Design of Pyrene-Fatty Acid Conjugates for Real-Time Monitoring of Drug Delivery and Controllability of Drug Release.

Fluorescence probes are usually employed to analyze pharmacokinetics of drug carriers; however, this method using usual probes is not suitable to monitor drug carriers in detail because fluorescence spectra do not change by the disruption of drug carriers. In this study, pyrene-fatty acid conjugates were investigated as probes to monitor the state of drug carriers in real time. 1-Pyrenemethanol was conjugated with fatty acids, such as lauric acid, stearic acid, and behenic acid, and the conjugates were stirred in ethanol, resulting in the formation of submicron particles; these particles exhibited excimer emission. When J774.1 and Colon 26 cells were treated with these particles, the associated fluorescence spectra shifted from excimer emission to monomer emission. Moreover, the degree of change was controlled by the type of fatty acid. These results support the design of drug carriers that can be used to monitor pharmacokinetics in real time and to control the disruption time.

Introduction

Nowadays, fluorescent materials are being used in many devices, including lasers,[1] organic electroluminescence displays,[2] biosensors,[3,4] and bioimaging probes.[5] In this context, a few small organic molecules and fluorescent proteins are being studied specifically for application as fluorescence probes in bioscience. An advantage of small organic molecules is their design flexibility. Small organic molecules can easily be modified by organic chemical reactions to enable suitable exciting and emission wavelengths.[6−9] These molecules can also be conjugated with other substances, such as proteins,[10] peptides,[11−13] DNA,[14,15] lipids,[16−18] and silica particles,[19] which proves their applicability in bioscience.

Pyrene and pyrene derivatives are organic molecules typically used as fluorescence probes. A highly concentrated pyrene solution[20] or pyrene in the solid state[21] shows an excimer emission at ∼475 nm because of π–π stacking.[22] This phenomenon can be applied to monitor inter and intramolecular interactions. For example, the interaction of a single strand of DNA was monitored using interstrand stacked pyrenes.[23] The formation of double-strand DNA was revealed by the change in the fluorescence spectrum of pyrene from monomer emission to excimer emission. Further, the interaction between dipeptidyl ureas was investigated based on the conjugation of dipeptidyl urea and pyrene.[24] However, excimer emission is dependent not only on the molecules but also on molecule aggregation. Pyrene and pyrene derivatives are also used as probes to monitor the formation of micellar aggregates[25,26] and gel matrices.[27] Intriguingly, pyrene derivatives were applied to monitor ion species and their concentration.[28] Roy et al. developed phospholipid vesicles (liposomes) containing lipid molecules with both pyrene (in the tail) and metal ion chelators (in the head group); the liposomes exhibited pyrene excimer fluorescence in the presence of a copper ion. Ion monitoring was accomplished not only by the change to excimer emission but also by the change to monomer emission.[29] Two pyrene moieties linked with O–Si–Si–O– or O–Si–O– chains show excimer emission in a tetrahydrofuran (THF)/H2O (v/v, 50/50) solution. This molecule was incubated with a fluorine ion, resulting in a change from excimer emission to monomer emission because of bond cleavage.

π–π stacking is also expected to facilitate pharmacokinetic analysis. In general, to analyze the pharmacokinetics of drug carriers, fluorescence probes containing drug carriers are used. However, this method cannot track the drug carrier itself but tracks only the fluorescence probes. Therefore, it is difficult to detect “when” and “where” a drug capsule is disrupted and the encapsulated drug is released. To control the pharmacological effect of drug carriers, it is necessary to control the retention time (such as the development of PEGylated liposomes to achieve long circulation times in the blood[30,31]). In recent years, the focus of research shifted from drug delivery at the disease site to organelle-specific targeting.[32−34] Thus, it is important to develop fluorescence probes to detect when and where drug carriers are disrupted and drugs are released not only at the organ level but also at the organelle level.

