pH-Dependent Behavior of Novel 5-FU Delivery System in Environmental Conditions Comparable to the Gastro-Intestinal Tract

Abstract

A biogenic carrier for 5-fluorouracil (5-FU) loading and subsequent tableting as a new drug formulation for slow release has been proposed using the biomineral from blue crab carapace. Due to its highly ordered 3D porous nanoarchitecture, the biogenic carbonate carrier could achieve increased effectiveness in colorectal cancer cure provided that the formulation would successfully pass through the gastric acid conditions. Following the recently proven viability of the concept by demonstrating the slow release of the drug from the carrier using the highly sensitive SERS technique, here we investigated the 5-FU release from the composite tablet drug in pH conditions replicating the gastric environment. The released drug from the tablet was studied in solutions with three relevant pH values, pH 2, pH 3, and pH 4. The 5-FU SERS spectral signature for each pH value was used to build calibration curves for quantitative SERS analysis. The results suggested a similarly slow-releasing pattern in acid pH environments to that in neutral conditions. Although biogenic calcite dissolution was expected in acid conditions, the X-ray diffraction and Raman spectroscopy showed preservation of calcite mineral along with the monohydrocalcite during acid solution exposure for two hours. The total released amount in a time course of seven hours, however, was lower in acidic pH solutions, with a maximum fraction of ~40% of the total amount of loaded drug, for pH 2, as opposed to ~80% for neutral values. Nonetheless, these results clearly prove that the novel composite drug retains its slow-releasing character in environmental conditions compatible with the gastrointestinal pH and that it is a viable and biocompatible alternative for oral delivery of anticancer drug to reach the lower gastro-intestinal tract.

Keywords: SERS; drug carrier; novel pharmaceutical formulation; pH-dependent release.

Figure 1

Schematic design of the biogenic calcite drug carrier for loading the 5-FU and the pH effect on the tablet and its slow-releasing active ingredient.

Figure 2

Micro-Raman spectrum of 5-FU powder (lower) compared to the SERS signal of the 5-FU solution (0.02 μg/mL) on AgNPs at various pH values: pH 7 (ultrapure water), pH 4, pH 3, and pH 2. Raman and SERS spectra were acquired using 1 s exposure time, 1 acquisition, 220 mW laser power, and a 20× magnification microscope objective. Excitation: 532 nm.

Figure 3

Normalized SERS spectra of 5-FU at concentrations ranging from 0.2 to 20 μg/mL, with the main SERS bands tagged for pH 2 (a), pH 3 (c), and pH 4 (e). Intensity ratio of the 5-FU band at 1336 cm⁻¹ and the water band at 3400 cm⁻¹ plot against the concentration for pH 2 (b), pH 3 (d), and pH 4 (f).

Figure 4

Scanning electron microscopy (SEM) image of the raw biogenic fragments of blue crab shells (a) compared to the tablet exposed to acidic pH: pH 2 (b), pH 3 (c), and pH 4 (d).

Figure 5

XRD pattern of the tablets exposed to acidic pH revealing the crystalline calcite signal (marked with C on the graph), as well as the monohydrocalcite signal (marked with MHC) and traces of quartz, probably from embedded environmental sand grain in native shell roughness.

Figure 6

Confocal Raman spectra of the tablets exposed to acidic pH revealing a crystalline calcium carbonate signal as well as the monohydrocalcite signal, on the 700–1600 cm-1 spectral range (a) and 1000–1200 cm-1 spectral range (b).

Figure 7

Averaged SERS spectra of the samples collected from the solutions containing the tablet over several hours of experiment for each pH value: pH 2 (a), pH3 (c), pH 4 (e). Released amount of FU from the tablet as a function of time for each pH value: pH 2 (b), pH3 (d), and pH 4 (f).