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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5345_Библиотеки_им_академика_М_И_Перельмана.pdf
Ligand- and Structure-Based Drug Design of NSAIs in Breast Cancer
cancers. Interestingly, it is found that patients with ER+/HER2+ get more benefit from drugs compared
+
with ER/HER2
breast cancers by inhibiting PIK3/AKT pathway. The HER2/neu is an important target
for monoclonal antibody trastuzumab (herceptin) (Piccart-Gebhart et al., 2005). Herceptin is effective
only in breast cancer when HER2/neu receptor is overexpressed. Overexpression of HER2 gene may be
suppressed by the amplification of other genes as well as use of trastuzumab. Another monoclonal antibody, pertuzumab is in clinical trials. It can inhibit the dimerization of HER2 and HER3 receptors. The
expression of HER2/ErbB2 protein is generally regulated by ERs. Again, estradiol and tamoxifen acting
through ERs are found to downregulate HER2/ErbB2 expression. When the coactivator AIB-3 exceeds
the corepressor PAX2, the HER2/ErbB2 is upregulated in presence of tamoxifen. It leads to tamoxifen
resistant breast cancer (Hurtado et al., 2008; Vogel, Cobleigh, & Tripathy, 2002; Salmon, Leyland-Jones,
+
& Shak, 2001). Lapatinib is responsible for delay in disease progression in HER2
+
to trastuzumab. New generation anti-HER2
targeted therapies are in development (Bedard, Azambuja,
breast cancer resistant
& Cardoso, 2009). Bevaxizumab retards metastasized tumor growth by blocking angiogenesis (Early
Breast Cancer Trialists’ Collaborative Group, 2005).
As far as the molecular targeted therapy of breast cancer is concerned, there are several molecular
targets, and inhibiting the functionality of the targets is effective for treating breast cancer, like HER-2
inhibitors (Brufsky, 2014), kinase inhibitors of phosphatidyl-inositol 3-kinase (PI3K), protein kinase B
(Akt), mammalian target of rapamycin (mTOR) and tyrosine, IGF1R inhibitors, cyclin dependent kinase
inhibitors (CDKs), heat shock protein 90 (HSP90) inhibitors, telomerase inhibitors, vascular endothelial
growth factor (VEGF) inhibitors, poly-(ADP-ribose) polymerase (PARP) inhibitors, inhibitors of apoptosis proteins (IAPs) (Schlotter et al., 2008), nuclear transcription factor (NF-κB), ubiquitine-proteasome
system and farnesyl transferase as well as p53 targeted gene therapy may be effective for development of
anti-breast cancer drugs (Schlotter et al., 2008). Apart from that, selective estrogen receptor modulators
(SERMs) and aromatase inhibitors (AIs) play vital role in prevention of breast cancer. The IAPs prevent
uncontrolled and excessive cell death in apoptotic signaling. Survivin, supposed to be a possible target
of breast cancer, is detected in almost 90% of breast tumors (Schlotter et al., 2008). Increased level of
survivin is found to be related in HER2 overexpression, VEGF overexpression and high urokinase type
plasminogen activator levels (Schlotter et al., 2008; Ryan et al., 2006) It plays a key role in multi-drug
resistance (MDR) in presence of p-glycoprotein, and this may be a novel strategy for modulating MDR
resistant cancer cells (Liu et al., 2007). Activation of NF-κB may be responsible for progression of
hormone dependent breast cancer (Nakshatri et al., 1997). The NF-κB is also found to inhibit extracellular signal-regulated kinase (ERK) activation to enhance cell survival during the development of
tumour adaptive radioresistance (Ahmed et al., 2006). The ubiquitine-proteasome system was reported
to regulate the p53 cyclins, cyclin-dependent kinases (CDKs) and the bcl family proteins. Inhibitors of
the proteasome system may be responsible for the accumulation of pro-apoptotic proteins, which deliver
cytochrome from mitochondria and thereby activating intrinsic apoptotic signal transduction (Schlotter
et al., 2008). Inactivation of NF-κB by proteasome inhibition may be responsible for increased apoptosis by histone deacetylase (HDAC) inhibitors induction. The HDAC inhibitors are a novel class of
anti-cancer agents, play important roles like cell growth arrest, cellular differentiation and apoptosis in