In this study, particles composed of pyrene–fatty acid conjugates were investigated as trackable drug carriers or excipients. Fatty acids are typical biomolecules found in cell membranes and are considered biocompatible. It is widely recognized that it is important to design biomaterials depending on the purpose.[35−38] In this context, it is easy to control the physicochemical properties of biomaterials composed of fatty acids by tailoring the length of the fatty acid. Herein, lauric acid, stearic acid, and behenic acid were conjugated with 1-pyrenemethanol. Solid-state pyrene–fatty acid conjugates were characterized in terms of their physicochemical properties and fluorescence spectra. Later, suspended pyrene–fatty acid conjugates in ethanol (EtOH) were added to murine cells. The conjugates were immediately taken up by the cells and subsequently disrupted. Finally, the relationship between the time to disruption and acyl chain lengths of the fatty acids is discussed.

Results and Discussion

Physicochemical Properties of Pyrene–Fatty Acid Conjugates

Pyrene–fatty acid conjugates synthesized with lauric acid, stearic acid, and behenic acid are designated as P–C12, P–C18, and P–C22, respectively, and their chemical structures are shown in Figure a. They were further characterized by fluorescence microscopy, and the results are shown in Figure b. The conjugates were found to be crystalline in nature; Figure a shows their powder X-ray diffraction (PXRD) patterns. Sharp peaks could be clearly observed, and the recorded patterns were different from those of pyrene. This result suggests that these pyrene–fatty acid conjugates exhibit different crystalline structures. According to differential scanning calorimetry (DSC) analysis, some of the fatty acid molecules show not only a melting point but also a glass transition (Figure S1).[39]Figure b shows the DSC curves of the synthesized pyrene–fatty acid conjugates. Although no glass transition could be observed, sharp peaks ascribed to melting were apparent; melting peaks corresponding to P–C12, P–C16, and P–C22 were found at 67.30, 82.02, and 89.50 °C, respectively. Thus, these pyrene–fatty acid conjugates remain in the crystalline form at 37 °C (normal body temperature). Moreover, Raman spectroscopy also supported the formation of stable crystalline structures (Figure c). The peaks at 1074 and 1130 cm–1 are assigned to gauche C–C stretch (υ(C–C)G) and trans C–C stretch (υ(C–C)T), respectively, and the ratio of these peak intensities indicates the ordering of pyrene–fatty acid conjugates.[40] Peaks at 1130 cm–1 could be clearly observed in the case of P–C18 and P–C22. This result indicates that the hydrocarbon chains of P–C18 and P–C22 are crystalline in nature. The peak intensities at 1074 and 1130 cm–1 were very weak in the case of P–C12. This might be ascribed to the difficulty in detecting short acyl chains. Moreover, it has been reported that at wavenumbers smaller than 600 cm–1, C–C–C out-of-plane bending vibrations occur, whereas in the wavenumber range of 1250–1000 cm–1, C–H in-plane bending and rocking occur.[41] At the same time, in the wavenumber range of 1650–1300 cm–1, aromatic C–C stretching vibrations occur.[41] These wavenumbers of peaks were not completely same, although these peaks were observed in all samples. This result suggests that the stacking states of the pyrene–fatty acid conjugates were different from each other.

Figure 1

(a) Chemical structure of pyrene–fatty acid conjugates (lauric acid (C12), stearic acid (C18), and behenic acid (C22)). (b) Fluorescence microscopy images of the pyrene-modified fatty acids.

Figure 2

Characterization of pyrene–fatty acid conjugates—(a) PXRD, (b) DSC, and (c) Raman spectroscopy.

Optical Properties of Pyrene–Fatty Acid Conjugates

To analyze the applicability of the synthesized pyrene–fatty acid conjugates, their optical properties, especially excimer emission properties, were investigated. Figure a,b shows the absorption and fluorescence spectra of pyrene, P–C12, P–C18, and P–C22 solubilized in hexane, respectively. In solution, the absorbance and fluorescence spectra of the pyrene–fatty acid conjugates were similar to each other but different from those of pyrene. The absorbance spectra of pyrene–fatty acid conjugates slightly red-shifted to longer wavelengths (300–350 nm). At 0.01 mM pyrene concentration, the peaks of monomer emission were obviously different. The difference was significant in the fluorescence spectra in the wavelength range of 350–425 nm (inset of Figure b). This may be caused by the ester bond between pyrene and fatty acids. However, at high concentrations (2.5 mM), the pyrene–fatty acid conjugates exhibited excimer emission peaks (Figure S2). Thus, the optical function of pyrene, that is, monomer/excimer fluorescence transition depending on the relative distance between fluorophores, was maintained in the solution form after conjugation with fatty acid molecules.

