Difference between revisions of "Photostable Compounds"

Revision as of 00:23, 11 December 2017

Dr. Susana Encinas

Departamento de Química/Instituto de Tecnología Química UPV-CSIC, Universitat Politècnica de València, Camino de Vera s/n, 46022 Valencia, Spain.

Light can change the properties of different materials and products. This is often observed as bleaching of coloured compounds like paint and textiles. Photostability has for many years been a main concern within several fields of industry, e.g. the textile, paint, food, cosmetic and agricultural industries. In the field of pharmacy, the number of drugs found to be photochemically unstable is steadily increasing. In this context, the term photostability is used to describe how a compound responds to light exposure and includes not only degradation reactions but also other processes such as formation of radicals, energy transfer and luminescence.

Each organism is a product of its response to the pressures of the environment, in order to establish a balance between the organism and its environment. This balance causes changes in the subject, which can be harmful or beneficial to their existence. Therefore, the possibility that several compounds in combination with sunlight may be beneficial or harmful to the patient should be considered.

1 Photophysical and Photochemical Aspects of Photostability

The fact that a substance absorbs radiation in the ultraviolet or visible region of the electromagnetic spectrum means that it is absorbing energy that is sufficient to break a bond in the molecule. Thus the property of absorption is a first indication that the substance may be capable of participating in a photochemical process leading to its own decomposition. According to the Grotthus’ law of photochemistry, no photochemical (or subsequent photobiological) reaction can occur unless electromagnetic radiation is absorbed. The absorption spectrum of a compound is therefore an immediate way of determining the wavelength range to which the substance may be sensitive.

There are two important factors to ponder in relation to the potential of a compound to be degraded following absorption of electromagnetic radiation. First, the absorption spectrum is normally described by the maximum absorption wavelength and the molar absorptivity at that wavelength, but the spectrum can be broad and any overlap of the absorption spectrum with the output of the photon source impinging upon it has the potential to lead to photochemical change. Second, the decomposition may be initiated by another close compound (for example, in a drug formulation) that has the absorption characteristics that overlap with the incident radiation while the main compound does not. This process is the photosensitization and the absorbing compound is the photosensitizer. [1] (see Photosensitization entry[[1]]).

Photochemical damage to a substance is initiated by the absorption of energy by the compound itself of by a photosensitizer. Many photochemical reactions are complex, and may involve a series of competing for reaction pathways in which oxygen may play a significant role. In fact, the great majority of photoreactions in biological systems involve the consumption of molecular oxygen and are photosensitized oxidation processes.[2]

The photophysical processes are usually well described by the Jablonsky diagram (see Photosensitization entry) and they could lead to a wide photochemical reactivity. Among the most common reaction types that a compound might experience under photon absorption are addition, cyclization, N-dealkylation, decarbonylation, decarboxylation, dehalogenation, dimerization, oxidation, reduction, isomerization, rearrangement, and/or hydrolysis [3].

Furthermore, the concept of photostability covers a very wide field and it could be applied to many issues such as drugs, dyes, labels, solar filters, etc. Next, a few explicative examples like drugs and solar filters are presented.

2 Drugs Photostability

Drugs are agents designed to produce a specific and beneficial effect in the body, but they can lead to adverse reactions, of very different nature, which are assumed in terms of the risk-benefit ratio of their use. In general, precautions are taken to prevent photodegradation of drugs during storage, but changes induced by light in a patient who has been treated with a drug can lead to significant side effects. Within the adverse reactions caused by drugs is the photosensitization, i.e. the drug cause light-induced side effects after administration to the patient by interaction with endogenous substances [1] (see Photosensitization entry[[2]]).

Until now, most reports on the biological activity of drugs come from dermatologists, and show adverse effects such as erythema, edemas, followed by hyperpigmentation and desquamation. Although drugs are the major cause of skin photosensitization, the incidence of the photosensitivity they cause is not easy to know, since there is a great variability, probably due to local variations in skin type, light exposure and drug doses [4].

In this sense, there is great diversity at the level of therapeutic groups, structural formula, etc., which makes it difficult to predict a priori the same photobiological activity. The difficulty of establishing a satisfactory structure-activity relationship is evident. Agents acting as photoallergic agents, as well as those that cause contact dermatitis, are usually liposoluble and with a low molecular weight. Like phototoxic agents, they tend to have conjugated structures capable of absorbing light radiation [5].

Therefore, it is important to know the photobiological activity of drugs because a) allowsvarying the molecular structure to minimize the side effects conserving the desired pharmacological effects, and b) allowstaking advantage of the light interaction with the biological processes for therapeutical purposes.