The short interest in biotech and emerging pharmaceutical companies was mixed again in the final two weeks of November. Shares sold short in Alexion Pharmaceuticals (ALXN), Illumina (ILMN) and Regeneron Pharmaceuticals (REGN) grew somewhat between the November 15 and November 31 settlement dates.

But short interest in Amgen (AMGN), Arena Pharmaceuticals (ARNA), Dendreon (DNDN), Gilead Sciences (GILD), Onyx Pharmaceuticals (ONXX), Pharmacyclics (PCYC) and Vertex Pharmaceuticals (VRTX) declined during that time.

Three stocks saw larger swings in short interest between the November 15 and November 31 settlement dates. They were Biogen Idec (BIIB), Celgene (CELG) and VIVUS (VVUS).

Biogen Idec

This S&P 500 component saw short interest rise more than 12 percent to 2.65 million shares, its third period of growth and the highest it has been since June. But short interest remains a little more than one percent of the float.

The company develops and markets therapies for the treatment of neurodegenerative diseases, hemophilia and autoimmune disorders. In late November, Biogen Idec opened a new facility at its Research Triangle Park Campus in North Carolina. The company has a market capitalization of about $36 billion and a return on equity of more than 21 percent. The long-term earnings per share (EPS) growth forecast is about 15 percent.

Fourteen of the 25 analysts surveyed by Thomson/First Call recommend buying shares. But they believe the stock has little room to run as their mean price target is a little over three percent higher than the current share price. But that target would be a new multiyear high. Over the past six months, the stock has narrowly underperformed larger competitor Pfizer (PFE) but it has outperformed the S&P 500.

Celgene

Shares sold short in this biopharmaceutical company jumped about 33 percent to 6.12 million. That was the highest number of shares sold short since August. The short interest is less than two percent of the float.

This maker of therapies to treat cancer and immune-inflammatory related diseases has a market cap of more than $34 billion. Celgene is an S&P 500 component, and in November one of its directors was charged with insider trading. The company's return on equity is more than 28 percent, and the long-term EPS growth forecast is more than 13 percent.

Out of 27 analysts polled, 12 rate the stock at Strong Buy and seven others also recommend buying shares. The mean price target indicates upside potential of about seven percent and would be a new multi-year high. Shares are up about 30 percent in the past six months. In that time, the stock has outperformed the likes of Johnson & Johnson (JNJ), as well as the S&P 500.

VIVUS

Short interest in this Mountain View, California-based company increased more than nine percent to 22.03 million shares. That was the fourth consecutive period of increases and the largest number of shares sold short this year. Short interest is about 22 percent of the float.

VIVUS therapies address obesity, sleep apnea, diabetes and male sexual health. One of its largest shareholders called for a sale of the company in mid-November. The company now has a market cap of about $1.2 billion. The long-term EPS growth forecast is about 51 percent, but the return on equity is in negative territory.

Seven of the 12 surveyed analysts who follow the stock recommend buying shares. Their mean price target, or where they expect the share price to go, is almost 50 percent higher than the current share price. But that may be because shares have retreated more than 61 percent from the 52-week high back in July. Over the past six months, VIVUS has underperformed larger competitors Eli Lilly (LLY) and GlaxoSmithKline (GSK), as well as the broader markets.

ETFs

Exchange traded fund iShares Nasdaq Biotechnology (IBB) includes Biogen Idec and Celgene among its top 10 holdings. Shares are up about 34 percent year to date, despite a pullback in October. SPDR S&P Biotech (XBI) also includes Celgene as a top holding, and it is up more than 36 percent year to date, despite also pulling back in October.

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