An investment firm for Thomas and Andreas Struengmann disclosed sales of about $110 million of BioNTech SE shares this year and filed last month to offload almost double that sum in one of the billionaires’ biggest selling sprees for the New York-listed stock, according to regulatory disclosures.

The brothers, who were seed investors in BioNTech, are still the German company’s biggest shareholders with a roughly 43% stake ahead of the upcoming sales. BioNTech partnered with Pfizer Inc. to develop the first Covid vaccine to be approved by the US Food and Drug Administration.

A representative for the 73-year-old Struengmanns — who have a combined net worth of about $23.4 billion, according to the Bloomberg Billionaires Index – didn’t respond to a request for comment. A representative for BioNTech declined to comment.

The sales show the brothers are increasingly realizing the gains they made from their early bet on BioNTech, which saw its share price boom to record highs during the pandemic.

The company’s stock has tumbled since then as demand wanes for the Covid shot, leading it to slash its forecast for this year’s vaccine sales by 20%. Still, the brothers are poised to lock in major gains after first providing BioNTech with €150 million ($186 million) in seed money when it was founded in 2008. The shares have climbed more than 550% since its US initial public offering in 2019, and were up as much as 9.1% Monday after it reported an unexpected profit. 

The Struengmanns are ramping up stock sales in their biggest listed asset as they lead the buyout of Schuelke & Mayr GmbH in a deal set to close this quarter that valued the German hand sanitizer-maker at about $1.5 billion. They have also allocated funds to at least three early-stage companies, including Danish biotechnology firm Bactolife and German silk-protein business AMSilk, in addition to recently boosting their real estate holdings. 

The sales signal the billionaires will keep making big bets on the pharmaceutical, life science and real estate sectors that primarily underpin the strategy for managing their fortune.

They do so through Athos, one of the world’s largest family offices, which was set up soon after Novartis AG announced in 2005 it was buying the brothers’ drug maker, Hexal, along with their stake in affiliate EON Labs for a combined €5.7 billion.

Athos was led for more than a decade by BioNTech Chairman Helmut Jeggle, but he stepped back from running the Munich-based firm in 2021 to focus on his own ventures. That year, it recruited general manager Wolfgang Kessler, a veteran investment professional with experience in real estate.

The brothers have since disclosed the purchase of a £26 million building in one of London’s financial districts and are also constructing about two dozen luxury houses at a lakeside location in their homeland. This year, they boosted their holding in German biotechnology firm Formycon AG and bought a stake in NexWafe, a solar-wafer producer that counts divisions of Mukesh Ambani’s Reliance Industries Ltd. and Saudi Aramco as investors.

Athos has about 40 employees and advertised this year for an investment role and a managerial position to implement a sustainability strategy. Those seeking to work at the Munich-based family office were encouraged in the postings to have a loyal mindset and humble appearance, in keeping with the low profile of the Struengmann brothers.