Abbott Laboratories’ anti-TNF drug Humira has been approved for the treatment of the autoimmune disease ankylosing spondylitis in the USA, adding to its existing indications in rheumatoid arthritis and psoriatic arthritis.

The new indication is expected to add momentum to Humira’s growth. The drug brought in sales of $1.1 billion last year, but has been tipped to accelerate to nearly $2 billion in 2006, helped by the approval in ankylosing spondylitis.

While less well-known that rheumatoid arthritis, ankylosing spondylitis is almost as common, although because it is often hard to diagnose – with the main symptom being back pain - it is chronically under-treated. The disease has a prevalence of around 0.5% of the population, versus 0.5% to 0.8% for rheumatoid arthritis.

Humira (adalimumab) was cleared for ankylosing spondylitis in Europe on June 1 on the back of six-month data from the ATLAS study, which showed that the drug could reduce symptoms by 20% (a so-called ASAS20 response) in 60% of patients, while around half of the patients on Humira saw a 40% improvement or better (ASAS40).

One-year data, reported at the European League against Rheumatism (EULAR) meeting in June, showed that the proportion of patients experiencing an ASAS20 response rose to 74%, with ASAS40 at around 55%.

There are two other TNF blockers - Johnson & Johnson/Schering-Plough's Remicade (infliximab) and Wyeth's Enbrel (etanercept) - cleared for use in ankylosing spondylitis, as well as various other indications including rheumatoid arthritis.

Abbott is hoping that the dosing regimen for Humira - a subcutaneous injection every two weeks - will help its drug win market share in the ankylosing spondylitis sector versus its more established rivals. Remicade is given by infusion at weeks 0, 2 and 6 and every 6-8 weeks thereafter, while Enbrel is given once or twice a week by subcutaneous injection.