Figure 3

(a) Absorption and (b) fluorescence spectra of pyrene–fatty acid conjugates and pyrene in solution form (0.01 mM in hexane). (c) Absorption and (d) fluorescence spectra of pyrene–fatty acid conjugates and pyrene in the solid state.

The absorption and fluorescence spectra of pyrene, P–C12, P–C18, and P–C22 in the solid state are shown in Figure c,d. It could be observed that the monomer emission of pyrene–fatty acid conjugates was different compared to that of pyrene. It has been reported that the modification of functional groups with pyrene can influence the appearance of their fluorescence spectra.[6,42] Further, the excimer emission spectra of pyrene and pyrene–fatty acid conjugates were also different from each other. It has been reported that absorbance peaks above 410 nm are related to excimer emission.[43] The absorbance above 410 nm of pyrene was the highest within the pyrene derivatives tested in this study, and the excimer fluorescence intensity ratio of pyrene was certainly the highest. This observation suggests that the acyl chains of pyrene–fatty acid conjugates restricted the stacking of pyrene by steric hindrance. However, all pyrene–fatty acid conjugates showed excimer emission peaks similar to the spectra of pyrene–fatty acid conjugates solubilized in hexane at high concentrations. Therefore, these pyrene–fatty acid conjugates can potentially be applied as fluorescence probes in pharmacokinetic analysis.

Application of Pyrene–Fatty Acid Conjugates as Trackable Drug Carriers or Excipients

To treat cells with pyrene–fatty acid conjugates, the conjugates were initially suspended in EtOH. The spherical-equivalent diameters of the suspended particles and particle size distribution are shown in Table and Figure S3, respectively. Particle size analysis is very important because it controls the endocytic pathway.[44,45] The sphere-equivalent diameters of P–C12 and P–C18 particles were 94.0 and 122.8 nm, respectively. On the other hand, the sphere-equivalent diameter of P–C22 was 354.8 nm, which is much larger than those of P–C12 and P–C18. When pyrene–fatty acid conjugate suspensions were added to phosphate-buffered saline (PBS), fluorescence spectra were measured. The fluorescence spectra of the pyrene–fatty acid conjugate particles are not completely similar to those of pyrene–fatty acid conjugates in the solid state (Figures d and 4). However, excimer emission peaks were observed. This result suggests that the suspended particles can be used as a fluorescence probe to analyze pharmacokinetic behaviors in detail.

Table 1

Sphere-Equivalent Diameters of Pyrene–Fatty Acid Conjugates

sample	size (nm)
P–C12	94
P–C18	123
P–C22	355

Figure 4

Fluorescence spectra of pyrene–fatty acid conjugate particles.

To analyze the uptake pathway, the fluorescence intensities of the pyrene–fatty acid conjugates absorbed in cells were measured after incubation at 4 °C or after treatment with an endocytosis inhibitor (Figure a). When J774.1 cells were incubated with pyrene–fatty acid conjugates at 4 °C, the fluorescence intensities of the conjugates reduced. Similarly, the fluorescence intensities decreased when the conjugates were treated with endocytosis inhibitors, such as phenylarsine oxide (PAO), wortmannin, cytochalasin B, cytochalasin D, and ethyl isopropyl ether (EIPA). However, in the case of methyl-β-cyclodextrin (MβCD), there was no reduction in the fluorescence intensities of the pyrene–fatty acid conjugates. Therefore, the J774.1 cells took up pyrene–fatty acid conjugates by an endocytic pathway; the contribution of caveola-mediated endocytic pathways is little. No size-dependent differences could be observed in the endocytic pathways. On the other hand, when Colon 26 cells were treated with the pyrene–fatty acid conjugates, the reduction in their fluorescence intensities was not adequate enough to determine the endocytic pathway (Figure b). This suggests that the Colon 26 cells absorbed a part of the pyrene–fatty acid conjugates through the plasma membrane by diffusion. From this discussion, it is obvious that the uptake mechanism is dependent on the cell line.

