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Androgen receptor targeted conjugate for bimodal photodynamic therapy of prostate cancer in vitro Greta Varchi, Valentina Rapozzi, Daniele Ragno, Gabriella Castoria, marzia DI DONATO, Emilia Della Pietra, Valentina Benfenati, Claudia Ferroni, Andrea Guerrini, Daniela Cesselli, and Emanuela Saracino Bioconjugate Chem., Just Accepted Manuscript • DOI: 10.1021/acs.bioconjchem.5b00261 • Publication Date (Web): 24 Jun 2015 Downloaded from http://pubs.acs.org on July 1, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Bioconjugate Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
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Androgen receptor targeted conjugate for bimodal photodynamic therapy of prostate cancer in vitro Valentina Rapozzi,‡ Daniele Ragno,∫ Andrea Guerrini,† Claudia Ferroni,† Emilia della Pietra,‡ Daniela Cesselli,‡ Gabriella Castoria,§ Marzia Di Donato,§ Emanuela Saracino,£ Valentina Benfenati,† and Greta Varchi*† † Institute of the Organic Synthesis and Photoreactivity Italian National Research Council Via P. Gobetti, 101 40129 Bologna – IT E-mail: [email protected] ‡ Department of Medical and Biological Sciences University of Udine Piazzale Kolbe, 4 – 33100 Udine – IT ∫ Department of Chemistry University of Ferrara Via Fossato di Mortara, 17 - 44121 Ferrara – IT § Department of Biochemistry, Biophysics and General Pathology – II University of Naples Via L. De Crecchio, 7 -80138 Naples – IT £ Institute for the Study of Nanostructured Materials Italian National Research Council Via P. Gobetti, 101, 40129 Bologna – IT *Correspondence should be addressed to: Greta Varchi, PhD, Institute of Organic Synthesis and Photoreactivity, Italian National Research Council, Via Gobetti, 101, 40129 Bologna, Italy. Phone: +39 0516398283; Fax +39 0516398349; E-mail: [email protected]. Abstract Prostate cancer (PC) represents the most common type of cancer among males and is the second leading cause of cancer death in men in Western society. Current options for PC therapy remain unsatisfactory, since they often produce uncomfortable long-term side effects, such as impotence (70%) and incontinence (5-20%) even in the first-stages of the disease. Light-triggered therapies, such as photodynamic therapy, have the potential to provide important advances in the treatment of localized and partially metastasized prostate cancer. We have designed a novel molecular conjugate (DR2) constituted of a photosensitizer (pheophorbide a, Pba), connected to a non-steroidal antiandrogen molecule through a small pegylated linker. This study aims at investigating whether DR2 represents a valuable approach for PC treatment based on light-induced production of single oxygen and nitric oxide (NO) in vitro. Besides being able to efficiently bind the androgen receptor (AR), the 2-trifluoromethylnitrobenzene ring on the DR2 backbone is able to release cytotoxic NO under the exclusive control of light, thus augmenting the general photodynamic effect. Although DR2 is similarly internalized in cells expressing different levels of androgen receptor, the AR ligand prevents its efflux through the ABCG2-pump. In vitro photo-toxicity experiments demonstrated the ability of DR2 to kill cancer cells more efficiently than Pba, while no dark-toxicity was observed. Overall, the presented approach is very promising for further development of AR-photosensitizer conjugates in the multi-modal photodynamic treatment of prostate cancer.
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Introduction Prostate cancer (PC) is the leading cause of cancer death for men in Western countries and an emerging malignancy in developing nations.1 Depending on the diagnosis response, PC treatment options vary. Clinicians may decide for an active surveillance or for more invasive treatments such as surgical removal of the prostate gland, external or internal radiotherapy alone or in combination with androgen depletion therapy (ADT). Despite being effective in shrinking tumor burden and improving survival, these therapies frequently fail and castration-resistant prostate cancer (CRPC) frequently develops.2 At this stage, PC is almost incurable and often accompanied by metastatic spreading and poor prognosis, with a median survival range from 12 to 24 months.3 Unfortunately, few biomarkers predictive of metastatic phenotype have been identified yet and few therapeutic options are available for CRPC patients. Although chemotherapy remains the first choice of most clinicians at this stage, its overall benefit is very modest, since PC frequently escapes from chemotherapy at both primary and metastatic sites.4–6 Within the past years, several promising agents, including inhibitors of androgen synthesis or androgen receptor (AR) activation have improved CRPC patient survival.7–9 Nevertheless, CRPC has evolved mechanisms to reactivate AR despite continued ADT. Therefore, further progress, beyond the discovery of more potent and selective drugs, is strongly warranted. The multi-modal treatment involving the administration of different therapeutic agents acting synergistically and/or in combination with techniques has emerged as a successful approach.10 In this context, lighttriggered therapies, such as photodynamic therapy (PDT), have the potential to provide important advances in the treatment of localized and partially metastasized PC. PDT consists of three essential components: a photosensitizer (PS), light and oxygen that together initiate a photochemical reaction that culminates in the generation of highly reactive oxygen species (ROS) responsible of tumor cells death.11 Indeed, in June 2013 Tookad® has entered a randomized phase III clinical trial for vascular targeted PDT of localized PC (https://clinicaltrials.gov/ct2/show/NCT01875393?term=NCT01875393&rank=1). Interestingly, light can be delivered to the entire prostate gland and to near-by metastasized organs by using interstitial, cylindrically diffusing optical fibres.12 Unlike chemotherapy or radiotherapy, the cell killing mechanism triggered by PDT is not directly dependent on DNA damage, thus decreasing the chances of therapy cross-resistance and eliminating late normal tissue effects such as second malignancy.13 Based on these preliminary considerations, we focused our study on the development of a novel tricomponent conjugate composed of a bacteriochlorin (pheophorbide a, Pba, Figure 1),14 connected to a nonsteroidal antiandrogen compound (Figure 1, A) through a small pegylated linker, with the aim of investigating in vitro whether this compound represents a valuable approach for the targeted multi-modal treatment of PC.
