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Antibacterial Micelles with Vancomycin-Mediated Targeting and pH/lipase-Triggered Release of Antibiotics Maohua Chen, Songzhi Xie, Jiaojun Wei, Xiaojie Song, Zhenghua Ding, and Xiaohong Li ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b16092 • Publication Date (Web): 09 Oct 2018 Downloaded from http://pubs.acs.org on October 11, 2018

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ACS Applied Materials & Interfaces

Antibacterial Micelles with Vancomycin-Mediated Targeting and pH/lipase-Triggered Release of Antibiotics

Maohua Chen, Songzhi Xie, Jiaojun Wei, Xiaojie Song, Zhenghua Ding, Xiaohong Li* Key Laboratory of Advanced Technologies of Materials, Ministry of Education, School of Materials Science and Engineering, Southwest Jiaotong University, Chengdu 610031, P.R. China

*Corresponding author. E-mail: [email protected]. Tel: +8628-87634068, Fax: +8628-87634649.

KEYWORDS: Antibiotics-loaded micelle; Vancomycin-mediated targeting; Acid-liable deshielding; Lipase-sensitive release; In vivo antibacterial efficacy

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ABSTRACT: Antibiotic delivery systems play an important role in increasing the efficacy while reducing the off-target toxicity and antibiotic resistance. Though bacterial infections share pathophysiological pathways similar to tumor tissues, few delivery systems have achieved bacterial targeting and on-demand release of antibiotics. In the current study, amphiphilic poly(ethylene glycol)poly(ε-caprolactone) (PECL) copolymers are conjugated with vancomycin (VAN) as targeting ligands via pH-cleavable hydrazone bonds to obtain micelle carriers (Van-hyd-PECL). Subsequently, ciprofloxacin (CIP) is encapsulated to obtain Van-hyd-PECL/Cip micelles with an average size of 77 nm and a CIP loading amount of 4.5%. The poly(ethylene glycol) shells and the extension of VAN moieties on the micelle surface enhance the blood circulation and selective recognition of bacteria. The deshielding of VAN shells under acidic conditions disrupts the hydrophobic/hydrophilic balance leading to an increase in micelle sizes, which facilitates the degradation of poly(ε-caprolactone) by lipase overexpressed in the infection site and the release of encapsulated CIP for bacterial destruction. The micelle treatment has improved the survival of P. aeruginosa-infected mice and reduced the bacterial burdens and alveolar injuries in lungs, compared with free drugs and micelles without inoculation of VAN moieties. Three doses of Van-hyd-PECL/Cip micelles further extend the animal survival, decrease the bacterial colonization in lungs and almost restore the normal alveolar microstructure. In this regard, this study has demonstrated a strategy to enhance the bacterial targeting of micelles via an antibiotic (VAN) and to sequentially trigger the release of antibiotics (VAN and CIP) at the infection site.


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1. INTRODUCTION Bacterial infection is of great concern in public healthcare around the world and has caused various diseases such as sepsis, bacteremia, pneumonia, endocarditis, or even death.1,2 Antibiotics are regarded as the most efficient type of drugs to treat bacterial infection and have decreased the morbidity and mortality even saved lives of infected patients. However, the antibiotic treatment is lack of specificity and low bioavailability resulted from the quick metabolism and excretion by the circulating system before reaching the infection site. Therefore, frequent administration of high antibiotic doses is required to maintain therapeutic levels, causing severe side effects to liver and other normal tissues. In addition, the overuse of antibiotics leads to the appearance and prevalence of antibiotic resistance, which reduces the effectiveness of clinical antibacterial agents and has become a serious threat to public health security.3,4 Thus, the development of antibiotic delivery systems represents a practical strategy to increase the efficacy and lifespan of antibiotics while reducing the off-target toxicity and antibiotic resistance.5 Nanotechnology has significantly impacted on the outcome of cancer treatments and is showing impressive potential in the management of bacterial infection. It is well-known that nanoparticles can passively accumulated in tumor tissues via the enhanced permeation and retention (EPR) effect.6 The EPR phenomenon is not exclusive to solid tumors and also occurs at the infection and inflammation sites. Pathogenic bacteria and their secretions not only activate immune cells and inflammation mediators to enhance vascular permeability, but also cause lymphatic dysfunction.7 Similar to the tumor microenvironment, the infected tissues are characterized by low pH values as far as pH 5.5, due to the accumulation of lactic and acetic acids as a result of low oxygen-triggered anaerobic fermentation.8 As the first example of stimuli-responsive antibiotic release, Pornpattananangkul et al. attached chitosan-modified gold nanoparticles to the surface of liposomes to prevent the uncontrollable fusion of liposomes. The bound gold particles detached from liposomes at acidic pH values and the 3 ACS Paragon Plus Environment


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presence of bacterial toxins destroyed the liposome membranes to release antibiotics.9,10 In addition, the bacteria proliferation at the infection site secretes various enzymes, such as phosphatase, phospholipase, lipase and protease, which form a unique microenvironment in the infection area.11 Ji et al. coated hyaluronic acid on graphene-mesoporous silica with loaded ascorbic acid and ferromagnetic nanoparticles. Upon arrival at the infection site, hyaluronic acid was degraded by hyaluronidase that was secreted by bacteria, followed by the release and conversion of ascorbic acid into hydroxyl radicals to destruct the bacterial membranes.12 However, the integration of EPR effect, bacterial targeting and stimuli-responsive release have not yet been explored in the treatment of bacterial infections, despite sharing similar pathophysiological pathways to tumor tissues.7 Various nanoparticle platforms have been developed, including liposomes, silica, metal and polymeric nanoparticles. In particular, liposomes and lipid nanoparticles for amphotericin delivery, including Abelcet, Amphotec, AmBisome and Fungisome, have been approved for use in patients. Resulted from the self-assembly of amphiphilic copolymers, micelles

have manifested several

attractive features over other types of carriers, such as higher stability than liposomes and stronger responsiveness to stimuli than nanoparticles.13 The hydrophilic outer corona of a micelle is responsible for reducing the uptake by the reticuloendothelial system, prolonging the in vivo circulation duration, while the hydrophobic inner core is beneficial for a high loading capacity of poorly water-soluble antibiotics.14 Huang et al. designed poly(aspartic acid)-poly(ε-caprolactone) copolymer micelles with curcumin encapsulation and silver nanoparticle decoration. The cooperative antibacterial effects of silver nanoparticles and curcumin enhanced antibacterial activities toward P. aeruginosa and S. aureus .15 Though stimuli-responsive micelles have been investigated to enhance the efficiency of drug delivery to tumors, their use in antibacterial treatment is still very limited.16 Herein, bacterial targeting and pH/lipase-sensitive release are integrated in the design of antibioticloaded micelles that allow long systemic circulation and on-demand antibiotic release while alleviating 4 ACS Paragon Plus Environment

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the premature drug release in transit to infection sites. Figure 1a shows the synthetic process of micellar carriers.

