Photosensitizer (PS)-cyanine dye (CD) conjugates: Impact of the linkers joining the PS and CD moieties and their orientation in tumor-uptake and photodynamic therapy (PDT)

Recent Advances in the HPPH-Based Third-Generation Photodynamic Agents in Biomedical Applications

Photodynamic therapy has emerged as a recognized anti-tumor treatment involving three fundamental elements: photosensitizers, light, and reactive oxygen species. Enhancing the effectiveness of photosensitizers remains the primary avenue for improving the biological therapeutic outcomes of PDT. Through three generations of development, HPPH is a 2-(1-hexyloxyethyl)-2-devinyl derivative of pyropheophorbide-α, representing a second-generation photosensitizer already undergoing clinical trials for various tumors. The evolution toward third-generation photosensitizers based on HPPH involves structural modifications for multimodal applications and the combination of multifunctional compounds, leading to improved imaging localization and superior anti-tumor effects. While research into third-generation HPPH is beneficial for advancing PDT treatment, equal attention should also be directed toward the other two essential elements and personalized diagnosis and treatment methodologies.

Dual-Activated Nano-Prodrug for Chemo-Photodynamic Combination Therapy of Breast Cancer

Herein, we developed a dual-activated prodrug, BTC, that contains three functional components: a glutathione (GSH)-responsive BODIPY-based photosensitizer with a photoinduced electron transfer (PET) effect between BODIPY and the 2,4-dinitrobenzenesulfonate (DNBS) group, and an ROS-responsive thioketal linker connecting BODIPY and the chemotherapeutic agent camptothecin (CPT). Interestingly, CPT displayed low toxicity because the active site of CPT was modified by the BODIPY-based macrocycle. Additionally, BTC was encapsulated with the amphiphilic polymer DSPE-mPEG₂₀₀₀ to improve drug solubility and tumor selectivity. The resulting nano-prodrug passively targeted tumor cells through enhanced permeability and retention (EPR) effects, and then the photosensitizing ability of the BODIPY dye was restored by removing the DNBS group with the high concentration of GSH in tumor cells. Light-triggered ROS from activated BODIPY can not only induce apoptosis or necrosis of tumor cells but also sever the thioketal linker to release CPT, achieving the combination treatment of selective photodynamic therapy and chemotherapy. The antitumor activity of the prodrug has been demonstrated in mouse mammary carcinoma 4T1 and human breast cancer MCF-7 cell lines and 4T1 tumor-bearing mice.

Photodiagnosis and photodynamic effects of bacteriochlorin-naphthalimide conjugates on tumor cells and mouse model

Photo-induced cytotoxicity and antitumor activity of a series of dual function agents for photodynamic therapy (PDT) and fluorescent imaging based on bacteriochlorin photosensitizer conjugated with various naphthalimide fluorophores was studied in vitro using murine tumor cells of S37 sarcoma and in vivo on mice bearing murine S37 sarcoma. Upon irradiation at the absorption maximum of the photosensitizer, the activity of conjugates was as high as in the case of individual bacteriochlorin, while an additional excitation of the naphthalimide fragment led to an increase in the PDT efficacy due to resonance energy transfer from the fluorophore to photosensitizer. The fluorescence contrast and specific cytotoxic activity measurements indicate that the conjugate of bacteriochlorin with 3,4-dimethoxestyrene-substituted naphthalimide is the most promising agent for the application as theranostic in PDT.

Cyanine conjugates in cancer theranostics

Cyanine is a meritorious fluorogenic core for the construction of fluorescent probes and its phototherapeutic potential has been enthusiastically explored as well. Alternatively, the covalent conjugation of cyanine with other potent therapeutic agents not only boosts its therapeutic efficacy but also broadens its therapeutic modality. Herein, we summarize miscellaneous cyanine-therapeutic agent conjugates in cancer theranostics from literature published between 2014 and 2020. The application scenarios of such theranostic cyanine conjugates covered common cancer therapeutic modalities, including chemotherapy, phototherapy and targeted therapy. Besides, cyanine conjugates that serve as nanocarriers for drug delivery are introduced as well. In an additional section, we analyze the potential of these conjugates for clinical translation. Overall, this review is aimed to stimulate research interest in exploring unattempted therapeutic agents and novel conjugation strategies and hopefully, accelerate clinical translation in this field.

