Ruthenium and platinum-based anticancer metallotherapeutics from the perspectives of photodynamic therapy and bioimaging applications

Bioimaging is a well-known process for visualizing biological substances or biological pathways in the human body at a molecular level without any intrusion. It has been used in various domains biological of chemistry, drug discovery, and disease detection, due to its potential application attributes of high resolution, high sensitivity, high selectivity, and low cost. In recent years, the transition metal-based luminescent probes or photosensitizers have a distinctive feature in optical bioimaging and photodynamic therapy, in in vivo and in vitro, as compared to other types of probes and have a strong influence on discovering biological analytes. Photodynamic therapy, a non-systemic treatment, has attracted significant attention for its ability to localize the disease. This depends on the nature of the photosensitizers, light, and reactive oxygen species (ROS), produced through electron transport or energy transfer pathways. In PDT, a photosensitizer (PS) gets activated on exposure to light of a specific wavelength and provides energy to molecular oxygen on the tissue surfaces. This generates reactive oxygen species such as superoxide, singlet oxygen, and hydroxyl radicals causing damage to cellular membranes, lipids, proteins, and DNA. Further, it also results in cell death via apoptosis and necrosis on targeted cancer cells. The current review provides a literature overview on bioimaging and PDT applications of reported ruthenium and platinum complexes acting as luminescent probes. The extensively studied ligands, in this context, are bipyridine, phenanthroline, porphyrin, BODIPY, and ferrocene. These complexes have been studied for their potential anticancer activity against different cancer cells in both light and dark conditions. In addition, these probes also possess excellent photophysical characteristics of intense emission, with significant Stokes shifts, extended lifetimes, controllable toxicity, and excellent photostability.

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