Updated with correct short interest information.

SAN DIEGO ( TheStreet) -- The most recent breakdown of Arena Pharmaceuticals' ( ARNA - Get Report) shareholder base shows ownership still largely in the hands of retail investors. Institutional investors sat on their hands during the fourth quarter.

One hundred forty-two institutional investors filed 13Fs listing Arena among their stock holdings at the end of the fourth quarter, according to Whalewisdom.com. The aggregate number of Arena shares in these 13Fs totaled 99.4 million at Dec. 31, 2012, an increase of 400,000 shares over the Sept. 30, 2012 filings.

Arena's overall institutional investor ownership remained unchanged at 47% at the end of the fourth quarter, Likewise, public, or retail, ownership of the stock was fixed at 52%, according to CapitalIQ.

Arena's stock price rose 12% from $8 to $9 during the fourth quarter but has since fallen back. Arena shares closed Tuesday at $8.35.

Arena and marketing partner Eisai are expected to launch the weight-loss pill Belviq sometime in March, following the completion of the DEA scheduling process. Vivus ( VVUS - Get Report) continues to struggle with the marketing of its weight-loss pill Qsymia, so investors will be watching the Belviq launch closely to see if Arena and Eisai can do any better.

Thirty institutional investors initiated new positions in Arena during the fourth quarter 2012, with another 62 funds adding to existing positions, according to Whalewisdom.com. On the other side of the ledger, 21 funds closed out Arena positions and 60 funds reduced their exposure to the stock.

Short interest in Arena remains high at 55.1 million shares at the end of January, according to Nasdaq.

Next week, Vivus is expected to release financial results for the fourth quarter. On average, analysts expect Qsymia sales of $3.34 million.

-- Reported by Adam Feuerstein in Boston.

Adam Feuerstein writes regularly for TheStreet. In keeping with company editorial policy, he doesn't own or short individual stocks, although he owns stock in TheStreet. He also doesn't invest in hedge funds or other private investment partnerships. Feuerstein appreciates your feedback; click here to send him an email.