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Figure 1. Chemical structure of conjugate DR2.
To this purpose, the synthetically modified 2,5-piperazine derivative A (Figure 1) was selected for (i) its high affinity to AR and ability to antagonize the receptor and (ii) the presence of the nitro substituent for the possible light-induced release of nitric oxide (NO). 2,5-piperazine derivatives represent potent orally and peripherally selective non-steroidal AR antagonists exhibiting high affinity for the AR.15 In principle, these features could favor the overall activity of the conjugate by binding to the receptor following its internalization. Recently Dreaden and co-workers demonstrated that by binding to AR and the G-protein GPRC6A, antiandrogen-decorated gold nanoparticles exhibit enhanced therapeutic efficacy as compared to the reference drug.16 Additionally, the conjugation of an antiandrogen moiety with a histone deacetylase inhibitor (HDACi) displays higher HDACi efficacy and selectivity in PC treatment as compared to HDACi alone.17 In recent years NO has stimulated a great deal of interest not only for its essential role in various biological functions,18 but also for its promising anticancer activity.19 NO levels in the tumor and its microenvironment can directly influence the response of cancer cells to PDT, confirming a pivotal role of exogenous NO in the overall antitumor PDT response.20,21 As an example, the combination of the NO donor, DETANONOate, and PDT resulted in significant potentiation of the cytotoxic effect in vitro and in vivo.22 Moreover, several research groups have recently developed novel NO donors to promote PDT-mediated anti-tumor cytotoxicity. For instance, Carneiro and co-workers reported the synthesis and activity of a nitrosylphtalocyanin ruthenium complex [Ru(NO)(NO)(ONO)(pc)] and studied its effect on a murine melanoma cell line, B16F10, in the presence or absence of light irradiation. Their findings demonstrated that the complex was more effective in inhibiting B16F10 cell growth than the [Ru (pc)], thereby confirming the importance of NO release in PDT treatment.23 Among small molecules, the ones bearing a 2-trifluoromethylnitrobenzene ring have been established as effective NO donors as they are able to generate nitric oxide through a nitro-to-nitrite photo-rearrangement that occurs upon controlled irradiation with light at 400 nm wavelength (Figure 2).24,25
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CF3 Light
NO2
ONO NO CF3
CF3
O
OH
Figure 2. Light-induced NO-releasing mechanism of nitrobenzene compounds.
In this work we present in vitro findings obtained using the newly synthesized DR2 molecular conjugate. In particular, the presented experiments focus on the ability of DR2 to be efficiently internalized in different PC cell lines and to function as a multi-modal photosensitizer, thus being capable of generating ROS and NO upon light activation, thereby providing additive photo-toxicity. Results and Discussion Conjugates synthesis and characterization Scheme 1 shows the synthesis of conjugate DR2. Acid 1 was obtained according to a slightly modified literature procedure15,26 (see also Supporting Information for details) and reacted with N-hydroxy succinimide in presence of N,N'-Dicyclohexylcarbodiimide (DCC) in 1,4-dioxane to afford compound 2 in good yield. Derivative 2 was than coupled with N-Boc-2,2′-(ethylene-1,2-dioxy)bis(ethylamine) in the presence of pyridine in tetrahydrofuran (THF), followed by deprotection of the amino group with CF3COOH to provide compound 3 in quantitative yield. Coupling reaction between NHS-activated Pba (4) and derivative 3 was carried out in the dark and in the presence of pyridine at room temperature to provide conjugate DR2 in acceptable yield.
Scheme 1. Synthesis of conjugate DR2.a
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a
Reaction conditions and reagents: a) DCC, NHS, 1,4-dioxane, rt, 5 h; (45%); b) H2N-[(CH2)2O]2(CH2)2NH(CO)OtBu,
Py, THF, rt, 21 h; (100%); c) CF3COOH, rt, 2 h (46%); d) Py, THF, dark, rt, 22 h; (98%).
Whenever necessary intermediates and final compound were purified by silica gel column chromatography and characterized by 1H and 13C NMR, and HRMS. The electronic absorption spectra of DR2 was recorded in CH2Cl2 and compared with free Pba. The spectra of the novel conjugate show the typical features of non-aggregated Pba (Figures S1 in Supporting Information Section) and display a Soret band at 415 nm, slightly red-shifted with respect to Pba alone (404 nm) and an intense Q band at 670 nm. A shoulder responsible for the absorption of the antiandrogen ligand is detected at 400 nm.
DR2 displaces the androgen receptor ligand binding. We used the prostate cancer-derived LNCaP cells, since they represent a model to analyze the androgenresponsiveness and AR activation.27 Using a siRNA approach, we firstly depleted LNCaP cells of AR and control cells were transfected with non-targeting siRNA. Transfected cells were made quiescent and then incubated with 10 nM of the synthetic AR radio-ligand [3H]-methyltrienolone ([3H] R1881). Cells were collected and used for the ligand-binding assay. Results presented in Figure 3A show that the [3H] R1881 binding decreases when AR was silenced. By increasing siRNA AR concentration (from 125 to 250 picomol, respectively), a parallel decrease (85% and 93%) in AR ligand binding was observed. The Western blot from lysate proteins (inset in Figure 3B), together with the densitometric analysis (Figure 3B) show that AR levels ACS Paragon Plus Environment
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decrease as siRNA AR concentration increases. Therefore, our assay is specific for AR expressed in LNCaP cells.
