Novel Positron Emitting Radiopharmaceuticals
Over the past two decades, nuclear imaging has transformed cancer care. Positron emission tomography (PET) and single photon emission computed tomography (SPECT) can provide clinicians with functional and biochemical information about tumor tissue that complement the anatomical data acquired through magnetic resonance imaging (MRI) and computed tomography (CT). In this chapter, we highlight a number of emerging radiotracers in oncology that are currently employed in clinical trials in the USA and worldwide yet are awaiting regulatory approval in the USA. The radiotracers discussed range from small molecule probes that target cellular transport mechanisms and metabolic pathways to antibody-based agents that target cell-surface receptors. In order to help the reader appreciate the diversity and potential of each of these imaging agents, we present the underlying mechanisms of each agent’s targeting and trapping in tumor tissue and provide examples of clinical studies in diverse cancer types as well as descriptions of the utility of each tracer for staging and treatment monitoring.
KeywordsNuclear imaging Oncology Positron emission tomography, PET Single photon emission tomography, SPECT Radiotracer Imaging agent Fluorine-18 Carbon-11 Gallium-68 Zirconium-89 Technetium-99m Indium-111 Amino acid transport Lipid biosynthesis Cellular proliferation Somatostatin receptor Integrin Bombesin receptor Prostate-specific membrane antigen, PSMA Steroid hormone receptor Hypoxia
- 18F-alfatide II
Aromatic amino acid decarboxylase
Bombesin receptor subtype-2
Bombesin receptor subtype-2
Central nervous system
Fatty acid synthase
Gastrin-releasing peptide receptor
Intensity-modulated radiation therapy
L-type amino acid transporter 1
Magnetic resonance imaging
Medullary thyroid cancer
Mammalian target of rapamycin
Non–small cell lung cancer
Positron emission tomography
Positron emission tomography/magnetic resonance imaging
Prostate-specific membrane antigen
Steroid hormone-binding globulin
Single photon emission computed tomography
Standardized uptake value
Minimum time to peak
Over the past two decades, nuclear imaging has transformed cancer care. Both positron emission tomography (PET) and single photon emission computed tomography (SPECT) have empowered clinicians with imaging tools capable of providing functional and biochemical information about tissues that complement the anatomical maps created by magnetic resonance imaging (MRI) and computed tomography (CT). A remarkable array of radiotracers has been created that enables the visualization of targets ranging from the expression of biomarkers by individual cancer cells to the pH of the tumor microenvironment.
The most widely used radiotracer in oncology – [18F]2-fluoro-2-deoxyglucose, [18F]FDG – is only one of a few PET radiopharmaceuticals that has been approved by the US Food and Drug Administration. Yet despite its utility, [18F]FDG is limited in many cases and cannot possibly be expected to fulfill all of the imaging needs of clinicians as we enter the era of personalized medicine. As a result, recent years have played witness to ever-increasing efforts toward the development and translation of novel, molecularly targeted imaging agents. In this chapter, we will focus on radiopharmaceuticals that have reached a particularly critical point in their journey from bench to bedside: imaging agents that have been the subject of multiple clinical trials but have yet to be approved by regulatory agencies for routine clinical use. While a multitude of radiopharmaceuticals have reached the clinic in some capacity, we have chosen to discuss a handful of particularly promising agents as case studies. We truly believe that these radiotracers represent the next generation of clinical nuclear imaging agents. We have organized these imaging agents into three groups according to their target: (I) the machinery of cellular metabolism and biosynthesis, (II) cell surface receptors, and (III) the tumor microenvironment, with a focus on imaging hypoxia. Importantly, the intrinsic technological advantages of PET over SPECT – principally the former’s quantitative output and higher resolution – mean that over the past half decade (the time span covered by this chapter), the vast majority of newly translated radiopharmaceuticals have been PET tracers. As a result, this chapter will focus on these agents.
Targeting Cellular Transport, Metabolism, and Proliferation
The aberrant growth of cancer cells results in numerous metabolic changes that can be targeted for diagnostic imaging. A classic example of a metabolism-targeting radiotracer is [18F]2-fluoro-2-deoxyglucose ([18F]FDG), a radiolabeled glucose analog that is avidly taken into cancer cells trying to meet their augmented energy requirements by increasing the expression of glucose transporters. Unfortunately, however, because of the ubiquitous role of glucose as an energy source, the increased uptake of [18F]FDG is not a phenomenon specific to cancer cells. Needless to say, this deficiency has spurred significant interest in the development of new, more tumor-selective radiotracers. Over the last decade, several small-molecule radiotracers specific for various metabolic pathways have been developed and clinically evaluated with promising results. Broadly speaking, these tracers can be divided into two groups: (i) radiolabeled amino acids and amino acid analogs that target amino acid transport and metabolism and (ii) acetate, choline, and analogs thereof that target lipid biosynthetic pathways. It is important to note that many of these radiotracers were originally developed for other purposes and only recently have been reevaluated for oncologic imaging. For example, [11C]acetate was initially created for imaging cardiac oxidative metabolism, while L-3,4-dihydroxy-6-[18F]fluorophenylalanine ([18F]FDOPA) was developed for imaging dopaminergic neurons in the central nervous system. Lastly, we will highlight the potential for imaging proliferating tumor cells using 3′-deoxy-3′-[18F]fluorothymidine ([18F]FLT), a nucleoside analog that is taken up and retained in proliferating cells without incorporation into DNA.
