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OPEN
received: 22 April 2016 accepted: 08 June 2016 Published: 04 July 2016
Programmed activation of cancer cell apoptosis: A tumor-targeted phototherapeutic topoisomerase I inhibitor Weon Sup Shin1,*, Jiyou Han2,*, Rajesh Kumar1,*, Gyung Gyu Lee2, Jonathan L. Sessler3, Jong-Hoon Kim2 & Jong Seung Kim1 We report here a tumor-targeting masked phototherapeutic agent 1 (PT-1). This system contains SN-38—a prodrug of the topoisomerase I inhibitor irinotecan. Topoisomerase I is a vital enzyme that controls DNA topology during replication, transcription, and recombination. An elevated level of topoisomerase I is found in many carcinomas, making it an attractive target for the development of effective anticancer drugs. In addition, PT-1 contains both a photo-triggered moiety (nitrovanillin) and a cancer targeting unit (biotin). Upon light activation in cancer cells, PT-1 interferes with DNA re-ligation, diminishes the expression of topoisomerase I, and enhances the expression of inter alia mitochondrial apoptotic genes, death receptors, and caspase enzymes, inducing DNA damage and eventually leading to apoptosis. In vitro and in vivo studies showed significant inhibition of cancer growth and the hybrid system PT-1 thus shows promise as a programmed photo-therapeutic (“phototheranostic”). Considerable interest has been focused lately on the development of so-called targeted therapeutics. In the case of cancer drug development, this approach is attractive in that it may allow dose-limiting systemic toxicities to be overcome via increased delivery of an active agent to a neoplastic site. For this strategy to be most effective, we believe it is necessary not only to effect delivery, but also controlled release. Appealing also would be an ability to monitor uptake and localization through optical or spectroscopic means. Examples of cancer-targeted, site-activated, and readily visualized antitumor agents are rare. To our knowledge none involving photo-activated release of an active agent has been reported to date. Here we report such a “phototheranostic” system; it is based on inhibition of topoisomerase I. Topoisomerase I is a critical enzyme that helps control DNA topology, for example, during replication, transcription, repair, and recombination1. Human topoisomerase I reversibly mends single strand disruptions and relaxation in DNA, and is a crucial step in DNA replication in healthy cells2. An elevated level of topoisomerase I is found in many carcinomas. This has made DNA topoisomerase I a good target for the development of anticancer drugs. Two pentacyclic-quinolone-based analogues of camptothecin, topotecan and irinotecan, that function as topoisomerase I inhibitors, have been approved by the US Food and Drug Administration for the treatment of cancer3,4. Despite their clinical success, these two drugs suffer from limitations, including low target specificity to cancer cells, adverse effects on healthy cells (e.g., hematological toxicity, liver dysfunction, and anemia), drug resistance due to multidrug resistance transporters (MDRs), and a less-than-ideal therapeutic index. In order to overcome these limitations, a number of topoisomerase I inhibitor prodrugs, activated by certain tumor characteristics, such as reactive oxygen species, pH5,6, enzymes7,8, and intracellular thiols9–11, have been developed. Unfortunately, most of these agents display toxicity towards healthy cells. We were thus attracted to photo-activation12. Irradiation with light provides the promise of activation with high temporal and spatial resolution and with potentially minimal toxicity13. To date, light-activated compounds have been widely studied in a 1
Department of Chemistry, Korea University, Seoul 136-701, Korea. 2Department of Biotechnology, Laboratory of Stem Cells and Tissue Regeneration, College of Life Sciences & Biotechnology, Korea University, Seoul 136-713, Republic of Korea. 3Department of Chemistry, University of Texas at Austin, Austin, TX 78712-1224, USA. *These authors contributed equally to this work. Correspondence and requests for materials should be addressed to J.L.S. (email: [email protected]) or J.-H.K. (email: [email protected]) or J.S.K. (email: [email protected]) Scientific Reports | 6:29018 | DOI: 10.1038/srep29018
1
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Figure 1. Synthesis, fluorescence changes, and the proposed mechanism of activation of PT-1. (a) The synthesis of PT-1. (b) Fluorescence spectra of a 10 μM solution of PT-1 in PBS (pH 7.4) during a 3 h period in the presence of 365-nm ultraviolet (UV) light. A.U.: arbitrary units. (c) Time course of changes in fluorescence intensity at 365 nm. (d) Fluorescence spectra of a 10 μM solution of PT-1 in PBS at various pH levels after irradiation for 3 h. (e) Comparison of irradiated and non-irradiated samples. (f) Proposed drug release mechanism of PT-1 upon 365 nm photo-irradiation.