Figure 3. AR ligand binding displacement studies in LNCaP cells. A) LNCaP cells were transfected with siRNA nt (0 on x axis) or the indicated amounts of siRNA AR. Transfected cells were incubated with 10 nM [3H] R1881. Intracellular radioactivity was assayed and residual binding was calculated and expressed as % of total AR binding sites. Inset in B, shows the Western blot of AR in the different experimental conditions. The membrane was stripped and re-probed with anti tubulin antibody, as a loading control. B) Densitometric analysis of Western blot is shown. AR levels were normalized for tubulin amounts. Residual AR was expressed as % of the total level, which corresponds to the receptor level detected in LNCaP cells transfected with siRNA nt. C) Quiescent LNCaP cells were incubated with 10 nM [3H] R1881, in the absence or presence of the indicated excess (from 0 to 4 µM) of DR2 or unlabeled R1881, or bicalutamide, or enzalutamide. Intracellular radioactivity was assayed and AR binding sites x 10-3 cells were calculated. D)-F) LNCaP cells were transfected with nt (D), or AR siRNAs (125 pM or 250 pM in E and F, respectively). Cells were incubated with 10 nM [3H] R1881, in the absence or presence of the indicated excess (from 0 to 4 µM) of DR2 Intracellular radioactivity was assayed and AR binding sites x 10-3 cells were calculated. In D)-F), the different values on the y axis are due to the increase in siRNA AR concentration, and hence to the parallel decrease in AR binding sites. A)-F), data were collected from three different experiments. Means and SEM are shown.
The efficacy of DR2 in displacing the AR ligand binding activity was similar to that observed using unlabeled R1881, or bicalutamide, or enzalutamide, two non-steroidal antiandrogens largely used in PC treatment,7 in LNCaP cells (Figure 3C). Again, a very significant [3H] R1881 displacement was detected when the conjugate DR2 was used at 0,25 and 0,5 µM in LNCaP cells transfected with non-targeting siRNA (Figure 3D). By increasing the DR2 concentration (from 1 to 4 µM), an almost complete displacement of AR ACS Paragon Plus Environment
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ligand binding activity was detected (Figure 3D). Similar findings were observed by using unlabeled R1881 (at 1 µM). Bicalutamide or enzalutamide show a similar effect on AR ligand binding displacement when used at 1 µM (legend to Figure 3D). DR2 was still able to displace the [3H] R1881 binding in LNCaP cells depleted of different amounts of AR upon siRNA AR transfection (125 picomol and 250 picomol, respectively in Figure 3E and F). In sum, these findings show that DR2 specifically binds AR and is active in displacing the receptor ligand binding activity even when low AR amounts are expressed, as it often occurs in PC-associated fibroblasts.28 DR2 in vitro uptake and internalization studies in AR-expressing PC cells. We then evaluated the DR2 delivery/uptake in VCaP cells, which over-express the wild type AR.29 The extent of conjugate uptake over time was quantified by fluorescence-activated cell sorting (FACS). Pba displayed different behavior if administered in its free form or as DR2 conjugate. In particular, the uptake of free Pba was significant already after 3 h in VCaP [MF (mean fluorescence) = 188.89] (Figure 4A), while a longer time (6 h) was required for DR2 to reach a similar intracellular concentration [MF DR2 in VCaP = 206] (Figure 4B). Pba might passively translocate across cell membrane, while the more steric hindered DR2 conjugate likely engages a different route to enter the cell. Furthermore, confocal microscopy analysis in Figure 4 (D and E), demonstrates that DR2 exhibits a higher accumulation rate over time, as compared to Pba alone. This behavior might be due to its affinity for AR expressed in VCaP cells.
Figure 4. Uptake of Pba and DR2 on VCaP cells. A) - B) FACS analyses of VCaP cells treated with 1 µM of Pba and DR2. C) Values of Mean of Fluorescence (MF) at different time of photosensitizer incubation. Fluorescence was detected after 3, 6 and 24 h from compound incubation. Data were collected from three different experiments D) - E) High magnification (40X) single plane confocal images representative of fluorescence emission collected by VCaP cells after 6 h of incubation with 1µM Pba (D) and 1µM DR2 (E). Bar is 100 µm.
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DR2 in vitro uptake and internalization studies in malignant PC cells. Prostate cancer-derived PC3 cells are highly metastatic and androgen-insensitive as they express low levels of the AR, thus representing a valuable in vitro model of castration-resistant PC.30,31 In order to evaluate the ability of our conjugate to be internalized by these cells, we analyzed by FACS analysis the delivery/uptake of Pba and DR2 in PC3 cells. Data in Figure 5A show that as it occurs in VCaP cells (Figure 4A), the uptake of free Pba in PC3 cells is significant already after 3 h (MF 302.88), while 6 h were required for the uptake of DR2 (MF 170). Confocal microscopy images in Figure 5 (D and E) show that Pba and DR2 are both diffused in cytoplasm compartment after 6 h incubation. Interestingly, when comparing confocal microscopy images of DR2 in VCaP and PC3 cells after 6 h of incubation, we observed that while in PC3 cells DR2 is localized exclusively in the cytoplasm, in VCaP cells red fluorescence is intensively diffused also in the nucleus (cfr Figures 4E and 5E). This result might suggest that DR2 translocates into the nucleus upon
binding to the receptor, as also confirmed by the binding experiment performed with the radioligand assay (Figure 3). However, more studies are underway to verify the role of DR2 after the binding to the androgen receptor in terms of gene transcription activity.
