Amgen's (NASDAQ:AMGN) Enbrel hasn't been able to keep pace with AbbVie's (NYSE:ABBV) Humira, managing $4.2 billion in U.S. sales last year compared to Humira's $5.2 billion. So what's Amgen to do? Make a copycat of Humira, of course.

You're not seeing double. Source: AbbVie.

Unlike small-molecule generics, copycats of biologic drugs, called biosimilars, will have to undergo clinical trials to show that they're as efficacious as the original drug. This week, Amgen released the first data for ABP 501, its biosimilar of Humira, showing that the two drugs performed similarly in patients with plaque psoriasis.

Amgen didn't release any data, but noted that, "At week 16, the PASI percent improvement from baseline was within the prespecified equivalence margin for ABP 501 compared to adalimumab." PASI stands for Psoriasis Area and Severity Index, and is the standard measurement for severity of plaque psoriasis. Adalimumab is the chemical name for Humira.

Patients who responded well to ABP 501 after 16 weeks will continue on the drug for another 32 weeks. Those who responded to Humira will be rerandomized to take either Humira or ABP501. The extended trial will give Amgen more safety data for ABP501, and a chance to show that switching from Humira to ABP501 doesn't make patients' psoriasis worse. Given the large number of patients taking Humira, patient switching could be more lucrative than capturing new-to-therapy patients.

Amgen is also testing ABP 501 in rheumatoid arthritis, which Humira is also approved for. If that trial proves successful, look for Amgen to launch ABP 501 in 2017 after Humira's patents start expiring in late 2016.

That's if the FDA allows it
Biosimilars is a relatively new field in the U.S. Biologic drugs used to have de facto exclusivity after their patents expire because there wasn't a way to get a copycat drug approved without running full clinical trials equivalent to what was required for the brand-name drug. Considering the cost of the clinical trials, it wasn't worth the effort.

While some biosimilars have been approved in Europe for a few years, the Food and Drug Administration only recently wrote rules on how to get biosimilars approved after Congress included a pathway to approve biosimilars as part of the Affordable Care Act.

Small-molecule generic drugs generally only have to show equivalent dosing in a handful of patients before being approved. Because biosimilars are more complex, the Food and Drug Administration wants trials that actually show efficacy.

Fortunately, the trials are still much smaller than the originator had to run. Amgen enrolled 350 patients in the ABP 501 trial; to get Humira approved for plaque psoriasis, Abbott Labs (NYSE:ABT), which ran the trials before splitting off its drug division AbbVie, evaluated 1,212 patients in the pivotal trial, and another 147 patients in a second trial.

While I'm sure Amgen ran the clinical trial design by the FDA before performing the trials, investors should realize that Amgen and the rest of the companies developing biosimilars are entering uncharted territory that's apt to change as the FDA digs into the data that companies are generating to show equivalence.

Amgen is making a big move into biosimilars with five other molecules in development. The ABP 501 data is a good start; but it remains to be seen how easy it will be to gain FDA approval, and how much the biosimilars will be used by doctors.