A Bioorthogonal Antidote Against the Photosensitivity after Photodynamic Therapy

As an effective and non-invasive treatment modality for cancer, photodynamic therapy (PDT) has attracted considerable interest. With the recent advances in the photosensitizing agents, the fiber-optic systems, and other aspects, its application is extended to a wide range of superficial and localized cancers. However, for the few clinically used photosensitizers, most of them suffer from the drawback of causing prolonged photosensitivity after the treatment. As a result, post-PDT management is also a crucial issue. Herein, a facile bioorthogonal approach is reported that can effectively suppress this common side effect of PDT in nude mice. It involves the use of an antidote that contains a black-hole quencher BHQ-3 conjugated with a bicyclo[6.1.0]non-4-yne (BCN) moiety and a tetrazine-substituted boron dipyrromethene-based photosensitizer. By using tumor-bearing nude mice as an animal model, it is demonstrated that after PDT with this photosensitizer, the administration of the antidote can effectively quench the photodynamic activity of the residual photosensitizer by bringing the BHQ-3 quencher close to the photosensitizing unit through a rapid click reaction. It results in substantial reduction in skin damage upon light irradiation. The overall results demonstrate that this simple and facile strategy can provide an effective means for minimizing the photosensitivity after PDT.

Polymeric Phthalocyanine-Based Nanosensitizers for Enhanced Photodynamic and Sonodynamic Therapies

Photodynamic therapy and sonodynamic therapy are two highly promising modalities for cancer treatment. The latter holds an additional advantage in deep-tumor therapy owing to the deep penetration of the ultrasonic radiation. The therapeutic efficacy depends highly on the photo/ultrasound-responsive properties of the sensitizers as well as their tumor-localization property and pharmacokinetics. A novel nanosensitizer system based on a polymeric phthalocyanine (pPC-TK) is reported herein in which the phthalocyanine units are connected with cleavable thioketal linkers. Such polymer could self-assemble in water forming nanoparticles with a hydrodynamic diameter of 48 nm. The degradable and flexible thioketal linkers could effectively inhibit the π-π stacking of the phthalocyanine units, rendering the resulting nanoparticles an efficient generator of reactive oxygen species upon light or ultrasonic irradiation. The nanosensitizer could be internalized into cancer cells readily, inducing cell death by efficient photodynamic and sonodynamic effects. The potency is significantly higher than that of the monomeric phthalocyanine (PC-4COOH). The nanosensitizer could also effectively inhibit the growth of tumor in liver tumor-bearing mice by these two therapies without causing noticeable side effects. More importantly, it could also retard the growth of a deep-located orthotopic liver tumor in vivo by sonodynamic therapy.

Design and synthesis of a NAD(P)H:quinone oxidoreductase 1-activatable photosensitiser for controlled photodynamic therapy

The utilisation of enzymes as stimuli can activate theranostic agents in a highly specific manner. We report herein a far-red-absorbing boron dipyrromethene-based photosensitiser that is responsive towards the cancer-associated human NAD(P)H:quinone oxidoreductase 1, enabling the controlled restoration of photodynamic activity for selective elimination of cancer cells.

Glycosylated BODIPY- Incorporated Pt(II) Metallacycles for Targeted and Synergistic Chemo-Photodynamic Therapy

Pt(II)-BODIPY complexes combine the chemotherapeutic activity of Pt(II) with the photocytotoxicity of BODIPYs. Additional conjugation with targeting ligands can boost the uptake by cancer cells that overexpress the corresponding receptors. We describe two Pt(II) triangles, 1 and 2, built with pyridyl BODIPYs functionalized with glucose (3) or triethylene glycol methyl ether (4), respectively. Both 1 and 2 showed higher singlet oxygen quantum yields than 3 and 4, due to the enhanced singlet-to-triplet intersystem crossing. To evaluate the targeting effect of the glycosylated derivative, in vitro experiments were performed using glucose transporter 1 (GLUT1)-positive HT29 and A549 cancer cells, and noncancerous HEK293 cells as control. Both 1 and 2 showed higher cellular uptake than 3 and 4. Specifically, 1 was selective and highly cytotoxic toward HT29 and A549 cells. The synergistic chemo- and photodynamic behavior of the metallacycles was also confirmed. Notably, 1 exhibited superior efficacy toward the cisplatin-resistant R-HepG2 cells.

