Drug Targeting Organ-Specific Strategies

11.8 Recombinant DNA Constructs 297
(a) (b)
(c)
Intact antibody F(ab’) 2-fragment Fab-fragment
(e)
(h)
S
S
(ScFv) 2
(ScFv) 2
(f)
Leu-zipper
combinant protein size, and thereby in complexity of the construct can be achieved when
both the carrier and homing device functions are intrinsic properties of one protein.
Antibodies make up a group of proteins which can be considered to have the properties of
both a carrier and a homing device and, as a result, have been used in many drug targeting
studies [18,109]. However, the relatively large size (150 kDa) of whole IgG molecules hampers
tissue penetration of these molecules. Several modifications of the original antibody
structure can be carried out to reduce the size of the IgG molecule. For instance, in natural
antibodies (Figure 11.5a) the Fc-region is necessary to activate T-cells of the immune system.
Since this function of the antibody is not required in most drug targeting constructs, these domains
have been removed by recombinant cloning techniques [110]. The resulting F(ab′) 2,
and Fab fragments, with molecular weight of around 100–110 and 50–55 kDa respectively, are
2
(ScFv) 3
(ScFv) 2
(g)
(i) (j)
(ScFv) 4
Streptavidin
(d)
ScFv
Figure 11.5. Schematic representation of genetically engineered antibody constructs for drug targeting.
Intact antibodies consisting of two heavy and two light chains (a) can be converted into divalent F(ab′) 2
fragments (b) or to monovalent Fab fragments (c). These fragments are stabilized via disulfide bridges.
Alternatively, the variable heavy and light chain fragments are linked via a flexible linker resulting in a
monovalent ScFv (d). Di-, tri- and tetravalent scFv fragments can be constructed by connecting two,
three or four scFv fragments with peptide linkers (e–g) or by introducing a S–S bridge between the
individual scFv fragments (h). Non-covalent interactions between scFv fragments are created by
introducing leucine zipper sequences into the construct (i) or via streptavidin–biotin interactions (j).
3