Editorial [ Recent Advances in Cancer Chemotherapy- Part II Guest Editor: Diwan S. Rawat ]

Cancer has remained one of the biggest health threats to the human life and many approaches are being explored for its treatment. Photodynamic therapy (PDT) is one of such approach and it has cured thousands of patients so far. The photodynamic agents selectively enter to the cancerous cells, and exhibit biological response only after exposure to the light. The exact reasons by which photosensitizers are selectively retained in the tumor cells and the mechanisms by which they show tumoricidal effect are not very clear, but for its biological response, PDT requires three components: oxygen, photosensitizer and light. Photodynamic therapy differs from radiation therapy as they interact differently with molecular oxygen. Ionizing radiation produces highly reactive oxygen-derived radical, whereas photodynamic agents are first excited by light and excited sensitizer transfers energy to molecular oxygen which produces singlet oxygen and that reacts with the cellular targets. The ionizing radiations are considered to be very toxic as they target DNA, while photodynamic agents are considered to be relatively safer, as they target cytoplasm, nuclear, mitochondrial and organelle membranes. Efforts are also underway to increase the tumor-specificity by developing target-specific photosensitizers. The present issue deals with the recent advancement in the area of photodynamic therapy (PDT). First three articles deal with the recent progress on the photodynamic therapy. Prof. Pandey et al. summarizes the concept of PDT, the selection criteria for designing the effective photosensitizers and the utility of porphyrin-carbohydrate conjugates in PDT. Kokube et al. describes the recent advancement in the two-photon absorption (2PA) PDT, which allows a more selective treatment of deeper-locating cancers. Mark Wainbringt has made an elegant comparison in the first and second-generation PDT agents. Prof. Zaleski describes the role of diazo compounds in the development of phototherapeutic agents for biomedical applications. Authors emphasized that natural or synthetic diazo compounds are susceptible to redox activated N2 loss, so transition metal complexes containing redox-active excited states that absorb in the tissue transparent therapeutic window have potential to be used as new therapeutic agents. Interestingly, highly π- conjugated molecules such as porphyrins and chlorins, which serve as the primary pigment for current PDT, have been able to support conjugated diazoketone functionality at the macrocycle periphery. These synthetic advances have made diazo constructs to serve as in situ biological labels or as recognition elements to probe biochemical mechanisms. I sincerely thank to all authors and reviewers for their contribution in the preparation of this especial issue. I sincerely thank to Prof. Michelle Prudhomme, Editor-in-Chief, Anti-Cancer Agents in Medicinal Chemistry, for giving me an opportunity to act as a guest editor of this prestigious international journal. It is my believe that readers will enjoy this special issue and will stimulate interactive research with other specialist areas.