A hand-held fiber-optic implement for the site-specific delivery of photosensitizer and singlet oxygen

The good, the bad, and the ugly - controlling singlet oxygen through design of photosensitizers and delivery systems for photodynamic therapy

Singlet oxygen, although integral to photodynamic therapy, is notoriously uncontrollable, suffers from poor selectivity and has fast decomposition rates in biological media. Across the scientific community, there is a conscious effort to refine singlet oxygen interactions and initiate selective and controlled release to produce a consistent and reproducible therapeutic effect in target tissue. This perspective aims to provide an insight into the contemporary design principles behind photosensitizers and drug delivery systems that depend on a singlet oxygen response or controlled release. The discussion will be accompanied by in vitro and in vivo examples, in an attempt to highlight advancements in the field and future prospects for the more widespread application of photodynamic therapy.

S,S-Chiral Linker Induced U Shape with a Syn-facial Sensitizer and Photocleavable Ethene Group

There is a major need for light-activated materials for the release of sensitizers and drugs. Considering the success of chiral columns for the separation of enantiomer drugs, we synthesized an S,S-chiral linker system covalently attached to silica with a sensitizer ethene near the silica surface. First, the silica surface was modified to be aromatic rich, by replacing 70% of the surface groups with (3-phenoxypropyl)silane. We then synthesized a 3-component conjugate [chlorin sensitizer, S,S-chiral cyclohexane and ethene building blocks] in 5 steps with a 13% yield, and covalently bound the conjugate to the (3-phenoxypropyl)silane-coated silica surface. We hypothesized that the chiral linker would increase exposure of the ethene site for enhanced ¹ O₂ -based sensitizer release. However, the chiral linker caused the sensitizer conjugate to adopt a U shape due to favored 1,2-diaxial substituent orientation; resulting in a reduced efficiency of surface loading. Further accentuating the U shape was π-π stacking between the (3-phenoxypropyl)silane and sensitizer. Semiempirical calculations and singlet oxygen luminescence data provided deeper insight into the sensitizer's orientation and release. This study has lead to insight on modifications of surfaces for drug photorelease and can help lead to the development of miniaturized photodynamic devices.

Synthesis, Photophysics and PDT Evaluation of Mono-, Di-, Tri- and Hexa-PEG Chlorins for Pointsource Photodynamic Therapy

Pointsource photodynamic therapy (PSPDT) is a newly developed fiber optic method aimed at the delivery of photosensitizer, light and oxygen to a diseased site. Because of a need for developing photosensitizers with desirable properties for PSPDT, we have carried out a synthetic, photophysical and phototoxicity study on a series of PEGylated sensitizers. Chlorin and pheophorbide sensitizers were readily amenable to our synthetic PEGylation strategy to reach triPEG and hexaPEG galloyl pheophorbides and mono-, di-, triPEG chlorins. On screening these PEG sensitizers, we found that increasing the number of PEG groups, except for hexaPEGylation, increases phototoxicity. We found that three PEG groups but not less or more were optimal. Of the series tested, a triPEG gallyol pheophorbide and a triPEG chlorin were the most efficient at generating singlet oxygen, and produced the highest phototoxicity and lowest dark toxicity to Jurkat cells. A detailed kinetic analysis of the PEGylated sensitizers in solution and cell culture and media is also presented. The data provide us with steps in the development of PSPDT to add to the PDT tools we have in general.

Efficient activation of a visible light-activatable CA4 prodrug through intermolecular photo-unclick chemistry in mitochondria

Mitochondria-targeted and visible light-activatable CA4 prodrug was efficiently activated through intermolecular photo-unclick chemistry in mitochondria for the combination therapy.

Photoactive fluoropolymer surfaces that release sensitizer drug molecules

We describe a physical-organic study of two fluoropolymers bearing a photoreleasable PEGylated photosensitizer that generates (1)O2((1)Δg) [chlorin e6 methoxy tri(ethylene glycol) triester]. The surfaces are Teflon/poly(vinyl alcohol) (PVA) nanocomposite and fluorinated silica. The relative efficiency of these surfaces to photorelease the PEGylated sensitizer [shown previously to be phototoxic to ovarian cancer cells (Kimani, S. et al. J. Org. Chem 2012, 77, 10638)] was slightly higher for the nanocomposite. In the presence of red light and O2, (1)O2 is formed, which cleaves an ethene linkage to liberate the sensitizer in 68-92% yield. The fluoropolymers were designed to deal with multiple problems. Namely, their success relied not only on high O2 solubility and drug repellency but also on the C-F bonds, which physically quench little (1)O2, for singlet oxygen's productive use away from the surface. The results obtained here indicate that Teflon-like surfaces have potential uses in delivering sensitizer and singlet oxygen for applications in tissue repair and photodynamic therapy (PDT).