Synthesis, Cellular Uptake, and Photodynamic Activity of Oligogalactosyl Zinc(II) Phthalocyanines

Invited for this month's cover is the group of Prof. Dennis K. P. Ng at The Chinese University of Hong Kong. The cover picture shows the selective internalization of molecules of a di-galactosyl zinc(II) phthalocyanine into a cancer cell. Upon light irradiation, these molecules are excited and interact with the endogenous oxygen to generate highly reactive singlet oxygen, which oxidatively damages the cellular components, leading to cell death eventually. More information can be found in the Research Article by Dennis K. P. Ng, and co-workers.

Synthesis, Cellular Uptake, and Photodynamic Activity of Oligogalactosyl Zinc(II) Phthalocyanines

A series of di-α-substituted zinc(II) phthalocyanines with different number of galactose moieties, ranging from 1 to 8, namely Pc-gal_n (n=1, 2, 4, and 8) were designed and synthesized. The synthesis involved the copper-catalyzed azide-alkyne cycloaddition reaction of a mono- or dialkynyl zinc(II) phthalocyanine with an acetyl-protected galactosyl azide or its dendritic derivative with four acetyl-protected galactosyl groups, followed by removal of the acetyl protecting groups via alkaline hydrolysis. In N,N-dimethylformamide, these oligogalactosyl phthalocyanines were non-aggregated as shown by the strong Q-band absorption and fluorescence emission. Owing to the di-α-substitution, they also behaved as efficient singlet oxygen generators upon light irradiation with a singlet oxygen quantum yield of 0.84. The spectroscopic and photophysical properties were not affected by the number of galactosyl units. In contrast, the compounds became significantly aggregated and quenched in phosphate-buffered saline. Their cellular uptake was then studied using a range of cell lines, which generally followed the order Pc-gal₁ >Pc-gal₂ ≈Pc-gal₄ >Pc-gal₈ . Interestingly, the di-galactosyl analogue exhibited selective uptake against HeLa human cervical carcinoma cells through an energy-dependent pathway instead of the expected asialoglycoprotein receptor. Upon light irradiation, it could effectively kill the cells with a half-maximal inhibitory concentration of 0.58 μM.

A tetrazine-responsive isonitrile-caged photosensitiser for site-specific photodynamic therapy

We report herein a versatile and efficient bioorthogonal strategy to actualise targeted delivery and site-specific activation of photosensitisers for precise antitumoural photodynamic therapy. The strategy involved the use of an isonitrile-caged distyryl boron dipyrromethene-based photosensitiser, labelled as NC-DSBDP, of which the photoactivities could be specifically activated upon conversion of the meso ester substituent to carboxylate initiated by the [4 + 1] cycloaddition with a tetrazine derivative. By using two tetrazines conjugated with a galactose moiety or the GE11 peptide, labelled as gal-Tz and GE11-Tz, we could selectively label the cancer cells overexpressed with the asialoglycoprotein receptor and the epidermal growth factor receptor respectively. Upon encountering the internalised NC-DSBDP, these tetrazines triggered the "ester-to-carboxylate" transformation of this compound, activating its fluorescence and reactive oxygen species generation inside the target cells. The bioorthogonal activation was also demonstrated in vivo, leading to effective photo-eradication of the tumour in nude mice.

Site-Specific Displacement-Driven Activation of Supramolecular Photosensitizing Nanoassemblies for Antitumoral Photodynamic Therapy

The delivery and activation of photosensitizers in a specific manner is crucial in photodynamic therapy. For an antitumoral application, it can confine the photodynamic action on the cancer cells, thereby enhancing the treatment efficacy and reducing the side effects. We report herein a novel supramolecular photosensitizing nanosystem that can be specifically activated in cancer cells and tumors that overexpress epidermal growth factor receptor (EGFR). It involves the self-assembly of the amphiphilic host-guest complex of a β-cyclodextrin-conjugated phthalocyanine-based photosensitizer (Pc-CD) and a ferrocene-substituted poly(ethylene glycol) (Mn = 2000) (Fc-PEG) in aqueous media. The resulting nanosystem Pc-CD@Fc-PEG with a hydrodynamic diameter of 124-147 nm could not emit fluorescence and generate reactive oxygen species due to the self-quenching effect and the ferrocene-based quencher. Upon interactions with molecules of adamantane substituted with an EGFR-targeting peptide (Ad-QRH*) in water and in EGFR-positive HT29 and A431 cells, the ferrocene guest species were displaced, resulting in disassembly of the nanoparticles and restoration of these photoactivities. The half-maximal inhibitory concentration values were down to 1.24 μM (for HT29 cells). The nanosystem Pc-CD@Fc-PEG could also be activated in an Ad-QRH*-treated HT29 tumor in nude mice, leading to increased intratumoral fluorescence intensity and effective eradication of the tumor upon laser irradiation. The results showed that this two-step supramolecular approach can actualize site-specific photosensitization and minimize nonspecific phototoxicity in a general photodynamic treatment.