number of chemical and biological contexts13–17. Furthermore, light-sensitive nanoparticles, such as polymeric up-conversion nanoparticles have been utilized in various practical applications18–21. In the case of cancer treatment, the use of a light-activated prodrug may allow for more precise control of drug release at the tumor site, along with the potential for both modulating the therapeutic activity. To our knowledge, however, the use of light-activated compounds for cancer drug release and the concurrent monitoring of targeted drug delivery has not been explored. As detailed below, we have now constructed a tumor-targeting masked phototherapeutic agent 1 (PT-1) that contains 7-ethyl-10-hydroxycamptothecin (SN-38)—a prodrug of irinotecan—and nitrovanillin as a phototriggered moiety (Fig. 1a). This phototheranostic consists of three moieties. The first is a biotin unit, a cancer-targeting unit that draws the antitumor agent to cancer cells selectively22. The second component is a light-activated unit, o-nitrobenzyl (nitrovanillin), which provides a fluorescent signal after photo-cleavage. The third moiety is an anticancer prodrug, SN-38, which also acts as a fluorescent group for imaging (monitoring). SN-38 is an active metabolite of irinotecan, a topoisomerase I inhibitor that inhibits DNA unwinding23. Upon light activation in a cancer cell, PT-1 interferes with DNA religation, thus causing DNA damage that leads to apoptosis24. In vitro and in vivo studies reveal significant inhibition of cancer growth by PT-1 upon irradiation with 405 nm laser light.
Results and Discussion
Synthesis, fluorescence changes, and the mechanism of activation of PT-1. The synthesis of
PT-1 is shown in Fig. 1a. 2-Azidoethanol 2 and compound 4 were synthesized by previously reported methods25,26. Compound 2 was coupled with biotin in the presence of EDCI to produce compound 3 in moderate yield. Reaction of compound 4 with propargyl bromide yielded alkyne 5, which on reduction with sodium borohydride gave the functionalized benzyl alcohol 6. The latter compound was then treated with 4-nitrophenyl chloroformate with SN-38 to give intermediate 7. A “click” reaction between compounds 7 and 3 then yielded the desired target
Scientific Reports | 6:29018 | DOI: 10.1038/srep29018
2
www.nature.com/scientificreports/ PT-1. All new compounds (PT-1 and 2–7) were characterized by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy and electrospray ionization mass spectrometry (Fig. S6–19 in Supporting Information). PT-1 incorporates an o-nitrobenzyl (nitrovanillin) subunit that was designed to act as a light-triggered unit to permit photo-activation in cancer cells. Photocaged molecules derived from o-nitrobenzyl are especially valuable for cellular studies because they and their daughter species typically induce negligible damage to biomolecules upon irradiation with 365-nm ultraviolet light. Therefore, to activate PT-1 it was irradiated at 365 nm using a commercially available UV hand lamp. PT-1 is stable on the laboratory time scale and, in the absence of UV irradiation, is characterized by a weak fluorescence emission whose maximum falls at 550 nm (Fig. 1c). Upon irradiation at 365 nm, a strong fluorescence emission feature at 550 nm is observed (Fig. 1b,c). The increase in fluorescence emission intensity is consistent with photoinduced cleavage of the fluorescent SN-38 subunit from the PT-1 core under the influence of light. The photo-activation of PT-1 was explored as a function of time. On the basis of these experiments we conclude that essentially complete drug release occurs within 2 h after the irradiation process is initiated (Fig. 1c). The proposed mechanism of activation and release of SN-3827 is shown in Fig. 1f. Intracellular thiols are present in excess in cancer cells. Their ability to interfere with the photo-release of SN-38 was evaluated by subjecting PT-1 to irradiation at 365 nm in the presence of various thiols (cysteine, homocysteine, or glutathione). The same changes in fluorescence were observed as those in their absence (Fig. S3 in Supporting Information), leading us to discount thiol-based interference as a cause for concern. Given the disparities in pH between cancer cells and most normal healthy cells, we evaluated whether the fluorescent behavior of PT-1 (10 μM aqueous) was pH dependent. As shown in Fig. 1d,e, there was no appreciable change in the fluorescence intensity of PT-1 over the 3 to 9 pH range in the absence of irradiation at 365 nm. This finding leads us to suggest that PT-1 would display high stability over the full biologically relevant pH range when applied in vitro or in vivo. After irradiation, however, the fluorescence intensity of PT-1 increased over the 3 to 9 pH range, with the greatest increase being seen once a pH of ca. 7 was reached. We conclude that PT-1 undergoes photo-activation over a wide pH range. The activation of PT-1 and release of SN-38 was analyzed by high performance liquid chromatography (HPLC) and mass spectrometry (MS). The HPLC chromatogram showed one peak corresponding to PT-1 eluting at 27 min in the absence of irradiation. A new peak corresponding to SN-38 appeared at 17.5 min after UV irradiation (Fig. S4 in Supporting Information). These findings served to confirm that photo-irradiation of PT-1 leads to the clean production of SN-38 (Fig. S5 in Supporting Information).
