Big drugmakers looking for places to grow generally look to diseases that have certain characteristics: They have a fair-size patient population, they’re chronic and they’re inadequately treated by current medicine. Psoriasis meets all those qualifications.

Not surprisingly, then, some of the biggest names in pharma have late-stage candidates in the works, including Amgen (AMGN), Eli Lilly (LLY), Pfizer (PFE), Novartis (NVS) and Celgene (CELG). All have potential, says Morningstar analyst Damien Conover.

“Everybody’s trying to attack psoriasis in a different way, because the penetration rate is pretty low,” Conover told IBD. “It definitely opens up a potential window to new treatment.”

Psoriasis is a condition in which, for mysterious reasons, the immune system attacks the body’s own skin cells as if they were intruders. The most common form causes scaly plaques on the skin, usually associated with itching and pain. Of the 6 million Americans suffering plaque psoriasis, most have plaques on less than 3% of their bodies, which is considered the mild form. About 15% have a moderate or severe form, covering up to 10% of the body.

For most patients it’s a recurring problem, though with treatment it goes into remission about 25% of the time.

Doctors generally use a succession of treatments of increasing strengths and prices, depending on how the patient responds. They start out with topical products like corticosteroid creams. Exposure to sunlight has long been known to help psoriasis, so doctors also use UV lamps for treatment.

If those efforts don’t work, the patient moves on to oral drugs. The most common second line of offense is methotrexate, a generic immunosuppressant. After that come the more effective but more expensive biologics. Four biologic drugs dominate: Amgen’s Enbrel, Johnson & Johnson‘s (JNJ) Stelara and Remicade, and AbbVie‘s (ABBV) Humira.

Seek PASI 75 Grade

The efficacy of all these drugs is measured by the PASI score, short for Psoriasis Area and Severity Index. The gold standard for drugmakers is a 75% reduction in the PASI score, known in the industry as “PASI 75.”

Since methotrexate is an older drug, the data are rather patchy, with PASI 75 rates ranging from 36% to 60% depending on the study. Humira’s and Remicade’s PASI 75 rates run over 70%, while Stelara’s is 67%. Enbrel’s runs in the 40s, but nonetheless many patients try it because you never know which patient is going to react to which drug.

Last month, Celgene reported phase-three trial results for its candidate apremilast, which it has been hyping as a potential blockbuster with sales topping $1.5 billion by 2017. Wall Street’s consensus was only about half that, based on the phase-two data. The phase-three results seemed to confirm their suspicions: apremilast’s PASI 75 score was similar to Enbrel’s.