Betaine host-guest complexation with a calixarene receptor: enhanced in vitro anticancer effect

Abstract

p-Sulfonatocalix[n]arenes have shown excellent potential for accommodating chemotherapeutic drugs through host-guest complexation and enhancing their anticancer activity. Betaine has been reported to exert an anticancer effect at high concentrations. In order to increase its concentration in cancer cells, we have complexed it with p-SC4, which releases its content in an acidic environment typical of cancer tissue. In this work, a host-guest complex of the chemically stable, natural, and safe active methyl donor (betaine) and p-sulfonatocalix[4]arenes (p-SC4) was designed and characterized using ¹H NMR, UV, Job's plot analysis, DFT calculations, and molecular modeling for use in cancer therapeutics. The peak amplitude of the prepared host-guest complexes was linearly proportional to the concentration of betaine in the range of 1.0 × 10^-5 M^-1 to 2.5 × 10^-4 M^-1. The reaction stoichiometry between p-SC4 and betaine in the formed complex was 1 : 1. The stability constant for the complex is 8.9 × 10⁴ M^-1 which corresponds to a complexation free energy of -6.74 kcal mol^-1. Complexation between betaine and p-SC4 was found to involve the insertion of the trimethylammonium group of betaine into the p-SC4 cavity, as supported by the experimental data. The complex displayed enhanced cytotoxic activities against breast adenocarcinoma cells (MCF-7) and cervical cancer cells (HeLa) compared to free betaine. In conclusion, the host-guest complexation of betaine with p-SC4 increases its concentration in cancer cells, which warrants further investigation for cancer therapy.

Fig. 1. Absorbance spectra of 0.2 mM betaine, 0.2 mM p-SC4, and a mixture encompassing 0.2 mM of each betaine and p-SC4 all in aqueous media.

Fig. 2. Plot of peak amplitudes at 264 nm attained from the mixtures containing sequentially increasing concentrations of betaine (0.01–0.25 mM) and a fixed concentration of 0.2 mM p-SC4.

Fig. 3. Normalized Job's plot where each of the amplitude values, S, of the first derivative ratio spectra peaks were divided by the maximum amplitude, S_max, are plotted against the mole fraction of the equivalent p-SC4 molar fraction.

Fig. 4. B97-D optimized structures of p-SC4: (a) side view and (b) top view. Upper (d₁) and lower (d₂) rim diameters of the calixarene are reported.

Scheme 1. Computationally investigated inclusion modes of betaine into the p-SC4 cavity.

Fig. 5. B97-D optimized geometrical structures of 1_H-G and 2_H-G complexes are reported together with the most relevant geometrical parameter (Å), calculated complexation free energies, and BCPs present.