Acidic Stress testing of Voriconazole

In the pharmaceutical industry, an important consideration to make before sending a drug into market is determining its shelf life. To determine a drug’s shelf life, it often goes through stress testing which involves exposing a drug to various environments to observe potential degradation. With the information gained from these experiments, scientists can predict how a drug will behave under normal storage conditions. In addition, scientists can uncover the mechanisms on how a drug degrades which allows them to set a drug’s expiration date and optimal conditions for storage.

To monitor the degradation of drugs, NMR is a useful method in determining degradation products without the need of physical separation.¹ To illustrate this, an acidic stress test of the drug, voriconazole, was conducted via ¹H NMR. Voriconazole was selected as the candidate of study because it is a fluorinated drug, thus it opens an avenue for further discovery into analyzing degradation pathways via ¹⁹F (Figure 1). Additionally, since Barbosa et al. previously conducted similar studies using high-field instrumentation, it was of interest to determine whether the results reported in the literature were consistent with results obtained using our benchtop spectrometer.

Figure 1. The structure of the drug of study, voriconazole.

Acidic conditions

A Pharmaceutical secondary standard of voriconazole was purchased from Sigma Aldrich and all NMR experiments were conducted using our 100 MHz benchtop NMR spectrometer.

Initially starting with a 25 mM solution of voriconazole in methanol-d₄ in an NMR tube, a drop of hydrochloric acid (50 mM) was added to the NMR tube to observe any form of degradation. Based on the ¹H analysis, no degradation was observed, however it can be inferred that the hydrochloride salt was formed after the addition of hydrochloric acid based on the spectra (Figure 2). As after the addition of hydrochloric acid, we observe a significant shift of one of the aromatic protons towards a higher frequency in the triazole ring suggesting it is adjacent to the supposedly protonated nitrogen (Figure 2).¹

Figure 2. Effect of the adding HCl to voriconazole with ¹H NMR spectra showing before (top) and after (middle – without suppression and bottom – with suppression) the addition of hydrochloric acid (50 mM) to voriconazole (25 mM) at 33°C. The shift of the triazole proton adjacent to the nitrogen before and after protonation is denoted with a green circle.

Although no degradation of voriconazole was observed in the ¹H analysis, we can conclude based on the analysis that voriconazole is stable in acidic conditions which is consistent with the results of Barbosa et al. Using NMR, other chemical environments such as basic, oxidative, and thermal can be explored to observe potential degradation. As these environments mimic storage and external conditions, a drug can face. This in turn, can aid estimates for shelf life ensuring the safety of consumers. Stay tuned because we will be publishing an application note soon applying a few more stress tests to voriconazole.

References

[1] Barbosa, T. M.; Morris, G. A.; Nilsson, M.; Rittner, R.; Tormena, C. F. RSC Adv. 2017, 7, 34000-34004