KER-065 — Safety, Research, Risks & Legal Status

Not medical advice. KER-065 is discussed here for informational and harm-reduction purposes only. We do not endorse use, and any dosing context is informational, not a protocol.

Also known as	ActRII-Fc fusion; Keros Therapeutics development code KER-065; hybrid activin receptor ligand trap
Category	Peptide (research chemical)
Drug class	Soluble hybrid ActRIIA/IIB-Fc fusion protein (biologic ligand trap)
Developer	Keros Therapeutics
Current development stage	Phase 1 complete (March 2025 topline); Phase 2 in DMD planned
FDA Orphan Drug Designation	Granted August 20, 2025, for Duchenne muscular dystrophy
Primary ligand targets	Myostatin (GDF-8), activin A, activin B, GDF-11
Reduced-affinity engineering	Designed with lower BMP9 binding vs. native ActRIIB-Fc to limit off-target BMP signaling
US legal status	Investigational drug under IND status in the United States. Not approved by the FDA for any indication. Not available for human use outside of authorized clinical trials. Granted FDA Orphan Drug Designation for Duchenne muscular dystrophy (DMD) in August 2025, which confers development incentives but does not constitute approval. Not a supplement and not legally available for consumer purchase or self-administration.

What is KER-065?

KER-065 is an engineered soluble fusion protein developed by Keros Therapeutics. It combines ligand-binding domains from both activin receptor type IIA (ActRIIA) and type IIB (ActRIIB) linked to a human IgG Fc region, functioning as a decoy receptor - a so-called "ligand trap" - that sequesters circulating proteins in the TGF-beta superfamily. The design is intended to simultaneously inhibit myostatin, activin A, activin B, and GDF-11, all of which are negative regulators of muscle mass and bone formation. Compared to native ActRIIB-Fc fusions studied previously in this class, KER-065 was engineered to reduce affinity for BMP9 with the goal of limiting off-target signaling through bone morphogenetic protein pathways. It is administered by injection as a biologic, not an oral small molecule or peptide in the conventional sense.

How it works

KER-065 acts as a soluble decoy receptor that binds and neutralizes myostatin (GDF-8), activin A and B, and GDF-11 before they can engage membrane-bound ActRIIA/IIB receptors on muscle and bone cells. By preventing ligand-receptor engagement, it blocks downstream SMAD2/3 signaling that would otherwise suppress muscle protein synthesis and promote muscle catabolism. Simultaneously, reduced activin signaling shifts the SMAD1/5/8 pathway balance toward bone anabolism, increasing osteoblast activity and bone formation markers. The hybrid receptor architecture captures a broader ligand profile than single-receptor constructs, while intentional reduced BMP9 affinity is intended to narrow the off-target risk seen with earlier pan-ActRIIB agents in this drug class.

Background & history

Keros Therapeutics, a clinical-stage biopharmaceutical company, developed KER-065 as part of a platform focused on modulating the TGF-beta superfamily for musculoskeletal and neuromuscular diseases. The compound entered a Phase 1 randomized, double-blind, placebo-controlled dose-escalation trial in healthy male volunteers. Initial topline Phase 1 results were announced in March 2025, reporting general tolerability and pharmacodynamic activity in bone, muscle, and adipose tissue. The FDA granted Orphan Drug Designation for DMD in August 2025. As of mid-2026, KER-065 is in the transition from Phase 1 to Phase 2 planning, with DMD as the primary indication and obesity announced as a secondary development program. No regulatory approval has been granted in any jurisdiction.

What the research says

Phase 1 human data (healthy male volunteers) indicates the compound was generally well-tolerated across evaluated dose levels, with adverse events described as predominantly mild-to-moderate and reversible. No serious adverse events or dose-limiting toxicities were reported in Phase 1. One Grade 4 elevation in creatine kinase was observed but was attributed to a cause unrelated to the study drug. Pharmacodynamic signals were observed in multiple tissue compartments: bone formation marker bone-specific alkaline phosphatase (BSAP) increased, bone resorption marker C-terminal telopeptide (CTX) decreased, and bone mineral density gains were sustained through Day 141 of observation. Body composition assessments showed changes consistent with muscle mass effects. In preclinical animal models (DMD mouse model and glucocorticoid-induced muscle and bone loss models), KER-065 increased muscle mass, improved functional strength metrics, enhanced bone formation, and attenuated corticosteroid-induced musculoskeletal deterioration. These are early-phase findings from a single Phase 1 trial in a healthy volunteer population and should not be extrapolated to disease populations or to efficacy conclusions without further controlled trials.

Reported effects

Increased bone formation marker BSAP observed in Phase 1 healthy volunteers (pharmacodynamic signal, not a therapeutic efficacy claim)
Decreased bone resorption marker CTX in Phase 1 human data
Bone mineral density gains sustained to Day 141 in Phase 1
Body composition changes consistent with altered muscle mass (exploratory Phase 1 endpoint, magnitude not publicly detailed)
Increased skeletal muscle mass and improved functional strength in DMD mouse preclinical models
Counteracted glucocorticoid-induced bone and muscle loss in animal models
Broad TGF-beta ligand neutralization covering myostatin, activin A/B, and GDF-11

Dosing & administration (informational)

Dose levels evaluated in Phase 1 have not been publicly disclosed in granular detail in available press materials. The trial used a two-part dose escalation design in healthy male volunteers with single and multiple dose cohorts; specific dose ranges, intervals, and routes have not been released to the public record reviewed. This information is provided for orientation only. No dosing guidance can be given - consult a licensed clinician and refer to registered clinical trial protocols for any clinical context.

