Application of Physicochemical and Analytic Techniques to the Study of Drug Interactions with Biological Systems

Photodegradation of 2-chloro substituted phenothiazines in alcohols

The mechanisms that trigger the phototoxic response to 2-chlorophenothiazine derivatives are still unknown. To better understand the relationship between the molecular structure of halogenated phenothiazines and their phototoxic activity, their photophysics and photochemistry were studied in several alcohols. The photodestruction quantum yields were determined under anaerobic conditions using monochromatic light (313 nm). Absorption- and emission-spectroscopy, (1)H- and (13)C-NMR and GC-MS were used to characterize the photoproducts and reference compounds. An electron transfer mechanism had been previously proposed by Bunce et al. (J. Med. Chem. 22, 202-204) to explain the large difference between the photodestruction quantum yield of 2-chlorpromazine (phi = 0.46) and 2-chlorphenothiazine (phi = 0.20). According to these authors, the alkylamino chain transfers an electron to the phenothiazine moiety. Our results demonstrate that this mechanism is incorrect, because the photodestruction quantum yields of all chlorinated derivatives of this study are the same under the same conditions of solvent and irradiation wavelength. The quantum yield has no dependence on the 10-substituent, but it depends on the solvent. The percentage of each photoproduct, on the other hand, strongly depends on that substituent, but not very much on the solvent. Finally, it is demonstrated that the phototoxic effect of chlorinated phenothiazines is not related to the photodechlorination, although both processes share the same transient.

Laboratory of Chemical Pharmacology, National Heart, Lung, and Blood Institute, NIH: a short history

The Laboratory of Chemical Pharmacology (LCP) began in 1950 as the Section of Pharmacology within the National Heart Institute, the National Institutes of Health. Its first chief was Bernard B. Brodie, considered by many to be one of the fathers of modern pharmacology. Since its inception, LCP has made many significant contributions to the fields of pharmacology and toxicology. LCP was among the first to study (a) the effects of drugs on the turnover of serotonin and norepinephrine in brain and other tissues, (b) the absorption of drugs from the gastrointestinal tract and their passage across the blood-brain barrier, (c) the oxidation and reduction of drugs and other foreign compounds by liver microsomal enzymes (later known as the cytochrome P450 enzymes) and inhibitors and inducers of these enzymes, (d) the formation of toxic chemically reactive metabolites of drugs and other foreign compounds, and (e) mechanisms of immunological responses. Approximately 300 scientists worked in LCP during its existence, and they and their collaborators published more than 1,300 papers. This is a short history of the people who worked in it and of their contributions to biomedical sciences.

Binding of radioiodinated human beta-endorphin to serum proteins from rats and humans, determined by several methods

Binding of immunoreactive radioiodinated human beta-endorphin (125I-beta-EP) to rat serum was demonstrated by gel filtration of 125I-beta-EP in pooled rat serum on Sephadex G-200. Two radioactive peaks associated with proteins eluted from the column. The first peak eluted at the void volume containing lipoproteins, alpha 2- and beta 2-macroglobulins, and the second peak at the fraction of albumin. Binding of 125I-beta-EP to albumin was directly proved by gel filtration of 125I-beta-EP in buffer containing 4% human serum albumin on Sephadex G-200. Equilibrium dialysis was not applicable to investigating the interaction of 125I-beta-EP with serum proteins, because of the intense nonspecific adsorption to the semipermeable membrane and the degradation of the peptide during dialysis. Therefore, in order to quantitatively evaluate the binding of 125I-beta-EP in sera from rats and humans, we utilized four other methods (ultrafiltration, charcoal adsorption, polyethylene glycol precipitation and equilibrium gel filtration). These methods corresponded well with each other and indicated 35-44% binding of 125I-beta-EP in rat serum. Binding of 125I-beta-EP in normal human serum was 36%, determined by ultrafiltration. Serum protein binding of 125I-beta-EP was concentration independent over the concentration range studied (1-1000 nM).

Improved procedure for the determination of protein binding by conventional equilibrium dialysis

The binding of drugs to plasma proteins has been studied extensively using a variety of methods, including equilibrium dialysis. Published information on controls used in these studies is frequently inadequate; in other cases, there are deficiencies in the experimental design for the controls. A method is described that eliminates many of the problems associated with artifactual errors in dialysis studies. Multiple replicated controls are performed at the same time as the test, under identical conditions. The controls are used to correct for concentration-dependent binding of drug to the membrane or other equipment. The method was used to determine the binding of sulfadimethoxine to CF-IV-1 alpha-globulin at therapeutic concentrations. The level of binding was low (9-13%), but the stringent control technique permitted statistical analysis which showed each mean test value to be significantly different from its corresponding control. Furthermore, there was a linear relationship between the control-corrected percentage binding values and total drug concentration, whereas there was no correlation between total drug concentration and the uncorrected percentage binding values.

Interaction of povidone with aromatic compounds I: Evaluation of complex formation by factorial analysis

In a study of complex formation between macromolecules and small ligands such as drugs, it appeared that the association constants must be calculated with more care (i.e., after a thorough investigation of the influencing parameters such as buffer composition, ionic strength, and temperature) to allow meaningful interpretations of the phenomena. For this purpose, factorial analysis seems to be the method of choice; it offers the advantage of evaluating the influence of several variables and their interactions at the same time with a minimum of experiments. The method was applied to the association of povidone with two ligands, salicylic acid and benzoic acid. Parameters such as buffer composition and ionic strength, which affect binding, could be distinguished. Especially at pH 7.00, a great positive influence of buffer ions (phosphate buffer) and a relative positive interaction between temperature and ionic strength were noted. Knowledge of the influences of these parameters allowed comparison of the effects of the functional groups attached to the ligand molecules, as well as their degree of dissociation, on adsorption to permit more meaningful interpretation of thermodynamic constants.

Binding of codeine, morphine, and methadone to human serum proteins

The binding properties of codeine, morphine (as representative opium alkaloids), and methadone (a synthetic pharmacologically similar compound) were studied with selected human serum proteins. The methodology involved equilibrium and dynamic dialysis using 3H-and/or 14C-labeled compounds. For estimation of the percent binding with equilibrium dialysis, concentrations of the ligand used were approximately therapeutic blood levels and another concentration 30-60 times higher. The percent binding to whole human serum ranged from about 20% for morphine to almost 60% for methadone. Of the human serum proteins investigated, the highest percent binding was found with albumin, except for methadone for which it was beta-globulin III. The affinity for other serum proteins varied with the ligand. In studies with albumin using dynamic dialysis, the plots of nubar divided by free concentration versus nubar were similar for all three ligands studied and had positive slopes, unlike those reported for acidic compounds for which the slope is always negative. In studies of binding of one ligand in the presence of another, significant competition was demonstrated, suggesting that the same binding sites were involved.