In vitro bioimaging, cytotoxicity, and the mode of action of PT-1. Biotin is essential for normal cellular growth, function, and proliferation. Various cancer cell lines, including breast cancer cells (MDA-M231, MCF7), human lung cancer cells (A549), cervical cancer cells (HeLa), hepatic cancer cells (HepG2, Huh7, Hep3B), stomach cancer cells (NCI-N87, AGS), prostate cancer cells (Du145, PC3), pancreatic cancer cells (Panc-1), leukemic cells (L1210FR), mastocytoma (P815), lung (M109), renal (RENCA, RD0995), colon (Colo-26), and ovarian cancer cells (Ov2008, ID8), express greater levels of biotin receptors than do healthy cells28; therefore, biotin uptake is much higher in cancer cells. Accordingly, we utilized biotin as a cancer cell-targeting agent in the design of PT-1. For in vitro analysis of PT-1 in the presence and absence of irradiation, we utilized 12 biotin receptor-positive cancer cell lines (A549, HeLa, MDA-M231, A549, Huh7, Hep3B, Du145, NCI-N87, AGS, HepG2, MCF7, PC3, and Panc-1) and 3 biotin receptor negative normal cell lines (BJ, WI-38 and 293T, Fig. S21 and Table S1 in Supporting Information). To test the localization of PT-1 in vitro, biotin receptor-positive cells (A549 and HeLa) and biotin receptor negative cells (WI-38 and BJ cells) were studied in detail. The A549 and HeLa cells were incubated with PT-1 (10 nM) for 1 h and then irradiated with a 405-nm laser. In the absence of irradiation, no significant fluorescence was observed (Fig. 2a–c). However, the emission intensity increased considerably upon irradiation (Fig. 2d–f). Cell viability assays were also performed to evaluate the cytotoxicity of PT-1 and two reference compounds (ref. 7: without a biotin unit, and ref. 8: without a photo-releasing unit) toward various cancer and healthy cells (Figs S21–23 and Table S1 in Supporting Information)28. Statistically significant higher cytotoxicity was observed for the cancer cells than in the case of healthy cells upon incubation with 10 nM PT-1 followed by irradiation at 405 nm (Figs 2g,h, and S21). Viabilities for both cancer and healthy cells were similar for refs 7 and 8, a finding that is consistent with the cancer targeting effect of biotin (see Fig. S23 in Supporting Information). The phototherapeutic potential of PT-1 was assessed by examining the population of A549 cancer cells before and after photo-irradiation. Large numbers of apoptotic cancer cells were found in the irradiated areas after 1 h but not in the untreated areas (Fig. 2j,l). Additionally, 24 h live imaging of the A459 cells was performed in the presence (Movie 2) and absence of irradiation (Movie 1). In the apoptosis-positive area no movement of A459 cells is seen, as would be expected for an apoptosis process that occurs after unmasking of the active SN-38 subunit from the PT-1 core. Cancer cells originally present in the non-irradiated area (left side) showed the hallmarks of efficient survival. However, those that migrated into the irradiated area were found to undergo apoptosis (Movie 2). On this basis, we believe that PT-1 is selectively taken up by cancer cells via biotin targeting and induces apoptosis upon photo-irradiation. Further analyses were carried out using fluorescence-activated cell sorting (FACS) in conjunction with PI (propidium iodide) staining (Fig. 3a–d). The number of apoptotic cells induced by SN-38 (12.5 nM) was found to be 10%29. In the absence of irradiation, the number of apoptotic cells induced by PT-1 (10 nM) was 1.20% (Fig. 3b). This number increased to 13.19% upon irradiation of the PT-1 (10 nM) cells for 1 h (Fig. 3d). Thus, PT-1 becomes a stronger chemotherapeutic than SN-38 upon irradiation. This is as expected for a system with cancer-localizing capability. In agreement with the FACS results, the fluorescent and phase contrast images of cells revealed evidence of effective apoptosis in the case of the A459 cells induced by PT-1 after 24 h (Fig. S24 in Supporting Information). In general, SN-38, an active metabolite of irinotecan, inhibits DNA topoisomerase I by intercalating into the DNA strand and disrupting DNA replication. To validate and compare the mode of action of PT-1 and SN-38, we Scientific Reports | 6:29018 | DOI: 10.1038/srep29018