Folate receptor-mediated enhanced and specific delivery of far-red light-activatable prodrugs of combretastatin A-4 to FR-positive tumor

We examined the concept of a novel prodrug strategy in which anticancer drug can be locally released by visible/near IR light, taking advantage of the photodynamic process and photo-unclick chemistry. Our most recently formulated prodrug of combretastatin A-4, Pc-(L-CA4)₂, showed multifunctionality for fluorescence imaging, light-activated drug release, and the combined effects of PDT and local chemotherapy. In this formulation, L is a singlet oxygen cleavable linker. Here, we advanced this multifunctional prodrug by adding a tumor-targeting group, folic acid (FA). We designed and prepared four FA-conjugated prodrugs 1–4 (CA4-L-Pc-PEG_n-FA: n = 0, 2, 18, ∼45) and one non-FA-conjugated prodrug 5 (CA4-L-Pc-PEG₁₈-boc). Prodrugs 3 and 4 had a longer PEG spacer and showed higher hydrophilicity, enhanced uptake to colon 26 cells via FR-mediated mechanisms, and more specific localization to SC colon 26 tumors in Balb/c mice than prodrugs 1 and 2. Prodrug 4 also showed higher and more specific uptake to tumors, resulting in selective tumor damage and more effective antitumor efficacy than non-FA-conjugated prodrug 5. FR-mediated targeting seemed to be an effective strategy to spare normal tissues surrounding tumors in the illuminated area during treatment with this prodrug.

Far-red light activatable, multifunctional prodrug for fluorescence optical imaging and combinational treatment

We recently developed “photo-unclick chemistry”, a novel chemical tool involving the cleavage of aminoacrylate by singlet oxygen, and demonstrated its application to visible light-activatable prodrugs. In this study, we prepared an advanced multifunctional prodrug, Pc-(L-CA4)₂, composed of the fluorescent photosensitizer phthalocyanine (Pc), an SO-labile aminoacrylate linker (L), and a cytotoxic drug combretastatin A-4 (CA4). Pc-(L-CA4)₂ had reduced dark toxicity compared with CA4. However, once illuminated, it showed improved toxicity similar to CA4 and displayed bystander effects in vitro. We monitored the time-dependent distribution of Pc-(L-CA4)₂ using optical imaging with live mice. We also effectively ablated tumors by the illumination with far-red light to the mice, presumably through the combined effects of photodynamic therapy (PDT) and released chemotherapy drug, without any sign of acute systemic toxicity.

"Pointsource" delivery of a photosensitizer drug and singlet oxygen: eradication of glioma cells in vitro

We describe a pointsource sensitizer-tipped microoptic device for the eradication of glioma U87 cells. The device has a mesoporous fluorinated silica tip which emits singlet oxygen molecules and small quantities of pheophorbide sensitizer for additional production of singlet oxygen in the immediate vicinity. The results show that the device surges in sensitizer release and photokilling with higher rates about midway through the reaction. This was attributed to a self-amplified autocatalytic reaction where released sensitizer in the extracellular matrix provides positive feedback to assist in the release of additional sensitizer. The photokilling of the glioma cells was analyzed by global toxicity and live/dead assays, where a killing radius around the tip with ~0.3 mm precision was achieved. The implication of these results for a new PDT tool of hard-to-resect tumors, e.g. in the brain, is discussed.

