Detection of Chiral Drugs Using HPLC with CD Detection

The need for chiral detection in the development of pharmaceuticals is increasing due to regulatory changes and the demand for single-isomer pharmaceutical drugs. These shifts require a means for chiral detection and compound separation because there is little control over which chiral form of a chemical compound is formed during the drug's production. At the molecular level chiral partners have identical physical properties and therefore are very difficult and expensive to separate.

Both Prilosec (omeprazole) and Nexium (esomeprazole) are proton pump inhibitors used to treat ulcers, heartburn, acid reflux, and Zollinger-Ellison syndrome. Nexium's active ingredient, esomeprazole, is the S-enantiomer of omeprazole, the active ingredient in Prilosec. Nexium was one of the first pharmaceuticals to be repatented and marketed as a pure enantiomer of a formerly mixed enantiomer drug. Nexium has a superior clinical efficacy due to its higher and more consistent bioavailability. We aimed to test the enantiomeric purity of both drugs using an HPLC system with circular dichroism (CD), fluorescence (FP), and optical rotation (OR) detectors.

Experimental Conditions

Prescription Nexium and Prilosec were examined by grinding the tablets and dissolving them in ethanol, and sonicating for 20 min. These solutions were brought to a final concentration of 1 µg/µL with an injection size of 2 µL. Separations were accomplished using a ChiralPak AD column (4.6 × 250 mm) with a 100% EtOH mobile phase and a flow rate of 0.9 mL/min. A JASCO HPLC system with CD, OR and FP detectors was used to perform the analysis.

Figure 1: Prilosec chromatograms Top: CD detector / Middle: FP detector / Bottom: OR detector.

Results and Discussion

The Nexium and Prilosec chromatograms are shown in Figures 1 and 2. As expected, the Prilosec chromatograms showed the inclusion of both enantiomers while in the Nexium results only the esomeprazole enantiomer was detected.

Figure 2: Nexium chromatograms Top: CD detector / Middle: FP detector / Bottom: OR detector

The results are clearest in the CD chromatograms. While the FP detector was as sensitive as the CD detector with respect to concentration, it was not able to clearly identify which enantiomer was which. On the other hand, while the OR detector was capable of discriminating the enantiomers, it was far less sensitive than the CD detector with respect to concentration.

Conclusion

An HPLC system outfitted with a CD detector is able to effectively determine the enantiomeric purity of certain chiral drugs more effectively than similar systems with either FP or OR detectors. JASCO currently offers the world's only CD detector for HPLC.

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