Christopher M. Riley

AQUEOUS SOLUBILITIES OF SOME VARIOUSLY SUBSTITUTED QUINOLONE ANTIMICROBIALS

The fluorinated 4-quinolones are a group of orally active antimicrobials, which are active against a wide range of gram-negative organisms and gram-positive cocci. These compounds are essentially zwitterions at physiological pH and, therefore, in their lowest state of solubility. The objective of this study was to evaluate the solubility of a series of the fluoroquinolones as a function of pH, temperature, and salt concentration. The aqueous solubilities (μ = 0.15 with NaCI) at 25 and 37°C, the apparent macroscopic dissociation constants (μ = 0.15 with NaCl), and the melting points of a series of fluoroquinolone antimicrobials were determined. The intrinsic solubility (μ = 0.15 with NaCl) at 25°C for this group of compounds was found to range from 1.28 × 10−4 M (0.0297 mg ml−1) to 7.64 × 10−3 M (2.75 mg ml−1). At pH values between 5 and 7, if the solubility of quinolone was greater than 5 mg ml−1, the observed solubility was greater than that predicted by theory. At pH values below 5, there was evidence of the salt forms of the quinolones limiting the solubilities. The effect of NaCl on the solubility of lomefloxacin mesylate was studied and showed that increasing ionic strength may suppress the solubility more than could be explained by the common-ion effect alone. The apparent macroscopic dissociation constants (μ = 0.15 with NaCl) were found to range from 5.46 to 6.30 for the carboxylic acid function. The range of apparent macroscopic dissociation constants for the piperazinyl function was found to be 9.0 ± 0.3 for all compounds except those with a methyl substituent on the piperazinyl nitrogen, in which case, the range was 7.8 ± 0.4.

Upward slurry packing of liquid chromatography columns

Abstract Some factors affecting the efficiency of packing microparticulate liquid chromatography columns are discussed with some experimental evidence on the most critical factors. Upward packing of a dilute slurry is considered to have some advantages.

Dermal Microdialysis Sampling in Vivo

Microdialysis sampling of the dermis in vivo was accomplished using a linear microdialysis probe. In contrast to previous studies using a commercial cannula-style microdialysis probe, the linear probe had no effect on the flux of drug through the skin in vitro. The extent of tissue damage in vivo due to probe implantation was evaluated by histological examination and microdialysis delivery studies. Tissue damage due to implantation of the linear probe was minimal with no bleeding or edema observed. Infiltration of lymphocytes into the tissue was observed beginning 6 hours after probe implantation with scar tissue beginning to form after approximately 32 hours. The infiltration of lymphocytes had no effect on the behavior of implanted microdialysis probes. Delivery of 5-fluorouracil was between 20 and 25% for six different probes implanted in six different animals demonstrating good probe-to-probe and implantation-to-implantation reproducibility. Constant delivery was maintained for at least 24 hours in all cases indicating that experiments of at least 24 hour duration are feasible. The dermal concentration of topically applied 5-FU cream, Efudex®, was continuously monitored by an implanted microdialysis probe demonstrating the feasibility of this technique as for monitoring skin drug levels in vivo. The dermal concentration of 5-FU following topical application was approximately 40-fold higher for in vitro excised skin than for in vivo intact skin.

Physicochemical properties of the fluoroquinolone antimicrobials. II. Acid ionization constants and their relationship to structure

The quinolone antimicrobials contain one acidic and one basic functional group and at physiological pH they exist as a mixture of the neutral and zwitterionic forms. The ratio of the neutral to zwitterionic species for a given compound is important because it is likely to determine the distribution properties of the drug in vivo. The present study was conducted to determine the relationship between the dissociation constants of the quinolones and their chemical structures. Regression analyses indicated that the apparent pKa value associated with the carboxylic acid function was influenced by the number of fluorines in the molecule, while the pKa value associated with the piperazinyl nitrogen was influenced mainly by the presence of an N-methyl substituent.

Analysis of anticancer drugs in biological fluids : determination of taxol with application to clinical pharmacokinetics

Taxol, a novel antimitotic, antitumor agent is currently undergoing Phase 1 clinical trials for the treatment of various tumors. An isocratic HPLC method has been developed for the determination of taxol in human plasma and urine. The method was then applied to the clinical pharmacokinetics of taxol following 6-h intravenous (i.v.) infusions at doses of 175 and 225 mg m-2. A mobile phase of methanol-acetate buffer (0.02 M, pH 4.5) (65:35, v/v) was used to elute a C8 column with detection at 227 nm. The sample preparation involved extraction with t-butyl methyl ether followed by further clean-up of the sample by solid-phase extraction. The method was linear from 0.10-10 microM injected, with a chromatographic run time of 6 min. The results obtained from the clinical study indicate that the plasma pharmacokinetics of taxol are best characterized by a two compartment open body model. Additionally, the present study resulted in the detection of a previously unreported peak which may be a metabolite of taxol.

