URGENT: Pfizer secretly halted a pivotal trial on a major new drug for sickle cell disease

In December, Pfizer quietly halted much of its work on the pivotal clinical trial of one of its most highly touted drugs, a new medicine for sickle cell disease — barely a month after telling investors it believed the drug was worth over $4 billion.

The drug, osivelotor, was in a Phase 3 trial, the final step in proving a medicine is safe and works and can be approved. Few drugs for sickle cell exist, and Pfizer has emphasized its work on new treatments, as well as their profit potential.

There was only one problem.

Even as the company talked up osivelotor, regulators in the United States and Europe were moving against it. In December, the Food and Drug Administration ordered Pfizer not to enroll more patients in the Phase 3 trial. Pfizer kept that fact quiet until late February, when it disclosed the halt in one sentence on page 48 of a 120-page financial filing.

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In the filing, Pfizer downplayed the FDA’s move, calling it a “partial clinical hold” and claiming that although it was not recruiting new patients for the trial, patients already taking osivelotor in the trial could remain on it.

But it is not clear how many, if any, patients still are receiving the drug.

Two physicians who specialize in treating sickle cell said Pfizer had told them the trial is indefinitely suspended and said nothing about patients currently receiving the drug.

“All we’ve been told is they’ve been reviewing all the data that they were going to review internally and talk to the FDA and EMA [the European Medicines Agency],” said Dr. Alan Anderson, a South Carolina physician who oversees the treatment of over 400 sickle cell patients. “No clinicians have been given any timetable as to whether there might be a report from the FDA or EMA, or when the trial might be restarted.”

In addition, the EMA’s Website refers to the trial’s three German locations simply as “suspended.” And a manager at Medpace, a third-party research organization which Pfizer had hired to monitor and manage the trial and recruit researchers, said in an email that “Medpace is no longer working on this trial.”

Pfizer did not respond to emailed questions asking for an interview. (I am suing Pfizer’s chief executive officer, Dr. Albert Bourla, in Berenson v Biden, so it is likely Pfizer has a blanket policy not to comment on or even acknowledge communications from me.)

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(Overall trial status: Suspended)

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The FDA’s move followed the deaths of patients taking another and very similar Pfizer drug for sickle cell called Oxbryta. (I am separately investigating Pfizer’s actions involving Oxbryta, what it knew about that drug’s dangers, and when it learned of them, and will have more on that story soon.)

Pfizer abruptly stopped selling Oxbrtya in September, a decision it called voluntary. But it said publicly it would keep developing osivelotor. Pfizer acquired both medicines in 2022, when it spent $5.4 billion on Global Blood Therapeutics, a California drugmaker that specialized in sickle cell treatments.

On a conference call with investors on Oct. 29, Dr. Mikael Dolsten, Pfizer’s then-chief scientific officer, said that osivelotor “has more potency” than Oxbryta and “looked really nice in tolerability.”

Dolsten added Pfizer had believed osivelotor represented up to 80 percent of the value of the Global Blood Therapeutics deal — or about $4.3 billion at the time of the purchase.

“I'm optimistic,” Dolsten said. “And as a final end here, please remember that when we did the GBT [Global Blood Therapeutics] deal, our eyes were really on osivelotor.”