3
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Figure 2. In vitro bioimaging and cytotoxicity of PT-1. (a–f) Intracellular fluorescence intensity recorded for A549 cells 1 h after incubation with of PT-1 in the absence (a–c) of and (d–f) after irradiation with 405 nm light for 1 h over the area delineated by the yellow dashed line. (g) Cytotoxicity of PT-1 observed in two cancer cell lines (A549 and HeLa) and two human normal fibroblast cell lines (WI-38 and BJ) after photoactivation. (h) Cell viability (relative percentage) observed for the A549, HeLa, WI-38, and BJ cell lines after treatment with PT-1 (incubation for 1 h) followed by irradiation at 405 nm for 1 h. Viability was determined via a 3-(4,5-dimethythiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay performed 24 h after photoirradiation. *P
OPEN
received: 22 April 2016 accepted: 08 June 2016 Published: 04 July 2016
Programmed activation of cancer cell apoptosis: A tumor-targeted phototherapeutic topoisomerase I inhibitor Weon Sup Shin1,*, Jiyou Han2,*, Rajesh Kumar1,*, Gyung Gyu Lee2, Jonathan L. Sessler3, Jong-Hoon Kim2 & Jong Seung Kim1 We report here a tumor-targeting masked phototherapeutic agent 1 (PT-1). This system contains SN-38—a prodrug of the topoisomerase I inhibitor irinotecan. Topoisomerase I is a vital enzyme that controls DNA topology during replication, transcription, and recombination. An elevated level of topoisomerase I is found in many carcinomas, making it an attractive target for the development of effective anticancer drugs. In addition, PT-1 contains both a photo-triggered moiety (nitrovanillin) and a cancer targeting unit (biotin). Upon light activation in cancer cells, PT-1 interferes with DNA re-ligation, diminishes the expression of topoisomerase I, and enhances the expression of inter alia mitochondrial apoptotic genes, death receptors, and caspase enzymes, inducing DNA damage and eventually leading to apoptosis. In vitro and in vivo studies showed significant inhibition of cancer growth and the hybrid system PT-1 thus shows promise as a programmed photo-therapeutic (“phototheranostic”). Considerable interest has been focused lately on the development of so-called targeted therapeutics. In the case of cancer drug development, this approach is attractive in that it may allow dose-limiting systemic toxicities to be overcome via increased delivery of an active agent to a neoplastic site. For this strategy to be most effective, we believe it is necessary not only to effect delivery, but also controlled release. Appealing also would be an ability to monitor uptake and localization through optical or spectroscopic means. Examples of cancer-targeted, site-activated, and readily visualized antitumor agents are rare. To our knowledge none involving photo-activated release of an active agent has been reported to date. Here we report such a “phototheranostic” system; it is based on inhibition of topoisomerase I. Topoisomerase I is a critical enzyme that helps control DNA topology, for example, during replication, transcription, repair, and recombination1. Human topoisomerase I reversibly mends single strand disruptions and relaxation in DNA, and is a crucial step in DNA replication in healthy cells2. An elevated level of topoisomerase I is found in many carcinomas. This has made DNA topoisomerase I a good target for the development of anticancer drugs. Two pentacyclic-quinolone-based analogues of camptothecin, topotecan