3020
`
`Current Pharmaceutical Design, 2008, 14, 3020-3032
`
`
`
`PET Imaging of Steroid Receptor Expression in Breast and Prostate
`Cancer
`
`G.A.P. Hospers1, F.A. Helmond2, E.G.E. de Vries1, R.A. Dierckx3 and E.F.J. de Vries3,*
`
`1Department of Medical Oncology, University Medical Center Groningen, University of Groningen, Groningen, the
`2Organon, a part of Schering. Plough Corporation, Roseland, NJ, USA and 3Department of Nuclear Medi-
`Netherlands;
`cine and Molecular Imaging, University Medical Center Groningen, University of Groningen, Groningen, the Nether-
`lands
`
`Abstract: The vast majority of breast and prostate cancers express specific receptors for steroid hormones, which play a
`pivotal role in tumor progression. Because of the efficacy of endocrine therapy combined with its relatively mild side-
`effects, this intervention has nowadays become the treatment of choice for patients with advanced breast and prostate can-
`cer, provided that their tumors express hormone receptors. However, in case of breast cancer it is well known that part of
`the patients have hormone receptor-negative tumors at diagnosis, whereas other patients have discordant receptor expres-
`sion across lesions. In addition, receptor expression can change during therapy and result in resistance to this therapy. Be-
`sides several lines of hormonal treatments, also other strategies to affect the hormone receptors are currently under inves-
`tigation, namely histone deacetylases (HDAC) and heat shock protein (HSP) inhibitors. Knowledge of the actual receptor
`status can support optimal treatment decision-making and the evaluation of new drugs. Positron emission tomography
`(PET) is a non-invasive nuclear imaging technique that allows monitoring and quantification of hormone receptor expres-
`sion across lesions throughout the body. Several PET tracers have been developed for imaging of the most relevant hor-
`mone receptors in breast and prostate cancer: i.e. the estrogen, progesterone and androgen receptors. Some of these PET
`tracers have been successfully applied in early clinical studies. This review will give an overview of the current status of
`PET imaging of hormone receptors in breast and prostate cancer.
`
`Key Words: Breast cancer, prostate cancer, estrogen receptor, progesterone receptor, androgen receptor, endocrine therapy,
`positron emission tomography, imaging.
`
`1. INTRODUCTION
`
` Worldwide, breast and prostate cancer are common
`causes of death among women and men, respectively. Breast
`and prostate cancer are the best known examples of hormone
`dependent tumors, although other tumors like ovarian tumors
`and endometrial cancer are also frequently characterized by
`hormone dependency. In hormone dependent tumors, the
`hormone receptors play a key role in tumor proliferation and
`disease progression. The primary signal for the activation of
`steroid hormone receptors (SR) is binding of the hormone
`(Fig. 1). In
`the absence of hormone, steroid recep-
`tor monomers are associated with heat shock protein (HSP)
`complexes and as a rule are only phosphorylated to a small
`extent. Upon binding of the hormone, receptors dissociate
`from the HSPs and form dimers. These hormone receptor
`dimers, translocate from the cytoplasm to the nucleus, bind
`to target gene-specific sites containing hormone response
`elements (HRE) and recruit a series of co-activator com-
`plexes to regulate target gene transcription [1]. Since they
`can activate oncogenes and inhibit the expression of tumor-
`suppressor genes, the steroid hormone receptors are impor-
`tant intermediates in the progression of breast and prostate
`
`
`*Address correspondence to this author at the Department of Nuclear Medi-
`cine and Molecular Imaging, University Medical Center Groningen, Univer-
`sity of Groningen, Hanzeplein 1, P.O. Box 30.001, 9700 RB Groningen,
`The Netherlands; Tel: +31-50-3613599; Fax: +31-50-3611687;
`E-mail: e.f.j.de.vries@ngmb.umcg.nl
`
`cancer and therefore are key targets for treatment in patients
