This page at the company website may have been updated since the last time I read it -- I didn't remember the sentence about Ziagen.

http://www.ipharminc.com/prurisol-1/

Quote:

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This regulatory approach helps expedite a drug’s potential approval as it allows the FDA to rely, in part, on existing long-term safety data of an already approved drug, in this instance, Ziagen—thereby shortening development time and reducing costs.

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