DEVELOPMENT OF EXPERIMENTAL TARGETED TOXIN THERAPIES FOR MALIGNANT GLIOMA - Fusion Protein Technologies for Biopharmaceuticals: Applications and Challenges - page 307

Binding
site

Human interleukin-13

IL-13-PE38QQR fusion toxin

PE38QQR

Pseudomonas exotoxin A

FIGURE 20.1 Schematic and 3D-ribbon representations of the structure of the recombinant fusion
toxin IL-13-PE38 (IL-13-PE38QQR). A derivative of the native Pseudomonas exotoxin A (PE),
PE38QQR, in which domain Ia (amino acids 1-252) has been completely deleted and domain Ib
(amino acids 365-380) has been partially deleted, lysine residues at positions 590 and 606 have been
replaced by glutamine, and at position 613 by arginine, has been fused to the carboxy terminus of the
human IL-13 to produce the fusion toxin IL-13-PE38QQR (IL-13-PE38).

benefit of control over undesired toxicity that may occur
with alternative delivery methods depending on large con-
centration gradients for tissue transport. There was a signifi-
cant penetration advantage of high-flow (convective) over
low-flow (diffusive) microinfusion. Using the same example
as described earlier, a 12-h high-flow microinfusion of a
180-kDa protein would provide 5-10-fold increases in
volume over low-flow infusion, with total treatment volumes
> 10 cm 3 . Degradation rates of the protein would play a role
in the final treatment volume, and more rapid degradation
rates would reduce the volume [29].
Tf labeled with 111 In (80 kDa) was intracerebrally infused
in cats and compared with the distribution of a 0.4-kDa 14 C-
sucrose molecule [30]. Systemic concentrations of 111 In-Tf
were determined in blood, liver, kidneys, and muscle. A
pressure gradient was maintained during infusion so as to
generate bulk flow in the interstitial space of the brain. The

volume of distribution was shown to increase linearly with
the infusion volume for both compounds. The systemic
concentrations of 111 In-Tf were less than 1% of the brain
concentrations for all infused volumes. These data indicated
that by using convection (bulk flow) instead of diffusion,
increased distribution of large and small molecules could be
achieved in the brain with minimal peripheral losses of
infusate [30].
A subsequent study in primates used single-photon emis-
sion computerized tomography (SPECT) to measure brain
distribution of 111 In-DTPA-Tf (81 kDa) by CED [31]. 111 In-
DTPA-Tf was infused at a rate of 114 m L/h for 87 h, and CT
and SPECT scans were taken in regular intervals. Distribu-
tion of the protein ranged between 2 and 3 cm from the
needle tip and infusion volumes were 3.9-6.7 cm 3 . These
data for time of infusion and area of distribution compared
favorably with the calculated diffusion of an IgG molecule,