Nucleic-acid- and cell-based therapeutics - Pharmaceutical Biotechnology: Concepts and Aapplications - page 455

obviously prevents translation. One potential advantage of ribozymes is that, as catalytic agents, a
single molecule could likely destroy thousands of copies of the target mRNA. Such a drug should,
therefore, be very potent. Again, however, ribozymes suffer from similar complications to anti-
sense-based products in terms of their development as biopharmaceuticals, and no such product is
likely to gain approval for some time to come.
14.9 Aptamers
Aptamers are single-stranded DNA or RNA-based sequences that fold up to adopt a unique three-
dimensional structure, allowing them to bind a specifi c target molecule.
Binding displays high specifi city, and aptamers capable of distinguishing between closely re-
lated isoforms or different conformational states of the same protein have been generated. Binding
affi nity is also high. It is in the low nanomolar to picomolar range, which is comparable to the
binding affi nity of an antibody for the antigen against which it was raised.
Aptamer technology was fi rst developed in 1990. It entails the initial generation of a large
aptamer library, with subsequent identifi cation of individual aptamers binding a target ligand via
an appropriate selection strategy. DNA aptamer libraries are usually generated via direct chemi-
cal synthesis and amplifi ed by PCR (Chapter 3). RNA libraries are usually generated by in vitro
transcription of synthetic DNA libraries. Identifi cation of specifi c aptamers binding the target
molecule is most easily undertaken by an automated in vitro selection approach known as system-
atic evolution of ligands by exponential enrichment (SELEX). Most libraries contain up to 10 15
species.
Because of their high binding specifi city and affi nity, aptamers (like antibodies) are/may prove
useful for affi nity-based purifi cation, target validation and drug discovery, diagnostics and thera-
peutics. One such product (Macugen, Box 14.4) has been approved for general medical use to date.
A modest number of additional aptamers are in clinical trials, aimed at treating conditions includ-
ing infectious diseases, cancer and haemophilia.
Aptamers appear to display low immunogenicity; but, when administered systemically, they
are quickly excreted via size-mediated renal clearance. In order to prevent renal removal, such
aptamers are usually conjugated to PEG. PEG may also help further protect the aptamers from
degradation by serum nucleases; native aptamers are prone to nuclease attack, but their half-lives
can most effectively be extended via chemical modifi cation, as discussed earlier in the context of
antisense agents.
14.10 Cell- and tissue-based therapies
Recent progress relating to the identifi cation, isolation and manipulation of stem cells has made
this cell type a focus of enormous attention, both within the scientifi c and general communities.
Although stem cells harbour great medical potential, their routine application to the treatment
of medical conditions (so-called regenerative medicine) remains a distant prospect, as discussed
below. In contrast, fully differentiated cells or groups of cells (organs and tissues) are currently in
routine medical use. Such products include cells or tissues used for the purposes of transplantation,
as well as a small number of engineered cell-based products.