English

• 1.   Introduction

  Supramolecular chemistry with directional, tunable and reversible molecular recognition motifs has been widely used to prepare supramolecular biomaterials in recent years.^[1~3] Over the past several decades, the fascinating features of supramolecular biomaterials have attracted considerable attention in various research fields, including the improvement of the bioavailability and patient-specific therapeutic function, as well as the reduction of side effects and dose or dosing frequency.^[4~8] Among them, a variety of macrocyclic hosts, including cyclodextrins, calix[n]arenes, porphyrins, and pillar[n]arenas, have been used to fabricate supramolecular biomaterials.^[9~32]

  The cucurbit[n]urils (Q[n], n=5~8, 10, 13~15), ^[33~36] a new class of macrocyclic hosts, have received increasing attention in recent years due to their special structure, which are composed of a rigid hydrophobic cavity and two identical carbonyl-laced portals, which can encapsulate small organic molecules, biomacromolecules, metal ions and even nanoparticles.^[37~48] In addition, cucurbit[n]urils also show limited toxicity at doses as high as 600 mg•kg^-1 (oral) and 200 mg•kg^-1 (intravenous) in rodents.^[49] Accordingly, numerous studies have suggested a promising role for various cucurbituril homologues as formulation excipients to facilitate improved drug stability and/or solubility in oral tablets, topical creams, eye drops, and nasal sprays, among other possible dosage forms.^{[50, 51]} Also, a wide range of structural diverse small molecule drugs, including enzyme inhibitors, antidiabetic agents, antineoplastic agents, beta blockers and anaesthetics, have been found to interact with members of the Q[n] family.^{[52, 53]} However, although all these findings support the potential use of Q[n]s in pharmaceutical and medicinal chemistry applications at non-toxic concentrations, a more water-soluble and biocompatible form of Q[n] is always highly sought after. With superior water-solubility, α', δ, δ'-tetramethylcucurbit^[6]uril (TMeQ[6]) has been suggested as a good candidate.^[54~56]

  Currently, capecitabine (CAP) is among the essential and most effective anticancer drug, taken orally, which is taken orally and is converted in vivo into the active compound 5-fluorouracil (5-FU).^[57] 5-FU and its precursor CAP are pyrimidine analogues characterized as antimetabolites that inhibit cell division and interfere with RNA and protein synthesis and can help many cancer patients extend their overall survival. CAP is mainly applied for the treatment of advanced primary or metastatic breast cancer, colorectal cancerand gastric cancer. Due to its better clinical curative effect and higher safety, CAP is popular with cancer patients.^[58] In the present paper, we used a supramolecular approach to chemically investigate the binding behaviour of the host molecule TMeQ[6] with the guest CAP (Figure 1) by NMR spectroscopy, MALDI-TOF mass spectrometry, fluorescence spectroscopic and isothermal titration calorimetry (ITC) analyses.

  Figure 1

  

  Figure 1.  Structures of hosts of TMeQ[6] and guest CAP

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  2.   Results and discussion

  2.1   NMR spectroscopy analysis

  In order to investigate the complexation of TMeQ[6] with CAP in solution, ¹H NMR titration experiments were first performed by adding increasing amounts of TMeQ[6] into the solution of CAP in D₂O. The results revealed, as shown in Figure 2, that in the presence of TMeQ[6] the peaks for all alkyl chain protons of CAP display a sizeable upfield shift compared with those of the free guest, the methylene protons H^g, H^h, Hⁱ and H^j and the methyl proton H^k experience significant upfield shift from δ 4.06, 1.57, 1.20, 1.20 and 0.73 to 3.92, 0.79, 0.31, 0.31 and 0.04, respectively. Meanwhile, the other protons (H^a-H^e) do not show any clear change, indicating that the alkyl chain moiety of CAP was inserted into the cavity of TMeQ[6], whereas the foran ring was prevented from entering the cavity, generating the host-guest inclusion complex TMeQ[6]-CAP. Additional support for this point comes from the observed in the COSY spectrum of the complexation of TMeQ[6] with CAP.

