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

Aug. 16

Weekly Loss

Vical (NASDAQ:VICL)

$1.40

61%

MannKind (NASDAQ:MNKD)

$5.77

23%

Orbitz Worldwide (NYSE:OWW)

$9.68

18%

Portugal Telecomm (NYSE:PTGCY)

$3.76

10%

Kandi (NASDAQ:KNDI)

$4.44

10%

Source: Barron's.

Let's start with Vical. The biotech shed more than half of its value after abandoning its once promising Allovectin program. Data from the potential cancer tumor-shrinking treatment's late-stage trial failed to meet the study's efficacy goals.

Vical will turn its attention to infectious disease vaccine programs, but the rigorous and lengthy approval process in getting a new treatment to market isn't going to fly with investors who were hoping for Vical to have a product on the market sooner rather than later.

MannKind is another company that took a hit on late-stage trial news, but this announcement was initially taken as a positive. Shares of MannKind popped higher announcing positive efficacy results for its inhalable insulin treatment for both primary types of diabetes.

It could have been a case of "sell on the news" kicking in. Impatient investors also may have gotten cold feet, since regulatory approval won't be completed until next year at the earliest.

There were also a couple of small negatives. The type 2 diabetes study found the majority of its Afrezza-inhaling test subjects experiencing low blood sugar. The type 1 diabetes results achieved target levels that weren't as effective as injection-based insulin that's already on the market.

In the end, these may be just minor setbacks given the clear appeal of needle-free insulin. It wouldn't be a surprise to see MannKind bounce back after the largely positive report.

Orbitz shares went on a steep descent after a large investor moved to sell a sizable chunk of its stake in the online travel portal. PAR Capital Management unloaded 8.1 million of its 24.6 million shares, and that's a tenth of the total Orbitz outstanding shares.

PAR Capital Management made the move to diversify. It's still holding on to most of its position. However, the market doesn't appreciate the dramatic expansion of more shares floating around.

Portugal Telecomm disconnected with the market after dramatically slashing its once beefy 11% yield. The telco is struggling on its home turf, but things are faring even worse in Brazil, where eroding currencies and cost-control concerns are weighing on the bottom line.

Kandi powered down after posting ho-hum quarterly results. The Chinese maker of ATVs, go-karts, and, more recently, electric vehicles posted a small adjusted profit as revenue climbed 10% to $12.2 million. That's not a lot, and it's easy to get concerned on learning that just $2 million of those sales were for electric vehicles.

That's fine. Investor interest in Kandi has increased since receiving government approval for its first Geely co-developed pure electric sedan, and the market will have to wait and see that play out.

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Longtime Fool contributorĀ Rick Munarriz and The Motley Fool have no position in any of the stocks mentioned. Try any of our Foolish newsletter services free for 30 days. We Fools don't 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.