Free drug measurements: methodology and clinical significance

Further studies on the localization of the reactive lysyl residue of pyruvate carboxylase

We have shown the increase in the acetyl-CoA-independent activity of sheep liver pyruvate carboxylase following trinitrophenylation of a specific lysine residue (designated Lys-A) to be the result of a large stimulation of the first partial reaction and a slight stimulation of the second partial reaction catalysed by this enzyme. Like acetyl-CoA, the activators adenosine 3',5'-bisphosphate and CoA did not stimulate the catalytic activity of the trinitrophenylated enzyme in either the overall reaction or the first partial reaction. Conversely, trinitrophenylation had no effect on activation of the overall reaction and the second partial reaction by acetyl-phosphopantetheine. Protection experiments demonstrated that the presence of both acetyl-CoA and adenosine 3',5'-bisphosphate decreased the rate of loss of activity during exposure of sheep liver pyruvate carboxylase to trinitrobenzenesulphonic acid (TNBS), whereas acetyl-phosphopantetheine did not. 5'-AMP and acetyl-dephospho-CoA did not protect the enzyme against loss of activity, whereas the presence of adenosine 2',5'-bisphosphate only slightly decreased the rate of modification. This suggests that Lys-A interacts with the adenosine nucleotide portion of the acetyl-CoA molecule, specifically the 3'-phosphate moiety. Acetyl-CoA and adenosine 3',5'-bisphosphate were shown to protect pyruvate carboxylase from Saccharomyces cerevisiae against inhibition by TNBS. A [14C]acetyl-CoA-binding assay demonstrated that modification of Lys-A inhibits the binding of acetyl-CoA to S. cerevisiae pyruvate carboxylase, indicating that Lys-A is at or near the acetyl-CoA-binding site.

Fetal and maternal placental and nonplacental clearances of metoclopramide in chronically instrumented pregnant sheep

The placental and nonplacental clearances of metoclopramide were studied in nine chronically instrumented, near-term pregnant sheep using a two-compartment open model. Metoclopramide was administered to the ewe and fetus on separate occasions as an initial iv bolus loading dose followed by a constant-rate infusion, with steady-state maternal and fetal plasma concentrations being obtained by 45 min. Following the maternal infusions, metoclopramide reached average steady-state concentrations of 50.0 +/- 20.2 ng/mL in the ewe and 27.1 +/- 8.6 ng/mL in the fetus, with a mean fetal-to-maternal concentration ratio of 0.57 +/- 0.14. The ability of the fetus to eliminate metoclopramide by nonplacental routes appears to be responsible for this ratio being less than unity, rather than differential protein binding and ion-trapping effects. Mean steady-state concentrations were 13.8 +/- 4.5 and 253.7 +/- 92.1 ng/mL in the ewe and fetus, respectively, after fetal drug administration. Metoclopramide was bound significantly less to fetal (39.5 +/- 8.9%) than to maternal (49.5 +/- 7.9%) plasma proteins, with values similar to that reported for humans (approximately 40%). Clearance of metoclopramide across the placenta from the fetus to the ewe (6.2 +/- 2.4 L/h/kg) was significantly greater than that in the reverse direction (4.3 +/- 1.3 L/h/kg) and accounted for approximately 80% of total fetal drug elimination. This may be explained by the higher percentage of fetal cardiac output to the placenta and the flow-limited transfer of this compound.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of drugs and other toxic substances in biological samples for pharmacokinetic studies

The importance of the role of analysis of drugs and other toxic substances in biological samples (bioanalysis) in medicine, toxicology, pharmacology, forensic science, environmental research and other biomedical disciplines is self-evident. Among these disciplines, bioanalysis plays a special pivotal role in pharmacokinetics. The pharmacokinetic parameters, such as half-life, volume of distribution, clearance and bioavailability, of drugs and other compounds are derived from the concentrations of these analytes assayed in the biological samples collected at specified time points. The capability of analysts to develop sensitive and specific analytical methods for the assay of low concentrations of drugs and other toxic compounds in small amounts of biological samples has contributed significantly to the theoretical advances in pharmacokinetics and its applications in clinical pharmacology and the management of drug therapy in patients. The increased demands for pharmacokinetic applications in turn have stimulated the innovation and improvement in bioanalytical technologies. The reliability of the pharmacokinetic conclusions depends on the accuracy and precision of the analytical methods employed to assay the biological samples. Factors that affect the integrity of the bioanalytical data should therefore be controlled in analysis of biological samples for pharmacokinetics studies. The biological samples for drug concentration determination should be collected as specified in the study protocol with respect to the time and site of sampling. These samples should be processed to avoid extraneous interactions between the analytes and sampling devices or additives resulting in the redistribution of the analytes between components of the biological samples, such as displacement of drug binding and changes in the distribution of the analytes between plasma and red blood cells. The stability of the drugs and other analytes in the samples should also be evaluated to establish the conditions suitable for the transportation and storage of the samples to avoid chemical, photochemical and enzymatic degradation of the analytes. Various technologies have been utilized to assay biological samples for pharmacokinetic studies. The most frequently used are chromatography (high-performance liquid chromatography, gas chromatography and thin-layer chromatography), immunoassays and mass spectrometry.(ABSTRACT TRUNCATED AT 400 WORDS)

A micromethod for the estimation of free levels of anticonvulsant drugs in serum

A micromethod for estimating free levels of phenobarbitone, phenytoin and carbamazepine in patients' sera is described. Serum samples are subjected to a process of ultrafiltration, the filtrates treated with acetonitrile and the drug concentration quantified using high performance liquid chromatography. The stability of free levels in specimens before and after storage is investigated. The method is reproducible and mean recovery exceeds 98.5% showing that there is no significant absorption of drug onto the filters used. There is no interference from other substances normally present in patients' sera and there is a good correlation between results obtained by this method and a fluorescence polarisation immunoassay with correlation coefficient between 0.975 and 0.999. Serum samples can be stored for a lengthy period before ultrafiltration without adverse effects. The relevance of the method to patient care is discussed.

