Correlation of serum binding of penicillins with partition coefficients

Physicochemical properties of beta-lactam antibiotics: oil-water distribution

Apparent partition coefficients Papp, of beta-lactam antibiotics were determined in octanol-water and 2-methylpropanol-water systems at various pH values. The pKa values also were determined by potentiometry under the conditions of partition experiments. The intrinsic partition coefficients for the unionized form, Pu, and the ionized form, Pi, of beta-lactam antibiotics were calculated from the equation Papp = Pu[aH+/(Ka + aH+)] + Pi[Ka/(Ka + aH+)]. The correlation between Pu and Pi values and lipophilic parameters of penicillins measured in other systems was examined.

Studies of the thermodynamics and nature of interaction between serum albumin and penicillins

The possibility of a calorimetric determination of the number of homogeneous independent binding sites on the protein molecule surface is presented, together with the possibility of the determination of interaction heat and association constants for the binding of small molecules to one such site. Thermodynamic characteristics of interaction of 10 penicillins and methyl orange with primary and secondary binding sites of bovine serum albumin have been obtained using this technique. The data show that hydrogen bonds between the component parts of the complexes studied are absent. The main contribution to free energy change at forming these complexes is made by hydrophobic interactions. Electrostatic forces are also of some importance, their contribution into the complex formation is more significant in the case of quinacillin and carbenicillin which have an additional charged carboxyl group.

Novel method for determination of partition coefficients of penicillins and cephalosporins by high-pressure liquid chromatography

Newly defined lipophilic indexes, log k', of a series of penicillins and cephalosporins were rapidly and reliably determined by reversed-phase high-pressure liquid chromatography (HPLC) on bonded octadecylsilane supports. The log k' values obtained from their retention times exhibited a linear relationship with methanol concentration (v/v%) in the mobile phase. The extrapolated log k' values to zero and those at 30% correlated well with the partition coefficients, log P, in 1-octanol-water and with Rm values from TLC. This HPLC technique provided some new log P and Rm values for highly ionizable beta-lactam antibiotics. The HPLC method for the determination of partition coefficients of drugs has some advantages and is a useful alternative for the determination of log P and Rm.

Linear relationships between plasma binding and lipophilicity of disopyramide derivatives

The extent of plasma binding and the partition coefficient of disopyramide and 20 disopyramide derivatives were determined. Structural variations on the four functional groups around the tetrahedral carbon in the disopyramide molecule was found to influence both parameters to varying degrees. Three linear equations were developed to correlate the observed effects, depending on the type of chemical modification. The linear correlation between drug-plasma interaction and lipophilic character was analyzed theoretically. A simple model was derived to relate quantitatively the variation in the extent of plasma binding to the change in lipophilicity of disopyramide derivatives.

Hydrophobic interactions responsible for unspecific binding of morphine-like drugs

The unspecific binding of four narcotic analgesics 3H-dihydromorphine, 14C-morphine, 3H-etorphine and 3H-fentanyl to human albumin, human plasma, rabbit plasma and several tissue homogenates from rabbits was investigated using equilibrium dialysis and ultrafiltration. At a drug concentration of 10(-7) M in human plasma, dihydromorphine is bound to an extent of 14%, morphine to 23%, etorphine to 88% and fentanyl to 70%. These differences in binding are due to different degrees of hydrophobic interaction between the drugs investigated and the plasma or tissue components. The hydrophobic interactions are due to the unionized form of the drugs. The ionized form is bound to a negligible extent with all four compounds, possibly in part by ionic mechanism. Binding increased with increasing ionic strength of the protein solution, with raising temperature between 0 degrees C and 37 degrees C and with increasing pH values of the protein solution, features which are characteristic of hydrophobic interactions. Scatchard plots of the binding data, from which the total binding constants nk were derived, indicated high concentrations of binding sites compared with drug concentrations found analgesically effective in vivo.

Binding of antibiotics to bovine and ovine serum

The degree of binding of 37 antibiotics to bovine and ovine serum, after treatment at therapeutic doses, was determined by equilibrium dialysis and ultrafiltration methods. In general, binding values obtained by the two methods were comparable. The extent of binding varied from 0% for cephaloridine and kanamycin to >95% for novobiocin and fusidic acid. Of the 37 drugs studied, one-fourth were less than 25% bound, one-fourth were more than 75% bound, and the percentage binding of about half of the antibiotics ranged from 25 to 75%. Animal to animal variations in the extent of binding of a particular antibiotic were very small. The capacity of bovine or ovine serum to bind antibiotics was, with a few exceptions, similar to the reported capacity of human serum. At drug concentration ranges usually achieved during therapy, variations in drug levels in serum did not influence the degree of binding except with cephalexin, lincomycin, clindamycin, and chloramphenicol. With these antibiotics, the extent of binding increased two- to sevenfold with the decrease in drug concentration in serum.

Physico-chemical studies of analgesics. The protein-binding of some p-substituted acetanilides

The binding of fifteen p-substituted acetanilides to bovine serum albumin is examined at pH 7·2. An excellent correlation is obtained between the binding enthalpy and Hammett’s substituent constant, s̀. This is interpreted to mean that the binding is non-specific in nature. A very good correlation is also obtained between the entropy of binding and s̀, which suggests that the extent of hydration of unbound drug is a function of the charge separation within the drug molecule. Of the compounds examined, those that have been used clinically as analgesics possess the best thermodynamic properties, being neither so fully bound as to give low free drug concentrations in the bloodstream, nor so little bound that there is no sustained action.