Figure 5. Uptake of Pba and DR2 on PC3 cells. A) - C) FACS analyses of PC3 cells treated with 1 µM of Pba and DR2. C) Values of Mean of Fluorescence after different time of photosensitizer incubation. Fluorescence was detected after 3, 6 and 24 h from compound incubation. Data were collected from three independent experiments D) - E) High magnification (40X) single plane confocal images representative of fluorescence emission collected by PC3 cells after 6 h of incubation with 1µM Pba (D) and 1µM DR2 (E). Bar is 100 µm.
Despite DR2 internalization seems to be slightly slower in PC3 cells as compared to VCaP cells (cfr. Figure 4C and 5C), DR2 is persistently present after 24 h of incubation in PC3 cells (MF 422). In contrast, the amount of Pba is significantly reduced (MF 249) at that incubation time (Figure 5C), likely because of its efflux. This behavior draw our attention to the emerging problem of PDT resistance,32 and in particular to ACS Paragon Plus Environment
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Pba as substrate for the ABCG2 pump efflux.33–35 The ABCG2 gene encodes a membrane transporter belonging to the ATP-binding cassette (ABC) superfamily of membrane transporters, which being involved in the trafficking of a variety of biological molecules and drugs across the plasma membrane, also plays a role in multidrug resistance. To investigate more deeply this aspect and the possible beneficial role of conjugating the photosensitizer with specific targeting antenna, we performed an experiment with A549 human lung adenocarcinoma cells that express higher levels of ABCG2, as compared with PC3 cells (Figure 6A-B).
Figure 6. A) Western Blot of ABCG2 and β-actin protein levels in A549 and PC3 cell lines. B) The histograms report the values of the ABCG2 protein band intensity determined by densitometric analysis and normalized for β-actin. C) Functional assay for ABCG2 substrate. The A549 cells were incubated with 5 µM of Pba or DR2 for 30 min with or without 10 µM reserpine, washed, and then incubated for 60 min in PS-free media with (red line) or without (blu line) reserpine. Intracellular fluorescence was determined using a flow cytometer equipped with a 633 nm red diode laser. Representative results from two independent experiments are shown.
Robey and co-workers described a functional assay to evaluate if different photosensitizers are substrates for this efflux pump, using an ABCG2 inhibitor, e.g. reserpine.34,36,37 Therefore, in order to determine whether DR2 is a substrate for ABCG2 pump, we incubated A459 cells with 5µM of Pba or DR2 in the presence or absence 10 µM of reserpine for 30 min. Subsequently, the cells were washed and incubated for 1 h in a photosensitizer-free medium in the presence of reserpine. Graphs in Figure 6C report the fluorescence of Pba (left) and DR2 (right), alone (blue lines) or plus reserpine (red lines), respectively. Reserpine increases the accumulation of Pba in the PC3 cells, while no shift of the fluorescence peak is observed by using DR2, thus suggesting that DR2 is not a substrate for the efflux pump. In sum, data in Figures 4, 5 and 6 show that DR2 conjugate binds with high affinity AR and escapes the ABCG2 efflux pump in PC3 cells. These properties make DR2 a promising candidate for PDT in PC, since it
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is retained in PC cells expressing different levels of AR. In this latter setting, DR2 consistently accumulates within PC cells, preventing the onset of PDT resistance.32 In vitro photodynamic activity The in vitro photodynamic activity of DR2 was investigated against LNCaP, VCaP and PC3 cells. Twentyfour h after light irradiation, cell proliferation was determined by resazurin assay using free Pba as reference. Pba presents a Q band absorption peak at 670 nm allowing irradiation with a red light, while the o-CF3 nitrobenzene group (AR ligand moiety) should release NO upon selective irradiation at 400 nm. Therefore, in order to stimulate both compounds, cells were initially irradiated with a white halogen light covering the entire visible spectrum. According to the results of the uptake experiments, cells were incubated in the dark with increasing concentrations of Pba and DR2 for 6 h and subsequently irradiated with a metal white halogen lamp for 30 min (7 J/cm2). Data shown in Figure 7 (A-C) account for a higher photo-toxicity of DR2 in all cell lines as compared to Pba alone. Remarkably, the IC50 values (defined as the compound concentration to inhibit 50% of the cells) of DR2 were lower than Pba in all cell lines (Table 1), when using white light. This might be due to the longer retention of DR2 inside the cells and to the ability of the NO-donor moiety to induce the release of NO species that are synergistic to 1O2 generated by Pba. Importantly, no dark toxicity was observed for the conjugate 24 h after treatment (Figure S2). In order to evaluate the contribution of NO to the overall toxicity of the new conjugate, we performed a second set of metabolic assays using a halogen lamp equipped with a red filter, with the aim to excite only Pba. Results on PC3, LNCaP and VCaP cells are reported in Figure 7 (D-F) and Table 1, confirming that no difference occurs in the photo-toxic behavior of the two compounds when irradiated with red light, thereby supporting the additive action of the NO released only when white lamp is used as light source.