`with breast and prostate cancer. At diagnosis, 70% of the
`breast cancer patients are positive for estrogen receptor (ER)
`and/or progesterone receptor (PR) expression, whereas the
`androgen receptor (AR) is expressed in 80-90% of the pa-
`tients with prostate cancer. The vast majority of hormone
`receptor expressing tumors are sensitive for endocrine treat-
`ment, which aims to inhibit the hormone receptor-mediated
`pathway for tumor proliferation. Endocrine therapy has be-
`come the essential part of treatment for hormone receptor
`positive patients with primary and metastatic breast cancer
`and patients with advanced prostate cancer, with a favorable
`benefit-to-toxicity ratio. Despite significant advances in pri-
`mary cancer treatment, many patients will develop a sys-
`temic relapse. At the time of systemic relapse and during
`treatment, receptor expression can change [2-5]. To achieve
`an effective hormone receptor-mediated treatment, knowl-
`edge of the actual receptor status of the primary tumor and
`metastases would be benificial (Cancer Information Summa-
`ries: Adult treatment; http://www.cancer.gov/cancertopics/
`pdq/adulttreatment). It is however often hard to obtain fresh
`tumor tissue due to the location of systemic metastasis. Non-
`invasive molecular imaging techniques to monitor the actual
`status or occupancy of the steroid receptors could therefore
`be of additional value for therapy management of patients
`with breast and prostate cancer. This review will give an
`overview of the current status of the application of the mo-
`
`
`
`1381-6128/08 $55.00+.00
`
`© 2008 Bentham Science Publishers Ltd.
`
`MYLAN - EXHIBIT 1033
`
`

`
`PET Imaging of Steroid Receptor Expression in Breast and Prostate Cancer
`
`Current Pharmaceutical Design, 2008, Vol. 14, No. 28 3021
`
`lecular imaging technique, PET, for visualization of steroid
`hormone receptor expression in breast and prostate cancer.
`
`2. STEROID HORMONE RECEPTORS IN BREAST
`CANCER
`
` Endocrine therapy has become the treatment of choice
`for many patients with metastatic breast cancer. The most
`frequently applied endocrine drugs are tamoxifen (antagonist
`of the ER, but with agonistic properties at low concentra-
`tions), fluvestrant (a pure ER antagonist; induces degradation
`of the receptor) and aromatase inhibitors (inhibit the produc-
`tion of estrogens). The importance of steroid hormone recep-
`tors in breast cancer was already recognized over 40 years
`ago, when radiolabeled estrogens were found to concentrate
`preferentially in the estrogen-influenced target organs of
`animals and in human breast cancers. At diagnosis, 70% of
`the breast cancer patients have tumors with positive ER or
`PR expression. Out of these patients, 50% to 60% will re-
`spond to anti-hormonal treatment, whereas the remain frac-
`tion will not (probably due to intrinsic resistance). For breast
`tumors that do not express these hormone receptors, endo-
`crine therapy is not effective. Among the patients with ster-
`oid hormone receptor expressing tumors, the ER–/PR+ phe-
`notype represents only 3% to 5% of patients, which would
`suggest that determination of the ER status is of primary
`importance for therapy management. However, determina-
`tion of the PR status could still be relevant because it is a
`reflection of an intact ER signaling pathway. For example,
`expression of the PR distinguishes two subsets of ER-
`positive tumors that may require different treatment strate-
`gies. ER-positive tumors without PR expression were found
`to respond less likely to the selective ER modulator ta-
`moxifen than the ER-positive tumors that also expressed PR
`[6, 7]. Because of their relative resistance to tamoxifen, the
`ER+/PR- tumors should therefore preferably be treated ini-
`tially with an aromatase inhibitor.
`
` After an initial response to tamoxifen or aromatase in-
`hibitor treatment, all patients will eventually become resis-
`tant to anti-hormonal treatment (acquired resistance) [8-10].
