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Improving the Membrane Permeability of 5‑Fluorouracil via Cocrystallization Xia-Lin Dai,† Song Li,† Jia-Mei Chen,*,† and Tong-Bu Lu†,‡ †

School of Pharmaceutical Sciences, Sun Yat-Sen University, Guangzhou 510006, China MOE Key Laboratory of Bioinorganic and Synthetic Chemistry, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China

‡

S Supporting Information *

ABSTRACT: Pharmaceutical cocrystallization is proposed as a new method to enhance membrane permeability of a BCS class III model drug, 5-fluorouracil (5FU). Three cocrystals of 5FU, 5FU/3-hydroxybenzoic acid (1), 5FU/4aminobenzoic acid (2), and 5FU/cinnamic acid (3), were successfully synthesized by a slurry method or a liquid-assisted grinding process. Spectroscopic methods, thermal analysis, and X-ray diffraction were used to characterize these new forms. The permeability was studied using a Franz diffusion cell and silicone membrane. All of the cocrystals showed improved membrane permeability compared to free 5FU. The cumulative amount per unit area of permeated 5FU in the first 10 h for 1−3 was increased by 41, 70, and 83%, and the steady penetration rates of 1−3 were increased by 38, 66, and 79%, respectively, as compared to the pure drug. Structure−permeability correlation study finds a link between intermolecular interactions and molecular packing in cocrystals and their permeability behavior and has important implications for use of a cocrystallization approach to improve drugs’ permeability in the pharmaceutical field.

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INTRODUCTION The transdermal route is considered as an effective drug delivery system and has made great contribution to medical practice.1 The transdermal drug delivery system (TDDS) offers an ascendant alternative except for oral administration and injection, which shows a variety of advantages, such as avoiding first-pass effect, reducing pharmacokinetic peaks and troughs, increasing both patient compliance and duration of action, etc.2 However, TDDS has not completely achieved its potential as oral dosage form or injection because only a limited number of drugs which show good permeability can penetrate through the stratum corneum of skin, which is considered as the primary barrier in the transdermal route. Pharmaceutists have developed many methods to improve a drug’s permeability such as prodrugs, chemical enhancers, iontophoresis, noncavitational ultrasound, electroporation, etc. All of these methods have respective weaknesses; for instance, the complexity design of prodrugs, irritation or toxicity of chemical enhancers, pain and damage of tissue caused by iontophoresis or noncavitational ultrasound, complex device design of electroporation, etc.3−6 Pharmaceutical cocrystals can be defined as a stoichiometric multiple component supramolecular structure combined by an active pharmaceutical ingredient (API) and one or more unique cocrystal formers with noncovalent bonds.7 Formation of pharmaceutical cocrystals can dramatically ameliorate the physicochemical properties of the API without any disruption of covalent bonds, including melting point, solubility, dissolution rate, stability, mechanical behavior, etc.8−14 © 2016 American Chemical Society

Recently, pharmaceutical cocrystallization was proposed as a new method to enhance permeability of APIs.15−17 It is free of tissue damage, well reproducible, and can be produced on a large scale compared to the other above-mentioned permeability improved methods. In the present study, 5-fluorouracil (5FU, Scheme 1) was selected as a model drug to perform such a research. 5FU is an antineoplastic drug and can be used by a transdermal drug delivery system to treat several skin cancers including squamous cell carcinoma and superficial basal cell carcinoma as well as a series of intractable skin diseases, such as acuteness wet wart, vitiligo, psoriasis, etc.18−20 However, 5FU is a BCS class III drug with good aqueous solubility (about 12 mg/mL) and poor permeability (log P = −0.89).21 The transdermal permeation of 5FU through the lipophilic stratum corneum is very low and marginal. Many approaches, including prodrug,22 chemical enhancers,18 physical methods,6,23,24 etc., have been evaluated to enhance the permeability of 5FU with different degrees of success. For example, the use of chemical enhancers, such as isopropyl myristate, lauryl alcohol, and azone, exhibited 3-, 4-, and 24-fold increases in the permeability of 5FU.18 The effect of a series of physical methods, namely, iontophoresis, electroporation, erbium:yttrium−aluminum−garnet laser, and Received: April 11, 2016 Revised: June 23, 2016 Published: June 24, 2016 4430