Physicochemical properties of the fluoroquinolone antimicrobials. III. Complexation of lomefloxacin with various metal ions and the effect of metal ion complexation on aqueous solubility

Abstract The complexation of lomefloxacin with five metal ions (Al 3+ , Ca 2+ , Mg 2+ , Bi 3+ , and Fe 3+ ) commonly found in antacid or vitamin preparations has been studied at 25°C. The stability constants and stoichiometries were determined by measuring the change of aqueous solubility of lomefloxacin as a function of metal ion concentration. The concentration of lomefloxacin was determined by liquid chromatography. The stoichiometry of binding was confirmed independently by FAB-MS analysis of aqueous solutions of the complexes. The binding constants were dependent on the nature of the metal ion and were related to the charge density. The pharmaceutical and biopharmaceutical implications of the effects of metal ion complexation on the aqueous solubility of lomefloxacin are discussed.

Physicochemical properties of the fluoroquinolone antimicrobials: V. Effect of fluoroquinolone structure and pH on the complexation of various fluoroquinolones with magnesium and calcium ions

Abstract The complexation of nalidixic acid and nine 7-piperazinylfluoroquinolone antimicrobials with magnesium and calcium ions was studied at room temperature (22 ± 1°C) by spectrofluorometry. Scatchard plots were used to determined the stoichiometry of binding and the effects of pH were investigated to determine which Bjerrum species of the compounds bound most tightly to the metal ions. Multiple regression analysis was used to determine the relationship between the association constants and the binding to magnesium ions. The results of that analysis were then used to propose a structure for the 2:2 complex of magnesium and lomefloxacin.

Physicochemical properties of the fluoroquinolone antimicrobials. III. 1-Octanol/water partition coefficients and their relationships to structure

Abstract The distribution of nalidixic acid and nine fluoroquinolones between 1-octanol and aqueous buffers has been studied at 25°C, pH 5, 7 or 9 and an ionic strength of 0.15. The partition coefficients were determined by analysis of both the organic and aqueous phases using either liquid chromatography or spectrophotometry. The partitioning of the compounds studied was generally consistent with that of zwitterions so that the highest partitioning was observed when the pH of the aqueous solution was close to that of the isoelectric point (pH 7) and decreased at both higher (9) and lower (5) pH values. However, deviation from ideal partitioning behavior was observed at pH 5 that was attributed to ion-pairing between the cationic form of the solute and anionic buffer species. Relationships between the theoretical partition coefficients obtained by a functional group approach and the experimental values obtained here were perturbed by the complex ionic equilibria of the compounds.

Microdialysis sampling for the investigation of dermal drug transport

Microdialysis perfusion in vivo has the potential to be a powerful sampling technique in dermal and transdermal drug delivery studies. Characterization of a commercially available microdialysis probe in vitro considering relevant physiological parameters is a vital first step in the evaluation of microdialysis as a dermal sampling technique. In previous microdialysis studies, analyte concentration and neutrality have been implicated in altering microdialysis recovery. The recovery of a model compound 5-fluorouracil (5-FU) was investigated at several pH values and donor concentrations. The relative recovery of 5-FU by the microdialysis probe was affected by pH but not by donor concentration. To confirm further that the changing concentration and pH profile presented by the flux of 5-FU was not significantly altering microdialysis recovery, an experiment comparing direct and microdialysis sampling of a Franz diffusion cell receptor compartment was performed. Although the 5-FU concentration (0-686 ng/ml) and pH (7.40-7.24) changed substantially, the recovery of 5-FU was not adversely affected. To demonstrate the feasibility of dermal microdialysis, the flux of a commercial preparation of 5-fluorouracil was monitored utilizing a microdialysis probe implanted in excised rat skin in vitro. The results from the dermally implanted probe demonstrate the potential of the technique while establishing the limitations of the current microdialysis system.

Characterization of the complexation of fluoroquinolone antimicrobials with metal ions by nuclear magnetic resonance spectroscopy

The complexation of the fluoroquinolone antimicrobials is important because it has been implicated in reduced oral bioavailability and reduced antimicrobial activity when the drugs are co-administered with antacids or multi-vitamin preparations containing iron. The complexation of two model compounds, lomefloxacin and norflaxacin was studied using NMR. With aluminum ions, exchange between free and bound drug molecules was slow on the NMR time-scale. Two complexes, proposed to have stoichiometries of 2:1 and 3:1 (drug:metal) based on peak widths and variable temperature studies, were observed. The crystal structure of lomefloxacin, which shows intermolecular self association previously reported to be crucial to the drugs mode of action, is also reported. Because the metal ion complexes could not be crystallized, the crystal structure of uncomplexed lomefloxacin together with the NMR data on the aluminum complexes were used in the molecular modelling of the lomefloxacin-aluminum complexes.