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Figure 7. %Metabolic activity of PC3, VCaP and LNCaP cells treated with increasing amounts of Pba and DR2. After 6 h incubation in the dark the cells were irradiated A-C) with a white halogen lamp (fluence 7.2 J/cm2) and D-F) with the same lamp equipped with a red filter (fluence 0.84 J/cm2). A resazurin assay was carried out 24 h after irradiation. Data represent mean values ± SD of three independent experiments. T-Test analysis of different treatments (Pba and DR2) vs NT a: p
Article
Androgen receptor targeted conjugate for bimodal photodynamic therapy of prostate cancer in vitro Greta Varchi, Valentina Rapozzi, Daniele Ragno, Gabriella Castoria, marzia DI DONATO, Emilia Della Pietra, Valentina Benfenati, Claudia Ferroni, Andrea Guerrini, Daniela Cesselli, and Emanuela Saracino Bioconjugate Chem., Just Accepted Manuscript • DOI: 10.1021/acs.bioconjchem.5b00261 • Publication Date (Web): 24 Jun 2015 Downloaded from http://pubs.acs.org on July 1, 2015
Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.
Bioconjugate Chemistry is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.
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Androgen receptor targeted conjugate for bimodal photodynamic therapy of prostate cancer in vitro Valentina Rapozzi,‡ Daniele Ragno,∫ Andrea Guerrini,† Claudia Ferroni,† Emilia della Pietra,‡ Daniela Cesselli,‡ Gabriella Castoria,§ Marzia Di Donato,§ Emanuela Saracino,£ Valentina Benfenati,† and Greta Varchi*† † Institute of the Organic Synthesis and Photoreactivity Italian National Research Council Via P. Gobetti, 101 40129 Bologna – IT E-mail: [email protected] ‡ Department of Medical and Biological Sciences University of Udine Piazzale Kolbe, 4 – 33100 Udine – IT ∫ Department of Chemistry University of Ferrara Via Fossato di Mortara, 17 - 44121 Ferrara – IT § Department of Biochemistry, Biophysics and General Pathology – II University of Naples Via L. De Crecchio, 7 -80138 Naples – IT £ Institute for the Study of Nanostructured Materials Italian National Research Council Via P. Gobetti, 101, 40129 Bologna – IT *Correspondence should be addressed to: Greta Varchi, PhD, Institute of Organic Synthesis and Photoreactivity, Italian National Research Council, Via Gobetti, 101, 40129 Bologna, Italy. Phone: +39 0516398283; Fax +39 0516398349; E-mail: [email protected]. Abstract Prostate cancer (PC) represents the most common type of cancer among males and is the second leading cause of cancer death in men in Western society. Current options for PC therapy remain unsatisfactory, since they often produce uncomfortable long-term side effects, such as impotence (70%) and incontinence (5-20%) even in the first-stages of the disease. Light-triggered therapies, such as photodynamic therapy, have the potential to provide important advances in the treatment of localized and partially metastasized prostate cancer. We have designed a novel molecular conjugate (DR2) constituted of a photosensitizer (pheophorbide a, Pba), connected to a non-steroidal antiandrogen molecule through a small pegylated linker. This study aims at investigating whether DR2 represents a valuable approach for PC treatment based on light-induced production of single oxygen and nitric oxide (NO) in vitro. Besides being able to efficiently bind the androgen receptor (AR), the 2-trifluoromethylnitrobenzene ring on the DR2 backbone is able to release cytotoxic NO under the exclusive control of light, thus augmenting the general photodynamic effect. Although DR2 is similarly internalized in cells expressing different levels of androgen receptor, the AR ligand prevents its efflux through the ABCG2-pump. In vitro photo-toxicity experiments demonstrated the ability of DR2 to kill cancer cells more efficiently than Pba, while no dark-toxicity was observed. Overall, the presented approach is very promising for further development of AR-photosensitizer conjugates in the multi-modal photodynamic treatment of prostate cancer.
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Introduction Prostate cancer (PC) is the leading cause of cancer death for men in Western countries and an emerging malignancy in developing nations.1 Depending on the diagnosis response, PC treatment options vary. Clinicians may decide for an active surveillance or for more invasive treatments such as surgical removal of the prostate gland, external or internal radiotherapy alone or in combination with androgen depletion therapy (ADT). Despite being effective in shrinking tumor burden and improving survival, these therapies frequently fail and castration-resistant prostate cancer (CRPC) frequently develops.2 At this stage, PC is almost incurable and often accompanied by metastatic spreading and poor prognosis, with a median survival range from 12 to 24 months.3 Unfortunately, few biomarkers predictive of metastatic phenotype have been identified yet and few therapeutic options are available for CRPC patients. Although chemotherapy remains the first choice of most clinicians at this stage, its overall benefit is very modest, since PC frequently escapes from chemotherapy at both primary and metastatic sites.4–6 Within the past years, several promising agents, including inhibitors of androgen synthesis or androgen receptor (AR) activation have improved CRPC patient survival.7–9 Nevertheless, CRPC has evolved mechanisms to reactivate AR despite continued ADT. Therefore, further progress, beyond the discovery of more potent and selective drugs, is strongly warranted. The multi-modal treatment involving the administration of different therapeutic agents acting synergistically and/or in combination with techniques has emerged as a successful approach.10 In this context, lighttriggered therapies, such as photodynamic therapy (PDT), have the potential to provide important advances in the treatment of localized and partially metastasized PC. PDT consists of three essential components: a photosensitizer (PS), light and oxygen that together initiate a photochemical reaction that culminates in the generation of highly reactive oxygen species (ROS) responsible of tumor cells death.11 Indeed, in June 2013 Tookad® has entered a randomized phase III clinical trial for vascular targeted PDT of localized PC (https://clinicaltrials.gov/ct2/show/NCT01875393?term=NCT01875393&rank=1). Interestingly, light can be delivered to the entire prostate gland and to near-by metastasized organs by using interstitial, cylindrically diffusing optical fibres.12 Unlike chemotherapy or radiotherapy, the cell killing mechanism triggered by PDT is not directly dependent on DNA damage, thus decreasing the chances of therapy cross-resistance and eliminating late normal tissue effects such as second malignancy.13 Based on these preliminary considerations, we focused our study on the development of a novel tricomponent conjugate composed of a bacteriochlorin (pheophorbide a, Pba, Figure 1),14 connected to a nonsteroidal antiandrogen compound (Figure 1, A) through a small pegylated linker, with the aim of investigating in vitro whether this compound represents a valuable approach for the targeted multi-modal treatment of PC.
