102
D.C. Hill et al.
4.2.3 Inhibition of Glandular Estrogen Biosynthesis
Aromatase is an NADPH-dependent monoxygenase which catalyses the conversion of androgen to estrogen, and is therefore a key enzyme in glandular
estrogen biosynthesis. Aromatase is found in breast cancer tissue, and specific
inhibitors may lower circulating estrogen levels without interfering with biosynthesis of other steroids.
An aromatase assay has been used to screen microbial metabolites. Human
placental or rat ovarian aromatase was assayed by measuring the conversion of
[113, 213-3H]-androstenedione to 3H20 and unlabelled estrone. After incubating
the enzyme with the substrate, the reaction mixture was extracted with chloroform, residual steroid in the aqueous fraction further adsorbed with activated
charcoal, and the activity of the supernatant determined radiometrically. This
work led to the identification of TAN-931, a substituted benzophenone derivative produced by Penicilliumfuniculosum [123, 124].
The inhibitory activity of TAN-931 was more potent against human than rat
enzyme, and was less potent than 4-hydroxyandrostenedione (4-OHA) against
either enzyme. The mechanism of aromatase inhibition by TAN-931 was different from that of 4-OHA. TAN-931 was shown to be more potent than 4-OHA in
vivo where it suppressed ovarian growth, reduced plasma 1713-estradiol levels,
and inhibited aromatase activity. TAN-931 did not displace 1713-estradiol from
the high affinity binding sites on MCF-7 cells, indicating that it does not have
affinity for estrogen receptors.
4.2.4 Testosterone Metabolism lnhibitors
Development of prostatic hyperplasia is mediated via androgen metabolism.
One of the key steps in testosterone metabolism is formation of 5-a-dihydrotestosterone. A testosterone 5a-reductase inhibition assay was used to discover
WS-9659A and B, novel phenazine derivatives with a monoterpenoid substituent produced by a Streptomyces sp. [125, 126]. WS-9659 A and B inhibited
testosterone 5a-reductase in a dose-dependent manner, and were competitive
inhibitors. WS-9659 A also inhibited testosterone 5a-reductase from dog and
human prostates. The compound did not, however, inhibit aldose reductase and
lactate dehydrogenase. WS-9659 A inhibited growth of ventral prostates induced by testosterone propionate in immature castrated rats.
4.2.5 Other Screen Methodologies and Targets
Increased glutathione S-transferase (GST) activity may be important in cellular
resistance to anti-cancer drugs. Screening programmes for GST inhibitors have
led to the discovery of a number of microbial metabolites including benastatins
A-D [127-129] and bequinostatins A-D [130, 131], all of which possess a
D.C. Hill et al.
4.2.3 Inhibition of Glandular Estrogen Biosynthesis
Aromatase is an NADPH-dependent monoxygenase which catalyses the conversion of androgen to estrogen, and is therefore a key enzyme in glandular
estrogen biosynthesis. Aromatase is found in breast cancer tissue, and specific
inhibitors may lower circulating estrogen levels without interfering with biosynthesis of other steroids.
An aromatase assay has been used to screen microbial metabolites. Human
placental or rat ovarian aromatase was assayed by measuring the conversion of
[113, 213-3H]-androstenedione to 3H20 and unlabelled estrone. After incubating
the enzyme with the substrate, the reaction mixture was extracted with chloroform, residual steroid in the aqueous fraction further adsorbed with activated
charcoal, and the activity of the supernatant determined radiometrically. This
work led to the identification of TAN-931, a substituted benzophenone derivative produced by Penicilliumfuniculosum [123, 124].
The inhibitory activity of TAN-931 was more potent against human than rat
enzyme, and was less potent than 4-hydroxyandrostenedione (4-OHA) against
either enzyme. The mechanism of aromatase inhibition by TAN-931 was different from that of 4-OHA. TAN-931 was shown to be more potent than 4-OHA in
vivo where it suppressed ovarian growth, reduced plasma 1713-estradiol levels,
and inhibited aromatase activity. TAN-931 did not displace 1713-estradiol from
the high affinity binding sites on MCF-7 cells, indicating that it does not have
affinity for estrogen receptors.
4.2.4 Testosterone Metabolism lnhibitors
Development of prostatic hyperplasia is mediated via androgen metabolism.
One of the key steps in testosterone metabolism is formation of 5-a-dihydrotestosterone. A testosterone 5a-reductase inhibition assay was used to discover
WS-9659A and B, novel phenazine derivatives with a monoterpenoid substituent produced by a Streptomyces sp. [125, 126]. WS-9659 A and B inhibited
testosterone 5a-reductase in a dose-dependent manner, and were competitive
inhibitors. WS-9659 A also inhibited testosterone 5a-reductase from dog and
human prostates. The compound did not, however, inhibit aldose reductase and
lactate dehydrogenase. WS-9659 A inhibited growth of ventral prostates induced by testosterone propionate in immature castrated rats.
4.2.5 Other Screen Methodologies and Targets
Increased glutathione S-transferase (GST) activity may be important in cellular
resistance to anti-cancer drugs. Screening programmes for GST inhibitors have
led to the discovery of a number of microbial metabolites including benastatins
A-D [127-129] and bequinostatins A-D [130, 131], all of which possess a