How Hanmi Pharmaceutical’s UCN2–CRFR2 approach could reduce fat while preserving or increasing lean mass during obesity treatment.
Related Analysis
For the deal and strategic context, read my main BP analysis: [Genentech Bets $2.3 Billion on Hanmi’s Muscle-Preserving Obesity Drug]
Science at a Glance
| Company / Asset | Modality / Target | Core Mechanism | Development |
|---|---|---|---|
| Hanmi / HM17321 | Urocortin-2 (UCN2) analog / corticotropin-releasing factor receptor 2 (CRFR2) agonist | CRFR2 activation → fat loss + muscle preservation/growth | Phase 1 |
| Eli Lilly / Bimagrumab | Monoclonal antibody / activin type II receptors A and B (ActRIIA / ActRIIB) | Blocks activin/myostatin signaling → muscle growth | Clinical development |
| Novo Nordisk / obesity R&D | Multiple next-generation programs | No directly comparable muscle-preserving mechanism publicly established | Research / clinical development |
HM17321 and bimagrumab both aim to improve the composition of weight loss, but through fundamentally different biology: HM17321 activates CRFR2, whereas bimagrumab removes inhibitory activin/myostatin signaling.
1. Why This Mechanism Matters
Lean mass loss can accompany pharmacologic weight loss, creating interest in obesity therapies that improve body composition rather than body weight alone.
Hanmi is developing HM17321 around this concept of “high-quality weight loss,” aiming to reduce fat while preserving or increasing lean mass. (Hanmi HM17321 pipeline overview)
2. Mechanism & Technology — How HM17321 Works
HM17321 is a long-acting urocortin-2 (UCN2) analog that selectively activates corticotropin-releasing factor receptor 2 (CRFR2) and is being developed to reduce fat mass while preserving or increasing lean mass. (Hanmi HM17321 pipeline overview)
[CRFR2 signaling in adipose tissue and skeletal muscle]
In adipose tissue, preclinical studies suggest increased energy expenditure and sustained fat-mass reduction.
In skeletal muscle, preclinical proteomic and functional studies suggest pathways associated with muscle hypertrophy, metabolic function and sustained muscle growth.
In Hanmi’s animal studies, this produced a different body-composition profile from semaglutide, with fat mass decreasing while lean mass increased and energy expenditure remained higher. (Hanmi HM17321 EASD preclinical poster)
The key idea is therefore simple:
HM17321 → CRFR2 activation → fat mass ↓ + muscle mass/function ↑
3. A Different Strategy From Incretin-Based Weight Loss
Incretin therapies primarily reduce body weight through effects on appetite, satiety and energy intake.
HM17321 instead targets a non-incretin metabolic pathway and is being developed around body recomposition rather than appetite suppression alone.
This does not necessarily mean that one mechanism will produce greater total weight loss. The potential advantage of HM17321 would instead be a higher proportion of fat loss while retaining more functional lean tissue, if its preclinical profile translates into humans.
4. HM17321 vs Bimagrumab
Eli Lilly is pursuing the same broad objective through a very different mechanism.
Bimagrumab blocks activin type II receptors A and B (ActRIIA / ActRIIB), preventing activin and myostatin signaling. Because these pathways normally restrain muscle growth, their inhibition is intended to promote muscle gain while also supporting fat-mass reduction. (Lilly bimagrumab mechanism overview)
The mechanistic difference can be summarized simply:
HM17321 activates a pro-growth metabolic pathway through CRFR2, whereas bimagrumab removes an inhibitory brake on muscle growth through ActRII blockade.
This difference could also affect how the drugs are ultimately used. HM17321 may influence both adipose and skeletal-muscle metabolism through one receptor pathway, whereas bimagrumab more directly targets the regulatory machinery controlling muscle growth.
5. Where Does Novo Nordisk Fit?
Novo Nordisk is also advancing multiple next-generation obesity programs, including agents such as amycretin, as part of a broader effort to improve obesity treatment beyond current therapies. (Novo Nordisk R&D pipeline)
However, a directly comparable dedicated muscle-preserving mechanism has not yet been publicly established in the same way as HM17321’s CRFR2 agonism or bimagrumab’s ActRII blockade.
For now, HM17321 and bimagrumab provide the clearest mechanistic examples of two very different ways to pursue fat loss while protecting muscle.
BP Science View
HM17321 is interesting because it does not simply aim to increase the magnitude of weight loss. It aims to improve what that weight loss consists of.
The contrast with bimagrumab highlights how differently this goal can be approached: HM17321 activates the UCN2–CRFR2 axis, while bimagrumab removes an activin/myostatin brake on muscle growth.
If HM17321’s preclinical body-composition profile translates into humans, its broader metabolic action could potentially support both fat reduction and muscle function, while bimagrumab may offer a more direct muscle-building strategy. Which approach produces the better balance of efficacy, safety and functional benefit will ultimately have to be determined clinically.
About BP Science
BP Science explains the biology, mechanisms, modalities and technology landscapes behind emerging biopharma therapies.
Leave a Reply