Biomedical Engineering Reference
In-Depth Information
Insulin analogs with changes of ProB28, as the above-mentioned insulin analogs aspart and
lispro, primarily exist in the monomeric form, which is known to be more vulnerable for unfolding
and subsequent i bril formation. To avoid this problem, mutations were limited to the neighboring
LysB29 and to residues in the N-terminus of the B-chain. It was already known that changes of the
B1-B8 residues primarily affected the stabilization of the dimer form of insulin. The basic LysB29
was exchanged for the acidic, polar, and hydrophilic Glu, and the neutral, polar, and hydrophobic
AsnB3 was exchanged for the basic, polar, and hydrophilic Lys. This double mutant has a slightly
lower p
I
of 5.1 compared with human insulin with a p
I
of 5.5, and accordingly its solubility is also
enhanced. The double mutant, AsnB3Lys + LysB29Glu, is especially interesting because higher-
order forms dominate relative to the monomeric form, i.e., it is more stable, while still maintaining
rapid dissociation to monomeric insulin. This double mutant is normally referred to as insulin glu-
lisine and is marketed as Apidra.
Thus, the design of rapidly absorbed, fast-acting insulin analogs must be characterized as a clear
success. The design of the above-mentioned insulin analogs were made possible because the large
number of insulin structures provided the researchers with a detailed information about the molecu-
lar interactions responsible for receptor binding and the hexamer-dimer-monomer equilibrium.
2.6 NEWMETHODS
The experimental and computational methods used in biostructure-based drug designs are constantly
evolving. Here, we will only mention a few of the methods that have and are expected to have a great
impact on the design process.
The availability of an increasing number of experimentally determined structures of relevant
targets is crucial. Crystallization, data collection, and structure determination processes are today
partially or fully automated. Centers for high throughput determination of 3D structures have been
established in several countries. These developments and initiatives are rel ected in an increase in
the number of experimentally determined structures deposited at the Protein Data Bank. Many
pharmaceutical companies also have in-house groups doing structure determinations, but the num-
ber of structures determined here is difi cult to estimate.
Computationally, the developments are also considerable. Improved methods for determination
of models of structures based on structures of related proteins, i.e., homology modeling or com-
parative modeling, are being developed. Faster computers allow longer and thereby more realistic
simulations of proteins where l exibility and solvation can be considered. Ligand docking programs
are being improved by considering not only the l exibility of the ligand but also the l exibility of
the protein. The prediction of binding energies have always been a problem in docking methods,
but improved quantum mechanics-based or quantum mechanics-derived methods combining speed
and accuracy are being developed. Better description and handling of the solvation vs. desolvation
processes are also crucial for the correct prediction of binding afi nities.
2.7 CONCLUSION
Biostructure-based drug design is being used to efi ciently develop new therapeutic candidates and
incorporates multiple scientii c disciplines, including medicinal chemistry, pharmacology, struc-
tural biology, and computer modeling.
The examples described earlier illustrate that biostructure-based drug design in cases where
structural information of the targets is available is a powerful method for the design of new or
improved drugs.
The DuPont Merck example on biostructure-based design of the cyclic urea HIV-1 protease
inhibitors nicely illustrates that in many cases it is possible based on the 3D structures of a target to
design ligands to control or regulate a biological system. Unfortunately, the example also illustrates
that several other features have to be considered in order for a ligand to become a successful drug.