Preanalytical considerations in drug assays in clinical pharmacokinetic studies

Many hospital pharmacy laboratories undertake drug analysis in biological fluids for the production of pharmacokinetic data. The success of such an undertaking very much depends on the selection of a suitable analytical method and a proper approach to sample collection and handling. This paper surveys the main types of biological specimens taken from the patients for pharmacokinetic drug analysis and discusses factors that affect them during or subsequent to their removal. Guidelines are provided in specimen handling and dealing with many problems which could arise prior to actual analysis. By its very nature this paper brings in many disciplines, the full details of which are well beyond its scope, however, some discussion on pharmacokinetic and bioavailability methods in relation to sampling procedures is included to put the matter into a proper perspective.

Total and free disopyramide by fluorescence polarization immunoassay and relationship between free fraction and alpha-1 acid glycoprotein

Disopyramide is an antiarrhythmic drug with concentration dependent protein binding within the therapeutic range. We found good agreement between fluorescence polarization immunoassay (FPIA, Abbott TDX) and high performance liquid chromatography (HPLC) for total disopyramide among 27 admission patients' sera (FPIA = 1.12 (HPLC) -0.002, r = 0.982, SEE = 0.378 mg/l). Agreement between FPIA and HPLC for free disopyramide was similarly good (FPIA = 0.867 (HPLC) -0.003, r = 0.986, SEE = 0.09 mg/l, n = 27). Serum alpha 1 acid glycoprotein concentration (AAG) and free fraction (FF) percent of disopyramide correlated inversely (FF = -0.0112 (AAG) + 31.3 r = 0.707, SEE = 6.41 mg/l, n = 24), and the free disopyramide fraction varied greatly. For two pooled sera with 12 different disopyramide concentrations (range from 0.5-20.0 mg/l), the proportion of free fraction ranged from 6.5 to 73.2%. Overall, we found the free disopyramide fraction variable with each individual, total drug, and protein concentration. Therefore, free drug concentration should be monitored in disopyramide therapy, and FPIA is reliable for free as well as total drug assay.

Salicylate measurement: clinical usefulness and methodology

The salicylates are the most commonly used analgesic, antipyretic, and anti-inflammatory drugs. They are available in hundreds of preparations, many of which are over-the-counter medications. The easy access to large quantities of the drug and the widespread perception that the drug is harmless have contributed to salicylate intoxication becoming a serious and common problem, particularly among the pediatric and geriatric populations. Salicylate is still the major drug for the treatment of rheumatic diseases. The use of salicylate in high doses for the management of these patients requires close monitoring of serum salicylate levels because of the large interindividual variation in dose-serum level relationships and the narrowness of the therapeutic range. Thus, both for the management of patients intoxicated with salicylate and patients who are on high-dose salicylate therapy, the measurement of serum salicylate levels is an important clinical laboratory service. Recent research on the inhibitory effect of aspirin on platelet aggregation has led to the prophylactic use of aspirin in low doses as an antithrombotic drug. This new therapeutic use of aspirin can be aided by monitoring low serum levels of salicylate and perhaps aspirin itself. This article reviews the current state of the knowledge of the pharmacokinetics and clinical toxicology of salicylate, the clinical usefulness of salicylate measurement by the clinical laboratory, and recent development in the analytical technology for salicylate analysis.

Use of unbound drug concentrations to determine neonatal anticonvulsant exposure

Unbound and total concentrations of several anticonvulsant drugs were measured by liquid chromatography in maternal and neonatal cord serum collected at birth from 16 women being treated for epilepsy and their newborns. Maternal and neonatal unbound drug concentrations agreed closely for phenobarbital (n = 6), phenytoin (n = 7), carbamazepine (n = 8), and its epoxide metabolite. Mean maternal total drug concentrations were higher than neonatal concentrations in the cases of phenobarbital, carbamazepine, its epoxide and diol metabolites. The differences were due to greater protein binding in maternal serum. Measurement of total anticonvulsant concentrations in newborns may be misleading, because of altered protein binding in the neonate. For the medications tested, neonatal and maternal exposures to unbound drug appear to be equivalent.

Non-isotopic spectrophotometric determination of the unbound fraction of drugs in serum

A spectrophotometric method to measure the free fraction of highly-bound drugs in serum has been established for a range of non-steroidal anti-inflammatory drugs (NSAIDs) and for frusemide. Spectrophotometry is used to measure fractional transit of drug from a large volume of dialysate to a small volume of serum during dialysis to equilibrium. The method, which depends on the principle that drug transit from dialysate to serum is proportional to serum binding, requires neither isotopic drug preparations nor specific drug assays, is independent of extraction efficiency from the dialysate and requires no measurements from the serum compartment. Estimates of percent unbound fraction (% UF) for aspirin (6.0 +/- 0.9%), phenylbutazone (0.9 +/- 0.2%), and frusemide (1.8 +/- 0.2%) were comparable with those obtained with 14C drug preparations. Values for % UF were determined for eleven additional NSAIDs. The method was valid for a four-fold change in serum: dialysate ratio. Kinetics of frusemide binding to serum were comparable using [14C]frusemide and the test method. This technique may have general application in establishing the % UF for substances that are extensively bound to serum proteins and for identifying sera that show abnormal binding.