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The photokinetic behaviour of drugs driven by polychromatic light is an area of pharmaceutics that has not received a lot of attention. Most often, such photokinetic data is treated by thermal kinetic models (i.e., the classical 0th-, 1st- or 2nd-order equations). Such models were not analytically derived from the rate-laws of the photodegradation reactions. Polychromatic light kinetic modelling is hence of importance, as a means to providing adequate toolkits and metrics. This paper aims at proposing two reliable drug-actinometers useful for polychromatic UVA range. The general actinometric methodology offered here is also useful for any drugs/materials obeying a primary photoprocess where both reactant and photoproduct absorb the incident light, of the $$AB{(1\Phi )}_{{\varepsilon }_{B}\ne 0}$$ A B ( 1 Φ ) ε B ≠ 0 type. The present method has been consolidated by the η-order kinetics. This framework further demonstrated the lamp-specificity of actinometers. Overall, Dacarbazine and Nifedipine photodegradations obeyed η-order kinetics, and stand as effective actinometers that can be recommended for the ICH Q1b photostability testing.

The photokinetic behaviour of materials driven by polychromatic light is an area that has not received a lot of attention in the literature. Most often, such photokinetic data is treated by thermal kinetic models (i.e., the classical 0th -, 1st - or 2nd -order equations). Such models were not analytically derived from the rate-laws of the photodegradation reactions. Polychromatic light kinetic modelling is hence of importance, as a means to providing adequate toolkits and metrics. This paper aims at proposing two reliable drug-actinometers useful for polychromatic UVA range. The general actinometric methodology offered here is also useful for any drugs/materials obeying a primary photoprocess where both reactant and photoproduct absorb the incident light, of the \(AB{\left(1{\Phi }\right)}_{{\epsilon }_{B}\ne 0}\) type. The present method has been consolidated by the η-order kinetics, a mathematical model analytically derived from the photosystem’s rate-law. This represents the first ever equation in the literature, to model polychromatic light of this reactive system. This framework further demonstrated the lamp-specificity of actinometers. Overall, Dacarbazine and Nifedipine photodegradations obeyed η-order kinetics, and stand as effective actinometers that can be recommended for the ICH Q1b photostability testing.

AbstractΦ-order kinetics has been shown to be the best tool to study photokinetics [1 -6]. Various species have been studied using this approach including photochromes [1,2] and drugs [3 -6]. The unimolecular photodegradation processes of the form AB (1Φ) and photoreversible systems AB (2Φ), where A, the initial species phototransforms into a product B (A→B) which itself can return to A (A→B) via two dis nct photoprocesses, each characterized each by an individual efficiency (Φ), have been mathematically described by integrated rate-law equations which express the so called 'Φ-order' kinetics [1 -6]. The application of this novel approach was found to be useful to determine a number of photoreaction attributes. The reaction's photochemical quantum yield(s), the effects of initial species concentration, or the impact of competitive absorbers on the rate-constant of the photoactive species were all made readily accessible by simple methods. More importantly, the Φ-order kinetics has proven to facilitate the development of a wide range of actinometers from AB (1Φ) and AB (2Φ) systems [2 -6]. This novel Φ-order kinetic method clearly has the potential for application in various fields including the photodegradation of natural products. The details of Φ-order elucidation kinetic methods will be discussed here and examples of application presented.