176 Heterocycles in Medicine
Metronidazole is used for both bacterial (commonly for dental infections) and some protozoan infections such as amoe-
bic dysentery. Benzimidazoles, such as mebendazole, are an important group of anthelmintics. The most common anti-
fungal agents are triazoles such as fl uconazole.
Antibacterial drugs
Among antibacterial agents, the simplest must be the pyridine-hydrazide, isoniazid, an important antituberculosis drug.
Heterocyclic contributions to mainstream antibiotics include a group of quinolones, such as the broad spectrum antibac-
terial ciprofl oxacin (perhaps best known in connection with recent terrorist use of anthrax), and the diamino-pyrimidine
trimethoprim. Many of the sulfonamides, the fi rst synthetic antibiotics, contain heterocyclic residues. Of course, penicil-
lins and cephalosporins are saturated heterocycles and many aromatic heterocycles are included in a multitude of side-
chain variants. Co-trimoxazole is a well-known combination of a sulfonamide (sulfamethoxazole) with trimethoprim.
This combination inhibits enzymes at two points of folic acid utilisation – the sulfonamide inhibits incorporation of
p-aminobenzoic acid during bacterial folic acid synthesis, and trimethoprim inhibits its conversion into tetrahydrofolate.
The overall result is synergistic, that is, there is a greater activity than the sum of the two components.