Comparison of the Effects of a
`Pure Steroidal Antiestrogen
`With Those of Tamoxifen in a
`
`Model of Human Breast Cancer
`
`C. Kent Osborne, Ester B.
`
`Coronado-Heinsohn, Susan G.
`
`Hr’lsenbeck, Bryant L. McCue,
`Alan E. Wakeling. Richard A.
`McClelland, David L. Manning,
`Robert I. Nicholson*
`
`non-
`a
`Tamoxifen,
`Background:
`is the
`steroidal estrogen antagonist,
`most prescribed drug for the treatment
`of breast cancer. The use of tamoxifen
`
`is limited, however, by the development
`of resistance to this compound in most
`patients. Although tamoxifen behaves
`primarily as an estrogen antagonist, it
`has agonist (or growth-stimulatory) ac-
`tivity as well. 1C] 182,780 is a 7a-alkyl-
`sulfinyl analogue of estradiol lacking
`agonist activity. The absence of agonist
`activity may make this steroidal an-
`tiestrogen superior to tamoxifen in sup-
`pressing tumor cell growth. Purpose:
`We compared the inhibitory effects of
`[Cl 182,780,
`tamoxifen, and estrogen
`withdrawal on the growth of estab-
`lished tumors and on tumorigenesis in
`a model system that uses estrogen-dc.
`pendent, human MCFJ breast tumor
`cells growing in athymic nude mice.
`We also studied the hormonal respon-
`siveness of tumors that became resis—
`
`tant to the two estrogen antagonists
`and the effects of
`these drugs on
`estrogen-regulated gene
`expression.
`Methods: MCF-7 cells were injected
`subcutaneously into the
`flanks of
`castrated,‘female nude mice. The ef-
`. fects of repeated doses of tamoxifen
`and [CI 182,780 (500 pg and 5 mg,
`respectively) on the growth of estab-
`lished tumors (8-10 mm in size) were
`determined after supplemental estro-
`gen was removed. The effects of anti-
`estrogen treatments on the process of
`tumorigenesis, in the absence of estro-
`gen supplementation, were determined
`by initiating drug administration on
`
`the same day as tumor cell inoculation.
`To evaluate the hormonal responsive-
`ness of tumors resistant to tamoxifen
`and [CI 182,780, l-mmJ segments of
`the tumors were transplanted onto the
`flanks of new recipient mice, which
`were then treated with estrogen or the
`antiestrogens—alone or in combina—
`tion. Tumor growth was monitored by
`measuring tumor volumes
`twice a
`week. Expression of
`the estrogen-
`responsive genes, pLI‘Vl and p82,
`in
`the tumors of treated animals was
`
`analyzed using blots of total cellular
`RNA and complementary DNA probes.
`Results: Treatment with ICI 182,780
`suppressed the growth of established
`tumors twice as long as treatment with
`tamoxifen or
`estrogen withdrawal.
`Tumorigenesis, in the absence of sup-
`plemental estrogen, was delayed to a
`greater extent in ICI 182,780-treated
`mice than in tamoxifen-treated mice.
`lCl 182,780 was found to be more ef.
`fective than tamoxifen in reducing the
`expression of estrogen-regulated genes.
`Most tumors eventually became resis-
`tant to IC! 182,780 and grew inde-
`pendently of estrogen. Conclusions: ICI
`182,780 is a more effective estrogen an-
`tagonist than tamoxifen in the MCF-7
`tumor cell/nude mouse model system.
`[J Natl Cancer Inst 87:746-750, 1995]
`
`Tamoxifen, a nonstcroidal antiestro-
`gen, is the most prescribed drug for the
`treatment of breast cancer. When used in
`
`the adjuvant setting after surgery for
`primary breast cancer, about one fifth of
`the deaths at 10 years are avoided by 2
`years or more of treatment (I). Tamox-
`ifen is also effective in inducing remis-
`siOns in women with estrogen receptor
`(ER)-positive metastatic breast cancer.
`Invariably, however, tumors become to-
`sistant to tamoxifen, and tumor progres-
`sion and death ensue. The evolution to
`tamoxifen resistance in metastatic breast
`
`cancer occurs after an average treatment
`duration of only lO-l2 months, severely
`limiting the usefulness of this approach.
