`
`Cancer Epidemiology, Biomarkers & Prevention
`
`65
`
`Review
`
`Aromatase Inhibitors as Potential Cancer Chemopreventives
`
`
`
`Gary J. Kelloff,! Ronald A. Lubet, Ronald Lieberman,
`Karen Eisenhauer, Vernon E. Steele, James A. Crowell,
`Ernest T. Hawk, Charles W. Boone, and
`Caroline C. Sigman
`Chemoprevention Branch, Division of Cancer Prevention and Control,
`National CancerInstitute, Bethesda, Maryland 20852 [G.J. K.. R. A. L., R. L..
`V.E.S., J. A.C., E. T. H., C. W. B.J; and CCS Associates, Mountain View,
`California 94043 [K. E., C. C.S.]
`
`Abstract
`
`Epidemiological and experimental evidence strongly
`supports a role for estrogens in the development and
`growth of breast tumors. A role for estrogen in prostate
`neoplasia has also been postulated. Therefore, one
`chemopreventive strategy for breast and prostate cancers
`is to decrease estrogen production. This can be
`accomplished by inhibiting aromatase, the enzyme that
`catalyzes the final, rate-limiting step in estrogen
`biosynthesis. The use of aromatase inhibitors is of clinical
`interest for cancer therapy, andselective, potent
`aromatase inhibitors have been developed. Several of
`these agents have demonstrated chemopreventive efficacy
`in animal models.
`Therationale for the use of aromatase inhibitors as
`chemopreventives and identification of inhibitors to serve
`as potential chemopreventive agents are the subjects of
`this review. After background information regarding
`aromatase is presented, the data for each inhibitor are
`summarized separately. The discussion focuses on those
`inhibitors that are clinically available or in clinicaltrials,
`including: aminoglutethimide (Cytadren), rogletimide,
`fadrozole hydrochloride, liarozole hydrochloride,
`anastrozole (Arimidex), letrozole, vorozole, formestane,
`exemestane, and atamestane. On the basis of results from
`preclinical studies, aromatase inhibitors may be
`promising agents for clinical trials in populations at high
`risk for developing estrogen-dependent cancers.
`Total suppression of aromatase may have adverse
`effects, as is evident in postmenopausal women(increased
`osteoporosis, cardiovascular disease, and urogenital
`atrophy). However, on the basis of preclinical studies of
`chemopreventive efficacy and chemotherapeutic
`applications of aromatase inhibitors showing dose-
`responseefficacy, it may be possible to obtain
`chemopreventive effects without total suppression of
`aromatase and circulating estrogen levels. Suppressing
`
`Received 6/6/97; revised 9/29/97, accepted 10/15/97.
`The costs of publication of this article were defrayed in part by the payment of
`page charges. This article must therefore be hereby marked advertisement in
`accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
`'To whom requests for reprints should be addressed, at Chemoprevention
`Branch, Division of Cancer Prevention and Control, National Cancer Institute,
`Executive Plaza North, Suite 201, 6130 Executive Boulevard, Rockville, MD
`20852.
`
`local estrogen production may be an alternative strategy,
`as suggested by the discovery of a unique transcriptional
`promoter of aromatase gene expression,I.4, in breast
`adipose tissue. The development of drugs that target this
`promoter region may bepossible.
`
`Strategies in Development of Cancer Chemopreventive
`Agents
`This paper is the third in a series on strategies used by the
`Chemoprevention Branch of the National Cancer Institute to
`develop cancer chemoprevention drugs (1-3). One chemopre-
`ventive strategy for hormone-dependentcancersis to interfere
`with the hormonesthat stimulate cellular proliferation in these
`tumors. Among the most important of these targets for inter-
`vention are estrogen-responsive tumors. Estrogen production
`can be decreased by inhibiting aromatase, the enzyme cata-
`lyzing the final, rate-limiting step in estrogen biosynthesis. The
`use of aromatase inhibitors is of clinical
`interest for cancer
`therapy, and selective, potent aromatase inhibitors have been
`developed. The rationale for use of aromatase inhibitors as
`chemopreventives and identification of inhibitors to serve as
`potential cancer chemopreventive agents are the subjects of this
`review.
