Anomalous relationship between free drug fraction and its total concentration in drug–protein systems

Reactions of (Arylimido)vanadium(V)-Trialkyl Complexes with Phenols: Effects of Arylimido Ligands and Phenols for Formation of the Vanadium Phenoxides

Reactions of a series of (arylimido)vanadium(V) trialkyl complexes, V(NAr')(CH₂SiMe₃)₃ (Ar' = C₆H₅, 2-MeC₆H₄, 2,6-Me₂C₆H₃, 2,6-Cl₂C₆H₃), with various phenols (ArOH, Ar = 2,6-F₂C₆H₃, 2,6-Cl₂C₆H₃, 2,6-Me₂C₆H₃, 2,6- ⁱ Pr₂C₆H₃, 2- ^t BuC₆H₄, 2,6- ^t Bu₂C₆H₃; 1.0 equiv) affording V(NAr')(CH₂SiMe₃)₂(OAr) were conducted in C₆D₆ at 25 °C, and the effects of both arylimido ligands and phenols on the substitution rate were explored. Sterically hindered arylimido ligands showed lower reactivity, and the reaction proceeded in the order: Ar' = 2,6-Me₂C₆H₃ < 2,6-Cl₂C₆H₃ < 2-MeC₆H₄ < C₆H₅. This order is somewhat different from that obtained from the chemical shifts in V(NAr')(CH₂SiMe₃)₃ in the ⁵¹V NMR spectra. The conversions with various disubstituted phenols increased in the order: 2,6- ⁱ Pr₂C₆H₃OH < 2,6-Me₂C₆H₃OH < 2,6-Cl₂C₆H₃OH < 2,6-F₂C₆H₃OH, irrespective of the kind of arylimido ligands. The reactions of V(NAr')(CH₂SiMe₃)₃ with 2,6- ^t Bu₂C₆H₃OH (1.0 or 3.0 equiv) did not take place even upon heating at 60 °C. These results suggest that the reactions proceed via coordination of ArOH toward vanadium, and the reactivity is highly dependent on steric bulk of both the arylimido ligand and the phenol.

Plasma protein binding: from discovery to development

The importance of plasma protein binding (PPB) in modulating the effective drug concentration at pharmacological target sites has been the topic of significant discussion and debate amongst drug development groups over the past few decades. Free drug theory, which states that in absence of energy-dependent processes, after steady state equilibrium has been attained, free drug concentration in plasma is equal to free drug concentration at the pharmacologic target receptor(s) in tissues, has been used to explain pharmacokinetics/pharmacodynamics relationships in a large number of cases. Any sudden increase in free concentration of a drug could potentially cause toxicity and may need dose adjustment. Free drug concentration is also helpful to estimate the effective concentration of drugs that potentially can precipitate metabolism (or transporter)-related drug-drug interactions. Disease models are extensively validated in animals to progress a compound into development. Unbound drug concentration, and therefore PPB information across species is very informative in establishing safety margins and guiding selection of First in Human (FIH) dose and human efficacious dose. The scope of this review is to give an overview of reported role of PPB in several therapeutic areas, highlight cases where PPB changes are clinically relevant, and provide drug metabolism and pharmacokinetics recommendations in discovery and development settings.

Monitoring free drug concentrations: challenges

Measurement of drug concentrations in biological samples is of utmost importance in many research areas. The information about the amount of drug in a biological sample can be given as either total concentration, which ignores the interaction of the drug with the sample matrix, or as free concentration, which shows the portion of molecules able to diffuse through membranes and exert biological activity. Although the historical trend has been towards determining total concentrations, measurement of free concentrations is becoming more important since it correlates better with pharmacological and toxicological effects. This review will discuss the most popular experimental approaches for monitoring free drug concentrations, based on the type of sample to be investigated and the kind of information to be collected. It is shown that the current challenges in measuring free concentrations are: convenience, accuracy, precision, wide applicability, availability of accurate and precise reference methods, ruggedness, and standardized sample conditions.