The Role of PET/CT and SPECT/CT in Oncology Drug Development

Draft Date: October 25, 2022 PET and SPECT/CT

The Role of PET/CT and SPECT/CT in Oncology Drug Development

Authors:

*Karen A Kurdziel, MD; [email protected]; 301-443-0622

Liza Lindenberg, MD; [email protected]

Esther Mena, MD; [email protected]

Baris Turkbey MD; [email protected]

Peter Choyke, MD; [email protected]

Affilliation (all)

Molecular Imaging Program, Center for Cancer Research

National Cancer Institute, National Institutes of Health

10 Center Drive; B3B

Bethesda, MD 20892

*Corresponding Author

i

mailto:[email protected]






Keywords:

Molecular Imaging

Oncology

PET/CT

Prostate Cancer

Radioimmunoscintigraphy

SPECT/CT

ii


Abbreviations

18F-FACBC18F-anti-1-amino-3-fluorocyclobutane-1-carboxylic acid

18F-FDG 2-deoxy-2-[18F]fluoro-D-glucose

5-FU 5-fluorouracil

ADCC antibody-dependent cell cytotoxicity

BPH Benign Prostate hypertrophy

CT Computed Tomography

FdUrd Fluoro-2’-deoxyuridine

Furd Fluorouridine

IHC immunohistochemical

MRI Magnetic Resonance Imaging

PET Positron Emission Tomography

PSMA Prostate Specific Membrane Antigen

SPECT Single Photon Emission Tomography

SUV Standardized Uptake Value

TCA TriCarboxylic Acid

THU tetrahydrouridine

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Abstract

Understanding the hallmarks of cancer is valuable in advancing drug development to fight the

disease. With the advent of molecular imaging, non-invasive in vivo visualization of physiology

and functional pathways allow for multiple levels of disease and therapy assessment beyond

tumor size measurements offered by conventional anatomic scans. Radiolabeling targeted

molecules can provide insight into tumor interactions within the body and may help guide

therapeutic constructs. Traditionally, radiolabeled probes have been useful for staging and

diagnosis but they can also be created to monitor and identify drug targets and pharmaceutical

biodistribution among other novel approaches. Radiotracers can be imaged either through

single photon emission computed tomography (SPECT) or positron emission tomography (PET),

with advantages and limitations to both. Adding CT to either modality is an added benefit for

anatomic localization and attenuation correction. In this review, we will briefly discuss several

molecular imaging agents, which target metabolism (18F-FDG, 18F-FACBC, 11C acetate),

proliferation (18F-FLT, 18F-5FU, 18F-FdCyd), and cancer specific drug receptors (111In-morab009,

111In-trastuzumab, 89Zr-panutumumab). Molecular imaging is a powerful tool that one day, may

be employed to personalize treatment and dosing strategies and predict anticancer therapy

outcomes.

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Introduction

In 2000, Hanahan and Weinberg proposed six “Hallmarks of Cancer”, which

include: achieving limitless replicative potential, sustaining

angiogenesis, evading apoptosis, establishing self-sufficiency in growth

signals and insensitivity to anti-growth signals, and developing tissue

invasion and metastasis capabilities[1].These hallmarks identified

features of cancer cells that set them apart from normal cells. In 2011,

Hanahan and Weinberg updated their original hallmarks with additional

features, reflecting advances in knowledge since the original article

[2]. Most cancer therapies target one or more of these processes, yet

response is still typically assessed simply by anatomic changes (i.e.

changes in tumor diameter). While conventional anatomic imaging depicts

these architectural changes with high fidelity, these are often late

findings and provide no insight into the mechanism by which the physical

changes were achieved. Molecular Imaging offers the possibility of non-

invasive in vivo visualization of physiology and functional pathways at

the cellular level. This provides a means for real time assessment of

the molecular techniques by which tumors succumb to and evade targeted

therapy.

Radiolabeling targeted molecules is a highly sensitive method of

detecting specific attributes of a tumor. Moreover, the biodistribution

and kinetics of the uptake of the targeted agent carry additional

information. There are two types of radioactive isotopes commonly used

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for in vivo molecular imaging probes. Single photon emission decay

results in the production of charateristic gamma rays that can be

detected either with a planar “gamma camera” or single photon emission

computed tomography (SPECT), which provides cross sectional imaging.

