Tumor Receptor Imaging
`
`David A. Mankoff1–3, Jeanne M. Link1, Hannah M. Linden2,3, Lavanya Sundararajan2,3, and Kenneth A. Krohn1
`
`1Division of Nuclear Medicine, University of Washington, Seattle, Washington; 2Department of Medicine, University of Washington,
`Seattle, Washington; and 3Seattle Cancer Care Alliance, Seattle, Washington
`
`Tumor receptors play an important role in carcinogenesis and tu-
`mor growth and have been some of the earliest targets for tumor-
`specific therapy, for example, the estrogen receptor in breast
`cancer. Knowledge of receptor expression is key for therapy di-
`rected at tumor receptors and traditionally has been obtained by
`assay of biopsy material. Tumor receptor imaging offers comple-
`mentary information that includes evaluation of the entire tumor
`burden and characterization of the heterogeneity of tumor recep-
`tor expression. The nature of the ligand–receptor interaction
`poses a challenge for imaging—notably, the requirement for a
`low molecular concentration of the imaging probe to avoid satu-
`rating the receptor and increasing the background because of
`nonspecific uptake. For this reason, much of the work to date
`in tumor receptor imaging has been done with radionuclide
`probes. In this overview of tumor receptor imaging, aspects of re-
`ceptor biochemistry and biology that underlie tumor receptor
`imaging are reviewed, with the estrogen–estrogen receptor sys-
`tem in breast cancer as an illustrative example. Examples of
`progress in radionuclide receptor imaging for 3 receptor systems—
`steroid receptors, somatostatin receptors, and growth factor
`receptors—are highlighted, and recent investigations of receptor
`imaging with other molecular imaging modalities are reviewed.
`Key Words: tumors; receptors; imaging
`
`J Nucl Med 2008; 49:149S–163S
`DOI: 10.2967/jnumed.107.045963
`
`Cancer therapy is becoming increasingly directed and
`
`taking advantage of biologic targets that are
`specific,
`uniquely expressed or markedly overexpressed in tumors.
`Tumor receptors have been some of the earliest targets for
`cancer therapy, with notable successes in the treatment of
`endocrine-related cancers such as breast, prostate, and
`thyroid cancers (1–3). Advances in molecular cancer
`biology have revealed an ever-increasing number of tumor
`targets, many of which are receptors, such as the epidermal
`growth factor (EGF) receptor (EGFR) (4). The ability to
`measure the expression of tumor receptors is essential for
`selecting patients for receptor-targeted therapy (2). Al-
`though this information traditionally has been obtained by
`
`Received Sep. 26, 2007; revision accepted Jan. 2, 2008.
`For correspondence or reprints contact: David A. Mankoff, Radiology,
`G-2100, Seattle Cancer Care Alliance, 825 Eastlake Ave. East, P.O. Box 19023,
`Seattle, WA 98109-1023.
`E-mail: dam@u.washington.edu
`COPYRIGHT ª 2008 by the Society of Nuclear Medicine, Inc.
`
`in vitro assay of biopsy material, recent studies have
`highlighted the complementary value of tumor receptor
`imaging for measuring regional tumor receptor expression,
`which can be quite heterogeneous.
`Tumor receptor imaging emphasizes important emerging
`themes in molecular imaging: characterizing tumor biol-
`ogy, identifying therapeutic targets, and delineating the
`pharmacodynamics of targeted cancer therapy (5,6). The
`advantages of imaging include noninvasiveness, the ability
`to measure receptor expression for the entire disease burden
`and thereby to avoid the sampling error that can occur with
`heterogeneous receptor expression, and the potential for
`serial studies of the in vivo effects of a drug on the target. A
`very practical consideration is that
`imaging can assess
`receptor expression at sites that are challenging to sample
`and assay, such as bone metastases. This review discusses
`receptor pharmacology, the biology of tumor receptors, and
`special considerations for tumor receptor imaging, with the
`estrogen receptor (ER) as an illustrative example. This
`discussion is followed by highlights of recent work in the
`imaging of
`steroid receptors,
`somatostatin receptors
`(SSTRs), and growth factor receptors, as examples of 3
`different
`types of tumor receptors. These topics were
`selected from a broad range of investigations in tumor
`receptor imaging (Table 1).