DOI: 10.1021/acs.cgd.6b00552 Cryst. Growth Des. 2016, 16, 4430−4438

Crystal Growth & Design

Article

Scheme 1. Structures of 5-Fluorouracil and Coformers

Table 1. Slurry Parameters, Yields, and Elemental Analyses of 1−3 slurry solvent slurry temperature yield (%) chemical formula anal. (%) calcd anal. (%) found a

1

2

3

water rta to 50 °C 82.6 C11H9FN2O5 C, 49.26; H, 3.38; N, 10.45 C, 49.23; H, 3.37; N, 10.39

water rta to 50 °C 80.1 C11H10FN3O4 C, 49.44; H, 3.77; N, 15.73 C, 49.58; H, 3.81; N, 15.75

water and ethanol (1:1) rta 79.4 C13H11FN2O4 C, 56.01; H, 4.00; N, 10.06 C, 56.01; H, 3.98; N, 10.07

Room temperature.

Table 2. Crystallographic Data and Refinement Parameters for 1−3 chemical formula formula wt temperature (K) crystal size (mm3) crystal system space group a (Å) b (Å) c (Å) α (deg) β (deg) γ (deg) volume (Å3) Z density (g/cm3) 2θ range F (000) index ranges

no. of reflns no. of unique reflns no. of params Rall, Robsa wR2,all, wR2,obsa goodness-of-fit on F2 a

1

2

3

C11H9FN2O5 268.20 150(2) 0.10 × 0.08 × 0.05 triclinic P1̅ 7.0551(4) 7.5754(6) 10.7143(7) 105.869(6) 96.217(5) 97.652(5) 539.57(6) 2 1.651 4.3367−66.6667 276 −8 ≤ h ≤ 6 −8 ≤ k ≤ 9 −12 ≤ l ≤ 12 4288 1904 185 0.0607, 0.0475 0.1377, 0.1253 1.056



R1 = ∑∥Fo| − |Fc∥/∑|Fo|. wR2 = [∑[w(Fo2 − Fc2)2]/∑w(Fo2)2]1/2, w = 1/[σ2(Fo)2 + (aP)2 + bP ], where P = [(Fo 2) + 2Fc2]/3.

From the perspective of crystallography, there are N−H hydrogen bonding donors and CO hydrogen bonding receptors in the molecular structure of 5FU which could form robust hydrogen bonding synthons with carboxylic groups. Therefore, a series of carboxylic acids, including, 3hydroxybenzoic, 4-aminobenzoic, cinnamic, phthalic, isophthalic, fumaric, and acetyl salicylic acids, were selected as coformers to react with 5FU. Finally, only 3-hydroxybenzoic, 4aminobenzoic, and cinnamic acids could successfully cocrystallize with 5FU through a slurry method or a liquid-assisted grinding process. Several solid-state material characterization techniques, such as X-ray powder diffraction, differential scanning calorimetry, thermogravimetric analysis, and infrared

their combination, on the permeability of 5FU has been studied and demonstrated significant enhancement.6 Herein, we attempt to improve the permeability of 5FU by cocrystallization. Up to now, a dozen cocrystals of 5FU have been reported, including cocrystals with cytosine,25 1methylcytosine,26 theophylline,27 urea, thiourea, 2,2′-bipyridine, 4,4′-bipyridine,28 acridine, phenazine, 4,4-bispyridylethene,29 and adipic, succinic, terephthalic, benzoic, malic,30 and 4hydroxybenzoic acids,31 etc. However, these researches mainly focus on synthesis, characterization, and/or crystal structures of 5FU cocrystals, and none of them are concerned about the permeability behavior of 5FU. 4431



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Table 3. Hydrogen Bonding Distances and Angles of 1−3 hydrogen bond 1 N(1)−H(1B)···O(5) N(2)−H(2A)···O(5) O(2)−H(2)···O(3) O(1)−H(1)···O(4) 2 N(1)−H(1)···O(2) N(2)−H(2)···O(2) O(3)−H(3)···O(5) N(3)−H(3B)···O(1) 3 N(3)−H(3)···O(2) N(4)−H(4)···O(2) O(7)−H(7)···O(6) C(12)−H(12)···F(1)

∠D−H···A (deg)

symmetry

2.892(2) 2.833(2) 2.608(2) 2.816(2)

