Feng, Xiaodong_ Xie, Hong-Guang - Applying pharmacogenomics in therapeutics-CRC Press (2016)

80 Applying Pharmacogenomics in Therapeutics
of “knocking-in” or “knocking-out” the genetic alteration on associated signaling
pathways and on physiological processes can be assessed, but more importantly, the
impact on disease initiation and/or progression can be measured. Disadvantages of
animal studies include time and cost, as well as ethical concerns. If the cause-andeffect
relationship is established, the cell lines and animal models generated for target
validation experiments can subsequently be used to test drug efficacy and toxicity in
preclinical studies of potential drug candidates. It is important that data from target
validation studies can also provide insights into whether targeting a genetic alteration
is likely to cause significant toxicity. For example, toxicity is more likely if the
genetic alteration is found to have a broad tissue distribution. In addition, target validation
studies often improve our understanding of disease etiology because the focus
is placed on understanding the impact of the genetic alteration on related pathways.
They can even result in the identification of additional drug and more suitable drug
targets. 28,29
Cell lines and animal models were instrumental in validating the involvement
of Bcr-Abl, the genetic alteration that results from a chromosomal translocation in
hematopoietic progenitor cells, in driving CML and for the development and testing
of drugs to treat this disease. 30–33 Some of the cell lines used for these studies
were generated from CML patient samples that harbored the Bcr-Abl gene fusion,
and others were generated by genetically engineering cells to include this genetic
alteration. 31,33 Phenotypic analyses of these cell lines, combined with manipulations
that allowed for direct targeting of the Bcr-Abl fusion gene and/or targeting
of upstream and/or downstream components of the associated signaling pathway,
demonstrated that Bcr-Abl is able to drive cell proliferation. To confirm the ability
of Bcr-Abl to mediate leukocyte proliferation in vivo and drive CML, several mouse
models were developed, including xenograft models (human cell lines harboring
the Bcr-Abl fusion gene were implanted into immunodeficient mice) and transgenic
models (the hematopoietic progenitor cells of these mice were engineered to harbor
the Bcr-Abl fusion gene). 30,32 In addition to confirming the importance of Bcr-Abl in
driving CML, these mouse models were also subsequently used to test the efficacy
of developmental drugs designed to target Bcr-Abl.
ASSESSING “DRUGGABILITY” OF A TARGET
AND LEAD DRUG DISCOVERY STRATEGIES
Once a drug target has been identified and validated, then drugs may be developed
to “hit” this target (by either inhibiting protein function and/or altering expression
levels) and thereby negate its effect on mediating disease initiation and/or progression.
A usual first step is to determine whether the target is in fact “druggable”:
that is, whether high affinity binding between the target and a pharmacological
agent is possible, and therefore whether or not a suitable drug can be developed. 34–37
Currently, it is estimated that only 10% of the human genome is druggable. 38 Binding
of a drug to its target can be affected by many factors, including protein structure,
size, charge, and polarity. Additional genetic alterations in the target gene (genetic
alterations other than those that are causing dysfunction and thereby disease, typically
polymorphisms) can potentially affect all of these factors and thereby affect