The other type of radioactive isotope is a positron emitter, which emits

positron that rapidly undergoes an annihilation reaction with an

electron resulting in two gamma rays per radioactive decay,

approximately 1800 apart. The coincidence circuitry of a positron

emission tomography (PET) camera takes advantage of these physical decay

properties, giving it intrinsic advantages in sensitivity and resolution

compared with SPECT, but both are still in wide use. They are both

often combined with computed tomography (CT) to provide both molecular

and anatomic imaging fused into one study. The differences between

SPECT and PET will be discussed in more detail in a later section.

Unlike drugs that are administered at therapeutic levels, the high

sensitivity of PET and SPECT cameras means that radiolabeled probes can

be administered at doses 1/100th to 1/1000th of the therapy dose, a

phenomenon that has given rise to the term “tracer” to describe these

agents. The chemical composition of the radiotracer determines what

functional process can be monitored. Routine clinical radiotracer

imaging studies using single photon emitting nuclides include: 99mTc

sestamibi or tetrofosimin for cardiac perfusion, 99mTc-

methylenediphosphonate (MDP) for bone imaging, 99mTc DTPA or MAG3 for

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renal function, 99mTc mebrofenin for hepatobiliary imaging, 123I for

thyroid disease, 111In octreotide and 123/131I MIBG for neuroendocrine tumor

imaging and 111In-granulocyte imaging for infection, among others. For

PET imaging, 18F-fluorodeoxyglucose (FDG), a glucose analog, has become

part of the routine clinical work-up for many cancers. 18F-Sodium

Fluoride bone scans are also emerging as PET tracers that provide highly

sensitive information on the status of bone metastases.

While most radiotracers are directed at diagnosis and staging, more

recently, radiotracers are being developed to monitor specific pathways,

identify potential drug targets, determine if a drug is hitting the

desired target, discriminate between premalignant and malignant states,

guide appropriate therapy and risk stratify patients. In this review, we

will briefly review several molecular imaging agents, which target

metabolism (18F-FDG, 18F-FACBC, 11C acetate ), proliferation (18F-FLT, 18F-

5FU, 18F-FdCyd), and cancer specific drug receptors (111In-morab009, 111In-

trastuzumab).

PET or SPECT?

As previously mentioned, radiotracers used clinically are generally

categorized as either gamma emitters or positron emitters. Radiotracers

which undergo gamma decay, emitting one or more characteristic gamma

photons, which can be imaged with a gamma camera (also known as the

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Anger camera, named after its inventor, Hal Anger [3], who designed it

in 1957 at the University of California, Berkeley). The gamma camera’s

circuitry is able to estimate the location from which gamma photons are

released and differentiate among the energies of the photons, allowing

it to selectively image photons of a pre-defined energy distribution.

This permits simultaneous imaging of several gamma isotopes, or the

detection of only photons with a characteristic energy arising from a

radioisotope of interest. A gamma camera consists of one or more flat

scintillation detectors optically coupled to an array of photomultiplier

tubes (which amplify the signal). A major limitation of gamma cameras is

their inability to accurately localize photons that are not

perpendicular to the detector. To improve photon localization, lead

collimators are used to restrict access to the detector, allowing only

perpendicular photons to reach the detector face. The tradeoff is a

large reduction in the number of detectable photons (<1% of the emitted

photons reach the detector) and a limited spatial resolution of 1-2 cm.

The development of tomographic gamma cameras, designated single-photon

computed tomography (SPECT), enabled 3-dimensional imaging with

tomographic sectioning of the body, similar to computed tomography but

at much lower spatial resolution.

