methoxy polyethylene glycol-epoetin beta is made from erythropoietin by chemically linking the n-terminal amino group or the ε-amino group of any lysine present in the protein with methoxy polyethylene glycol butanoic acid. the average molecular weight is approximately 60 kda. the drug stimulates erythropoiesis by interacting with the erythropoietin receptor on progenitor cells in the bone marrow. it has a reduced receptor binding activity compared to other esas and but retains in vivo activity due to an extended serum half-life. it has an "in vivo" half-life of around 135 hours (5.6 days) as compared to darbepoetin alfa which has a half life of around 21 to 70 hours, the half life of which is three times that of the naturally occurring erthropoietin in the body.