Mapping fluorescence resonance energy transfer parameters of a bifunctional agent using time-domain fluorescence diffuse optical tomography

We present a method to map fluorescence resonance energy transfer (FRET) parameters of a bifunctional photodynamic therapy agent, (2-[1-hexyloxyethyl]-2-devinyl pyropheophorbide-a)-cyanine dye (HPPH-CD) conjugate, which consists of a photosensitizer (HPPH) and a fluorescent agent CD. We utilized time-domain fluorescence diffuse optical tomography, the normalized Born ratio model in the Fourier-domain, and an iterative algorithm to map depth-resolved spatial heterogeneities of FRET parameters. Our results exhibited depth-resolved changes of fluorophore's lifetime and the distance maps due to FRET between HPPH and CD. Our model suggests a potential approach of using FRET parameters to monitor efficacies of multifunctional photodynamic therapy agents in deep tissue.

Core-shell polymeric nanoparticles co-loaded with photosensitizer and organic dye for photodynamic therapy guided by fluorescence imaging in near and short-wave infrared spectral regions

BACKGROUND

Biodistribution of photosensitizer (PS) in photodynamic therapy (PDT) can be assessed by fluorescence imaging that visualizes the accumulation of PS in malignant tissue prior to PDT. At the same time, excitation of the PS during an assessment of its biodistribution results in premature photobleaching and can cause toxicity to healthy tissues. Combination of PS with a separate fluorescent moiety, which can be excited apart from PS activation, provides a possibility for fluorescence imaging (FI) guided delivery of PS to cancer site, followed by PDT.

RESULTS

In this work, we report nanoformulations (NFs) of core-shell polymeric nanoparticles (NPs) co-loaded with PS [2-(1-hexyloxyethyl)-2-devinyl pyropheophorbide-a, HPPH] and near infrared fluorescent organic dyes (NIRFDs) that can be excited in the first or second near-infrared windows of tissue optical transparency (NIR-I, ~ 700-950 nm and NIR-II, ~ 1000-1350 nm), where HPPH does not absorb and emit. After addition to nanoparticle suspensions, PS and NIRFDs are entrapped by the nanoparticle shell of co-polymer of N-isopropylacrylamide and acrylamide [poly(NIPAM-co-AA)], while do not bind with the polystyrene (polySt) core alone. Loading of the NIRFD and PS to the NPs shell precludes aggregation of these hydrophobic molecules in water, preventing fluorescence quenching and reduction of singlet oxygen generation. Moreover, shift of the absorption of NIRFD to longer wavelengths was found to strongly reduce an efficiency of the electronic excitation energy transfer between PS and NIRFD, increasing the efficacy of PDT with PS-NIRFD combination. As a result, use of the NFs of PS and NIR-II NIRFD enables fluorescence imaging guided PDT, as it was shown by confocal microscopy and PDT of the cancer cells in vitro. In vivo studies with subcutaneously tumored mice demonstrated a possibility to image biodistribution of tumor targeted NFs both using HPPH fluorescence with conventional imaging camera sensitive in visible and NIR-I ranges (~ 400-750 nm) and imaging camera for short-wave infrared (SWIR) region (~ 1000-1700 nm), which was recently shown to be beneficial for in vivo optical imaging.

CONCLUSIONS

A combination of PS with fluorescence in visible and NIR-I spectral ranges and, NIR-II fluorescent dye allowed us to obtain PS nanoformulation promising for see-and-treat PDT guided with visible-NIR-SWIR fluorescence imaging.