`Remarkably, loss of ER expression (ER-) has only been
`demonstrated in 17-28% of patients with acquired resistance
`[11] and mutations of the ER are rare and have mainly been
`found in tumors that were immunohistochemically classified
`as ER- [12]. Thus, the majority of the tumors with acquired
`anti-hormonal resistance still expresses the ER. A major
`mechanism of acquired resistance to endocrine therapy is the
`development of increased sensitivity of breast cancer cells
`for estrogen or (partial) ER agonists like tamoxifen [8]. This
`first phase of acquired resistance to tamoxifen is character-
`ized by tamoxifen or estrogen-stimulated tumor growth. Re-
`moval of estrogen with an aromatase inhibitor or blocking
`the ER with the pure antagonist fulvestrant prevents tumor
`growth and provides an alternative therapy for patients that
`have become resistant to tamoxifen. Laboratory studies have
`demonstrated that a second phase of acquired resistance can
`occur after prolonged estrogen deprivation. In this phase of
`acquired resistance, tumor cells have become hypersensitive
`to estrogens and endocrine treatment is ineffective. In fact, at
`this stage tumor cells have become so hypersensitive to es-
`trogens that they are killed by physiological concentrations
`of the hormone. This mechanism was first demonstrated in
`
`in-vitro experiments, in which ER expressing tumor cells
`that were maintained estrogen-free for years showed a switch
`in response to estradiol from stimulation of proliferation into
`induction of apoptosis [13, 14]. Similarly, estradiol caused a
`rapid regression of ER expressing tumors in athymic mice
`that had been treated with tamoxifen for a long period of
`time [15]. Interestingly, estrogen-hypersensitivity of ta-
`moxifen-resistant tumor cells is accompanied by a 4 to 10-
`fold increase in ER expression. A clinical parallel to the
`aforementioned
`laboratory observations of
`estrogen-
`hypersensitivity is shown in a number of studies in respec-
`tively 523, [16], 143 [17] and 32 patients [18], which dem-
`onstrated that treatment with estrogen showed objective re-
`sponses in 30 to 42% of the breast cancer patients that were
`previously treated with one or more lines of hormonal treat-
`ment. Another mechanism of acquired tamoxifen-resistance
`in breast tumors is cross-talk of the ER pathway with other
`signal transduction pathways, such as growth factor receptor
`signaling pathways (e.g. EGFR, HER-2). In this situation,
`tumor growth is stimulated by growth factor receptor signal-
`ing, whereas the classic ER genomic function is repressed.
`[19].
`
`Since the steroid receptor pathway can be affected in
`
`several manners during treatment, different alternative treat-
`ments would be applicable in therapy resistant patients. Se-
`lection of the most suitable treatment of therapy-resistant
`tumors could be based on the actual ER expression levels in
`the tumor. If steroid receptor expression is lost, the treatment
`of choice would be chemotherapy. When the ER density is
`increased, estrogen treatment may be a good alternative. For
`resistant tumors with intermediate ER density, treatment
`with an aromatase inhibitor or fulvestrant would be the best
`option. Moreover, exiting new avenues may be openend by
`the development of new drug as HDAC and HSP inhibitors,
`which affect by their action by modulation of the expression
`of the ER [20-22]. Molecular imaging techniques, such as
`PET, to monitor the presence and to quantify expression lev-
`els of the ER, PR and AR could be of additional value for
`therapy management.
`
`3. PROSTATE CANCER: THE ANDROGEN RECEP-
`TOR.
`
`Since approximately 80-90% of prostate cancers is an-
`
`drogen-dependent at initial diagnosis [23], endocrine therapy
`of prostate cancer is always directed toward the reduction of
`serum androgens and inhibition of the AR signaling. How-
`ever, the initial response to anti-androgen therapy is almost
`always followed by a relapse to an unresponsive, hormone-
`refractory stage. In prostate cancer, the same mechanism of
`anti-hormonal therapy resistance have been found as in
`breast cancer [24]. In contrast to ER expression in breast
`cancer, the hormone-refractory stage in prostate cancer is
`rarely associated with a loss of AR expression [25]. On the
`contrary, the AR gene is over-expressed in approximately 20
`to 30% of the hormone refractory tumors [23, 26]. Edwards
`et al. investigated AR protein expression in hormone-
`sensitive and hormone-refractory tumors from the same pa-
`tient [26]. They found that AR expression levels were higher
`in hormone-resistant tumors than in matched hormone-
`sensitive tumors. Others found that 30-40% of men whose
`disease progresses during anti-androgen therapy experienced
`
`

`
`3022 Current Pharmaceutical Design, 2008, Vol. 14, No. 28
`
`Hospers et al.
