Boron Agent Delivery Platforms Based on Natural Products for Boron Neutron Capture Therapy

Boron neutron capture therapy (BNCT) is one of the most promising modalities for cancer treatment due to its minimal invasiveness. Although two types of boron agents are clinically used, several issues persist in their delivery, including poor water solubility, instability in aqueous media, selectivity toward cancer cells, accumulation in cancer cells, retention time in tumor tissue, and efficiency in achieving the boron neutron capture reaction. Addressing these challenges, numerous groups have explored various boron agents to enhance the therapeutic benefits of BNCT. This review summarizes delivery platforms based on natural products for BNCT.

"On-Off" Switching of Functional Guest Molecules via Exchange of Natural Product Solubilizing Agents

For the development of delivery systems, the solubilization of hydrophobic guest molecules in water is an important yet challenging task. This can be achieved by preparing stable aqueous solutions with a high concentration of guest molecules using a natural product as a solubilizing agent and a mechanochemical high-speed vibration milling apparatus as a solubilizing method. Various solubilizing agent-guest molecule complexes can be obtained via the exchange between solubilizing agents, which enables the "on-off" switching of the properties of functional guest molecules, such as fluorescence intensity, and photodynamic activity. In the exchange method, guest molecules can transfer into cell membranes such as lysosomes and exosomes. Therefore, the exchange method of the solubilizing agents not only creates novel solubilizing agent-guest molecule complexes but also is applied to drug delivery systems.

Selective Photodynamic Activity of Tetrakis(4-aminophenyl)porphyrins with and without Acetyl Protecting Groups on Cancer and Normal Cells

Tetrakis(4-aminophenyl)porphyrin (1) and tetrakis(4-acetamidophenyl)porphyrin (2) were dissolved in water with the incorporation of a polysaccharide (λ-carrageenan (CGN)) as a water-solubilizing agent. Although the photodynamic activity of the CGN-2 complex was considerably lower than that of the CGN-1 complex, the selectivity index (SI; IC50 in a normal cell/IC50 in a cancer cell) of the CGN-2 complex was considerably higher than that of the CGN-1 complex. This is because the photodynamic activity of the CGN-2 complex was significantly affected by the intracellular uptakes by the normal and cancer cells. During in vivo experiments, the CGN-2 complex inhibited tumor growth under light irradiation with high blood retention compared with the CGN-1 complex and Photofrin, which exhibited lower blood retention. This study showed that the photodynamic activity and SI are influenced by substituent groups of arene in the meso-positions of porphyrin analogs.

Fluorescence Turn-on of Tetraphenylethylene Derivative by Transfer from Cyclodextrin to Liposomes, HeLa Cells, and E. coli

Herein, trimethyl-β-cyclodextrin (TMe-β-CDx) and γ-cyclodextrin (γ-CDx) could dissolve a tetraphenylethylene derivative (TPE-OH₄ ) in water through high-speed vibration milling. The fluorescence intensity of the TMe-β-CDx-TPE-OH₄ complex was much higher than that of the γ-CDx-TPE-OH₄ complex, as the rotation of the central C=C double bond of TPE-OH₄ after photoactivation was inhibited in a smaller TMe-β-CDx cavity in comparison with the γ-CDx cavity. In contrast, the fluorescence intensity of the γ-CDx-TPE-OH₄ complex was very weak; nevertheless, it increased after the addition of liposomes due to the transfer of TPE-OH₄ from the γ-CDx cavity to the lipid membrane as a "turn-on" phenomenon. Furthermore, to apply temperature sensor, it was demonstrated that the fluorescence intensity in the liposomes depended on the phase-transition temperature. By using the fluorescence turn-on phenomenon, TPE-OH₄ could detect the presence of HeLa cells and E. coli by fluorescence.

Mechanism toward Turn-on of Polysaccharide-Porphyrin Complexes for Fluorescence Probes and Photosensitizers in Photodynamic Therapy in Living Cells

β-(1,3-1,6)-D-Glucan, λ-carrageenan, tamarind gum, and pullulan can dissolve various porphyrin derivatives via the formation of complexes in water using a high-speed vibration milling method. The aqueous solutions of the resulting complexes exhibit long-term stability. Despite the adverse effects of the self-quenching process, notable fluorescence and improved photodynamic activity of the polysaccharide-complexed porphyrin derivatives were observed in the presence of liposomes, micelles, cyclodextrins, and HeLa cells. It was noted that the type of porphyrins was more important than the type of polysaccharides present in the complex. Porphyrin self-aggregates were monodispersed in the lipid membranes of the liposomes and lysosomes. The polysaccharide-complexed porphyrin derivatives showed increased photodynamic activity toward HeLa cells under photoirradiation between 610 and 740 nm.

Time-Resolved Monitoring of Intracellular Processes with a Cyclical On-Off Photoswitchable Nanoprobe

Fluorescent microscopic imaging with the help of small-molecule probes (chemoprobes) is one of the most feasible approaches for noninvasive sensing of intracellular molecules. However, the "always on" property of current chemoprobes failed to achieve time-resolved monitoring. Here, we report the development of a supramolecular nanoassembling strategy to integrate multiple functions on one nanoscale probe (nanoprobe) with a cyclical on-off switchable sensing ability. The reversal of the nanoprobe can be rapidly achieved by converting the single-wavelength near-infrared (NIR) laser to two-way emissions by a lanthanum nanoparticle core that is sensitive to the light power density. Through regulating the NIR power density, the azobenzene derivative, which was doped in the surface of the lipid bilayer of the nanoprobe, can act as an "impeller" and "brake" for bio-benign activation and deactivation, respectively, of the nanoprobe in biological applications. A reduced nicotinamide adenine dinucleotide nanoprobe was constructed as the model to demonstrate precise and time-resolved monitoring of intracellular processes including cancerous glycolysis and ligand-induced enzymatic processes. We envision that this cyclical on-off switchable nanoprobe strategy will hold great promise for endowing universal chemoprobes with high precision and temporal resolution.

Stabilisation of lipid membrane-incorporated porphyrin derivative aqueous solutions and their photodynamic activities

A porphyrin derivative which exists on the hydrophilic surface of the liposomes showed high photodynamic activity.

Incorporation of large guest molecules into liposomes via chemical reactions in lipid membranes

The incorporation of hydrophobic guest molecules into lipid membranes by the exchange of the guest molecule from a cyclodextrin (CDx) complex to a liposome is limited to guest molecules that can be included in CDxs. To solve this problem, large guest molecules were incorporated into liposomes by chemical reactions of guest molecules in lipid membranes. Stable lipid-membrane-incorporated fullerene derivatives with large substituent(s) were prepared by Diels-Alder reactions in lipid membranes.

Lipid-membrane-incorporated arylboronate esters as agents for boron neutron capture therapy

An arylboronate ester bearing a methyl group at both of its ortho positions was stably incorporated into lipid membranes at high concentrations without hydrolysis.

Formation of lipid membrane-incorporated small π-molecules bearing hydrophilic groups

Lipid membrane-incorporated π-conjugated guest molecules (LMIGs) have been classified into four categories, including stable LMIGs, the precipitation or dissolution of some of the guest molecules from the LMIGs and the formation of small aggregates.

Porphyrin-uptake in liposomes and living cells using an exchange method with cyclodextrin

A preparation of a lipid-membrane-incorporated tetraphenylporphyrin was achieved from the corresponding tetraphenylporphyrin·cyclodextrin complexes using an exchange method in both liposomes and cells.