In contrast to single photon emitters, a positron (anti-electron) of a

characteristic energy is emitted as anti-matter from the nucleus of a

PET radioisotope. While the energies of the positrons differ among PET

4


isotopes, this energy determines the distance it can move before

annihilating, hence influencing the achievable spatial resolution for a

particular PET emitter. The emitted positron is short lived and rapidly

encounters an electron, which causes matter-anti-matter annihilation

resulting in the production of two 511-keV photons, emitted in

approximately opposite directions. It is the detection of these photons

which creates a PET image. A positron emission detection camera

(positron emission tomography or PET) incorporates a coincidence

detection circuitry that only collects photons of ~511 keV which reach

the detector more or less simultaneously. This creates a series of lines

of response that can be used to generate an image but does not require

collimation. This results in a substantial (~10 fold) improvement in

sensitivity for PET imaging. While positron emitting radioisotopes can

be imaged with gamma cameras (using high energy collimators which

further limit the number of photons reaching the detector), the use of

PET technology provides for higher sensitivity and resolution imaging

when compared with SPECT. This does however, come at the expense of more

costly equipment. The cost of detector materials in PET scanners is also

high and thus, in order to reduce costs, a smaller field of view is

utilized compared to SPECT. Nonetheless, the increased sensitivity of

PET allows for faster counting at each “station” allowing PET to be even

more efficient in scanning times than SPECT.

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Despite the benefits of PET technology, gamma camera imaging retains its

strengths. The choice of modality is dependent on the clinical question,

radiotracer availability, and the chemistry involved in radiolabeling

the desired compound. The inclusion of CT within the camera gantry is

available for both modalities and allows for both tumor localization and

a means of attenuation correction [4, 5], which are both necessary for

quantitating tumor uptake.

Imaging Tumor Metabolism

2-deoxy-2-[18F]fluoro-D-glucose (18F-FDG)

2-deoxy-2-[18F]fluoro-D-glucose (18F-FDG) is the most widely used

clinical PET radiopharmaceutical. As a glucose analog, its uptake

correlates with glucose transport and metabolism. Due to the high

glycolysis rates of many tumors, 18F-FDG PET/CT has enabled oncologists

to better diagnose, stage and monitor many malignancies. However many

benign conditions (i.e. inflammation) also show increased 18F-FDG

uptake, and not all malignancies demonstrate up-regulated glucose

metabolism, thus are not well served by 18F-FDG PET/CT. Prostate cancer,

in particular, exhibits variable uptake of 18F-FDG making it unreliable.

Several investigational PET radiotracers targeting alternative metabolic

pathways, including 11C-acetate [6, 7] and 18F-anti-1-amino-3-

fluorocyclobutane-1-carboxylic acid (18F-FACBC) [8-10], have shown some

promise in prostate cancer imaging.

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11C-acetate

As a common metabolic intermediate, radiolabeled acetate has been used

as a biomarker of lipid and cholesterol synthesis for many years. [11]

{Howard, 1977 #31} Recent studies have shown that 11C-acetate PET/CT

imaging may be useful in patients with certain malignancies, including

prostate cancer [12]. 11C-acetate is a naturally occurring compound that

is converted to acetyl-CoA, which is a substrate for the tricarboxylic

acid (TCA) cycle. It then becomes incorporated into cholesterol and

fatty acids. The mechanism by which 11C-acetate is taken up in cancer

cells is still unclear, but it is postulated that acetate is involved in

cytoplasmic lipid synthesis and becomes incorporated into the cell

membrane of tumor cells. Because prostate cancers rely more on fatty

acid metabolism than glycolysis, 11C-acetate is a promising method for

identifying prostate cancer. In a pilot study, Oyama et al. [13] imaged

22 prostate cancer patients with 11C-acetate, and showed accumulation of

the tracer in all primary tumors, albeit with variable uptake values

(Standardized Uptake Value (SUV) ranging from 3.27 to 9.87), and 11C-

acetate was more sensitive than 18F-FDG PET (SUV ranging from 1.97 to

6.34). Furthermore, 11C-acetate was superior for the detection of lymph

node metastases, demonstrating increased uptake in five patients. 18F-

FDG was positive in only two of the five patients. However, using

immunohistochemistry, a recent study found [7], no correlation between

11C-acetate uptake and fatty acid synthase, which is the enzyme that

7


converts acetate to acetyl-CoA, casting some doubt on the mechanism by

which 11C-acetate is taken up in prostate cancer.