many tumour cells. They may also regulate the activity of the anti-apoptotic transcription factor NF-κB
(Domingo-Domenech et al., 2007). The PI3K is reported to play an important role in survival, proliferation, motility and neoangiogenesis in cancer cells. Due to the disregulation of the cell cycle, PI3K is
overexpressed and may activate Akt. By means of phosphorylation of NF-κB, bad and caspase 9, Akt
may have anti-apoptotic influences. The EGFR and HER2 may also activate PI3K and responsible to
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Ligand- and Structure-Based Drug Design of NSAIs in Breast Cancer
enhance cell growth via Akt. Therefore, PI3K may be a valuable target for the breast cancer (Schlotter
et al., 2008). The mTOR protein is activated by phosphorylated Akt. Hence, mTOR protein is found to
promote cancer cell proliferation and neoangiogenesis. It may also promote tumor spread and metastasis
and inhibit cancer cell apoptosis. Inhibition of mTOR by temsirolimus may have negatively influences on
these mechanisms, investigated in clinical trials (Schlotter et al., 2008). The Hsp90, a chaperone protein,
is reported to promote protein folding and stabilization, hence prevent rapid protein degradation. The
Hsp90 inhibitors, like tanespimycin, retaspimycin and ganetespib are proved to be beneficial against
HER2 resistant breast cancer in clinical trials. However, the clinical study for tanespimycin was terminated because of corporate decision, whereas the investigation was discontinued due to lack of efficacy
(Modi et al., 2011; Modi et al., 2013). The expression of telomerase is observed in cellular proliferation
that has a relation with tumorogenesis. Inhibition of telomerase may cause apoptosis (Goldblatt et al.,
2009). Telomerase inhibitor, imetelstat is reported to restore trastuzumab sensitivity in trastuzumab
resistant cell line. However, phase 1 study of imetelstat showed no response in metastatic breast cancer
(Miller, 2012). A crosstalk between IGF1R and HER2 in breast cancer may lead to heterodimerization
and may cause tumor cells to escape trastuzumab toxicity. However, in clinical trials, it was proved that
IGF1R inhibitor and trastuzumab combination resulted better activity than trastuzumab alone ((Nahta et
al., 2005). Treatment with linsitinib, an IGF1R inhibitor, in metastatic breast cancer was discontinued in
phase 2 study due to lack of efficacy and severe toxicity (Brufsky, 2014). Cyclooxygenase-2 (COX-2)
inhibitors were also a promising target for breast cancer. Celecoxib was found to be effective in ERnegative breast cancer and triple negative breast cancer (TNBC) by delaying tumor progression (Howe
et al., 2002). Unfortunately, due to increased risk of heart failure,clinical trials of COX-2 inhibitors were
halted by FDA though there were positive breast cancer prevention clinical trials (Hollander et al., 2013).
Chemotherapy helps to stop the growth of cancer cells either by killing or by arresting their multiplication. It is dependent on the breast tumor size, number of lymph nodes involved, presence of estrogen
and/or progesterone receptors as well as the amount of HER2/neu protein of cancer cells. It is observed
that combination of drugs (combination chemotherapy) is more effective than single drug for breast
cancer treatment (Hortobagyi, 2006; Mauri et al., 2008). The common drugs used in combination are
florouracil, methotrexate, doxorubicin, cyclophosphamide, paclitaxel and docetaxel.
Hormonal therapy is useful as it blocks the effect of estrogen in breast cancer cells. Anti-estrogenic
drugs act either by blocking the hormone receptor or destroy ovaries or suppress the estrogen production. The growth stimulatory effects of estrogens may be reduced either by using SERMs or by using
AIs. The SERMs may compete for binding to ERs (competitive inhibition) and reduce the number of
ERs available for binding to endogenous estrogen. This approach is an effective anticancer strategy. It
leads to the development of antiestrogenic drugs, such as tamoxifen, raloxifene and tormifene (Carmichael, 1998; Jordan, 1995). Inhibition of aromatase enzyme is most effective approach to reduce growth
stimulatory effects of estrogen.
