The US Court of Appeals for the Federal Circuit has upheld the decision of the Vermont District Court that Alza's patent (US 4,588,580) covering transdermal-patch administration of the narcotic painkiller fentanyl is infringed by Mylan Laboratories, not invalid and not unenforceable. On appeal, Mylan argued that the claims were not correctly construed and that the patent was unenforceable due to inequitable conduct.

The patent was unforeseeable due to inequitable conduct.

Prior to the '580 patent, transdermal patches used huge excesses of drug with high solubility to maintain the necessary concentration gradient for prolonged delivery. This design was inappropriate for a narcotic due to the large excesses of controlled substance that remained in discarded patches. The inventors of the '580 patent discovered that the skin permeability of fentanyl was highly dependent on the chemical form of the drug, and that incorporating the drug in the patch in the form of a base, rather than the more common fentanyl citrate, achieved satisfactory delivery rates. The patch is sold by Janssen as Duragesic.

Did a prior-art patent (US 4,470,962), which claimed the possibility of using a fentanyl citrate patch, render the '580 patent invalid? The district court ruled that it was clear from the prosecution history that the claims contained in the '580 patent referred to the base form of fentanyl and excluded the citrate form, therefore the '962 patent did not anticipate or render obvious the '580 patent. The court also focussed on the construction of the claim term 'skin permeable'. Both the prosecution history and the specification disclaimed fentanyl citrate, because it was unsuitable for transdermal administration and therefore not classed as a 'skin-permeable form' of fentanyl. Finally, the district court found that although during the prosecution history of the patent one statement had the potential to be misleading, there was no evidence of the requisite intent to deceive in order to conclude inequitable conduct.