Pharmaceutical Ireland-headquartered specialty pharma firm Shire plc (LSE: SHP) has filed a lawsuit in the US District Court for the Northern District of California against Impax Laboratories (Nasdaq: IPXL) and Watson Pharmaceuticals (NYSE: WPI) for the infringement of US Patent numbers. 5,854,290, 6,287,599 and 6,811,794, covering the firm's attention-deficit hyperactivity disorder (ADHD) drug Intuniv (guanfacine HCl) extended release tablets. 6 December 2010