11.3.2 Ligand Selection
In addition, ligands and chelators needed to be added to the termini of PEGs or the
surface of dextran [66]. Ligands should have been chosen from the molecules
already known to chase the targets, which included small molecules or biomacromolecules. The well-known biomacromolecules are either mouse (momab), chimeric (ximab), humanized (zumab), or human (mumab) monoclonal antibodies.
Their recently introduced competitors are affibodies [67], aptamers [68]—either
peptide aptamers or nucleic acid aptamers—and the even more recently aptides [69]
and avibodies [70, 71]. Radiolabeled antibodies have been used for imaging and/or
therapy. Both
90 Y-labeled ibritumomab tiuxetan (Zevalin
® , Biogen Inc.,
Cam-bridge, MA, USA), an anti-CD20 monoclonal antibody, and
131 I-labeled
tositumomab (Bexxar, GlaxoSmithKline, UK), which is a murine anti-CD20
mono-clonal antibody were approved by the FDA for the treatment of non-Hodgkin
lymphoma [72, 73]. However,
131 I-tositumomab was discontinued in February
2014 because of low sales. The effort to develop clinically applicable radiolabeled
antibodies continued and in an example report, Hong et al. reported a dual labeling
method of an anti-CD105 monoclonal antibody with
89 Zr and near-infrared
fluorescent dye [74].
Small molecular ligands are recognized by their target infrequently, while they
sometimes are recognized as opsonins by their target cells or tissues. In that they are
recognized more effectively if they are bound to the surface of the NPs, the aptamers or affibodies can be advantageous as ligands. The advantage of these newly
introduced aptamers and the likes is derived from the fact that they are produced
based on combinatorics, which selects the most desirable macromolecules with
optimal affinity and avidity for the target from a large pool of candidates.
Surface modification of NPs with monoclonal antibodies needs the correct
combination of the best antibody with the correct direction of conjugation. If the
binding motif itself is bound to the surface of NPs and hidden from the open space,
the modified NPs will not bind their target. The most commonly used conjugation
method is the coupling of the primary amine-group present on the Fab fragment of
the active site of the antibody molecule with the carbodiimide and/or
N-hydroxysuccinimide derivatives of the acid-group on the surface of the NPs.
Using this method, the antibody-NP conjugate has a limited number of active
groups for efficient targeting [75, 76]. To overcome this limitation, site directional
conjugation methods have been developed to target the Fc fragment of the antibody
through its hydroxyl or disulfide-groups [75, 77]. Recently, Jeong et al. reported a
new method for site-specific and orientation controlled antibody conjugation on the
NP surface using copper-free click chemistry [78].
Some investigators have even attempted to label protein A, which has high
affinity for the Fc region of IgG, onto the entire surface of the NPs [79]. Then they
further labeled the epitope of protein A with monoclonal antibodies [80–82]. This
approach was successful for in vitro diagnostics but was not employed for in vivo
use because of the increased size of the NPs and the fact that the two-step
214
D. S. Lee et al.
In addition, ligands and chelators needed to be added to the termini of PEGs or the
surface of dextran [66]. Ligands should have been chosen from the molecules
already known to chase the targets, which included small molecules or biomacromolecules. The well-known biomacromolecules are either mouse (momab), chimeric (ximab), humanized (zumab), or human (mumab) monoclonal antibodies.
Their recently introduced competitors are affibodies [67], aptamers [68]—either
peptide aptamers or nucleic acid aptamers—and the even more recently aptides [69]
and avibodies [70, 71]. Radiolabeled antibodies have been used for imaging and/or
therapy. Both
90 Y-labeled ibritumomab tiuxetan (Zevalin
® , Biogen Inc.,
Cam-bridge, MA, USA), an anti-CD20 monoclonal antibody, and
131 I-labeled
tositumomab (Bexxar, GlaxoSmithKline, UK), which is a murine anti-CD20
mono-clonal antibody were approved by the FDA for the treatment of non-Hodgkin
lymphoma [72, 73]. However,
131 I-tositumomab was discontinued in February
2014 because of low sales. The effort to develop clinically applicable radiolabeled
antibodies continued and in an example report, Hong et al. reported a dual labeling
method of an anti-CD105 monoclonal antibody with
89 Zr and near-infrared
fluorescent dye [74].
Small molecular ligands are recognized by their target infrequently, while they
sometimes are recognized as opsonins by their target cells or tissues. In that they are
recognized more effectively if they are bound to the surface of the NPs, the aptamers or affibodies can be advantageous as ligands. The advantage of these newly
introduced aptamers and the likes is derived from the fact that they are produced
based on combinatorics, which selects the most desirable macromolecules with
optimal affinity and avidity for the target from a large pool of candidates.
Surface modification of NPs with monoclonal antibodies needs the correct
combination of the best antibody with the correct direction of conjugation. If the
binding motif itself is bound to the surface of NPs and hidden from the open space,
the modified NPs will not bind their target. The most commonly used conjugation
method is the coupling of the primary amine-group present on the Fab fragment of
the active site of the antibody molecule with the carbodiimide and/or
N-hydroxysuccinimide derivatives of the acid-group on the surface of the NPs.
Using this method, the antibody-NP conjugate has a limited number of active
groups for efficient targeting [75, 76]. To overcome this limitation, site directional
conjugation methods have been developed to target the Fc fragment of the antibody
through its hydroxyl or disulfide-groups [75, 77]. Recently, Jeong et al. reported a
new method for site-specific and orientation controlled antibody conjugation on the
NP surface using copper-free click chemistry [78].
Some investigators have even attempted to label protein A, which has high
affinity for the Fc region of IgG, onto the entire surface of the NPs [79]. Then they
further labeled the epitope of protein A with monoclonal antibodies [80–82]. This
approach was successful for in vitro diagnostics but was not employed for in vivo
use because of the increased size of the NPs and the fact that the two-step
214
D. S. Lee et al.