FUSION PROTEINS FOR HALF-LIFE EXTENSION - Fusion Protein Technologies for Biopharmaceuticals: Applications and Challenges - page 89

IIA TIME: FUSION PROTEIN STRATEGIES
FOR HALF-LIFE EXTENSION
FUSION PROTEINS FOR HALF-LIFE EXTENSION

S TEFAN R. S CHMIDT

Rentschler Biotechnologie GmbH, Laupheim, Germany

6.1 Introduction
6.2 Half-life extension through size and recycling
6.3 Half-life extension through increase of hydrodynamic radius
6.4 Aggregate forming peptide fusions
6.5 Other concepts
6.6 Conclusions and future perspective
References

from lysosomal degradation. A schematic view on the fate of
a therapeutic protein can be seen in Figure 6.1.
This chapter introduces the various fusion protein based
concepts to modify the excretion of proteins in order to
extend their plasma half-life. Neglecting absorption and
metabolism, the half-life is primarily dependent on the
hydrodynamic radius of the molecule from a strictly bio-
physical point of view. Globular proteins below 70 kDa are
removed quite rapidly by glomerular filtration in the kidney.
The other important parameter regulating excretion of
macromolecules besides the pore diameter in the glomeruli
of 60 A is the negative charge of the cell surface that repels
anionic macromolecules [1]. However, all these designs to
increase the hydrodynamic radius must take into account the
other side of the metal. Large size limits tissue penetration.
This is a major obstacle in the treatment of solid tumors.
Particularly, in the case of antibody variants there seems to
be an ideal tumor-targeting zone with a size above the renal
cutoff and 120 kDa. This area represents a good compromise
between penetration and half-life [2]. It has been described
that size, charge, valency, and affinity determine the ability
of molecule to penetrate solid tumors. Some of these param-
eters also contribute to the pharmacokinetic profile of the
molecule. Therefore, a good balance must be achieved to
design the optimal therapeutic protein [3]. It seems that
uncharged molecules have a better penetration profile
because they do not suffer from charge repulsion.
An important mechanism that prolongs circulation time is
the recycling of molecules through receptors. This is the
case for some very abundant plasma molecules, albumin,
and immunoglobulins and to a lesser extent for transferrin
(Tf) as well. The impact of receptor-mediated recycling was

6.1 INTRODUCTION

Therapeutic proteins are part of our standard medical care
for many years. But most of the attractive biological drugs
from the classes of hormones, cytokines, coagulation, and
growth factors are small and disappear quickly from the
blood stream. In the past, this small therapeutic window was
compensated by frequent administration, which makes ther-
apy cumbersome for the patients and also creates concen-
tration spikes that might limit dosing.
The following part is dedicated to fusion protein based
concepts to extend the half-life of therapeutic proteins. The
bioavailability of drugs in general is dependent on factors
such as absorption, distribution, metabolism, and excretion
(ADME). In Chapter 1 mutagenesis strategies were described
to avoid proteolytic degradation. Metabolic removal of pro-
teins is dependent on their cellular uptake through receptor-
mediated endocytosis or pinocytosis. As explained later,
receptor binding can also serve the purpose of recycling,
thus keeping protein longer in circulation and protecting it