`
`RECEPTOR PHARMACOLOGY
`
`Common to all receptors is the interaction of a ligand
`and the receptor, in which specific binding of the ligand to
`the receptor results in downstream biochemical or physio-
`logic changes (7,8). Ligand–receptor binding can activate
`or inhibit downstream processes through a variety of
`mechanisms, such as G-protein activation (e.g., PAR1),
`tyrosine kinase activation (e.g., EGFR), or activation of
`transcription (e.g., ER) (9–11). Ligands that cause physio-
`logic changes with receptor binding, typically the naturally
`occurring ligands, are called agonists. Ligands that bind to
`the receptor and block the binding of agonists but that do
`not activate changes are known as antagonists. Drugs
`directed toward tumor receptor systems most frequently
`use a receptor antagonist, for example, tamoxifen, which
`blocks estrogen binding to the ER. Alternatively, some drugs
`lower the level of an agonist to decrease ligand–receptor
`interactions, for example, levothyroxine, which suppresses
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`Target
`
`Small-molecule ligands
`AR
`ER
`
`PR
`Sigma-2 receptor
`
`EGFR (tyrosine kinase)
`Peptide ligands
`SSTR
`
`EGFR
`
`avb5 Integrins
`
`Bombesin receptors
`
`Monoclonal antibodies and fragments
`HER2
`
`EGFR
`
`TABLE 1
`Selected Examples of Tumor Receptor Imaging Agents
`
`Imaging probe
`
`Modality
`
`18F-FDHT (64)
`11-b-Methoxy-17-a-123I-iodovinylestradiol (179)
`18F-FES (52)
`21-18F-fluoro-16-a-ethyl-19-norprogesterone (65)
`N-(4-(6,7-dimethoxy-3,4-dihydroisoquinolin-2-1H-yl)butyl)
`2-(2-18F-fluoroethoxy)-5-methylbenzamide (180)
`11C-gefitinib (181)
`
`PET
`SPECT
`PET
`PET
`PET
`PET
`PET
`
`111In-octreotide (101)
`68Ga-DOTA-octreotide (97)
`64Cu-TETA-octreotide (100)
`111In-DTPA-EGF (121)
`68Ga-DOTA-EGF (182)
`Cy5.5-EGF (156)
`18F-galacto-RGD (183)
`RGD-USPIO (155)
`RGD-Cy5.5 (184)
`[111In-DTPA-Pro1,Tyr4]bombesin (185)
`64Cu-DOTA-[Lys3]bombesin (186)
`Bombesin-CLIO (Cy5.5) (187)
`
`68Ga-DOTA-F(ab9)2-trastuzumab (146)
`111In-DTPA-trastuzumab (133)
`Polylactic acid nanoparticle–trastuzumab (161)
`RhodG-trastuzumab (164)
`(Avidin-Gd)–biotinylated anti-HER2/neu
`monoclonal antibody (159)
`64Cu-DOTA-cetuximab (131)
`Cy5.5-cetuximab (162)
`
`SPECT
`PET
`PET
`SPECT
`PET
`Optical imaging
`PET
`MRI
`Optical imaging
`SPECT
`PET
`Optical imaging, MRI
`
`PET
`SPECT
`Ultrasound
`Optical imaging
`MRI
`
`PET
`Optical imaging
`
`USPIO 5 ultra-small superparamagnetic iron-oxide nanoparticles; CLIO 5 cross-linked iron oxide nanoparticles.
`
`thyroid-stimulating hormone levels in the treatment of
`thyroid cancer (12).
