Irish drug giant Alkermes PLC filed a trademark infringement lawsuit against a four-year old Boston-based biotech with a very similar name. Alkermes, which operates a research facility in Waltham that makes drugs for diseases of the central nervous system, sued Alkeus Pharmaceuticals Inc. The suit, filed in the US District Court in Massachusetts, says the name Alkeus could be easily mistaken for Alkermes, especially considering they are both involved in clinical drug development in the Boston area.