Natural Product-Based Photosensitizers for Photodynamic Therapy: Translational Potential and Pharmaceutical Strategies for Clinical Translation

Photodynamic therapy (PDT) is a clinically established, minimally invasive modality that relies on the interaction between a photosensitizer (PS), light, and molecular oxygen to generate cytotoxic reactive oxygen species (ROS). Despite decades of development, the clinical performance of many photosensitizers remains limited—not primarily due to insufficient photodynamic activity, but rather to unfavorable physicochemical and biopharmaceutical properties that impair in vivo efficacy. Natural products represent a structurally diverse and biologically relevant source of photosensitizers with intrinsic photochemical potential. However, their translation into clinically viable PDT agents has remained disproportionately limited. This discrepancy highlights a critical and often underappreciated bottleneck: pharmaceutical incompatibility. In this mini-review, we provide a pharmaceutics-centered perspective on natural product-based photosensitizers, shifting the focus from molecule discovery toward translational feasibility. We critically examine the key barriers that restrict clinical progression—including poor aqueous solubility, aggregation-induced quenching, instability, and suboptimal biodistribution—and assess the formulation strategies that enable their resolution. Particular emphasis is placed on nanotechnology-enabled delivery systems, targeted carriers, and hybrid platforms that enhance solubility, stability, and tissue selectivity. Representative compounds are discussed within a translational context, highlighting the contrast between advanced candidates such as hypericin and chlorophyll-derived chlorins and more limited systems such as curcumin. Collectively, this work demonstrates that the success of natural photosensitizers in PDT is determined less by intrinsic photodynamic efficiency and more by their compatibility with pharmaceutical engineering strategies. This perspective provides a concise framework to guide the rational development of clinically relevant natural photosensitizer systems.