`
`
`
`Fig. (1). Mechanism of steroid hormone action as published by Weigel and Moore [1] (reproduction is allowed). In the absence of hormone,
`steroid receptor monomers (SR) are associated with heat shock protein complexes (HSP) and are typically basally phosphorylated. Upon
`binding hormone (1), receptors dissociate from HSPs and dimerize (2). The dimer binds to target gene-specific sites containing hormone
`response elements (HRE) (3), and recruits a series of coactivator complexes to regulate target gene transcription (4). Site-specific phosphory-
`lation of receptors increases subsequent to hormone binding, with some increases occurring rapidly, and others with delayed kinetics. Upon
`steroid binding, some receptors also interact with Src (steroid receptor coactivator) and MNAR (modulator of nongenomic action of estrogen
`receptor) (5), activating Src and downstream MAPK (mitogen-activated protein kinases) (6). Membrane-associated steroid receptors (mSR)
`also bind hormone and initiate signaling cascades (7). While some of these are classical steroid receptors, others bear no homology to the
`steroid receptor superfamily.
`
`a fall in serum prostate-specific antigen (PSA) after discon-
`tinuation of therapy [27]. These results suggest the develop-
`ment of hypersensitivity to androgens, in analogy to the
`phase II type of resistance to anti-estrogen therapy in breast
`cancer. In animal model hormone refractory prostate cancer
`cells became highly sensitive to androgens. Besides andro-
`gen hypersensitivity that is accompanied by enhanced AR
`expression, there are several other mechanisms that lead to
`resistance to androgen ablation [28]. For example, increased
`5-alpha reductase activity causes conversion of the less po-
`tent AR agonist testosterone to its metabolite dihydrotestos-
`terone, which is 10 times more potent and therefore can
`more efficiently induce AR signaling [29, 30]. Other mecha-
`nisms could involve enhanced levels of AR transcriptional
`co-activators and cross-talk between the AR signaling path-
`way and other signal transduction pathways that activate the
`Src/MAPK kinase cascade (Fig. 1).
`
` As follows from the above, the AR density can change
`during treatment. Depending on the kind of changes in AR
`expression different follow-up treatments may be required.
`In rare cases where AR expression is lost, the treatment of
`choice will be chemotherapy. In case of increased AR ex-
`pression (hypersensitivity), discontinuation of anti-androgen
`therapy or even supplementation of androgens has the pref-
`erence. If normal AR levels are maintained when resistance
`develops, alternative treatment could be aimed at further
`decreasing the AR agonist concentrations in blood, blocking
`the receptor with full AR antagonists or preventing cross talk
`between the AR pathway and other signal transduction
`pathways that activate the Src/MAPK kinase cascade. Thus,
`
`for optimal treatment decision-making, molecular imaging
`techniques to monitor the actual density of the AR and to
`visualize the effect of the intervention could be of additional
`value.
`
`In addition to clinical decision-making, imaging may also
`
`play a role in the development of new drugs. Currently, new
`drugs are being developed that can modulate hormone recep-
`tor expression. Promising candidates among these drugs are
`histone deacetylases (HDAC) and heat shock protein (HSP)
`inhibitors. HDACs are enzymes that deacetylate the amino-
`teminal tails of histones, causing structural changes in chro-
`matin that regulated transcription. HDAC inhibitors can also
`destabilize the AR by interfering with the binding of HSP90
`to the AR. HSP90 inhibitors cause AR degradation by com-
`peting with ATP for binding. Thus, HDAC and HSP90 in-
`hibitors induce the breakdown of the hormone receptors and
`thus interrupt the hormone receptor signaling pathway that is
`responsible for tumor growth. The HSP90 inhibitors and
`HDCAs are currently in early clinical phase I/II trials [31].
`Imaging of AR density by quantitative PET imaging could
`help the development of these drugs.