`The mechanisms by which tumors ac-
`quire resistance to tamoxifen are poorly
`understood. Loss of ER from the tumor
`
`can occur by selection of an Ell-negative
`clane or by suppression of receptor ex-
`pression. but
`this loss explains only a
`minority of cases (2). Growing experi-
`
`mental and clinical evidence suggests that
`resistance in some patients may be caused
`by the intrinsic estrogen agonist proper-
`ties of tamoxifen. Although tamoxifen is
`predominantly an estrogen antagonist in
`breast cancer cells, acquisition of increas-
`ingly dominant agonist activity over time
`may result in clinical resistance because
`of the acquired ability of the drug to
`stimulate, rather than to inhibit. tumor
`growth (3-7). The mechanisms for ta-
`moxifen-stimulated tumor growth are net
`clear, but
`these data suggest
`that an-
`tiestrogens with pure antagonist proper-
`ties might
`have
`superior
`antitumor
`activity.
`[CI 182,780 is a 7a-alkylsulfinyl ana-
`logue of estradiol that differs substantial-
`ly from tamoxifen in terms of
`its
`chemical. pharmacologic. and biologic
`prOperties. This agent has no intrinsic
`estrogen-agonist activity and,
`thus,
`is
`considered a “pure” antiestrogen (8.9). it
`has potent
`antiestrogenic
`activity in
`preclinical in vitro and in vivo model sys-
`tems (10). We recently reported (7) that
`treating nude mice with lCl 182,780 in-
`hibits the grewth of MCF—7 human breast
`tumor
`implants that had acquired ta-
`moxian resistance through the mechanism
`of tamoxifen-stimulated growth. Similar
`results were
`obtained with
`another
`
`ICl 164.384, studied earlier
`analogue,
`(ll). These data suggest the possibility
`that pure steroidal anticstrogens may be
`effective
`in
`some
`tamoxifen-resistant
`patients.
`In the present study, we have inves-
`tigated the preclinical activity of [Cl
`182,780 in more detail. We compared the '
`inhibitory effects of ICI 182,780, tamox-
`ifen, and estrogen withdrawal on the
`
`‘Aflilialions of authors: C. K. Osborne, E. B.
`Coronado-Hem. S. G. Hilsenbeck. B. L.
`McCue, Department of Medicine. Division of Medi-
`cal Oncology. The University of Texas Health
`Science Center at San Antonio. San Antonio. Tex.
`A.
`E. Wakeling, knees
`Pharmaceuticals,
`Mereside Alderley Park. Macclesfield. Cheshire.
`England. UK.
`R. A. McClelland. D. L. Manning. R. l. Nichol-
`son. Tenovus Cancer Research Center. University of
`Wales College of Medicine, Heath Park, Cardiff,
`Wales,U.K.
`'
`Correspondence to: C. Kent Osborne, Mn.
`Department of Medicine, Division of Medical On-
`cology, The University of Texas Health Science
`Center at San Antonio, 7703 Floyd Curl Dr., San
`Antonio, TX 73284-7884.
`See “Notes” section following "References."
`
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`growth of established tumors and on
`tumorigenesis in a model system that uses
`estrogen-dependent.
`human MCF-7
`breast
`tumor cells growing in athymic
`nude mice. We also studied the hormonal
`
`responsiveness of tumors that had be-
`come resistant to the two estrogen an-
`tagonisrs and the effects of these drugs on
`estrogen-regulated gene expression.
`
`Materials and Methods
`
`Nude Mouse Model System
`
`Ell-positive MCF-7 human breast cancer cells
`(passage 100-200) were cultured as described pre-
`viously (I2). The athymic nude mice used in these
`experiments were 4- to 5-week-old female castrated
`BALB/c-nu‘lnu’ mice purchased from Harlan
`SpragueDawley.
`Inc.
`(Madison. Wis). The
`methods for maintenance and housing of the mice
`and for growing MCF-‘l tumors from cell suspen-
`sions and from tumor transplants have been pub-
`lished in detail (3.7). Animal care was in accordance
`with institutional guidelines.