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Figure 1. Chemical structure of conjugate DR2.
To this purpose, the synthetically modified 2,5-piperazine derivative A (Figure 1) was selected for (i) its high affinity to AR and ability to antagonize the receptor and (ii) the presence of the nitro substituent for the possible light-induced release of nitric oxide (NO). 2,5-piperazine derivatives represent potent orally and peripherally selective non-steroidal AR antagonists exhibiting high affinity for the AR.15 In principle, these features could favor the overall activity of the conjugate by binding to the receptor following its internalization. Recently Dreaden and co-workers demonstrated that by binding to AR and the G-protein GPRC6A, antiandrogen-decorated gold nanoparticles exhibit enhanced therapeutic efficacy as compared to the reference drug.16 Additionally, the conjugation of an antiandrogen moiety with a histone deacetylase inhibitor (HDACi) displays higher HDACi efficacy and selectivity in PC treatment as compared to HDACi alone.17 In recent years NO has stimulated a great deal of interest not only for its essential role in various biological functions,18 but also for its promising anticancer activity.19 NO levels in the tumor and its microenvironment can directly influence the response of cancer cells to PDT, confirming a pivotal role of exogenous NO in the overall antitumor PDT response.20,21 As an example, the combination of the NO donor, DETANONOate, and PDT resulted in significant potentiation of the cytotoxic effect in vitro and in vivo.22 Moreover, several research groups have recently developed novel NO donors to promote PDT-mediated anti-tumor cytotoxicity. For instance, Carneiro and co-workers reported the synthesis and activity of a nitrosylphtalocyanin ruthenium complex [Ru(NO)(NO)(ONO)(pc)] and studied its effect on a murine melanoma cell line, B16F10, in the presence or absence of light irradiation. Their findings demonstrated that the complex was more effective in inhibiting B16F10 cell growth than the [Ru (pc)], thereby confirming the importance of NO release in PDT treatment.23 Among small molecules, the ones bearing a 2-trifluoromethylnitrobenzene ring have been established as effective NO donors as they are able to generate nitric oxide through a nitro-to-nitrite photo-rearrangement that occurs upon controlled irradiation with light at 400 nm wavelength (Figure 2).24,25
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CF3 Light
NO2
ONO NO CF3
CF3
O
OH
Figure 2. Light-induced NO-releasing mechanism of nitrobenzene compounds.
In this work we present in vitro findings obtained using the newly synthesized DR2 molecular conjugate. In particular, the presented experiments focus on the ability of DR2 to be efficiently internalized in different PC cell lines and to function as a multi-modal photosensitizer, thus being capable of generating ROS and NO upon light activation, thereby providing additive photo-toxicity. Results and Discussion Conjugates synthesis and characterization Scheme 1 shows the synthesis of conjugate DR2. Acid 1 was obtained according to a slightly modified literature procedure15,26 (see also Supporting Information for details) and reacted with N-hydroxy succinimide in presence of N,N'-Dicyclohexylcarbodiimide (DCC) in 1,4-dioxane to afford compound 2 in good yield. Derivative 2 was than coupled with N-Boc-2,2′-(ethylene-1,2-dioxy)bis(ethylamine) in the presence of pyridine in tetrahydrofuran (THF), followed by deprotection of the amino group with CF3COOH to provide compound 3 in quantitative yield. Coupling reaction between NHS-activated Pba (4) and derivative 3 was carried out in the dark and in the presence of pyridine at room temperature to provide conjugate DR2 in acceptable yield.
Scheme 1. Synthesis of conjugate DR2.a
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a
Reaction conditions and reagents: a) DCC, NHS, 1,4-dioxane, rt, 5 h; (45%); b) H2N-[(CH2)2O]2(CH2)2NH(CO)OtBu,
Py, THF, rt, 21 h; (100%); c) CF3COOH, rt, 2 h (46%); d) Py, THF, dark, rt, 22 h; (98%).
Whenever necessary intermediates and final compound were purified by silica gel column chromatography and characterized by 1H and 13C NMR, and HRMS. The electronic absorption spectra of DR2 was recorded in CH2Cl2 and compared with free Pba. The spectra of the novel conjugate show the typical features of non-aggregated Pba (Figures S1 in Supporting Information Section) and display a Soret band at 415 nm, slightly red-shifted with respect to Pba alone (404 nm) and an intense Q band at 670 nm. A shoulder responsible for the absorption of the antiandrogen ligand is detected at 400 nm.
DR2 displaces the androgen receptor ligand binding. We used the prostate cancer-derived LNCaP cells, since they represent a model to analyze the androgenresponsiveness and AR activation.27 Using a siRNA approach, we firstly depleted LNCaP cells of AR and control cells were transfected with non-targeting siRNA. Transfected cells were made quiescent and then incubated with 10 nM of the synthetic AR radio-ligand [3H]-methyltrienolone ([3H] R1881). Cells were collected and used for the ligand-binding assay. Results presented in Figure 3A show that the [3H] R1881 binding decreases when AR was silenced. By increasing siRNA AR concentration (from 125 to 250 picomol, respectively), a parallel decrease (85% and 93%) in AR ligand binding was observed. The Western blot from lysate proteins (inset in Figure 3B), together with the densitometric analysis (Figure 3B) show that AR levels ACS Paragon Plus Environment
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decrease as siRNA AR concentration increases. Therefore, our assay is specific for AR expressed in LNCaP cells.
