Drug Targeting Organ-Specific Strategies

302 11 Development of Proteinaceous Drug Targeting Constructs
structs, the recombinant protein could function as a competitive substrate for HIV-1 protease.This
approach made use of the fact that surface-exposed loops of globular proteins can
often tolerate insertions of additional amino acids without altering the function of the protein
[142]. Molecular modelling was used to select candidate insertion sites in surface-exposed
loops of transferrin that were distant from the biologically active domains.
Evidently, resolution of the three-dimensional structures of proteins will aid in the design
of rational approaches for constructing drug conjugates, as demonstrated by the above-mentioned
examples. Evaluation of molecular structure at this level may prove to be one of the
more successful approaches used in the design of recombinant drug conjugates.
11.9 Recombinant Domains as Building Blocks for Drug
Targeting Constructs
With the growing knowledge about protein structure–function relationships and the availability
of new techniques like phage display, we are now able to select small proteinaceous sequences
that could function as building blocks for recombinant targeting constructs. Such
building blocks can function as a targeting moiety (homing device), a membrane translocation
moiety and/or an active drug substance. In order to facilitate the construction of recombinant
preparations, it seems reasonable to assemble the final construct from smaller subunits.
11.9.1 Targeting Domain
Probably the smallest sequence known to be responsible for receptor recognition is the
RGD-tripeptide, initially discovered in fibronectin [143]. However, the specificity of the interaction
with different integrins, the counter receptors of RGD sequences on the cell surface,
is established by the flanking sequences of the RGD motif and the conformation of the
tripeptide. In other words, the presentation of the RGD sequence is important for specific
recognition by individual integrins.
Insertion of RGD sequences as targeting domains into protein carriers, is an attractive approach
for integrin-directed targeting. Studies with RGD sequences of viral origin cloned
into solvent-exposed regions of β-galactosidase demonstrated binding and internalization of
the active chimeric enzyme into mammalian cells [144]. Likewise, introduction of an RGD
motif into the capsid protein of adenoviruses was shown to increase the cell-specific delivery
of adenoviral vectors as gene delivery vehicles [145]. Fusion proteins containing RGD sequences
are likely to be effective delivery systems but the clinical relevance thereof awaits
further analysis.The RGD motif can form the targeting domain, but at the same time can also
function as the active drug, since its binding to the receptor may result in prevention or disruption
of the natural ligand–integrin interaction, and consequently in a therapeutic response.
The use of phage-display techniques has identified peptide ligands with specific affinity to
cell surface receptors or specific tissues (see Table 11.2). Such peptide homing devices can be