`
`4. DEVELOPMENT OF PET TRACERS FOR STER-
`OID RECEPTORS
`
`PET is a nuclear imaging technique that could be an at-
`
`tractive alternative for repeated biopsies of tumor tissue as a
`tool for guiding of treatment. PET can visualize and quantify
`physiological and biochemical parameters in-vivo by admin-
`istering a radioactive tracer to a patient. The distribution of
`the tracer is monitored over time using a dedicated PET
`
`

`
`PET Imaging of Steroid Receptor Expression in Breast and Prostate Cancer
`
`Current Pharmaceutical Design, 2008, Vol. 14, No. 28 3023
`
`camera. The data that are acquired by the PET camera are
`subsequently converted in quantitative 3D-images of the
`tracer distribution as a function of time. With pharmacoki-
`netic modeling paradigms, the dynamic PET data can pro-
`vide quantitative in-vivo measures of biochemical and
`physiological parameters. This technique is basically non-
`invasive and allows monitoring of the whole body in a single
`session. Nuclear imaging techniques like PET could there-
`fore offer the unique opportunity to detect and quantify the
`steroid receptor expression levels in patients with hormone
`responsive tumors, provided that a suitable tracer for the
`receptor of interest is available. Thus, imaging-based tumor
`characterization may provide the required input for guiding
`systemic therapy in patients and may assist drug develop-
`ment. In the following sections, we will describe the current
`status of PET imaging methods for the most relevant steroid
`receptors in breast and prostate cancer: ER, PR and AR.
`
`4.1. PET Tracers for Imaging of Estrogen Receptors
`In the 1980’s, the estrogen derivative 16(cid:1)-[18F]fluoro-
`
`17(cid:2)-estradiol ([18F]FES) was developed for imaging of the
`ER (Fig. 2). To date, [18F]FES is still the most frequently
`applied PET tracer for ER imaging. A simplified, two-step
`procedure for labeling [18F]FES in high radiochemical yields
`was described by Römer et al. [32] and subsequently adapted
`for automated clinical productions [33-35]. [18F]FES proved
`stable upon storage in aqueous ethanol solution for up to 24
`h. In-vivo, [18F]FES showed favorable characteristics as an
`imaging agent for the ER in tumor-bearing rat and mouse
`models [36-40]. Highest tracer uptake was found in the
`uterus and ovaries, both organs with high ER expressions.
`The ER consists of 2 subtypes, called ER(cid:1) and ER(cid:2). Studies
`in ER(cid:1) and in ER(cid:2) knock-out mice demonstrated that
`[18F]FES preferentially binds to the ER(cid:1)-subtype [36]. In
`rats, co-injection of estradiol resulted in a dose-dependent
`reduction in tracer uptake in these organs at an injected dose
`
`of 1 (cid:1)g and higher [41]. In addition, receptor occupancy by
`tamoxifen after pretreatment of the rats with the drug could
`be measured by titration of [18F]FES uptake in relevant or-
`gans. In rodents, [18F]FES is also able to detect ER expres-
`sion in positive breast tumors, either by ex-vivo biodistribu-
`tion or microPET imaging [38, 40].
`In rat plasma, [18F]FES is converted into a hydrophilic
`
`metabolite with a metabolic half-life of approximately 30
`min [42]. [18F]FES is also fairly rapidly metabolized into
`glucuronides and sulfates in human plasma [43]. The unme-
`tabolized [18F]FES is mainly reversibly bound to albumin
`and sex hormone binding globulin (SHBG) in plasma, where
`it is largely protected from metabolism.
` The feasibility of in-vivo quantification of [18F]FES bind-
`ing parameters, using either equilibrium analysis or graphical
`analysis was demonstrated in rat brain [42]. Both analysis
`approaches allowed quantitative measurement of ER in re-
`ceptor-rich brain regions, such as pituitary and hypothala-
`mus, but not in brain areas with low receptor levels like hip-
`pocampus. In contrast, [18F]FES uptake was found to be
`flow-dependent in tissues with very high ER concentrations,
`such as uterus and ovaries and therefore tracer uptake may
`underestimate ER levels in these organs [41]. Since receptor
`density in breast tumors is substantially lower than in uterus,
`quantification of ER density in tumors by [18F]FES PET
`should be feasible. In fact, FES uptake was found to corre-
`late with the ER concentration (Bmax) in the tumor from in-
`vitro assays, although the correlation coefficient was rela-
`tively low (r = 0.45, p<0.05) [38]. [18F]FES has already been
`used to monitor ER expression in several patient studies (see
`section 4.1).
