Microbiological effect of photodynamic therapy (PDT) in healthy volunteers: a comparative study using methyl aminolaevulinate and hexyl aminolaevulinate cream.

Background: Acne vulgaris is a common skin problem that affects up to 90% of adolescents. Colonization of the duct with Propionibacterium species is one of the factors implicated in the development of acne. Owing to the increasing incidence of antibiotic resistance, there has been an greater interest in the development of new methods to treat acne. Early studies have shown that photodynamic therapy (PDT) with aminolaevulinic acid (ALA) can lead to prolonged improvement in acne. Newer derivatives of ALA such as methyl aminolaevulinate hydrochloride (MAL) and hexyl aminolaevulinate hydrochloride (HAL) have been developed for use in PDT, with the potential benefits of higher lipophilicity and penetration potential.

Objectives: To determine the microbiological effect and tolerability of a single application of HAL-PDT and to compare it with MAL-PDT in healthy volunteers.

Methods: This was a randomised double-blind study to examine the microbiological effects and safety of a single application of MAL-PDT and HAL-PDT on normal skin in 18 healthy volunteers. Bacterial skin samples for Propionibacterium spp. and Micrococceae were obtained at baseline and 2, 4, 7 and 14 days.

Results: Following PDT with MAL and HAL, a statistically significant transient reduction in mean density of Propionibacterium spp. 2 days after treatment using each agent (P < 0.05 for both) was found. There were no significant changes in mean number of Micrococceae for the duration of the study period. Treatment with HAL-PDT and MAL-PDT was well tolerated. Overall, HAL-PDT was associated with fewer side-effects compared with MAL-PDT (P < 0.01) over the 14 day study period.

Conclusion: HAL-PDT and MAL-PDT transiently reduce density of Propionibacterium spp. density to a similar degree in normal healthy individuals. The transient reduction in Propionibacterium spp. suggests that the prolonged antiacne effect of PDT relies on factors independent of bacterial density. HAL-PDT appears to be better tolerated than MAL-PDT.