Emerging uses of 5-aminolevulinic-acid-induced protoporphyrin IX in medicine: a review of multifaceted, ubiquitous, molecular diagnostic, therapeutic, and theranostic opportunities

Significance: 5-Aminolevulinic acid (5-ALA) is a medical pro-drug used to induce the intracellular production of protoporphyrin IX (PpIX) via the heme synthesis pathway. Discoveries in mechanisms and developments in novel applications still continue with this uniquely endogenous intracellular optical system. Aim: Understanding and exploiting the growing uses can be advanced through a survey of knowledge on the mechanisms and biokinetics of 5-ALA administration, partitioning, PpIX production, localization changes, clearance mechanisms, biological interactions, and methods for unique activation methods in both diagnostic and therapeutic applications. Approach: The current medical uses of PpIX are reviewed, separating into therapeutic and diagnostic areas, and the expansion and lateral growth areas are outlined. Results: Initially approved for photodynamic therapy of skin lesions, fluorescence diagnostic indications later developed to guide surgical resection in bladder cancer and glioma. Today, the 5-ALA-PpIX system's spatial-temporal complexity in photophysics and pharmacokinetics continues to lead to more uses, such as photodynamic priming to alter tissue, fast intracellular tissue oxygen sensing, infection, and burn imaging and therapy. Conclusions: The 5-ALA-PpIX system has broad potential partly because of the ubiquity of the heme synthesis across many cell/tissue types, combined with natural selectivity, unique pharmacokinetics, bright fluorescence, and sufficiently strong singlet oxygen production.