Humira is the world's best-selling drug, bringing in $9.6 billion last year. The way drug information specialist EvaluatePharma sees it, AbbVie's ($ABBV) gem is going to remain the world's best selling drug for the next 5 years. That is despite the fact that its patent expires in 2016. According to EvaluatePharma's "World Preview 2013, Outlook to 2018," sales of Humira should grow 56.5% through 2018, when it is forecast to have sales of $12.8 billion. And if it is the world's best-selling drug that year, then it obviously will be the world's best-selling rheumatoid arthritis (RA) drug in 2018. In fact, it is expected to dominate that market in 2018, holding 23.5% share.

What EvaluatePharma defines as the anti-rheumatics market currently makes up the second largest treatment area by sales, with worldwide revenues of $41.1 billion. Now that is quite a bit smaller than the oncology category, which last year racked up $68 billion in revenues, but still very significant and growing. Anti-rheumatics will remain strong over the next 5 years, despite expected biosimilar competition. The market is forecast to churn out a compound annual growth rate of 4% and reach $52.1 billion in revenues in 2018. It will be outpaced by anti-diabetics, which should have $60.6 billion in sales by then, and oncology treatments, which are projected at $114.4 billion. But it will remain one of the most lucrative markets for the drugmakers that target it.

It will hold that position in part because of some of the current best-sellers in the category that are expected to remain strong sellers through 2018. The three current best-sellers, based on IMS Health figures, are Humira; Enbrel, which Amgen ($AMGN) and Pfizer ($PFE) share; and Remicade, the drug that Johnson & Johnson ($JNJ), Merck ($MRK) and Mitsubishi Tanabe are partnered on. These three are forecast to hold the same top spots in 2018 as well.

Some newcomers will be creeping up the list, however, including Xeljanz, the new drug from Pfizer; it's expected to be the fourth-best-selling drug in the category by 2018 with projected sales of $2.7 billion. It was approved in November 2012 and earned $30.6 million in a couple of months. The drug is an entirely new type of RA drug--a Janus kinase inhibitor, or JAK inhibitor. It's approved for patients who can't tolerate treatment with methotrexate, or haven't fully responded to it. There were big expectations for this one when it was building to a launch, given its novel approach to treating the condition and the fact that it is a twice-daily pill instead of an injection. A Pfizer exec has acknowledged the drug isn't exactly tearing up the category so far, but it is a category with a lot of established players, and so it may just take time. Analysts have predicted it could reach peak sales of $3 billion.

That is all looking forward, however. What about the here-and-now? Of course, Humira is the top dog, and as I mentioned, Enbrel and Remicade round out the top three. We give you the rest of the top 10 in this report as well. Take note, the list is based on sales for rheumatoid arthritis for the 12 months ended June 30, as determined by IMS Health, the healthcare technology and information company. The time period chosen is to give you the most current data. But some of the drugs on the list are approved for a variety of treatment areas and so their RA sales make up just a portion of the total revenues they generate.

And while this report is about the top 10 branded RA drugs, I would be remiss in not also mentioning methotrexate. Why? Because the generic is usually the first med doctors turn to for RA patients. Total sales of methotrexate for RA were $130 million in the 12-month period, IMS Health says. That is way more than the final three drugs on the list earned individually and very nearly as much as the combined sales for those three.

So please read the report that follows and let us know what your thoughts are. Also, you can join our LinkedIn group and expound there as well. -- Eric Palmer (email | Twitter)