Of the four fusion products approved, two are Fc-fusion based (Eylea and Nulojix) with the antibody Fc portion of the products
serving to extend the serum half life of the overall molecule. Nulojix, used to prevent organ rejection following kidney transplant,
is interesting in that it evolved from Orneca, which was approved in 2005 for the treatment of rheumatoid arthritis. Orneca
negatively regulates the activation of a range of T lymphocytes. Nulojix is a protein engineered version of Orneca, identified
through mutagenesis and screening, which binds with greater avidity to a range of T lymphocytes. This engineering apparently
underpins a more potent immunosuppresive effect.
Eylea and Nulojix join several previously approved Fc fusion products. The additional two fusion products (Elonva and Provenge)
are more novel in this context. Elonva, used to achieve controlled ovarian stimulation in some fertility treatments, is a
modified recombinant human follicle stimulating hormone (FSH) in which the carboxy-terminal peptide of the β subunit of human
chorionic gonadotropin (hCG) is fused to the FSH β chain. The resulting fusion product retains its ovarian stimulation activity,
but displays an improved blood elimination half life.
Provenge, used to treat hormone-refractory prostate cancer, is a more complex product in that it consists of twin active components:
autologous antigen-presenting dendritic cells and the recombinant fusion protein, PAP-GM-CSF. The latter consists of human
prostatic acid phosphatase (PAP), an antigen expressed in some 95% of prostate cancer tissue, linked to human granulocyte-macrophage
colony-stimulating factor (GM–CSF) through a dipeptide (glycine-serine) linker sequence. The treatment approach consists of
three phases: an initial collection of dendritic cells from the blood of the prostate cancer patient (dendritic cells are
the only antigen-presenting cells that can prime native T lymphocytes to initiate an immune response); in vitro exposure of the collected cells to the fusion product, which results in their activation (the GM–CSF portion of the protein
helps to target the PAP protein to the dendritic cells and activate those cells; the PAP portion provides the tumor specificity
that will eventually direct the immune system to target prostate cancer) and the final phase, that is, the re-infusion of
the activated cells back into the original donor patient.
In addition to Adcetris (described earlier), two additional products approved throughout 2010 and 2011 are engineered using
an in vitro conjugation process post protein synthesis. In both instances, the purpose of conjugation is to extend product serum half
life. Krystexxal (a recombinant urate oxidase enzyme used to treat gout) is PEGylated with a 10kDa monomethoxy-polyethylene
glycol (m-PEG) moiety whereas Victoza (used to treat type 2 diabetes) consists of glucagon like peptide 1 to which a C-14
fatty acid is conjugated.
Although the cumulative biopharmaceutical market value for 2011 is not readily available as yet, cumulative global sales of
recombinant therapeutic proteins reached a notable milestone in 2010 (5). In that year, recombinant proteins surpassed the
$100 billion mark, reaching an estimated $107 billion. Moreover, that same year 30 individual biopharmaceutical products recorded
sales of more than $1 billion each, with Enbrel (etanercept) retaining the top spot and boasting sales value of almost $7.3
In terms of APIs, 2010 figures equate to approximately 21 metric tons (21,000 kg) of pure protein product. Almost two-thirds
of that amount—approximately 13.5 tons—is produced in microbial systems (mainly insulin) with the remaining third being produced
almost exclusively using animal cell culture (mainly mAbs) (6).