Determination of free concentration of piroxicam and naproxen in plasma. The influence of experimental conditions in equilibrium dialysis

The pH dependency of the binding of drugs to plasma proteins in man

An analysis of pH-induced changes of drug binding may contribute to the understanding of the mechanisms involved and the clinical relevance. A literature search was performed, and acceptance criteria set up, to select reported data for quantitative evaluation. The relationship between percentage of unbound drug, fu, and pH was analyzed, and the relevance of physicochemical characteristics of the ligand drugs and the importance of hydrogen ion-induced changes in plasma proteins for the pH sensitivity of the binding were evaluated. With all basic and the majority of acidic drugs, fu depended linearly on pH. Basic drugs showed a consistent behavior with fu decreasing with increasing pH. Acidic compounds behaved differently: With some, fu increased, and with others fu decreased, with pH, and with a third group of acids fu was pH independent. Large differences in the pH sensitivity of the plasma protein binding among individual compounds were found. The fu in plasma for some bases and acids increased up to 136% and 95%, respectively, at pH values seen in severe acidemia or alkemia. These changes in fu could be clinically relevant with narrow-therapeutic-range drugs. Physicochemical properties and other characteristics of the ligands affect the pH sensitivity of the interaction with plasma proteins, but there was clear evidence indicating that pH-induced changes in the plasma proteins are also involved in the observed pH-dependent interaction with ligands. It is generally accepted that the unbound, free fraction in whole blood or plasma is an important determinant of the pharmacokinetics and pharmacodynamics of drugs. pH-dependent protein binding and consequent changes in the free fraction have been reported for many drugs. From a basic science point of view, the systematic study of pH-induced perturbations of the drug-protein interaction may provide insight into the mechanism and forces involved in the binding of drugs to plasma proteins. From a clinical viewpoint it may be of interest to know the extent of pH-induced changes in the unbound fraction of drugs under extreme acidemic or alkalemic conditions. Arterial blood pH values compatible with life reportedly range between 6.7 and 8.0. pH values as low as 6.3 have been measured in survivors of drowning accidents. To the best knowledge of the authors, a review and interpretation of pH-associated changes in the protein binding of drugs has not been attempted to date. The goals of this investigation were to (1) review published results of studies that determined the impact of pH changes on the protein binding of drugs in man, (2) select representative data using predetermined criteria, (3) determine relevant factors impacting the pH sensitivity of the drug-protein interaction, and (4) attempt to interpret the results and their clinical relevance.

Spectrofluorometric determination of piroxicam

The spectrofluorometric determination of piroxicam [4-hydroxy-2-methyl-N-(2-pyridyl)-2II-1,2-benzothiazine-3-carboxam ide-1, 1-dioxide] in pharmaceutical tablets is described. It involves excitation at 330 nm of an acid solution (HNO3 0.5 M) of the drug, and measurement of the fluorescence intensity at 440 nm. The linear range is 0.01-1.25 micrograms ml-1.

Standards of laboratory practice: analgesic drug monitoring

Analgesics are the most commonly consumed over-the-counter preparations in the United States. They are used in the treatment of various pain syndromes and other medical conditions. Although analgesics are generally perceived to be safe agents, serious toxicity may occur in the setting of acute overdose, chronic abuse, or overuse. The indications for therapeutic drug monitoring in patients using these medications appropriately is as yet not well defined. The emphasis of this discussion, therefore, is on recommendations for monitoring in situations where toxicity is suspected. Preanalytical, analytical, and practice issues including drug interactions, frequency of monitoring, pertinent ancillary tests, reporting, and special patient groups at risk for toxicity are reviewed. Recent information from a major manufacturer of evacuated tubes arguing against the use of gel tubes for blood collection for drug monitoring is included. Colorimetric/enzymatic/immunoassays for the routine/stat monitoring of acetaminophen and salicylate and diflunisal cross-reactivity with most of the currently used salicylate assays are presented. Achiral and chiral chromatographic assays and newly introduced columns such as restricted access media and/or automated chromatographic systems are reviewed for the analysis of ibuprofen, naproxen, and the recently introduced tramadol. Finally, concepts regarding future directions including drug chirality and chiral analysis are presented.