Lipase-sensitive

poly(ε-caprolactone)

(PCL)

is

synthesized

through

ring-opening

polymerization of ε-caprolactone (ε-CL) initiated by poly(ethylene glycol) (PEG) with a pedant acetyl group, followed by conjugating vancomycin (VAN) as a targeting ligand via a pH-cleavable hydrazone bond to obtain micelle carriers (Van-hyd-PECL). Fluoroquinolone antibiotic ciprofloxacin (CIP) is encapsulated in the micelle cores through hydrophobic interactions. As shown in Figure 1b, Van-hydPECL/Cip micelles could effectively accumulate at the infection site via the EPR effect and selectively recognize bacteria. Once VAN shells are removed via the cleavage of hydrazone bonds under acidic conditions, the PCL cores are degraded by lipase that is overexpressed at the infection site, leading to in situ release of CIP for bacteria destruction. The first highlight of the carrier design is the initial use of a first-line antibiotic (VAN) as targeting moieties of micelles. Different ligands have been used to recognize bacteria for biosensing and detection,17 and few studies have include antibodies to target delivery of antibacterial agents.18 VAN can recognize both Gram-positive and Gram-negative bacteria.19 Another highlight is the strong adaptivity of micelle size and drug release at the infection site while maintaining the stability during the blood circulation. Once arriving at the infection site, the VAN layer is peeled off via acid-labile breakage of hydrazone bonds, resulting in the disruption of hydrophobic/hydrophilic balances. The increase in micellar sizes can facilitate the lipase-catalyzed degradation of PCL cores to promote the release of encapsulated antibiotics. Therefore, a novel micelle system is developed to achieve bacterial targeting via an antibiotic (VAN) and on-demand release of antibiotics (VAN and CIP) for combating bacterial infections. 2. EXPERIMENTAL SECTION Materials. VAN, CIP hydrochloride, ε-CL, (4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT), lipase from P. cepacian and dialysis membranes were obtained from Sigma−Aldrich (St. Louis, MO). E. coli Top10 and P. aeruginosa PA01 were procured from the Chinese Medical 5 ACS Paragon Plus Environment


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Culture Collection Center (Beijing, China). PEG (Mw: 2 and 3.5 kDa) and other chemicals were of analytical grade and received from Changzheng Regents Company (Chengdu, China), unless otherwise indicated. Fabrication and Characterization of CIP-Loaded Micelles. Copolymers of PEG and PCL (PECL) were synthesized though ring-opening polymerization of ε-CL using PEG (Mw: 2 and 3.5 kDa) as initiator to obtain PECL2k and PECL3.5k. Van-hyd-PECL was obtained by conjugation of VAN to PECL2k via a hydrazone linker. The synthesis procedures and characterizations are included in the Supporting Information. Van-hyd-PECL/Cip micelles were fabricated by a modified solvent evaporation method.20 Briefly, 20 mg of Van-hyd-PECL copolymers were dissolved in 5 mL of tetrahydrofuran, followed by adding dropwise into 10 mL of deionized water under vigorous stirring. After evaporation of tetrahydrofuran and filtration through syringe filters, Van-hyd-PECL micelles were obtained in a final concentration of 2 mg/mL. For the preparation of Van-hyd-PECL/Cip micelles, 1.0 mg of CIP hydrochloride and 10 mg of Van-hyd-PECL were added into 5 mL of tetrahydrofuran and water mixtures (v/v: 1/1), and the pH was adjusted to 7.4 using 0.1 M NaOH under vigorous stirring. The resulting mixture underwent dialysis against deionized water and filtration to remove free CIP. PECL2k/Cip and PECL3.5k/Cip micelles and Nile red-loaded micelles were prepared in similar ways. The micelle morphology was observed by a transmission electron microscope (TEM, JEOL JEM2100F, Japan) after staining with tungstic acid. The average size, polydispersity index (PDI) and zeta potential of micelles were determined by dynamic light scattering (DLS, Malvern Nano-ZS90, UK). The loading content and efficiency of CIP in micelles were determined by dissolution in methylene dichloride and extraction with deionized water as described previously.21 The CIP concentration was measured with an ultraviolet-visible spectrophotometer (Shimadzu UV-2550, Japan) at 277 nm. The drug loading amount indicated the amount (in milligrams) of CIP encapsulated per 100 mg of micelles, 6 ACS Paragon Plus Environment

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while the loading efficiency indicated the percentage of CIP encapsulated with respect to the total amount used for micelle preparation. The critical micelle concentrations (CMC) were determined by fluorescence spectroscopy using pyrene as a fluorescent probe.20 Briefly, the concentrations of the micelle carriers varied from 1 × 10−4 to 0.25 M and the final concentration of pyrene was fixed at 1.0 µM. The fluorescence intensity at the emission wavelength of 390 nm was recorded by a fluorescence spectrophotometer (Hitachi F-7000, Japan). The CMC value was estimated from the cross-point when extrapolating the fluorescence intensity ratios at the excitation wavelengths of 333 and 339 nm (I339/I333) at low and high concentration regions. In Vitro Stability and Sensitivity of Micelles. To study the sensitivities of Van-hyd-PECL micelles to acid and lipase, their size changes were measured and compared with PECL3.5k micelles. Briefly, the micelles were dissolved in phosphate-buffered saline (PBS) of pH 7.4 or 6.0 and incubated at 37 °C for over 15 h. Subsequently, lipase (1 mg/mL) was added into the buffers and incubated for 4 h at 37 °C.22 At the predetermined time, 1 mL of sample was withdrawn and the micelle sizes were measured by DLS. The in vitro drug release from Van-hyd-PECL/Cip, PECL2k/Cip and PECL3.5k/Cip micelles was determined after incubation in buffers of different pH values in the presence of lipase as described previously.23 Briefly, micelle suspensions were transferred into dialysis bags (1 kDa cutoff) and immersed into 30 mL of release media to achieve a sink condition, which were kept in a thermostated incubator at 37 °C. The tested media were PBS of pH 7.4 and 6.0 with or without lipase (1 mg/mL). At predetermined time intervals, 1 mL of media outside the dialysis bag was collected and 1 mL of release media was added back for continued incubation. The CIP concentration in the release media was detected as above. To determine the release of CIP from Van-hyd-PECL/Cip micelles in the presence 7 ACS Paragon Plus Environment