`
`Association of Estrogen with Carcinogenesis
`Breast. Aromatase, the enzymethat catalyzes the rate-limiting
`step in estrogen formation (4), is expressed in severaltissues in
`women. In premenopausal women,the granulosacells of ovar-
`ian follicles produce the majority of circulating estrogen, pri-
`marily in the form of estradiol. Estrogen is also produced
`extragonadally in liver, muscle, and fat by aromatization of
`adrenal androgens. After menopause, adipose tissue is the ma-
`jor source ofcirculating estrogens (5). Extragonadal production
`of estrogen primarily involves aromatization of adrenal andro-
`stenedione, resulting in estrone, a weaker estrogen than estra-
`diol (6). Epidemiological evidence strongly supports a role for
`estrogens in the development and growth of breast neoplasia.
`The most consistently documented epidemiologicalrisk factors
`for breast cancer, early age at menarche, late age at menopause,
`late age at
`first
`full-term pregnancy, and postmenopausal
`weight gain, all increase cumulative endogenous estrogen ex-
`posure (7-9). Experimental evidence also strongly favors a role
`for estrogens in the development and growth of breast cancers
`(10, 11). Estrogens promote the development of mammary
`cancerin rodents and exert both direct andindirectproliferative
`effects on cultured breast cancer cells (11). Induction of en-
`zymes and proteins involved in nucleic acid synthesis (e.g.,
`DNA polymerase and thymidine kinase) and oncogenes may
`account for their direct mitogenic effects. Indirect effects of
`these hormones also occur via induction of pituitary prolactin
`secretion and expression of various growth factors (e.g.. trans-
`forming growth factor a and epidermal growth factor) and
`non-growth factor peptides (e.g., plasminogen activators).
`It
`has been estimated that 30% of breast cancers are dependent on
`estrogen for their proliferation (12, 13). Although the available
`
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`66
`
`Review: Aromatase Inhibitors as Cancer Chemopreventives
`
`data are not conclusive, there is also evidence that estrogens
`may be directly genotoxic, for example, by DNA alkylation or
`oxidation, leading to free radicals, which, in turn, bind to and
`damage DNA (e.g., Ref. 14).
`The estrogen that stimulates tumor growth can be derived
`from extratumoral or tumor sources, and the relative impor-
`tance of each is controversial. Within the breast, adipose tissue
`is the major extratumoral source of aromatase, although aro-
`matase was also detected immunocytochemically in normal
`breast epithelial cells in one study (15). Aromatase activity is
`higher in adipose tissue from breast cancer patients than it is
`from those with benign breast disease (16). In breasts with
`cancer, aromatase expression (17) and activity (18) are higher
`in quadrants bearing tumors compared with those without tu-
`mors. The exact cellular localization of aromatase expression in
`breast cancertissue is also somewhat controversial. Immuno-
`cytochemical studies have detected aromatase in breast carci-
`nomacells (15, 19) and in stromal spindle cells in breast tumors
`(20). At the present time, it appears that both adipose and breast
`tumorcells contribute to locally high estrogen production, and
`determination of their relative importance requires further
`study.
`The effects of estrogens are mediated by the ER.” Al-
`though only a few studies have been carried out, 100% of
`precancerous atypical hyperplastic ductal breast lesions that
`have been studied express ER (21), and ER levels are higher in
`atypical ductal hyperplasia than they are in normal breast epi-
`thelium (22). Changes in ER expression during tumor progres-
`sion are not well established, but it has been reported that
`approximately 60% of breast DCISs are ER positive (23).
`These and other epidemiological and experimental data suggest
`that all breast cancers are estrogen responsive during some
`portion of their natural history. Therefore, limiting estrogen
`exposure should be a feasible chemopreventive approach. Not-
`withstanding the observation that not all ER-positive breast
`cancers are responsive to antiestrogen therapy,
`loss of ER
`during tumor progression implies that the premalignant stages
`of the disease may be more sensitive to estrogen deprivation
`than are later stages, when some tumors have lost estrogen
`responsiveness.
`The prophylactic potential of limiting estrogen exposure
`has been demonstrated in both clinical and experimental set-
`tings. Administration of the ER blocker tamoxifen to women
`with previous breast cancer significantly decreases the risk of
`developing a new cancerin the contralateral breast (24). Nu-
`merous studies have demonstrated the chemopreventive activ-
`ity of tamoxifen (e.g., Refs. 25 and 26) and other antiestrogens
`(27) in experimental breast cancer models. A disadvantage of
`many antiestrogensis their partial estrogen agonist effects.