In radiotracer imaging, the physiological distribution of the tracer

plays a large role in the clinical utility. In humans, 11C-acetate PET

imaging shows accumulation/excretion of tracer in the pancreas and

intestines and thus, has minimal bladder uptake, facilitating pelvic

imaging, unlike 18F-FDG which is excreted by the kidney into the

bladder.

While 11C-acetate uptake appears to be greater in the tumor than in the

normal tissue, acetate is not cancer specific. In one study, [7] there

was a significant difference (p<0.001) between tumor uptake (mean SUV of

4.4) and normal prostate tissue (mean SUV 2.1) but there was also

significant overlap in uptake values between benign prostate hypertrophy

(BPH) and tumor foci; therefore focal uptake does not necessarily

indicate tumor. Despite this non-specificity, most of the 11C-acetate

false positive findings were in regions of BPH which were correctly

categorized by MR. Fig. 1 Thus, a combination of these technologies (via

sequential imaging or using the recently developed PET/MR camera) may

overcome this limitation. In addition to accumulation in BPH, Kato et

al. [14] found the physiologic uptake of 11C-acetate to increase with

age. Metabolic changes in prostate tumors following androgen ablation

have also been studied in vitro with a marked suppression of 11C-acetate

uptake observed after anti-androgen therapy. [15]

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In the evaluation of prostate cancer recurrence, 11C-acetate showed a

higher sensitivity than 18F-FDG for local and regional lymph node

metastases. In the study by Sandblom et al. [16] in 20 patients with

increasing PSA after radical prostatectomy, recurrence was detected with

11C-acetate in 75% of the patients while 15% of the cases showed false-

positive uptake. While11C-acetate PET can detect both primary and

metastatic prostate cancer, its widespread clinical use is greatly

hindered by the short half-life of 11C (~20 minutes), which limits its

availability and restricts its use to facilities with on-site cyclotrons

and radiochemistry laboratories.

18F FACBC

Amino acids are clearly an important part of tumor metabolism and are

required for growth of the tumor. Several synthetic peptides have been

synthesized and radiolabeled in an attempt to exploit the increased

transport of amino acids in cancer for imaging purposes. The active

pharmaceutical ingredient of [18F]-FACBC is 1-amino-3-fluorocyclobutane-

1-carboxylic acid in the ‘anti’ configuration. This is a synthetic

amino- acid L-leucine analogue [17, 18] whose uptake by prostate

carcinoma cells is likely mediated via sodium-dependent amino acid

transporters. [19] 18F -FACBC, is not incorporated into proteins, but is

thought to accumulate in cancer cells, making it a promising molecular

imaging agent. It was initially reported to have utility in the

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evaluation of primary and metastatic cancers in the brain, [20, 21] due

to its low brain background uptake and high uptake by brain tumors. More

recently, it has been reported to be useful in assessing metastatic or

recurrent prostate cancers in the prostate bed, lymph nodes and bone.

[8-10, 22-24] Nye et. al. [25] reported the biodistribution of 18F -

FACBC in 6 healthy volunteers, showing slow bladder accumulation, with

only about 3.2% (range: 1.1% to 7.4%) urinary tract excretion. Unlike

18F-FDG which rapidly accumulates in the bladder and ultimately

demonstrates much higher uptake than the prostate (leading to

reconstruction artifacts), the concentration of 18F -FACBC in the

bladder approaches that of the prostate (at ~15 minutes), enabling good

differentiation between the two(Fig. 2).

A pilot study of 18F-FACBC conducted by Emory University included

15 subjects with recently diagnosed or recurrent prostate cancer. [10]