177(2) 172(3) 178(5) 164.3

−x + 1, −y + 1, −z + 3 −x, −y + 1, −z + 3 −x + 2, −y + 2, −z + 1

1.91(3) 1.89(3) 1.77 2.18(3)

2.818(2) 2.824(2) 2.579(2) 3.055(3)

174(2) 171(2) 169.9 147(2)

−x, −y + 2, −z + 1 −x, −y + 1, −z + 1 −x + 1, −y + 4, −z − 1

1.95 1.96 1.628(19) 2.50

2.8029(17) 2.8062(18) 2.6236(18) 3.3997

168.6 170.1 174(3) 162.8

−x, −y + 3, −z + 1 −x + 1, −y + 3, −z + 1 −x, −y, −z + 1

H···A (Å)

D···A (Å)

2.04(3) 1.98(3) 1.727(11) 2.02

Gemini A Ultra system with graphite monochromated Cu Kα radiation (λ = 1.54178 Å). Cell refinement and data reduction were applied using the program CrysAlisPro.32 The structures were solved by the direct methods using the SHELX-97 program33 and refined by the full-matrix least-squares method on F2. All non-hydrogen atoms were refined with anisotropic displacement parameters. The hydrogen atoms were placed in calculated positions with fixed isotropic thermal parameters and included in the structure factor calculations in the final stage of full-matrix least-squares refinement. Crystallographic data and details of refinements of 1−3 are listed in Table 2, and selected hydrogen bonding distances and angles are given in Table 3. In Vitro Permeation Experiments. The permeation study of 5FU and its cocrystals was conducted using Franz-type diffusion cells through a silicon membrane (120 μm thickness). The silicon membrane was pretreated with 95% ethanol for 30 min and dried under air before use. The silicon membrane was then mounted between the donor chamber and the receptor chamber with an effective surface area of 3.14 cm2. The receptor chamber was filled with 8 mL of degassed phosphate buffer (pH 7.4) and maintained at 37.0 ± 0.1 °C, and the receptor solution was stirred at 250 rpm for 30 min to achieve a system balance. Excess of 5FU (39 mg) and its cocrystal materials (containing 39 mg of pure 5FU) was added to 1 mL of IPM, respectively, and kept volute for 1 min. Then, the resulting suspensions were placed on the silicon membrane as donor chamber. During the experiment, the receptor solution was maintained at 37.0 ± 0.1 °C and was continuously stirred at 250 rpm. An aliquot of 0.2 mL of the receptor solution was withdrawn from the receptor chamber and filtered with 0.22 μm nylon filters at predetermined time intervals. The receptor chamber solution volume was kept constant by replacing fresh medium. Permeation study of 5FU and its cocrystals was carried out in triplicate (n = 3). The concentration of 5FU for each sample was analyzed by HPLC. Solubility Determination. The solubility of 5FU and its cocrystals in phosphate buffer (pH 7.4) was determined. All the samples were milled to powder and sieved by standard mesh sieves to keep the particle size ranges within 75−150 μm. Excess of 150 mg of 5FU and its cocrystal materials (containing 150 mg of 5FU) was added to 5 mL of buffer solution, and the resulting slurry was stirred at 37 °C and 250 rpm. An aliquot of the slurry was withdrawn after 5 h and filtered through a 0.22 μm nylon filter. The filtered aliquot was appropriately diluted and measured by HPLC to quantify 5FU concentrations. Solubility determination was carried out in triplicate (n = 3). HPLC Analysis Method. The concentration of 5FU was determined by a Shimadzu LC-20A HPLC system at a UV detection wavelength of 266 nm using a C18 column (Inertsil ODS-3, 5 μm × 4.6 mm × 150 mm column, GL Sciences Inc., Japan). The mobile phase consisted of a mixture of methanol and aqueous phosphoric acid solutions (pH 2.4). The gradient elution was used to analyze cocrystals

spectroscopy, were utilized to characterize these new cocrystals. Their crystal structures were also resolved from single crystal Xray diffraction data. Finally, in vitro permeation experiments using a Franz diffusion cell and silicon membrane as well as solubility determination and stability performance test of the cocrystals were also carried out.

