Aqueous solubility and degradation kinetics of the phytochemical anticancer thymoquinone; probing the effects of solvents, pH and light

Abstract

Thymoquinone (TQ) is a potent anticancer phytochemical with confirmed in vitro efficacy. Its clinical use has not yet established, and very few reports have documented its formulation. There also are no reports about the aqueous solubility and stability of this valuable drug, despite their direct correlation with the in vivo efficacy. In the current research, we have established and validated a stability-indicating HPLC method for the detection of TQ and its degradation products under different conditions. We then investigated the solubility and stability profiles of TQ in aqueous solutions. The stability study was aimed to determine the effect of pH, solvent type and light on the degradation process of TQ, along with the investigation of the kinetics of this degradation. The solubility of TQ varied in different aqueous solvents, and might be compromised due to stability issues. However, these findings confirm that the aqueous solubility is not the major obstacle for the drug formulations mainly due to the considerable water solubility (>500 μg/mL) that may be enough to exert pharmacologic effects if administered via parenteral route. Stability study results showed a very low stability profile of TQ in all the aqueous solutions with rapid degradation that varied with solvent type. The study of the degradation kinetics showed a significant effect of pH on the degradation process. The process followed first order kinetics at more acidic and alkaline pH values, and second order kinetics at pH 5-7.4, regardless of the solvent type. The results also expressed that light has a greater impact on the stability of TQ as a shorter period of exposure led to severe degradation, independent of the solution pH and solvent type. Our results also addressed some discrepancies in previously published researches regarding the formulation and quantification of TQ with suggested solutions. Overall, the current study concludes that TQ is unstable in aqueous solutions, particularly at an alkaline pH, in addition to presenting severe light sensitivity. This data indicates the inappropriateness of aqueous solutions as pharmaceutical vehicles for TQ preparations. To the best of our knowledge, this is the first study describing TQ aqueous solubility and stability that may lead to the development of a stable and effective TQ formulation.

Figure 1

The UV-VIS spectrum of TQ (10 μg/mL) in ethanol (A) and the chemical structure of the drug (B).

Figure 2

HPLC chromatograms of the forced degradation study of TQ in acid, base, UV, and oxidation stress conditions compared to the standard solution of TQ prepared in mobile phase.

Figure 3

Residual TQ concentration in different solutions at variable time intervals of the stability study estimated by HPLC, Time in hours, (error bars represent the standard deviation).

Figure 4

HPLC chromatograms of TQ before and after four days of the stability study in different solutions, showing the decrease in the peak intensity of the drug and the appearance of new degradation peaks.

Figure 5

Degradation kinetics of TQ in aqueous media, zero order, First order, and second order kinetics models.

Figure 6

TQ Light degradation kinetics plotted for zero, first and second order models.

Figure 7

Initial HPLC chromatographs of TQ (A), after 48 h light stability study (B) and the initial UV-VIS spectrum of TQ (C), after 48 h light stability study (D).