For years, AbbVie's (NYSE:ABBV) powerhouse product Humira has held the top-dog position of best-selling drug in the world. In 2016, this anti-inflammatory drug brought in over $16 billion in global sales and is expected to remain the world's top-selling drug out to 2020.  However, being lead husky means the competition is always nipping at your heels, and competitors are itching to knock Humira off its perch. Luckily, AbbVie may have an ace up its sleeve -- namely, it's developing a sequel to Humira. Here's what you need to know.

A "JAK" of all trades

AbbVie's sequel to Humira is a drug called upadacitinib. Unlike Humira, which belongs to class of drug called "anti-TNF's," upadacitinib is what's known as a "JAK" inhibitor. While the science is a little heavy, you can think of JAK (short for janus kinase) as an enzyme that activates the cell's immune/inflammatory response. By inhibiting the activation of JAK, drugs like upadacitinib are able to lower the body's immune response. 

Why lower the body's immune response you might ask? This function is very useful in autoimmune diseases, or diseases in which the body's immune system mistakenly identifies its own host as a foreign invader and attacks. Humira has already been approved for a laundry list of autoimmune indications including rheumatoid arthritis, psoriatic arthritis, ankylosing spondylitis, plaque psoriasis, and Crohn's disease.  Upadacitinib is currently being studied as a potential treatment for many similar indications.

Money and pills on a scale

Image source: Getty Images.

Phase 3 data

Earlier this month, AbbVie release positive top-line data from a phase 3 study of upadacitinib as a treatment for rheumatoid arthritis (RA). In this study, which compared upadacitinib vs placebo in RA patients who had already failed on conventional therapy, upadacitinib met its primary endpoint of a 20% improvement in patient symptoms as measured by what's known as an "ACR20" score.

Furthermore, upadacitinib also hit key secondary endpoints, including higher ACR scores (indicating more patient improvement) and around 30% of trial patients achieved clinical remission. The safety profile for upadacitinib was also encouraging, with no deaths reported and an adverse event profile consistent with earlier phase 2 trials.

Will it be enough?

While the phase 3 data for upadacitinib is encouraging, it is unlikely that this drug will ever come close to matching Humira in sales. For one, once Humira loses its patent protection (which the company estimates will be in 2022), payors will likely require potential patients to first fail on the cheaper biosimilar version of Humira prior to approving payment for upadacitinib.

In addition, upadacitinib will not the first JAK inihibitor approved for the treatment of autoimmune conditions. Should upadacitinib achieve FDA approval, the drug would face stiff competition from the likes of Pfizer's Xeljanz as well as other next-generation autoimmune therapies.

AbbVie's future

With Humira's patent expiration looming, AbbVie has not been one to sit on its haunches. In addition to upadacitinib, AbbVie sports an incredibly rich pipeline with 12 pivotal trials reading out in 2017 alone. Keep an eye on future readouts from risankizumab in psoriasis as well as phase 3 data from Venclexta's MURANO trials in relapsed/refractory chronic lymphocytic leukemia.

In addition, while future sales of upadacitinib may never touch those of Humira, analysts have projected upwards of $3.5 billion in peak sales for this product.  With AbbVie's background, distribution network, and proven brand within the autoimmune space, I believe this number may even be an underestimate.

While I wouldn't recommend buying AbbVie on the potential of upadacitinib alone, this company is definitely one I'm keeping my eye on.

This article represents the opinion of the writer, who may disagree with the “official” recommendation position of a Motley Fool premium advisory service. We’re motley! Questioning an investing thesis -- even one of our own -- helps us all think critically about investing and make decisions that help us become smarter, happier, and richer.