Chemotherapeutic Agents   Chemotherapy is a general term referring to the use of a drug to kill or weaken invading cells or organisms without harming the patient. Such invading organisms may be bacteria, protozoa, worms, and viruses or tumors. Hence, if we wish to be more precise in designating a chemotherapeutic agent, we may speak of antibacterial, antiprotozoal, anthelmintic, antiviral, or antitumor drugs.  All clinically useful chemotherapeutic agents are more toxic to the organism than to the patient and may be used internally to treat systemic infections.

Antibacterial Agents  -The fight against bacterial infection is one of the great success stories of medicinal chemistry. Antibiotic agent is an antibacterial agent derived from a natural source. Antibacterial agent is a synthetic or naturally occurring agent which can kill or inhibit the growth of bacterial cells.

 The bacterial cell  The success of antibacterial agents owes much to the fact that they can act selectively against bacterial cells rather than animal cells. This is largely due to the fact that bacterial cells and animal cells differ both in their structure and in the biosynthetic pathways which proceed inside them.

 The differences between the bacterial cell and the animal cell:  1-The bacterial cell does not have a defined nucleus, whereas the animal cell does.  2-Animal cells contain a variety of structures called organelles (e.g. mitochondria, etc.), whereas the bacterial cell is relatively simple.  3-The biochemistry of a bacterial cell differs significantly from that of an animal cell. For example, bacteria may have to synthesize essential vitamins which animal cells can acquire intact from food. The bacterial cells must have the enzymes to catalyse these reactions. Animal cells do not, since the reactions are not required

 4-The bacterial cell has a cell wall, as well as a cell membrane, whereas the animal cell has only a cell membrane.  Mechanism of antibacterial actions:  There are five main mechanisms by which antibacterial agents act.  1-Inhibition of bacterial cell wall synthesis: leads to bacterial cell lysis (bursting) and death. Agents operating in this way include penicillins and cephalosporins and vancomycin

 2- Inhibition of cell metabolism: Antibacterial agents which inhibit cell metabolism are called antimetabolites. These compounds inhibit the metabolism of a microorganism, but not the metabolism of the host. They do this by inhibiting an enzyme-catalysed reaction which is present in the bacterial cell, but not in animal cells. The best known examples of antibacterial agents acting in this way are the sulfonamides

 3- Interactions with the plasma membrane: Some antibacterial agents interact with the plasma membrane of bacterial cells to affect membrane permeability. This has fatal results for the cell. Polymyxins and tyrothricin operate in this way.  4- Disruption of protein synthesis: means that essential enzymes required for the cell's survival can no longer be made. Agents which disrupt protein synthesis include the rifamycins, aminoglycosides, tetracyclines, and chloramphenicol.  5- Inhibition of nucleic acid transcription and replication: Inhibition of nucleic acid function prevents cell division

 and/or the synthesis of essential enzymes. Agents acting in this way include nalidixic acid and proflavin.

Antimicrobial agents  Antimicrobial agents fall into two general classes:  I-Antibiotics: Are chemicals that are produced by micro- organisms having the capacity to inhibit, at high dilutions, the growth of or to kill, bacteria and other microorganisms, such as penicillin, cephalosporin, tetracycline and chloramphenicol.  II-Synthetic chemothernpcutic Agents: such as sulfonamides, isonicotinic acid hydrazide and the quinolones.

 Antimicrobial agents: are also frequently classified according to their effect on the microbe as either bacteriostatic (inhibitory) or bactericidal (lethal) agents. Bacterostatic : are drugs inhibit the growth and multiplication of bacteria, but do not directly kill them (e.g. sulphonamides, chloramphenicol.  Bactericidal : are drugs irreversibly damage and kill bacteria, usually by attacking the cell wall or plasma (e.g. penicillin, cephalosporins, aminoglycosidcs, rifumpicin