`The ligand–receptor interaction is a bimolecular chem-
`ical reaction. The concentration of the receptor is typically
`quite low; for example, the level of ER expression in breast
`cancer is in the range of 3–100 fmol per milligram of
`protein (13). Furthermore, receptor-specific ligands bind to
`receptors with a high affinity and often with a very low
`ligand–receptor dissociation rate (7). The combination of a
`low receptor concentration and a high ligand–receptor
`affinity leads to a low overall capacity for ligand–receptor
`binding. This property is helpful for drug therapy, in which
`the goal is to saturate the receptor with an antagonist to
`prevent receptor activation by an agonist. However, the
`high-affinity, low-capacity ligand–receptor binding reaction
`presents a challenge for imaging in that the number of
`molecules that can contribute to the specific receptor image
`is small. Furthermore, nonspecific binding of ligands to
`plasma proteins and nontarget tissues can limit imaging
`agent delivery and contribute to nontarget image back-
`ground. For these reasons, imaging of receptor binding, as
`opposed to imaging of an enzymatic reaction, such as
`
`glucose phosphorylation, in which it is difficult to saturate
`the uptake mechanism, is challenging. It is important for
`receptor imaging probes to have very low molecular con-
`centrations. Even small molar quantities of imaging agents
`may saturate receptors, limiting the ability to visualize
`receptor expression and increasing the background of
`nonspecific binding (14). Therefore, molecular imaging of
`tumor receptors has been mainly confined to radionuclide
`imaging (PET and SPECT), with which it is possible to
`generate images with micromolar to picomolar concentra-
`tions of imaging probes.
`It is important to note that the criteria for a suitable
`receptor imaging agent are different from those for a
`receptor-targeted drug. Although selectivity for the drug
`requires an effect on the tumor in the absence of apprecia-
`ble toxicity from nontarget tissue drug action, the require-
`ment for high target uptake and low image background in
`imaging places constraints on radiopharmaceutical selec-
`tivity and background clearance that can be even more
`stringent than those for therapeutic drugs.
`The estrogen–ER system is an illustrative example of a
`receptor system with relevance to cancer (15). The ER is
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`important in female reproductive physiology and is selec-
`tively expressed in a variety of normal
`tissues—most
`notably, breast, uterus, ovary, bone, and pituitary tissues
`(9). Estradiol is a naturally occurring agonist ligand for the
`ER. The molecular mechanism of estradiol action through
`the ER has been well studied (3,9). Estradiol is lipophilic,
`allowing access across cell membranes to the ER, a nuclear
`receptor. The ER has 2 receptor subtypes: ER-a and ER-b.
`ER-a serves mainly as an activator of downstream events
`related to breast and female sex organ function. The
`function of ER-b is less well understood; in some situa-
`tions, ER-b may inhibit ER-a by forming a heterodimer
`with ER-a (16). Estradiol binding to ER-a in the nucleus
`results in dimerization of the receptor and allows interac-
`tions with specific DNA sequences known as the estrogen
`response elements (15), leading to the selective regulation
`of target gene transcription.
`ER activation leads to different physiologic actions in
`different tissues. Much of the tissue specificity appears to
`be attributable to coregulators that interact with the ER
`homodimer and the estrogen response elements and that
`can affect the pattern of gene transcription (15). In the
`uterus, estrogens bound to the ER stimulate endometrial
`growth and are critical in maintaining a functioning uterine–
`placental unit during pregnancy. Estradiol promotes new
`bone formation and is important
`in maintaining bone
`mineral density, especially in women (9,17). Also, estro-
`gens affect the cardiovascular system, mainly through their
`beneficial effect on serum lipids. In breast tissue, estradiol
`promotes ductal epithelial cell proliferation and is a key
`component stimulating lactation. Estrogens are established
`growth factors for endometrial and many breast cancers.
`Over 70% of breast cancers express the ER, and estradiol
`and other estrogens provide a key stimulus for tumor
`growth and a target for endocrine system–based therapy
`(endocrine therapy) (2,3,18–21). Tissue-specific coregula-
`tors interact with ligand–ER dimers and may affect the
`physiologic actions of different ligands when they bind to
`the ER. For example, drugs known as selective ER modula-
`tors (SERMs) exhibit various degrees of either ER agonist
`or antagonist behavior in different tissues, an effect thought
`to be based on the differential expression of ER coregula-
`tors in different tissues (9).