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of bacteria, micelles were incubated with bacteria suspensions in Luria−Bertani (LB) media as described previously.22 P. aeruginosa in the exponential growth phase with an optical density at 600 nm (OD600) of 0.2 or 1.0 were introduced in the release media, using E. coli suspensions and LB media without bacteria as control. The micelle suspensions in dialysis bags were immersed in the bacteria suspensions. After incubation at 37 °C for different time periods, 0.3 mL of the LB media were removed and filtrated with Millipore’s Amicon Ultra-0.5 centrifugal filters. The concentration of CIP in the filtrate was determined as above. In Vitro Bacterial Targeting of Micelles. The bacterial targeting was determined after incubation of P. aeruginosa with Nile red-loaded micelles as described previously with slight changes.19_23 Briefly, P. aeruginosa were harvested from LB media by centrifugation, washed three times with deionized water, and re-suspended in 10 mL of PBS at a concentration of 2.5 × 108 CFU/mL. Then 200 µL of micelles (2.0 mg/mL) was added into 300 µL of P. aeruginosa suspensions in pH 7.4 buffers, which were kept shaking at 150 rpm for 1 h at 37 °C. The suspensions were centrifuged and washed three times with buffers to remove unbound micelles, and bacteria were observed by a confocal laser scanning microscope (CLSM, Nikon A1+, Japan) at the exciting/emission wavelengths of 490/636 nm. For comparison, buffers containing 400 µg/mL of free VAN were used as control. In Vitro Antibacterial Testing of Free Drugs and Micelles. The antibacterial efficacy of micelles and free drugs was determined against P. aeruginosa using the serial dilution method as described previously.24 Briefly, 96-well tissue culture plates (TCPs) were seeded with 20 µL of P. aeruginosa suspensions in LB media at the concentration of 5 × 108 CFU/mL. CIP or VAN solutions (180 µL) were added into TCPs following serial dilutions by 2 folds with fresh LB media. After shaking at 150 rpm for 18 h at 37 °C, the absorption value of suspensions in each well was determined by using a microplate reader (Elx-800, Bio-Tek Instrument Inc., Winooski, VT) at 600 nm, using LB broth 8 ACS Paragon Plus Environment

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containing bacteria alone as negative control. The minimum inhibitory concentrations (MICs) were defined as the lowest concentration of an agent that inhibited the visible growth of bacteria after overnight incubation. The combined antibacterial effect of VAN and CIP was determined from the bacterial growth after incubation with VAN at 1/2, 3/4 and 1 fold of MICs and CTP at serial dilutions by 2 folds as described previously.25 The effect of the corresponding single drug at the same dose and the control growth in LB media were used to compare with the paired drugs. The OD600 value of bacterial suspensions after each treatment was determined as above and the antagonistic or synergistic effect of VAN and CIP was estimated from the synergic index as described previously.26 The inhibitory effect of Van-hyd-PECL/Cip micelles was determined after incubation of bacteria with different micelle concentrations for different time periods as described previously.27 Briefly, P. aeruginosa suspensions of 50 µL were added into 96-well TCPs at 1 × 106 CFU/mL per well. Bacteria were treated with Van-hyd-PECL/Cip micelles of serial CIP concentrations from 0 to 512 µg/mL for 12 h. In another batch of experiment, bacteria were treated with Van-hyd-PECL/Cip micelles of 512 µg/mL for up to 12 h. The OD600 values of bacterial suspensions were determined by using a microplate reader as above, using broth containing bacteria alone as control. After treatment for 8 h, bacteria were collected by centrifugation and then fixed with 4% glutaraldehyde for 4 h, followed by water washing and dehydration with serial ethanol solutions. The bacterial morphology was observed by a scanning electron microscope (SEM, FEI Quanta 200, The Netherlands) equipped with accelerating voltage (20 kV) and Robinson detector after 2 min of gold coating to minimize the charging effect. Cytotoxicity Testing of Micelles. The cytotoxicities of Van-hyd-PECL/Cip and PECL3.5k/Cip micelles were tested on 293T and RAW 264.7 cells. Briefly, 293T and RAW 264.7 cells were from the 9 ACS Paragon Plus Environment


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American Type Culture Collection (Rockville, MD) and maintained in DMEM medium containing 10% fetal bovine serum (FBS, Gibco BRL, Grand Island, NY) and 1% penicillin–streptomycin (Sigma, St. Louis, MO). To evaluate the cytotoxicities of Van-PECL/Cip micelles, cell suspensions (100 µL) were seeded into a 96-well TCP at a density of 3000 cells/well and incubated for 24 h, followed by treatment with Van-hyd-PECL/Cip micelles of up to 1024 µg/mL. After incubation for another 24 h, the cell viabilities were determined by using MTT assay as described previously.28 The hemolysis effect of micelles was determined after incubation with red blood cells (RBCs) as described previously with some modifications.15 Briefly, fresh blood was collected from rats and RBCs were harvested by centrifugation, followed by PBS wash for several times to ensure no hemoglobin release. After re-suspension in cold PBS at a concentration of 5% (v/v), 0.5 mL of RBC suspensions were mixed with 0.5 mL micelles of up to 1024 µg/mL, using treatment with deionized water and PBS as positive and negative controls, respectively. After incubation at 37 °C for 60 min, the mixtures were centrifuged at 4000 rpm for 5 min, and the absorbance of the supernatant was measured using a microplate reader at 570 nm. The hemolysis rate was calculated from the absorbance difference between samples with negative control, in comparison with that between positive and negative control. In Vivo Antibacterial Efficacy of Micelles. The in vivo antibacterial efficacy of Van-hyd-PECL/Cip micelles was evaluated on the P. aeruginosa infection model. Briefly, 6−8-week-old female BALB/c mice (18–22 g) were received from Sichuan Dashuo Biotech Inc. (Chengdu, China), and all animal procedures were approved by the University Animal Care and Use Committee. The infection model was established by intraperitoneal injection of 1 × 105P. aeruginosa cells into the enterocoelia as described previously.27 After 4 h of incubation, mice were treated with micelles and free drugs by intravenous injection through the tail vein, using PBS injection as control. Mice were treated with a single dose of Van-hyd-PECL/Cip micelles (1.0, 3.0 and 5.0 mg/Kg), PECL3.5k/Cip (5.0 mg/Kg), CIP (0.22 mg/Kg) and mixture of CIP (0.22 mg/Kg) and VAN (1.0 mg/Kg). In another batch of experiment, 10 ACS Paragon Plus Environment

