Tectonic rebuilt relaxin to last. Now it is testing it in a heart-failure subtype with no approved therapy.

Tectonic Therapeutic didn't rediscover relaxin. It re-engineered the pregnancy hormone into a molecule the body can actually keep around long enough to treat chronic disease. That distinction is the entire bet behind TX45, a single-chain, Fc-fused relaxin analog now in the home stretch of a 191-patient Phase 2 readout in a pulmonary-hypertension subtype of heart failure with no approved disease-modifying therapy.

Native relaxin is a two-chain peptide the body clears in minutes. Novartis's serelaxin, the last big attempt to drug the pathway, carried that same pharmacokinetic ceiling into a large acute heart failure program and failed on outcomes in 2013, partly because the molecule wasn't sticking around long enough to do sustained work on the vasculature. Tectonic's bet, originating in Andrew Kruse's structural-biology lab at Harvard Medical School around 2017, is that the biology was right and the molecule was wrong. The fix was a protein-engineering trick, not a new indication. After the lab stabilized the engineered molecule, Harvard's Office of Technology Development helped spin the program out into a company that is now publicly traded on Nasdaq as TECX.

The trick has two parts. First, the two chains of native relaxin were stitched into a single continuous chain, so the receptor still gets activated but the peptide doesn't fall apart the way it does in serum. Second, that single chain was fused to the Fc fragment of an antibody, the same half-life-extension scaffold used in many approved biologics, so the drug survives long enough to dose once weekly instead of by continuous infusion. Kruse told the Harvard Gazette his lab had spent years trying to find a way to "make this molecule behave like a real drug." The single-chain Fc fusion is the answer.

That engineering now determines what the APEX Phase 2 trial can and cannot tell us. APEX is a 24-week, randomized, double-blind, placebo-controlled study in PH-HFpEF, a Group 2 pulmonary hypertension phenotype sitting on top of HFpEF (heart failure with preserved ejection fraction) and carrying a particularly high mortality. Roughly 1 million U.S. patients live with HFpEF, and the CpcPH (combined pre- and post-capillary) subset that APEX enriches for is the slice whose pulmonary vasculature is actively remodeling rather than passively reflecting high left-sided filling pressures. Enrollment closed June 10, 2026 at 191 patients across 14 countries; 137 of them, about 72%, sit in the enriched CpcPH subgroup with baseline pulmonary vascular resistance above 3 Wood units. Mean baseline PVR is 4.2 WU overall and 5.2 WU in the CpcPH subset. The primary endpoint is change from baseline in PVR in the PVR>3 WU population. Topline is expected early in the first quarter of 2027.

If TX45 moves PVR in the enriched arm, the result supports the engineering thesis: a relaxin molecule with a long enough half-life to engage the right receptor chronically can do the vascular work the old two-chain version could not, and the underlying biology (relaxin's role in softening vascular tone, damping fibrosis, and remodeling extracellular matrix during pregnancy) finally has a molecule durable enough to test it. If it misses, the explanation gets harder. Either the biology doesn't carry, half-life alone wasn't the binding constraint, or the dose, the patient selection, or the 24-week window wasn't right. Serelaxin's failure left an open question about whether the cardiovascular field had given the relaxin pathway a fair shot, and APEX is now the test of whether the rebuilt molecule earned a different answer.

Two design choices in the trial are worth flagging. Enriching for PVR>3 WU, rather than enrolling all comers with PH-HFpEF, narrows the population to patients whose pulmonary vasculature is genuinely the problem and gives the molecule its cleanest shot at a hemodynamic effect. Using PVR as the primary endpoint, rather than a symptoms or outcomes measure, is a faster, smaller readout that the company and its investors can use to decide whether to run a larger outcomes trial, but it leaves the symptom-relief and mortality questions for the next study. Both are reasonable calls for a Phase 2 program; they also mean a positive APEX will not, by itself, answer whether TX45 makes PH-HFpEF patients feel better or live longer.

The next test date is the topline. Watch the PVR change in the CpcPH subgroup first; that is where the engineering bet is on the line.