biomedicines Review Current Limitations and Recent Progress in Nanomedicine for Clinically Available Photodynamic Therapy Jooho Park 1, Yong-Kyu Lee 2, In-Kyu Park 3 and Seung Rim Hwang 4,* 1 Department of Biomedical Chemistry, College of Biomedical and Health Science, Konkuk University, Chungju 27478, Korea; [email protected] 2 Department of Chemical and Biological Engineering, Korea National University of Transportation, Chungju 27469, Korea; [email protected] 3 Department of Biomedical Sciences, Chonnam National University Medical School, Hwasun 58128, Korea; [email protected] 4 College of Pharmacy, Chosun University, Gwangju 61452, Korea * Correspondence: [email protected]; Tel.: +82-62-230-6365 Abstract: Photodynamic therapy (PDT) using oxygen, light, and photosensitizers has been receiving great attention, because it has potential for making up for the weakness of the existing therapies such as surgery, radiation therapy, and chemotherapy. It has been mainly used to treat cancer, and clinical tests for second-generation photosensitizers with improved physicochemical properties, pharmacokinetic profiles, or singlet oxygen quantum yield have been conducted. Progress is also being made in cancer theranostics by using fluorescent signals generated by photosensitizers. In order to obtain the effective cytotoxic effects on the target cells and prevent off-target side effects, photosensitizers need to be localized to the target tissue. The use of nanocarriers combined with photosensitizers can enhance accumulation of photosensitizers in the tumor site, owing to preferential extravasation of nanoparticles into the tumor vasculature by the enhanced permeability and retention effect. Self-assembly of amphiphilic polymers provide good loading efficiency and sustained release of hydrophobic photosensitizers. In addition, prodrug nanomedicines for PDT can be activated by Citation: Park, J.; Lee, Y.-K.; Park, I.-K.; Hwang, S.R. Current stimuli in the tumor site. In this review, we introduce current limitations and recent progress in Limitations and Recent Progress in nanomedicine for PDT and discuss the expected future direction of research. Nanomedicine for Clinically Available Photodynamic Therapy. Keywords: nanomedicine; photodynamic therapy; photosensitizer; nanocarrier; self-assembly; prodrug Biomedicines 2021, 9, 85. https:// doi.org/10.3390/biomedicines9010085 Received: 20 December 2020 1. Introduction Accepted: 13 January 2021 The discovery of dyes and their use in combination with light since the end of the Published: 16 January 2021 nineteenth century led to the idea of modern photodynamic therapy (PDT) for the treatment of cancers, infections, and other diseases [1,2]. PDT has been receiving great attention, Publisher’s Note: MDPI stays neutral because it has potential for making up for the weakness of the existing therapies such as with regard to jurisdictional claims in surgery, radiation therapy, and chemotherapy. In order to destroy malignant cells, PDT published maps and institutional affil- is done in two stages involving administration of a light-responsive agent known as a iations. photosensitizer and activation of the photosensitizer by light irradiation, usually using a laser [3]. The wavelength of near-infrared (NIR) light is useful for tissue penetration without interference by endogenous chromophores [4]. When photosensitizer molecules are excited by absorption of light, they can react with substrates to form free radicals, or Copyright: © 2021 by the authors. the absorbed photon energy in the photosensitizer molecules is transferred to molecular Licensee MDPI, Basel, Switzerland. triplet oxygen after intersystem crossing between the two different electronic states to This article is an open access article generate reactive oxygen species (ROS) [5]. The distance diffused by ROS is estimated to distributed under the terms and 1 be 0.01–0.02 µm. Namely, the lifetime of ROS, mainly of O2, in the cells is estimated to be conditions of the Creative Commons 0.01–0.04 µs [6]. Attribution (CC BY) license (https:// The singlet oxygen with high reactivity can diffuse across the cell membrane and creativecommons.org/licenses/by/ 4.0/). induce intracellular signaling, resulting in organelle damage and cell death [7]. Biomedicines 2021, 9, 85. https://doi.org/10.3390/biomedicines9010085 https://www.mdpi.com/journal/biomedicines Biomedicines 2021, 9, x FOR PEER REVIEW 2 of 17 Biomedicines 2021, 9, 85 2 of 17 The singlet oxygen with high reactivity can diffuse across the cell membrane and induce intracellular signaling, resulting in organelle damage and cell death [7]. In order to obtain the effective cytotoxic effects on tumor cells (larger than 10 μm), photosensitizersIn order to obtainadministered the effective in vivo