and irinotecan, that function as topoisomerase I inhibitors, have been approved by the US Food and Drug Administration for the treatment of cancer3,4. Despite their clinical success, these two drugs suffer from limitations, including low target specificity to cancer cells, adverse effects on healthy cells (e.g., hematological toxicity, liver dysfunction, and anemia), drug resistance due to multidrug resistance transporters (MDRs), and a less-than-ideal therapeutic index. In order to overcome these limitations, a number of topoisomerase I inhibitor prodrugs, activated by certain tumor characteristics, such as reactive oxygen species, pH5,6, enzymes7,8, and intracellular thiols9–11, have been developed. Unfortunately, most of these agents display toxicity towards healthy cells. We were thus attracted to photo-activation12. Irradiation with light provides the promise of activation with high temporal and spatial resolution and with potentially minimal toxicity13. To date, light-activated compounds have been widely studied in a 1
Department of Chemistry, Korea University, Seoul 136-701, Korea. 2Department of Biotechnology, Laboratory of Stem Cells and Tissue Regeneration, College of Life Sciences & Biotechnology, Korea University, Seoul 136-713, Republic of Korea. 3Department of Chemistry, University of Texas at Austin, Austin, TX 78712-1224, USA. *These authors contributed equally to this work. Correspondence and requests for materials should be addressed to J.L.S. (email: [email protected]) or J.-H.K. (email: [email protected]) or J.S.K. (email: [email protected]) Scientific Reports | 6:29018 | DOI: 10.1038/srep29018
1
www.nature.com/scientificreports/
Figure 1. Synthesis, fluorescence changes, and the proposed mechanism of activation of PT-1. (a) The synthesis of PT-1. (b) Fluorescence spectra of a 10 μM solution of PT-1 in PBS (pH 7.4) during a 3 h period in the presence of 365-nm ultraviolet (UV) light. A.U.: arbitrary units. (c) Time course of changes in fluorescence intensity at 365 nm. (d) Fluorescence spectra of a 10 μM solution of PT-1 in PBS at various pH levels after irradiation for 3 h. (e) Comparison of irradiated and non-irradiated samples. (f) Proposed drug release mechanism of PT-1 upon 365 nm photo-irradiation.
number of chemical and biological contexts13–17. Furthermore, light-sensitive nanoparticles, such as polymeric up-conversion nanoparticles have been utilized in various practical applications18–21. In the case of cancer treatment, the use of a light-activated prodrug may allow for more precise control of drug release at the tumor site, along with the potential for both modulating the therapeutic activity. To our knowledge, however, the use of light-activated compounds for cancer drug release and the concurrent monitoring of targeted drug delivery has not been explored. As detailed below, we have now constructed a tumor-targeting masked phototherapeutic agent 1 (PT-1) that contains 7-ethyl-10-hydroxycamptothecin (SN-38)—a prodrug of irinotecan—and nitrovanillin as a phototriggered moiety (Fig. 1a). This phototheranostic consists of three moieties. The first is a biotin unit, a cancer-targeting unit that draws the antitumor agent to cancer cells selectively22. The second component is a light-activated unit, o-nitrobenzyl (nitrovanillin), which provides a fluorescent signal after photo-cleavage. The third moiety is an anticancer prodrug, SN-38, which also acts as a fluorescent group for imaging (monitoring). SN-38 is an active metabolite of irinotecan, a topoisomerase I inhibitor that inhibits DNA unwinding23. Upon light activation in a cancer cell, PT-1 interferes with DNA religation, thus causing DNA damage that leads to apoptosis24. In vitro and in vivo studies reveal significant inhibition of cancer growth by PT-1 upon irradiation with 405 nm laser light.
