Abcuro has raised $66 million in Series D financing to fund another potentially registrational study of ulviprubart in inclusion body myositis, making an unusually clear investor bet on a prespecified subgroup after the company’s previous Phase 2/3 trial failed its primary and key secondary endpoints. New Leaf Venture Partners led the financing, with participation from existing investors including Bain Capital Life Sciences, Samsara BioCapital, Redmile Group, RA Capital Management, Sanofi Ventures, Foresite Capital and NEA, while Rock Springs Capital joined as a new investor. Abcuro expects the next study to begin in the fourth quarter of 2026, with topline results targeted for the second half of 2028 and a potential BLA filing if the trial succeeds.
The financing is notable precisely because MUSCLE did not produce a conventional late-stage win. The 272-patient study randomized participants to ulviprubart at 0.5 mg/kg, ulviprubart at 2 mg/kg or placebo, with dosing every eight weeks and change in the Inclusion Body Myositis Functional Rating Scale at week 76 serving as the primary endpoint. In the overall population, IBMFRS declined by 1.7 points with the lower dose, 2.1 points with the higher dose and 2.4 points with placebo. The respective comparisons did not reach statistical significance, with P values of 0.086 and 0.373.
Why is Abcuro trying again after a Phase 2/3 failure?
The rationale rests on a prespecified analysis of patients entering MUSCLE with less severe disease. Among participants with baseline IBMFRS scores of at least 29, both ulviprubart doses produced an average 1.3-point decline at week 76 compared with a 2.6-point decline on placebo. Abcuro characterizes that difference as a 50% slowing of functional deterioration, although the subgroup comparisons still did not cross conventional statistical-significance thresholds, with P values of 0.066 and 0.075 for the two doses.
That distinction is central to interpreting the new financing. The subgroup was prespecified, making it more credible than an entirely retrospective search for patients who happened to respond after a failed trial, but it remains a subgroup whose result needs independent prospective confirmation. Investors are effectively financing a hypothesis generated and partially supported by MUSCLE rather than financing a drug that has already demonstrated pivotal efficacy.
This is a legitimate clinical-development strategy when disease stage plausibly influences treatment biology. A drug designed to prevent immune-mediated muscle destruction might be more effective before substantial irreversible muscle damage has accumulated. The next trial can now test that proposition prospectively by enrolling the population in which Abcuro believes sufficient salvageable muscle remains.
How does ulviprubart differ from broad immune suppression?
Ulviprubart, previously known as ABC008, is a monoclonal antibody targeting killer cell lectin-like receptor G1, or KLRG1. Abcuro’s therapeutic thesis is that highly differentiated KLRG1-positive cytotoxic T cells contribute to chronic muscle injury in inclusion body myositis and can be depleted selectively while sparing much of the broader immune system.
That approach differs from generalized immune suppression, which can reduce inflammation but also affect wide populations of immune cells. IBM has historically proved resistant to many conventional immunomodulatory approaches, contributing to the absence of an approved pharmacological therapy despite clear inflammatory features within affected muscle.
The mechanistic question is whether the cytotoxic T-cell population Abcuro removes is sufficiently central to disease progression. MUSCLE provided pharmacological evidence that ulviprubart can engage its intended immune-cell population, but target engagement is only useful if it translates into preservation of muscle function.
Why might earlier-stage IBM be biologically easier to modify?
IBM combines inflammation with progressive muscle degeneration. Once substantial muscle tissue has been lost and replaced by fatty or fibrotic tissue, stopping an immune driver may not recreate functioning muscle that has already disappeared.
This creates an asymmetry common to neurodegenerative and muscular diseases. A therapy can successfully suppress the mechanism causing ongoing damage yet produce only a modest functional effect if treatment begins after too much irreversible injury has occurred. The subgroup selected by higher baseline IBMFRS scores may therefore represent patients with more remaining functional reserve.
That theory is clinically plausible, but the next study needs to distinguish biological truth from statistical variation. The less severe patients were only part of the 272-person MUSCLE population, so effect estimates carry more uncertainty than the full randomized comparison.
What did MUSCLE show about safety?
Abcuro reported broadly similar tolerability between ulviprubart and placebo. Falls were the most common adverse event and occurred in roughly half of participants across treatment and placebo groups, which is not unexpected in a progressive muscle disease characterized by weakness and impaired mobility. Chills, headache, fever and nasopharyngitis were among treatment-emergent events occurring more frequently with ulviprubart. No participant receiving ulviprubart discontinued treatment because of a treatment-emergent adverse event, according to the company’s detailed presentation.
That safety profile matters because the next development strategy targets patients earlier in their disease. Physicians may tolerate greater treatment risk when function has already deteriorated severely, whereas an intervention intended to preserve function over several years must justify exposing less impaired patients to long-term immune-cell depletion.
The next trial will therefore have to confirm both the functional signal and the sustainability of selective KLRG1-positive T-cell depletion across longer treatment.
Why does inclusion body myositis remain commercially attractive despite being rare?
IBM is relentlessly progressive and lacks an approved pharmacological treatment, leaving patients facing declining grip strength, difficulty rising from chairs, falls and eventual substantial mobility impairment. Abcuro estimates approximately 40,000 diagnosed patients in the United States and another roughly 35,000 across major European countries and Japan, although those figures include company assumptions about diagnosis and prevalence.
A therapy capable of slowing progression rather than improving symptoms temporarily could therefore occupy a relatively open market. The absence of an established disease-modifying standard also means a successful pivotal study would not need to outperform a highly effective incumbent drug.
The challenge is proving that the disease can in fact be modified pharmacologically. IBM has generated numerous disappointing clinical programmes, and Abcuro’s first large trial now joins that history at the overall-population level.
What does the $66 million financing really say about the programme?
The new capital does not erase the MUSCLE failure. It says sophisticated healthcare investors believe the prespecified less-severe subgroup produced enough biological and clinical coherence to justify a second controlled experiment.
That makes Abcuro’s next trial unusually clean analytically. If a prospectively selected less-severe population reproduces something close to the 50% relative slowing observed in MUSCLE, the earlier failed trial could ultimately become the experiment that identified the correct treatment window. If the signal disappears, it will look more like a chance subgroup in a negative study.
Abcuro has now raised enough capital to force that distinction. The company is not simply rerunning MUSCLE with a new statistical argument; it is narrowing the disease stage around the one hypothesis the previous trial left alive. That makes the next study both a development opportunity and a direct test of whether precision in disease timing can rescue a therapy that failed when used too broadly.