Parenteral Delivery of Peptides and Proteins - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 538

avidin to the monoclonal antibody (MAb), which can then be attached to a biotin moiety
linked to the 'to be transported' drug by disulfide bridges [74,77] .
A fusion protein approach has also been suggested as a possible protein formu-
lation for brain targeting, whereby, for instance, glial-derived neurotrophic factor
(GDNF), is fused with the carboxyl terminus of the chimeric MAb to the human
insulin receptor (HIR). Such a fusion protein, HIRMAb-GDNF, is bifunctional, and
both bind the HIR to trigger receptor-mediated transport across the BBB, and bind
the GDNF receptor (GFR)-1 to activate GDNF neuroprotection pathways behind
the BBB [78] . Similar results have been drawn from Human TNFR ECD (tumor
necrosis factor-alpha receptor extracellular domain). A fusion protein (HIRMAb-
TNFR ECD) and the erythropoietin (EPO) fusion protein (HIRMAb-EPO) have been
prepared by fusing the proteins to the carboxyl terminus of the chimeric MAb to the
HIR to facilitate BBB permeation.
A similar concept was applied in one of the studies where proteins targeting the
brain were achieved when lentivirus, having the ability to express fusion proteins con-
sisting of the avidin and endocytotic part of the low-density lipoprotein receptor, was
delivered in the liver and spleen. The lentivirus expressed and secreted the therapeutic
modified fusion protein, capable of binding to the low-density lipoprotein receptor on
the BBB. The binding then results in a transcytosis to the albuminal side of the barrier,
where, presumably, the apolipoprotein is released to be taken up by neurons and/or
astrocytes in the brain [66,68] . Pardridge [79] noted that only those peptides that are
able to cross the BBB to an appreciable extent (e.g., insulin, insulin-like growth factor,
cationized albumin, and transferrin) are utilized for receptor-mediated endocytosis.
Modulation of osmotic pressure of the BBB can be used to increase permeability
of drugs through the BBB, but this strategy has drawbacks such as weakened or dam-
aged defenses of the BBB. Biochemical openings of the BBB can be achieved with
various entities such as vasoactive agents like bradykinin or histamine, leukotriene
C4, solvents such as high-dose ethanol or dimethylsulfoxide (DMSO), detergents
such as sodium dodecyl sulfate (SDS) or polysorbate 80 (also known as Tween-80),
alkylating agents like etoposide and melphalan, immune adjuvant, cytokines, and
combinations of high-dose air bubbles and high-dose ultrasound. All have been used
to disrupt the BBB [47,48] . However, many of these approaches are unacceptably
toxic and therefore not clinically useful. Ultrasound and electromagnetic radiation
also modify the integrity of the tight junction by inducing high temperatures and
intense mechanical activity, although these are the reasons for various side effects.
11.3 Techniques for Stabilizing Aqueous P/P Formulation
For protein pharmaceuticals, the term “stability” needs to be defined with more accu-
racy. Chemical stability typically involves the integrity of the amino acid sequence (pri-
mary structure) and the reactivity of the side chains. Most of the time, the activity of
peptides depends on the primary and possibly secondary structures, whereas proteins
possess an additional tertiary and sometimes even a quaternary structure that allows the
protein chains to fold and adopt a three-dimensional conformation [80,81] . The chemical