Modulation of Reactive Oxygen Species Photogeneration of Bacteriopheophorbide a Derivatives by Exocyclic E-Ring Opening and Charge Modifications

Pyrobacteriopheophorbide-a derivatives possessing various hydrophilic esterifying groups at the C17-propionate residues for photodynamic therapy

Aiming at the application to photodynamic therapy, natural bacteriochlorophyll-a was converted to chemically stable free-base derivatives possessing different kinds of hydrophilic C17-propionate residues. These semi-synthetic bacteriochlorins were found to have self-assembling ability in an aqueous environment and formed stable J-type aggregates in a cell culture medium containing 0.2% DMSO. The electronic absorption spectra of all the sensitizers showed Qy absorption maxima at 754 nm in DMSO as their monomeric states, while a drastic shift of the red-most bands to ca. 880 nm was observed in the aqueous medium. The circular dichroism spectra in the medium showed much intense signals compared to those measured in DMSO, supporting the formation of well-ordered supramolecular structures. By introducing hydrophilic side chains, the bacteriochlorin sensitizers could be dispersed in the aqueous medium as their J-aggregates without the use of any surfactants. Cellular uptake efficiencies as well as photodynamic activities were evaluated using human cervical adenocarcinoma HeLa cells. Among the 11 photosensitizers investigated, the best result was obtained for a charged derivative possessing trimethylammonium terminal (17-CH2CH2COOCH2CH2N+(CH3)3I-) and photocytotoxicity of EC50 = 0.09 μM was achieved by far-red light illumination of 35 J/cm2 from an LED panel (730 nm).

Photochemical synthesis of fluorescent Au16(RGDC)14 and excited state reactivity with molecular oxygen

Aqueous metal nanoclusters have emerged as effective materials for biomedical imaging and therapy. Among them, gold nanoclusters (AuNCs) have been widely studied due to their unique electronic structures. These nanoclusters are often optically impure, comprising a mixture of fluorescent clusters with different metal/ligand compositions. The polydispersity of nanoclusters makes it challenging to isolate the most stable structure, and poses further risks for eventual clinical applications. Herein, Au16L14 clusters are reported which are optically pure as assessed by fluorescence excitation-emission matrix (EEM) spectroscopy and parallel factor (PARAFAC) analysis. The reactivity of their excited state with molecular oxygen was also probed, demonstrating that the Au16L14 clusters generate type I reactive oxygen species (ROS), which can make them effective sensitizers for photodynamic therapy.

BODIPY-Based Photodynamic Agents for Exclusively Generating Superoxide Radical over Singlet Oxygen

Developing Type-I photosensitizers is considered as an efficient approach to overcome the deficiency of traditional photodynamic therapy (PDT) for hypoxic tumors. However, it remains a challenge to design photosensitizers for generating reactive oxygen species by the Type-I process. Herein, we report a series of α,β-linked BODIPY dimers and a trimer that exclusively generate superoxide radical (O₂ ^-. ) by the Type-I process upon light irradiation. The triplet formation originates from an effective excited-state relaxation from the initially populated singlet (S₁ ) to triplet (T₁ ) states via an intermediate triplet (T₂ ) state. The low reduction potential and ultralong lifetime of the T₁ state facilitate the efficient generation of O₂ ^-. by inter-molecular charge transfer to molecular oxygen. The energy gap of T₁ -S₀ is smaller than that between ³ O₂ and ¹ O₂ thereby precluding the generation of singlet oxygen by the Type-II process. The trimer exhibits superior PDT performance under the hypoxic environment.

