Protoporphyrin IX formation and photobleaching in different layers of normal human skin: Methyl‐ and hexylaminolevulinate and different light sources

Topical photodynamic therapy (PDT) is used for various skin disorders, and selective targeting of specific skin structures is desirable. The objective was to assess accumulation of PpIX fluorescence and photobleaching within skin layers using different photosensitizers and light sources. Normal human skin was tape‐stripped and incubated with 20% methylaminolevulinate (MAL) or 20% hexylaminolevulinate (HAL) for 3 h. Fluorescence microscopy quantified PpIX accumulation in epidermis, superficial, mid and deep dermis, down to 2 mm. PpIX photobleaching by light‐emitting diode (LED, 632 nm, 18 and 37 J/cm²), intense pulsed light (IPL, 500–650 nm, 36 and 72 J/cm²) and long‐pulsed dye laser (LPDL, 595 nm, 7.5 and 15 J/cm²) was measured using fluorescence photography and microscopy. We found higher PpIX fluorescence intensities in epidermis and superficial dermis in HAL‐incubated skin than MAL‐incubated skin (P < 0.001). In mid and deep dermis, fluorescence intensities were higher (37%) in HAL‐treated skin than MAL‐treated skin, although not significant (P = ns). At skin surface, photobleaching was significantly higher (90–98%) after LED illumination (18 and 37 J/cm²) than IPL (29–53%, 36 and 72 J/cm²) and LPDL (43–62%, 7 and 15 J/cm²) (P < 0.001). Within the skin, photobleaching was steady from epidermis to deep dermis by LED illumination (37 J/cm², P = ns), but declined from epidermis to mid and deep dermis for IPL‐treated skin and LPDL‐treated skin (IPL 72 J/cm²: 26–15%; LPDL 15 J/cm²: 37–23%) (P < 0.04). Clinically, erythema correlated linearly with MAL and HAL‐induced photobleaching (r² = 0.175, P < 0.001). In conclusion, selective PpIX accumulation indicates HAL as an alternative to MAL for epidermal‐targeted PDT. In clinically relevant doses, PpIX photobleaching throughout the skin was more profound following LED than LPDL and IPL exposure.