`Approximately 5 x 106 MCF-7 cells were in-
`jected subcutaneouslyinto the flanks. just under the
`forelimb. of female nude mice to initiate tumor for-
`mation. Estrogen supplementation was provided in
`the form of a 0.25-mg estradiol (5;) pellet (Innova-
`tive Research. Rockville, Md.) placed subcutaneous-
`ly in the interscapular region of the mice. The
`effects of tamoxifen and ICI 182.780 on the growth
`of established tumors were studied after the tumors
`had reached a size of 8‘10 mm (3-5 weeks). At this
`time, the animals were randomly allocated into four
`treatment groups: 1) continued estrogen supplemen-
`tation. 2) removal of the E; pellet. 3) removal of the
`E2 pellet plus treatment with Silo-pg tamoxifen
`citrate (Zeneca Pharmaceuticals, Wilmington. Del.)
`in peanut oil (injected subcutaneously each day.
`Monday through Friday). or 4) removal of the E,
`pellet and treatment with the indicated doses of
`ICI 182.780 (Zeneea Pharmaceuticals. Macclesfield.
`England) in castor oil (subcutaneous injections once
`a week).
`Initial dose-response studies with ICI
`182.780 were performed in the presence of con-
`tinued estrogen supplementation. Tumor growth was
`assessed. and tumor volumes were measured twice a
`weelt as described previously ([2).
`In tumorigenesis experiments. various treatments
`were begun on the same day tumor cells were in—
`jected. Inoculated mice were randomly allocated im-
`mediately into four treatment groups: l) estrogen
`supplementation. 2) 500 pg tamoxifen once a day.
`Monday through Friday, 3) 5 mg ICI 182.780 once a
`weelt. or 4) drug vehicle (peanut oil and/or castor
`oil). Tumor volumes were measured twice a week.
`To investigate the hormonal responsiveness of
`tumors that had become resistant to ICI 182.780.
`mice with resistant tumors were killed by cervical
`dislocation. and the tumors were resorted and cut
`into 1mm) fragments. The fragments were then
`uansplanted subcutaneoust on the flank just under
`the forelimb of new 4- to 5-week-old recipient mice
`that were then treated with estrogen. tamoxifen. ICI
`1 82.780. or vehicle alone.
`
`Estrogen and Progesterone Receptor
`Assays
`ER content was determined in tumors homo-
`
`genized in 0.4 M KCl—Tn's buffer. usingthe ER an-
`tibody kit (ER-Em; Abbott Laboratories. North
`Chicago. Ill.). Progesterone receptor (PgR) levels
`were measured by a ligand-binding. dextrancoatcd
`charcoal method (3).
`
`mg to 10.0 mg. Inhibitory activity was
`modest with doses of 0.5 mg or 1.0 mg.
`while more dramatic—but approximately
`equivalent—inhibitory effects were ob-
`served with 5.0-mg and 10.0-mg doses
`(data not shown). For subsequent experi-
`ments, a dose of 5.0 mg per mouse, given
`once a week, was used.
`
`Estrogen-Regulated Gene Expression
`
`Expression of the estrogen-responsive genes.
`leVI and p82. was determined by northern blot
`
`using mmzplementary DNA (cDNA)
`analysis.
`probes labeled with [3 Pldooxycytidine triphosphate
`(30W Cilmmol; Amcrsham Ltd. Amcrsham.
`England. U.K.) by the random-printing method as
`described previously (13). Briefly. total RNA was
`obtained from the tumors of treated mice by cell
`lysis in 4 M guanidinium thiocyanate and 1% 2-mer-
`captoethanol and centrifugation through 5.7 M
`caesium chloride (Beckman L-30 ultracentrifuge.
`SW50 rotor. 34000 rpm at 20 ‘C for 17 hours).
`Purified samples were stored in RNase-free water at
`-70 ‘C before elccuophoresis (10 ugllane). blotting.
`and hybridization. Densitometric analysis of auto-
`tadiographs was performed using a model 620 video
`densitomerer
`(Bio-Rad Labmatories. Richmond.
`Calif). and values obtained were con-acted for
`equivalence of RNA loading by comparison with the
`signals generated using a cDNA probe to human
`glyceraldeyhyde
`3-phosphate
`dehydrogenase
`(G3PDH) (Clontech Laboratories. Inc.. Palo Alto.
`Calif.)
`Recorded densitometry values represent the area
`of peak values obtained. following background sub-
`traction. from equivalently exposed autoradiographs
`(where x = band width in mm and y = optical den-
`sity value). Hybridizations of each set of filters in
`the study were carried out simultaneously with the
`same labeled probes. The reported values represent
`means of groups, and at
`least
`two separate
`hybridization: of different filters were performed for
`each probe (stripping the previous probe with high-
`stringency washes and checking for clearance by
`autoradiography).
