acid sequences against a target, and the resultant high-affinity sequences were
termed as “aptamers” (a sobriquet coalescing the Latin word aptus (“to fit”) and
the Greek word meros (“part”)) [4, 5]. The development of aptamers includes
designing a library of random nucleic acid sequences, typically 60–90 bp long,
forming secondary structures (these structures may range from or include stem, loop,
bugle, pseudoknot, G-quadruplex, and kissing hairpin) [6, 7] which in turn is
functionally analogous to the binding site of antibodies (Fab fragment). Sequential
cycles of repetitive selection against a target and PCR-assisted enrichment follow.
The final product of SELEX is aptamers which can bind their cognate targets with
high affinity and specificity.
Consider the following points to understand how aptamers are at least an ideal
surrogate, if not better, for antibodies:
• First of all, antibodies are generated in biological systems, typically in horses or
sheep. This poses a limitation, as antibodies cannot be generated against toxins
that cannot be tolerated by the animal system. Further, antibodies cannot be
generated against non-immunogenic (targets that do not illicit immune response
in the host systems) entities. Conversely, as aptamers are generated chemically
and the selection is in vitro, the intended number of targets can be theoretically
endless.
• The synthesis procedure of aptamers is rapid and cheap and suffers only
minimally from batch to batch variations. Antibody synthesis is opposite to it
on every account. Furthermore, the selection process for monoclonal antibodies is
far more time-consuming and costly than aptamer selection.
• Both offer comparable range of affinity (low nanomolar to picomolar range,
though aptamers have gone to zeptomolar (10
À21 M) [8]) and selectiveness.
Because aptamers are chemically synthesized and selected in vitro, aptamers
offer greater room for modifications than antibodies. Features like the ability to
be chemically modified as per requisite, selection of target epitope, even pharmacokinetic (PK) parameters can be tailored according to needs, and simply do
not exist with antibodies.
• Aptamers have the ability to refold into their functionally active native state after
high-temperature exposures and thus have less stringent storage conditions than
antibodies which only remain functional when stored in refrigerated conditions.
Because of such technical superiority aptamers were destined for the diagnostics
and therapeutics market. Despite being only discovered in 1990, the industry’s worth
estimate runs as high as $2.1bn by 2018 [9] and is poised to grow at unprecedented
rate in the coming years. With such optimism and opportunities, a profound understanding of the requirements from end users and the aptamer’s intended use becomes
of paramount importance. Thoughtful consideration of facts, such as (1) what is the
specific use of the espoused product and if it aligns properly with the needs of the
end users or the target populations and (2) what features to be incorporated so that
the product can compete with the contemporary gold standards, facts that concern
the investors and the remaining stakeholders, is indispensable for a viable market
product. Luckily such motley but germane questions can be conjoined in a document
Defining Target Product Profiles (TPPs) for Aptamer-Based Diagnostics
197
termed as “aptamers” (a sobriquet coalescing the Latin word aptus (“to fit”) and
the Greek word meros (“part”)) [4, 5]. The development of aptamers includes
designing a library of random nucleic acid sequences, typically 60–90 bp long,
forming secondary structures (these structures may range from or include stem, loop,
bugle, pseudoknot, G-quadruplex, and kissing hairpin) [6, 7] which in turn is
functionally analogous to the binding site of antibodies (Fab fragment). Sequential
cycles of repetitive selection against a target and PCR-assisted enrichment follow.
The final product of SELEX is aptamers which can bind their cognate targets with
high affinity and specificity.
Consider the following points to understand how aptamers are at least an ideal
surrogate, if not better, for antibodies:
• First of all, antibodies are generated in biological systems, typically in horses or
sheep. This poses a limitation, as antibodies cannot be generated against toxins
that cannot be tolerated by the animal system. Further, antibodies cannot be
generated against non-immunogenic (targets that do not illicit immune response
in the host systems) entities. Conversely, as aptamers are generated chemically
and the selection is in vitro, the intended number of targets can be theoretically
endless.
• The synthesis procedure of aptamers is rapid and cheap and suffers only
minimally from batch to batch variations. Antibody synthesis is opposite to it
on every account. Furthermore, the selection process for monoclonal antibodies is
far more time-consuming and costly than aptamer selection.
• Both offer comparable range of affinity (low nanomolar to picomolar range,
though aptamers have gone to zeptomolar (10
À21 M) [8]) and selectiveness.
Because aptamers are chemically synthesized and selected in vitro, aptamers
offer greater room for modifications than antibodies. Features like the ability to
be chemically modified as per requisite, selection of target epitope, even pharmacokinetic (PK) parameters can be tailored according to needs, and simply do
not exist with antibodies.
• Aptamers have the ability to refold into their functionally active native state after
high-temperature exposures and thus have less stringent storage conditions than
antibodies which only remain functional when stored in refrigerated conditions.
Because of such technical superiority aptamers were destined for the diagnostics
and therapeutics market. Despite being only discovered in 1990, the industry’s worth
estimate runs as high as $2.1bn by 2018 [9] and is poised to grow at unprecedented
rate in the coming years. With such optimism and opportunities, a profound understanding of the requirements from end users and the aptamer’s intended use becomes
of paramount importance. Thoughtful consideration of facts, such as (1) what is the
specific use of the espoused product and if it aligns properly with the needs of the
end users or the target populations and (2) what features to be incorporated so that
the product can compete with the contemporary gold standards, facts that concern
the investors and the remaining stakeholders, is indispensable for a viable market
product. Luckily such motley but germane questions can be conjoined in a document
Defining Target Product Profiles (TPPs) for Aptamer-Based Diagnostics
197