AROMATASE BIOCHEMISTRY
Effective aromatase inhibitors (AIs) are developed as the therapeutic agents for estrogen-dependent breast
cancer. Development of novel AIs started in early 80s of the last century and continued greatly in last four
decades (Brueggemeier, Hackett, & Diaz-Cruz, 2005; Murthy, Rao, & Sastry, 2004; Recanatini, Cavalli,
& Valenti, 2002; Geisler & Lonning, 2005; Harvey, Lipton, & Santen, 1982; Covey, 1988; Johnston &
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407
Ligand- and Structure-Based Drug Design of NSAIs in Breast Cancer
Metcalf, 1984; Banting et al., 1988; Banting et al., 1989; Cole & Robinson, 1990; Brueggemeier, 1990;
Brueggemeier, 1994; Brodie & Njar, 1996, 2000; Brodie, Lu, & Long, 1999; Santen & Harvey, 1999;
Seralini & Moslemi, 2001; Geisler, 2003; Smith & Dowsett, 2003; Altundag & Ibrahim, 2006; Benson
& Ravisekar, 2007). Estrogen biosynthesis in postmenopausal women occurs principally in extragonadal
sites where adipose tissue plays an important role. The peripheral estrogen production is important. This
may be the reason for occurrence of breast tumors in postmenopausal women. Furthermore, it is observed
that 17β-estradiol may be produced in breast tissue, and thereby estrogen concentration may be maintained at an appropriate level for stimulating breast cancer growth. Even biosynthesis of small amount
of estrogen may cause breast tumor development (Labrie et al., 1997). After menopause, biologically
important estrogen, i.e., 17β-estradiol is generated from the circulating androstendione from adrenal
gland. Androstenedione is transformed to estrone. This is converted into estrone sulfate as a reservoir
of estrogen hormone. Estrone may be derived from the aromatization of androgen or from the sulfate
metabolite. After removal of the sulfate group, it is converted to 17β-estradiol. Aromatase catalyzes the
syntheses of estrogens. This is done via the aromatization of the A ring of androgen precursors, namely
androstenedione and testosterone. Aromatization is considered to be responsible for the biosyntheses of
estrogens from androgens (Figure 3) (Schuster et al., 2006).
Figure 3. Biosynthetic pathway of estrogens and androgens
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Ligand- and Structure-Based Drug Design of NSAIs in Breast Cancer
Estradiol is the most important endogenous estrogen which is biosynthesized from androgens by
the cytochrome P450 enzyme complex known as ‘aromatase’ (Simpson et al., 1994). Aromatase is a
multienzymatic complex which is mostly expressed in the ovary of premenopausal women, placenta,
peripheral adipose tissues, breast tissue and in the brain (Simpson et al., 1994). It is overexpressed
in breast cancer tissue and may be responsible for local estrogen production and proliferation of
breast tumors (Millet et al., 2008; Miller, 1997; James et al., 1987; Miller & O’neill, 1987; Reed
et al., 1989; Bulun et al., 1993). Adipose stromal cells surrounding the cancer cells are also reported to contain higher aromatase mRNA level than adipose stromal cells in noncancerous areas
(Dickson & Lippman, 1995; Harada, 1997). Again, estrogen concentration in breast cancer tissue
is found to be several-fold higher than plasma in postmenopausal women. This is supported by the
synthesis of estrogens with the aromatase enzyme (Pasqualini et al., 1996). Out of 70 breast tumor
specimen, aromatase mRNA is also found in 67 specimens (Zhou et al., 1996). The presence of
aromatase in breast cancer epithelial and stromal cells is also found by immunocytochemical analysis
(Esteban et al., 1992). Gene transcriptional studies reveal that aromatase promoter switches from
a glucocorticoid stimulated promoter I.4 in normal tissue to cAMP stimulated promoters I.3 and II
in cancereous tissue. Prevention of aromatase expression suppresses in situ estrogen biosynthesis.
