Photosensitizing Herbs as Potential Therapeutics: A Prospective Insights into their Mechanisms for the Development of Novel Drug Leads in War with Cancer and Other Human Diseases

In recent years, photodynamic therapy (PDT) has been accepted as an alternative option for the treatment of a wide spectrum of human ailments. It is a minimally invasive treatment that involves the interaction of a non-toxic photosensitizer. In PDT, combining photosensitizing (PS) agent that absorbs specified wavelength of light, which in turn produces free radical molecules to eliminate unwanted cells and tissues. The photosensitization process is activated by the light-induced excitation of molecules within the tissue. Bioactive principles acquired from plants documented as nature-inspired potential photosensitizers with varied properties against microbes, insects, or tumor cells. PDT is a promising method for removing diverse types of cancers but needs to be recognized in therapy as conventional chemotherapy. At present, natural compounds with PS properties are being continuously unearthed and identified. As of now, hundreds of photosensitive drugs or drug leads identified from natural sources with reduced or no toxicity to healthy tissues and no side effects encourage investigators to pursue natural PS for PDT. Although existing PS was developed years back, only a handful of them are engaged in human clinical applications. The main classes of natural photosensitizers discussed in this review are chlorophylls (hypocrellin A and B), hypericin, chlorins (Chlorin e6), and other emerging ones such as curcumin. Hence, the present review aimed to explore the efficacious PS properties of a few herbal-derived PS, preferably the potential ones in terms of specificity, and mechanism of action, inducing less or no toxicity to normal cells but their other medicinal applications.