`A second approach to achieve estrogen deprivation in-
`volves direct inhibition of estrogen biosynthesis via aromatase
`inhibitors. As described below, aromatase inhibitors are clini-
`cally available for breast cancer therapy. Support for the fea-
`sibility of preventing breast cancer by inhibiting aromatase
`comesfrom clinical studies in which aromatase inhibitors cause
`tumorregression in postmenopausal breast cancer patients. In
`experimental mammary cancer models, aromatase inhibitors
`
`> The abbreviations used are: ER, estrogen receptor; DCIS, ductal carcinoma in
`situ, BPH, benign prostatic hyperplasia; AG, aminoglutethimide; 4-OHA,4-hy-
`droxyandrost-4-ene-3,17-dione: DMBA,7,12-dimethylbenz(a)anthracene; MNU,
`N-methyl-N-nitrosourea,; FDA, Food and Drug Administration; CNS, central
`hervous system; PMSG, pregnant mare’s serum gonadotropin; ACTH, adreno-
`corticotropin; FSH,follicle-stimulating hormone; LH,luteinizing hormone.
`
`induce regression of established tumors(e.g., 28, 29) and, more
`importantly for the purposes of this discussion, prevent cancer
`development(27, 30, 31).
`Prostate. Etiological and risk factors for prostate cancer in-
`clude age of >50 years, family history, high serum testoster-
`one, high-fat diet, prostatitis, and geographical background
`(prevalence being highest in the United States, Canada, and
`northwest Europe; Ref. 32). As for breast cancer, significant
`risk for prostate cancer appears to be associated with exposure
`to steroid hormones(i.e., the high serum testosterone and high
`fat consumption). Besides testosterone, estrogens have also
`been postulated to play a role in prostate cell proliferation and
`have been implicated in BPH and prostate cancer. BPH is a
`nonmalignant enlargement of the prostate due to cellular hy-
`perplasia and hypertrophy of both the epithelial and stromal
`elements of the gland, whereas prostate cancer involves the
`epithelial tissue. Studies on the role of estrogen in BPH may be
`informative about the regulation of prostate cell proliferation,
`but it should be emphasized that prostatic intraepithelial neo-
`plasia, not BPH,is the most likely precursor of prostate carci-
`noma (33, 34).
`In men, 10-25% of estrogen is synthesized locally in the
`testes, and 75-90% arises from extraglandular aromatization of
`testosterone and androstenedione (5, 35). The estrogen:andro-
`gen ratio increases with age, presumably dueto greater estrogen
`synthesis, accompanied by unchanged or decreased androgen
`production. Whether the prostate is a source of estrogen is
`controversial. Some studies have reported an absence of aro-
`matase in normal prostates (36) and BPH (37). Others have
`reported aromatase activity in normal prostate and BPH (38)
`and in both BPH andprostate cancercells (39).
`ERs have been found in normal, BPH, and cancerous
`human prostate tissue (40). Estrogens target the normal prostate
`stroma, and the involvementof estrogen and stroma-epithelial
`interaction in BPH hasrecently been studied (41, 42). In dogs,
`estrogens synergize with androgensandresult in complex stro-
`mal and glandular hyperplasia. This effect appears to be due to
`an estrogen-mediated increase in stromal and epithelial andro-
`gen receptor levels (43), although the induction of BPH as a
`result of injury by estrogen metabolites, followed by 5a-dihy-
`drotestosterone-stimulated growth of altered prostatic cells, has
`also been postulated (44). Increased levels of estrogen have
`been found in BPH stroma compared with BPH epithelium and
`normal prostate epithelium and stroma (45). In both normal
`prostate and BPH,stromal estrogen levels increase with age,
`resulting in an increased estrogen:androgenratio. These results
`suggest that estrogen is involved in the development of BPH,
`but the clinical trials of aromatase inhibitors in BPH patients
`have not been encouraging (41, 46).