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EXPERIMENTAL SECTION

Materials and General Methods. 5FU was purchased from Suizhou Hongqi Chemical Co. Ltd. 3-Hydroxybenzoic, 4-aminobenzoic, cinnamic, phthalic, isophthalic, fumaric, and acetyl salicylic acids as well as isopropyl myristate (IPM) were purchased from Aladdin Reagent Inc. All other reagents and chemicals were commercially available and used directly. Elemental analyses were determined using an Elementar Vario EL elemental analyzer. X-ray powder diffraction (XRPD) patterns were obtained on a Bruker D2 Phaser with Cu Kα radiation (30 kV, 10 mA). Differential scanning calorimetry (DSC) was recorded on a Netzsch DSC 200 F3 instrument and aluminum sample pans in a nitrogen atmosphere, with a heating rate of 10 °C/min from 30 °C. Thermogravimetric (TG) analyses were recorded on a Netzsch TG-209 instrument and alumina crucibles in a nitrogen atmosphere, with a heating rate of 10 °C/min from 30 to 500 °C. Infrared (IR) spectra were recorded using KBr pellets and a Bruker EQUINOX 55 spectrometer. A total of 64 scans were collected over a range of 4000−400 cm−1 with a resolution of 0.2 cm−1 for each sample. Preparation of Cocrystals. 5FU/3-hydroxybenzoic acid cocrystal (1:1) (1), 5FU/4-aminobenzoic acid cocrystal (1:1) (2), and 5FU/ cinnamic acid cocrystal (1:1) (3) were prepared by both liquid-assisted grinding and slurry methods. (1) The liquid-assisted grinding experiments were performed by addition of an equimolar of 5FU (130 mg, 1 mmol) and corresponding coformers with two drops of deionized water to a 25 mL stainless steel grinding jar. The mixture was then ground at a frequency of 25 Hz for 30 min. (2) An equimolar of 5FU (130 mg, 1 mmol) and corresponding coformers were mixed in a test tube. A 1 mL portion of solvent was added and stirred for 24 h. The suspension was filtered, and the filter cake was dried under vacuum. The slurry parameters, yields, and elemental analyses of all cocrystal materials prepared are summarized in Table 1. Preparation of Single Crystals. Single crystals of 2 (columnshaped) were obtained when the filtrate of the slurry was left to slowly evaporate at room temperature. Single crystals of 1 (column-shaped) and 3 (needle-like) were obtained by stirring an excess of the corresponding powder sample in methanol and deionized water, respectively; then the resulting suspensions were filtered and the filtrates were left to slowly evaporate at room temperature. Single Crystal X-ray Diffraction. Single crystal X-ray diffraction data of cocrystals 1−3 were collected on an Agilent Technologies 4432



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Figure 1. (a) The asymmetric unit, (b) 2D sheet structure, (c) side view, and (d) vertical view of 3D structure in 1. 1 and 3. It was started with 5% (v/v) methanol (5 min), followed by an increase to 50% (v/v) methanol (22 min), reserved at 50% (v/v) methanol (27.5 min), and returned immediately to 5% (v/v) methanol (28 min), and then reserved at 5% (v/v) methanol (35 min) for cocrystal 1. It was started with 5% (v/v) methanol (5 min), followed by an increase to 80% (v/v) methanol (24 min), and returned immediately to 5% (v/v) methanol (25 min), and then reserved at 5% (v/v) methanol (32 min) for cocrystal 3. An isocratic elution was used to analyze cocrystal 2, with 5% (v/v) methanol and 95% (v/v) phosphoric acid solution. The gradient or isocratic elution was used with a flow rate of 0.7 mL/min. Stability Test. A vial of each sample (1−3) was subjected to the accelerated ICH condition (40 °C/75% RH) for durations of 1, 2, 3, and 6 months. Upon the duration terminated, the samples were immediately analyzed by XRPD.

