Source: Sanofi via Facebook.

Quarterly sales of Sanofi's (NASDAQ:SNY) oral multiple-sclerosis drug Aubagio have surged by 78.6% in the past year. If that growth continues, then Aubagio could become a big threat to Biogen (NASDAQ:BIIB) and Novartis' (NYSE:NVS) competing oral-MS drugs in 2016.

Transforming care
Historically, relapsing multiple sclerosis has been treated with injection-based drugs, including Biogen's Avonex; however, drugs that are taken orally have become increasingly prescribed during the past two years.

These oral-MS therapies reduce patient burden while also reducing the number of MS relapses at rates that are similar to, or better than, Avonex. As a result, Biogen's Tecfidera and Novartis' Gilenya are now outselling Avonex, which posted sales of $685 million in Q3. During the same period, Biogen's Tecfidera delivered sales of $937 million, up 19% year over year, and Novartis' Gilenya racked up sales of $696 million, up 16% year over year.

Tecfidera and Gilenya's third-quarter performance shows that they're reshaping how doctors treat patients, but Biogen and Novartis may soon begin losing market share to Sanofi's Aubagio, another oral-MS drug that's growing even more quickly. Aubagio's sales of $247 million in Q3 increased Aubagio's sales during the first nine months, to $660 million, and that's up an impressive 81.9% from last year.

Considering options
Aubagio could be catching up because of reports in the past year of a rare brain disease occurring in Tecfidera and Gilenya patients. That rare disease, which is known as PML, can be life threatening. While it's not unheard of PML occurring in patients that are receiving MS therapies, the fact that no cases of PML have been reported by patients taking Aubagio (yet) may have some prescribers leaning toward it.

Doctors could also be embracing Aubagio because Sanofi has released data during the past year that suggests Aubagio can significantly delay MS progression. In October 2014, Sanofi reported that Aubagio patients participating in its Tower study had a statistically significant reduction in annual relapse rate and relative risk of sustained disability progression versus placebo.

At that time, Sanofi also reported that newly diagnosed patients who have suffered their first MS symptoms were less likely to have a relapse when treated with Aubagio than patients taking a placebo. That finding makes Aubagio the first of these oral MS drugs to deliver positive results in early MS patients.

Looking ahead
In October, Sanofi further strengthened its case for using Aubagio rather than other MS drugs when it reported that Aubagio had a positive impact on brain atrophy during trials. Specifically, results from Sanofi's Temso phase 3 study showed that patients taking 14 mg of Aubagio saw brain volume loss of 0.90% at 24 weeks, and that's significantly less than the 1.29% loss reported for people in the placebo arm of the study.

That finding could give doctors yet another reason to prescribe Aubagio over Biogen's Tecfidera and Novartis' Gilenya, but we'll have to wait a couple of quarters to see if that proves to be the case.

Regardless, with Aubagio nipping at Tecfidera's and Gilenya's heels, and combined sales of Tecfidera and Gilenya running at a pace of more than $6.5 billion per year, investors might want to pay close attention to Sanofi's quarterly earnings releases during the next year to see if Aubagio makes even greater headway in the indication.

Todd Campbell has no position in any stocks mentioned. Todd owns E.B. Capital Markets, LLC. E.B. Capital's clients may have positions in the companies mentioned. The Motley Fool recommends Biogen. 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.