Figure 5

(a,b) Evaluation of the uptake mechanism (treatment with endocytosis inhibitors) of J774.1 cells and Colon 26 cells, respectively. (c,d) Measurement of the disruption time (ratio of excimer emission) and (e,f) cytotoxicity (MTT assay) of pyrene–fatty acid conjugates with respect to J774.1 cells and Colon 26 cells, respectively.

Figure c,d shows the disruption times of the excimer fluorescence of pyrene–fatty acid conjugates used to treat J774.1 and Colon 26 cells, respectively. They were evaluated using the IM/IE ratio. IM and IE show the maximum fluorescence intensity of monomer and excimer emission, respectively. When the J774.1 and Colon 26 cells were treated with P–C12, the fluorescence spectra changed smoothly from an excimer emission mode to a monomer emission mode. After uptake by the cells, the excimer emission of P–C12 decreased drastically, indicating that the assembly of pyrene–fatty acid conjugate particles was disrupted. Therefore, the particles of P–C12 might be disrupted smoothly, independent of the cell line and uptake pathway. On the other hand, the changes in the fluorescence spectra of P–C18 and P–C22 were smaller than those in P–C12. Therefore, it can be theorized that the particles of pyrene–fatty acid conjugates with longer acyl chains are not disrupted significantly.

The acyl chain lengths of pyrene–fatty acid conjugates also affected their cytotoxicity. Figure e,f illustrates the cytotoxicity of pyrene–fatty acid conjugates toward J774.1 and Colon 26 cells, respectively. The cytotoxicity of P–C12 was less than that of P–C18 and P–C22. As described previously, the particles of P–C12 were disrupted smoothly after uptake, independent of the cell line. Thus, it seems that pyrene–fatty acid conjugate particles were taken by the endocytic pathway, and P–C12 was metabolized more smoothly compared to P–C18 and P–C22, resulting in a lower cytotoxicity (Figure ).

Figure 6

Schematic image of the uptake of pyrene–fatty acid conjugates by J774.1 cells.

Conclusions

The fluorescence spectra of pyrene changed from excimer emission to monomer emission because of the disruption of particles. On the basis of this property, pyrene–fatty acid conjugates can be used to analyze pharmacokinetics. Moreover, the particle size, disruption time, and cytotoxicity of pyrene–fatty acid conjugates are controlled by the length of the used fatty acid. When J774.1 cells were treated with small particles of P–C12 or P–C18 or large particles of P–C22, the particles were taken up by the cells according to clathrin-dependent endocytosis and/or micropinocytosis. The P–C12 particles were smoothly disrupted and showed low cytotoxicity. On the other hand, particles of P–C18 and P–C22 did not exhibit any significant disruption and consequently exhibited high cytotoxicity. Thus, considering the differences in the disruption time, it is expected that the release of drugs cocrystallized with pyrene–fatty acid conjugates can be controlled by manipulating the choice of fatty acids.

Experimental Section

Materials

1-Pyrenemethanol was purchased from Tokyo Chemical Industry Co. Ltd. (Tokyo, Japan). The fatty acids—(lauric acid (C12), stearic acid (C18), and behenic acid (C22))—and 1,1′-carbonyldiimidazole (CDI) were purchased from Sigma-Aldrich (St. Louis, MO, USA). THF, chloroform, hexane, ethanol, and dimethyl sulfoxide (DMSO) were purchased from Wako Pure Chemical Industries (Osaka, Japan).

Synthesis and Crystallization of Pyrene–Fatty Acid Conjugates

Pyrene–fatty acid conjugates were synthesized using CDI as the condensation reagent (Figure S4). Equimolar amounts of the fatty acid and CDI were dissolved in THF, followed by stirring for 8 h at 60 °C. An equimolar amount of 1-pyrenemethanol was added to this solution, which was then subjected to further stirring for 24 h at 60 °C. The synthesized pyrene–fatty acid conjugates were purified by silica gel chromatography [ethyl acetate/hexane = 1:5, silica gel 60N (Kanto Chemical Co. Inc., Tokyo, Japan)], followed by washing with ethanol overnight. The purity of the obtained pyrene–fatty acid conjugates was evaluated by liquid-state 1H nuclear magnetic resonance [1H NMR; JNM-ECX-400 (JEOL Co. Ltd., Tokyo, Japan)] spectroscopy (Figure S5).