perspective. Form I of 5FU (refcode FURACL14) adopts a hydrogen bonded sheet structure exploiting all of the available uracil donors and acceptors (including a doubly hydrogen bonded R22(8) homosynthon) and exhibiting regions where the fluorine atoms are in close proximity, approaching within 3.2 Å (Figure S1, Supporting Information). The structure has a much higher calculated density (1.78 g/cm3, 150 K) than most organic crystals demonstrating uncommonly dense and efficient packing in its crystal lattice, which suggests that 5FU is in a low energy state, and this may be one of the causes to its poor permeability. When 5FU cocrystallizes with a guest carboxylic acid, the hydrogen bonding sites of two components would adjust to achieve a balance. Thus, some supramolecular homosynthons and dense packing in a pure drug must be interrupted, whereas some new drug−coformer heterosynthons and packing mode must be generated. This modification of forms and crystal structures may change 5FU’s activity and subsequently improve its permeability when across a membrane. The crystal structure of 1 belongs to the triclinic crystal system, P1̅ space group (Table 2). The asymmetric unit contains one 5FU and one 3-hydroxybenzoic acid molecule (Figure 1a). 5FU molecules are connected through two doubly hydrogen bonded R22(8) homosynthons (N(2)−H(2A)···O(5)

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RESULTS AND DISCUSSION Crystal Structures. Single crystal X-ray diffraction is considered as the “gold standard” of the structural characterization of a cocrystal. It can determine the crystal form, give a 3D representation of the structure, and elucidate the supramolecular synthons and the crystal packing details. To compare molecular packing and noncovalent interactions, in particular, supramolecular synthons before and after cocrystallization, the structure of 5FU is analyzed from a crystal engineering 4433



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Figure 2. (a) The asymmetric unit, (b) 2D sheet structure, (c) side view, and (d) vertical view of 3D structure in 2.

permeability of 5FU. In addition, all the calculated densities of 1−3 from single crystal data decrease to a different extent, suggesting that the dense packing of 5FU is disrupted and a looser molecular packing mode is formed after cocrystallization. That might be helpful for increasing the activity and subsequently improving the permeability of 5FU. XRPD Analysis and Thermal Analysis. The XRPD was used to check the crystalline phase purity of the three cocrystals (Figure S2, Supporting Information). The results show that the XRPD patterns of 1−3 are different from either API or those of the corresponding coformers, indicating the formation of new crystalline phases. In addition, all the peaks displayed in the measured patterns closely match those in the simulated patterns generated from single crystal diffraction data, confirming the formation of the corresponding cocrystals. The thermal behaviors of these new crystal forms were investigated by DSC and TG analysis (Figure S3, Supporting Information). 1 and 2 begin to decompose at approximate 184 °C, whereas 3 begins to decompose at 145 °C, and no evidence of a phase transformation in the DSC curve can be observed before decomposition of 1−3. IR Analysis. IR spectra of 5FU and 1−3 are presented to further verify the hydrogen bonding changes involved in the bulk cocrystal production (Figure S4, Supporting Information). 5FU shows characteristic absorption peaks at 3069 and 1660

and N(1)−H(1B)···O(5)) to form a molecular ribbon. The two adjacent 3-hydroxybenzoic acid molecules are linked through an acid−acid R22(8) homosynthon (O(2)−H(2)··· O(3)) to form a carboxylic dimer. The 5FU molecular ribbons are further connected by 3-hydroxybenzoic acid dimers via hydrogen bonds (O(1)−H(1)···O(4)) to generate a twodimensional (2D) sheet (Figure 1b). The 2D sheets are packed along the b-axis via interlayer π···π interactions (3.571 Å) between the rings of 5FU and 3-hydroxybenzoic acid to form the three-dimensional (3D) structure (Figure 1c,d). Similarly, 2 and 3 also crystallized in the triclinic P1̅ space group with one 5FU and one coformer molecule (Figures 2a and 3a). Molecular arrangements in the lattice of 2 and 3 show similar structures depicted for structure 1. The 5FU molecule forms molecular ribbons through two doubly hydrogen bonded R22(8) homosynthons, which extend as 2D sheet structures with carboxylic dimers through N−H···O hydrogen bonding interactions in 2 and C−H···F interactions in 3, respectively (Figures 2b and 3b). The 2D sheets are further packed via π···π interactions (3.647 Å for 2 and 3.530 Å for 3) to generate the 3D molecular arrangements (Figures 2c,d and 3c,d). Our crystal structure analysis shows that some N−H···O hydrogen bonding interactions found in pure 5FU are replaced by drug−coformer heterosynthons in the cocrystals. This modification might lead to alteration of the membrane 4434



