Correction: Enbrel was approved in 1998. An earlier version of this story stated Enbrel was approved in 1989.

NEW YORK (TheStreet) -- Amgen (AMGN) - Get Amgen Inc. Report raised the price of its rheumatoid arthritis drug Enbrel -- again.

The 9.9% hike in Enbrel's list price pushed through by Amgen on May 1 illustrates once more how biotech and drug companies profit by raising the cost of their older medicines to offset falling prescription volume. Enbrel doesn't work any better for rheumatoid arthritis patients today than the drug did when first approved in 1998 -- but its does cost a lot more.

The same Enbrel with a list price of $10,000 per year in 1998 now costs almost $42,000 per year. Inflation only accounts for about one-third of Enbrel's price increase.

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Amgen relies on regular price hikes to maintain sales growth of its older drugs, which are being used by fewer patients. As I reported last month, Enbrel sales grew 13% to just over $1.1 billion in the first quarter, but only because Amgen raised the price of the drug 19% to offset a 2% drop in prescription volume.

The most recent uptick in Enbrel's list price follows two similar price increases last November (7.9%) and June (6.9%), according to Cowen.

Abbvie (ABBV) - Get AbbVie, Inc. Report also raised the price of Humira, a competing rheumatoid arthritis drug, by 9.9% on April 1. (And Amgen's hike matched that exactly one month later. Interesting.) Humira's patents in the U.S. and Europe expire in 2016 and 2018, respectively, paving the way for the approval and commercial launch of cheaper "biosimilar" versions.

The roll-out of less expensive but equally effective treatments for rheumatoid arthritis patients is likely to put a dent in revenue generated by Amgen and Abbvie. But until that day of reckoning comes, the prices of Enbrel and Humira will surely march higher.

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.