The present invention relates to novel modified proteins having N-terminal free thiols that can be produced by recombinant methods and are ready for further chemical
derivatization. In particular, the invention relates to
erythropoietin conjugate compounds having altered biochemical, physiochemical and pharmacokinetic properties. More particularly, one embodiment of the invention relates to
erythropoietin conjugate compounds of the formula: (M)n-X-A-cys-EPO (I) where EPO is an erythropoeitin
moiety selected from
erythropoietin or an erythropoietin variant having at least one
amino acid different from the wild-type human EPO, or any pharmaceutical acceptable derivatives thereof having biological properties of causing
bone marrow cells to increase production of red blood cells; cys represents the
amino acid cysteine and occurs at position −1 relative to the
amino acid sequence of the erythropoietin
moiety; A indicates the structure of the residual
moiety used to chemically attach X to the
thiol group of −1Cys; X is a
water soluble polymer such as a polyalkylene glycol or other
polymer; M is an organic molecule (including peptides and proteins) that increases the circulating half-life of the construct; and N is an integer from 0 to 15.