Spectrum Pharmaceuticals (Nasdaq: SPPI) had a blowout performance, registering its sixth straight quarter of profitability as revenue grew 37%. Yet shares trade lower than they did before the results were announced last week.

What gives? Let's look at the sales numbers for the last few quarters to see whether we can parse it out.

Drug

Q2 2011

Q3 2011

Q4 2011

Q1 2012
Fusilev $34 $41 $43 $51
Zevalin $8 $7 $7 $6

Source: Company releases. Sales in millions.

It's all about Fusilev these days. There's nothing wrong with having one drug take over sales. Amylin Pharmaceuticals (Nasdaq: AMLN) sells Symlin, but the growth story is Byetta and its new extended release formulation, Bydureon. Elan (NYSE: ELN) sold a bunch of drugs and collected royalties from others, but investors only cared about its multiple sclerosis blockbuster Tysabri, which it sells with Biogen Idec (Nasdaq: BIIB). The growth stories were so different that Elan sold off the unit to Alkermes (Nasdaq: ALKS).

The problem for Spectrum is that Fusilev's nice run is thanks to a related generic drug that's been in short supply. Investors seem worried about what will happen when the generic drug comes back.

Doctors don't seem to think that's going to happen anytime soon. The number of accounts that ordered Fusilev more than doubled in the first quarter compared with the fourth quarter. Assuming those accounts eventually hit the average sales that the fourth-quarter accounts had, there's a ton of upside.

Normally, you'd expect that sales wouldn't hold once the supply of generic drugs was back at full strength, but the generics aren't the same product and aren't substituted directly. Doctors can specifically prescribe the generic instead of Fusilev, but since they treat cancer and are administered by the doctor, I think there's less of a motivation to save money than there would be for a typical drug for a chronic disease that patients pick up at a retail pharmacy. My guess is the longer a doctor uses Fusilev, the less likely the doctor will be to switch back.

Now if Spectrum could only get those Zevalin sales numbers up, too.

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