Anti-aromatase drugs are a second-line treatment in patients who fail anti-estrogen therapy. The
patients (20-30%) failed to anti-estrogen therapy, may respond to aromatase inhibitors (Chen, 1998).
Aromatase enzyme complex is located in the endoplasmic reticulum of cells. This is composed
of a cytochrome P450 heme protein (CYP19). The CYP19 carries out the aromatization reaction.
Simultaneously, a NADPH-cytochrome P450 reductase (a flavoprotein) is required for the electron
transfer from NADPH to the cytochrome P450 enzyme (Brodie & Njar, 1998; Simpson & Davis,
2001). The heme protein (CYP19) is responsible for converting the alicyclic A ring of C19 steroids
(androgens) to C18 steroids (estrogens) containing the phenolic A ring (Kellis & Vickery, 1987),
whereas NADPH-cytochrome P450 reductase is responsible for transferring reducing equivalents
to cytochrome P450 (CYP19). In the three step process, aromatase converts androstendione, testosterone and 17α-hydroxytestosterone to estrone, 17β-estradiol and 17β, 16α-estriol, respectively by
coupling with 1 mol O
, 1 mol NADPH and cytochrome P450 reductase (Simpson et al., 1994). The
2
first two steps involve C19 methyl hydroxylation. The third step is responsible for the aromatization
of the ring A which is a unique characteristic of aromatase enzyme.
STRUCTURE OF AROMATASE
Aromatase is the only known enzyme in vertebrates capable of catalyzing the aromatization reaction of a
six-membered alicyclic ring. The functional human aromatase is monomeric. It is comprised of a heme
group as well as a single polypeptide chain of 503 amino-acid residues. It is an integral membrane-bound
protein of the endoplasmic reticulum. It is attached to the membrane by an amino-terminal transmembrane domain, in addition to other membrane-associating regions (Shimozawa et al., 1993; Amarneh
et al., 1993). It shows high substrate specificity for syntheses of estrogens from androgen precursors.
The crystal structure of placental aromatase showed key aspects regarding the properties of aromatase
(Ghosh et al., 2009, 2010, 2011; Jiang & Ghosh, 2012). A ribbon diagram of the overall crystal structure
of human placental aromatase is shown in Figure 4.
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Ligand- and Structure-Based Drug Design of NSAIs in Breast Cancer
Figure 4. The structure of human placental aromatase (ribbon diagram) along with the heme group
and the bound androstenedione molecule at the active site; the α-helices are labeled from A to L and
β-strands are numbered from 1 to 10.
(Ghosh et al., 2009).
The tertiary structure of human aromatase is made up of 12 major α-helices as well as 10 β-strands.