`
`In order to develop an improved tracer, moxestrol (17(cid:2)-
`
`ethynyl-11(cid:3)-methoxy-estradiol), which is one of the most
`potent estrogens, has been labeled with fluoro-18 for PET
`imaging of ER. 16(cid:3)-[18F]fluoromoxestrol ([18F](cid:3)FMOX) was
`
`OH
`
`18F
`
`
`
`H3CO
`
`OH
`
`CH
`
`18F
`
`HO
`
`[18F]FES
`
`OH
`
`[18F]beta-FMOX
`
`O
`
`OH
`
`Et2N
`
`18F
`
`H3CO
`
`OH
`
`18F
`
`HO
`
`F
`
`HO
`
`4F-M[18F]FES
`
`18F
`
`HO
`
`(CH2)9SO(CH2)3CF2CF3
`
`18F
`
`HO
`
`[18F]fluorofulvestrant
`
`[18F]fluorotamoxifen
`
`C3-[18F]fluoroethylcyclofenil
`
`
`
`Fig. (2). Structures of PET tracers for imaging of the estrogen receptor.
`
`

`
`3024 Current Pharmaceutical Design, 2008, Vol. 14, No. 28
`
`Hospers et al.
`
`evaluated in animal experiments and displayed the most
`promising characteristics for a PET tracer among a series of
`other 17-ethynylestradiols
`[44-46]. Uterine uptake of
`[18F](cid:3)FMOX in immature rats was approximately twofold
`higher than that of [18F]FES. In contrast to [18F]FES,
`[18F](cid:3)FMOX also exhibited specific binding in organs with
`low levels of ER expression, such as kidney, muscle and
`thymus. This suggests that [18F](cid:3)FMOX could be a more
`sensitive tracer of ER than [18F]FES. The improved tracer
`uptake of [18F](cid:3)FMOX in target organs is most likely due to
`its higher metabolic stability, which results in an extended
`bioavailability [46]. Dosimetry in immature female rats indi-
`cated that the radiation burden of [18F](cid:3)FMOX was within
`acceptable limits for clinical application [46]. Despite the
`encouraging results in rat studies, [18F](cid:3)FMOX proved un-
`able to detect ER-positive lesions in breast cancer patients
`[47]. This lack of specific uptake was ascribed to fast me-
`tabolism of the tracer in humans. In contrast to [18F]FES,
`[18F](cid:3)FMOX had low affinity for SHBG and consequently is
`not protected against metabolic degradation.
` Besides [18F](cid:3)FMOX, several other modifications in the
`structure of [18F]FES have been investigated in order to im-
`prove the binding affinity and/or stability of the tracer, such
`as the introduction of an 11(cid:3)-methoxy or a 7(cid:2)-methyl sub-
`stituent [39, 48]. Among these tracers, 11(cid:3)-methoxy-4,16(cid:2)-
`[16(cid:2)-18F]difluoroestradiol (4F-M[18F]FES) showed the high-
`est uterine uptake and uterus-to-background ratios. This
`compound has low binding affinity for SHBG. Its properties
`remain to be further investigated in humans.
`
`In addition to tracers that were derived from estradiol, a
`
`few tracers have been based on drugs that are used in hor-
`monal therapy. For example, the ER antagonist fulvestrant
`was labeled at the 16(cid:2)-position with fluorine-18 [49]. How-
`ever, introduction of the [18F]fluorine atom strongly reduced
`the binding affinity of the compound and uterine uptake in
`immature rats, making the tracer unsuitable for PET imag-
`ing. Also the selective ER modulator tamoxifen was labeled
`with fluoro-18 [50]. In rats, [18F]fluorotamoxifen exhibited
`specific uptake in uterus and mammary tumors that could be
`blocked by co-administration of estradiol. However, uterine
`uptake and the percentage of displaceable binding of
`[18F]fluorotamoxifen was much lower than that of fluoro-18
`labeled steroids like [18F]FES. Still, [18F]Fluorotamoxifen
`was evaluated in a pilot PET study in 10 women with ER-
`positive breast tumors [51]. Twenty-three lesions were
`evaluated, of which 2 out of 3 lesions were scored as true
`negative and 16 out of 20 lesions as true positive. Tracer
`
`uptake did not correlate with ER concentration in the pri-
`mary lesion. It was suggested that [18F]fluorotamoxifen
`might have some use in predicting response to treatment, as
`tracer uptake in tumors that responded well to tamoxifen
`treatment was higher than in poorly responding tumors, but
`this was only the case when bone lesions were excluded.