Fast, facile, base-free microwave-assisted metallation of bacteriochlorophylls and corresponding high yield synthesis of TOOKAD

Naturally-derived metallo-bacteriochlorophylls have attracted much attention since their clinical approval for cancer photodynamic therapy. Their therapeutic properties are rooted in the metal complexation of bacteriochlorophylls, which endows them with optical properties favourable for biophotonic and biomedical applications, including near-infrared light-activated reactive oxygen species generation at therapeutic levels. Despite these advantages, the utility of these chromophores has been limited by synthetic challenges associated with bacteriochlorophyll metallation; specifically, a slow reaction rate and necessity of complex purification procedures remain barriers towards metalated bacteriochlorophyll synthesis. Here, these limitations are overcome through the development of a new fast, facile, efficient, base-free microwave heating metallation method for the synthesis of a series of metallo (Pd, Cu, Zn, Cd, Sn, In, Mn, Co) bacteriopyropheophorbides. The preparation and structural and optical spectral characterization of these complexes are presented. This microwave-enabled synthetic method is then applied to generate the clinical photosensitizer agent Pd-bacteriopheophorbide (TOOKAD) effectively and efficiently, followed by validation of its metallation-enhanced ROS generation.

Rational design of an "all-in-one" phototheranostic

We report here porphodilactol derivatives and their corresponding metal complexes. These systems show promise as "all-in-one" phototheranostics and are predicated on a design strategy that involves controlling the relationship between intersystem crossing (ISC) and photothermal conversion efficiency following photoexcitation. The requisite balance was achieved by tuning the aromaticity of these porphyrinoid derivatives and forming complexes with one of two lanthanide cations, namely Gd3+ and Lu3+. The net result led to a metalloporphodilactol system, Gd-trans-2, with seemingly optimal ISC efficiency, photothermal conversion efficiency and fluorescence properties, as well as good chemical stability. Encapsulation of Gd-trans-2 within mesoporous silica nanoparticles (MSN) allowed its evaluation for tumour diagnosis and therapy. It was found to be effective as an "all-in-one" phototheranostic that allowed for NIR fluorescence/photoacoustic dual-modal imaging while providing an excellent combined PTT/PDT therapeutic efficacy in vitro and in vivo in 4T1-tumour-bearing mice.

Trafficking of a Single Photosensitizing Molecule to Different Intracellular Organelles Demonstrates Effective Hydroxyl Radical-Mediated Photodynamic Therapy in the Endoplasmic Reticulum

Photodynamic therapy (PDT) is often used in preclinical and clinical treatment regimens. Reactive oxygen species (ROS) generated by photosensitizers (PSs) upon exposure to light induce cell death via diverse mechanisms. PSs can exert therapeutic effects in different cellular organelles, although the efficacy of organelle-specific PDT has yet to be determined as most previous studies use different PSs in different organelles. Here, we explored how a single PS, chlorin e6 (Ce6), targeted to different organelles altered the effectiveness of PDT. Ce6 intrinsically localizes to the ER after 4 h of incubation. Modification of Ce6 via conjugation with an octapeptide (LS765), a monosubstituted triphenylphosphonium (TPP) derivative (LS897), or a disubstituted TPP derivative (LS909) altered the intrinsic localization. We determined that LS765 and LS9897 predominantly accumulated in the lysosomes, but LS909 trafficked equally to both the mitochondria and the lysosomes. Moreover, the conjugation altered the type of ROS produced by Ce6, increasing the ratio of hydrogen peroxide to hydroxyl radicals. Irradiation of identical concentrations of the PSs in solution with 650 nm, 0.84 mW/cm² light for 10 min showed that the TPP conjugates nearly doubled the hydrogen peroxide production from ∼0.2 μM for Ce6 and LS765 to ∼0.37 μM for LS897 and LS909. In contrast, Ce6 produced ∼1.5-fold higher hydroxyl radicals than its conjugates. To compare the effect of each PS on cell death, we normalized the intracellular concentration of each PS. Hydrogen peroxide-producing PSs are effective PDT agents in the lysosomes while the hydroxyl-generating PSs are very effective in the ER. Compared to the PSs that accumulated in the lysosomes, only the ER-targeted Ce6 exerted >50% cell death at either low light power or low intracellular concentration. By delineating the contributions of cellular organelles and types of ROS produced, our work suggests that targeting hydroxyl radical-producing PSs to the ER is an exciting strategy to improve the therapeutic outcome of PDT.