`Estrogen mayalso play a role in prostate cancer, as sug-
`gested by studies involving rat models. In the Noble rat, pros-
`tate dysplasia can be induced by simultaneous treatment with
`testosterone and estradiol for 16 weeks but not by treatment
`with testosterone or Sa-dihydrotestosterone alone (47). Long-
`term treatment of Noble rats with testosterone induces a low
`incidence of adenocarcinomasin the dorsolateral prostate (48),
`whereas treatment with testosterone and estrogen significantly
`increases carcinomaincidence and decreases tumorlatency (49,
`50). The mechanism of estrogen action on the rat prostate is
`unknown. No effect of estrogen on prostate androgen receptor
`levels was observed in estradiol- and testosterone-treated
`Wistar rats (51). In Wistar rats, estrogen treatment increased
`nuclear 5a-reductase activity, although microsomal reductase
`activity was decreased (52). Treatment of Noble rats with
`
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`Cancer Epidemiology, Biomarkers & Prevention
`
`67
`
`CtHLOH
`Chs
`c=o1,o=0
`OO
`ee
`sce
`3BHSD _
`
`HO
`
`HO
`
`oO
`
`Cholesterol
`
`Pregnenolone
`
`Progesterone
`
`Aldosterone
`
`| Cco
`
`3BHSD v
`Cortisol—
`re
`
`Androstt-ene-3,17-dione
`
`Estrone
`
`
`
`EnzymeAbbreviations
`
`17-Ketosteroid Reductase
`
`17HSD l|| 17sR
`
`1BH80| | 17sR
`
`Side-Chain Cleavage Enzyme
`scc:
`36-Hydroxysteroid Dehydrogenase
`SBHSD:
`TIBHSD: 17fB-Hydroxysteroid Dehydrogenase
`17KSR:
`
`Testosterone
`
`Estradiol
`
`Fig. ]. Steroidogenic pathway.
`
`estradiol and testosterone was shownto result in a unique DNA
`adductin the dorsolateral prostate (the tissue where carcinomas
`originate in this model), coincident with the appearance of
`putative preneoplastic lesions, but the structure and mechanism
`of its formation are unknown(53). This adduct mayplay a role
`in carcinogenesis in this model. Interestingly, Spencer et al.
`(54, 55) also identified DNA adducts in normal human prostate
`and prostate tumor biopsies. The role of estrogen in human
`prostate cancer has not yet been fully elucidated. Estradiol has
`been shown to stimulate the growth of an androgen-responsive
`human prostate cancer cell line (LNCaP) and to inhibit the
`growth of an androgen nonresponsive line (PC3) in vitro (56).
`Treatment with an aromatase inhibitor resulted in pain relief in
`some patients with prostate cancer; however,
`there was no
`correlation between clinical response andestradiol levels (57).
`
`Aromatase Activity and Regulation
`Aromatase is an enzyme complex that is localized in the en-
`doplasmic reticulum and consists of a specific cytochrome
`P450 heme protein and a flavoprotein NADPH cytochrome
`P450 reductase. It catalyzes the synthesis of estradiol from
`testosterone and estrone from androstenedione. Three separate
`hydroxylation steps are catalyzed by the enzyme, which re-
`quires 3 mol of molecular oxygen for the conversion of 1 mol
`of C,, androgen to C,, estrogen (6). Importantly, because
`aromatization is the last step in steroid biosynthesis, selective
`inhibition of the enzyme should not disrupt production of other
`steroids such as adrenal corticoids (Fig. 1).
`
`Although the translated region of the aromatase gene is
`identical among different tissues, the untranslated regions and
`regulatory control appear to be tissue specific. At least four
`major promoter sites have been identified that respond to go-
`nadotropins, glucocorticoids, growth factors, and cytokines. A
`unique transcriptional promoter of aromatase gene expression,
`1.4, has been identified in breast adipose tissue (6, 17).
`
`Aromatase Inhibitors
`Both nonsteroidal and steroidal aromatase inhibitors have been
`developed, and the characteristics of each class have been
`reviewed (58-68). Nonsteroidal inhibitors act by binding to a
`prosthetic heme group on the enzyme. However, because this
`hemegroupis present on all members of the cytochrome P450
`superfamily, these inhibitors may lack specificity and, thus,
`inactivate other steroidogenic enzymes. Unlike steroidal inhib-
`itors, nonsteroidal inhibitors lack hormonal agonist or antago-
`nist activity and are more likely to be p.o. absorbed. Structures
`of the nonsteroidal inhibitors discussed below appear in Figs. 2
`(AG,rogletimide, fadrozole, and liarozole) and 3 (anastrozole,
`letrozole, and vorozole).
