There are numerous formulations of recombinant human erythropoietin, but only a few of which are used in the U.S. Generally speaking, all of the formulations have the exact same 165 amino acid core sequence which encodes the human erythropoietin protein; the main aspect in which they differ is in their degree of glycosylation.
Glycosylation refers to the enzymatic attachment of sugar molecules onto a protein via Asparigine residues (N-linked glycosylation) or via Threonine or Serine residues (O-linked glycosylation). It turns out to be extremely important for stability of EPO, as non-glycosylated recombinant erythropoietin has an extremely short (and therefore not clinically useful) half-life. A breakthrough in the manufacture of recombinant human erythropoietin came with the manufacture of transfected EPO within Chinese hamster ovary (CHO) cells, a cell line which is avidly glycosylates proteins.
Epoetin-alpha (either Procrit or Epogen in the U.S., and Eprex in Europe) is grown in CHO cells and consists of about 40% carbohydrate. These medications are all about 30kD and have a half-life of about 7-8 hours.
Darbepoeitin-alpha (Aranesp) has a slightly modified amino acid sequence which adds an additional 2 N-linked glycosylation sites, which increases the drug to 51% carbohyrate content, 37.1 kD in size, and a half-life of between 21-24 hours.
There are other recombinant human erythropoietins (e.g., epoetin-beta, epoetin-delta, epoetin-omega) but these are not commonly used in the U.S.