Photoactivated biscarbocyanine dye with two conjugated chromophores: complexes with albumin, photochemical and phototoxic properties

Complexes of photosensitizers with blood proteins play an essential role in their delivery to the cell, as well as in the efficacy of photodynamic therapy. Biscarbocyanine dye non-covalently binds human serum albumin (HSA), the dissociation constant of the dye with albumin being Kd = (1.7 ± 0.1) × 10-5 M. According to time correlated single photon counting (TCSPC) fluorescence lifetime spectroscopy data, two types of complexes with lifetimes of 1.0 ns and 2.5 ns are formed between the dye and HSA. Confocal fluorescence microscopy has unambiguously shown the penetration of biscarbocyanine into endoplasmic reticulum, lysosomes, mitochondria and nuclei of the cells. The dye demonstrates photocytotoxicity towards the colon carcinoma HCT116 cells with IC50 = 0.3 μM. Hydrophobicity of the polymethine chain and the presence of two positive charges on the dye molecule contribute to the effective binding of the dye with HSA and the penetration into cells. These facts allow considering the biscarbocyanine dye as a promising agent for the photodynamic therapy of cancer.

Enhancement of Photodynamic Cancer Therapy by Physical and Chemical Factors

The viable use of photodynamic therapy (PDT) in cancer therapy has never been fully realized because of its undesirable effects on healthy tissues. Herein we summarize some physicochemical factors that can make PDT a more viable and effective option to provide future oncological patients with better-quality treatment options. These physicochemical factors include light sources, photosensitizer (PS) carriers, microwaves, electric fields, magnetic fields, and ultrasound. This Review is meant to provide current information pertaining to PDT use, including a discussion of in vitro and in vivo studies. Emphasis is placed on the physicochemical factors and their potential benefits in overcoming the difficulty in transitioning PDT into the medical field. Many advanced techniques, such as employing X-rays as a light source, using nanoparticle-loaded stem cells and bacteriophage bio-nanowires as a photosensitizer carrier, as well as integration with immunotherapy, are among the future directions.

Porphyrinoid-based photosensitizers for diagnostic and therapeutic applications: An update

Porphyrin-based molecules are actively studied as dual function theranostics: fluorescence-based imaging for diagnostics and fluorescence-guided therapeutic treatment of cancers. The intrinsic fluorescent and photodynamic properties of the bimodal molecules allows for these theranostic approaches. Several porphyrinoids bearing both hydrophilic and/or hydrophobic units at their periphery have been developed for the aforementioned applications, but better tumor selectivity and high efficacy to destroy tumor cells is always a key setback for their use. Another issue related to their effective clinical use is that, most of these chromophores form aggregates under physiological conditions. Nanomaterials that are known to possess incredible properties that cannot be achieved from their bulk systems can serve as carriers for these chromophores. Porphyrinoids, when conjugated with nanomaterials, can be enabled to perform as multifunctional nanomedicine devices. The integrated properties of these porphyrinoid-nanomaterial conjugated systems make them useful for selective drug delivery, theranostic capabilities, and multimodal bioimaging. This review highlights the use of porphyrins, chlorins, bacteriochlorins, phthalocyanines and naphthalocyanines as well as their multifunctional nanodevices in various biomedical theranostic platforms.

A novel bacteriochlorin-styrylnaphthalimide conjugate for simultaneous photodynamic therapy and fluorescence imaging