After dynamic acquisition of 18F-FACBC images, correct identification of

focal malignancy was achieved in 40 of 48 prostate sextants: moreover,

recurrence was successfully detected in all four subjects with recurrent

disease. Pelvic nodal status correlated with 18F-FACBC findings in 7 of

9 subjects. Even though occasional low-level uptake was seen in benign

inguinal lymph nodes, (as it can also be seen in 11C-acetate imaging),

the maximum SUV of malignant adenopathy was significantly higher than

that of uninvolved nodes. Fig.3 depicts uptake in benign lymph nodes in

patients who were imaged with 11C-acetate or 18F-FACBC. With regard to

evaluating localized prostate cancer, similar to 11C-acetate, there is

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overlap in the uptake in BPH nodule and tumor, making it impossible to

differentiate using PET alone. When spatially compared with MRI; however

BPH nodules can be clearly identified. Fig.4

More recent data from Schuster et al., [9] in the setting of biochemical

failure (rising PSA following prostatectomy or definitive radiation),

suggests that 18F-FACBC (sensitivity of 89% in the prostate bed)

outperforms the currently FDA-approved SPECT tracer Prostascint

(sensitivity of 69% in the prostate bed), which binds to an

intracellular prostate specific membrane antigen (PSMA) receptor.

Imaging Proliferation

Unlimited proliferation is a hallmark of cancer that has been the target

of therapy development for decades. Thus, there is great interest in

assessing cellular proliferation. Most methods focus on increased DNA

synthesis. The radiotracer most widely used to study proliferation is

3'-deoxy-3'-18F fluorothymidine (18F-FLT) [26-28].

18F- FLT is a structural analog of the DNA nucleotide, thymidine.

Normally, thymidine is taken up by cells and phosphorylated, a process

mediated by thymidine kinase, before being incorporated into DNA.

Analogs of thymidine, such as FLT, behave similarly but instead of being

incorporated into DNA, accumulate in the cytoplasm. As such, 18F-FLT

has the potential to reflect tumor proliferation. Studies have shown

that 18F-FLT uptake correlates with Ki-67 (rho=0.83) [29] and, to a

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lesser extent with thymidine kinase, rho=0.65 the putative enzyme for

trapping 18F- FLT within the cell; however it does not correlate well

with thymidine kinase enzymatic activity. [30]

Data suggest that 18F-FLT uptake is reduced in inflammation, potentially

making it a useful adjunct for 18F-FDG-positive lesions suspected of

being false positives [31, 32]. Thus, 18F-FLT PET/CT is being studied in

a variety of cancer types to monitor therapy and differentiate benign

inflammatory conditions from residual malignancy. Preliminary data shows

that 18F- FLT PET/CT has a higher specificity, when compared with 18F-

FDG, in lymphoma [33], lung cancer [34, 35], brain tumor [36, 37] and

head and neck cancers [38-40]. Preliminary data from patients with

lymphoma supports this concept. Fig. 5 depicts a patient with a history

of lymphoma and an 18F-FDG-positive lesion, which was negative on 18F-FLT

PET/CT and was confirmed by biopsy to be inflammation.

As cancer therapies often target DNA synthesis, radiopharmaceutical

imaging provides a method to visualize the distribution of such drugs.

The goal of drug therapy is to deliver an adequate amount of active drug

to the tumor while minimizing exposure to normal tissue. Once the active

drug is in the bloodstream, delivery to the tumor and normal tissues is

initially defined by relative perfusion. Drug penetration in solid

tumors can be limited by factors influencing perfusion such as high

interstitial pressure, vascular heterogeneity and regional necrosis.

Once the drug reaches the tumor, it may or may not specifically

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accumulate. Imaging radiolabeled drugs provide a means to non-invasively

visualize the drug distribution based on perfusion and retention over

time and can thereby, estimate tumor drug concentrations.

5-fluorouracil (5-FU)

5-fluorouracil (5-FU) was introduced as a member of a class of antitumor

fluoropyrimidines in 1957 [41] and remains in use today. Its major

cytotoxic mechanisms include inhibition of thymidylate synthetase and

interference with DNA and protein synthesis, as a result of conversion

to a ribose nucleoside and substitution into RNA [42].

5-FU was radiolabeled with 18F and its uptake correlated with response

to 5- FU therapy [43]; however, 5-FU is rapidly catabolized in vivo into

metabolites that also demonstrate efficacy. Because PET imaging cannot

differentiate between the parent radiopharmaceutical and its

radiolabeled metabolites 18F-5-FU is limited as an imaging agent [44,

45].