These β-strands are distributed into 1 major and 3 minor sheets. These show the characteristic cytochrome
P450 fold (Ghosh et al., 2010). The amino terminus starts at residue 45, whereas the carboxyl terminus
is ending at residue 496. Androstenedione binds at the heme distal site. This is the active site of the
enzyme. The 3-keto and 17-keto oxygens of androstenedione are responsible for two H-bond contacts
with Asp309 and Met374 respectively. Among the 12 major α-helices, helices I (amino acid residue 293-
324), F (residue 210-227), G (residue 242-267), H (residue 278-287), C (residue 138-152), D (residue
155-174), E (residue 187-205), J (residue 326-341), K (residue 354-366) and L (440-455) are similar in
most of the cytochrome P450s. Other α-helices, i.e., A′ (residue 57-68), A (residue 69–80), B (residue
100-109), B′ (residue 119-126), G′ (residue 232-236), H′ (residue 271-274), J′ (residue 346-349), K′
(residue 398-404) and K′′ (residue 414-418) are 1-4 turns long. These consist of more variability among
P450s in their locations, lengths and orientations (Ghosh et al., 2010). The major β-sheet is a 4-stranded
sheet. The first two strands begin near the amino terminus (β1:83-88 and β2:93-97). These end in two
strands from the carboxyl terminal half of the polypeptide chain (β3:373–376 and β6:393-396) (Ghosh
et al., 2010). Amino terminal residues 47-50 of the aromatase structure are having one backbone Hbond with β1. Each of the three minor sheets is made up of two anti-parallel strands. These are scattered
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Ligand- and Structure-Based Drug Design of NSAIs in Breast Cancer
over the polypeptide chain (sheet2: β4:381-383 and β5:386-388; sheet3: β8:473-475 and β9:479-481;
sheet4: β7:458-461 and β10:491-494) (Ghosh et al., 2010). Long loops interconnect with the secondary structure in tertiary aromatase structure. Long loops between helices like B′ and C, β7 and β8, and
β9 and β10 are either in active site residues or are responsible for participation in the scaffolding of
functionally important elements. These loops are having intra-loop H-bonding interaction. This is essential for their structural stabilities. All cytochrome P450s are found to bind the heme group through
its propionate moieties. These are done by arginine and tryptophan side chains via ionic and hydrogen
bonding. Residues in aromatase are Arg115, Trp141, Arg145, Arg375 and Arg435, homolog to those in
3A4, 2D6 and other proteins (Ghosh et al., 2010). The active site of aromatase is having the distal cavity
of the heme binding pocket. The heme iron is observed at the reaction center of the enzyme. The active
site is buried deep inside the roughly spherical molecule near its geometrical center. Androstenedione
binds with its β-face oriented towards the heme group. The C19 methyl group is located at a distance
4.0 Å from the Fe-atom (Figure 5).
The refined Fe-position is displaced around 0.2 Å away from the heme plane towards the amino acid
residue Cys437 of aromatase enzyme. The oxygen atom of 17-keto group of the substrate is located at
a distance 2.8 Å from the backbone amide nitrogen of Met374. This helps to accept a proton as well as
makes a hydrogen bond. The 3-keto oxygen on the other end is at 2.6 Å away from the carboxylate oxy-
Figure 5. Close up view of the human placental aromatase active site bound androstenedione molecule
within its unbiased electron density surface along with important side chains, heme and water molecules
are shown (distances in Å)
(Ghosh et al., 2010).
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Ligand- and Structure-Based Drug Design of NSAIs in Breast Cancer
Figure 6. Androstenedione-aromatase interactions (close view of Ala306CO…HOThr310 pair that play
key role in aromatization of androstenedione A ring)
(Ghosh et al., 2009).
gen of the Asp309 side chain. This suggests protonation of the carboxylate moiety and the formation of
a hydrogen bond. The geometries of these two hydrogen bonds are in a fashion such that 3-keto oxygen
and the water oxygen atom lie approximately in the carboxylate plane. Again, this water molecule is
situated at 3.6 Å from the guanidinium group of the Arg192 side chain. This is salt-bridged to Glu483
(not shown). Two water molecules are found hydrogen-bonded to each other and also to the hydroxyl
group of Ser478. This, in turn, donates its proton to His480 nitrogen atom from the active site. Again, the
Ser478 side chain is linked through these two water molecules to Arg192 by a weak hydrogen bond of
length 3.4 Å (Figure 5). The hydroxylation step involves the aromatization of the A ring via the involvement of Ala306, Thr310, Asp309 and the heme iron of the aromatase enzyme. In the first two steps, the
C-19 methyl is transformed into aldehyde derivative of androstenedione (Figure 6) (Ghosh et al., 2009).
Thereafter, nucleophilic attack on H2β by the Ala306 CO moiety as well as electrophilic attack on the
C3 keto oxygen by a protonated Asp309 side chain help the H2β abstraction by catalytic water molecule
(stabilized by hydrogen bond interactions with Thr310, heme iron and H2β of androstenedione) and
2,3-enolization by protonated Asp309 (which is again protonated by bound water molecule) (Ghosh et
al., 2009) (shown in Figure 7). The aromatization is over when the C-19 aldehyde of androstenedione
intermediate is abstracted by ironperoxy residue of heme to form formic acid and water.