  Figure 2

  

  Figure 2.  ¹H NMR spectra (400 MHz, D₂O) of TMeQ[6] (a), TmeQ^[6] in the presence of 1.0 equiv. of CAP (b), and neat CAP (c)

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  2.2   Isothermal titration calorimetry (ITC)

  To gain a better insight into the host-guest interactions between TMeQ[6] and CAP, we performed the ITC experiments at 25 ℃. The equilibrium association constants (K_a) and thermodynamic parameters for the interaction system between TMeQ[6] and CAP are shown in Table 1 and Figure 3, in particular that data shows that the determined binding molar ratio value (N) was 1.001. This result is very close to the expected value of 1.0, suggesting that the interaction ratio of TMeQ[6] to CAP is 1:1. Furthermore, the experimental results revealed that the association constant (K_a) of TMeQ[6] with CAP was (7.55±0.45)×10⁴ L•mol^-1, suggesting the formation of stable inclusion complex in aqueous solution. In addition, the relatively large negative enthalpy values (ΔH°=-45.47±0.78 kJ•mol^-1) and small negative entropy values (TΔS°=-10.21±0.67 kJ•mol^-1) indicate that the intermolecular complexation interactions between TMeQ[6] and CAP appear to be driven by favourable enthalpy changes, accompanied by small negative (unfavourable) entropy changes.

  Table 1

  

  Table 1.  ITC measurements of the thermodynamics of TMeQ[6] and CAP interactions at 298.15 K

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  Experiment	TMeQ[6]-CAP
  Model	Independent
  Ka/(L•mol-1)	(7.55±0.45)×104
  ΔH°/(kJ•mol-1)	-45.47±0.78
  N	1.001±0.022
  TΔS°/(kJ•mol-1)	-10.21±0.67

  Figure 3

  

  Figure 3.  ITC data for the binding of TMeQ[6] with CAP in aqueous solution at 298.15 K

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  2.3   Fluorescence emission and UV-vis absorption spectra

  The interaction of TMeQ[6] with CAP was also evaluated by fluorescence spectroscopy and UV absorbance spectrophotometry analysis. According to the fluorescence emission spectroscopic results shown in Figure 4a, the guest, CAP, had an emission peak at 395 nm, with the excitation at 299.07 nm. Also, the gradual addition of TMeQ[6] to CAP in an aqueous solution, leading to an increase in the ratio TMeQ[6]/CAP, gradually decreased the fluorescence intensity of the system, which is attributed to the formation of the supramolecular complex between TMeQ[6] and CAP. Moreover, when the molar ratio of TMeQ[6] to CAP was close to 1:1, the fluorescence intensity of the system tended to be weaker (Figure 4b). Furthermore, Job's plots (Figure 4c) based on the continuous variation method clearly showed that the fluorescence spectra fitted well with the 1:1 stoichiometry of the host-guest inclusion complex, which was consistent with the data obtained in the ITC analysis. Also, according to the UV absorption spectroscopic results, the gradual addition of TMeQ[6] to CAP was accompanied by a significant increase in the intensity at 240 nm due to the strong interaction between TMeQ[6] and CAP.

  Figure 4

  

  Figure 4.  Fluorescence spectra of CAP (4.0×10^-5) with increasing concentrations (0.0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0 equiv.) of TMeQ[6] (a) the corresponding F vs N_TMeQ[6]/N_CAP curves (b), and the Job's plot (c)

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  2.4   MALDI-TOF mass spectrum

  The inclusion complex between TMeQ[6] and CAP was also confirmed by the MALDI-TOF mass spectrometric analysis. The results revealed, as clearly shown in Figure 5, an intense signal at m/z=1412.56, which corresponds to TMeQ[6]-CAP (calculated 1411.71), thereby providing direct support for the formation of the 1:1 stoichiometry for the host-guest inclusion complex TMeQ[6]-CAP.