`Steroidal inhibitors bind either very tightly orirreversibly
`to the active site of the enzyme and are so-called “mechanism-
`based”or “suicide” inhibitors; that is, these inhibitors compete
`with androstenedione and testosterone for the active site of
`aromatase and are then converted to reactive alkylating species
`by the enzyme, which form covalent bondsator near the active
`site, thereby irreversibly inactivating aromatase. Recovery of
`
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`68
`
`Review: Aromatase Inhibitors as Cancer Chemopreventives
`
`H.C
`
`NH,
`
`N Aminoglutethimide
`
`/ \
`"
`cH
`
`cl
`
`‘
`Liarozole
`
`NH
`N =/
`
`
`
`Z N
`N~Y
`
`c
`tl
`N
`Fadrozole
`
`Fig. 2.
`
`Piperidinedione and imidazole nonsteroidal inhibitors.
`
`noN
`N
`|
`n~/
`
`MS
`
`/
`n=\
`Noy”
`
`H,¢
`
`c
`yw
`N
`
`CH, H3C
`
`Arimidex
`
`CH;
`
`c
`“s—N
`NN
`
`Vorozole
`
`¢ti
`"
`Letrozole
`
`CH,
`Ny
`N
`
`Fig. 3. Traizole nonsteroidal inhibitors.
`
`enzymatic activity depends on the rate of de novo enzyme
`synthesis. The potential advantages of using suicide inhibitors
`include potent and sustained inhibitory activity, with the pos-
`sibility of intermittent dosing schedules. Disadvantages include
`unwanted hormonal agonist (particularly androgen) or antago-
`nist effects. Structures of some of the steroidal inhibitors dis-
`cussed below appear in Fig. 4 (4-OHA, exemestane, atames-
`tane, and plomestane).
`
`Representative Agents
`Data pertinent to the potential development of these aro-
`matase inhibitors as chemopreventive drugs are summarized
`in Table 1.
`
`Nonsteroidal Inhibitors
`
`3-(4-aminophenyl)-3-ethy1l-2,6-piper-
`AG. AG [Cytadren,
`idinedione] is structurally related to phenobarbital and was
`originally developed as an anticonvulsive. AG was the first
`aromatase inhibitor used clinically. It is available from Ciba-
`Geigy and is approved for use in breast cancer therapy. AG has
`demonstrated chemopreventive efficacy in rat models for breast
`cancer. Administration of diets containing AG decreased
`
`cu, °
`
`cH,
`
`é
`
`OH
`
`4-OHA
`
`o
`
`CH;
`
`cH,
`
`cH;
`
`o
`
`Atamestane
`
`o
`
`cH,
`
`cH,
`
`o
`
`CH
`
`a
`
`HC &
`
`o
`
`cH,
`Exemestane
`
`0
`
`Plomestane
`
`Fig. 4.
`
`Steroidal inhibitors.
`
`DMBA-induced mammary tumorincidence and multiplicity in
`Holtzman rats (30). Here, tumor incidence was decreased from
`70% to 28% and 16% with 0.05% and 0.1% AGin the diet,
`respectively. Multiplicity was decreased from 3.4 tumors/rat to
`2.3 tumors/rat and 2.0 tumors/rat, respectively. In a study
`sponsored by the Chemoprevention Branch of the National
`Cancer Institute, a diet containing 400 mg/kg AG also de-
`creased mammary tumor multiplicity from 10.6 tumors/rat to
`5.5 tumors/rat and increased latency 30 days in MNU-treated
`Sprague-Dawleyrats (27). The efficacy in the latter study was
`accomplished in the presence of increased androgenactivity, as
`reflected in a significantly increased body weightin rats treated
`with AG.
`AGis currently approved for the treatment of postmeno-
`pausal breast cancer, and objective responses in approximately
`33% of patients have been reported (58). However,
`it also
`inhibits a number of other steroidogenic cytochrome P450-
`dependent enzymes, including cholesterol side-chain cleavage
`enzymes, 118-hydroxylase, 18-hydroxylase, and 21-hydroxy-
`lase, necessitating glucocorticoid replacement therapy. Further-
`more, it has significant toxicities, especially CNS effects. Nu-
`merousefforts to optimize AG activity toward aromatase while
`reducing inhibition of other enzymes and diminishing toxic
`effects have been attempted, including elongation of the ethyl
`substituent and replacementofthe ethyl group by a cycloalkalyl
`moiety (58, 65). Although these modifications improved aro-
`matase inhibitory activity, the AG derivatives are still not very
`potent compared to other compounds discussed below (58).