`
`Statistical Analysis
`
`Analyses were performed using either the Krus-
`kal-Wallis one-way analysis of variance (when
`there were more than two groups) or the Wilcoxon
`signed rank test for two samples. All statistical tests
`were two-sided.
`
`Results
`
`ICI 182,780 Dose—Response
`
`lCl 182,780 inhibited estrogen-induced
`growth of MCF—7 tumors in a dose-de-
`pendent manner. Estrogen-supplemented
`mice with established MCF—7 tumOrs
`
`were randomly allocated to receive either
`continued estrogen treatment or estrogen
`treatment plus injections of ICI 182,780
`once a week in doses ranging from 0.5
`
`Effect of Estrogen Withdrawal,
`Tamoxifen, and [CI 182,780 on
`MCF-‘I Tumor Growth
`
`Treatment of mice by removal of the
`F4 pellet alone or with tamoxifen or ICI
`182.780 significantly inhibited MCF-7
`tumor growth (Fig. 1). In this experiment,
`tumor volumes remained stable for nearly
`100 days after estrogen withdrawal before
`progression ensued.
`In contrast.
`tumor
`volumes decreased slightly with tamoxi-
`fen and ICI 182.780 treatment, and tumor
`size remained stable for variable periods
`of time. A consistent observation was the
`
`delayed time to progression that was evi—
`dent
`in mice treated with ICI 182,780.
`With estrogen withdrawal alone or with
`tamoxifen, tumors developed resistance,
`and progression was evident in all mice
`after 3-4 months of treatment (median, 97
`and 104 days, respectively). However, the
`median time to progression was nearly
`twice as long with ICI 182.780, and the
`growth of some tumors remained con-
`trolled for extended periods of time
`(median, 200 days). In fact, two of the 10
`tumors from ICI 182,780-treated mice
`still had not progressed after 11 months
`and one small tumor (4 mm diameter)
`completely regressed and did not reap-
`pear during the course of the experiment
`(data not shown).
`
`Effect of ICI 182,780 on Tumorigenesis
`
`ICI 182,780 also had a greater impact
`on tumor formation in mice in which drug
`treatments were begun on the day of
`tumor cell
`inoculation (Fig. 2). Tumors
`grew rapidly in mice treated with es-
`trogen. Tumor growth was substantially
`delayed in mice treated with tamoxifen,
`but after 2 months. the growth rate in-
`creased. Tumors grew very slowly. or not
`at all, in mice treated with ICI 182.780—
`similar to the growth pattern observed in
`estrogen-deprived mice (12). By day 70,
`barer measurable tumors were present in
`the majority of mice. In another experi-
`ment (data not shown), three of six mice
`
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`
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`
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`l
`3
`
`ICI 182,780
`
`
`
`
`
`TumorVolume(mm
`
`.‘
`
`I V,‘":{L 1 "a “L
`'
`1‘ "
`(‘4‘
`
`I
`
`,. ii
`i Ji’ ”
`.“ i c i'
`Tamoxifen
`I
`
`'1
`
`I
`
`ICI 182,780 were transplanted into new
`castrated, recipient mice that were then
`treated with estrogen,
`tamoxifen,
`ICI
`182,780, tamoxifen plus ICI 182,780, or
`vehicle alone. This experiment was con-
`ducted five times with different
`tumor
`
`transplants, and a representative result is
`shown in Fig. 3. Transplanted tumor frag-
`ments. grew well in all mice, even those
`treated with vehicle alone (—54), suggest-
`ing estrogen independence. However, in
`four of five experiments, tumor growth
`was slightly increased by estrogen treat-
`ment (+5)),
`indicating continued sen-
`sitivity to the hormone. As expected,
`growth of these transplanted ICI 182,780-
`resistant
`tumors was also observed in
`
`recipient mice treated with [Cl 182,780.
`IntereStingly, in four of the five experi-
`ments, treatment of recipient mice with
`tamoxifen
`alone
`or
`tamoxifen
`plus
`ICI 182,780 resulted in a slight retarda-
`tion of tumor growth compared with
`treatment
`using
`vehicle
`alone
`or
`ICI 182,780 alone. although the observed
`differences in the individual experiments
`were modest and not statistically sig-
`nificant. A total of six of the 25 mice in
`
`.
