AstraZeneca's Nexium is the magnesium salt of the (S)-enantiomer of omeprazole. When the company originally applied for patent protection in 1995, it claimed particular salt forms — namely, sodium, magnesium, lithium, potassium, calcium or ammonium salts — of single enantiomers of omeprazole. Later, the company amended its patents to focus on the (S)-enantiomer (known as esomeprazole) and also changed the wording of the claims. In particular, reference to specific salt forms in the claims was removed, being replaced instead by the term “alkaline salts”. However, the written description — the part of the patent that describes the invention — still contained information about the six salt forms.
Hanmi's product — known as Esomezol — is the strontium salt of esomeprazole. Because this salt was not listed in the written description and in other parts of AstraZeneca's patents, Hanmi asserted that its product did not infringe on the patents. Conversely, AstraZeneca argued that the written description only provided examples of alkaline salts and that its patents covered a broader range of salts, including Hanmi's strontium salt of esomeprazole.