`Rogletimide. Rogletimide [pyridoglutethimide; 3-ethyl-3-(4-
`pyridyl)piperidine-2,6-dione] is an AG analogue that strongly
`inhibits aromatase (K,; = 1.1 4m)butis less potent than AG (K;
`= 0.6 pm;Ref. 66). However, rogletimide is more specific and
`does not block cholesterol side-chain cleavage.It also has fewer
`adverse CNSeffects. The administration of 50 mg/kg/day ro-
`gletimide to rats previously treated with DMBA prevented
`testosterone-induced increases in mammary tumorsize (67).
`Several clinical studies in postmenopausal breast cancer
`patients have been reported. Doses of 200-1200 mg b.i.d. sig-
`nificantly suppressed estradiol levels without having effects on
`
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`Cancer Epidemiology, Biomarkers & Prevention
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`69
`
`Table ]
`Aromatase inhibitor
`
`Preclinical and clinical of aromatase inhibitors studies that are relevant to cancer chemoprevention
`Preclinical studies
`Clinical studies
`
`Potential as chemopreventive drug
`
`Nonsteroidal inhibitors
`AG [Cytadren, 3-(4-aminophenyl)-3-
`ethyl-2,6-piperidinedione]
`
`Rogletimide [Pyridoglutethimide;
`3-ethyl-3-(4-pyridy])piperidine-2,6-
`dione]
`
`Fadrozole [CGS 16949A; 4-(5,6,7,8-
`tetrahydromidazo(1,5-a)pyridin-5-
`yl)benzonitrile]
`
`Liarozole hydrochloride [R 75 521,
`(+/—)-5-(m-chloro-a-imidazole-]-
`ylbenzyl)benzimidazole
`monohydrochloride]
`
`Inhibition of DMBA-induced (30) and
`MNU-induced (27) mammary gland
`tumors inrats
`
`FDAapproved for treatment of
`postmenopausal breast cancer
`
`Prevented testosterone-induced
`Mammary tumor growth in rats (67)
`
`In rats bearing DMBA-induced
`mammary tumors, suppressed the
`appearance of new tumors (28) and
`prevented growth of established
`tumors (28, 74, 75); also prevented
`benign and malignant spontaneous
`mammary neoplasmsin rats (73)
`Inhibited growth of slow growing,
`well-differentiated androgen
`dependent, as well as androgen-
`independent prostate cancers in rats
`(84, 85)
`
`Early clinical studies show suppressed
`serum estradiol levels in
`postmenopausal breast cancer
`patients (68)
`Early clinical studies show suppressed
`serum estradiollevels in
`postmenopausal breast cancer
`patients (77, 78)
`
`Clinical trials for treatment of
`advanced prostate cancer ongoing
`(1, 52, 83, 86)
`
`Anastrozole [ICI D1033; ZD1033;
`a,or,cc',r’-tetramethy]-5-(1H-1,2,4-
`triazole-1-ylmethy])-1,3-
`benzenediacetonitrile, Arimidex]
`
`Letrozole [CGS 20267, 4,4’-(1H-
`1,2,4-triazol-1-ylmethylene)bis-
`benzonitrile]
`
`Suppressed prowth and inhibited new
`mammary tumor developmentin
`DMBA-induced rats (93, 94).
