PHOTOTHERAPY

Various forms of phototherapy are under investigation for their use in treating acne vulgaris. Up to 70% of patients report that sun exposure improves their acne (109). This reported benefit may be due to camouflage by UV radiation-induced erythema and pigmentation, although it is likely that the sunlight has a biologic effect on the pilosebaceous unit and P. acnes. Porphyrins are formed endogenously by P. acnes, and are also acquired by exogenous sources. Protoporphyrin IX is taken up via cell wall receptors (110), and coproporphyrin III is the major endogenous porphyrin. Coproporphyrin III can absorb light at the near-UV and blue-light spectrum of 415 nm (111). In vitro irradiation of P. acnes with blue light leads to photoexcitation of endogenous bacterial porphyrins, singlet oxygen production, and subsequent bacterial destruction (112). Although UVB can also kill P. acnes in vitro, it is clinically insignificant since it has low skin penetration, and only high doses causing sunburn have be shown to improve acne (113,114). UV radiation may have anti-inflammatory effects by inhibiting cytokine action (115).