Biliary excretion of penicillins in the rat

Algal toxicity of antibacterial agents used in intensive farming

The growth inhibiting effects of eight antibiotics used either therapeutically or as growth promoters in intensive farming on two species of micro algae, Microcystis aeruginosa (freshwater cyanobacteria) and Selenastrum capricornutum (green algae) were investigated. The effects of the antibiotics benzylpenicillin (penicillin G) (BP), chlortetracycline (CTC), olaquindox (O), spiramycin (SP), streptomycin (ST), tetracycline (TC), tiamulin (TI) and tylosin (TY) were tested in accordance with the ISO 8692 (1989) standard protocol. Algal growth was measured as increase in chlorophyll concentration by extraction with ethanol followed by measurement of fluorescence. Results were quantified in terms of growth rates using the Weibull equation to describe the concentration response relationship. The toxicity (EC50 value, mg/l) in alphabetic order were BP (0.006); CTC (0.05); O (5.1); SP (0.005); ST (0.007); TC (0.09); TI (0.003) and TY (0.034) for M. aeruginosa. BP (NOEC = 100); CTC (3.1); O (40); SP (2.3); ST (0.133); TC (2.2); TI (0.165) and TY (1.38) for S. capricornutum. In this investigation M. aeruginosa is found to be about two orders of magnitude more sensitive than S. capricornutum. It was observed that most of the compounds were unstable during the test period due to hydrolysis and photolysis.

Hydrophobicity of beta-lactam antibiotics. Explanation and prediction of their behaviour in various partitioning solvent systems and reversed-phase chromatography

beta-Lactam antibiotics tend to undergo self-association in hydrophilic organic solvents, which leads to a strong dependence of their experimentally observable log P values on the partitioning conditions. As a result, most of the earlier obtained log P values for beta-lactam antibiotics cannot be applied as a common hydrophobicity measure, but they proved to be linearly related to each other and to a large body of reversed-phase chromatographic data. The retention of cephalosporins on reversed-phase liquid chromatographic columns is complicated by silanophilic interactions. However, under elution conditions that eliminate these silanophilic interactions, good correlations with log P data are observed, and a unified hydrophobicity scale for 90 penicillin and cephalosporin compounds could be evaluated. The Hansch and Leo additive scheme was shown to be valid for the calculation of hydrophobicities for penicillin and cephalosporin C-6(7) substituents, but it failed when applied to the prediction of cephalosporin C-3-substituent hydrophobicities. The hydrophobic increments for the sixteen most common cephalosporin C-3-substituents were empirically evaluated from literature data, and a simple equation was derived for an overall beta-lactam antibiotic hydrophobicity calculation. The proposed scale is valid for predicting the partitioning of most beta-lactam antibiotics in both hydrophilic and lipophilic organic-water systems, although it should be used with caution when applied to antibiotics containing additionally charged side-chains.

Drug-protein conjugates--XIII. The disposition of the benzylpenicilloyl hapten conjugated to albumin

The disposition and metabolic fate of benzylpenicillin conjugated to a protein, human serum albumin (HSA), were compared with those of free penicillin in the rat. The conjugate was prepared by in vitro incubation of [3H]-benzylpenicillin and HSA at pH 10.8 for 24 hr at 37 degrees, conditions which favour the formation of penicilloyl-lysine residues. The synthetic conjugate was cleared more slowly from plasma than free penicillin after intravenous administration; thus at 3 hr, concentrations of 5.08 +/- 0.50% dose/ml of the conjugate (0.31 microCi; 2.92 mg protein) was obtained. In an earlier study a concentration of 0.03 +/- 0.01% dose/ml was obtained after administration of free BP (2.7 mmol kg-1). During this time, 1.41 +/- 0.50% of the conjugate dose was excreted in urine while 5.0 +/- 0.2% of the dose was excreted in bile. Tissue analysis indicated that the liver contained 15.3 +/- 0.9% of the dose, while other tissues contained less than 6% of the dose. In long term metabolism studies it was found that 39.5 +/- 1.0% and 46.5 +/- 0.9% of the dose (0.43 microCi; 6.33 mg protein) was excreted in the urine after 3 and 7 days respectively. The principal metabolite (63-68%) excreted in both bile and urine was identified on the basis of cochromatography and fast atom bombardment mass spectrometry as benzylpenicilloic acid, indicating that the conjugate undergoes specific cleavage at the bond between the benzylpenicilloyl moiety and the protein. In vitro degradation studies indicate that the metabolism occurs primarily in the liver. Therefore benzylpenicilloic acid excreted in urine, after administration of free BP, may be formed either by direct hydrolysis of the beta-lactam ring, and/or result from catabolism of protein conjugates formed in vivo.

Biliary elimination of beta-lactam antibiotics by the isolated perfused rabbit liver

Biliary elimination of 15 beta-lactams was studied using a model of isolated perfused rabbit liver. During the perfusion period, the main liver functions were well preserved. The highest biliary concentrations were reached by antibiotics of the penicillin group, and the highest biliary recovery was observed with metampicillin, apalcillin and mezlocillin (46.5, 30.7 and 20.3%, respectively, of the dose administered). This technique was also suitable for estimating the degree of bio-transformation in the liver of the antibiotics studied.

Quantitative relationships between structure and pharmacokinetic parameters using molecular connectivity chi indices I: Substituted 2-sulfapyridines

Quantitative relationships between the structure and pharmacokinetic parameters of several compounds have been derived using the molecular connectivity approach. The correlation coefficients of equations obtained using the Chi indices (chi) as predictor parameters were compared favorably with those obtained by the linear free energy approach, where physicochemical parameters have been used as predictor variables. High correlation coefficients (r) for the first-order elimination rate constant (r = 0.86), total body clearance (r = 0.89), and protein binding association constant (r = 0.78) for substituted 2-sulfapyridines were obtained. However, including the pKa (indicative of electronic effects) of the compounds within the equation as a predictor variable caused an increase in the correlation 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.

Relation between lipophilicity and pharmacological behavior of minocycline, doxycycline, tetracycline, and oxytetracycline in dogs

Four tetracyclines were studied in dogs to determine the relation between their lipophilicity and various other pharmacological characteristics. Lipid solubility correlated inversely with the mean concentration of drug in arterial plasma and renal uptake and excretion, and directly with the biliary concentration gradient (level in bile/level of free drug in serum). Only the more lipophilic congeners minocycline and doxycycline passed the blood-brain and blood-ocular barriers in detectable concentrations. Mean levels of minocycline in the brain exceeded those of doxycycline by almost threefold; the difference was of borderline statistical significance (P = 0.05 to 0.1). Lipophilicity correlated inversely with the concentration of antibiotic in renal medulla but not in renal cortex or in the liver. When intestinal loops containing saline, milk, or 10% Gelusil were studied, the only combination exhibiting striking intraluminal accumulation was doxycycline in milk. These results indicate that lipophilicity correlates with many, but not all, of the transport characteristics of tetracycline antibiotics.

Chromatographic hydrophobic parameters in correlation analysis of structure-activity relationships

The use of RM and delta RM parameters as indices of hydrophobicity for inclusion in quantitative structure activity relationships has been studied. The relationship between these parameters and other free-energy related parameters is illustrated theoretically and experimentally. It is suggested that the chromatographically obtained parameters could find a wider applicability in structure-activity relationships, and that their use would result in improved correlation of data.