Thus, the crystal structure of human aromatse reveals a finely orchestrated molecular machine that
produces estrogen from androgens. The molecular basis of enzyme-substrate and enzyme-drug interactions, more effective intervention of estrogen production may lead to the discovery of potent inhibitors.
EXPRESSION OF GENE OF AROMATASE
The aromatase gene is designated as CYP19. It encodes the cytochrome P450-aromatase. It is found at
the chromosome 15q21.1. The coding region is nearly 30 kb in size, whereas the regulatory region is
approximately 93 kb (Simpson et al., 1993; Bulun et al., 2004). Aromatase gene consists of 10 exons.
Its full length cDNA (3.4 kb) encodes the protein consists of 503 amino acids. The aromatase enzyme is
a glycosylated cytochrome P450 protein having a molecular mass of 58000 Da approximately (Gartner
et al., 2001). Regulation of aromatase enzyme is a complex mechanism in tissues and tissue-specific
promoter regions identified from the CYP19 gene (Simpson et al., 1993, 2002; Zhao et al., 1997). The
tissue specific promoters consist of PI.1, PI.3, PI.4, PI.6 and PI.7 as well as PII. The tissue specific
aromatase gene and promoter regions are shown in Figure 8.
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Ligand- and Structure-Based Drug Design of NSAIs in Breast Cancer
Figure 7. Possible mechanism for H2β abstraction and 2,3-enolization of androstenedione
(Ghosh et al., 2009).
Figure 8. Tissue specific aromatase gene and promoter regions
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Ligand- and Structure-Based Drug Design of NSAIs in Breast Cancer
Promoter I.1 (PI.1) is one of the major promoters utilized in placental tissues. The PII promoter is
utilized in the ovary and breast cancer tissues. It has a cAMP response element. Promoters PI.3, PI.4,
PI.6 and PI.7 are utilized in extraglandular sites. The PI.4 is the primary promoter and it is utilized in
normal adipose tissue. It is responsive to glucocorticoids and cytokines, e.g., IL-1, IL-6 and TNF. Promoter PI.3 is available in breast adipose tissues. It is elevated in breast cancer.
AROMATASE INHIBITORS (AIs)
Aromatase inhibitors are mainly categorized into three generations based on their chronology of
clinical trials. These are also classified as type-I and type-II inhibitors depending on their mechanisms
of action. Aminoglutethimide (AGT) (3) (Figure 1) is the first generation AI, whereas the second
generation AIs are formestane (FMS) (1) and fadrazole (FDZ) (4) (Figure 1). The second generation
AIs are more potent than the first generation AIs but these have either poor pharmacokinetic profile
or can affect other steroidogenic enzymes. The third generation AIs, such as anastrazole (ANZ) (5),
letrozole (LTZ) (6) and exemestane (EXM) (2) (Figure 1) are extremely potent. These are specific
in inactivating aromatase at nanomolar concentrations. Again, AIs are also divided chemically into
steroidal (SAIs) and non-steroidal (NSAIs) molecules. The use of first generation AIs like AGT
(3) and second generation AIs like FMS (1) and FDZ (4) are limited (Ariazi et al., 2006), whereas
third generation AIs are now used to treat breast cancer. The third generation AI candidates were
developed in early 1990s of the last century. These include the triazole derivatives [e.g., ANZ (5)
and LTZ (6)] and steroidal derivatives [e.g., EXM (2)] (Smith & Dowsett, 2003). SAIs are again
classified into two types depending on the mechanism of action. These are termed as competitive
enzyme inhibitors and mechanism-based enzyme inhibitors. Competitive inhibitors compete with
androstenedione through non-covalent binding to the aromatase active site and thereby, block its
action. Investigations of the AIs started with the synthesis and biochemical evaluation of this type
of inhibitors (Schwarzel, Kruggel, & Brodie, 1973; Sitteri & Thompson, 1975; Brueggemeier,
Floyd, & Counsell, 1978). Steroidal competitive enzyme inhibitors are developed on the basis of
the androstenedione nucleus and the substitutions were done at the varying positions on the steroid
(Figure 9).