Figure 3. AR ligand binding displacement studies in LNCaP cells. A) LNCaP cells were transfected with siRNA nt (0 on x axis) or the indicated amounts of siRNA AR. Transfected cells were incubated with 10 nM [3H] R1881. Intracellular radioactivity was assayed and residual binding was calculated and expressed as % of total AR binding sites. Inset in B, shows the Western blot of AR in the different experimental conditions. The membrane was stripped and re-probed with anti tubulin antibody, as a loading control. B) Densitometric analysis of Western blot is shown. AR levels were normalized for tubulin amounts. Residual AR was expressed as % of the total level, which corresponds to the receptor level detected in LNCaP cells transfected with siRNA nt. C) Quiescent LNCaP cells were incubated with 10 nM [3H] R1881, in the absence or presence of the indicated excess (from 0 to 4 µM) of DR2 or unlabeled R1881, or bicalutamide, or enzalutamide. Intracellular radioactivity was assayed and AR binding sites x 10-3 cells were calculated. D)-F) LNCaP cells were transfected with nt (D), or AR siRNAs (125 pM or 250 pM in E and F, respectively). Cells were incubated with 10 nM [3H] R1881, in the absence or presence of the indicated excess (from 0 to 4 µM) of DR2 Intracellular radioactivity was assayed and AR binding sites x 10-3 cells were calculated. In D)-F), the different values on the y axis are due to the increase in siRNA AR concentration, and hence to the parallel decrease in AR binding sites. A)-F), data were collected from three different experiments. Means and SEM are shown.
The efficacy of DR2 in displacing the AR ligand binding activity was similar to that observed using unlabeled R1881, or bicalutamide, or enzalutamide, two non-steroidal antiandrogens largely used in PC treatment,7 in LNCaP cells (Figure 3C). Again, a very significant [3H] R1881 displacement was detected when the conjugate DR2 was used at 0,25 and 0,5 µM in LNCaP cells transfected with non-targeting siRNA (Figure 3D). By increasing the DR2 concentration (from 1 to 4 µM), an almost complete displacement of AR ACS Paragon Plus Environment
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ligand binding activity was detected (Figure 3D). Similar findings were observed by using unlabeled R1881 (at 1 µM). Bicalutamide or enzalutamide show a similar effect on AR ligand binding displacement when used at 1 µM (legend to Figure 3D). DR2 was still able to displace the [3H] R1881 binding in LNCaP cells depleted of different amounts of AR upon siRNA AR transfection (125 picomol and 250 picomol, respectively in Figure 3E and F). In sum, these findings show that DR2 specifically binds AR and is active in displacing the receptor ligand binding activity even when low AR amounts are expressed, as it often occurs in PC-associated fibroblasts.28 DR2 in vitro uptake and internalization studies in AR-expressing PC cells. We then evaluated the DR2 delivery/uptake in VCaP cells, which over-express the wild type AR.29 The extent of conjugate uptake over time was quantified by fluorescence-activated cell sorting (FACS). Pba displayed different behavior if administered in its free form or as DR2 conjugate. In particular, the uptake of free Pba was significant already after 3 h in VCaP [MF (mean fluorescence) = 188.89] (Figure 4A), while a longer time (6 h) was required for DR2 to reach a similar intracellular concentration [MF DR2 in VCaP = 206] (Figure 4B). Pba might passively translocate across cell membrane, while the more steric hindered DR2 conjugate likely engages a different route to enter the cell. Furthermore, confocal microscopy analysis in Figure 4 (D and E), demonstrates that DR2 exhibits a higher accumulation rate over time, as compared to Pba alone. This behavior might be due to its affinity for AR expressed in VCaP cells.
Figure 4. Uptake of Pba and DR2 on VCaP cells. A) - B) FACS analyses of VCaP cells treated with 1 µM of Pba and DR2. C) Values of Mean of Fluorescence (MF) at different time of photosensitizer incubation. Fluorescence was detected after 3, 6 and 24 h from compound incubation. Data were collected from three different experiments D) - E) High magnification (40X) single plane confocal images representative of fluorescence emission collected by VCaP cells after 6 h of incubation with 1µM Pba (D) and 1µM DR2 (E). Bar is 100 µm.
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DR2 in vitro uptake and internalization studies in malignant PC cells. Prostate cancer-derived PC3 cells are highly metastatic and androgen-insensitive as they express low levels of the AR, thus representing a valuable in vitro model of castration-resistant PC.30,31 In order to evaluate the ability of our conjugate to be internalized by these cells, we analyzed by FACS analysis the delivery/uptake of Pba and DR2 in PC3 cells. Data in Figure 5A show that as it occurs in VCaP cells (Figure 4A), the uptake of free Pba in PC3 cells is significant already after 3 h (MF 302.88), while 6 h were required for the uptake of DR2 (MF 170). Confocal microscopy images in Figure 5 (D and E) show that Pba and DR2 are both diffused in cytoplasm compartment after 6 h incubation. Interestingly, when comparing confocal microscopy images of DR2 in VCaP and PC3 cells after 6 h of incubation, we observed that while in PC3 cells DR2 is localized exclusively in the cytoplasm, in VCaP cells red fluorescence is intensively diffused also in the nucleus (cfr Figures 4E and 5E). This result might suggest that DR2 translocates into the nucleus upon
binding to the receptor, as also confirmed by the binding experiment performed with the radioligand assay (Figure 3). However, more studies are underway to verify the role of DR2 after the binding to the androgen receptor in terms of gene transcription activity.