Serum protein binding of nonsteroidal antiinflammatory drugs: a comparative study

The unbound fraction in serum, fu, is a critical parameter in describing and understanding the pharmacokinetics of NSAIDs. We compared fu for 6 different NSAIDs using ultrafiltration of pooled serum at pH 7.4 and 24C. Measurements covered a wide concentration range in order to define binding affinity and number of binding sites. HPLC was used to measure drug concentrations in serum and ultrafiltrate. Direct injection of ultrafiltrate and serum (diluted 250 x) permitted quantitation down to approximately 70 nM for most of the NSAIDs, i.e., approximately 15-20 ng/ml. Assuming binding only to albumin, the data were fitted to a model of two classes of binding sites with dissociation constants K1 and K2. The lowest K1 (highest affinity) was found with flurbiprofen, 0.0658 microM, the highest with ketoprofen, 5.23 microM, an 80-fold difference. At low drug concentrations, fu becomes virtually constant and approaches a lower limit, fumin. The following fumin values were calculated: diclofenac 0.21%; fenoprofen 0.25%, flurbiprofen 0.022%, ketoprofen 0.52%, naproxen 0.039%, and tolmetin 0.37%. Thus the least bound NSAID, ketoprofen, had a value 24-fold that of the most highly bound, flurbiprofen. The NSAIDs also differed widely with regard to the extent of variation in fu within the range of therapeutic concentrations, and hence with regard to their potential as displacers of other drugs.

Free plasma concentrations of piroxicam in patients with osteoarthritis: relation to age, sex and efficacy

Steady state piroxicam plasma samples from 85 patients suffering from osteoarthritis of the hip and/or the knee, and treated with piroxicam 20 mg daily for at least four weeks, were obtained. Twenty-seven of these patients had a newly diagnosed osteoarthritis and had not been treated with a nonsteroidal antiinflammatory drug (NSAID) previously. The plasma samples were subjected to equilibrium dialysis, analyzed by HPLC and total and free drug concentrations measured and unbound fractions were calculated. In the 27 newly diagnosed patients with free piroxicam concentrations ranging from 0.004 to 0.117 micrograms/ml, there was no correlation between free concentration and change in any of the clinical response variables from pretrial to week 4. In the total patient population free concentrations were 0.057 +/- 0.038 micrograms/ml (mean +/- standard deviation). Females had a 79% higher free concentration than males (p < 0.0002) and there was a statistically significant (p < 0.01) increase of free concentration with increasing age in females. The unbound fraction was 0.87% +/- 0.36% (mean +/- SD). There was no difference in unbound fraction between the sexes, nor could we detect any change with increasing age.

Total and free plasma and total synovial fluid piroxicam concentrations: relationship to antiinflammatory effect in patients with reactive arthritis and other arthritides

Twenty-five patients (14 males, 11 females), sixteen with reactive arthritis and 9 with other arthritides were treated with piroxicam, 40 mg at day 1 and 20 mg daily the following 9 days. Both local knee joint and general disease activity were assessed at day 0 and day 10. At day 10, plasma and synovial fluid were drawn. Total and free (determined after equilibrium dialysis) plasma concentrations and total synovial fluid concentrations were measured by HPLC. Total plasma piroxicam concentrations (micrograms/ml) were 4.1 (0.5-8.3) [median (range)], free piroxicam plasma concentrations were 0.051 (0.012-0.118), total synovial fluid concentrations were 2.2 (0.3-4.6) and ratios total synovial to total plasma concentration were 0.51 (0.39-0.90). The effect of piroxicam treatment varied among the patients. We found no relationship between drug concentration and change in any of the disease activity parameters nor any correlation between the ratio of synovial fluid to plasma piroxicam concentration and the local knee joint disease activity parameters.