Raheela Bano

Photodegradation of levofloxacin in aqueous and organic solvents: A kinetic study

The kinetics of photodegradation of levofloxacin in solution on UV irradiation in the pH range 2.0-12.0 has been studied using a HPLC method. Levofloxacin undergoes first-order kinetics in the initial stages of the reaction and the apparent first-order rate constants are of the order of 0.167 to 1.807×10-3 min-1. The rate-pH profile is represented by a curve indicating the presence of cationic, dipolar and anionic species during the reaction. The singly ionized form of the molecule is non-fluorescent and is less susceptible to photodegradation. The increase in the degradation rate in the pH range 5.0-9.0 may be due to greater reactivity of the ionized species existing in that range. The rate appears to vary with a change in the degree of ionization of the species present in a particular pH range and their susceptibility to photodegradation. Above pH 9, the decrease in the rate of photodegradation may be a result of deprotonation of the piperazinyl group. The levofloxacin molecule is more stable in the pH range around 7, which is then suitable for formulation purposes. The photodegradation of levofloxacin was found to be affected by the dielectric constant and viscosity of the medium

Photodegradation of Moxifloxacin in Aqueous and Organic Solvents: A Kinetic Study

The kinetics of photodegradation of moxifloxacin (MF) in aqueous solution (pH 2.0–12.0), and organic solvents has been studied. MF photodegradation is a specific acid-base catalyzed reaction and follows first-order kinetics. The apparent first-order rate constants (kobs) for the photodegradation of MF range from 0.69 × 10−4 (pH 7.5) to 19.50 × 10−4 min−1 (pH 12.0), and in organic solvents from 1.24 × 10−4 (1-butanol) to 2.04 × 10−4 min−1 (acetonitrile). The second-order rate constant (k2) for the [H+]-catalyzed and [OH−]-catalyzed reactions are 6.61 × 10−2 and 19.20 × 10−2 M−1 min−1, respectively. This indicates that the specific base-catalyzed reaction is about three-fold faster than that of the specific acid-catalyzed reaction probably as a result of the rapid cleavage of diazabicyclononane side chain in the molecule. The kobs-pH profile for the degradation reactions is a V-shaped curve indicating specific acid-base catalysis. The minimum rate of photodegradation at pH 7–8 is due to the presence of zwitterionic species. There is a linear relation between kobs and the dielectric constant and an inverse relation between kobs and the viscosity of the solvent. Some photodegraded products of MF have been identified and pathways proposed for their formation in acid and alkaline solutions.

Formulation and stabilization of norfloxacin in liposomal preparations.

A number of liposomal preparations of norfloxacin (NF) containing variable concentrations of phosphatidylcholine (PC) (10.8-16.2mM) have been formulated and an entrapment of NF to the extent of 41.7-56.2% was achieved. The values of apparent first-order rate constants (kobs) for the photodegradation of NF in liposomes (pH7.4) lie in the range of 1.05-2.40×10(-3)min(-1) compared to a value of 8.13×10(-3)min(-1) for the photodegradation of NF in aqueous solution (pH7.4). The values of kobs are a linear function of PC concentration indicating an interaction of PC and NF during the reaction. The second-order rate constant for the photochemical interaction of PC and NF has been determined as 8.92×10(-2)M(-1)min(-1). Fluorescence measurements on NF in liposomes indicate a decrease in fluorescence with an increase in PC concentration as a result of formation of NF(-) species which exhibits poor fluorescence. Dynamic light scattering has shown an increase in the size of NF encapsulated liposomes with an increase in PC concentration. The stabilization of NF in liposomes is achieved by the formation of a charge-transfer complex between NF and PC.

Stability-Indicating Photochemical Method for the Assay of Riboflavin: Lumichrome Method

A stability-indicating photochemical method for the assay of riboflavin (RF) in photodegraded samples and aged vitamin preparations has been developed. It is based on photochemical conversion of RF to lumichrome (LC) in alkaline solution under controlled conditions of light intensity, temperature, pH, time of exposure, and distance. Under these conditions about two-thirds of RF is converted to LC and on the basis of the RF : LC ratio the concentration of RF can be determined in degraded solutions. The method involves the extraction of photolyzed solutions of RF (pH 2.0) with chloroform and determination of LC along with lumiflavin (LF) by a two-component spectrometric method at 356 and 445 nm. The method has been validated and the results of the assay of RF in photodegraded solutions compare well with those of the standard USP fluorimetric method. The recovery of the method is 99–101% and the precision is within 2%. The method is stability-indicating and can be applied to the assay of RF in photodegraded solutions and aged vitamin preparations. The method is specific compared to that of the USP fluorimetric method in which the degraded LC may interfere with the fluorescence emission of RF.

Solvent Effect on Photoinitiator Reactivity in the Polymerization of 2-Hydroxyethyl Methacrylate

Efficacy of photoinitiators such as riboflavin (RF), camphorquinone (CQ), and safranin T (ST) and triethanolamine as a coinitiator has been compared in carrying out the polymerization of 2-hydroxyethyl methacrylate (HEMA) in aqueous and organic solvents. HEMA solutions were polymerized in the presence of RF, CQ, and ST using a low intensity visible radiation source. HEMA was assayed by a UV spectrophotometric method during the initial stages of the reactions (i.e., ~5% change). A comparison of the efficacy of photoinitiators in causing HEMA polymerization showed that RF is more efficient than CQ and ST. The rate of polymerization is directly related to solvent dielectric constant and inversely related to the solvent viscosity. RF is the most efficient photoinitiator in the polymerization of HEMA and the highest rate of reaction occurs in aqueous solutions. A general scheme for the polymerization of HEMA in the presence of photoinitiators is presented.