound fraction in serum. The unbound clearance (CLui) of three thiobarbiturates were similar (approximately 11 mL h-1). The unbound fraction of drug in serum appears to be a factor determining their CLi. Barbituric acid and salicylic acid, two acidic compounds, showed a low CLi (less than 1 mL h-1).(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacokinetics and cardio-respiratory effects of oral theophylline in exercised horses

The pharmacokinetics of theophylline at rest and the effects on cardio-respiratory and blood lactate responses to exercise were investigated after repeated oral administrations in six healthy Standardbred horses. A dose of 5 mg/kg body weight was administered every 12 h. The binding of theophylline to plasma protein was also determined. There was good agreement between predicted and observed plasma concentrations of theophylline at steady state. The mean half-life of elimination was shown to be 17.0 +/- 2.5 h, the mean half life of absorption was 1.6 +/- 1.8 h, the apparent volume of distribution was 852 +/- 99.0 ml/kg and total plasma clearance 0.61 +/- 0.08 ml/kg/min. Theophylline showed very low plasma protein binding (12%). The heart rate and blood lactate levels, during and after exercise, were significantly increased during theophylline-treatment. There was an increase of the arterial oxygen tension after exercise and the arterial carbon dioxide values before and after exercise were significantly lower than the premedication values. No severe adverse effects of the drug were noted. The recommended oral dose is therefore 5 mg/kg every 12 h but due to inter-individual variation, an adjustment of the dose may be necessary. The changes in the studied exercise parameters indicate that the performance capacity may be impaired by theophylline in the healthy horse.

Possible effect of lactational period on the milk-to-plasma drug concentration ratio in lactating women: results of an in vitro evaluation

The fat and protein composition of human milk changes dramatically in the first several weeks postpartum. In order to investigate the possible effect of this compositional change on the milk-to-plasma drug concentration ratio (M/P), the following experiment was performed. Milk samples were collected from five healthy lactating women on days 3, 5, 7, and 14 postpartum; blood samples were obtained on these days, as well as on day 1. Serum and skim milk unbound fractions (fp and fm, respectively) and the skim milk-to-whole milk drug concentration ratio (S/M) were determined in vitro in the above samples for diazepam, propranolol, and etretin, an aromatic retinoid. In addition, the composition of these milk and serum samples was also assessed. Using a previously proposed mathematical model for the distribution of drugs between milk and plasma, M/P was calculated from values for fm, fp, S/M, milk pH, and literature values for the pKa values of the compounds. The M/P was calculated for each subject on each day of sample collection. Total serum protein and alpha-1-acid glycoprotein (AAG) levels increased in the first two weeks postpartum. Mean diazepam fp values fell from 0.020 to 0.014 during this period, while propranolol fp values changed inversely with serum AAG levels. Milk whey and total proteins decreased as lactation progressed, but changes in fat levels were not statistically significant. Calculated propranolol M/P increased during the study period, predominately due to changes in milk pH and propranolol S/M.(ABSTRACT TRUNCATED AT 250 WORDS)