Antibiotics - Drug Design and Discovery - page 421

salts of penicillin. In the cephalosporins, whose structure was determined in 1961, the thiazolidine cycle
is replaced by a hydrothiazine ring. It should be noted that not only the double ring structure but also the
coni guration at C 3 , C 5 , and C 6 , respectively C 6 and C 7 , are essential for activity.
25.2.1.2 Penicillins
The production was and still is performed by fermentation. The original Penicillium notatum of
Fleming was replaced by Penicillium chrysogenum , whose production has been increased more
than a 1000-fold by the isolation of mutants. Instead of surface culture, already in 1943 deep fer-
mentation was developed in the United States. Large steel cylinders of 100.000 L and more, which
are stirred and aerated are used. Culture media usually contain mineral salts, a carbohydrate source
and soy or peanut meal, and/or cornsteep liquor. It was found in 1944 in a Government laboratory in
Peoria, that addition of cornsteep liquor increased penicillin production. The use of cornsteep liquor
was the origin of the difference between the American penicillin (penicillin G) that has a benzyl
side chain and the English penicillin (penicillin F) that has a pentenyl side chain. The difference is
due to the presence of phenylethylamine (formed from phenylalanine), which is transformed into
phenylacetic acid. Since then, this acid is always added during penicillin G fermentation. On this
basis, Behrens et al. in the Lilly laboratories, examined in 1944-1945 several acids as precursors
for penicillin production. They obtained 11 new penicillins. All these penicillins were active, but
apparently had no interesting new properties. In 1954, Brandl and Margreiter in Austria observed
that a new penicillin, which was acid stable, was formed when phenoxyethanol was added to the
culture medium. This penicillin that was identii ed as phenoxymethyl penicillin (penicillin V), which
unexpectedly was acid stable, was introduced as oral penicillin. It is produced by adding phenoxyacetic
acid to the fermentation medium of Penicillium chrysogenum .
The production of new penicillins by this method, however, is very limited because the mold
incorporates only monosubstituted acetic acids (R-CH 2 -COOH). The possibility of preparing other
penicillins was vastly increased by the discovery of 6-aminopenicillanic acid (6-APA) by Batchelor,
Doyle, and Rolinson in 1959. They could isolate this substance from the fermentation media of
Penicillium chrysogenum to which no precursor acid was added. The yield was very low but for-
tunately another method of preparation was found. The Beecham group and other laboratories in
the United States and Germany (in 1960) discovered that the side chain of penicillin G could be
removed by an enzyme from some bacteria. Many bacteria produce this enzyme, but only certain
strains of Escherichia coli are used in the industry. It was also found that this enzyme, called
penicillin acylase, did not cleave penicillin V. But an enzyme present in other microorganisms like
Fusarium or Erwinia was able to transform penicillin V into 6-APA. Still another practical pro-
cedure is the chemical cleavage of penicillin to 6-APA, discovered in 1970 by Weissenburger and
Vanderhoeven.
The i rst semisynthetic penicillins, phenethicillin, and propicillin, obtained by reaction of
6-APA with a-phenoxypropionic or a-phenoxybutyric acid, had a spectrum of activity very similar
to that of phenoxymethylpenicillin (Table 25.1). A more important advance was the discovery of ampi-
cillin = a-aminobenzylpenicillin by Doyle et al. in 1962 (Table 25.2). This penicillin had a broader
spectrum of activity than the previous products, i.e., it is active against some Gram-negative bac-
teria like E. coli, Proteus, Enterococcus but not against Klebsiella, Enterobacter, Pseudomonas
aeruginosa . The antibacterial spectrum of cyclacillin is similar to that of ampicillin but its activity
is lower.
The p -hydroxy derivative of ampicillin, amoxicillin, unexpectedly presents a much better oral
absorption than ampicillin (1971). Similarly, a better oral absorption occurs also with esters of
ampicillin, pivampicillin, talampicillin, and bacampicillin (1970-1975). These prodrugs are hydro-
lyzed to ampicillin, partially through the action of esterases, partially spontaneously. It also should
be noticed that in these penicillins, phenyl glycine or derivative, has the d-coni guration because
this epimer is more active than the epimer with an l-side chain.