`
`these experiments showed slower tumor
`growth with tamoxifen treatment, indicat-
`ing some heterogeneity among the trans-
`planted
`fragments
`in
`response
`to
`tamoxifen. However, most mice resistant
`
`to ICI 182,780 showed cross—resistance to
`tamoxifen.
`
`Resistance to ICI 182,780 was not due
`to a complete loss of tumor ER, although
`treatment with this drug reduced expres-
`sion of both ER and PgR. Tumors har-
`vested 4 weeks after initiating treatment
`with ICI l82,780 (ER = 37 t 3 fmol/mg
`protein; PgR = 27 :i: 7 fmol/mg protein)
`as well as those harvested at the time of
`resistance to [Cl 182,780 (ER = 16 :l: 4
`fmollmg protein; PgR = 17 :l: 8 fmol/mg
`protein) expressed both ER and PgR at
`markedly reduced levels compared with
`estrogen-treated controls (ER = 208 :l: 81
`fmol/mg protein; PgR = 103 :l: 20
`fmol/rng protein) (P = .024).
`Expression of two estrogen-responsive
`genes, p82 and leVI, was also mea-
`sured in these tumors (Table 1). ps2 and
`leVl messenger RNA (mRNA) expres-
`sion was reduced by 20%-74% in tumors
`from tamoxifen-treated mice (P = .013).
`It is interesting that p82 and pLIVl ex-
`pression remained suppressed even after
`
`Fig. 1. Effects ofestrogen (esuadiol [E2]) withdrawal. tamoxifen. and [Cl 182.780 on MCF-7 tumor growth.
`Estrogen-supplemented mice were inoculated with MCF-7 cells. On day 36 when tttmors had formed. mice
`were randomly allocated to treatment by withdrawal of esrmgen (-E2: 43-); Withdrawal of estrogen and
`treatment with 500 pg tamoxifen given once a day, Monday through Friday (-0—); or 5 mg ICI 182,780
`given once a week (I) Tumor volumes were determined at the times shown. it = to mice per group; means
`iSE.
`
`
`
`
`
`TumorVolume(mm3)
`
`ICI 182,780
`
`Tamoxifen
`
`Fig. 2. Effect of estrogen. tamoxifen, and 1C] I82,780 on MCF-7 ntmorigenesis. Mice were inoculated with
`MCF-7 cells on day 0 and randomly allocated immediately to receive treatment with a 175 estradiol (Ea) pel-
`let (+E2; - -); 500 pg tamoxifen given once a day, Monday through Friday (-6-); or 5 mg [Cl l82,780
`given once a week (I). Tumor volumes were determined at the times shown. n = 8 mice per group; means
`:55.
`
`treated with ICI 182,780 failed to grow
`measurable tumors even after 6 months of
`treatment.
`
`ICI 182,780-Resistant Tumors
`
`As indicated above, tumor resistance
`eventually occurred in most, but not all,
`
`mice treated with ICI 182,780. This resis-
`tance was manifested by regrowth of
`tumors. usually after many months of
`treatment. To investigate the hormonal
`sensitivity of these resistant tumors, frag-
`ments of a tumor that had progressed
`after months
`of
`treatment with
`
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`than treatment by estrogen withdrawal
`alone or with tamoxifen. Finally, expres-
`sion of the estrogen—regulated genes p82
`and pLIVl was nearly abolished by heat-
`ment with [Cl 182,780.
`
`Previous reports by us and by other in-
`vestigators (7,11,14-16) have also shown
`that the growth of tumors with acquired
`tamoxifen resistance can be inhibited or
`
`blocked by treatment with a pure an—
`tiestrogen such as 1C1 182,780, suggest-
`ing that the pure antiestrogens work by a
`different mechanism of
`action
`titan
`
`tamoxifen and other similar antiestrogens. ‘
`Tamoxifen resistance in our model sys-
`tem is associated with drug—induced
`tumor growth stimulation that occurs
`after an initial period of growth suppres-
`sion (3). The ability of tamoxifen alone to
`stimulate the growth of these tumors is
`less than that of esu'ogen. Interestingly,
`when combined with estrogen, tamoxifen
`can
`still
`inhibit
`estrogen-stimulated
`growth,
`indicating that
`it continues to
`possess both estrogen-agonist and an-
`tagonist properties (7). The increasingly
`dominant agonist properties of tamoxifen
`that develop after prolonged treatment
`can be blocked by the addition of pure
`antiestmgens
`(7,“).