A self-assembled subphthalocyanine-based nanophotosensitiser for photodynamic therapy

A subphthalocyanine substituted with nine tetra(ethylene glycol) chains on the periphery has been synthesised. This novel amphiphilic and cone-shaped compound can self-assemble in water into spherical nanoparticles with a hydrodynamic diameter of 154 nm. These nanoparticles can be taken up readily by cancer cells and localised predominately in lysosomes where they disassemble gradually, leading to activation in fluorescence emission and, photocytotoxicity, showing IC₅₀ values of as low as 1.2 μM.

Targeted Delivery and Site-Specific Activation of β-Cyclodextrin-Conjugated Photosensitizers for Photodynamic Therapy through a Supramolecular Bio-orthogonal Approach

Targeted delivery of photosensitizers using hydrophilic and tumor-directing carriers and site-specific activation of their photocytotoxicity are two common strategies to enhance the specificity of anticancer photodynamic therapy. We report herein a novel supramolecular bio-orthogonal approach to integrate these two functions. A β-cyclodextrin-substituted aza-boron-dipyrromethene-based photosensitizer was first complexed with a ferrocene-substituted black-hole quencher to inhibit its photosensitizing ability. Upon encountering the adamantane moieties that had been delivered to target cancer cells through specific binding of the conjugated peptide to the overexpressed epidermal growth factor receptor, the ferrocene-based guest species were displaced due to the stronger binding interactions between β-cyclodextrin and adamantane, thereby restoring the photodynamic activity of the photosensitizer. Hence, this two-step process enabled targeted delivery and site-specific activation of the photosensitizer, as demonstrated through a series of experiments in aqueous media, in a range of cancer cell lines and in tumor-bearing nude mice.

Nanoparticles for Triple Drug Release for Combined Chemo- and Photodynamic Therapy

A pH-responsive drug delivery system (DDS) based on mesoporous silica nanoparticles (MSNs) has been prepared for the delivery of three anticancer drugs with different modes of action. The novelty of this system is its ability to combine synergistic chemotherapy and photodynamic therapy. A photoactive conjugate of a phthalocyanine (Pc) and a topoisomerase I inhibitor (topo-I), namely camptothecin (CPT), linked by a poly(ethylene glycol) (PEG) chain has been synthesized and then loaded into the mesopores of MSNs. Doxorubicin (DOX), which is a topoisomerase II inhibitor (topo-II), has also been covalently anchored to the outer surface of the MSNs through a dihydrazide PEG linker. In the acidic environment of tumor cells, selective release of the three drugs takes place. In vitro studies have demonstrated the endocytosis of the system into HeLa and HepG2 cells, and the subsequent release of the three drugs into the cytoplasm and nucleus. Furthermore, the cytotoxic effect of DOX, CPT and Pc has been assessed in vitro before and upon light irradiation.

Synthesis and biological evaluation of an epidermal growth factor receptor-targeted peptide-conjugated phthalocyanine-based photosensitiser

A peptide-conjugated zinc(ii) phthalocyanine containing the epidermal growth factor receptor-targeted heptapeptide QRHKPRE has been prepared. The conjugate labelled as ZnPc-QRH* can selectively bind to the cell membrane of HT29 human colorectal adenocarcinoma cells in 10 min followed by internalisation upon prolonged incubation via receptor-mediated endocytosis, leading to localisation in lysosomes eventually. By manipulating the incubation time, the subcellular localisation of the conjugate can be varied and the cell-death pathways induced upon irradiation can also be altered. It has been found that photosensitisation initiated at the cell membrane and in the lysosomes would trigger cell death mainly through necrosis and apoptosis respectively. Intravenous administration of the conjugate into HT29 tumour-bearing nude mice resulted in higher accumulation in the tumour than in most major organs. The selective binding of this conjugate to tumour has also been demonstrated by comparing the results with those of the analogue with a scrambled peptide sequence (EPRQRHK). The overall results indicate that ZnPc-QRH* is a promising EGFR-targeted photosensitiser for photodynamic therapy.