Vol. 7. 65—78. January I998
`
`Cancer Epidemiology. Biomarkers 8: Prevention
`
`65
`
`Review
`
`Aromatase Inhibitors as Potential Cancer Chemopreventives
`
`
`
`Gary J. Kelloff,l 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 Cancer Institute, Bethesda. Maryland 20852 [G. .I. K.. R. A. L.. R. L,.
`V. E. S.. J. A. C” E. T. H.. C. W. 8.]: 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, and selective, potent
`aromatase inhibitors have been developed. Several of
`these agents have demonstrated chemopreventive efficacy
`in animal models.
`The rationale 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 clinical trials,
`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-
`response efficacy, 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 pan by the payment of
`page charges. This article must therefore be hereby marked udverlisemen! in
`accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
`lTo 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, 1.4, in breast
`adipose tissue. The development of drugs that target this
`promoter region may be possible.
`
`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—dependent cancers is to interfere
`with the hormones that 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
`revrew.
`
`Association of Estrogen with Carcinogenesis
`Breast. Aromatase, the enzyme that catalyzes the rate—limiting
`step in estrogen formation (4), is expressed in several tissues in
`women. In premenopausal women, the granulosa cells of ovar-
`ian follicles produce the majority of circulating estrogen. pri~
`marin in the forrn 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 of circulating 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 epidemiological risk 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.
`ll). Estrogens promote the development of mammary
`cancer in rodents and exert both direct and indirect proliferative
`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: Aromntase Inhibitors as Cancer Chernopreventives
`
`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 cancer tissue is also somewhat controversial. Immuno-
`cytochemical studies have detected aromatase in breast carci-
`noma cells (15, 19) and in stromal spindle cells in breast tumors
`(20). At the present time, it appears that both adipose and breast
`tumor cells contribute to locally high estrogen production, and
`determination of their relative importance requires further
`study.
`The effects of estrogens are mediated by the ER.2 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 cancer in 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 antiestrogens is 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
`comes from clinical studies in which aromatase inhibitors cause
`tumor regression in postmenopausal breast cancer patients. In
`experimental mammary cancer models, aromatase inhibitors
`
`2 The abbreviations used are: ER, estrogen receptor; DCIS. ductal carcinoma in
`situ; BPH, benign prostatic hyperplasia; AG, aminoglutethimide; 4-0HA. 4—hy—
`droxyandrost—4—ene—3.17-dione: DMBA. 7,lZ—dimemylbenz(a)anthracene: MNU.
`N—methyl-N—nitrosourea; FDA, Food and Drug Administration: CNS, central
`nervous 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 estrogenzandro-
`gen ratio increases with age, presumably due to 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 and prostate cancer cells (39).
`HRS have been found in normal, BPH, and cancerous
`human prostate tissue (40). Estrogens target the normal prostate
`stroma, and the involvement of estrogen and stroma-epithelial
`interaction in BPH has recently been studied (41, 42). In dogs,
`estrogens synergize with androgens and result 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 estrogenzandrogen ratio. 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 may also 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 5a-dihydrotestosterone alone (47). Long-
`term treatment of Noble rats with testosterone induces a low
`incidence of adenocarcinomas in the dorsolateral prostate (48),
`whereas treatment with testosterone and estrogen significantly
`increases carcinoma incidence and decreases tumor latency (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 Sat—reductase activity, although microsomal reductase
`activity was decreased (52). Treatment of Noble rats with
`
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`Cancer Epidemiology, Biomarkers & Prevention
`
`67
`
`9H;(:0
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`Fig. l. Steroidogenic pathway.
`
`estradiol and testosterone was shown to result in a unique DNA
`adduct in 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 may play 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 and estradiol 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 C1.9 androgen to C18 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, l7).
`
`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
`heme group is 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 or irreversibly
`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 bonds at or near the active
`site, thereby irreversibly inactivating aromatase. Recovery of
`
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`

