When it comes to biotechs, Amgen (NASDAQ:AMGN) qualifies for the senior citizen discount. The company, founded way back in 1980, ranks as one of the pioneers of the industry.

Amgen announced its latest quarterly results after the market close on Tuesday. Investors were clearly disappointed, with shares falling 6% in after-hours trading. Should you buy stock in Amgen after this pullback -- or is this oldie no longer a goodie?

Crunching the numbers
Amgen announced adjusted earnings of $1.96 per share, a 22% increase year-over-year. This easily beat analyst estimates of $1.84 per share.

GAAP earnings per share numbers also looked solid, coming in at $1.88 per share. That reflects an increase of 27% over 2012 first quarter's GAAP earnings of $1.48 per share.

With good earnings results, why did shares drop so much in trading after the market close? One answer comes from the top line. Amgen reported revenue of $4.2 billion in the first quarter, up 5% from the same period in 2012. However, this total missed the consensus analyst expectation of $4.37 billion.

Also, part of the improvement in adjusted earnings stemmed from tax benefits and share buybacks. That makes the earnings growth somewhat less impressive. Amgen's adjusted net income for the quarter still grew 16% compared to 2012, though.

The biotech also provided earnings guidance for full-year 2013 above the midpoint of the range from $7.05 to $7.35 per share. This projection comes in near or perhaps higher than the $7.21 per share average analyst estimate.

On revenue, though, Amgen reiterated previous guidance of $17.8 billion to $18.2 billion. Analysts expect revenue of $18.07 billion.

Looking ahead
If shares stay down 6% or so after these quarterly results, is Amgen a buy? I think so.

Enbrel still runs behind AbbVie's (NYSE:ABBV) Humira in the rheumatoid arthritis market. Humira chalked up 2012 sales of $9.3 billion, while Amgen made $4.2 billion from Enbrel. Pfizer (NYSE:PFE), Amgen's partner with the drug, recorded $3.7 billion revenue for Enbrel.

However, Enbrel is still showing solid growth with first quarter 2013 sales up 11% year-over-year. Amgen stands to benefit beginning later this year as Pfizer yields its royalties for the North American sales.

Also, while AbbVie loses patent exclusivity for Humira at the end of 2016, Amgen received patent approval in 2011 that should give Enbrel protection through 2028. This could allow Amgen to gain the lead in the rheumatoid arthritis market, although the competition should continue to be fierce.

The real key for Amgen's long-term health will be in replacing lost revenue from its Neulasta/Neupogen franchise. Teva Pharmaceuticals (NYSE:TEVA) reached an agreement with Amgen in 2011 that allows marketing of a biosimilar in the U.S. beginning this November. Teva already competes against Neupogen in Europe with its Neutroval biosimilar.Sales for Neulasta/Neupogen were flat in the first quarter, but don't expect that to hold up for much longer.

Several of Amgen's products are on the move, though. For example, Xgeva/Prolia sales jumped 51% in the first quarter of 2013 and now total $365 million. Expectations are for the franchise to reach $1 billion or more in peak annual sales.

The biotech should report news later this year on several other promising drugs in its pipeline. Amgen plans to discuss additional findings from a phase 3 study of Talimogene laherparepvec near the end of the year. Results from a late-stage study of Trebananib in treating ovarian cancer are expected for mid-2013.

Plenty of kick left
I suspect that this pullback will only be temporary. The first-quarter results weren't that bad, even with disappointing revenue figures. Amgen might be something of an old codger in the biotech world, but I think it has plenty of kick left. This stock remains a buy in my view.