Photo-biology is the analysis of the effect of visible, UVA and/or UVB, and IR radiation on living systems. According to the first law of photochemistry (Grotthaus-Draper Law) for a photochemical event to happen, light should be absorbed. Photochemical reactions target the DNA in dermal tissue and chromophores in drug products. Photo-allergy and/or photo-irritation occur upon entry of a photoactive chemical into the skin through systemic circulation or dermal penetration and the subsequent excitation of the chemical by appropriate visible or UV light photons.
The test method proposed for chemicals absorbing light in the range of 290-700 nm (Table 1) for topically applied substances, or substances that can reach the eyes and/or skin by systemic exposure (intravenous or oral), is called photo-safety testing.
Table 1. Relative cumulative erythemal effectiveness for light absorbance of chemicals
| Spectral Range (nm) |
Measured % RCEE |
| Lower Limit |
Upper Limit |
| <290 |
|
<0.1 |
| 290-300 |
1 |
8.0 |
| 290-310 |
49.0 |
65.0 |
| 290-320 |
85.0 |
90.0 |
| 290-330 |
91.5 |
95.5 |
| 290-340 |
94.0 |
97.0 |
| 290-400 |
99.0 |
100.0 |
| UVA II (320-340) |
≥ 20.0 |
|
| UVA I (340-400) |
≥ 60.0 |
|
Photo-safety testing addresses the following four basic endpoints or results:
- Photo-toxicity, or photo-irritation, is an acute light-induced response of the skin to a photo-reactive chemical.
- Photo-allergy is as an immune reaction to a chemical that is initiated by photo-products formation. It can also be a byproduct of antigen exposure.
- Photo-genotoxicity is a genotoxic response which occurs after exposure to a chemical photo-activated by visible or UV light.
- Photo-carcinogenicity is the ability of a chemical to induce a skin tumor formation when combined with UV light exposure.
In the case of cosmetic components or pharmaceuticals, basic photo-safety testing is performed to rule out the possibility of the above-mentioned endpoints upon exposure to UV radiation by in-vitro or in-vivo or testing of cell cultures or animals. The test results are compared with those of comparable samples in the absence of UV radiation exposure, and with controls where the chemical is exposed to a similar UV light dose. Different classes of drugs - such as anticonvulsants, NSAIDs, antihypertensives, antimicrobials, diuretics, and antidepressants -are known to induce photoirritation in humans.
Sol-UV Series Solar Simulators
Most of the test methods involve the use of an irradiation spectrum to approximate the solar spectrum, utilizing the appropriate filters to eliminate the UVC component but allow the UVA and UVB components to pass through. Further guidance is not provided by a majority of the methods, and so the variation in the intensity between the UVA and UVB components can vary significantly according to the light source used. The Oriel Sol-UV Series solar simulators (Figure 1) can produce a defined spectral output that is compliant with the following standards: FDA CFR Part 201.327; ISO 24444:2010(e) First Edition; the International SPF Test Method (CTFASA/COLIPA/JCIA/CTFA: May 2006) for Spectral Match.
Figure 1. Oriel Sol-UV Series solar simulator
When a standardized light source is used, the variables associated with the use of multiple undocumented light sources with very different spectral profiles in the UV are eliminated, facilitating the experimental data to be easily compared. In addition to spectral consistency, irradiance uniformity over the work area is also maintained, preventing the formation of so-called hot spots that can cause errors in delivered dosage (Figure 2).
Figure 2. Colipa irradiance response curve
The spatial uniformity performance standard of the Oriel Sol-UV Series solar simulator is designed to maintain the non-uniformity to less than 5% across the entire work area (Table 2). Temporal stability also plays a vital role in reducing the dosage errors. Newport’s expertise in designing feedback controllers and ultra stable power supplies is useful to ensure the best long-term and short-term stabilities and ensures that the output light is stable over a long time, reducing the impact on the desired dosage.
Table 2. Sol-UV performance specification
| Simulator Model |
SOL-UV-2 |
SOL-UV-4 |
SOL-UV-6 |
| Collimation Angle |
(half angle)
< ±4 |
(half angle)
< ±4 |
(half angle)
< ±3 |
| Output Power Adjustment Range |
10 - 100% of Maximum |
10 - 100% of Maximum |
10 - 100% of Maximum |
| Output Power Adjustment Method |
Manual |
Manual |
Manual |
| Uniformity Classification |
< 5% nonuniformity |
< 5% nonuniformity |
< 5% nonuniformity |
| Typical time to reach SED (Standard Erythemal Dose) @ Max Output Power |
26 seconds |
26 seconds |
59 seconds |
| Working Distance |
4 inches (50 mm) |
4 inches (50 mm) |
6 inches (150 mm) |
The Sol-UV Series enables the dosage to be varied using an integrated variable aperture, and can be used in combination with an optional light intensity control system to enable specific dosages to be delivered as a function of the exposure time.
Photo-Toxicity
In the case of photo-toxicity testing, the phototoxic potential of the test substance that is induced by the chemical being excited after exposure to light is identified using the in-vitro 3T3 NRU photo-toxicity test, which evaluates photo-cytotoxicity by relatively reducing the viability of cells exposed to the chemical in the absence versus presence of light. Substances that are identified through this test are probably photo-toxic in vivo, upon systemic application and distribution into the skin.
Recently, evaluation of the relevance and reliability of the in-vitro 3T3 NRU photo-toxicity test indicated that the test is predictive of acute photo-toxicity effects in humans and animals in vivo. However, the test does not predict other unfavorable effects caused due to the combined action of a chemical and light. It does not predict photo-allergy, photo-genotoxicity, or photo-carcinogenicity, and does not enable assessment of phototoxic potency.
The 3T3 method indicates that the simulation of sunlight by using solar simulators is the optimal artificial light source. Here, the irradiation power distribution of the filtered solar simulator must be nearly equal to that of outdoor daylight. As xenon-based solar simulators emit notable quantities of UVB, they must be appropriately filtered for the highly cytotoxic UVB wavelengths to be attenuated.
To suppress the UVB, the Sol-UV Series solar simulator is equipped with appropriate filters. The method suggests that the spectrum recorded for these filters must not deviate from the standardized outdoor daylight, irrespective of the measures taken to attenuate parts of the spectrum through filtering or through unavoidable filter effects of the equipment. These requirements are satisfied by the COLIPA compliant output of the Sol-UV solar simulators.
Photo-Allergy
As photo-allergy testing is performed in-vivo as it requires the measurement of the immune response to photo-products that are produced as a result of exposure to visible or UV light. Sol-UV exposes the animal to UV light with wavelength similar to the wavelength range used in sunscreen protection factor (SPF) as reported in the COLIPA monograph.
Photo-Genotoxicity
Photo-genotoxicity testing evaluates the ability of a compound to be transformed into a genotoxic product upon exposure to and activation by visible or UV light. Results such as inhibition of the normal DNA repair mechanisms and damage to DNA, i.e. strand breaks or mutagenesis, indicate the occurrence photo-genotoxic response.
Photo-Carcinogenicity
The evaluation of photo-carcinogenicity is usually carried out through in-vivo testing of albino hairless mice to study the effect of UV light on promoting various types of skin cancers. The mechanism through which photo-carcinogenicity occurs may be immunosuppression or photo-genotoxicity. It may also be caused by photo-induced by-products displaying either of the mechanism. This test involves using light sources that produce output similar to the UV output from the sun, resembling the COLIPA standard, and may also involve accelerated dosages.
This information has been sourced, reviewed and adapted from materials provided by Oriel Instruments.
For more information on this source, please visit Oriel Instruments.