Results and Discussion
Synthesis, fluorescence changes, and the mechanism of activation of PT-1. The synthesis of
PT-1 is shown in Fig. 1a. 2-Azidoethanol 2 and compound 4 were synthesized by previously reported methods25,26. Compound 2 was coupled with biotin in the presence of EDCI to produce compound 3 in moderate yield. Reaction of compound 4 with propargyl bromide yielded alkyne 5, which on reduction with sodium borohydride gave the functionalized benzyl alcohol 6. The latter compound was then treated with 4-nitrophenyl chloroformate with SN-38 to give intermediate 7. A “click” reaction between compounds 7 and 3 then yielded the desired target
Scientific Reports | 6:29018 | DOI: 10.1038/srep29018
2
www.nature.com/scientificreports/ PT-1. All new compounds (PT-1 and 2–7) were characterized by 1H and 13C nuclear magnetic resonance (NMR) spectroscopy and electrospray ionization mass spectrometry (Fig. S6–19 in Supporting Information). PT-1 incorporates an o-nitrobenzyl (nitrovanillin) subunit that was designed to act as a light-triggered unit to permit photo-activation in cancer cells. Photocaged molecules derived from o-nitrobenzyl are especially valuable for cellular studies because they and their daughter species typically induce negligible damage to biomolecules upon irradiation with 365-nm ultraviolet light. Therefore, to activate PT-1 it was irradiated at 365 nm using a commercially available UV hand lamp. PT-1 is stable on the laboratory time scale and, in the absence of UV irradiation, is characterized by a weak fluorescence emission whose maximum falls at 550 nm (Fig. 1c). Upon irradiation at 365 nm, a strong fluorescence emission feature at 550 nm is observed (Fig. 1b,c). The increase in fluorescence emission intensity is consistent with photoinduced cleavage of the fluorescent SN-38 subunit from the PT-1 core under the influence of light. The photo-activation of PT-1 was explored as a function of time. On the basis of these experiments we conclude that essentially complete drug release occurs within 2 h after the irradiation process is initiated (Fig. 1c). The proposed mechanism of activation and release of SN-3827 is shown in Fig. 1f. Intracellular thiols are present in excess in cancer cells. Their ability to interfere with the photo-release of SN-38 was evaluated by subjecting PT-1 to irradiation at 365 nm in the presence of various thiols (cysteine, homocysteine, or glutathione). The same changes in fluorescence were observed as those in their absence (Fig. S3 in Supporting Information), leading us to discount thiol-based interference as a cause for concern. Given the disparities in pH between cancer cells and most normal healthy cells, we evaluated whether the fluorescent behavior of PT-1 (10 μM aqueous) was pH dependent. As shown in Fig. 1d,e, there was no appreciable change in the fluorescence intensity of PT-1 over the 3 to 9 pH range in the absence of irradiation at 365 nm. This finding leads us to suggest that PT-1 would display high stability over the full biologically relevant pH range when applied in vitro or in vivo. After irradiation, however, the fluorescence intensity of PT-1 increased over the 3 to 9 pH range, with the greatest increase being seen once a pH of ca. 7 was reached. We conclude that PT-1 undergoes photo-activation over a wide pH range. The activation of PT-1 and release of SN-38 was analyzed by high performance liquid chromatography (HPLC) and mass spectrometry (MS). The HPLC chromatogram showed one peak corresponding to PT-1 eluting at 27 min in the absence of irradiation. A new peak corresponding to SN-38 appeared at 17.5 min after UV irradiation (Fig. S4 in Supporting Information). These findings served to confirm that photo-irradiation of PT-1 leads to the clean production of SN-38 (Fig. S5 in Supporting Information).
