Modeling and Elucidation of the Kinetics of Multiple Consecutive Photoreactions AB(4Φ) With Φ-order Kinetics. Application to the Photodegradation of Riboflavin.

New semi-empirical rate-law system of equations is proposed for the first time for consecutive photoreactions that involve up to 4 photoreaction steps, AB(4Φ). The equation system was developed, tested, and validated against synthetic kinetic traces generated by fifth-order Runge-Kutta calculations. The model accurately fitted the kinetic traces of Riboflavin photodegradation in ethanol which decomposes via the AB(2Φ) mechanism involving 2 consecutive photoreaction steps.

Quantitative assessment of photostability and photostabilisation of Fluvoxamine and its design for actinometry.

Despite the numerous concerns that have been raised in relation to considering 0(th), 1(st) and 2(nd)-order kinetic treatments for photodegradation characterisation and assessment of drugs, they still are employed, as they are the only tools available for these types of studies. The recently developed Φ-order kinetic models have opened new perspectives in the treatment of photoreaction kinetics and stand as the best known alternative to the classical approach. The Φ-order kinetics have been applied here to Fluvoxamine (Fluvo) with the aim of setting out a detailed and comprehensive procedure capable of rationalising photodegradation/photostability of drugs and proposing a platform for photosafety studies.

Ф-order kinetics of photoreversible-drug reactions.

Background: Drug photodegradation data are usually treated by zero-, first- or second-order kinetic equations. Such treatments would lack reliability since the aforementioned equations have been originally developed for pure thermal reactions. In this respect, it has recently been shown that unimolecular photodegradations obey Ф-order kinetics (Maafi and Maafi, 2013).

Montelukast photodegradation: elucidation of Ф-order kinetics, determination of quantum yields and application to actinometry.

A recently developed Ф-order semi-emperical integrated rate-law for photoreversible AB(2Ф) reactions has been successfully applied to investigate Montelukast sodium (Monte) photodegradation kinetics in ethanol. The model equations also served to propose a new stepwise kinetic elucidation method valid for any AB(2Ф) system and its application to the determination of Monte's forward (Ф(λ(irr))(A-->B)) and reverse (Ф(λ(irr))(B-->A)) quantum yields at various irradiation wavelengths. It has been found that Ф(λ(irr))(A-->B) undergoes a 15-fold increase with wavelength between 220 and 360 nm, with the spectral section 250-360 nm representing Monte effective photodegradation causative range.

Modelling nifedipine photodegradation, photostability and actinometric properties.

Background: The photodegradation of drugs obeying unimolecular mechanisms such as that of nifedipine (NIF) were usually characterised in the literature by zero-, first- and second-order kinetics. This approach has been met with varying success. This paper addresses this issue and proposes a novel approach for unimolecular photodegradation kinetics.