  Figure 5

  

  Figure 5.  MALDI-TOF mass spectrum of the complex TMeQ[6]-CAP

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  3.   Conclusions

  In summary, we have demonstrated the supramolecular host-guest complexation between macrocyclic TMeQ[6] and the well-known anticancer drug CAP by a variety of methods, including NMR spectroscopy, fluorescence spectroscopy, MALDI-TOF mass spectrometry and ITC analyses. The result reveals that the alkyl chain moiety of the guest CAP is inserted into the cavity of the macrocyclic TMeQ[6]. Our studies represent an important strategy to manipulate the physicochemical properties of pharmaceutical agents, improve bioavailability and reduce side effects.

  4.   Experimental

  4.1   Materials and measurements

  Capecitabine was obtained from Aldrich and used without further purification. TMeQ[6] was prepared as described previously. All other reagents were of analytical grade and used as received. Double-distilled water was used for all experiments.

  4.2   ¹H NMR measurements

  All ¹H NMR spectra, including those for the titration experiments, were recorded at 298.15 K on a JEOL JNM-ECZ400S spectrometer in D₂O. D₂O was used as a field-frequency lock.

  4.3   Isothermal titration calorimetry (ITC) experiments

  Microcalorimetric experiments were performed using a Nano ITC isothermal titration calorimeter (TA, USA). A typical ITC titration was carried out by adding a CAP solution (1.0×10^-3 mol•L^-1, 10 μL aliquots) to a TMeQ[6] solution at 298.15 K. The concentration of TMeQ[6] in the sample cell (1.3 mL) was 1.0×10^-4 mol•L^-1. The heat of reaction was corrected for the heat of dilution of the guest solution determined in separate experiments. All solutions were degassed by sonication prior to titration experiments. Computer simulations (curve fittings) were performed using the Nano ITC analyze software.

  4.4   Fluorescence spectroscopy measurements

  All fluorescence spectra were recorded from samples in 1 cm quartz cells on a Varian RF-540 fluorescence spectrophotometer. The host and guests were dissolved in distilled water. fluorescence spectra were obtained at 298.15 K at a concentration of 4.00×10^-5 mol/L CAP with different TMeQ[6] concentrations.

  4.5   MALDI-TOF mass spectra measurements

  MALDI-TOF mass spectra were taken on a Bruker BIFLEX Ⅲ ultra-high resolution Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometer with α-cyano-4-hydroxycinnamic acid as matrix.

  Supporting Information  2D COSY NMR spectrum of guest CAP, 2D COSY NMR spectrum and UV-Vis absorption spectra of the interactions between TMeQ[6] and CAP. The Supporting Information is available free of charge via the Internet at http://sioc-journal.cn

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• 

  Figure 1  Structures of hosts of TMeQ[6] and guest CAP

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  Figure 2  ¹H NMR spectra (400 MHz, D₂O) of TMeQ[6] (a), TmeQ^[6] in the presence of 1.0 equiv. of CAP (b), and neat CAP (c)

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  Figure 3  ITC data for the binding of TMeQ[6] with CAP in aqueous solution at 298.15 K

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  Figure 4  Fluorescence spectra of CAP (4.0×10^-5) with increasing concentrations (0.0, 0.2, 0.4, 0.6, 0.8, 1.0, 1.2, 1.4, 1.6, 1.8, 2.0 equiv.) of TMeQ[6] (a) the corresponding F vs N_TMeQ[6]/N_CAP curves (b), and the Job's plot (c)

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  Figure 5  MALDI-TOF mass spectrum of the complex TMeQ[6]-CAP

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  Table 1.  ITC measurements of the thermodynamics of TMeQ[6] and CAP interactions at 298.15 K

  Experiment	TMeQ[6]-CAP
  Model	Independent
  Ka/(L•mol-1)	(7.55±0.45)×104
  ΔH°/(kJ•mol-1)	-45.47±0.78
  N	1.001±0.022
  TΔS°/(kJ•mol-1)	-10.21±0.67

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•