Paclitaxel (PTX) is one of the most useful chemotherapeutic agents approved for several cancers, including ovarian, breast, pancreatic, and non-small cell lung cancer. However, it causes systemic side effects when administered parenterally. Photodynamic therapy (PDT) is a new strategy for treating local cancers using light and photosensitizer. Unfortunately, PDT is often followed by recurrence, due to incomplete ablation of tumors. To overcome these problems, we prepared the far-red light-activatable prodrug of PTX by conjugating photosensitizer via singlet oxygen-cleavable aminoacrylate linker. Tubulin polymerization enhancement and cytotoxicity of prodrugs were dramatically reduced. However, once illuminated with far-red light, the prodrug effectively killed SKOV-3 ovarian cancer cells through the combined effects of PDT and locally released PTX. Ours is the first PTX prodrug that can be activated by singlet oxygen using tissue penetrable and clinically useful far-red light, which kills the cancer cells through the combined effects of PDT and site-specific PTX chemotherapy.

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.

Although tissue-penetrable light (red and NIR) has great potential for spatiotemporally controlled release of therapeutic agents, it has been hampered because of the lack of chemistry translating the photonic energy to the cleavage of a chemical bond. Recently, we discovered that an aminoacrylate group could be cleaved to release parent drugs after oxidation by SO and have called this "photo-unclick chemistry". We demonstrate its application to far-red-light-activated prodrugs. A prodrug of combretastatin A-4 (CA4) was prepared, CMP-L-CA4, where CMP is dithiaporphyrin, a photosensitizer, and L is an aminoacrylate linker. Upon irradiation with 690 nm diode laser, the aminoacrylate linker of the prodrug was cleaved, rapidly releasing CA4 (>80% in 10 min) in CDCl3. In tissue culture, it showed about a 6-fold increase in its IC50 in MCF-7 after irradiation, most likely because of the released CA4. Most significantly, CMP-L-CA4 had better antitumor efficacy in vivo than its noncleavable (NC) analog, CMP-NCL-CA4. This is the first demonstration of the in vivo efficacy of the novel low-energy-light-activatable prodrug using the photo-unclick chemistry.

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-PEGn-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.

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