`The circulating levels of agonists for the ER are variable
`but are tightly regulated in normal human physiology (17).
`The agonist estradiol has 2 sources: synthesis in the ovary
`in premenopausal women and conversion from adrenal
`steroids, mainly through aromatization (and aromatase
`enzymes) present in a variety of tissues—most notably,
`fat, breast tissue, and breast cancers (22,23). Premeno-
`pausal levels of estradiol vary, depending on the phase of
`the menstrual cycle, reaching levels as high as 500 pg/mL
`(1.7 nM) at midcycle (17). In postmenopausal women and
`men, the levels are generally less than 30 pg/mL (0.1 nM).
`Estradiol
`is very lipophilic and is generally present at
`slightly higher concentrations in tissues with higher fat
`
`contents, providing an opportunity for nonspecific uptake.
`Circulating estradiol is mainly protein bound. This binding
`occurs with a high affinity but a low capacity to sex
`hormone–binding globulin (SHBG or SBP) and with a
`low affinity but a high capacity to albumin (24,25). Much
`of circulating estradiol is bound to SHBG, and the remain-
`der is bound to albumin (24). Binding to both SHBG and
`the ER appears to be important for normal estrogen phys-
`iology and also appears to be important for ER imaging
`agents (25–27). One of the physiologic roles of SHBG
`appears to be the regulation of estrogen metabolism (28).
`Estradiol is highly metabolized in the liver to estrone and
`conjugates of both estradiol and estrone (29) and enters a
`cycle of enterohepatic circulation (30–32). Binding to
`SHBG, extraction by the liver, and reabsorption of conju-
`gated estrogens in the small intestine all play important
`roles in regulating estradiol levels in normal physiology
`(24,28).
`Considerations for imaging of the ER include the need
`for low injected molar doses to remain below physiologic
`levels (typically 30 pg/mL or higher), the need for imaging
`agents to bind to both the ER and the transport protein
`(SHBG), normal routes of estrogen metabolism and excre-
`tion, and nonspecific binding in the blood and lipophilic
`tissues (33). These considerations are discussed in more
`detail in the following sections as we examine other aspects
`of tumor receptor imaging. Overall, the complex nature of
`ER physiology emphasizes the need for a detailed under-
`standing of receptor pharmacology and physiology in de-
`veloping tumor receptor imaging approaches.
`
`BIOLOGY AND PHYSIOLOGY OF TUMOR RECEPTORS
`
`The biologic role of most receptor systems important in
`cancer is derived from their role in the tissue of cancer
`origin. In general, tumor receptors are expressed in the
`parent cell lineage and have an established physiologic
`function. For example, ER expression is essential to the
`function of normal mammary gland epithelial cells (9,15).
`When estradiol binds to ductal epithelial ER, it stimulates
`mammary gland growth, maintenance, and physiologic
`function (9,15). Many tumor receptors also play an impor-
`tant role in promoting carcinogenesis or tumor growth, as is
`the case for steroid receptors in breast and prostate cancer
`(34). The dependence on the receptor pathway for tumor
`growth makes the receptor an ideal
`target for therapy,
`because interruption of the receptor-initiated signal will
`result in a cessation of tumor growth and often tumor cell
`death (3,10).
`For many tumor receptors expressed in the normal parent
`cell lineage, the receptors are expressed at levels compa-
`rable to those in normal cells, and tumor growth stimulation
`occurs in concert with other factors contributing to the
`dysregulation of tumor cell growth. This mechanism ap-
`pears to be the case for steroid receptors, although gene
`amplification may occur in some cases (35). For other
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`receptors, overexpression of the receptors leads to aberrant
`stimulation of the signaling pathway. Examples of this
`mechanism include EGFR in lung cancer and HER in
`breast cancer (3,10,36). In this situation, aberrant expres-
`sion (overexpression) becomes an important marker for the
`activation of the pathway and predicts the likely efficacy of
`therapy directed against the target. For example, the over-
`expression of HER2 is highly predictive for a response
`to trastuzumab, a monoclonal antibody directed against
`HER2 (37). In all cases, knowledge of the levels of recep-
`tor expression, which may vary considerably in different
`tumors and even in different sites in the same tumor, is
`required to infer the likelihood that receptor-directed ther-
`apy will be effective.