Crystal Growth, Analysis of the Crystal Structure, and Characterization of the Physicochemical Properties of Pyrene–Fatty Acid Conjugates

Crystals of pyrene and pyrene–fatty acid conjugates were obtained by evaporation in a chloroform solution (50 mM). The obtained crystals were analyzed using a fluorescence microscope (IX51) with a WU filter (Ex: 330–385 nm, Em: 420 nm) (Olympus, Tokyo, Japan). The crystallinities of the samples (15 mg) were analyzed using an XRD instrument (XRD-6100, Shimadzu, Kyoto, Japan). The diffraction patterns were generated in the 2θ range of 10°–50° at a scanning rate of 4.0° min–1 using Cu Kα radiation at 40 kV and 30 mA. DSC analysis of these crystals was conducted using an X-DSC7000 differential scanning calorimeter (Hitachi High-Tech Science, Tokyo, Japan). The samples were placed in aluminum pans and hermetically sealed; they were then cooled to 30 °C, followed by heating at 5 °C/min to 200 °C. The Raman spectra of pyrene and pyrene–fatty acid conjugate crystals were recorded using a confocal Raman microscope (LabRAM HR-800, Horiba Ltd., Kyoto, Japan). A YAG laser (785 nm) was used for excitation, and a 20× objective lens was used for the measurement.

Absorption and Fluorescence Measurements of Pyrene–Fatty Acid Conjugates

The absorption spectra of pyrene and pyrene–fatty acid conjugates solubilized in hexane (0.01 mM) or in the solid state were measured using an ultraviolet–visible (UV–vis) spectrophotometer (UV-3600, Shimadzu, Kyoto, Japan). The fluorescence spectra of pyrene and pyrene–fatty acid conjugates solubilized in hexane (0.01 or 2.5 mM) or in the solid state were recorded using an RF-5300PC fluorometer (Shimadzu, Kyoto, Japan) at an excitation wavelength of 336 nm. During the fluorescence spectral analysis of pyrene and pyrene–fatty acid conjugates solubilized in hexane, the temperature was maintained at 60 °C to achieve complete dissolution.

Cells and Cell Culture

Murine J774.1 macrophage-like cells and murine cells derived from rectal cancer (Colon 26 cells) were provided by the RIKEN BRC through the National BioResource Project of MEXT, Japan. These cells were cultured in Eagle’s minimum essential medium (E-MEM) containing 10% fetal bovine serum (FBS) in a humidified atmosphere of 5.0% CO2 at 37 °C.

Preparation of Pyrene–Fatty Acid Conjugate Particles and Measurement of Fluorescence Spectra

The synthesized pyrene–fatty acid conjugates were suspended in EtOH because it is difficult to add pyrene–fatty acid conjugates in the solid state to cells directly. The particle preparation procedure is as follows. The conjugates (50 μmol) were added to 1 mL of EtOH and stirred for 24 h. Assuming complete dissolution, the final concentration of the pyrene–fatty acid conjugates was 50 mM. The sizes of the conjugate particles in EtOH were measured using Zetasizer nanoZS (Malvern Instruments Ltd., Malvern, Worcestershire, UK). These suspensions were first diluted with EtOH until they could be analyzed. To generate the fluorescence spectra of the conjugate particles, 30 μL of the suspended particles was added to 3 mL of PBS (137 mM NaCl, 2.7 mM KCl, 10 mM Na2HPO4, and 2 mM KH2PO4; pH 7.3) and fluorescence spectra of these solutions were measured.

