Self-assembling surfactant-like peptide A₆K as potential delivery system for hydrophobic drugs

Abstract

Background

Finding a suitable delivery system to improve the water solubility of hydrophobic drugs is a critical challenge in the development of effective formulations. In this study, we used A₆K, a self-assembling surfactant-like peptide, as a carrier to encapsulate and deliver hydrophobic pyrene.

Methods

Pyrene was mixed with A₆K by magnetic stirring to form a suspension. Confocal laser scanning microscopy, transmission electron microscopy, dynamic light scattering, atomic force microscopy, fluorescence, and cell uptake measurements were carried out to study the features and stability of the nanostructures, the state and content of pyrene, as well as the pyrene release profile.

Results

The suspension formed contained pyrene monomers trapped in the hydrophobic cores of the micellar nanofibers formed by A₆K, as well as nanosized pyrene crystals wrapped up and stabilized by the nanofibers. The two different encapsulation methods greatly increased the concentration of pyrene in the suspension, and formation of pyrene crystals wrapped up by A₆K nanofibers might be the major contributor to this effect. Furthermore, the suspension system could readily release and transfer pyrene into living cells.

Conclusion

A₆K could be further exploited as a promising delivery system for hydrophobic drugs.

Keywords:

• pyrene
• self-assembling peptide
• micelles
• nanofibers
• drug delivery

Supplementary data

Calibration curve for pyrene at higher concentrations in ethanol

Standard pyrene solutions in ethanol were prepared at concentrations ranging from 1×10⁻⁴ mM to 2×10⁻² mM. The fluorescence spectrum of each standard solution was measured to obtain its value at 374 nm. The parameters for measurement of fluorescence were set as follows: exciting wavelength 336 nm, scan speed 240 nm/min, excitation slit width 5 nm, emission slit width 5 nm, and photomultiplier tube voltage 400 V. The calibration curve was obtained as a concentration-versus-fluorescence plot (Figure S1), from which the following equation could be obtained:

$y=0.00507*\exp (0.00832x)-0.00504$(1)

where y is the pyrene concentration and x is the fluorescence value at 374 nm. The adjusted R-square value is 0.99874.

Calibration curve for pyrene at lower concentrations in phosphate-buffered saline

Standard pyrene solutions in phosphate-buffered saline (0.02 M) were prepared by diluting a stock pyrene solution (1 mM in ethanol) with phosphate-buffered saline, at concentrations ranging from 1×10⁻⁵ mM to 6×10⁻⁴ mM. The fluorescence spectrum of each standard solution was measured to obtain its value at 374 nm. Parameters for fluorescence measurement were set as follows: exciting wavelength 336 nm, scan speed 240 nm/min, excitation slit width 20 nm, emission slit width 10 nm, and photomultiplier tube voltage 400 V. The calibration curve was obtained as a concentration-versus-fluorescence plot (Figure S2), according to which the following equation could be obtained:

$y=3.9591\times {10}^{-6}x-4.48259\times {10}^{-5}$(2)

where y is the pyrene concentration and x is the fluorescence value at 374 nm. The adjusted R-square value is 0.99937.

Figure S1 Calibration curve for pyrene at higher concentrations in ethanol.

Figure S2 Calibration curve for pyrene at lower concentrations in phosphate-buffered saline.

Figure S3 Fluorescent pyrene particles in the initial mixture of pyrene and A₆K.

Notes: (A) Normal light, (B) fluorescence, and (C) merged image. Large pyrene particles could be seen in these images. Scale bar, 5 μm.

Figure S4 Transmission electron microscopic image of the initial mixture of pyrene and A₆K. Large pyrene particles with an irregular shape can be seen in this image.

Figure S5 Fluorescence spectra for the pyrene-A₆K mixture at different storage times. In the first 4 days, the fluorescence value between 440–500 nm dropped markedly, indicating the decreasing of pyrene eximers. After 4 days, the spectra reached an equilibrium state, indicating that a stable suspension had been obtained.

Acknowledgments

This work was financially supported by the National Natural Science Foundation of China (81000658 and 31100565).

Disclosure

The authors report no conflicts of interest in this work.