(Bloomberg) -- Bristol-Myers Squibb Co.’s leading cancer treatment fought a serious blood cancer better than Seagen Inc.’s in a study, raising questions about the biotech’s portfolio just weeks after Pfizer Inc. agreed to buy it.

Bristol’s blockbuster Opdivo halted the progression of advanced Hodgkin’s lymphoma in 94% of patients who also received chemotherapy, compared with 86% of those who received Seagen’s Adcetris plus the standard treatment. Four patients who received Opdivo died during the one-year study of 976 people, versus 11 who took Adcetris, according to findings released Sunday at the American Society of Clinical Oncology’s annual meeting in Chicago. 

Seagen is a leader in designing, testing and making antibody-drug conjugates — precision therapies that deliver cancer-killing drugs so potent they might otherwise be too toxic to use. The comparison to Bristol’s immune therapy could raise investor concerns as Pfizer Inc. prepares to acquire Seagen for $43 billion. 

The head-to-head trial is “practice-changing,” said Alex Herrera, an oncologist at the City of Hope National Medical Center in Duarte, California who led it. Opdivo plus chemotherapy “is poised to become the way we treat patients with stage 3 and 4 Hodgkin’s lymphoma.”

Pfizer Chief Executive Officer Albert Bourla is under increasing pressure to show the company can replace billions in revenue from Covid-19 products whose sales are falling as the pandemic eases. The Seagen deal is part of its bid to expand aggressively into oncology, one of the industry’s biggest growth opportunities. Adcetris is expected to generate $1 billion in sales this year, mainly for treatment of Hodgkin’s lymphoma as well as other lymphomas.

Opdivo was better tolerated by the teens and adults studied than Adcetris, according to Herrera. Patients who took the Seagen drug needed injections to raise levels of white blood cells, he said, and the therapy can cause bone pain. Opdivo patients also had less numbness and pain in the fingers and toes than the group that took Adcetris, which saw twice as many participants drop out due to side effects.

Longer follow-up is needed to assess the overall survival rate and patient outcomes, Herrera said. Whether the drugs could be used together as part of initial treatments is “a question for the future,” he said. 

The study was conducted by SWOG, formerly called the Southwest Oncology Group, a government-supported research organization.

Opdivo is among a group of drugs including Merck & Co.’s bestselling Keytruda that allow the immune system to recognize and kill cancer cells. Adcetris operates by a different mechanism, with similar drugs using antibodies to deposit a strong concentration of medication directly at a tumor site.

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