Figure 9. Steroidal competitive enzyme inhibitors
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Ligand- and Structure-Based Drug Design of NSAIs in Breast Cancer
Figure 10. Steroidal mechanism-based aromatase inhibitors
The mechanism-based inhibitors mimic the substrate and are transformed by the enzyme to reactive
intermediates. These result in inactivation of aromatase enzyme. These inhibitors, in other terms are
called as ‘enzyme-activated irreversible inhibitors’ or ‘suicide substrates’ or ‘suicide inactivators’. This
type of AIs is also structurally similar to the natural substrate androstenedione (Figure 10).
As far as the NSAIs are concerned, these consist of a heteroatom as a common chemical feature.
These interfere with steroid hydroxylation by the binding of the heteroatom with the heme iron of aromatase enzyme. AGT (3) (Figure 1) is the prototype for NSAIs (Cocconi, 1994) since it is first studied in
patients. It should be administered with corticosteroids because of the inhibitory effects on cortisol and
aldosterone biosyntheses (Cocconi, 1994; Hoffken, 1993). FDZ (4) (Figure 1), an imidazole derivative,
acts as a potent second generation inhibitor of aromatase enzyme both in vivo and in vitro (Trunet et al.,
1997). It is more selective than AGT (3). It is 700 times more potent than AGT (3). The FDZ (4) has
some nonselective inhibitory activity in aldosterone, progesterone and corticosterone biosyntheses. The
‘R’ isomer of FDZ (4) is less potent than the ‘S’ isomer (Furet et al., 1993). The triazole analog ANZ
(5) (Figure 1) is an achiral triazole derivative having potent aromatase inhibitory activity (IC
= 15 nM)
50
(Plourde, Dyroff, & Dukes, 1994). The LTZ (6) is also a potent aromatase inhibitor in human placental
aromatase (IC
= 11.5 nM) (Bhatnagar et al., 1990). It is found to be orally active. It causes regression
50
of tumors in 7, 12-dimethylbenz [a] anthracene (DMBA) induced hormone dependent rat tumor model
(Demers, 1994). The ANZ (5) and LTZ (6) are highly specific drugs. These are 100-3000 times more
potent than AGT (3). It is a challenge to design higher degree of specificity and more selective aromatase inhibitors still now. Researchers are trying to design and synthesize more effective and selective
imidazolylmethyl, triazolylmethyl and pyridylmethyl derivatives of NSAIs (Neves et al., 2009; Jackson
et al., 2008; Furet et al., 1993; Leonetti et al., 2004; Cavalli et al., 2005; Le´ze´ et al., 2004, 2006, 2008;
Marchand et al., 2003; Le Brogne et al., 1999, 2007; Vinh et al., 1999; Saberi et al., 2006; Khodarahmi
et al., 2001; Whomsley et al., 1993; Woo et al., 2003, 2007, 2010; Bubert et al., 2008; Wood et al.,
2005, 2008, 2010; Okada et al., 1996, 1997a, 1997b, 1997c; Serpentini et al., 2004; Jones et al., 1990;
Taylor et al., 1987; Lang et al., 1993; Recanatini et al., 2001; Gobbi et al., 2006, 2007, 2010; Pouget
et al., 2002a, 2002b, 2004; Yahiaoui et al., 2004; Hackett et al., 2005; Su et al., 2005; Kim, Hackett, &
Bruggemeier, 2004; Hartmann et al., 1995, 1996; 2004; Wachter et al., 1996; Jacobs et al., 2000; Gupta
et al., 2004; Bayer et al., 1991; Hartmann, Bayer, & Grun, 1994). Some of the representative molecules
having potent aromatase inhibitory activity are shown in Figure 11.
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415