`Evidence
`for
`tamoxifen-stimulated tumor growth as a
`mechanism for acquired tamoxifen resis-
`tance in patients has also been presented
`(5,6.17). On the basis of these preclinical
`studies, it has been suggested that treat-
`ment with 1C1 182,780 might
`induce
`tumor regression in some patients who
`have developed tamoxifen resistance.
`One recent study (18) has shown that
`short-term ICI
`182,780 treatment of
`patients who have [ER-positive tumors
`causes statistically significant reductions
`in the Ki67 labeling index and reductions
`in the expression of estrogen-regulated
`genes such as PgR and p82. In addition,
`remissions have now been reported in
`tamoxifen-resistant patients treated with
`this drug ([9).
`Although ICI 182,780 controls MCF—7
`tumor growth for longer durations than
`tamoxifen, eventual
`resistance to this
`agent
`is common. MCF-7 tumors that
`progress after prolonged treatment are
`estrogen-independent (grow in the ab-
`sence of estrogen supplementation) al-
`though they are still estrogen-sensitive
`(growth is enhanced by estrogen). The
`mechanisms
`by which resistance
`to
`
`Fig. 3. Hormonal sensitivity of 1C1 182,780-resistant tin-nuts. Fragments of a tumor that had developed resis-
`tance in a mouse treacd with ICI 182,780 were transplanted into new recipient ferrule castrated nude mice.
`The recipient mice were then randomly allocated to receive vehicle alone (--52; 42'! -). an esttadiol (E2) pellet
`(+E2: 4-); tamoxifen alone (-A-); ICI 182.790 alone (4). or a combination of tamoxifen plus ICI 182,780
`(-I-). Tumor volqu were calculated on the days shown. is = 6 mice per group; means :55.
`
`evolution to tamoxifen resistance when
`the drug was stimulating tumor growth
`(3). In fact, p82 was significantly lower
`in tamoxifen-resistant
`tumors than in
`
`tamoxifen-sensitive tumors (1’ = .012).
`This finding suggests that the agonist ac»
`tivity of tamOxifen,
`if responsible for
`tamoxifen-stimulated tumor growth, may
`be specific to genes associated with cell
`proliferation, while its antagonist activity
`continues to suppress the activity of
`genes less crucial for tumor survival. In
`contrast
`to the results obtained with
`
`tamoxifen, mRNA expression was nearly
`
`Table l. Exprmion of estrogen-mime genes‘
`
`Gene, relative
`mRNA level
`
`Treatment group
`(No. of blots analyzed)
`
`p52
`
`leVl
`
`12.2 1 0.6
`12.2 1 0.7
`Estrogen (4)
`6.0 d: l .5
`9.8 :t 0.5
`Tamxifen—senaitive (5)
`7.5 :t 1.8
`3.2 1 0.4
`Tamoxifen—resistant (5)
`0 t 0
`0.3 :1: 0.05
`ICI-sensitive (S)
`
`
`0.6 10.23ICI-resistant (8) 2.3 1 1.3
`
`‘rnRNA expession was mounted by northern
`blot analysis of total RNA extracted from MCF-7
`tumors taken from mice treated with estrogen (con-
`trols). tamoxifen for 3 weeks (tamoxifen-sensitive).
`tamoxifen until
`the time of tumor progression
`(tamoxifen-resistant), ICI 182. 780 for 4 weeks (ICI-
`sensitivc). or [Cl 182,780 until tumor progression
`(lCl-resistant). Values shown are the means 1 SE of
`scanning densitcmetry units corrected for RNA
`loading.
`
`abrogated by treatment with ICI 182,780
`(P<.004), and there was no difference be-
`tween sensitive and resistant tumors. It is
`
`unlikely, therefore, that ICI 182,780 resis-
`tance is caused by metabolic conversion
`of the drug to E2, since expression of
`these estrogen-regulated genes remained
`low.
`
`Discussion
`
`Clinical data demonstrate that, in some
`patients, the current endocrine therapies
`for breast cancer result
`in temporary
`tumor regression or growth stabilization.
