the receptor. Furthermore, in order to bind to the receptor, the conformations
and configuration of the new antagonist should be complementary to the
structure of the receptor. If sufficient data is available, molecular modelling
docking techniques (see section 5.5) can yield valuble insight into these aspects
of the binding of the potential antagonist to the receptor.
7.4.3 Partial agonists
Partial agonists are compounds that act as both agonists and antagonists. They
are believed to act as antagonists by preventing the endogenous ligand binding
to the receptor but at the same time weakly activating the receptor. This is
thought to cause a weak signal to be sent to the appropriate domain of the
receptor. The net result of these opposing effects is that a much higher dose of
the agonist is required to obtain the maximum response. Furthermore, this
response is less than that of pure agonists with similar structures. Most drugs
are partial agonists.
7.5 Drugs that target nucleic acids
Drugs that target DNA and RNA (see Chapter 1) either inhibit the synthesis of
DNA and RNA or act on existing nucleic acid molecules. Those that inhibit the
synthesis of nucleic acids usually act as antimetabolites and enzyme inhibitors.
Those that act on existing nucleic acid molecules may for convenience by
broadly classified as intercalating agents, alkylating agents and chain cleaving
agents. In both cases, the net result is the prevention or slowing down of cell
growth and division. Consequently, the discovery of new drugs that target
existing DNA and RNA is a major line of approach for the development of
new drugs for the treatment of cancer (see Appendix 10) and bacterial and other
infections due to microorganisms.
7.5.1 Antimetabolites
Antimetabolites are compounds that block the normal metabolic pathways
operating in cells. They act by either replacing an endogenous compound in
the pathway by a compound whose incorporation into the system either results
in a product that can no longer play any further part in the pathway or inhibits
DRUGS THAT TARGET NUCLEIC ACIDS 147