LONDON-- GlaxoSmithKline PLC on Wednesday cut the amount of money it plans to return to shareholders under an asset swap with Switzerland's Novartis AG as it reported a rise in first-quarter net profit, boosted by the deal.

Glaxo also said it would keep its stake in Viiv Healthcare, its HIV medicine joint venture with Pfizer Inc. and Shionogi & Co., after recent drug launches beat expectations. It had previously considered spinning off the unit.

The U.K.-based company said profit attributable to shareholders leapt to GBP8.09 billon ($12.28 billion), compared with GBP668 million a year earlier. Revenue edged up to GBP5.62 billion from GBP5.61 billion in the first quarter of 2014.

Profit was boosted by proceeds from the deal with Novartis, under which Glaxo sold its oncology unit and bought the Swiss company's vaccines business. The two companies also merged their consumer-health businesses.

Core operating profit, GSK's preferred measure of earnings, fell 14% on a constant exchange rate basis to GBP1.31 billion. Analysts surveyed by The Wall Street Journal had expected revenue of GBP5.65 billion and core profit of GBP1.31 billion.

However, the company cut the amount of money it will return to shareholders from the proceeds of the Novartis transaction to GBP1 billion from GBP4 billion, saying it wanted to maintain financial flexibility. The sum to be returned to shareholders will be paid via a special dividend alongside its fourth-quarter payment.

GSK said it would pay a first-quarter dividend of 19 pence a share and planned to pay an annual ordinary dividend of 80 pence a share for each of the next three years, 2015-2017.

The results were complicated by the closing of GSK's $20 billion asset swap with Novartis in the quarter, meaning they reflected a mix of the "old" and "new" company.

"With the completion of the Novartis transaction, we have reviewed future prospects for the newly shaped group, including the opportunities offered through the integration and our cash allocation strategy," Chief Executive Andrew Witty said as he set out the group's medium-term expectations.

While 2015 earnings will be diluted by the Novartis deal, Mr. Witty said earnings and revenue are expected to grow over the next five years from 2016 to 2020.

He expects group revenue to grow in the low-mid single digits over the five year period on a constant currency exchange basis, but earnings to grow faster than sales at mid-to-high single digits over the five years 2016-2020.

Write to Denise Roland at Denise.Roland@wsj.com and Ian Walker at ian.walker@wsj.com

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