`For tumor
`receptor systems, considerable variability
`exists in both ligands and receptors. Ligands may be
`naturally occurring small molecules, such as estradiol and
`testosterone, synthesized and secreted in endocrine organs,
`such as the adrenal gland, ovary, and testis, or peptides
`assembled and secreted by different types of endocrine
`cells (38). Receptors can be located on the cell membrane,
`for example, EGFR and SSTRs (36), or localized within the
`cell, for example, steroid receptors in the nucleus (9). In
`some cases, such as steroid receptors, the binding of the
`agonist ligand to the receptor is well understood, and there
`is a clear causal relationship between binding and path-
`way activation (15). In other cases, such as HER2, the
`pharmacologic significance of ligand–receptor binding in
`activating the pathway is less well understood; however,
`receptor-targeted therapy can still be effective at interrupt-
`ing the signaling pathway through antagonism (10,39).
`The approaches to tumor receptor–targeted therapy vary.
`In many situations, receptor-targeted antagonists bind to the
`receptor and block the normal agonist ligand from binding
`and activating the pathway; for example, the drugs tamox-
`ifen and flutamide block the ER and the androgen receptor
`(AR), respectively (9,40,41). Many receptor-blocking agents
`have structures relatively similar to that of the natural ligand
`and are designed to bind to the same site as the agonist; ex-
`amples are antiestrogens and antiandrogens. Blocking agents
`can also bind through immune recognition; for example, the
`agents trastuzumab and cetuximab bind to HER2 and EGFR,
`respectively (36).
`An alternative therapeutic strategy is to deplete the
`ligand. This approach has been extraordinarily effective
`for estrogens and the ER in breast cancer, for which the use
`of aromatase inhibitors has met with considerable success
`(22,23). With aromatase inhibitors, the drug is not a re-
`ceptor antagonist ligand but rather is an enzyme inhibitor
`that blocks the synthesis of the naturally occurring agonist
`ligands estradiol and estrone (22). Aromatase inhibitors
`block the conversion of adrenal steroids to estrogens, the
`major source of estrogen in postmenopausal women, low-
`ering estrogen concentrations both in the serum and in the
`local tumor environment (42). Aromatase inhibitors are
`now first-line adjuvant and primary metastatic breast cancer
`
`treatments in postmenopausal patients with ER-expressing
`tumors (43).
`Despite the importance of tumor receptors in carcino-
`genesis and tumor growth, tumor receptors are not always
`effective targets for cancer treatment, because some cancers
`can sustain growth independently of receptor activation. In
`some situations, growth independence is accompanied by a
`loss of or a reduction in receptor expression, such as in ER-
`negative breast cancers (3). In such situations, the absence
`of receptor expression indicates a negligible chance of
`success of receptor-targeted therapy. In other situations,
`even though a receptor is still present, receptor pathway
`activation is not required for growth. For example, although
`70% of breast cancers express the ER, only 50%–75% of
`ER-expressing primary breast cancers respond to endocrine
`tumors respond (3).
`therapy, and even fewer recurrent
`Redundant growth pathways may make the receptor system
`no longer necessary to sustain tumor growth. The latter
`situation appears to occur with breast cancers that both
`express ER and overexpress HER2 and that are often
`resistant to endocrine therapy, even when ER expression
`is preserved (44). These examples illustrate that although
`tumor receptor expression is necessary for a functional
`pathway, receptor expression does not necessarily guaran-
`tee the success of receptor-targeted therapy. The approach
`to patient selection is therefore sequential. The first step is
`the assay of receptor expression, because the absence of
`receptor expression invariably indicates that the receptor
`pathway is not a suitable target for therapy. After the
`selection of patients with receptor expression, the next step
`is to show that the pathway is functional and required,
`typically by assessing the response to receptor-targeted
`therapy. This clinical paradigm sets the stage for the goals
`of tumor receptor imaging.