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Figure 3. (a) The asymmetric unit, (b) 2D sheet structure, (c) side view, and (d) vertical view of 3D structure in 3.

cm−1, which are assigned to N−H and CO stretching vibrations, respectively. After formation of cocrystals 1−3, the N−H stretching frequency were observed at 3091, 3089, and 3079 cm−1 and the CO stretching vibrations were observed at 1678, 1676, and 1691 cm−1, respectively. A hypsochromic shift in the N−H and CO stretching frequency verifies that the intrinsic hydrogen bonding interactions in 5FU are interrupted and some new hydrogen bonds are produced accompanying cocrystallization. Permeation Study of Cocrystals. Permeability study of 5FU cocrystals was carried out using a Franz diffusion cell with a silicone membrane as an alternative nonpolar membrane to human and animal skin, for a duration of 10 h.34−36 The cumulative amount per unit area permeated (Qn) of 1−3, with form I of 5FU as a control, is shown in Figure 4 in the form of Qn of 5FU (μg/cm2) vs time (h). The plots indicate that Qn’s of 5FU/cocrystals increase slowly with time. Qn’s of 1−3 in 10 h were 41, 70, and 83% higher than that of pure API (Table 4). Furthermore, Qn’s of pure 5FU and 1−3 all have a good linear relationship with diffusion time, demonstrating that the drug diffuses through the membrane at a nearly constant speed. Thus, the slope of the Qn plots can approximatively refer to their respective steady penetration rate, Js (Table 4). Js’s of 1−3 are increased by 38, 66, and 79% as compared to the pure drug, respectively. The plots of flux of 5FU/cocrystals against time are graphed as Figure 5. Figure 5 depicts that the flux of pure 5FU increases in the first 3 h and maintains a steady

Figure 4. Cumulative amount per unit area permeated (Qn) of 5FU and 1−3 vs time plot.

penetration rate in the rest of the time, whereas those of 1− 3 increase quickly within an hour and then drop slightly. The figure displays that the amount of drug flux is higher in cocrystals than in pure 5FU. A qualitative order of permeation/ flux/mass transport of 5FU across the artificial silicone membrane can be stated as follows: 5FU < 1 < 2 < 3. 4435



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Table 4. Cumulative Amount Per Unit Area Permeated (Qn) in 10 Hours, Steady Penetration Rate (Js), and Solubility of 5FU and 1−3 sample

Qn (μg/cm2)

Js (μg/cm2/h)

solubility (mg/mL)

5FU 1 2 3

17.88(1.73) 25.14(53) 30.32(1.58) 32.75(97)

1.81(2) 2.50(3) 3.00(2) 3.24(6)

19.1(1) 21.2(3) 18.8(8) 18.1(3)

Figure 6. Cumulative amount per unit area permeated (Qn) 5FU and physical mixture vs time plot.

crystal structures, which may help improve the predictability of pharmaceutical cocrystallization strategies for the aim of permeability enhancement. From in vitro permeability experiments, we can find that the permeability property of 5FU is improved after formation of the cocrystals with replacement of some drug−drug homosynthons by new drug−coformer heterosynthons. Although the three cocrystals are similar in their crystal structures, the rank order of the permeability is 5FU < 1 < 2 < 3. If we carefully examine their single crystal structures, we can see that 1, 2, and 3 combine drug and coformer molecules through strong O− H···O (2.816 Å, 164.3°) interactions, intermediate N−H···O (3.055 Å, 147°) interactions, and weak C−H···F (3.3997 Å, 162.8°) interactions, respectively. A weaker interaction between 5FU and coformer molecules is accompanied by a higher thermodynamic activity of 5FU molecules in the crystal lattice and a subsequent increase in permeability. Further, the thermodynamic activity of a molecule in a crystal lattice also depends upon its molecular packing efficiency. In general, a crystal with less efficient packing is always of lower density, lower lattice energy, and higher thermodynamic activity. Reduction of calculated density via single crystal data from 1.78 g/cm3 in 5FU to lower values in 1−3 (1.651 g/cm3 in 1, 1.596 g/cm3 in 2, and 1.526 g/cm3 in 3) is another possible reason for increasing the thermodynamic activity and consequently improving the permeability of the cocrystals. Figure 7 depicts the negative correlation between density and permeability (steady penetration rate, Js) of the cocrystals. Moreover, we also attempt to explore whether an interplay exists between lipophilicity of the coformers and permeability of corresponding cocrystals. The use of lipophilic components is accompanied by a high partition coefficient (log P) and a subsequent increase in membrane permeability. Figure 8 shows the correlation between log P of the coformers and permeability (, Js) of the cocrystals, indicating an approximately positive correlation. This result implies that selection of coformers with higher lipophilicity to cocrystallize with an API can be helpful when attempting to improve its permeability. Stability Test. The stability of 1−3 under accelerated ICH conditions was also monitored with different storage time points to confirm their stability. The results of XRPD