`
`(+)-Vorozole [R83842; (+)-6-((4-
`chlorophenyl)- 1 H-1,2,4-triazole-1-
`ylmethy])-1-methyl-1H-
`benzotriazole]
`
`Potent inhibition of MNU-induced (31)
`and DMBA-induced (101)
`mammary gland tumors in rats;
`suppressed growth and inhibited
`new mammary tumor development
`in DMBA-induced rats (101)
`
`ORG 33201 {((3aR)-trans-1-[3a-
`ethyl-9-(ethylthio)-2,3,3a,4,5,6-
`hexahydro- 1 H-phenalen-2-
`yl)methyl)-1H-imidazole HCI}
`Steroidal inhibitors
`Minamestane (FCE24928; 4-
`aminoandrosta-| ,4,6-triene-3,17-
`dione)
`
`FDAapproved for treatment of
`recurrent postmenopausal breast
`cancer in patients who have failed
`tamoxifen therapy (89, 90); highly
`potent suppressor of serum estradiol
`without affecting other steroids (89)
`Clinical trials for treatment of
`postmenopausal breast cancer
`ongoing (70); early clinical studies
`showed letrozole to be a potent and
`selective suppressor of serum
`estradiol levels in postmenopausal
`breast cancer patients (95-98)
`Clinical trials for treatment of
`postmenopausal breast cancer
`ongoing (29, 70, 102, 103); early
`clinical studies showed letrozole to
`be a potent and selective suppressor
`of serum estradiol levels in
`postmenopausal breast cancer
`patients
`
`Plomestane (MDL 18962; PED; 10-
`propargylestr-4-ene-3,17-dione)
`
`Prevented new mammary tumors (131)
`and caused regression of established
`mammary tumors (128, 131, 132) in
`DMBA-treated rats
`
`Limited clinical data shows that
`plomestane decreases serum
`estrogen in normal males (133); no
`clinical data
`
`Nonspecific cytochrome P450
`inhibitor; so significant toxicities,
`especially CNS, may precludeits
`use in a prevention setting
`Less potent inhibitor than AG, butit is
`more specific for aromatase and less
`toxic; no chemoprevention studies
`currently planned
`More potent inhibitor than AG or
`rogletimide; no chemoprevention
`studies currently planned
`
`Besides aromatase, liarozole inhibits
`the cytochrome P450 involved in
`retinoic acid metabolism (87); this
`suggests that liarozole may be
`synergistic with retinoids; no
`chemoprevention studies are
`currently planned, but there is
`interest in exploring the drug’s
`chemopreventive activity, alone and
`in combination with retinoids
`Very potent inhibitor with long half-
`life (once daily dosing possible); no
`chemoprevention studies are
`currently planned
`
`More potent than its analogue
`fadrazole (10 in animals, 100X in
`humans) (93); no chemoprevention
`studies are currently planned
`
`Potent aromatase inhibition, few side
`effects, and possibility of
`influencing estradiol levels in
`premenopausal women (29) are of
`interest for chemoprevention: Phase
`Il chemoprevention studies in breast
`and prostate are in early planning
`stage
`Less potent inhibitor than fadrazole
`(123) but more specific; data are
`insufficient for evaluating
`chemopreventive potential
`
`Analogue of exemestane with less
`potency, but also it is devoid of
`androgenic activity (125, 126);
`available data are insufficient for
`evaluating its chemopreventive
`potential; further consideration will
`depend on results of studies with
`exemestane
`Plomestane no longer being developed
`by manufacturer; no
`chemoprevention studies currently
`planned
`
`serum levels of cortisol (68). Rogletimide produced dose-de-
`pendent aromatase inhibition, but even at the maximum toler-
`ated dose of 800 mg b.i.d., it was not as effective as AG (69).
`Side effects of rogletimide include those normally associated
`with estrogen deprivation: gastrointestinal
`symptoms, hot
`flushes, dizziness, and lethargy (70).
`
`Fadrozole. Fadrozole [CGS 16949A; 4-(5,6,7,8-tetrahydromi-
`dazo(1,5-a]pyridin-5-yl)benzonitrile], an imidazole, is a potent,
`competitive aromatase inhibitor being developed by Novartis
`(Basel, Switzerland). Jn vitro, fadrozole demonstrated an IC.,
`of 4.5 nm toward human placental aromatase (71). In MCF-7
`cells, fadrozole inhibited aromatase activity and prevented tes-
`
`Downloaded from cebp.aacrjournals.org on October 9, 2015. © 1998 American Association for Cancer Research.
`
`AstraZeneca Exhibit 2026 p. 5
`
`
`
`70
`
`Review: Aromatase Inhibitors as Cancer Chemopreventives
`
`tosterone-induced MCF-7 growth (72). p.o. administration of
`0.260 mg/kg inhibited estrogen synthesis by >90% in imma-
`ture rats stimulated with PMSG (71).
`Fadrozole has also demonstrated chemopreventive activ-
`In intact Sprague-Dawley rats bearing DMBA-induced
`ity.
`mammary tumors, p.o. administration of 2.0 mg/kg/day fadro-
`zole completely suppressed the appearance of new tumors (28),
`In a 2-year study, treatment of Sprague-Dawley rats with 1.25
`mg/kg/day fadrozole completely prevented the appearance of
`benign and malignant spontaneous mammary neoplasms (73).
`At 0.25 mg/kg/day, no malignant mammary tumors and a
`decreased numberof benign tumors were seen, whereasat 0.05
`mg/kg/day, the incidences of malignant and benign mammary
`tumors were decreased.