`followed by tumor
`regrowth, usually
`within 6-18 months of treatment. We
`
`have developed an experimental in vivo
`model that mimics this clinical scenario.
`
`Our data suggest that, in this experimen-
`tal model system, ICI 182,780 possesses a
`greater ability to suppress estrogen—sensi-
`tive gene expression and greater an-
`titumor activity than the partial estrogen
`antagonist tamoxifen. In addition, MCF-7
`tumorigenesis was significantly delayed
`by ICI 182.780 when compared with
`tamoxifen. Moreover, a proportion of
`treated mice failed to develop tumors
`even after prolonged follow-up, an event
`rarely encountered in our experience
`treating mice with tamoxifen. ICl 182,780
`also suppressed growth of established
`tumors for a significantly longer duration
`
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`
`E 5a S
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`References
`
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`(I7) Wiebe VJ. Osborne CK. Fuqun SA. et al:
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`Cancer Res 542408414. 194
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`
`Notes
`
`[CI 182.780 develops are not clear. but
`reduced levels of ER and reduced expres-
`sion of estrogen-regulated genes (com-
`pared with tamoxifen-sensitive or with
`tamoxifen-resistant tumors) are evident.
`Reduced ER levels have also been seen in
`
`treated with
`from patients
`tumors
`10 182,780.
`in cultured breast cancer
`cells; and in mouse uterine tissue fol10w-
`ing the administration of the prototype
`pure antiestrogen ICl 164,384 (18-20).
`Other data suggest
`that
`the pure anti-
`estrogen-ER complex may be more
`fragile and more susceptible to receptor
`degradative pathways (16).
`in contrast,
`ER levels are high in tamoxifen-resistant
`tumors obtained with our model system
`(3). On the basis of our data. we would
`predict
`that most
`patients
`with
`ICI 182,780~resistant tumors would not
`respond well
`to subsequent
`treatment
`with tamoxifen.
`
`Even if pure antiestrogens are shown to
`have
`superior
`antitumor
`activity
`in
`women with breast cancer, they may not
`be the optimal antiestrogens for clinical
`use. The estrogenic properties of ta-
`moxifen in bone and on blood lipids may
`help to reduce bone loss and prevent car-
`diovascular disease. which are added
`benefits when treating breast cancer
`patients
`for prolonged periods
`after
`surgery for primary tumors or for breast
`cancer prevention (21,22). The effect of
`ICI 182,780 on these parameters is not
`yet known, but
`it might be deleterious
`given its lack of estrogenic qualities.
`However.
`treatment with [Cl 182.780
`might not be associated with the in-
`creased risk of endometrial cancer recent-
`ly attributed to tamoxifen (23). Further
`clinical study of pure antiestrogens in
`tamoxifen-resistant
`and in tamoxifen-
`
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`(4) Gonordis MM. Jordan VC: Development of
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`(5) Pritchard K1. Thomson DB. Myers RB. et al:
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`Cancer Chernother Pharmacol 34:89-95. 1994 '
`(8) Wakeling AE. Bowler .1: Novel antioestmgens
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`(9) Walteling AB. Dukes M. Bowler J: A potent
`specific pure antiestrogen with clinical poten-
`tial. Cancer Res 51:3867-3873. 1991
`(10) Wakeling AE: Steroids] pure antiestrogens. In
`Regulatory Mechanisms
`in Breast Cancer
`(Lippman M. Dickson R. eds). Boston: Kluwer
`Acad Pub]. 1991. pp 239-257
`([1) Gottardis MM. Jiang SY. Jeng MH. et al: In-
`hibition of tamoxifen-stimulated growth of an
`MCF-‘I
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`in athyrnic mice by
`novel
`steroidal atttiestrogens. Cancer Res
`49:4090-4093. 1989
`(I2) Osborne CK. Hobbs K. Clark CM: Effect of
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`Supported in part by Public Health Service grants
`CAJOZSI. CA30195. CA58183. and CA54174 from
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`the National Cancer Institute. National Institutes of
`(13) Manning DL. McClelland RA. Gee JM. et al:
`Health. Department of Health and Human Services.
`The role of four oestrogen-responsive genes.
`Manuscript received September 19. 1994; revised
`pLIVl. p52. pSYD3 and pSYD8. in predicting
`naive patients is clearly indicated.
`December 27. 1994; accepted March 6. 1995.
`responsiveness to endocrine therapy in pri-
`
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