Beyond Conventional ALA-PDT: Strategies for Deeper, Faster, and More Efficient Treatment of Non-Melanoma Skin Cancer

Background/Objectives: Photodynamic therapy (PDT) using 5-aminolevulinic acid (ALA) and its derivatives is an established non-surgical approach for non-melanoma skin cancer (NMSC), particularly basal cell carcinoma (BCC), squamous cell carcinoma (SCC) in situ, and premalignant lesions such as actinic keratosis. Despite favorable cosmetic outcomes and its ability to treat field cancerization, conventional topical ALA-PDT is better suited to superficial lesions because of limited precursor penetration, heterogeneous protoporphyrin IX (PpIX) accumulation, and insufficient light delivery to deeper tissue layers. This review critically examines the biological and physical barriers that limit ALA-based PDT efficacy in NMSC and discusses emerging multimodal strategies to address these challenges. Methods: This review examines the biological and physical factors that limit topical ALA-based PDT, drawing on current knowledge of heme biosynthesis, mitochondrial regulation of PpIX accumulation, skin optical properties, and oxygen dependence. It also highlights near-infrared photobiomodulation (PBM) and microneedle-assisted ALA delivery as complementary strategies that may help overcome these barriers. Results: Near-infrared PBM is a promising adjunct: in cell culture and exploratory human skin studies, it modulates endogenous PpIX production. Whether this translates into greater photosensitizer availability at depth, shorter incubation, or reduced treatment-related pain in NMSC remains hypothetical and untested clinically. Concurrently, dissolving microneedle arrays (MAs) have shown enhanced intradermal ALA delivery and increased PpIX accumulation and distribution in preclinical studies. These are independent strategies that address distinct barriers within the PDT pathway and may be combined to maximize therapeutic response. Conclusions: Overcoming the depth and delivery limitations of conventional ALA-PDT may be approached by improving precursor delivery, photosensitizer generation, and efficient irradiation. Near-infrared PBM and microneedle-assisted drug delivery are separate, mechanistically grounded strategies that could extend PDT efficacy to thicker, more challenging NMSC lesions; whether they also improve patient tolerability remains to be established. Future translational and clinical studies should prioritize optimizing these protocols and validating their impact on long-term PDT response, recurrence rates, and patient-reported outcomes.