`
`SPECIAL CONSIDERATIONS FOR TUMOR RECEPTOR
`IMAGING
`
`Tumor receptor imaging poses unique challenges for the
`design of radiopharmaceuticals and imaging approaches.
`Most receptors have high affinities for their ligands and are
`active at nanomolar concentrations of the ligands. For this
`reason, radiopharmaceuticals with high specific activity are
`essential. Even small molar quantities of an imaging agent
`may saturate a receptor and limit the ability to visualize
`receptor expression (14,33). For this reason, molecular
`imaging of tumor receptors has been most successful with
`radionuclide imaging (PET and SPECT), with which it is
`possible to generate images with nanomolar amounts of
`imaging probes. For larger molecules, such as peptides and
`monoclonal antibodies, other labels suitable for optical
`imaging, MRI, and ultrasound imaging are possible (Table
`1); however, for small-molecule receptor imaging agents,
`such as labeled steroids for steroid receptors, radionuclide
`imaging appears to be the only feasible approach. The need
`for high specific activity (ratio of radioactive to nonradio-
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`active molecules) also poses a challenge for radiopharma-
`ceutical quality control (14,33). Specific activity often
`needs to be measured before each radiopharmaceutical
`administration to ensure that a failure to visualize the
`receptor is not the result of poor tracer-specific activity.
`The choice of a label depends on the nature of the
`receptor imaging probe. For example, somatostatin imaging
`of neuroendocrine tumors entails the use of a labeled pep-
`tide closely related to the naturally occurring peptide hormone
`somatostatin (45). In this situation, the imaging molecule is
`large enough to enable the use of chelating groups and
`radiometal labels without a loss of receptor binding. A variety
`of somatostatin imaging agents have been successfully devel-
`oped with both single-photon emitters (111In and 99mTc) and
`positron emitters (68Ga and 64Cu) (46–48).
`For smaller molecules, the isotope label may signifi-
`cantly affect binding to the receptor, binding to transport
`proteins, and in vivo metabolism. In this situation, the
`choices of radionuclide and labeling position for imaging
`may be relatively limited, such as for ER imaging agents
`(14). Considerable work has been done with both single-
`photon–emitting and positron-emitting halides for ER
`imaging (49–51), but studies have suggested that 18F is
`the most attractive label for PET ER imaging (14). Fluorine
`is a small halogen in which substitutions can be made at
`several positions of the estrogen while preserving binding
`affinities for both ER and SHBG (52,53). Furthermore, 18F
`has a sufficiently long half-life to permit multistep synthe-
`sis of ligands (52,54) as well as uptake by target tissue and
`elimination by nontarget tissue during imaging (14,55); in
`addition, the use of PET permits quantitative imaging of
`regional receptor binding.
`For small molecules, the location of the label can also be
`important. For ER imaging, 18F substitution in the 16-a
`position for the steroidal analog estradiol to yield 18F-16-a-
`17-b-fluoroestradiol (18F-FES) resulted in highly selective
`uptake by target tissue, with a uterus-to-blood ratio of 39
`(52). Changing the molecule or labeling can have unex-
`pected results. For example, 18F-labeled moxestrol (18F-
`FMOX), another ER imaging agent, was developed with
`the goal of decreased metabolism and increased ER bind-
`ing. Preclinical studies in vitro and in rats demonstrated
`better ER binding in vitro and increased uterine uptake in
`immature rats for 18F-FMOX than for 18F-FES (26,56).
`However, 18F-FMOX performed poorly in human studies.
