WordNet sense 1 (someone who can be employed as a detective to collect information):

operative, shamus, sherlock, private detective, private eye, private investigator

WordNet sense 2 (the scientist in charge of an experiment or research project):

principal investigator

WordNet sense 3 (an antiviral drug used against HIV; interrupts HIV replication by binding and blocking HIV protease; often used in combination with other drugs):

protease inhibitor