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mice were treated with 2 (0 and 12 h) and 3 doses (0, 12, and 24 h) of Van-hyd-PECL/Cip micelles (1.0 mg/Kg).24 The survival rate of mice was monitored every day over a 14-day treatment period, and plotted in Kaplan−Meier survival curves. The lungs from sacrificed mice were washed with PBS and processed for bacterial counting and hematoxylin and eosin (H&E) staining. Briefly, the lungs were fixed in 4% glutaraldehyde for 24 h at 4 °C, and dehydrated by serial ethanol solutions. After routinely embedded into paraffin blocks and cut into 5 µm-thick sections, tissue sections were stained with H&E and observed under a microscope (Nikon Eclipse E400, Japan). In addition, another part of lungs were homogenized and the dilutions were spread onto LB agar plates for calculating CFUs of P. aeruginosa as described previously.29 Statistical Analysis. The data were expressed as mean ± standard deviation (SD). Whenever appropriate, comparisons among multiple groups were achieved using analysis of variance (ANOVA), while a two-tailed Student’s t-test was used to discern the statistical difference between two groups. Probability values (p) of less than 0.05 were used to show statistically significant. 3. RESULTS AND DISCUSSION Characterization of Van-hyd-PECL Carriers. A novel amphiphilic copolymer of Van-hyd-PECL was synthesized by conjugation of PECL with Van-hyd to have active targeting ability to bacteria and sensitivities to pH and enzyme signals at the infection site. The 1H NMR spectrum of acetyl-terminated PEG showed characteristic peaks of methylene adjacent to the ester and methyl of 4-acetylbutyric acid at 4.2 and 2.1 ppm, respectively (Figure S1a). The integral ratio of the two signals was 0.67, indicating that one of the hydroxyl groups of PEG formed an ester bond with 4-acetylbutyric acid. Amphiphilic polymer PECL2k was prepared by ring-opening polymerization of ε-CL using PEG as initiator. The average polymerization degree of PCL segment was approximately 50, as estimated from the integral 11 ACS Paragon Plus Environment


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ratio of peaks at 2.13 (methyl of 4-acetylbutyirc acid) and 4.06 ppm (methylene of PCL) (Figure S1b). Similarly, PECL3.5k indicated the average polymerization degree of 48 (Figure S2). Targeting ligand VAN was conjugated to PECL2k via a hydrazone linker, which was proved by the characteristic multiplets at 6.2−7.8 (benzene protons) and 0.7−1.4 ppm (methyl protons of VAN) in the 1H NMR spectrum of Van-hyd-PECL (Figure S3b). Furthermore, the sequential synthesis was also verified by GPC analysis, showing the weight-average molecular weights of 5680, 8080 and 6720 for PECL2k, PECL3.5k and Van-hyd-PECL , respectively (Figure S4). Characterization of Van-hyd-PECL/Cip Micelles. Amphiphilic Van-hyd-PECL copolymers could self-assemble into micelles, whose shells and cores were formed by the hydrophilic (VAN and PEG) and hydrophobic segments (PCL). Figure 2a shows a typical TEM image of Van-hyd-PECL/Cip micelles, indicating a well-defined spherical shape. Table 1 summarizes the properties of drug-loaded Van-hyd-PECL, PECL2k and PECL3.5k micelles. The DLS analysis indicated an average size of Vanhyd-PECL/Cip micelles at around 77 nm with a PDI of 0.1 (Figure S5a). The smaller micelle size of around 30 nm was observed in TEM, due to the shrinkage of hydrophilic shells after the drying process.

It was indicated that the molecular weight of amphiphilic copolymers and the ratio of

hydrophilic and hydrophobic segments affected the micelle properties.30 Van-hyd-PECL copolymers were synthesized from PECL2k, and the shorter hydrophilic segments of PECL2k led to a larger micelle size at around 94 nm. Van-hyd-PECL and PECL3.5k had the similar ratio of hydrophilic and hydrophobic segments and were self-assembled into micelles with similar sizes (Table 1). It should be noted that all the micelle sizes were inferior to 100 nm for an effective penetration into tissues.30 Figure S5b shows the plots of fluorescence intensity ratios (I339/I333) of pyrene versus polymer concentrations. The CMC values of Van-hyd-PECL and PECL3.5k were 1.02 and 1.23 µg/mL respectively, which were larger than that of PECL2k (0.81 µg/mL). The increase in length of hydrophobic blocks usually led to a decrease in CMC values.31 The low CMCs were beneficial for 12 ACS Paragon Plus Environment

