The FDA appears to have a category of New Chemical Entity (NCE)...

The FDA appears to have a category of New Chemical Entity (NCE) Exclusivity of 5 years following approval. See - https://www.fda.gov/drugs/cder-small-business-industry-assistance-sbia/small-business-assistance-frequently-asked-questions-new-drug-product-exclusivity

According to the FAQ, "A new chemical entity means a drug that contains no active moiety that has been approved by FDA in any other application submitted under section 505(b) of the Act."

My understanding is that Zantrene was never approved by the FDA, so in that context it would appear to qualify as a "new" drug despite its extensive history.

There's a bit more information in regards to other exclusivity periods there in the FAQ too, including around new formulations, dosages, etc. which require additional clinical trials to gain approval.

It's not quite as good as the 7 years for the orphan drug approved indication obviously, but it would appear there is at least some other exclusivity for other approved indications to back up the protection offered by RACs patents.