Light alone can activate the PDT reaction by stimulating porphyrins which accumulate within the sebaceous follicle in acne. Porphyrins, mainly coproporphyrin III, are produced by Propionibacterium acnes, the anaerobic bacterium that proliferates and causes inflammation in obstructed sebaceous follicles [7]. Endogenous porphyrins serve as photophores to mediate the PDT reaction following exposure to light, particularly blue light [8]. This reaction generates singlet oxygen and mediates bacterial destruction within the sebaceous follicles [8].
Treatment with blue light alone has been mildly to moderately effective in clinical trials, likely due to poor skin penetration by blue light. Blue light once weekly for two weeks in one study showed a 25% improvement in acne severity [9]. Another study of blue light 15 min per day resulted in a 30 and 15% mean reduction for inflammatory and comedonal acne lesions, respectively, after 4 weeks; with a final mean improvement of 63 and 45%, respectively, after 12 weeks [10]. Another study of blue light once weekly for 4 weeks yielded a 43% reduction [11].
While red wavelengths are less effective at porphyrin photoactivation, they do achieve greater penetration depth, thereby targeting the more deeply situated sebaceous follicles. Red light (660 nm) combined with blue light (415 nm) for 15 min daily increased efficacy to a mean lesional reduction of approximately 50% for inflammatory and 25% for come – donal acne after 4 weeks, and 76 and 58% respectively, after 12 weeks [10]. These values resulting from a single treatment with red and blue light combined, were superior to the 30 and 15% mean reduction for inflammatory and comedonal acne lesions, respectively, after 4 weeks; and the final mean improvement of 63 and 45%, respectively, after 12 weeks observed with blue light alone [10]. Recently, red light (635-670 nm) as monotherapy 15 min twice daily for 8 weeks was shown to reduce acne counts (59% decrease) as compared to controls (3% increase) in a split-face design study of 28 patients with acne vulgaris, followed to the 8 week post-treatment interval [12]. Thus, these findings suggest that red light alone demonstrates higher efficacy, as compared to blue light alone in mediating PDT and treating acne.