18F-5-Fluoro-2’-deoxycytidine (18F -FdCyd)

The nucleoside analogue 5-Fluoro-2’-deoxycytidine (FdCyd) is a DNA

methyltransferase inhibitor currently being evaluated clinically;

however, the therapeutic benefits are only observed in a minority of

patients. Most patients experience toxicity, so designing a patient

selection strategy that identifies likely responders is important [46].

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By establishing a method of evaluating the biodistribution of the drug

and its ability to be taken up by tumor cells, 18F -FdCyd may improve

patient selection.

PET imaging of the pyrimidines has been hindered by background

radioactivity produced by catabolism. In vivo, FdCyd is rapidly

metabolized into 5-Fluoro-2’-deoxyuridine (FdUrd), 5-Fluorouracil (FU),

and 5-Fluorouridine (FUrd) [47], all of which cause toxic side effects

and all of which are radiolabeled. Co-administration of

tetrahydrouridine (THU) inhibits the converting enzyme, cytidine

deaminase, [48] and impedes this catabolism.

By co-administering radiolabeled 18F-FdCyd and THU, the rapid metabolism

of FdCyd is inhibited and PET imaging can be used to estimate FdCyd

delivery to the tumor. As the specific activity (MBq/ug) of the

radiopharmaceutical injected is known, measuring the total tumor

activity (MBq) over time provides an estimate of the total drug

accumulation.

Imaging with 18F -FdCyd may confirm tumor uptake and thus help identify

patients who are responsive to the parent drug, FdCyd. In cases of

treatment failure, 18F -FdCyd can determine if failure is due to

inadequate drug delivery or poor tumor retention. Fig. 6 shows the

tumor uptake of FdCyd when co-administered with THU.

Tumor Targeted Agents

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111In-CHX-A”-DTPA-trastuzumab

The most common forms of tumor-targeted imaging are antibody based.

Trastuzumab (Herceptin, Roche Laboratories) was one of the first

receptor specific antibodies introduced as a therapy. Trastuzumab

targets the HER2 receptor, a 185 kD membrane-bound receptor which

interacts with members of the EGFR family and other ligands [49]. These

receptors stimulate intracellular signal transduction pathways involved

in the control of cell growth. HER2 is over-expressed in a variety of

epithelial tumors; approximately 25-30% breast and ovarian cancers, 35-

45% of all pancreatic adenocarcinomas and up to 80% of colorectal

carcinomas [49, 50]. In a study of >100 human tumor xenografts, over-

expression of HER2 was also shown to be common in uterine cervix (67%),

gastric (75%), prostate (50%) cancers and adenocarcinoma of the lung

(67%). Over-expression of this oncogene is associated with poor

prognosis and aggressive tumor attributes [51, 52]. As a humanized

monoclonal antibody targeting the HER2 receptor, trastuzumab gained FDA

approval in 1998 for the treatment of advanced stage HER2 over-

expressing tumors.

Trastuzumab as a monotherapy has resulted in a response rate of 12-20%

in metastatic breast cancer participants [53]. The therapeutic efficacy

of trastuzumab has been improved when combined with chemotherapy [54,

55]. In spite of these apparent benefits, large numbers of patients fail

to respond to treatment and/or relapse after a short period on therapy.

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Patient selection in these trials is based on immunohistochemical (IHC)

staining of biopsy samples for HER2 expression. Patients with a scoring

of 2+ or 3+ are selected for treatment with trastuzumab [49, 56]. A

consequence of this screening criterion is that only a low percentage of

patients may be eligible for trastuzumab therapy and tumors that don’t

express HER2 to a significant extent are unlikely to respond to therapy.

For example, of the 1480 patients screened in one clinical trial, only

222 patients (15%) met the 2+/3+ scoring criteria while 280 (18.9%)

patients presenting with a score of 1+ were ineligible. [57] Thus, in

this study approximately 30% of patients expressed IHC detectable HER2

within their tumors. But, the existing methods of establishing HER2

expression have limitations. The IHC assessment of tumor samples is

somewhat subjective and some patients with zero or 1+ expression

actually have higher expression levels than were previously thought.

Fluorescent in situ hybridization (FISH) results are stated

ratiometrically rather than subjectively by the 1+, 2+, 3+ designation.

The ability to image trastuzumab could define the extent of HER2

expression in primary lesions but also define secondary lesions with

HER2 expression.

