FTA:

There was evidence to suggest that tumors secrete activin, such that circulating levels of the protein rise in those with cancer. Activin is closely related to another protein, called myostatin, which is known to be important in muscle […]

Animals lacking myostatin or taking treatments that block it grow bigger muscles. There was some evidence to suggest that activin blockers might have a similar effect.

Based on that hunch, the researchers treated mice with cancer and associated cachexia with a recombinant and soluble version of the ActRIIB receptor (sActRIIB), a kind of molecular “decoy” that potently inhibited both activin and myostatin activity. That treatment reversed the animals' muscle loss and prolonged their survival by several weeks on average.

“In tumor-bearing mice with profound cachexia, blocking this pathway not only prevents muscle wasting but completely reverses the loss of muscle, strength and anorexia,” Han said. (Anorexia is another symptom of cachexia, but appetite stimulants and nutritional supplements don’t help much.)

note: cachexia = muscle wasting