News, Views, & Reviews. Antimicrobial Photodynamic Therapy: Applications Beyond Skin Cancer

Reactive oxygen species (ROS) generation by the innate immune system in response to pathogen invasion has spurred new efforts to utilize oxidative stress as an anti-infective strategy. In the “respiratory burst” process, neutrophils and macrophages engulf foreign cells, triggering the enzymatic production of ROS; myeloperoxidase generates hypochlorite (OCl^- ), NADPH oxidase generates superoxide (02 - ), and iNOS generates NO¹. This inactivates microbial organisms via damage to cell membranes, mutations to DNA, degradation of proteins and activation of pro-apoptotic factors. In the age of rising microbial resistance, pathogen destruction via ROS in the form of photodynamic therapy (PDT) is gaining more attention for the treatment of superficial, identifiable infection and is a promising new alternate to conventional therapies.

PDT is a technique that creates ROS by exciting a pharmacologically inert photosensitizer (PS) with light matched to its absorption wavelength, in the presence of oxygen. The PS molecule goes to the first excited singlet state, and while some of the energy is dissipated as heat or light (fluorescence), undergoes intersystem crossing to a longer lived, triplet state (Figure 1). This conversion is crucial and dyes without a significant triplet yield are not useful as only in this state can the PS survive long enough to undergo two photochemical reactions: Type I and Type II. The Type I mechanism involves the transfer of electrons to a substrate which can then react with oxygen to produce cytotoxic species like superoxide, hydroxyl and lipid-derived radicals. The Type II reaction is more common and involves the direct transfer of electrons to ground-state molecular oxygen (triplet) to produce excited-state singlet oxygen.^1,2 The singlet oxygen reacts rapidly with most biological constituents and induces the most oxidative damage. The short lifetime and reactivity of singlet oxygen prohibits its distribution in cells and necessitates the localization of PDT treatments to the exact point of interest.

PDT has been most extensively developed for oncologic and ophthalmologic uses, but the ease of skin access by a light source has brought renewed focus on PDT to treat superficial diseases.³ It has several favorable features for cutaneous infection including the ability to inactivate antibiotic-resistant strains, a broad spectrum of action, and the lack of selection for photo-resistant organisms due its multi- mechanistic approach.^4,5 In general, Gram-positive bacteria are more susceptible to PS due to their relatively permeable cell wall. Gram-negative bacteria and fungi, however, first require a sensitizer to weaken the cell well and allow for PS translocation. Initial reports utilized chemical methods to increase permeability, including pretreatment with polycationic peptide polymyxin B nonapeptide (PMBN) to expand the outer leaflet of the membrane,⁶ and ethylenediaminetetraacetic acid (EDTA) to release LPS.⁷ However, a simpler approach has been to use or synthesize molecules with an intrinsic cationic charge via conjugation to a positive carrier or addition of a quaternary ammonium or phosphonium.⁷ The added benefit of the cationic approach is that it has faster uptake by bacteria as compared to host cells, contributing to greater selectivity and less contiguous toxicity.⁵

Treatment regimens utilizing PDT can be designed by varying the type of PS, the route of administration and dose, and the incubation time before light exposure.⁸ The many permutations argue for the versatility and adaptability of PDT for different situations. Topical PDT has, to date, only been FDA approved for the treatment of non-melanoma skin cancers and precursors. Yet despite this narrow approval, multiple offlabel applications are being used.⁹

Types of PS
Porphyrinoid derivatives (eg, porphyrins, chlorins, bacteriochlorins, phthalocyanines) and precursors have been the most successful in producing requisite singlet oxygen.¹⁰ The most commonly employed agents for dermatologic use are 1,5-aminolevulinic acid (ALA, Levulan^® Kerastick or other ALA preparations) and the methyl ester of ALA, methyl aminolevulinate (MAL, Metvix^® cream). These molecules enter the endogenous metabolic pathway for heme synthesis and lead to the accumulation proptoporphyrin IX in cells (Figure 2). Although porphyrinoid structures comprise the majority of photosensitizers, several non-porphyrin chromogens have been investigated as topical PS agents, including anthraquinones (eg, hypericin), phenothiazines (eg, toluidine blue, methylene blue), cyanines (eg, indocyanine green), and curcuminoids.¹⁰

The altered barrier in many skin pathologies aids in percutaneous delivery of PS.⁸ Topical application allows for the precise localization of PDT effect, avoids the general toxicity associated with systemic agents, and more evenly distributes in superficial skin. MAL is lipophilic and may penetrate at a faster rate than ALA,¹¹ while ALA is a more efficient producer of PpIX in cells.¹² However these differences have not resulted in variances in oxidative damage as studies comparing these agents have failed to show a difference in clinical response¹³ and final penetration