Evaluation of Particle Uptake by Cells

The uptake pathway of pyrene–fatty acid conjugate particles was analyzed as described below. Cells were seeded on 6-well culture plates (2 mL, 2.0 × 105 cells/mL) and cultured for 24 h in E-MEM supplemented with 10% FBS in an incubator. Subsequently, the culture medium was replaced with 2 mL of fresh medium. The cells were then treated with endocytosis inhibitors, that is, 20 μL of 200 mM MβCD in PBS,[46] 20 μL of 0.30 mM PAO in DMSO,[47,48] 20 μL of 5.0 mM wortmannin in DMSO,[49,50] 1.92 μL of 20.85 mM cytochalasin B in DMSO,[47] 3.84 μL of 9.85 mM cytochalasin D in DMSO,[47,51] and 20 μL of 2.0 mM EIPA in DMSO.[52,53] The types of inhibitors, inhibitory pathways, and final concentrations of the inhibitors are listed in Table S1. After incubation for 2 h, 20 μL of 50 mM pyrene–fatty acid conjugate particles were added to the cells. The cells were then further incubated for 15 min and washed twice with PBS. They were then treated with trypsin, followed by washing twice with PBS. The fluorescence spectra of the pyrene–fatty acid conjugates in the cell suspension were measured using an RF-5300PC fluorometer (Shimadzu; Kyoto, Japan). The conjugates in the cell suspensions were excited at 336 nm, and spectra in the wavelength range of 350–600 nm were recorded. The maximum fluorescent intensities in the wavelength ranges of 350–430 and 430–600 nm were considered to be the fluorescent intensities corresponding to monomer emission (IM) and excimer emission (IE), respectively. The uptake of P–C12 particles and particles of pyrene–fatty acid conjugates with long acyl chains (P–C18 and P–C22) was evaluated in terms of IM and IE because only the P–C12 particles were disrupted smoothly in the cells (see Results and Discussion). The uptake quantity was evaluated by comparing the cells treated with pyrene–fatty acid conjugate particles with intact cells.

Monitoring of the Disruption of Particles by Fluorescence Spectral Analysis

The time to disruption of the pyrene–fatty acid conjugate particles was measured using a method similar to the one described in the previous section. Cells were seeded on 6-well culture plates (2 mL, 2.0 × 105 cells/mL) and cultured for 24 h in E-MEM supplemented with 10% FBS in an incubator. The culture medium was then replaced with 2 mL of fresh medium containing 20 μL of 50 mM of pyrene–fatty acid conjugate particles suspended in EtOH. The cells were incubated for 15 min, and the culture medium was then replaced with 2 mL of fresh medium to evaluate the disruption of initially taken pyrene–fatty acid conjugate particles. At the end of the incubation time, the cells were treated with trypsin and washed twice with PBS. The fluorescence spectra of the pyrene–fatty acid conjugates in cell suspensions were measured using an RF-5300PC fluorometer. The time to disruption was evaluated based on the ratio of IM and IE.

Evaluation of Cytotoxicity by 3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium Bromide Assay

3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assay was performed using a CellTiter 96 Non-Radioactive Cell Proliferation Assay kit (Promega, Fitchburg, WI, USA). Cells were seeded on 96-well culture plates (100 μL, 2.0 × 105 cells/mL) and cultured for 24 h in E-MEM supplemented with 10% FBS in the incubator. The culture medium was later replaced with 100 μL of fresh medium. Subsequently, 1 μL of ∼100 mM pyrene–fatty acid conjugate (final concentration: 5.0 × 10–4 to 1 mM) was added to the cells; the cells were then incubated for 24 h (37 °C, 5% CO2). A dye solution (15 μL) was added to the medium. After incubation for 4 h, 100 μL of solubilization solution/stop mix was added to the medium. After incubation for 1 h at 25 °C, the absorbance of each well was measured using a microplate spectrophotometer, iMark (Bio-Rad, Hercules, CA, USA). The ratio of cell viability was calculated using the following equationwhere A570 and A630 are the absorbance values at 570 and 630 nm, respectively, when cells were treated with pyrene–fatty acid conjugate particles. A570(0) and A630(0) refer to the absorbance values at 570 and 630 nm, respectively, when cells were treated with PBS. The absorbance at 570 nm was measured to determine the quantity of formazan dye. The absorbance at 630 nm was measured to reduce the effect of cell debris.