Other Routes of Protein and Peptide Delivery: Transdermal, Topical, Uterine, and Rectal - Challenges in Delivery of Therapeutic Genomics and Proteomics - page 631

is because aprotinin (MW 6500; p I  10.5) existed as a cation at pH 4.0 and was
probably delivered into the skin during iontophoresis. In contrast, soybean trypsin
inhibitor (MW 8000; p I  4.0-4.2) was anionic, or neutral, at the pH used and had
a higher molecular weight. Other polypeptides that have been delivered iontopho-
retically include an analogue of growth hormone-releasing factor with a molecular
weight of 3929 Da [66] .
Insulin Insulin, despite being a large molecule (MW ~6 kDa), has generated substan-
tial interest and has been widely investigated for iontophoretic delivery in different
animals [67-76] . The momentum for research is obviously the importance of insu-
lin for control of diabetes and the lack of any breakthrough drug-delivery systems
yet. In the first reported study on transdermal iontophoretic delivery of insulin for
systemic effect, Stephen et al. were able to deliver a highly ionized and monomeric
form of insulin to a pig and observed a turndown in blood glucose levels and a boost
in serum insulin levels [67] . Increased penetration of insulin with application of a
depilatory lotion, cream, or absolute alcohol, prior or in combination with iontopho-
resis, has been reported [76,77] . Another group of scientists reported the successful
transdermal delivery of a therapeutic dose of human insulin to diabetic albino rabbits
across the skin with intact SC [69] . Although iontophoresis overcomes the physical
barrier to some extent, the question remains whether enough insulin can be delivered
to be of therapeutic benefit to diabetic patients. A drug flux of 2-4 mg/cm 2 an hour
is necessary to meet the basal insulin needs for a diabetic patient. Although the pro-
teolytic activity of skin is low, skin contains both exo- and endopeptidases, which
causes degradation of insulin during delivery through skin.
Another problem is the formation of a depot in the skin. It was observed that
blood glucose levels were declining even after current was turned off, suggesting that
insulin forms a reservoir and releases gradually from the skin [70] . Similar observa-
tions on skin accumulation of insulin have been made by other researchers. Thus, it
would seem that iontophoresis serves to load up the skin tissue with insulin to form a
reservoir from which insulin molecules continue to creep slowly into the blood circu-
lation until several hours after the current application. This could be a potential dis-
advantage. In addition, most commercially available insulin products actually exist
in hexameric form, so attempts are being made to deliver a protein with a molecular
weight of about 36 kDa, which is most likely too high to be within the scope of ion-
tophoretic delivery. Iontophoretic delivery of monomeric insulin analogues has been
investigated with better success than regular hexameric insulin. Still, the isoelectric
point of insulin (5.3) falls in the region of skin p I (4.0-6.0). This poses a major hur-
dle to its delivery because insulin will lose its charge in the skin environment, and
the predominant impetus for delivery (electrical repulsion) would end. Because some
insulin then diffuses toward the blood or physiological interior (pH 7.4), it acquires
a negative charge and may be drawn back toward the anode. Therefore, despite the
considerable interest in iontophoretic delivery of insulin, it is not a model protein
for delivery. Its isoelectric point and tendency toward aggregation make successful
iontophoretic delivery of regular insulin unlikely. However, other enhancement tech-
niques discussed in this chapter may be more promising for insulin delivery.