Analysis of Commercial Pharmaceuticals Using PXRD (Powder X-ray Diffraction)

APPLICATION NOTE

OBJECTIVE

To demonstrate the use of X-ray diffraction (XRD) to investigate the crystal structure of ibuprofen in products from two different vendors.

INTRODUCTION

The presence of polymorphs (different crystal structures) as well as solvates and hydrates of active pharmaceutical ingredients (APIs) can have a huge impact on the bioavailability and effectiveness of pharmaceuticals. For this reason, it is critical to identify the polymorph (or polymorphs) present in raw APIs as well as in pharmaceutical preparations. It is also essential to understand if and how those polymorphs change over time, or after exposure to different storage conditions. XRD, with its ability to identify different crystal structures, is uniquely positioned to identify the polymorphs present in pharmaceuticals. This application note will demonstrate how XRD is used to identify polymorphs in pharmaceuticals.

Ibuprofen is a well-known analgesic and nonsteroidal anti-inflammatory drug (NSAID) that is used to relieve pain from various conditions such as headache, dental pain, muscle aches or arthritis etc. There are two enantiomers of Ibuprofen (molecular formula = C13H18O2): S(+)-ibuprofen (dexibuprofen) and R(-)-ibuprofen. These two enantiomers of ibuprofen are mirror images of each other and are not only different in crystal structure, but they also have differences in physical properties and chemical stabilities. Dexibuprofen, which is the pure S(+)-ibuprofen, has higher solubility, higher dissolution rates and a lower melting temperature compared to R(-)-ibuprofen. Racemic mixtures of ibuprofen contain both S(+)-ibuprofen and R(-)-ibuprofen.

In most of the commercially available over-the-counter formulations, ibuprofen is present as a racemic mixture, and only the racemic compound (ibuprofen-racemate) is in clinical use. In addition to the two enantiomers, there are two crystalline phases of racemic ibuprofen2: phase I (the conventional phase) is stable and phase II is metastable, based on different melting temperatures.

Historical powder x-ray diffraction patterns for Cu-Kα radiation for the pure S(+) enantiomer (dexibuprofen) and the racemic mixture are shown in Figure 1 and Figure 2, respectively. S(+)-ibuprofen is monoclinic, with space group P214 while racemic ibuprofen also has a monoclinic structure, but with space group P21/c4. As you can see, the two patterns are quite different and should be easy to differentiate.

Figure 1: X-ray reference pattern of dexibuprofen (S(+)-ibuprofen)1

Figure 2: X-ray reference pattern of Racemic ibuprofen1

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