To create an imaging agent from trastuzumab, the antibody is chelated

with a chemical linker such as CHX-A’’-DTPA [58] to enable the

attachment of radioisotope. Because of its long clearance time,

trastuzumab has been radiolabeled with a long acting radioisotope, 111In

(111In-CHX-A”-DTPA-trastuzumab). Initial human studies demonstrated high 16


uptake in metastatic foci for patients whose primary tumor expressed

HER2. Fig. 7 While the uptake is appreciated on planar views, it is

better localized on the SPECT images. Due to the limited quantitative

capabilities of SPECT, estimates of tumor drug concentration can only be

made if there is a “standard” aliquot of the radiolabel, of known

activity (measured in a dose calibrator), placed in the imaging field of

view. By measuring the number of counts in the tumor on the SPECT image,

it is possible to manually calculate the tumor activity using the

measured counts obtained from imaging the standard and the known

activity of the injected dose. Further normalization to the patient

weight results in a SPECT standard uptake value (SUV) equivalent

measure, which permits comparison between different imaging sessions and

among different patients. Using a very similar chelated antibody, it is

possible to treat such tumors with radioimmunoconjugates [59, 60] or

other receptor targeted cytotoxic “payloads”[58, 61] which may enable

unique treatment options for tumors with lower expression of HER2.

111In-MorAb009

The same chelating conjugate has also been applied to another targeted

IgG monoclonal antibody, amatuximab (Morphtek), which targets the cell

surface marker, mesothelin. Mesothelin is a cytoplasmic membrane

glycoprotein believed to be involved in cell adhesion and tumor

metastasis [62]. While low levels of this glycoprotein are also noted in

some normal tissues, marked over-expression is found in many tumors,

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particularly pancreatic adenocarcinomas and mesotheliomas. Malignancies

associated with mesothelin expression generally have poor prognosis.

Preclinical studies demonstrating antibody-dependent cell cytotoxicity

(ADCC) with amatuximab indicate strong potential as an anticancer agent,

and clinical studies are underway. The radiolabeled construct, 111In-CHX-

A”-DTPA-amatuximab, [63] demonstrated good tumor uptake in mice and it

is predicted that SPECT imaging of mesothelin distribution may provide

insight to the antibody’s efficacy and enhance dosing strategies for

amatuximab.

Antibody binding to shed antigen in the circulation can decrease

radiotracer availability for the target tumor, so initial studies were

performed by pre-injecting un-labeled (“cold”) antibody to saturate this

shed mesothelin. Based on prior experience with other antibodies, this

was expected to improve the likelihood of radiotracer binding to tumor

(as opposed to the shed fraction). The addition of small amounts of

cold antibody may also increase tumor specific binding by occupying non-

specific binding sites throughout the body. The amount of cold antibody

to be co-administered for optimal imaging is difficult to predict. Too

much cold antibody can saturate the tumor specific binding sites,

leaving none open for the tracer dose of radiolabeled antibody. Pre-

administration of cold antibody also confounds the measurement of

amatuximab tumor binding as only the radiolabeled antibody is imaged,

possibly resulting in a large fraction of the binding potential not

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imaged. In preliminary clinical studies, while high uptake was found in

some tumor sites, the uptake in other tumors known to express mesothelin

was low [Fig. 8] suggesting that too many of the tumor specific binding

sites are occupied by the non-radiolabeled drug.

Conclusion

Radiotracers have the potential to advance knowledge of the complex

methods cancer employs to survive in the body. Not only can imaging

provide valuable insight into the quantitative biodistribution of

anticancer therapies, it may allow for personalized treatment and dosing

strategies. Radiotracers allow non-invasive interrogation of the tumor.

The choice of radiolabeled drug, imaging modality and complexity of the

image data analyses are dependent on the question to be answered.

However, before radiotracers can be considered “surrogate” biomarkers

(i.e. have the ability to predict outcome), the imaging data they

provide must be shown to be reliably quantifiable, reproducible, and

relate to relevant physiological processes. There are numerous targeted

radiotracers under evaluation (just a handful were discussed in this

review) and progress is being made towards validating them as surrogate

imaging markers, which should lead to the successful integration into

the drug development paradigm.

