are introduced to protect the vulnerable 2
0 -position of the ribose-phosphate backbone
against endonucleases (2
0 -fluoro, 2
0 -amino, 2
0 -O-methyl) [45]. Terminal nucleotides
are also attached to prevent exonuclease susceptibility [46, 47]. A more extensive
strategy is the use of mirror-image configuration oligonucleotides. These aptamers,
built from nonnatural L-nucleotides, are known as a Spiegelmers
® and are not
recognized by the ubiquitous plasma nucleases. Thus, the mirror-image configuration confers plasma stability in vitro and immunological passivity. Their selection
process consists on the use of D-configured oligonucleotide libraries against the
mirror-image of the biological target of interest. Identified sequences are finally
synthesized using enantiomeric (L-)ribonucleotides [48].
High Contrast Ratio High image quality requires high contrast which is a consequence of a high target-to-background signal ratio. First, the target must uptake and
then retain the probe long enough. High target uptake is achieved by overcoming the
biological barriers and through effective tissue penetration. For tumoral and intracellular targets, additional barriers must be considered. Aberrant blood vessels,
heterogeneous microvascular perfusion, interstitial pressure, and pH are relevant in
pathologies, such as cancer [49]. The aptamers’ size, charge, and composition are
important characteristics to obtain tissue penetration and significant target uptake.
However, low signal on normal tissue is very important to decrease the background on images and depends on systemic clearance. Probe decay within the
biological environment is dependent on its pharmacokinetic properties. Route of
administration and physicochemical properties, such as pKa, molecular weight, and
logP, directly affect probe pharmacokinetics, including rates of adsorption, distribution, metabolism, and excretion. The small size of aptamers, between 25 and
40 nucleotides, results in a molecular weight of ~15,000 Da, which is excreted by
the kidneys in minutes [50]. In most of cases, this is a desirable characteristic for
in vivo probes.
Scale Production and Economic Feasibility In vivo applications need to be scaled
up, which can directly affect their application. Low manufacturing costs, shorter
generation time, and no batch-to-batch variability are fundamental aspects for
translation to clinic.
Aptamers’ versatility enables the combination of all these attributes in a unique
probe for in vivo application [12]. Additional characteristics conferred by attachment
of nanoparticles or multivalent structures might significantly improve the probes’
bioavailability. Simple building blocks of the same aptamer motif can significantly
improve the avidity due to multiple target binding sites. The combination of different
aptamer motifs offers opportunities to build multifunctional molecules that can serve
as a basis for diverse approaches [51]. Lei and colleagues designed a novel strategy
of nanotriangle-scaffolded multivalent split activatable aptamer probes, which combine advantages of programmable self-assembly, multivalent effect, and targetactivatable architecture [52, 53].
The in vivo diagnostic approaches join into the imaging development, and the
contribution of aptamers includes molecular target strategies in this field.
Aptamers in Diagnostic and Molecular Imaging Applications
149
0 -position of the ribose-phosphate backbone
against endonucleases (2
0 -fluoro, 2
0 -amino, 2
0 -O-methyl) [45]. Terminal nucleotides
are also attached to prevent exonuclease susceptibility [46, 47]. A more extensive
strategy is the use of mirror-image configuration oligonucleotides. These aptamers,
built from nonnatural L-nucleotides, are known as a Spiegelmers
® and are not
recognized by the ubiquitous plasma nucleases. Thus, the mirror-image configuration confers plasma stability in vitro and immunological passivity. Their selection
process consists on the use of D-configured oligonucleotide libraries against the
mirror-image of the biological target of interest. Identified sequences are finally
synthesized using enantiomeric (L-)ribonucleotides [48].
High Contrast Ratio High image quality requires high contrast which is a consequence of a high target-to-background signal ratio. First, the target must uptake and
then retain the probe long enough. High target uptake is achieved by overcoming the
biological barriers and through effective tissue penetration. For tumoral and intracellular targets, additional barriers must be considered. Aberrant blood vessels,
heterogeneous microvascular perfusion, interstitial pressure, and pH are relevant in
pathologies, such as cancer [49]. The aptamers’ size, charge, and composition are
important characteristics to obtain tissue penetration and significant target uptake.
However, low signal on normal tissue is very important to decrease the background on images and depends on systemic clearance. Probe decay within the
biological environment is dependent on its pharmacokinetic properties. Route of
administration and physicochemical properties, such as pKa, molecular weight, and
logP, directly affect probe pharmacokinetics, including rates of adsorption, distribution, metabolism, and excretion. The small size of aptamers, between 25 and
40 nucleotides, results in a molecular weight of ~15,000 Da, which is excreted by
the kidneys in minutes [50]. In most of cases, this is a desirable characteristic for
in vivo probes.
Scale Production and Economic Feasibility In vivo applications need to be scaled
up, which can directly affect their application. Low manufacturing costs, shorter
generation time, and no batch-to-batch variability are fundamental aspects for
translation to clinic.
Aptamers’ versatility enables the combination of all these attributes in a unique
probe for in vivo application [12]. Additional characteristics conferred by attachment
of nanoparticles or multivalent structures might significantly improve the probes’
bioavailability. Simple building blocks of the same aptamer motif can significantly
improve the avidity due to multiple target binding sites. The combination of different
aptamer motifs offers opportunities to build multifunctional molecules that can serve
as a basis for diverse approaches [51]. Lei and colleagues designed a novel strategy
of nanotriangle-scaffolded multivalent split activatable aptamer probes, which combine advantages of programmable self-assembly, multivalent effect, and targetactivatable architecture [52, 53].
The in vivo diagnostic approaches join into the imaging development, and the
contribution of aptamers includes molecular target strategies in this field.
Aptamers in Diagnostic and Molecular Imaging Applications
149