Biology Reference
In-Depth Information
[76], resulting in a measurable increase in hematocrit. Several concerns
became apparent from these studies, including inefficient and variable delivery
of the
EPO
gene and subsequent variations in EPO concentration. EPO expres-
sion also decreased over time. Expression systems and gene delivery methods
with improved efficiency have been reported [77-80]. Current
EPO
gene ther-
apy protocols require repeated administration of EPO genes. In addition there
is concern that the therapy may be irreversible or result in altered gene expres-
sion resulting in tumorigenicity. Over-expression of
EPO
genes could result in
polycythemia with little ability to correct the condition.
The efficiency and irreversibility concerns have been addressed by devel-
oping implantable capsules containing EPO-expressing cells [81]. The cap-
sules can be removed, halting EPO delivery. A further improvement would be
to construct vectors whereby EPO expression is controlled by a small mole-
cule such as tetracycline, enabling increased EPO expression in response to
oral administration of the gene activator [82]. Controlled expression of the
EPO
gene has been demonstrated in mice using tetracycline [77, 78, 83],
mifepristone [84], or rapamycin [85]. Additional advances are the develop-
ment of vectors where EPO expression is controlled by oxygen tension [82],
or methods that target the kidney for gene transfer [86]. Despite these
advances, safe and controlled EPO delivery using gene therapy methods suit-
able for human use remains a distant but tantalizing opportunity.
New formulations and devices
Endogenous EPO concentration is exquisitely controlled in the body by rapid
changes in expression. In contrast, protein therapeutics are placed in a non-
physiologic environment for extended periods of time, which in some
instances may be years. Safe storage in any formulation requires that condi-
tions and formulations be designed to minimize formation and accumulation
of unnatural breakdown products or alterations in EPO structure. Inappropriate
formulations that do not maintain the integrity of the product can risk expos-
ing the patient to an abnormal form of the protein.
Formulations containing rHuEPO have been successfully developed and
used safely and effectively for more than a decade. Despite the success of cur-
rent formulations, change is sometimes required to keep up with regulatory or
safety concerns or to allow for new technologies, such as new devices or deliv-
ery systems. Such manufacturing and formulation changes included removal
of excipients such as human serum albumin or bovine-derived products.
Prolonged stimulation of erythropoiesis is one desirable property that may
be addressed by new delivery systems, including devices that allow controlled
release of rHuEPO over a long time. This approach necessitates that the mol-
ecule remain stable in the device for a prolonged period of time. Another
approach is to introduce rHuEPO into a biodegradable matrix that degrades
slowly over time (slow release) [87, 88]. This strategy requires development of
