Bimagrumab (BYM338)

Bimagrumab (BYM338) is not a peptide. It is a fully human IgG monoclonal antibody (~150 kDa) selected from the HuCAL library in a Novartis–MorphoSys collaboration, and it behaves nothing like the short peptides it sits next to — not in size, not in pharmacokinetics, not in how it must be handled.

What makes it distinct

Most investigational myostatin inhibitors bind the ligand. Bimagrumab acts one level down: it occupies the activin type II receptors (ActRIIA and ActRIIB) so that none of their ligands can dock. That single move silences myostatin (GDF-8), activin A and GDF-11 together — an entire family of negative regulators of muscle mass rather than one molecule. Receptor-level, not ligand-level, blockade is the main reason bimagrumab produced substantially larger increases in muscle mass in studies than myostatin-only agents.

What the research actually showed

Bimagrumab is one of the few compounds in this category with a large published clinical record, and that record is mixed. It reliably increases muscle mass — demonstrated in sporadic inclusion body myositis, COPD and sarcopenia. But across three separate programmes the added mass did not translate into added function: the phase 2b RESILIENT trial (n=251) missed its primary endpoint of 6-minute walking distance. That is where Novartis ended the muscle-disease effort. Across these same programmes, bimagrumab carried a characteristic side-effect profile — most notably diarrhoea and muscle spasms — that limited tolerability for some participants.

The second chapter is metabolic. In a 48-week phase 2 trial in obesity with type 2 diabetes (n=75), ActRII blockade produced a −20.5% loss of fat mass alongside a +3.6% gain in lean mass. That unusual combination redirected the whole programme: Eli Lilly acquired Versanis Bio in 2023 and the compound became LY3985863. The full read of the data, the limitations and the open questions is in our reference monograph.

Listing status

This listing is coming soon — the item is not in stock. The form below registers interest so we can notify you if and when the reagent becomes available.

Related compounds are listed under Muscle Growth.