Autocatalytic-assisted photorelease of a sensitizer drug bound to a silica support

The photorelease of a sensitizer from a fluorinated silica surface occurs by a reaction of singlet oxygen with the vinyl ether bond linker with scission of a dioxetane intermediate. Irradiation of the released sensitizer generates singlet oxygen, which accelerates the release of more sensitizer via an autocatalytic reaction. Sigmoidal behavior of sensitizer release in n-butanol and n-octanol occurs at an optimal temperature of 20 °C. The photorelease efficiency was reduced at low temperatures, where the sensitizer was retained on the surface due to a long-lived dioxetane with inefficient scission, and also reduced at high temperatures, due to a slower reaction of (1)O2 with the vinyl ether bond. Immediate acceleration is a result of released sensitizer being used as a dopant to eliminate the induction step, further implicating an autocatalytic mechanism. However, the sigmoidal sensitizer release was not correlated to solvent viscosity, heat, or light from the dioxetane decomposition or to minor O2 solubility enhancements caused by the fluorinated silica. The mechanistic information collected here can be used to help control the pace of drug release; however, it remains to be seen whether an autocatalytic-based drug delivery system has an advantage to those with non-sigmoidal kinetics.

A fiberoptic (photodynamic therapy type) device with a photosensitizer and singlet oxygen delivery probe tip for ovarian cancer cell killing

A portable "fiber optic-based sensitizer delivery" (FOSD) device has been developed and studied. Before there might be success in photodynamic therapy (PDT) and antibacterial ambitions, an understanding of basic factors on device performance was needed. Thus, the device was examined for the localized delivery of sensitizer molecules in ovarian cancer cells and production of high concentrations of singlet oxygen for their eradication in vitro. The device tip releases stored pheophorbide by attack of singlet oxygen from sensitized oxygen gas delivered through the hollow fiber using 669 nm laser light. The performance of the device was enhanced when configured with a fluorosilane tip by virtue of its Teflon-like property compared with a conventional glass tip (greater sensitizer quantities were photoreleased and laterally diffused, and greater amounts of ovarian OVCAR-5 cancer cells were killed). No cell damage was observed at 2.2 N of force applied by the probe tip itself, an amount used for many of the experiments described here.

Fluorine end-capped optical fibers for photosensitizer release and singlet oxygen production

The usefulness of a fiber optic technique for generating singlet oxygen and releasing the pheophorbide photosensitizer has been increased by the fluorination of the porous Vycor glass tip. Singlet oxygen emerges through the fiber tip with 669-nm light and oxygen, releasing the sensitizer molecules upon a [2 + 2] addition of singlet oxygen with the ethene spacer and scission of a dioxetane intermediate. Switching from a nonfluorinated to a fluorinated glass tip led to a clear reduction of the adsorbtive affinity of the departing sensitizer with improved release into homogeneous toluene solution and bovine tissue, but no difference was found in water since the sensitizer was insoluble. High surface coverage of the nonafluorohexylsilane enhanced the cleavage efficiency by 15% at the ethene site. The fluorosilane groups also caused crowding and seemed to reduce access of (1)O(2) to the ethene site, which attenuated the total quenching rate constant k(T), although there was less wasted (1)O(2) (from surface physical quenching) at the fluorosilane-coated than the native SiOH silica. The observations support a quenching mechanism that the replacement of the SiOH groups for the fluorosilane C-H and C-F groups enhanced the (1)O(2) lifetime at the fiber tip interface due to less efficient electronic-to-vibronic energy transfer.

Generating singlet oxygen bubbles: a new mechanism for gas-liquid oxidations in water

Laser-coupled microphotoreactors were developed to bubble singlet oxygen [(1)O(2) ((1)Δ(g))] into an aqueous solution containing an oxidizable compound. The reactors consisted of custom-modified SMA fiberoptic receptacles loaded with 150 μm silicon phthalocyanine glass sensitizer particles, where the particles were isolated from direct contact with water by a membrane adhesively bonded to the bottom of each device. A tube fed O(2) gas to the reactor chambers. In the presence of O(2), singlet oxygen was generated by illuminating the sensitizer particles with 669 nm light from an optical fiber coupled to the top of the reactor. The generated (1)O(2) was transported through the membrane by the O(2) stream and formed bubbles in solution. In solution, singlet oxygen reacted with probe compounds (9,10-anthracene dipropionate dianion, trans-2-methyl-2-pentanoate anion, N-benzoyl-D,L-methionine, or N-acetyl-D,L-methionine) to give oxidized products in two stages. The early stage was rapid and showed that (1)O(2) transfer occurred via bubbles mainly in the bulk water solution. The later stage was slow; it arose only from (1)O(2)-probe molecule contact at the gas/liquid interface. A mechanism is proposed that involves (1)O(2) mass transfer and solvation, where smaller bubbles provide better penetration of (1)O(2) into the flowing stream due to higher surface-to-volume contact between the probe molecules and (1)O(2).