Propargyl-15²,17³-dimethoxy-13¹-amide of bacteriochlorin e (BChl) and a 4-(4-N,N-dimethylaminostyryl)-N-alkyl-1,8-naphthalimide bearing azide group in the N-alkyl fragment were conjugated by the copper(i)-catalyzed 1,3-dipolar cycloaddition to produce a novel dyad compound BChl-NI for anticancer photodynamic therapy (PDT) combining the modalities of a photosensitizer (PS) and a fluorescence imaging agent. A precise photophysical investigation of the conjugate in solution using steady-state and time-resolved optical spectroscopy revealed that the presence of the naphthalimide (NI) fragment does not decrease the photosensitizing ability of the bacteriochlorin (BChl) core as compared with BChl; however, the fluorescence of naphthalimide is completely quenched due to resonance energy transfer (RET) to BChl. It has been shown that the BChl-NI conjugate penetrates into human lung adenocarcinoma A549 cells, and accumulates in the cytoplasm where it has a mixed granular-diffuse distribution. Both NI and BChl fluorescence in vitro provides registration of bright images showing perfectly intracellular distribution of BChl-NI. The ability of NI to emit light upon excitation in imaging experiments has been found to be due to hampering of RET as a result of photodestruction of the energy acceptor BChl unit. Phototoxicity studies have shown that the BChl-NI conjugate is not toxic for A549 cells at tested concentrations (<8 μM) without light-induced activation. At the same time, the concentration-dependent killing of cells is observed upon the excitation of the bacteriochlorin moiety with red light that occurs due to reactive oxygen species formation. The presented data demonstrate that the BChl-NI conjugate is a promissing dual function agent for cancer diagnostics and therapy.

Exploring organic photosensitizers based on hemicyanine derivatives: a sustainable approach for preparation of amide linkages

Hemicyanine derivatives C1–C4 have been synthesized and show strong absorption in the visible region, good water solubility, efficient intersystem crossing, a high singlet oxygen quantum yield and high ability to transport electrons from the donor to acceptor. These hemicyanine derivatives were utilized as photocatalysts in additive/base free oxidative amidation of aromatic aldehydes in mixed aqueous media under visible light irradiation at low catalytic loading. The hemicyanine derivative C4 exhibited recyclability upto four cycles and reusability upto five cycles in oxidative amidation of aromatic aldehydes. Among all the hemicyanine derivatives, C4 shows a high photocatalytic efficiency due to a high singlet oxygen quantum yield. All the mechanism investigations showed involvement of reactive oxygen species generated by the organic triplet photosensitizer based on hemicyanine derivative for carrying out oxidative amidation of aldehyde. Our results will encourage the design of new “metal free” organic photosensitizers and their application in photocatalysis.

Hemicyanine derivatives C1–C4 have been synthesized and utilized as photocatalysts in additive/base free oxidative amidation of aromatic aldehydes in mixed aqueous media under visible light irradiation at low catalytic loading.

Controllable Photodynamic Therapy Implemented by Regulating Singlet Oxygen Efficiency

With singlet oxygen (1O2) as the active agent, photodynamic therapy (PDT) is a promising technique for the treatment of various tumors and cancers. But it is hampered by the poor selectivity of most traditional photosensitizers (PS). In this review, we present a summary of controllable PDT implemented by regulating singlet oxygen efficiency. Herein, various controllable PDT strategies based on different initiating conditions (such as pH, light, H2O2 and so on) have been summarized and introduced. More importantly, the action mechanisms of controllable PDT strategies, such as photoinduced electron transfer (PET), fluorescence resonance energy transfer (FRET), intramolecular charge transfer (ICT) and some physical/chemical means (e.g. captivity and release), are described as a key point in the article. This review provide a general overview of designing novel PS or strategies for effective and controllable PDT.

Manipulation of novel nano-prodrug composed of organic pigment-based hybrid network and its optical uses

Here we developed the first case of pyropheophorbide-a-loaded PEGylated-hybrid carbon nanohorns (CNH-Pyro) to study tumor targeting therapy. During incubation with living cells, CNH-Pyro exhibited very intense red emissions. The intracellular imaging results were carried out by flow cytometry based on four different kinds of cell lines (including three adherent cell lines and one suspension cell line). Compared with free pyropheophorbide-a, CNH-Pyro demonstrated enhanced photodynamic tumor ablation efficiency during in vitro experiments due to improved biocompatibility of the hybrid nanomaterial and the photothermal therapy effect derived from carbon-network structure. Trypan blue staining experiments supported that the cell fate was dependent on the synergistic effects of both CNH-Pyro and laser irradiations. These results indicated that the chlorin-entrapped carbon nanohorns could provide powerful delivery vehicles for increasing photodynamic efficacy and possess early identification of the disease.