ICH Q1B Photostability tests seen in a whole new light

The 1996 guideline CPMP/ICH/279/95 Q1B describes the process for carrying out photostability tests on new active substances and medicinal products. It stipulates the following: Cool white fluorescent lamps with an output similar to that stipulated in ISO 10977 (1993). UVA fluorescent lamps with a spectral distribution of 320 nm to 400 nm and a maximum energy emission of between 350 nm and 370 nm. A significant proportion of the waves must be within the ranges of 320 nm to 360 nm and 360 nm to 400 nm. The samples should be exposed to visible light (VIS) for at least 1.2 million lux hours and to UVA for at least 200 watt hours per square meter. The specifications listed above are well known. Less familiar, however, are the intricacies that need to be taken into account when implementing the ICH Q1B guideline in photostability chambers featuring ICH-compliant irradiation facilities.  This blog article introduces two light photometry methods for photostability tests that comply with ICH Q1B, discusses ICH-compliant light sources, and considers the key pros and cons. Which light sensors are better: planar or spherical? For us, there’s no competition Two types of light sensors are available for ICH Q1B photostability tests – detectors with a flat sensor surface (planar sensors) and detectors with a spherical sensor surface (spherical sensors). There are two major differences between them – in one case, the sensor calculates the intensity of the radiation, whereas the other measures the actual radiation intensity. Lambert’s cosine law states that radiation intensity decreases as angles become more oblique. In order to compensate for this reduction, planar sensors with diffusers use what is known as cosine correction. This means that only a small proportion of the radiation that hits the sensor surface is actually measured – the radiation that hits at a 90° angle. For all other angles of incidence (see Figure 1), the radiation intensity is calculated using a mathematical equation. This is why BINDER does not recommend using planar sensors with diffusers to test the photostability of new active substances and medicinal products. The same applies to planar sensors without diffusers, as they record even less of the actual radiation intensity.

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