deep cleft across the enzyme, and about a dozen amino acids are involved in binding
substrate or inhibitor.
The biochemical role of dihydrofolate reductase (DHFR) is enormously important in
a number of biosynthetic pathways. It reduces dihydrofolate to tetrahydrofolate, a
cofactor that accepts a one-carbon fragment in several forms and transfers it to sub-
strates during the synthesis of some amino acids, purines, and pyrimidines.
Tetrahydrofolate—or its precursor, folate—is a vitamin for humans, and must be
acquired from food. It is present abundantly in the green leaves of vegetables [folium
(Latin)=leaf]. Bacteria, on the other hand, cannot use external folic acid, relying
instead on an obligatory folate synthetic mechanism. In addition, variations in DHFR
isozymes in different organisms offer a number of pharmacological targets in accord
with the principle of antimetabolite therapy—that is, the use of competitive enzyme
inhibitors. Sulfanilamide DHFR inhibitors have found use as antibacterial agents which
act on the basis of folate synthesis inhibition. As mentioned above, bacteria must syn-
thesize their own folate. Interference with this process will inhibit nucleic acid biosyn-
thesis and result in bacteriostatic action because cell division ceases.
Discovered in 1935 by Domagk in Germany, these sulfa agents opened the modern
era of bacterial chemotherapy. A tremendously rapid development of new and improved
derivatives increased the antimicrobial spectrum and therapeutic ratio of these drugs
and eliminated many side effects. Although the introduction of penicillin and other
antibiotics led to some loss of interest in sulfanilamides, a new emphasis began when
their utility in coexistence with other antibiotics was recognized. Research in this area
also triggered the formulation of many contemporary concepts of drug action, metabo-
lism, and molecular mechanisms, specifically on competitive enzyme inhibitors.
The mode of action of sulfanilamides became known around 1947, when the structure
and biosynthesis of folic acid were elucidated. This compound is built by bacteria from
the heterocyclic pteroyl moiety,p-aminobenzoate, and glutamate. p-Aminobenzene-
sulfonamide (9.89, sulfanilamide) is a competitive inhibitor of the synthase enzyme,
acting as an “antimetabolite” of p-aminobenzoate. Occasionally, the sulfanilamide can
even be incorporated into the modified folate, resulting in an inactive compound and
thus an inactive enzyme. This theory, proposed by Woods and Fildes in 1940, became
the first molecular explanation of drug action.
The structure of sulfonamides is shown for representative purposes. Among the
several thousand compounds in existence, about 25–30 have found widespread use.
Sulfanilamide itself is, by present-day standards, very inactive. It was the development
of heterocyclic derivatives that produced the highly potent sulfathiazole (9.90). When a
succinyl or phthalyl group is attached to the aniline nitrogen, the inactive acylanilide
derivatives will not be absorbed from the intestinal tract. Slow deacylation by intestinal
578 MEDICINAL CHEMISTRY