Bioactive Agents from Natural Sources: Trends in Discovery and Application - Advances in Biochemical Engineering Biotechnology - page 133

nisms such as Yeast or Drosophila.Gene functions related to cell proliferation
can favorably be studied in Saccharomyces cerevisiae whilest Drosophila mela-
nogaster is preferred for examining signaling pathways [329]. DNA sequence
analysis of Saccharomyces cerevisiae has been completed recently, and gene
function analysis is rapidly progressing, thus contributing to facilitated func-
tion assignment of various novel human DNA sequences and genes.
In fact,the precise function of numerous human genes can only be studied
in a mammalian model.From the beginning ofanimal studies in the industrial
drug development process the mammal of choice has been the mouse.Homo-
logous proteins between mouse and man typically share over 90% of their
amino acid sequences.Physiology,embryology and genetics ofthe mouse have
been well characterized over many years.Referring to human genomic scien-
ces,one of the most important technologies developed so far is the generation
of transgenic mice by introducing any genetic change that can be transferred
into a live mouse and bred into subsequent generations [329]. However, the
basic techniques for target validation are accessible in principle,but it takes a
long time and a resources-consuming process to prove the impact of affecting
a selected target gene on a certain disease phenotype. Consequently, it is
actually speculated that target validation can be omitted. Rapid bioassay
development and transfer to the HTS process promise the identification of
appropriate binding low-molecular mass ligands in a short time.These ligands
are potential drug candidates, and thus can be used favorably to exploit the
relevance of the selected target.
Genomic sciences are expected to identify accessible targets for specific drug
interaction.However,the majority of diseases are multifactorial,with the con-
sequence that a defined phenotype is mostly due to a specific genotype. This
indicates that in future the therapeutics market has to diversify substantially to
provide tailor-made drugs relevant to the respective disease genotype. This
seems to contradict today's strategies of the global operating pharmaceutical
companies aiming at drugs with sales rates of at least several hundred million
US $ per year which are necessary for a return on their investment.

5.2
High-Throughput Screening Assays

Before the advent of molecular biology and gene technology, whole-cell and
whole-organism assays with non-specific endpoints were applied to discover
and evaluate bioactive compounds. Today, a target-based approach using
mechanism-based screens has replaced the non-specific assays. These target-
directed bioassays following the key-lock principle fall into two major cate-
gories:
i) receptor- or enzyme-based screens using a particular target of interest in a
purified system,and
ii) cell-based assays using engineered eukaryotic cells or microorganisms.
In the latter systems,introduction of reporter systems based on the transcrip-
tion of genes encoding proteins such as b -galactosidase, luciferase, alkaline