Figure 5. Uptake of Pba and DR2 on PC3 cells. A) - C) FACS analyses of PC3 cells treated with 1 µM of Pba and DR2. C) Values of Mean of Fluorescence after different time of photosensitizer incubation. Fluorescence was detected after 3, 6 and 24 h from compound incubation. Data were collected from three independent experiments D) - E) High magnification (40X) single plane confocal images representative of fluorescence emission collected by PC3 cells after 6 h of incubation with 1µM Pba (D) and 1µM DR2 (E). Bar is 100 µm.
Despite DR2 internalization seems to be slightly slower in PC3 cells as compared to VCaP cells (cfr. Figure 4C and 5C), DR2 is persistently present after 24 h of incubation in PC3 cells (MF 422). In contrast, the amount of Pba is significantly reduced (MF 249) at that incubation time (Figure 5C), likely because of its efflux. This behavior draw our attention to the emerging problem of PDT resistance,32 and in particular to ACS Paragon Plus Environment
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Pba as substrate for the ABCG2 pump efflux.33–35 The ABCG2 gene encodes a membrane transporter belonging to the ATP-binding cassette (ABC) superfamily of membrane transporters, which being involved in the trafficking of a variety of biological molecules and drugs across the plasma membrane, also plays a role in multidrug resistance. To investigate more deeply this aspect and the possible beneficial role of conjugating the photosensitizer with specific targeting antenna, we performed an experiment with A549 human lung adenocarcinoma cells that express higher levels of ABCG2, as compared with PC3 cells (Figure 6A-B).
Figure 6. A) Western Blot of ABCG2 and β-actin protein levels in A549 and PC3 cell lines. B) The histograms report the values of the ABCG2 protein band intensity determined by densitometric analysis and normalized for β-actin. C) Functional assay for ABCG2 substrate. The A549 cells were incubated with 5 µM of Pba or DR2 for 30 min with or without 10 µM reserpine, washed, and then incubated for 60 min in PS-free media with (red line) or without (blu line) reserpine. Intracellular fluorescence was determined using a flow cytometer equipped with a 633 nm red diode laser. Representative results from two independent experiments are shown.
Robey and co-workers described a functional assay to evaluate if different photosensitizers are substrates for this efflux pump, using an ABCG2 inhibitor, e.g. reserpine.34,36,37 Therefore, in order to determine whether DR2 is a substrate for ABCG2 pump, we incubated A459 cells with 5µM of Pba or DR2 in the presence or absence 10 µM of reserpine for 30 min. Subsequently, the cells were washed and incubated for 1 h in a photosensitizer-free medium in the presence of reserpine. Graphs in Figure 6C report the fluorescence of Pba (left) and DR2 (right), alone (blue lines) or plus reserpine (red lines), respectively. Reserpine increases the accumulation of Pba in the PC3 cells, while no shift of the fluorescence peak is observed by using DR2, thus suggesting that DR2 is not a substrate for the efflux pump. In sum, data in Figures 4, 5 and 6 show that DR2 conjugate binds with high affinity AR and escapes the ABCG2 efflux pump in PC3 cells. These properties make DR2 a promising candidate for PDT in PC, since it
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is retained in PC cells expressing different levels of AR. In this latter setting, DR2 consistently accumulates within PC cells, preventing the onset of PDT resistance.32 In vitro photodynamic activity The in vitro photodynamic activity of DR2 was investigated against LNCaP, VCaP and PC3 cells. Twentyfour h after light irradiation, cell proliferation was determined by resazurin assay using free Pba as reference. Pba presents a Q band absorption peak at 670 nm allowing irradiation with a red light, while the o-CF3 nitrobenzene group (AR ligand moiety) should release NO upon selective irradiation at 400 nm. Therefore, in order to stimulate both compounds, cells were initially irradiated with a white halogen light covering the entire visible spectrum. According to the results of the uptake experiments, cells were incubated in the dark with increasing concentrations of Pba and DR2 for 6 h and subsequently irradiated with a metal white halogen lamp for 30 min (7 J/cm2). Data shown in Figure 7 (A-C) account for a higher photo-toxicity of DR2 in all cell lines as compared to Pba alone. Remarkably, the IC50 values (defined as the compound concentration to inhibit 50% of the cells) of DR2 were lower than Pba in all cell lines (Table 1), when using white light. This might be due to the longer retention of DR2 inside the cells and to the ability of the NO-donor moiety to induce the release of NO species that are synergistic to 1O2 generated by Pba. Importantly, no dark toxicity was observed for the conjugate 24 h after treatment (Figure S2). In order to evaluate the contribution of NO to the overall toxicity of the new conjugate, we performed a second set of metabolic assays using a halogen lamp equipped with a red filter, with the aim to excite only Pba. Results on PC3, LNCaP and VCaP cells are reported in Figure 7 (D-F) and Table 1, confirming that no difference occurs in the photo-toxic behavior of the two compounds when irradiated with red light, thereby supporting the additive action of the NO released only when white lamp is used as light source.
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Figure 7. %Metabolic activity of PC3, VCaP and LNCaP cells treated with increasing amounts of Pba and DR2. After 6 h incubation in the dark the cells were irradiated A-C) with a white halogen lamp (fluence 7.2 J/cm2) and D-F) with the same lamp equipped with a red filter (fluence 0.84 J/cm2). A resazurin assay was carried out 24 h after irradiation. Data represent mean values ± SD of three independent experiments. T-Test analysis of different treatments (Pba and DR2) vs NT a: p
