Former Brevan Howard portfolio manager Robert Sicari is facing a lawsuit from a quantitative researcher who claims he was denied up to $3.5m in trading profits and an equity stake in a hedge fund they established after leaving the firm, according to a report by Bloomberg.

Wooyoung Hong, who previously worked under Sicari at Brevan Howard, filed a complaint in New York State Supreme Court alleging that Sicari failed to honour a compensation agreement at ThirtyPark Capital, a fund launched following their departures from Brevan Howard in 2024.

Hong claims he was promised 50% of the profits generated from certain trades and a 30% ownership stake in ThirtyPark in exchange for developing the firm's trading infrastructure and providing quantitative research support.

According to the lawsuit, Hong generated substantial gains through event-driven trades linked to several major corporate transactions, including Nippon Steel's acquisition of US Steel, Chevron's takeover of Hess and the Paramount-Skydance merger.

The US Steel trades alone reportedly generated returns of approximately 80%, with Hong claiming entitlement to between $1.2m and $2m from those positions. He estimates his total unpaid compensation from November 2024 to December 2025 could reach $3.5m.

Hong alleges that Sicari subsequently sought to renegotiate their agreement, proposing a $400,000 cash payment and a 30% profit share that would vest over time, citing the need to preserve capital at the fledgling fund.

After Hong rejected the revised terms, Sicari allegedly revoked his access to ThirtyPark's trading systems in January. Hong claims he received just $20,000 for more than a year of work.

The two had previously worked together at Brevan Howard, where Sicari managed a portfolio worth hundreds of millions of dollars. Hong, a Harvard-trained physicist and former Google employee, joined the hedge fund in 2023 on a reported $400,000 base salary.

Sicari told Bloomberg that ThirtyPark Capital is no longer operating but declined to comment further on the allegations. His lawyer also declined to comment, while Brevan Howard, which is not a party to the proceedings, declined to comment.

The case is being heard in New York State Supreme Court.