`The explanation for these findings was that poor binding of
`18F-FMOX to the steroid transport protein, SHBG, likely
`limited its utility in humans. Rats lack SHBG (25); there-
`fore, 18F-FMOX was an effective imaging agent in rats. In
`this example, a change in the imaging molecule that
`promoted increased ER binding unfavorably altered the
`binding to SHBG, resulting in a compound with poorer per-
`formance. This example illustrates the demanding nature of
`radiopharmaceutical design for tumor receptor imaging and
`the need for validation at each step of development, from
`the laboratory bench to the bedside.
`
`For these reasons, considerable preclinical work and early
`testing in patients are necessary to develop and validate
`receptor imaging agents (33). In vitro studies must confirm
`the high-affinity receptor binding of a radiopharmaceutical
`as a starting point for development. Subsequently, in vitro
`and in vivo animal models must demonstrate that binding is
`specific to the receptor and that an excess of the nonlabeled
`natural
`ligand, or a suitable substitute specific for the
`receptor, can displace or block the binding of the radiophar-
`maceutical. In vivo clearance, metabolism, and biodistri-
`bution in preclinical models and early patient studies must
`confirm sufficient tracer clearance to visualize uptake in
`tumors but sufficiently slow blood clearance and metabo-
`lism to permit uptake in receptor-rich tissues. Defining the
`nature of labeled metabolites is also important, because
`some metabolites may bind to the receptor, whereas others
`may not. For example, in humans, 18F-FES-labeled metab-
`olites are present mostly in the form of conjugates that do
`not bind to the receptor or have access to the nuclear
`receptor. These metabolites therefore contribute to nonspe-
`cific image background (27,55,57). In addition, assessment
`of nonspecific binding is important; nonspecific binding
`must be sufficiently low to avoid interference with the
`visualization and quantification of tumor uptake at target
`sites.
`Receptor imaging poses some additional challenges for
`image acquisition and analysis. Because the absence of
`receptor expression may be even more important than the
`presence of a receptor, the imaging approach must be able
`to quantify low levels of radiopharmaceutical uptake and
`reliably identify situations in which a tumor is present but
`the uptake of a receptor imaging agent is low or absent.
`This approach requires multimodality imaging. Combined
`functional imaging and anatomic imaging, such as PET/CT
`or SPECT/CT, may be essential for localizing tumor sites
`at which the uptake of a receptor imaging probe can be
`quantitatively interrogated. Because it may be difficult to
`identify active tumor sites by anatomic imaging alone, it
`may be necessary to align tumor receptor images with
`images obtained with another tumor imaging probe, such as
`18F-FDG, to identify viable tumor. We used this approach
`for the ER imaging of breast cancer; 18F-FDG PET iden-
`tified sites of active breast cancer at which to evaluate ER
`expression by 18F-FES PET (Fig. 1). It may be beneficial to
`coregister different functional
`images; for example, for
`PET/CT and SPECT, the anatomic images may be used as
`the basis for coregistration. Finally, because radiopharma-
`ceutical uptake may be at or close to background uptake in
`nontumor tissues, accurate correction for the imaging of
`physics-related background counts, such as scattered pho-
`tons, is critical.
`issue is separating the effects of
`Another potential
`imaging probe transport and binding in determining the
`overall image. It is possible that transport barriers will limit
`the in vivo access of the imaging probe to the site of
`tumor receptor expression. Therefore, it may be difficult to
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`A
`
`Before Therapy
`
`After Therapy
`
`"' • •
`
`B
`
`18F-FES
`
`18F-FDG
`
`18F-FDG
`
`FIGURE 1. Examples of different pat-
`terns of ER expression measured by 18F-
`FES PET. Both patients had bone
`metastases arising from ER-expressing
`primary tumors, and both were treated
`with endocrine therapy. (A) For this pa-
`tient, pretherapy 18F-FDG and 18F-FES
`PET scans showed 18F-FES uptake at all
`sites of active disease seen by 18F-FDG
`PET. Follow-up 18F-FDG PET scan
`showed response to treatment after ini-
`tiation of aromatase inhibitor
`therapy.
