Measurement and Analysis of Unbound Drug Concentrations

Abstract

The plasma protein binding of drugs has been shown to have significant effects on numerous aspects of clinical pharmacokinetics and pharmacodynamics. In many clinical situations, measurement of the total drug concentration does not provide the needed information concerning the unbound fraction of drug in plasma which is available for distribution, elimination, and pharmacodynamic action. Thus, accurate determination of unbound plasma drug concentrations is essential in the therapeutic monitoring of drugs. Many methodologies are available for determining the extent of plasma protein binding of drugs, however, in the clinical evaluation of drug therapy, equilibrium dialysis and ultrafiltration are the most routinely utilised methods. Both of these methods have been proven to be experimentally sound and to yield adequate protein binding data. Furthermore, the characterisation of the interactions between drug and protein molecules is essential for the assessment of the pharmacokinetic implications of drug-protein binding. Protein binding parameters which characterise the affinity of the drug-protein association, the number of classes of binding sites, the number of binding sites per class or protein and the binding capacity are useful for predicting unbound drug concentrations. Simple graphical methods have often been used to obtain protein binding parameters, but these methods have limitations and are not useful for drugs with more than 1 class of binding site. Therefore, the fitting of protein binding models which characterise the drug-protein binding interaction for experimental data is the preferred method of calculating binding parameters. Using the appropriate model, values for binding parameters are typically estimated by using nonlinear least-squares regression analysis.

6-Arylpyrazine-2-carboxamides: A New Core for Trypanosoma brucei Inhibitors

“…Risk for transmission has been associated with fetching water, domestic activities such as washing clothing or food (105) and fishing (106). In the transitional vegetation zones between forest and woodland savannah, which are often used for farming, the islands of vegetation provide tsetse habitats from which hosts are sighted, and farming has been implicated as a risk factor (29).…”

“…Incorrect mounting of the cover slip or replacement of the original cover slip by an ordinary microscope cover slip are errors that are often made with reusable counting chambers; these lead to an incorrect volume of CSF and thus inaccurate cell counts. Such manipulation errors can be avoided by using disposable plastic counting chambers, which retain a fixed volume of CSF (106). The filled counting chamber is placed on the microscope stage, and the counting grid is brought into focus at low magnification (objective 10x, oculars 10x or 15x).…”

“…The sensitivity is increased by centrifugation. As modified single centrifugation is more sensitive and easier to perform (106,107), it should be preferred over single and double (108) centrifugation of CSF. In modified single centrifugation, about 3.5 ml of CSF are centrifuged immediately (10 min, 1000 g) in a flame-sealed Pasteur pipette or in an mAECT collector tube (64,109).…”

Sulindac sulfide inhibits Ras signaling

“…In preliminary experiments the approximate time for drug equilibrium between the reservoirs separated by a dialysis membrane with a cuto of 10 000 kD was determined to be 18 h. After 24 h drug and protein concentrations were measured by spectroscopic analysis in all chambers and the concentrations of bound and unbound drugs were calculated (Wright et al, 1996). For dialysis against an excess of buer 50 mM p21rasGppNHp was incubated for 1 h with 250 mM sulindac sul®de and subsequently dialyzed against a 10006volume of PBS at 208C.…”

Why mammals more susceptible to the hepatotoxic microcystins than fish: evidences from plasma and albumin protein binding through equilibrium dialysis

“…Only the unbound drug is available for diffusion or transport across cell membranes, and interaction with specific targets (e.g. receptor, ion channel, transporter and enzyme) (Wright et al 1996;Baranczewski et al 2006). As a result, a full characterization of the binding property to plasma proteins has become essential for understanding the pharmacokinetic and toxicological profile of a drug.…”