Figure 5. Flux of 5FU and 1−3 with respect to time.

Solubility Study of Cocrystals. 5FU is a BCS class III drug with high hydrophilic nature. Table 4 summarizes the solubility of 5FU cocrystals in pH 7.4 phosphate buffer at 37 °C. The solubility values of all cocrystals are very close to that of the parent drug (within ±10%), indicating that cocrystallization has no remarkable impact on the solubility of API in the present study. The enhanced flux by cocrystals cannot be attributed to solubility improvement as other cocrystal systems,16 since these cocrystals exhibit similar solubility to 5FU. Some other reasons must contribute to the improved permeability of 5FU cocrystals. Permeability Study of Physical Mixtures. To understand the difference in permeability behavior of the cocrystals and physical mixtures of 5FU and corresponding coformers, the permeation experiments of the respective physical mixtures were also conducted under the same conditions. The physical mixtures of 5FU with 3-hydroxybenzoic acid and cinnamic acid show similar cumulative permeated amounts per unit area to pure 5FU, while the physical mixture of 5FU and 4aminobenzoic acid demonstrates a worse diffusion (Figure 6). According to our study, if carboxylic acids are used in 5FU formulations, they should be incorporated in the crystal lattice, and thus improve the permeability of 5FU by changing the intermolecular interactions and molecular packing of 5FU molecules. Structure−Permeability Correlation. It can be seen that incorporation of carboxylic acids to the crystal lattice can significantly improve the membrane permeability of 5FU, while the addition of carboxylic acids as physical mixtures cannot improve or even worsen the permeability. Thus, a key factor for permeability enhancement of 5FU is the formation of new drug−coformer heterosynthons and molecular packing. Herein, we attempt to correlate the permeability properties with the 4436



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confirm that all of these new crystal forms are stable as isolated solids and that they have the potential to envisage more efficient transdermal formulations of 5-fluorouracil.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.cgd.6b00552. The crystal structure of form I of 5-fluorouracil, DSC/ TG, IR, XRPD analysis with regard to preparation and stability test (PDF) Accession Codes

CCDC 1472837−1472838 and 1472847 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/ cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

Figure 7. Negative correlation between Js and density of cocrystals.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +86-20-84112921. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by NSFC (Nos. 21571194 and 21331007) and NSF of Guangdong Province (No. S2012030006240).

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REFERENCES

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Figure 8. Positive correlation between Js of cocrystals and log P of corresponding coformers.

measurements indicate that 1−3 retained their initial crystal forms for 6 months (Figure S5, Supporting Information). It demonstrates that 1−3 are stable as isolated solids for at least 6 months under 40 °C/75%.

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CONCLUSIONS 5-Fluorouracil is a BCS class III drug with good solubility and poor permeability. The present study demonstrates that cocrystallization of 5-fluorouracil with lipophilic carboxylic acids can effectively improve its permeability. Three cocrystals of 5-fluorouracil with 3-hydroxybenzoic acid, 4-aminobenzoic acid, and cinnamic acid were synthesized by a slurry or liquidassisted grinding method and characterized by XRD, DSC, TG, and IR measurements. Permeability study through a silicone membrane shows enhanced flux/permeability in almost all cases. Structure−permeability correlation study reveals that new supramolecular synthon formation, drug−coformer interactions, molecular packing in the crystals, and their interplay play a decisive role to modulate permeability of the cocrystals. Lipophilicity of the coformers is also an important contributor toward the desired permeability of the drug. From these facts, our findings have important implications for use of a cocrystallization approach to improve drugs’ permeability in the pharmaceutical field. The accelerated ICH condition tests 4437



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