The US Supreme Court have just handed down a decision which arguably will make it easier to challenge drug patents. The specific legal challenge was based on article 3 of the US constitution and IPKat has a nice explanation of the outcome and the implications.
MedImmune were paying Genentech licence fees to work on their patented antibodies technologies and also technologies that had a patent pending. When the latter patent was granted Genetech asked for more money but MedImmune though the second patent was invalid. They decided to pay the royalties to avoid being sued by Genetech but they also challenged the patent. Genetech argued that there was no "case in controversy" because MedImmune were paying royalties. MedImmune argued they should not have to stop paying royalties, thereby opening themselves up to being sued by Genetech, before they were allowed to challenge the patent. The Supreme Court agreed with MedImmune.
IPKat says: "in US Constitutional law terms the case extends the scope of the "case in controversy" test significantly, but for patent lawyers its effect is quite simple. You can apply to the court for a patent to be revoked while continuing to pay the licence fee: keeping the farm and all the animals safe."