SAN FRANCISCO — Two of the Bay Area’s heaviest legal hitters are trying to debunk an untested legal theory—with $1.6 billion on the line.

With legendary defense lawyers Cristina Arguedas and Allen Ruby in the mix, legal watchers say FedEx Corp.’s indictment for shipping drugs from illegal online pharmacies is shaping up to be one of the Northern District’s fiercest white-collar battles in years. FedEx pleaded not guilty earlier this week to 15 charges, slamming the government’s allegations that the company willfully broke the law to turn huge profits as the middle man in an online drug scheme.