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micellization and micellar stability in the blood circulation.30 As shown in Table 1, the drug loading content (DLC) and encapsulation efficiency (DEE) of CIP-loaded micelles were around 4.5% and 35%, respectively. pH and Lipase-Sensitivities of Van-hyd-PECL/Cip Micelles. The pH and lipase-sensitivities of Van-hyd-PECL/Cip micelles were determined from the size change and drug release and compared with those of PECL3.5k and PECL2k micelles. Figure 2b show the size changes of micelles after incubation at different pH values (7.4 or 6.0) and in the presence of lipase. There was no significant size change of Van-hyd-PECL and PECL3.5k micelles after incubation for 15 h in pH 7.4 buffers. Vanhyd-PECL micelles increased rapidly from around 73 to 91 nm during 6 h of incubation at pH 6.0, while PECL3.5k micelles showed a slight fluctuation in sizes. Under acidic conditions, the hydrophilic VAN molecules were removed from Van-hyd-PECL carriers via hydrazone cleavage, which disturbed the hydrophilic/hydrophobic balance and led to the changes in micelle sizes. After incubation for 15 h in pH 6.0 buffer, the size of Van-hyd-PECL micelles increased to around 96 nm, close to that of PECL2k (Table 1). As the hydrophobic PCL was susceptible to enzymatic degradation by lipase,32 variations in micelle size were analyzed in lipase-containing culture media. Both Van-hyd-PECL and PECL3.5k micelles had dramatic size decreases after incubation at pH 7.4 or 6.0 in the presence of lipase (Figure 2b). The hydrophilic/hydrophobic balance in the backbone of amphiphilic copolymers was destroyed by PCL degradation leading to the gradual collapse of micelles. Figure 2c summarizes the accumulated release of CIP from Van-hyd-PECL micelles in response to pH and lipase. To analyze the drug release kinetics, the results of CIP release profiles were plotted in a first order model (Figure S6).33 Approximately 29% of CIP was released after 60 h of incubation at pH 6.0, only 20% under physiological conditions. In comparison, the lipase incubation led to remarkable increases in the CIP release due to the catalyzed degradation of PCL cores (Figure S6a). After incubation with lipase for 12 h, the cumulative release amount of CIP at pH 7.4 and 6.0 was 55% and 13 ACS Paragon Plus Environment


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78%, respectively. This finding indicated that the deshielding of VAN shells under acidic conditions could accelerate the lipase-catalyzed degradation of PCL cores. In addition, the release rate of CIP was slower in the presence of lipase at a concentration of 0.5 mg/mL than that at 1 mg/mL, suggesting the dependence of the release rate on lipase concentrations. Figure 2d shows the CIP release profiles from PECL2k micelles, similar to those from Van-hyd-PECL micelles under lipase inoculation at pH 6.0 (Figure S6b). This could be attributed to that Van-hyd-PECL micelles underwent acid-triggered cleavage of hydrazone linkages and subsequent destruction of the hydrophilic/hydrophobic balance to form micelles similar to PECL2k micelles. Figure 2e summarizes the CIP release from PECL3.5k micelles in response to pH and lipase. After incubation at pH 7.4 or 6.0 for 60 h, less than 20% of CIP was released from PECL3.5k micelles. The addition of lipase enhanced the amount of released CIP to around 39% and 56% after 12 h of incubation at pH 7.4 and 6.0. In addition, higher CIP releases were detected from PECL3.5k micelles after incubation at pH 6.0 than those at pH 7.4 in the presence of lipase (Figure S6c). The pKa value of CIP was 6.1,34 and the lower water solubility at pH 7.4 led to a slower release from micelles. To further evaluate the responsive release of CIP from Van-hyd-PECL/Cip micelles, the release study was performed in bacteria-containing media. P. aeruginosa PA01 has strong capabilities of lipase secretions since these enzymes correlate with bacterial lipid metabolism,22 while E. coli Top10 is low lipase-secreting bacteria.35 As shown in Figure 2f, around 19% and 44% of CIP were released from Van-hyd-PECL/Cip micelles after 1 h of incubation in P. aeruginosa suspensions with OD600 values of 0.2 and 1.0, respectively. The cumulative release of CIP was over 80% after 24 h of incubation in both cases. It was indicated that the higher bacterial concentration and longer incubation time had a beneficial effect on the CIP release (Figure S6d). However, only 4.8% and 9.3% of total CIP releases were detected after incubation for 1 h with LB media or lipase-negative bacteria E. coli at an OD600 value of 1.0, respectively. It was indicated that Van-hyd-PECL micelles could be destructed by lipase14 ACS Paragon Plus Environment

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secreting bacteria. The size changes and drug release results revealed that the micelles had a great potential to reduce the premature drug release in the blood circulation while facilitating the on-demand release of antibiotics at the infection site. Bacterial Targeting of Van-hyd-PECL Micelles. Hydrophobic Nile red was encapsulated into micelles as a fluorescent probe to visualize the interactions between micelles and bacteria using CLSM. As shown in Figure 3a, the complete bacteria morphology was overlaid with strong corresponding red fluorescence after incubation with Van-hyd-PECL micelles. To further prove that the binding affinity was attributed to VAN, PECL3.5k micelles were used as control, while only weak fluorescence signals were observed. The addition of free VAN blocked the interactions with bacteria, resulting in lower cellular uptake of Van-hyd-PECL micelles. These results indicated that Van-hyd-PECL micelles could capture P. aeruginosa via VAN mediation. VAN was a well-known glycopeptide antibiotic and could recognize the peptidoglycan terminus D-Ala-D-Ala on the cell wall of Gram-positive bacteria, inhibiting the synthesis of bacterial walls.36 In addition, VAN was able to capture Gram-negative bacteria, due to either the unspecific binding with receptors on the pathogen surface or the break/deformity in the outer membrane exposing D-Ala-D-Ala groups on the interior surface.19 Though VAN offered lower specificity/selectivity than monoclonal antibodies for bacterial targeting, the orientation of VAN moieties on the surface and the exact functional groups that chemically bound with micelles were far easier to control. It was indicated that the extension of VAN moieties from the nanoparticle surface was essential for the capture of bacteria.19 Thus, in the current study, hydrophilic PEG segments were also act as spacers for VAN conjugation to enhance the consistency of bacterial capture. Antibacterial Activity of Van-hyd-PECL/Cip Micelles. In the current study, VAN and CIP were gradually released from Van-hyd-PECL/Cip micelles. VAN is a glycopeptide antibiotic to inhibit the biosynthesis of peptidoglycan in the bacterial wall,37 while CIP is a quinolone antibiotic to inhibit DNA 15 ACS Paragon Plus Environment