`However, the statistical power of this study was limited, be-
`cause only a small number of patients were included in the
`study.
`
` Cyclofenil derivatives form another class of non-steroidal
`ER ligands that have been labeled with 18F, 11C and 94Tc for
`PET imaging [52-55]. Despite high in-vitro binding affinities
`of these compounds, specific uptake in rats was disappoint-
`ingly low.
` Thus, [18F]FES remains the only validated PET tracer for
`ER that is currently used in clinical studies, although its
`characteristics are not ideal. So far, attempts to develop a
`tracer with better properties for ER imaging have yielded
`disappointing results. Therefore, the efforts to develop better
`alternatives for [18F]FES have increased.
`
`4.2. PET Tracers for Imaging of Progesterone Receptors
`
`So far, only a few tracers for PET imaging of the PR
`
`have been investigated (Fig. 3). Two decades ago, encourag-
`ing preclinical results have already been obtained with the
`high affinity PR ligand 21-[18F]fluoro-16(cid:1)-ethyl-19-norpro-
`gesterone ([18F]FENP) [56, 57]. In estrogen-primed rats,
`[18F]FENP showed high levels of specific uterine uptake,
`with uterus-to-blood ratios of 14 [57] to 26 [56] at 60 min
`after tracer injection. Uterine uptake could be blocked by
`pretreatment with unlabeled FENP ((cid:1) 83% reduction in up-
`take), indicating that uterine uptake was receptor-mediated.
`However, considerable [18F]FENP uptake was also observed
`in fat and bone, reflecting the high lipophilicity and meta-
`bolic defluorination rate of the compound, respectively [56].
`High tracer uptake in fat could hamper imaging of breast
`lesions, since the breast contains a high proportion of adi-
`pose tissue. In addition to the specific tracer uptake in the
`uterus, receptor-mediated uptake of [18F]FENP was also ob-
`served in PR-positive mammary carcinoma in mice, although
`the uptake in the tumor was substantially lower than in the
`uterus [57]. In a pilot study in 8 patients with PR-positive
`primary breast carcinoma, however, tumor-background ratios
`of [18F]FENP were low and consequently the tumors could
`only be detected in 50% of the patients [58]. Moreover,
`[18F]FENP uptake did not correlate with PR expression lev-
`
`18F
`
`O
`
`Et
`
`R1
`
`O
`
`R2
`
`
`O
`
`O
`
`O
`
`18F
`
`O
`
`18F
`
`O
`
`O
`
`R
`
`O
`
`O
`
`[18F]FENP
`
`4-fluorophenyldioxolane derivatives
`(R1 = H, CH3, R2 = OH, H)
`
`furanyldioxolane derivatives
`(R = H, CH3)
`
`
`
`O
`
`Fig. (3). Structures of PET tracers for imaging of the progesterone receptor.
`
`

`
`PET Imaging of Steroid Receptor Expression in Breast and Prostate Cancer
`
`Current Pharmaceutical Design, 2008, Vol. 14, No. 28 3025
`
`els in the tumors. Metabolite analysis demonstrated that
`these disappointing results in humans were the result of rapid
`metabolism of [18F]FENP into 20-dihydro-[18F]FENP by
`liver, blood and tumor cells [59]. This hydroxyl metabolite
`that was formed has much lower affinity for the PR than the
`parent compound and thus does not contribute to the recep-
`tor-mediated uptake, but only elevates the background sig-
`nal. In order to increase the metabolic stability of the tracer,
`the ketone function at C20 has been converted into a dioxo-
`lane moiety. Hitherto, several dioxolane derivatives of
`16(cid:1),17(cid:1)-dihydroxyprogesterone have been labeled with a
`positron emitter (Fig. 3). The 4-fluorophenyldioxolane de-
`rivatives did display some receptor-mediated uptake in the
`uterus of estrogen-primed rats, but the extent of specific
`binding was twofold lower than that of [18F]FENP [60]. De-
`fluorination of these compound was slower than for [18F]-
`FENP, as observed by the reduced bone uptake, but uptake
`in fat was equal or higher. The furanyldioxolane derivatives,
`on the other hand, have shown more promising results. They
`display equally high levels of specific binding in PR-rich
`organs like uterus and ovaries as [18F]FENP [61]. However,
`fat and bone uptake were substantially lower for these new
`tracers, suggesting slower metabolic defluorination and
`lower non-specific binding. Despite these encouraging re-
`sults in animal studies, to our knowledge, no data have been
`published on the behavior of these progestin furanyldioxo-
`lane tracers in humans in the past decade. As follows from
`the above, no validated tracer for the PR is available yet and
`as a consequence PET imaging of the PR status in patients
`remains a mirage.
