I think the OP just made a fly-by comment using very imprecise language. Most people assume drug exclusivity works through patents without giving generics with expired patents (like HCQ) much thought.

They wouldn't need to modify it for exclusivity via Abbreviated New Drug Application - I just don't see the FDA accepting any evidence short of a phase III clinical trial that HCQ helps with Alzheimers to fulfill the new use requirements. At best they'd be able to run a proper clinical trial for some cocktail of generic drugs and use ANDAs to prevent competitors from using those generics in their own cocktail for a few years.

It would if you sued to prevent the off-label use, which is something that happened with avastin and lucentis for wet AMD.