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synthesis in bacteria by binding to the DNA-modifying enzymes.38 Figure S7a shows the bacterial growth after treatment with different concentrations of VAN and CIP. The MIC was an indicator of non-detected growth of bacteria, and those of CIP and VAN were determined to be around 1 and over 400 µg/mL, respectively. To evaluate the interactions between VAN and CIP in the antibacterial effect, a synergy testing was performed under sub-inhibitory concentrations of antibiotics as described previously.39 Figure S7b summarizes the bacteria growth after treatment with serial CIP concentrations and the Van concentrations were set as 0, 200, 300 and 400 µg/mL. To reflect the drug interactions in the antibacterial activities, the synergic index was statistically calculated, where synergistic, additive and antagonistic interactions were indicated when the value fell in the range of -1/-0.5, -0.5/0.5, and 0.5/2, respectively.26 The synergic index of VAN and CIP was determined to be 0.34, indicating the additive antibacterial effect of VAN and CIP and no synergistic or antagonistic interactions between them. Figure 3b showed the antibacterial effect of Van-hyd-PECL/Cip and PECL3.5k/Cip micelles under different concentrations, in comparison with free CIP and blank micelles of Van-hyd-PECL and PECL3.5k. Both of the micelle carriers indicated no antibacterial effect. Compared with free CIP with an MIC of 0.8 µg/mL, Van-hyd-PECL/Cip and PECL3.5k/Cip micelles showed less significant growth inhibition, due to the gradual release of CIP from micelles. The VAN-mediated targeting enhanced the inhibition of bacterial growth, as the MICs of Van-hyd-PECL/Cip and PECL3.5k/Cip micelles were around 2 and 4 µg/mL, respectively. Figure 3c shows the antibacterial activities of Van-hyd-PECL/Cip and PECL3.5k/Cip micelles after incubation for different time periods. The free CIP treatment exhibited lower antibacterial efficacy after incubation for 4 h, while the sustained release of CIP from micelles led to persistent inhibition of bacterial growth during 12 h. The pH and lipase liability of Van-hydPECL/Cip micelles led to higher antibacterial abilities than that of PECL3.5k/Cip. To further explore the antimicrobial mechanism of CIP-loaded micelles, the morphological changes of P. aeruginosa were 16 ACS Paragon Plus Environment

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observed by SEM. As shown in Figure 3d, the pristine bacteria exhibited smooth surfaces with intact membrane structures, whereas treatment of CIP-loaded micelles resulted in distorted and corrugated surfaces of P. aeruginosa. Compared with PECL3.5k/Cip micelles, Van-hyd-PECL/Cip micelles caused higher surface roughness with fracturing and breaking appearance. Cytotoxicity and Hemolysis of Van-hyd-PECL/Cip Micelles. The biocompatibility of antibacterial nanocarriers was a major concern for in vivo administration.40 Figure 4a shows the viabilities of RAW 267.4 and 293T cells after incubation with Van-hyd-PECL/Cip and PECL3.5k/Cip. Both cells exhibited only a slight decrease in viability with increasing micelle concentrations and over 80% of cells remained viable at a concentration of 512 µg/mL (p > 0.05), suggesting negligible toxicities of both micelles to mammalian cells. In addition, the hemolytic behavior of these micelles was examined, as the blood compatibility was the first barrier for in vivo applications. As presented in Figure 4b, there were no obvious hemolytic activity for Van-hyd-PECL/Cip even at the highest concentration tested (1024 µg/mL), indicating that the micelles possessed good biocompatibility with RBCs. Therefore, Van-hyd-PECL/Cip micelles held the potential for in vivo treatment of bacterial infections. Survival of Bacteria-Infected Mice after Treatment with Micelles. Currently the antibiotic-loaded micelles are often tested in vitro with respect to drug release and antibacterial activity, while only few of them have been evaluated in an infection model in vivo. Chu et al. developed charge-adaptive micelles to prolong circulation time in blood and enhance their accumulation in infected tissues via acid-sensitive charge conversion. CLSM observations showed higher drug accumulations at the infection site compared to free drugs in a subcutaneous infection model by S. aureus.16 While the drug development against Gram-positive infections has achieved modest success, current therapeutic options for Gram-negative bacteria are quite limited.41 In the current study, the antibacterial activity of Vanhyd-PECL/Cip micelles was evaluated with an infection mouse model upon intraperitoneal injection of Gram-negative P. aeruginosa.27 The symptoms of P. aeruginosa infections were generalized by 17 ACS Paragon Plus Environment


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inflammation, sepsis, cell death or organ dysfunction. The in vivo therapeutic effect of micelles was examined in terms of animal survival, histology and bacterial counting in tissues after different treatments. Figure 5a summarizes the survival rates of P. aeruginosa-infected mice during two weeks of treatment. No mice survived after 2 days of PBS treatment, whereas the lethality rate of mice was remarkably decreased after treatment with CIP-loaded micelles. P. aeruginosa had two concentric membrane structures surrounding the degradative periplasmic compartment studded with efflux pumps, which became a formidable barrier for antibacterial agents. Thus, the free CIP treatment showed no beneficial effects on the survival of mice and all died after 5 days. The single dose of Van-hydPECL/Cip micelles extended the mouse survival and the mortality rate was in a dose-dependent manner. At the end of the monitoring period, treatment with Van-hyd-PECL/Cip micelles at a dosage of 5.0 mg/kg led to two fold higher survival rate (60%) than that at 3.0 mg/kg (28%). Treatment with PECL3.5k/Cip micelles showed a lower survival rate of 42%, due to the absence of VAN targeting ligands. It was suggested that bacteria targeting offered a means to specifically localize micelles to bacteria, either by ligand-mediated binding and/or other physical interactions with membranes.29 The therapeutic effect was also determined after injection of multiple doses of Van-hyd-PECL/Cip micelles at a lower concentration. As shown in Figure 5a, the survival rate was around 14% after 5 days of treatment with the single dose (1 mg/Kg) of Van-hyd-PECL/Cip micelles. The treatment with 2 doses obviously increased the survival efficiency up to 43% and a whopping 83% survival rate after 3 doses (p < 0.05). It was indicated that 3 doses of Van-hyd-PECL/Cip micelles (1.0 mg/Kg each) could prolong the mouse survival compared to a single dose of 3.0 mg/Kg. Antibacterial Activity of Van-hyd-PECL/Cip Micelles in Vivo. P. aeruginosa is a common opportunistic bacterial pathogen of human and is known to cause pulmonary tract infections, thus the bacteria contents were detected by agar plate counting after homogenization of lung tissues.42 Figure S8 shows the images of agar plates for bacterial counting, and Figure 5b summarizes the bacteria 18 ACS Paragon Plus Environment

