(Adds company comment from conference call, details on DoJ probe, analyst comment; updates shares)

By Manas Mishra and Trisha Roy

Nov 5 - Drugmaker Regeneron Pharmaceuticals Inc beat analysts' estimates for third-quarter profit on Tuesday, boosted by higher demand for its blockbuster eye drug, Eylea, and eczema therapy Dupixent.

Shares of the company rose 6.8% to $332, adding over $2 billion to Regeneron's market value after it said its board had authorized a $1 billion buyback of common stock.

Sales of Eylea, which has brought in over a $1 billion in quarterly revenue for more than two years, surged despite Roche Holding AG and Novartis AG's Lucentis, among others, snapping at its heels.

Quarterly sales of Eylea rose 14% to $1.92 billion, ahead of analysts' estimate of $1.61 billion, according to Refinitiv estimates. The drug treats age-related degeneration in eyes.

The sales were helped by a shortage of rival Roche Holding's Avastin, Regeneron commercial head Marion McCourt said on a conference call.

"We are hearing that patients that started on Eylea because of these shortages do continue on Eylea therapy," said McCourt.

Analysts, however, fear the recent approval of Novartis' anti-blindness drug Beovu could eat into sales of Eylea, adding to its troubles as it is expected to lose patents in the next five years.

Regeneron also disclosed in a filing on Tuesday that it had received a civil investigative demand from the U.S. Department of Justice into incentives paid to doctors that could violate certain federal laws.

The request for information relates to several of its drugs including Eylea and Dupixent, Regeneron said, adding that it was cooperating with the investigation. Meanwhile, Regeneron is expecting Dupixent, developed with Sanofi, to be its next growth driver and has massed up approvals for three major diseases including asthma and nasal polyps, since the drug was approved in 2017.

Dupixent's quarterly sales more than doubled to $633.1 million, beating expectations of $620.6 million.

Separately, the company also reported interim data from a late-stage trial of its drug Libtayo in lung cancer patients.

Guggenheim analyst Yatin Suneja said the rate of response for Libtayo seen in the trial was broadly in line with Merck & Co's Keytruda, which dominates the market for first-line treatment of advanced lung cancer.

The drug could be a potential threat to Keytruda if it continued to display similar responses, Suneja said in a client note.

Excluding one-time items, Regeneron earned $6.67 per share, above the average analyst estimate of $6.36.

Total revenue rose 23.1% to $2.05 billion. (Reporting by Manas Mishra and Trisha Roy in Bengaluru; Editing by Shinjini Ganguli)