There's never a shortage of losers in the stock market.

Let's take a closer look at five of this past week's biggest sinkers.

Company

Oct. 19

Weekly Loss

My Watchlist

Apollo Group (NASDAQ:APOL)

$20.39

(29%)

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Isis Pharmaceuticals (NASDAQ:IONS)

$9.35

(26%)

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Advanced Micro Devices (NASDAQ:AMD)

$2.18

(20%)

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Clearwire (UNKNOWN:CLWR.DL)

$1.85

(20%)

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MannKind (NASDAQ:MNKD)

$1.98

(16%)

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Source: Barron's.

Apollo Group flunked out after continuing softness for the for-profit post-secondary educator. The parent company of the University of Phoenix announced that it would close some of its smaller campuses and trim its payroll.

Isis may have received a nod of approval from an FDA advisory committee for its cholesterol-tackling drug, but the concern here is that a nasty potential side effect -- 3% of the patients taking Kynamro during the clinical study developed tumors -- will restrict the drug's recommended usage.

Advanced Micro Devices took a hit after the PC chip maker posted disastrous quarterly results. If the 25% plunge in revenue or the larger-than-expected deficit doesn't scare you, the desperate move to dismiss 15% of its workforce in a cost-shaving move should do the trick.

Clearwire has been volatile since Softbank announced that it would be taking a 70% stake in its ally Sprint Nextel (NYSE: S). Many speculated that Sprint would seek to acquire Clearwire outright, but now it seems as if the country's third-largest wireless carrier may be content with merely a majority stake in Clearwire.

MannKind took one giant leap down after announcing plans to sell as many as 140 million shares of stock. Investors seemed to disregard the fact that MannKind needs the money for new trials to try to get its inhaled insulin treatment moving again. The transaction will ease the company's leveraged burden by swapping debt for equity. All that investors saw last week was dilution, so down went MannKind.

Longtime Fool contributor Rick Aristotle Munarriz has no positions in the stocks mentioned above. The Motley Fool has no positions in the stocks mentioned above. Motley Fool newsletter services recommend MannKind. Try any of our Foolish newsletter services free for 30 days. We Fools may not all hold the same opinions, but we all believe that considering a diverse range of insights makes us better investors. The Motley Fool has a disclosure policy.