Biotech stocks couldn't hold up the sector's record highs of recent weeks, as the Nasdaq Biotechnology Index lost more than 4.3% over the past five days. While the sector suffered a terrible start to the week, investors can't blame all of biotech's losses on the Washington shutdown: A few key stocks made investors pull their hair out in frustration this week, led by Ariad Pharmaceuticals' (NASDAQ:ARIA) absolute implosion.

Ariad wasn't the only stock to suffer in a week where a meager few biotech picks managed to stay in the green. Let's catch up on what you need to know from the sector's woes this week.

Ariad's not the only biotech falling
Idenix Pharmaceuticals
(NASDAQ:IDIX) took a tough blow over the past five days, as shares lost nearly 24%. Idenix has already taken one big hit recently: Back in June, this stock nosedived by 35% on a single day after the FDA put a hold on the company's plans of human clinical trials for its developmental hepatitis-C drug, IDX20963. Regulators requested more safety data at the time, an unfortunate setback for a company that's already been hurt by safety concerns of earlier drugs.

Now Idenix has taken another hit after the company's partner, Johnson & Johnson, opted to acquire a rival hepatitis-C medication from GlaxoSmithKline. Idenix will have to seek a new partner for its drug now, and time's slipping away in its fight to capture a piece of the oral hepatitis-C market. AbbVie and Gilead are surging ahead in this race to bring the first all-oral hep-C drug to approval for a market that analysts say could reach $20 billion, and Idenix will need to move fast if it wants to hold its own in this race.

NPS Pharmaceuticals (NASDAQ:NPSP) has been one of the darlings of the biotech sector this year, but the stock dropped more than 14.5% over the past week. Still, investors hardly can be displeased by the stock in 2013: NPS's shares have jumped more than 225% year-to-date. With little news out from this company over the past five days, the weekly loss looks more like an example of profit-taking rather than anything seriously wrong with the stock.

In the long term, NPS looks like a winner. Between its successful launch of short bowel syndrome treatment Gattex so far to the company's ongoing development of hypoparathyroidism treatment Natpara, NPS is looking up. Even with the stock's big gains in 2013, I don't think this week's fall is an indication that NPS has hit its zenith -- especially if Gattex continues to roll.

The biggest biotech story this week, however, was Ariad. Shares collapsed to the tune of a 77.3% fall over the past five days. The biggest drop came Thursday, after the FDA initiated a partial hold on clinical trials of Ariad's developmental leukemia drug Iclusig when earlier study data showed an increase in heart problems and blood clots in patients.

Iclusig's still on the market and selling after being approved for two other indications in the recent past, but Ariad had hoped to expand the drug's market. Now with the FDA launching an investigation into Ariad's drug and any reported side effects from it, the future's looking murky. Wall Street's already notched several downgrades for Ariad's stock, and with the situation in flux, it's a dangerous time for Ariad shareholders. Caution should rule the day with this stock until the situation progresses, so keep your eye on Ariad in the near future in case the FDA comes up with more news -- either good or bad.

Fool contributor Dan Carroll has no position in any stocks mentioned. The Motley Fool recommends Gilead Sciences and Johnson & Johnson and owns shares of Johnson & Johnson. 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.