This is general research/context information, not medical advice or a recommended protocol.

Safety & side effects

No serious adverse events reported in the completed Phase 1 trial
No dose-limiting toxicities identified across evaluated dose levels
Adverse events characterized as predominantly mild-to-moderate and reversible
One Grade 4 creatine kinase elevation reported but attributed to a non-drug cause by investigators
Long-term safety data are not available; Phase 1 follow-up extended only to Day 141
Class-level concern: ActRII ligand traps have historically been associated with cardiovascular effects (telangiectasias, bleeding) in related compounds - KER-065 reduced-BMP9 affinity was engineered to mitigate this, but long-term human cardiovascular data do not yet exist
Not studied in women, pediatric populations, or disease populations at Phase 1 stage; safety profile in DMD patients is unknown pending Phase 2

Drug & supplement interactions

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Who should avoid it

Self-administration outside of a registered clinical trial - compound is not approved or available for consumer use
Any commercial source claiming to supply KER-065 - no legitimate consumer supply exists
Combination with other TGF-beta or ActRII pathway modulators outside of supervised clinical protocols
Use in individuals with cardiovascular disease, given class-level signals from related ActRIIB fusion proteins and the absence of long-term cardiac safety data for KER-065
Use in pediatric populations pending dedicated pediatric safety evaluation (Phase 2 DMD plans may address this under trial conditions only)

How it is commonly combined

No stacking data exist. KER-065 has not been studied in combination with any other agent in humans. Preclinical combination context (glucocorticoid plus KER-065 in animal models) suggests mechanistic interaction with corticosteroid pathways, but this is investigational data only and does not constitute a basis for self-directed combination use.

Legal & regulatory. Investigational drug under IND status in the United States. Not approved by the FDA for any indication. Not available for human use outside of authorized clinical trials. Granted FDA Orphan Drug Designation for Duchenne muscular dystrophy (DMD) in August 2025, which confers development incentives but does not constitute approval. Not a supplement and not legally available for consumer purchase or self-administration.

Quality & harm reduction

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Frequently asked questions

Is KER-065 available to purchase or use outside of a clinical trial?

No. KER-065 is an investigational biologic with no approved indication and no commercial supply. It exists only within Keros Therapeutics' regulated clinical development program. Any vendor claiming to sell it is not supplying a verified product.

What dosing protocol should I follow for KER-065?

No dosing protocol should be self-administered. KER-065 is not approved for any use outside of clinical trials conducted under investigational protocols. Specific dose ranges from the Phase 1 trial have not been publicly disclosed. If you are exploring enrollment in a registered trial, consult the sponsoring institution and a qualified clinician.

What were the Phase 1 results for KER-065?

Topline Phase 1 results announced in March 2025 reported general tolerability in healthy male volunteers with no serious adverse events and no dose-limiting toxicities. Pharmacodynamic signals were observed across bone (increased BSAP, decreased CTX, BMD gains sustained to Day 141), muscle, and body composition. These are early single-trial results in healthy subjects and do not establish efficacy in any disease.

What condition is KER-065 being developed for?

The primary clinical priority is Duchenne muscular dystrophy (DMD), for which FDA Orphan Drug Designation was granted in August 2025. Keros has also announced an obesity development program. Exploratory potential in bone health, cachexia, and fibrotic conditions has been discussed but no trials in those indications have been registered publicly as of available data.

How is KER-065 different from earlier myostatin inhibitors like bimagrumab or luspatercept?

KER-065 uses a hybrid ActRIIA/IIB ligand-binding domain intended to capture a broader range of TGF-beta ligands (myostatin, both activins A and B, and GDF-11) than single-receptor constructs. It was also engineered to reduce BMP9 affinity relative to native ActRIIB-Fc fusions, which is hypothesized to reduce the cardiovascular adverse effects seen with earlier pan-ActRIIB agents. Whether this design advantage translates to an improved clinical safety profile requires longer-term controlled data.

What are the known safety risks with KER-065?

Phase 1 data reported no serious adverse events and no dose-limiting toxicities. The main class-level concern with ActRII ligand traps as a group has been cardiovascular effects, including telangiectasias and bleeding, seen with related compounds. KER-065's reduced BMP9 affinity was engineered to address this risk, but long-term cardiovascular safety data in humans do not yet exist. One isolated Grade 4 creatine kinase elevation was reported but attributed to a non-drug cause by investigators. The compound has not been studied in disease populations or in women.

References & further reading

PubMed: KER-065 myostatin activin receptor fusion
PubMed: ActRIIA ActRIIB hybrid ligand trap muscle bone
PubMed: myostatin inhibition Duchenne muscular dystrophy clinical trial
ClinicalTrials.gov: Keros Therapeutics KER-065 Phase 1 healthy volunteers
ClinicalTrials.gov: Keros Therapeutics KER-065 Duchenne muscular dystrophy Phase 2