Last week, Gilead Sciences (NASDAQ:GILD) announced that it had submitted its Food and Drug Administration marketing application for its HIV cocktail pill containing rilpivirine, emtricitabine, and tenofovir alafenamide, abbreviated R/F/TAF. (If you're curious, emtricitabine got assigned "F" because Gilead is also developing other TAF-containing drugs, including multiple drugs beginning with "E," so emtricitabine drew the short straw in the nomenclature lottery.)

The announcement wouldn't be all that interesting for investors, except that Gilead said it had submitted the priority review voucher that it bought last November for $125 million along with the application. The voucher will allow R/F/TAF to be reviewed in six months after the application is accepted by the FDA instead of the standard 10 months.

Interestingly, when Gilead bought the voucher in November, it required the owner to give the FDA a 365-day heads-up that the voucher would be used. That restriction was changed a month later under the "Adding Ebola to the FDA Priority Review Voucher Program Act," so Gilead either wasn't planning on using it for this drug or was banking on having the change included in the act.

Innovative drugs with clear unmet needs automatically get a priority review, so it only makes sense for Gilead to use a priority review voucher if the commercial benefit from marketing for the extra four months of sales justifies the cost of the voucher.

R/F/TAF is arguably not all that innovative. Rilpivirine, emtricitabine, and a related drug, tenofovir disoproxil fumarate, or TDF, are already sold by Gilead as a single-pill cocktail called Complera.

Tdf Taf Structures

TDF (left) and TAF (right) have similar structures, but TAF requires a dose less than one-tenth of TDF. Source: Vaccinationist via Wikimedia Commons.

Replacing TDF with TAF offers some side-effect benefits for patients -- improvements in kidney (renal) function and bone mineral density -- but the main advantage for Gilead is that TAF has a longer patent life than TDF, which will see generic competition when Teva Pharmaceuticals launches its generic on Dec. 15, 2017.

R/F/TAF wasn't going to get a priority review, especially since Gilead's other TAF-swapping medications received standard reviews. The only way for Gilead to get the drug on the market earlier was to use a voucher.

And Gilead would like to start selling R/F/TAF as quickly as possible so it can begin switching patients from Complera to R/F/TAF. Starting at the end of 2017, patients who would have taken Complera could take Emtriva (emtricitabine), Edurant (rilpivirine), and generic TDF. Gilead would only get sales from Emtriva, because Edurant is sold by Johnson & Johnson.

When Emtriva goes generic in 2021, patients can take Edurant with both generics -- they're sold as a combo pill called Truvada -- and Gilead will get nothing.

So it isn't so much about the extra four months of sales of R/F/TAF -- it seems unlikely the combined sales of R/F/TAF and Complera in the first four months will be $125 million more than what Complera would have done on its own -- but the four months allow more time to get patients to switch from Complera to R/F/TAF, and those patients will be worth a lot to Gilead after 2018, and even more after 2021.

An earlier approval of R/F/TAF also pushes the launch closer to the two other TAF-containing cocktails already under FDA review. Gilead should hear about E/C/F/TAF on or before Nov, 5,  and about F/TAF on or before next April 7. Having multiple drugs to talk about will make it easier for the drug reps to promote the new combo pills to doctors.

Brian Orelli has no position in any stocks mentioned. The Motley Fool recommends Gilead Sciences, Johnson & Johnson, and Teva Pharmaceutical Industries. The Motley Fool owns shares of Gilead Sciences. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.