cytotoxic need to effectsbe localized on tumor to the cells target (larger tissue. than In 10theµ cur-m), photosensitizersrent era of nanotechnology, administered scientificin vivo studiesneedto on be the localized use of nanomaterials to the target tissue.in PDT Inarethe in- currentcreasing era forof the nanotechnology, treatment of a wide scientific range studies of diseases, on the especially use of nanomaterials cancer. Encapsulation in PDT are of increasingphotosensitizer for the molecules treatment into of athe wide nanocarrie range ofr diseases, targeting especially neoplastic cancer. endothelial Encapsulation cells can ofbe photosensitizerselectively delivered molecules to the intotumor the endothel nanocarrierium targetingby the enhanced neoplastic permeability endothelial and cells re- cantention be selectively (EPR) effect delivered [8]. Generation to the tumor of ROS endothelium by irradiation by of the ultrashort enhanced pulse permeability lasers induces and retentioncytotoxic effects (EPR) effecton vascular [8]. Generation endothelial of cells ROS and by widens irradiation intercellular of ultrashort spaces pulsein the lasersblood inducesvessels, improving cytotoxic effects the efficient on vascular accumulation endothelial and cells therapeutic and widens efficacy intercellular of anti-angiogenic spaces in thetherapies blood [9,10]. vessels, Since improving engineered the efficient nanomaterial accumulations with at and least therapeutic one dimension efficacy of around of anti- angiogenic100 nm have therapies shown great [9,10 ].pote Sincential engineered as nanocarriers nanomaterials with complementary with at least and one supplemen- dimension oftary around roles in 100 PDT, nm havetheir shownuse in combination great potential with as photosensitizers nanocarriers with has complementary increased over and the supplementarylast decade [11]. roles in PDT, their use in combination with photosensitizers has increased overPharmacokinetic the last decade [11 profiles]. of NIR photosensitizers together with the prognosis of dis- easesPharmacokinetic have also been studied profiles by of using NIR photosensitizers NIR imaging [12]. together As gold with nanorods the prognosis show the of sur- dis- easesface plasmon have also absorption been studied in the by NIR using region NIR imaging, gold nanorod–photosensitizer [12]. As gold nanorods complexes show the surface plasmon absorption in the NIR region, gold nanorod–photosensitizer complexes were invented as the multifunctional nanoplatform for photodynamic/photothermal ther- were invented as the multifunctional nanoplatform for photodynamic/photothermal ther- apy as well as fluorescence imaging [13]. Coating multifunctional gold nanorods with the apy as well as fluorescence imaging [13]. Coating multifunctional gold nanorods with the mesoporous silica shell enhances stability of the photosensitizer and the surface plasmon mesoporous silica shell enhances stability of the photosensitizer and the surface plasmon absorption band of gold nanorods [14]. The micellar nanoscale delivery system for a pho- absorption band of gold nanorods [14]. The micellar nanoscale delivery system for a tosensitizer was reported to show theranostic potential for brain tumors [15]. photosensitizer was reported to show theranostic potential for brain tumors [15]. Human clinical studies on PDT for the treatment of early-stage bladder, skin, lung, Human clinical studies on PDT for the treatment of early-stage bladder, skin, lung, esophagus, and stomach cancers showed promising therapeutic responses [16–19]. How- esophagus, and stomach cancers showed promising therapeutic responses [16–19]. How- ever, clinical improvements in cancer patient survival have been observed only at high ever, clinical improvements in cancer patient survival have been observed only at high doses. There are several challenges that must be overcome for the clinical use of PDT doses. There are several challenges that must be overcome for the clinical use of PDT agents. Appropriate dosage forms that improve solubility and stability of hydrophobic agents. Appropriate dosage forms that improve solubility and stability of hydrophobic photosensitizersphotosensitizers inin biologicalbiological fluidsfluids areare required.required. NanoparticlesNanoparticles formedformed byby self-assemblyself-assembly of amphiphilic copolymerscopolymers maymay provideprovide goodgood loadingloading efficiencyefficiency andand sustainedsustained releaserelease ofof hydrophobic photosensitizers [[20].20]. In addition,addition, prodrugprodrug nanomedicinesnanomedicines forfor PDTPDT cancan bebe activated byby stimulistimuli suchsuch asas enzymesenzymes inin thethe tumortumor sitesite (Figure(Figure1 1).). Figure 1. Schematic representation