In vitro bioimaging, cytotoxicity, and the mode of action of PT-1. Biotin is essential for normal cellular growth, function, and proliferation. Various cancer cell lines, including breast cancer cells (MDA-M231, MCF7), human lung cancer cells (A549), cervical cancer cells (HeLa), hepatic cancer cells (HepG2, Huh7, Hep3B), stomach cancer cells (NCI-N87, AGS), prostate cancer cells (Du145, PC3), pancreatic cancer cells (Panc-1), leukemic cells (L1210FR), mastocytoma (P815), lung (M109), renal (RENCA, RD0995), colon (Colo-26), and ovarian cancer cells (Ov2008, ID8), express greater levels of biotin receptors than do healthy cells28; therefore, biotin uptake is much higher in cancer cells. Accordingly, we utilized biotin as a cancer cell-targeting agent in the design of PT-1. For in vitro analysis of PT-1 in the presence and absence of irradiation, we utilized 12 biotin receptor-positive cancer cell lines (A549, HeLa, MDA-M231, A549, Huh7, Hep3B, Du145, NCI-N87, AGS, HepG2, MCF7, PC3, and Panc-1) and 3 biotin receptor negative normal cell lines (BJ, WI-38 and 293T, Fig. S21 and Table S1 in Supporting Information). To test the localization of PT-1 in vitro, biotin receptor-positive cells (A549 and HeLa) and biotin receptor negative cells (WI-38 and BJ cells) were studied in detail. The A549 and HeLa cells were incubated with PT-1 (10 nM) for 1 h and then irradiated with a 405-nm laser. In the absence of irradiation, no significant fluorescence was observed (Fig. 2a–c). However, the emission intensity increased considerably upon irradiation (Fig. 2d–f). Cell viability assays were also performed to evaluate the cytotoxicity of PT-1 and two reference compounds (ref. 7: without a biotin unit, and ref. 8: without a photo-releasing unit) toward various cancer and healthy cells (Figs S21–23 and Table S1 in Supporting Information)28. Statistically significant higher cytotoxicity was observed for the cancer cells than in the case of healthy cells upon incubation with 10 nM PT-1 followed by irradiation at 405 nm (Figs 2g,h, and S21). Viabilities for both cancer and healthy cells were similar for refs 7 and 8, a finding that is consistent with the cancer targeting effect of biotin (see Fig. S23 in Supporting Information). The phototherapeutic potential of PT-1 was assessed by examining the population of A549 cancer cells before and after photo-irradiation. Large numbers of apoptotic cancer cells were found in the irradiated areas after 1 h but not in the untreated areas (Fig. 2j,l). Additionally, 24 h live imaging of the A459 cells was performed in the presence (Movie 2) and absence of irradiation (Movie 1). In the apoptosis-positive area no movement of A459 cells is seen, as would be expected for an apoptosis process that occurs after unmasking of the active SN-38 subunit from the PT-1 core. Cancer cells originally present in the non-irradiated area (left side) showed the hallmarks of efficient survival. However, those that migrated into the irradiated area were found to undergo apoptosis (Movie 2). On this basis, we believe that PT-1 is selectively taken up by cancer cells via biotin targeting and induces apoptosis upon photo-irradiation. Further analyses were carried out using fluorescence-activated cell sorting (FACS) in conjunction with PI (propidium iodide) staining (Fig. 3a–d). The number of apoptotic cells induced by SN-38 (12.5 nM) was found to be 10%29. In the absence of irradiation, the number of apoptotic cells induced by PT-1 (10 nM) was 1.20% (Fig. 3b). This number increased to 13.19% upon irradiation of the PT-1 (10 nM) cells for 1 h (Fig. 3d). Thus, PT-1 becomes a stronger chemotherapeutic than SN-38 upon irradiation. This is as expected for a system with cancer-localizing capability. In agreement with the FACS results, the fluorescent and phase contrast images of cells revealed evidence of effective apoptosis in the case of the A459 cells induced by PT-1 after 24 h (Fig. S24 in Supporting Information). In general, SN-38, an active metabolite of irinotecan, inhibits DNA topoisomerase I by intercalating into the DNA strand and disrupting DNA replication. To validate and compare the mode of action of PT-1 and SN-38, we Scientific Reports | 6:29018 | DOI: 10.1038/srep29018
3
www.nature.com/scientificreports/
Figure 2. In vitro bioimaging and cytotoxicity of PT-1. (a–f) Intracellular fluorescence intensity recorded for A549 cells 1 h after incubation with of PT-1 in the absence (a–c) of and (d–f) after irradiation with 405 nm light for 1 h over the area delineated by the yellow dashed line. (g) Cytotoxicity of PT-1 observed in two cancer cell lines (A549 and HeLa) and two human normal fibroblast cell lines (WI-38 and BJ) after photoactivation. (h) Cell viability (relative percentage) observed for the A549, HeLa, WI-38, and BJ cell lines after treatment with PT-1 (incubation for 1 h) followed by irradiation at 405 nm for 1 h. Viability was determined via a 3-(4,5-dimethythiazol-2-yl)-2,5-diphenyl tetrazolium bromide (MTT) assay performed 24 h after photoirradiation. *P
