Patents infringing patents?

Classen Immunotherapies Inc. alleges that Elan Pharmaceuticals Inc., a unit of Elan Corp. filed patents on the muscle relaxant Skelaxin that included methods involving the disclosure of drug interaction events covered by two of Classen's patents, and that King Pharmaceuticals Inc. later purchased these patents from Elan.

Infringement under 35 USC 271 requires the alleged infringer to make, use, sell, or offer to sell the accused product or process. One can obtain a patent on a method that requires the use of a (separately) patented method. That by itself is not infringement. If one uses the method, then there are issues.