`68
`
`Review: Aromatase Inhibitors as Cancer Chemopreventives
`
` III 3 C
`
`N H 2
`
`Rogletimide
`
`N Aminoglutethirnide
`/ 3
`T
`c H
`
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`
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`
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`
`Liamzolc
`
`/ N
`N \//
`
`c
`"IN
`Fadrozole
`
`Fig. 2.
`
`Piper-idinedione and imidazole nonsteroidal inhibitors.
`
`c“: 0
`
`en,
`
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`
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`
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`
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`"\//u
`
`Fig. 4.
`
`Steroidal inhibitors.
`
`H36
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`//,
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`
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`
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`
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`
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`
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`[mule
`
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`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—
`tame, and plomestane).
`
`Representative Agents
`Data pertinent to the potential development of these aro-
`matase inhibitors as chemopreventive drugs are summarized
`in Table l.
`
`Nonsteroidal Inhibitors
`
`3—(4~aminophenyl)—3-ethyl-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
`
`DMBA-induced mammary tumor incidence and multiplicity in
`Holtzman rats (30). Here, tumor incidence was decreased from
`70% to 28% and 16% with 0.05% and 0.1% AG in 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-Dawley rats (27). The efficacy in the latter study was
`accomplished in the presence of increased androgen activity, as
`reflected in a significantly increased body weight in rats treated
`with AG.
`
`AG is 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, IIB-hydroxylase,
`l8-hydroxylase, and 21-hydr0xy-
`lase, necessitating glucocorticoid replacement therapy. Further-
`more, it has significant toxicities. especially CNS effects. Nu-
`merous efforts 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 replacement of the 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 [pyridoglutethimideg 3-ethyl-3-(4—
`pyridyl)piperidine-2,6-dione] is an AG analogue that strongly
`inhibits aromatase (Ki = 1.1 MM) but is less potent than AG (Ki
`= 0.6 [.LM; Ref. 66). However, rogletimide is more specific and
`does not block cholesterol side-chain cleavage. It also has fewer
`adverse CNS effects. The administration of 50 mg/kglday ro-
`gletirnide to rats previously treated with DMBA prevented
`testosterone-induced increases in mammary tumor size (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
`
`69
`
`Table 1
`Aromatasc 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 [Cytadrem 3—(4—aminophenyl)-3—
`ethyl-2,6—piperidinedione]
`
`Rogletimide [Pyridoglutethimide;
`3—ethyl-3-(4—pyridyl)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 in rats
`
`FDA approved 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 neoplasms in 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 estradiol levels in
`postmenopausal breast cancer
`patients (77, 78)
`
`Clinical trials for treatment of
`advanced prostate canCer ongoing
`(l. 52. 83, 86)
`
`Anastrozole [lCI D1033; ZD1033;
`u,a,a',a'-tetramethyl-5—(lH—l,2,4—
`triazole-l-ylmethyD-lj-
`benzenediacetonitrile, Arimidex]
`
`Letrozole [CGS 20267; 4,4’-(1H—
`1.2.4-tn'azol- l .ylmethylene)bis-
`benzonitrile]
`
`Suppressed growth and inhibited new
`mammary tumor development in
`DMBA-induced rats (93. 94).
`
`(+)-Vorozole [R83842; (+)-6—((4—
`chlorophenyl)— 1H- 1 .2,4—tn'azole- l-
`ylmethyl)-1»methy1-1H-
`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 l(((3aR)-trans-l-[3a-
`ethyl-9—(etliylthio)-2,3.3a,4.5.6.
`hexahydro— lH—phenalen-Z-
`yl)methyl)-1H-imidazole HCl)
`Steroidal inhibitors
`Minamestane (FCE24928; 4—
`aminoandrosta- 1 ,4,6—niene-3, 1 7-
`dione)
`
`FDA approved 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 (13])
`and caused regression of established
`mammary tumors (128. 131. 132) in
`DMBA-treated rats
`
`Limited clinical data shows that
`plomestane decreases scrum
`estrogen in normal males (133); no
`clinical data
`
`Nonspecific cytochrome P450
`inhibitor: so significant toxicities.
`especially CNS. may preclude its
`use in a prevention setting
`Less potent inhibitor than AG. but it is
`more specific for aromatase and less
`toxic; no chemoprevention studies
`currently planned
`More potent inhibitor than AC- 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
`fadrazolc (10X 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
`II 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(l,5—a]pyridin-5-yl)benzonitrile], an imidazole, is a potent,
`competitive aromatase inhibitor being developed by Novartis
`(Basel, Switzerland). In vitro, fadrozole demonstrated an 1C5,0
`of 4.5 nM toward human placental aromatase (71). In MCF—7
`cells, fadrozole inhibited aromatase activity and prevented tes-
`
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`

`

`70
`
`Review: Aromatase Inhibitors as Cancer Cbemopreventives
`
`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 number of benign tumors were seen, whereas at 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 and in 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-Dawley rats (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 1
`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- l -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 carcinomas in 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
`advanced prostate 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 honnone-refractory prostate cancer
`patients treated with 37.5—300 mg b.i.d. has been conducted.
`Four patients 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 in at 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 some of liaro—
`zole’s effects are due to modulation of retinoic acid metabo-
`lism. Liarozole has been shown to inhibit cytochrome P450
`isozymes that are responsible for retinoic acid catabolism (87),
`and, in fact, liarozole has been shown to increase endogenous
`levels of retinoic acid in a number of target tissues. 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 may involve inhibition of aromatase,
`retinoic acid catabolism, and, in some cases, androgen biosyn—
`thesis.
`
`Anastrozole. Anastrozole [ICI D1033; ZD1033; a,a,a’,a'-
`tetramethyl-S-(lH-1,2,4-triazole-l-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 pos