`(B) For this patient, there was no uptake
`at site of disease seen on 18F-FDG PET
`scan (small arrow). Follow-up 18F-FDG
`P E T s c a n s h o w e d s u b s e q u e n t
`disease progression (small arrow) with
`endocrine therapy. (Reprinted with per-
`mission of (84).)
`
`determine from a single static image whether the absence of
`radiopharmaceutical uptake at a tumor site is attributable to
`a lack of receptor expression or to a lack of delivery of the
`imaging probe. This is less likely to be an issue for small,
`lipophilic molecules, such as steroid imaging agents, but
`may pose an important consideration for peptides and
`especially monoclonal antibodies. In such situations, more
`sophisticated, dynamic imaging acquisitions may be re-
`quired, along with more detailed and kinetic analyses.
`
`EXAMPLES OF TUMOR RECEPTOR IMAGING
`
`Steroid Receptors
`Steroid receptor targets in cancer include the ER and the
`progesterone receptor (PR) in breast cancer and the AR in
`prostate cancer. We first briefly review experience in AR
`and PR imaging and then discuss ER imaging, for which
`the most experience with steroid receptor imaging has been
`obtained.
`Parallel to efforts for ER imaging, a variety of com-
`pounds have been developed for PET of the AR; these have
`been directed mainly toward prostate cancer imaging (58–
`64). AR imaging has proved to be somewhat more chal-
`lenging than ER imaging, perhaps because of the relatively
`tighter binding of androgens than of estrogens to SHBG;
`the latter property may limit the delivery of the imaging
`agent over the time scale of PET, even though tight binding
`
`of the AR imaging agent to SHBG appears to be important
`in generating AR images (25,58). Preclinical studies in
`baboons, which have SHBG similar to that in humans,
`16-b-18F-fluoro-5-a-dihydrotestosterone
`indicated
`that
`(18F-FDHT) was a promising compound for PET (58).
`Early studies with 18F-FDHT showed promise for the
`imaging of regional AR expression in prostate cancer
`(59,61). Larson et al. (61) reported significant 18F-FDHT
`uptake in 7 patients with prostate cancer. Like that of 18F-
`FES, 18F-FDHT metabolism was rapid, with 80% of radio-
`activity in the blood in the form of metabolites at 10 min
`after injection. Cancer treatment diminished 18F-FDHT
`uptake in the subset of patients re-imaged after therapy in
`that study. Dehdashti et al. (59) studied 20 patients with
`prostate cancer by 18F-FDHT PET and found evidence of
`uptake in 63% of the patients; 18F-FDHT PET revealed a
`substantial number of tumor sites that had not been iden-
`tified by conventional imaging. Furthermore, in a subset of
`12 patients who were re-imaged 1 d after the initiation of
`flutamide therapy, the imaging demonstrated a significant
`(.50%) average decline in 18F-FDHT uptake, indicating
`the ability to measure the pharmacodynamics of treatment
`with receptor antagonists. These encouraging results sup-
`port the feasibility of PET AR imaging, and ongoing studies
`support the promise of imaging of AR expression in local-
`izing prostate cancer and possibly predicting the response
`to antiandrogen therapy.
`
`154S
`
`THE JOURNAL OF NUCLEAR MEDICINE (cid:129) Vol. 49 (cid:129) No. 6 (Suppl) (cid:129) June 2008
`
`Petitioner GE Healthcare – Ex. 1028, p. 154S
`
`

`

`Efforts to image the PR have been less successful (56,65).
`In a study of 8 patients with primary breast cancer,
`found that 21-18F-fluoro-16a-ethyl-19-
`Dehdashti et al.
`norprogesterone was taken up in some tumors, but the level
`of uptake did not correlate with the level of PR expression
`(65). This result may have been attributable in large part to
`the relatively low affinity of progestins for the PR, with
`binding affinities that are orders of magnitude lower than
`those of androgens for the AR and of estrogens for the ER
`(56). Therefore, relatively high nonspecific binding com-
`pared with specific binding of imaging probes may limit
`their utility for PR imaging. Later studies also showed that
`the radiopharmaceutical
`tested had rapi