Serelaxin (Recombinant Human
Relaxin-2)

The Discovery

The Mystery Hormone

In 1926, an American scientist named Frederick Hisaw noticed something remarkable about pregnant ...

In 1926, an American scientist named Frederick Hisaw noticed something remarkable about pregnant guinea pigs. Their pubic bones, normally rigid, became flexible just before birth. Something in their blood was loosening the connective tissue to allow the baby to pass through. Hisaw spent years trying to isolate this mysterious substance. He finally purified it in 1930 and named it relaxin, because it relaxed the pelvic ligaments. For decades, relaxin was considered a simple pregnancy hormone — interesting but not particularly important outside of reproduction.

The Breakthrough

The Sleeping Giant Wakes

In the 1980s and 1990s, scientists discovered that relaxin was far more than a pregnancy hormone.

In the 1980s and 1990s, scientists discovered that relaxin was far more than a pregnancy hormone. It had powerful effects on blood vessels, the heart, and the kidneys. It made blood vessels wider, improved blood flow, and reduced fibrosis — the scarring that damages organs in chronic disease. Researchers found relaxin receptors throughout the body, not just in reproductive tissue. The hormone reduced inflammation and helped the kidneys filter blood more efficiently. By 2000, pharmaceutical companies were paying attention. A biotech company called Corthera began developing a synthetic version of relaxin-2 called serelaxin for treating acute heart failure.

The Trials

The Big Test Begins

In 2013, a large clinical trial called RELAX-AHF tested serelaxin in patients hospitalized with a...

In 2013, a large clinical trial called RELAX-AHF tested serelaxin in patients hospitalized with acute heart failure. The drug was given as a 48-hour IV infusion. The results were exciting: patients who received serelaxin felt better faster, had less organ damage, and — most importantly — were less likely to die within six months. The drug seemed to protect the heart, kidneys, and liver simultaneously. Novartis, which had acquired Corthera, poured resources into development. A massive Phase 3 trial called RELAX-AHF-2 enrolled over 6,600 patients at 370 hospitals across 35 countries.

The Crisis

The Moment of Hope

The medical community waited eagerly for RELAX-AHF-2 results.

The medical community waited eagerly for RELAX-AHF-2 results. Heart failure kills more people than most cancers, and no new drug had dramatically improved survival in years. Doctors hoped serelaxin would be the breakthrough. In March 2017, Novartis announced the results: serelaxin failed. It did not reduce deaths or worsening heart failure compared to placebo. The result was devastating for researchers who had spent decades studying relaxin. Novartis abandoned the program entirely. Some scientists questioned whether the trial design was flawed. Others wondered if the drug needed to be given differently.

The Legacy

The Unexpected Ending

Despite the clinical failure, relaxin research hasn't stopped.

Despite the clinical failure, relaxin research hasn't stopped. Scientists continue studying why the hormone protects organs so effectively in animal models but failed in the large human trial. New approaches include longer-acting versions of relaxin, gene therapy that makes the body produce its own relaxin, and targeting specific relaxin receptor subtypes. Researchers are also studying relaxin for fibrotic diseases like liver cirrhosis and lung fibrosis, where its anti-scarring properties might shine. The story of relaxin-2 is a humbling reminder that biology is more complex than any single trial can capture. The mystery hormone discovered in pregnant guinea pigs a century ago still has secrets to reveal.