`
`4.3. PET Tracers for Imaging of Androgen Receptors
`
`Several steroids have been radiolabeled as potential trac-
`
`ers for PET imaging of the AR (Fig. 4). The first labeled
`steroid that showed encouraging specific binding to the AR
`in animals was 20-[18F]fluoromibolerone (20-[18F]FMib)
`[62]. In rat that were treated with DES to suppress the en-
`dogenous androgen production, 20-[18F]FMib prostate-to-
`background (blood, muscle) ratios increased from 4 at 30
`min to 12 at 4 h. Prostate uptake of 20-[18F]FMib could be
`
`blocked by co-injection of testosterone and by endogenous
`androgens. In a subsequent study by the same group, 20-
`[18F]FMib was compared with six fluoro-18 labeled andro-
`gens [63]: 16(cid:2)-[18F]fluorodihydrotestosterone ([18F]FDHT),
`16(cid:2)-[18F]fluorotestosterone ([18F]FT), 16(cid:2)-[18F]FMib, 16(cid:1)-
`[18F]fluoro-7(cid:1)-methyl-19-nortestosterone (16(cid:1)-[18F]FMNT),
`16(cid:2)-[18F]FMNT and 20-[18F]fluorometribolone (20-[18F]F-
`R1881). All labeled androgens demonstrated specific recep-
`tor-mediated uptake in the prostate of DES-treated rats. In
`this animal model, rapid defluorination of the three 16(cid:2)-
`fluorine androgens ([18F]FDHT, [18F]FT and 16(cid:2)-[18F]FMNT)
`was observed, resulting in relatively low uptake in the pros-
`tate 4 h after tracer injection, as compared to the other com-
`pounds. However, target-to-background ratios of the 16(cid:2)-
`fluorine androgens were still higher than those of the other
`tracers (with the exception of 16(cid:1)-[18F]FMNT), because de-
`fluorination not only reduced tracer uptake in target tissue,
`but also increased the clearance of radioactivity from non-
`target tissues. Moreover, at earlier times that correspond bet-
`ter to imaging times in humans, the differences in prostate
`uptake and target-to-background ratio between the fluoro-18
`labeled androgens were only small. When interpreting these
`results, one should realize that the rat is not an ideal model
`for evaluating radiolabeled androgens, because rats do not
`express SHBG. SHBG is a glycoprotein that binds androgens
`in plasma of primates and thus shields the steroids from
`metabolic degradation. The binding affinity of [18F]FDHT
`for SHBG is much higher than those of the other fluoro-18
`labeled androgens [63]. In animal models with high SGHB
`levels, defluorination of [18F]FDHT was significantly re-
`duced, whereas metabolism of androgens with low affinity
`for SHBG was unaffected [64]. SHBG binding did not affect
`target tissue uptake of [18F]FDHT. Three of the fluoro-18
`labeled androgens ([18F]FDHT, 16(cid:2)-[18F]FMib and 20-[18F]-
`FMib) were evaluated in baboons [65]. Like in rats, these
`tracers also showed specific prostate uptake in baboon,
`which could be blocked by co-injection of testosterone.
`Highest prostate uptake, highest target-to-background ratios
`and highest metabolic stability were observed for [18F]FDHT.
`In fact, the defluorination rate of [18F]FDHT was 37 times
`
`OH
`
`18F
`
`OH
`
`18F
`
`OH
`
`R1
`
`R2
`
`OH
`
`