CAMBRIDGE, Mass. ( TheStreet) -- Vertex Pharmaceuticals ( VRTX) appears to have a late but competitive entry in the race to develop a pill for rheumatoid arthritis, a $12 billion treatment market now dominated by injected drugs. Results from a phase IIa study announced Monday demonstrated that Vertex's drug VX-509 significantly improved the signs and symptoms of the rheumatoid arthritis compared to a placebo after 12 weeks of treatment. Based on the results from this study, Vertex said it will move VX-509 into a larger six-month phase IIb study. Investors are keen to see whether Vertex has another potential blockbuster drug in its pipeline, following the launch this spring of the hepatitis C drug Incivek. But VX-509 is already playing catch up behind three competing and more advanced rheumatoid arthritis pills being developed separately by Pfizer ( PFE) and partnerships between Incyte ( INCY) and Eli Lilly ( LLY) and Rigel Pharmaceuticals ( RIGL) and AstraZeneca ( AZN). The convenience of treating painful and swollen joints with a pill has the potential to be a very attractive option for the 1.5 million U.S. rheumatoid arthritis patients who now rely on regular injections to treat their chronic disease. A pill that has comparable efficacy and safety to injections could generate billions of dollars in peak sales. VX-509 belongs to a class of oral drugs that work by blocking a family of enzymes known as Janus kinases, or JAK, known to cause inflammation. Vertex designed VX-509 to be a more selective inhibitor of JAK than similar drugs from Pfizer and Incyte/Lilly, which may translate into VX-509 improving symptoms of rheumatoid arthritis with fewer side effects, the company says. The Pfizer drug, tofacitinib, is already through phase III studies and the company is expected to seek U.S. approval by the end of the year. The results from VX-509's phase IIa study announced Monday are the first look at the drug's clinical profile in patients. The study enrolled 204 patients with moderate to severe arthritis that failed to respond to at least one prior therapy. The study includes four different doses of VX-509, each given twice a day plus a placebo arm for comparison. Efficacy of VX-509 was measured after 12 weeks of treatment using a so-called ACR20 score, which is the percentage of patients who achieve a minimum 20% improvement in symptoms. The change from baseline in improvement in DAS28, a measure of rheumatoid arthritis disease activity, is also being assessed in the VX-509 study.