Porphyrins and Phthalocyanines for Theranostics

The field of theranostics is growing rapidly as therapeutic regimes are increasingly being integrated and guided by diagnostic practices such as medical imaging. There is some room for discussion about the general definition of a theranostic agent, if there exists such a definition. Is it a diagnostic imaging agent that is used in the context to guide surgical procedures? Or rather, is it a therapeutic agent whose administration and dosing is guided by diagnostic metrics? Probably, theranostics encompasses both of these concepts and many additional ones, which are regularly discussed within this journal. However, it may be presumed that single agents which have the capacity for both imaging and therapy are particularly suited for theranostic applications. Porphyrins and phthalocyanines, which share an aromatic and planar tetrapyrrole backbone, are one class of such molecules which are ideally suited for intrinsic multifunctional imaging and therapeutic applications. In this special Theranostics issue, which is dedicated to porphyrins and phthalocyanines, a partial representation of the multifunctional theranostic capabilities of porphyrins and phthalocyanines is presented. The incredibly rich history of porphyrins and phthalocyanines as theranostics agents is presented in a review paper within this issue [1]. The endogenous bright red porphyrins in blood could be considered as the original old theranostic agent. Heme has maintained its use in new imaging techniques as the central molecule that enables functional magnetic resonance imaging. Porphyrins have been pioneering agents for modern theranostics. Heme derivatives have been used for cancer imaging as early as the 1920s, phthalocyanines were used for positron emission studies since the 1950s and porphyrins have been used as MR contrast agents since the 1980s. Of course, porphyrins have made a clinical mark on modern theranostics in the form of photodynamic therapy (PDT). Most porphyrins generate singlet oxygen upon irradiation, so they can be used for therapeutic applications involving a photosensitizer, light and oxygen. A definitive, comprehensive summary of the theranostic roles of porphyrins with respect to photodynamic therapy is presented as a review in this issue [2]. The intertwined fluorescent and photodynamic properties of porphyrins permit novel optical theranostic approaches. By using two-photon fluorescence lifetime imaging, Yeh et al. demonstrate that subcellular localization can be determined based on differences in porphyrin fluorescence lifetime properties in the cell membrane and the cytosol [3]. By developing such novel cutting-edge optical techniques, it is hoped that such methodologies and instrumentation may eventually enter clinical practice for improving routine PDT. PDT is a multi-factor treatment, with clinicians being able to control laser power, laser treatment time, drug dose, and number of laser treatments. Thus, developing feedback systems to guide therapy is critical. Glidden et al., use image-based quantification in a three dimensional tumor model to optimize PDT parameters [4]. By understanding the interplay between drug localization, Ivyspring