`
`Therapeutic effects of fadrozole have been demonstrated
`in preclinical models andin clinical trials. p.o. administration of
`fadrozole inhibited the growth of established DMBA-induced
`mammary tumors (28, 74, 75) in Sprague-Dawley rats. The
`combination of fadrozole and tamoxifen resulted in signifi-
`cantly greater tumor regression than did either treatment alone
`in intact Sprague-Dawleyrats (76). Several Phase I trials have
`been conducted in postmenopausal breast cancer patients, and
`a dose of 2 mg b.i.d. resulted in >90% aromatase inhibition
`(77, 78). In a multiple-dose study, p.o. administration of 0.6—
`8.0 mg b.i.d. rapidly suppressed blood and urine estrogen
`levels, but adrenal and cortical responses were also diminished
`at the highest dose levels (79). Two studies have suggested |!
`mg b.i.d. as an optimal dose (80, 81). The main side effects of
`fadrozole are nausea, hot flushes, and somnolence (82). Fadro-
`zole is currently in Phase III clinical
`trials as second-line
`endocrine therapy in postmenopausal breast cancer patients
`(82).
`Liarozole Hydrochloride. Liarozole hydrochloride [R75521,
`(+/—)-5-(m-chloro-a-imidazole-1-ylbenzyl)benzimidazole
`monohydrochloride], an imidazole derivative developed by
`the Janssen Research Foundation (Beerse, Belgium),
`is a
`potent
`inhibitor of aromatase activity in human placental
`microsomes and rat granulosa cells in vitro (83). However,
`it also inhibits other cytochrome P450 enzymes and de-
`creases androgen, progesterone, and cortisol synthesis in
`vitro. Several studies have investigated the effects of liaro-
`zole on the growth of prostate carcinomasin rats. Dietary
`administration of 80-160 mg/kg liarozole inhibited growth
`of slow-growing, well-differentiated, androgen-dependent
`Dunning-H tumors (84, 85), as well as androgen-indepen-
`dent prostate cancers (84, 85).
`Liarozole is currently in clinical trials for the treatment of
`advancedprostate cancer. In male volunteers, administration of
`a single p.o. dose of 300 mg significantly lowered plasma
`testosterone and estradiol concentrations for 24 h and blunted
`the normal cortisol response to ACTH (83). A Phase I dose-
`escalation trial involving 38 hormone-refractory prostate cancer
`patients treated with 37.5—300 mg b.i.d. has been conducted.
`Fourpatients had a >50% decrease in prostate-specific antigen
`levels. In patients with measurable soft-tissue disease, two had
`partial responses, as judged by a >50% decrease inat least one
`measurable lesion (86). There was no evidence of adrenal
`insufficiency. Side effects included lethargy, somnolence, and
`body rash. All patients enrolled in this study were under max-
`imum androgen suppression (orchiectomy or gondadotropin-
`releasing hormone therapy).
`in which liarozole was
`Results from rat model studies,
`effective in androgen-independent tumor models, and from the
`Phase I trial, in which responses were seen in androgen-sup-
`
`pressed patients, have led to the suggestion that someof liaro-
`zole’s effects are due to modulation of retinoic acid metabo-
`lism. Liarozole has been shown to inhibit cytochrome P450
`isozymesthat are responsible for retinoic acid catabolism (87),
`and,in fact, liarozole has been shownto increase endogenous
`levels of retinoic acid in a numberoftargettissues. In view of
`this, it is not surprising that liarozole exerts retinoid-mimetic
`effects in vivo (88). At the present time, the specific mecha-
`nisms responsible for liarozole’s inhibition of prostate tumor
`growth are unknown and mayinvolve inhibition of aromatase,
`retinoic acid catabolism, and, in some cases, androgen biosyn-
`thesis.
`
`Anastrozole. Anastrozole [ICI D1033; ZD1033; a,a,a’',a’-
`tetramethyl-5-(1H-1,2,4-triazole- 1-ylmethyl)-1,3-benzenedi-
`acetonitrile, Arimidex]
`is a triazole developed by Zeneca
`(Wilmington, DE), which has been approved by the FDA for
`treatment of recurrent postmenopausal breast cancerin patients
`whohave failed tamoxifen therapy. In vitro, anastrozole dem-
`onstrated an IC. of 15 nm toward human placental aromatase.
`In mature rats, the drug is maximally active at p.o. doses of
`about 0.1 mg/kg and is selective for aromatase.
`Several Phase I clinical
`trials have been conducted in
`postmenopausal