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counts in lungs after different treatments. Bacterial colonization counting of untreated mice revealed the existence of around 6.3 × 105 CFU per lung. The treatment with free CIP and VAN-CIP mixtures showed apparent decreases to around 4.1 × 104 and 3.7 × 104 CFU per lung, respectively (p < 0.05). The bacterial burden was significantly reduced after treatment with CIP-loaded micelles compared with free drugs. After administration with the single dose of Van-hyd-PECL/Cip micelles, the bacterial counts in lung tissues were significantly reduced by around 3.1-log10 folds compared to that of free VAN-CIP, which was more effective than PECL3.5k/Cip micelles at around 2.6-log10 folds of reduction compared to free CIP treatment (p < 0.05). These results demonstrated the strength of VAN-mediated bacterial targeting and stimuli-responsive release of antibiotics for the treatment of bacterial infections. In addition, the therapeutic effect of micelles was also depended on the treatment dose and schedule. When treated with triple doses of Van-hyd-PECL/Cip micelles at 1.0 mg/kg of each dose, the number of bacteria was decreased dramatically to around 1.2 × 102 CFU per lung (p < 0.05), compared with the single dose of 1.0 (2.1× 103 CFU per lung) and 3.0 mg/kg (1.5 × 103 CFU per lung). To further confirm the antibacterial efficacy, the lungs of infected mice were stained by H&E after treatment with micelles, using normal mice without infection as controls. As illuminated in Figure 5c, no inflammation and no alteration of the microstructure of alveolar or blood vessels were present in normal mice. The lung sections of infected mice after PBS treatment indicated heavy infiltrations of inflammatory cells, thickened alveolar interstitium and patchy hemorrhages. Bacteria-infected mice after treatment with Van-hyd-PECL/Cip micelles showed varying degrees of lung damage, accompanied with a few hemorrhage. According to the areas of infiltration and pulmonary interstitial edema, the injury degree of the alveolar space and capillary walls were significantly attenuated after treatment with CIP-loading micelles compared to free CIP and VAN-CIP mixtures, demonstrating that micelles could effectively prolong the circulation time and promote the accumulation of CIP in the infection site. In addition, the lungs of infected mice after treatment with Van-hyd-PECL/Cip micelles 19 ACS Paragon Plus Environment


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showed a dose-dependent recovery. The alveolar microstructure was almost normal after treatment with triple doses of Van-hyd-PECL/Cip micelles. It was suggested that multiple doses of CIP-loaded micelles could further improve the treatment efficacy and prolong the mouse survival. 4. CONCLUSIONS Van-hyd-PECL/Cip micelles are demonstrated to enable the VAN-mediated bacterial targeting as well as the CIP release in response to pH and lipase signals at the infection site. These micelles shows no apparent change in size and minimal drug release under physiological conditions, whereas the deshielding of VAN shells in response to acidic signals leads to an increase in size and thus accelerates the lipase-catalyzed destruction of micelles and the release of CIP. Compared with PECL3.5k/Cip micelles, the presence of VAN moieties enhances the bacterial capture and growth inhibition. The sustained release of CIP from micelles exhibits persistent inhibitions compared with free drug treatment. The Van-hyd-PECL/Cip micelle treatment results in higher survival of P. aeruginosainfected mice, fewer bacterial burdens and lower alveolar injuries in lungs, compared with PECL3.5k/Cip micelles and free drugs. Triple doses of Van-hyd-PECL/Cip micelles further extend the animal survival, decrease the bacterial colonization in lungs and almost restore the normal alveolar microstructure. Thus, Van-hyd-PECL/Cip micelles provide a feasible strategy to realize both antibioticmediated bacteria targeting and antibiotic release in response to the infection microenvironment. Supporting Information available: The synthesis and characterization of Van-hyd-PECL, the DLS analysis and CMC determination of micelles, the CIP release kinetics, the bacteria killing effect of VAN, CIP and VAN-CIP mixtures, and the bacterial counting of lung homogenates are included in the Supporting Information ACKNOWLEDGMENTS


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This work was supported by National Natural Science Foundation of China (31771034 and 31470922), the Key Research and Development Program of Sichuan Province (2018SZ0348) and Doctoral Innovation Fund Program of Southwest Jiaotong University. REFERENCES (1)

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Table 1 Characterization of PECL3.5k, PECL2k and Van-hyd-PECL micelles.

Sample

CMC

Size

(mg/mL)

(nm)

PECL3.5k

0.81

69.4

PECL2k

1.23

Van-hyd-PECL

1.02

PDI

Zeta

DLC

DEE

(mV)

(wt%)

(wt%)

0.128

-2.99

4.3

35.4

94.3

0.161

-0.39

4.8

37.3

76.9

0.104

+3.79

4.4

34.7



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Figure Legends: Figure 1. (a) Synthesis procedures of Van-hyd-PECL amphiphilic copolymers. Van-hyd-PECL is obtained by conjugation of CH3-CO-PECL with Van-hyd. CH3-CO-PECL is prepared by ring-opening polymerization of ε-CL using acetyl-terminated PEG (CH3-CO-PEG) as initiator. Van-hyd is synthesized by the reaction of carboxyl group of VAN with hydrazine. (b) Micelle formation, bacterial targeting and drug release from Van-hyd-PECL micelles. After VAN-mediated bacterial targeting, the VAN shell is removed from micelles via the cleavage of hydrazone bonds under acidic conditions, and the PCL core is degraded by lipase overexpressed at the infection site, followed by CIP release and bacterial destruction. Figure 2. (a) Typical TEM images of CIP-loaded PECL3.5k, PECL2k and Van-hyd-PECL micelles. (b) Size changes of PECL3.5k and Van-hyd-PECL micelles in pH 7.4 and 6.0 buffers in the presence of lipase (1.0 mg/mL) (n = 3). (c) Percent release of CIP from Van-hyd-PECL, (d) PECL2k and (e) PECL3.5k micelles after incubation at 37 °C in pH 7.4 or 6.0 buffers in the absence or presence of lipase at 0.5 or 1.0 mg/mL (n = 3). (f) Percent release of CIP from Van-hyd-PECL micelles after incubation with P. aeruginosa suspensions with OD600 value of 0.2 or 1.0, using E. coli (OD600: 1.0) and LB media without bacteria as control (n = 5; *: p < 0.05). Figure 3. In vitro bacterial targeting and antibacterial activity of micelles. (a) CLSM images of P. aeruginosa bacteria after incubation with Nile red-loaded PECL3.5k and Van-hyd-PECL micelles, using buffers containing free VAN as control; The merged CLSM and bright field images show the interactions between micelles and bacteria. (b) Bacterial viability after incubation for 12 h with CIP, PECL3.5k, Van-hyd-PECL, PECL3.5k/Cip and Van-hyd-PECL/Cip micelles at different CIP concentrations (n = 5; *: p < 0.05 compared with PECL3.5k and Van-hyd-PECL; #: p < 0.05 compared with CIP). (c) Bacterial viability after incubation with CIP, PECL3.5k/Cip and Van-hyd-PECL/Cip 28 ACS Paragon Plus Environment

