Drug Targeting Organ-Specific Strategies

11.8 Recombinant DNA Constructs 299
otin-labelled molecules can be attached to the recombinant protein via the biotin binding site
of the construct.
Recombinant drug targeting constructs utilizing the targeting moiety of antibodies have
predominantly involved toxins and cytokines as the active drug substance (Table 11.5). The
bacterial toxins most commonly used for immunotoxin constructs are diphtheria toxin (DT),
pseudomonas exotoxin (PE) and to a lesser extent shiga(-like) toxin (ST/SLT). These toxins
all have the same overall composition: a receptor binding moiety, a fragment involved in
membrane translocation and a toxic or catalytic domain. In order to avoid interactions with
cells that bear the ‘normal’ toxin receptor, the receptor-binding domain is removed, resulting
in so-called truncated toxins. Genetic coupling of such chemical ‘bombs’ to an antibody or
scFv, results in a highly selective and potent drug targeting construct.
A variant of the original immunotoxin approach is the so-called immunocytokines. In
these constructs the antibody targeting moiety is maintained, but the toxin as the effector
molecule is replaced by a cytokine. In contrast to toxins, cytokines are often proteins endogenously
produced in man. If both the antibody and cytokine are of human origin, then no
foreign proteins are introduced which could provoke an antibody response from the host immune
system when the drug targeting preparation is clinically applied.
Selective targeting of very potent cytokines may be an attractive approach to overcome
the many side-effects seen after general systemic administration of such compounds [122].
Most cytokines are LMWPs and as such rapidly eliminated by renal glomerular filtration.
Consequently, high doses are necessary to obtain locally effective concentrations.
The potential of immunocytokines was elegantly demonstrated in two separate studies using
IL-12 for anti-tumour therapy. IL-12, a potent stimulator of natural killer cells and cytotoxic
T-lymphocytes (CTL), activates the immune system to eradicate the cancer cells. IL-12
(a)
(b)
P35 P40 L chain H chain S-S bridge (Gly4Ser) 3 linker
Figure 11.6. Schematic diagram showing the assembly of IL-12 protein for antibody-based drug
delivery. (a) The mature sequences of the p35 subunit of IL-12 are fused to the C-terminus of the heavy
chain of a tumour-specific antibody and co-expressed with the antibody light chain and the p40 subunit
of IL-12. Formation of the final immunocytokine requires the creation of disulfide bridges between the
antibody chains and interactions of p35 and p40 subunits of IL-12 [119]. (b) Alternatively the IgG heavy
chain and both subunits of IL-12 can be linked via flexible linkers allowing for equimolar assembly of IL-
12 [120].