By John McCormick

Susan Rice, believed to be a top contender to be Joe Biden's vice presidential running mate, has sold a significant proportion of the Netflix Inc. shares she has acquired since becoming a company director in May 2018.

The former national security adviser and United Nations ambassador for President Obama exercised roughly a quarter of the more than 5,200 options she held as of this week, according to a Securities and Exchange Commission filing dated Thursday.

Mr. Biden, the presumptive Democratic presidential nominee, is expected to announce his running mate next week. Ms. Rice is viewed as a strong contender because she worked closely with him when he was vice president and he is known to value personal relationships.

The Wall Street Journal reported earlier this week that Ms. Rice held the options in an article about her having more experience in corporate America than any of the other women Mr. Biden is believed to be considering.

At her sale price of $508.68 per share, Ms. Rice netted about $305,000 after accounting for the various strike prices associated with the options awarded to her as compensation.

"Ambassador Rice's sale of a fraction of her Netflix stock has nothing to do with V.P. speculation," Erin Pelton, Ms. Rice's spokeswoman, said in a statement. "She filed a stock plan pursuant to SEC regulations over three months ago."

A Netflix spokeswoman declined to comment.

The company's shares have been trading higher this summer as the pandemic has forced people to spend more time at home and boosted subscriptions to the entertainment service.

Even as she sold shares this week, Ms. Rice acquired additional options. As part of monthly compensation to board members, she was awarded 125 shares at an exercise price of $498.62, a Wednesday SEC filing shows.

Ms. Rice, 55 years old, would be the first Black woman on a major-party presidential ticket if she is nominated. If she were to become vice president, it would entail a significant pay cut because she would be expected to resign her Netflix directorship.

Ms. Rice had a net worth of between $23.5 million and $43.5 million as of 2009, according to the nonpartisan Center for Responsive Politics in Washington. That made her the wealthiest member of the executive branch that year, narrowly beating then-Secretary of State Hillary Clinton, the center reported. The disclosures Ms. Rice filed during the Obama administration required only broad ranges for assets, making it impossible to determine exact figures for some holdings.

Ms. Rice's stockholdings drew scrutiny from progressives in 2012 when she was under consideration for secretary of state. If Mr. Biden selects her, Ms. Rice's corporate ties could get additional scrutiny, as would her role in foreign policy during the Obama administration.

Specifically, Republicans have criticized her for initially characterizing deadly 2012 attacks on Americans in Benghazi, Libya, as an episode of spontaneous protest hijacked by extremists instead of a premeditated terrorist attack. They have also accused her of spying on Michael Flynn, President Trump's first national security adviser.

A 2014 House of Representatives report on the Libya incident found that while public statements by her and other U.S. officials were proven wrong, the inaccuracies stemmed from poor intelligence.

Declassified records, meanwhile, have shown no evidence that she improperly accessed information about Mr. Flynn, and Ms. Rice has called for greater transparency surrounding what she has said are legitimate counterintelligence concerns about Mr. Flynn's communications with Russia.

Write to John McCormick at mccormick.john@wsj.com