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micelles at a CIP concentration of 512 µg/mL for different time periods (n = 5; *: p < 0.05 compared with other groups). (d) Typical SEM images of P. aeruginosa after treatment for 8 h with CIP, PECL3.5k/Cip and Van-hyd-PECL/Cip micelles at the CIP concentration of 512 µg/mL, using PBS treatment as control. Figure 4. Cytotoxicity of CIP-loaded micelles. (a) Cytotoxicity of PECL3.5k/Cip and Van-hydPECL/Cip micelles against 3T3 and RAW 264.7 cells after incubation for 24 h at different concentrations (n = 5; N.S.: p > 0.05). (b) Hemolysis assays of PECL3.5k/Cip and Van-hyd-PECL/Cip micelles after incubation for 60 min with RBCs in PBS, using deionized water and PBS as positive and negative controls, respectively (n = 5; *: p < 0.05; #: p < 0.05 compared with other groups). Figure 5. In vivo antibacterial efficacy of micelles against P. aeruginosa-infected mice. (a) Survival curves of infected mice after treatment with Van-hyd-PECL/Cip micelles at a single dose of 1.0, 3.0 and 5.0 mg/kg, 2 and 3 doses of 1.0 mg/kg, using a single dose of PECL3.5k/Cip (5 mg/Kg), free CIP, VAN-CIP mixtures, and PBS treatment as control (n = 6; *: p < 0.05 compared with other groups). (b) Bacterial counts of lungs retrieved from infected mice after different treatment (n = 6). (c) Typical H&E staining images of lung sections from infected mice after different treatment, compared with normal mice without infection.



Figure 1. (a) Synthesis procedures of Van-hyd-PECL amphiphilic copolymers. Van-hyd-PECL is obtained by conjugation of CH3-CO-PECL with Van-hyd. CH3-CO-PECL is prepared by ring-opening polymerization of εCL using acetyl-terminated PEG (CH3-CO-PEG) as initiator. Van-hyd is synthesized by the reaction of carboxyl group of VAN with hydrazine. (b) Micelle formation, bacterial targeting and drug release from Vanhyd-PECL micelles. After VAN-mediated bacterial targeting, the VAN shell is removed from micelles via the cleavage of hydrazone bonds under acidic conditions, and the PCL core is degraded by lipase overexpressed at the infection site, followed by CIP release and bacterial destruction. 98x81mm (300 x 300 DPI)

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Figure 2. (a) Typical TEM images of CIP-loaded PECL3.5k, PECL2k and Van-hyd-PECL micelles. (b) Size changes of PECL3.5k and Van-hyd-PECL micelles in pH 7.4 and 6.0 buffers in the presence of lipase (1.0 mg/mL) (n = 3). (c) Percent release of CIP from Van-hyd-PECL, (d) PECL2k and (e) PECL3.5k micelles after incubation at 37 °C in pH 7.4 or 6.0 buffers in the absence or presence of lipase at 0.5 or 1.0 mg/mL (n = 3). (f) Percent release of CIP from Van-hyd-PECL micelles after incubation with P. aeruginosa suspensions with OD600 value of 0.2 or 1.0, using E. coli (OD600: 1.0) and LB media without bacteria as control (n = 5; *: p < 0.05). 120x96mm (300 x 300 DPI)



Figure 3. In vitro bacterial targeting and antibacterial activity of micelles. (a) CLSM images of P. aeruginosa bacteria after incubation with Nile red-loaded PECL3.5k and Van-hyd-PECL micelles, using buffers containing free VAN as control; The merged CLSM and bright field images show the interactions between micelles and bacteria. (b) Bacterial viability after incubation for 12 h with CIP, PECL3.5k, Van-hyd-PECL, PECL3.5k/Cip and Van-hyd-PECL/Cip micelles at different CIP concentrations (n = 5; *: p < 0.05 compared with PECL3.5k and Van-hyd-PECL; #: p < 0.05 compared with CIP). (c) Bacterial viability after incubation with CIP, PECL3.5k/Cip and Van-hyd-PECL/Cip micelles at a CIP concentration of 512 µg/mL for different time periods (n = 5; *: p < 0.05 compared with other groups). (d) Typical SEM images of P. aeruginosa after treatment for 8 h with CIP, PECL3.5k/Cip and Van-hyd-PECL/Cip micelles at the CIP concentration of 512 µg/mL, using PBS treatment as control. 164x179mm (300 x 300 DPI)


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Figure 4. Cytotoxicity of CIP-loaded micelles. (a) Cytotoxicity of PECL3.5k/Cip and Van-hyd-PECL/Cip micelles against 3T3 and RAW 264.7 cells after incubation for 24 h at different concentrations (n = 5; N.S.: p > 0.05). (b) Hemolysis assays of PECL3.5k/Cip and Van-hyd-PECL/Cip micelles after incubation for 60 min with RBCs in PBS, using deionized water and PBS as positive and negative controls, respectively (n = 5; *: p < 0.05; #: p < 0.05 compared with other groups). 163x333mm (600 x 600 DPI)



Figure 5. In vivo antibacterial efficacy of micelles against P. aeruginosa-infected mice. (a) Survival curves of infected mice after treatment with Van-hyd-PECL/Cip micelles at a single dose of 1.0, 3.0 and 5.0 mg/kg, 2 and 3 doses of 1.0 mg/kg, using a single dose of PECL3.5k/Cip (5 mg/Kg), free CIP, VAN-CIP mixtures, and PBS treatment as control (n = 6; *: p < 0.05 compared with other groups). (b) Bacterial counts of lungs retrieved from infected mice after different treatment (n = 6). (c) Typical H&E staining images of lung sections from infected mice after different treatment, compared with normal mice without infection. 92x57mm (300 x 300 DPI)


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TOC graphic 36x16mm (600 x 600 DPI)


Antibacterial Micelles with Vancomycin-Mediated ... - ACS Publications

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