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20


Table 1 Comparison of the imaging properties of PET and SPECT

SPECT PET

Multiple Energy Imaging (multiple

isotopes)

Only images positron emitting

isotopes

Lower cost Higher cost

Larger FOV Limited FOV

~1.5 cm spatial resolution <1cm spatial resolution

Low sensitivity due to collimation Higher sensitivity (no collimators)

Attenuation Correction optional Attenuation Correction included

Lower isotope energiesless hand

dose to technologists

511 keV high energy photons, more

radiation safety concerns

Semi-quantitative (counts/pixel) Quantitative (Bq/ml)

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FIGURE 1:

MRI (top), 11C-acetate PET (middle) and Fused PET/CT (bottom) axial

images of the prostate in a patient with prostate cancer. Crosshairs and

red arrow show the histology confirmed tumor, while the blue arrow shows

uptake in BPH nodules. The overlap in tumor and BPH uptake make it

impossible to differentiate the 2 based on 11C-acetate PET/CT alone.

22


FIGURE 2

Representative coronal CT (upper row) and 18F-FACBC PET (bottom row)

images at 10 minutes and 26 minutes post injection, showing the ability

to distinctly differentiation between activity in the bladder and the

prostate. At the latter time points on the time activity curve (B), the

activity in the bladder is essentially equal to that in the prostate.

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Figure 3

58-year-old man with a serum PSA of 5.41ng/dl. Large field of view (A)

and zoomed (B) 18F-FACBC PET images demonstrate uptake in bilateral

central gland (blue arrows in B) and in the left mid peripheral zone

(orange arrow). Axial T2W MRI (C) demonstrates findings suggesting

benign prostate hyperplasia (BPH) in central gland bilaterally (blue

arrows in C) and a hypointense focus suspicious for tumor (orange arrow

in C). Histopathology revealed BPH in central gland bilaterally, however

left central gland also included a small Gleason 3+3 tumor focus (inked

in green in D) and right central gland included a small focus of atrophy

(inked in brown in D), which were not visualized either by PET or MRI.

Moreover, the left sided uptake area in PET, which was deemed as a tumor

at MRI, was found to include high grade prostatic intraepithelial

neoplasia at histopathology (inked in black in D).

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FIGURE 4

Uptake in benign inguinal lymph nodes in 2 different patients show mild

uptake on (A) 11C-acetate and (B) 18F-FACBC PET/CT. Blue solid arrows

indicate benign lymph nodes. Green arrow in (A) indicates prostate

uptake

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FIGURE 5

19 year old female status post therapy for peripheral T-cell lymphoma

presented with 18F-FDG positive (SUV 6.9) intercostal soft tissue mass

which shows little to no increased uptake on the 18F-FLT images. Biopsy

showed acute inflammation.

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FIGURE 6

Patient with squamous cell carcinoma in the right posterior paranasal

region following co- co-administration of tetrahydrouridine (THU) and

18F-FdCyd, a DNA methyltransferase inhibitor, showing mild focal uptake

(SUV 1.5) within the tumor. It is hoped that the amount of uptake can be

correlated with clinical response to therapy.

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FIGURE 7

68 year old female with post-menopausal left Stage IV breast

adenocarcinoma with metastasis to the left axilla (and lungs—not shown)

Histology of the primary tumor was ER/PR negative, HER-2 positive.

Axial and coronal SPECT (top) and CT (bottom) images performed 48 hours

after 111In-trastuzummab injection shows uptake in the left axillary node

(crosshairs)

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FIGURE 8

SPECT/CT imaging of 62- year old male diagnosed with epithelioid

malignant pleural mesothelioma following injection of 111In-CHA”-

amatuximab. Axial SPECT (right top), CT (left top) and fused SPECT/CT

(bottom) showing intense 111In-CHA”-amatuximab (Morab009) in a large

gastroesophageal node (green arrow), with only mild uptake in the

pleural based disease in the anterior left lung (gold arrow) in a

patient with mesothelioma

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The Role of PET/CT and SPECT/CT in Oncology Drug Development

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