Imaging Amino Acid Transport and Metabolism
As a source of both carbon and nitrogen, glutamine is a vital feedstock for a number of biosynthetic pathways in cancer cells. This efficient use of glutamine by cancer cells is achieved by the upregulation of glutamine transporters on the cell membrane as well as the increased expression of mitochondrial glutaminase that drives the synthesis of glutamate from glutamine to feed the TCA cycle . The creation of a PET tracer for glutamine-based imaging has required the careful consideration of radiolabeling strategies. The seemingly obvious choices for a radiolabel are 11C and 13N (t1/2 = 9.97 min) both of which could yield chemically identical isotopologues of glutamine. However, in addition to the synthetic challenges posed by the short physical half-lives of the two radioisotopes, 11C- and 13N-labeled glutamine radiotracers undergo rapid degradation and elimination of the radiolabel from the tracer in tumor cells as a result of glutaminolysis.
Imaging Lipid Biosynthesis
Imaging Cellular Proliferation with [18F]FLT
[18F]FLT PET has been utilized in dozens of clinical trials with a variety of cancers [56, 58]. One of the most common uses of [18F]FLT has been as a noninvasive tool for assessing the viability of tumors. Such scans can be administered following a cycle of chemotherapy or radiation therapy and have been repeatedly confirmed as an effective predictive measure of whether a patient will benefit from further treatment. For example, Contractor et al. found that a reduction in [18F]FLT uptake in the weeks following the treatment of breast cancer with docetaxel correlated significantly to lesion size evaluated via ultrasound and CT (P = 0.004) . Such results are mirrored in nearly every case: the overall uptake of [18F]FLT decreases significantly in response to therapy, providing an effective method to monitor the progression of treatment .
Interestingly, in some instances, it has been noted that a “flare effect” can occur immediately following the administration of therapeutic agents, during which there is a marked increase in [18F]FLT uptake at the tumor. This may be due to the inhibition of de novo thymidine synthesis pathways . A greater flare response was found to be associated with poorer patient prognosis and survival rate . Such flares may be useful indicators to evaluate the efficacy of chemotherapy on a patient-by-patient basis.
A significant mechanistic obstacle to the widespread use of [18F]FLT is the ability of some cancers to convert deoxyuridine monophosphate to thymidine monophosphate . Therefore, [18F]FLT uptake does not necessarily correlate to overall cellular proliferation but rather reflects the activity of the thymidine salvage pathway. Going forward, clarifying the relationship between cellular viability, cellular proliferation, and [18F]FLT uptake is crucial to the further clinical implementation of this imaging agent.
Targeting Cell Surface Receptors
Imaging the Expression of Somatostatin Receptors
Naturally occurring SST and a subset of SST2-targeting peptides used to deliver PET radioisotopes to tumor sites. Also shown are two common chelators conjugated to these peptides
Peptide sequence a
Diethylene triamine pentaacetic acid
Imaging the Expression of Integrins
Several review articles covering clinical imaging with radiolabeled RGD peptides have cataloged the achievements of these radiopharmaceuticals [83, 84, 85, 86]. A variety of modifications to the core RGD structure have been tested to improve the in vivo behavior of the radiotracers. For example, the introduction of a sugar onto a single RGD peptide sequence resulted in improved in vivo stability while maintaining favorable tumor-to-background ratios [87, 88]. Studies with [18F]galacto-RGD showed variation in uptake between patients, indicating differences in tumor vascularization and cancer stage . In a different investigation, in order to decrease the labeling time, a click reaction was used to conjugate an 18F-labeled prosthetic group to galacto-RGD to form [18F]flotegatide-RDG (or [18F]RGD-K5) . In initial human studies, [18F]RGD-K5 uptake did not correlate with that of [18F]FDG, a result similar to that observed with [18F]galacto-RGD. However, [18F]RGD-K5 PET was determined to be an effective tool for the delineation of integrin expression in breast cancers . Thus, the principal utility of [18F]RGD-K5 may be in monitoring response to therapy rather than tumor identification . Yet another RGD-based radiotracer features a polyethylene glycol (PEG) spacer that is radiolabeled with 18F. This compound, [18F]fluciclatide (18F-AH111585), was injected into a small number of patients with metastatic breast cancer and displayed uptake in lesions that were also visible via CT. Unfortunately, in these patients, the liver uptake of the tracer was too high to identify liver metastases. Further [18F]fluciclatide imaging studies in patients with brain tumors, lung cancer, squamous cell carcinoma of the head and neck, differentiated thyroid carcinoma, sarcoma, and melanoma will focus on the ability of the tracer to delineate angiogenesis . In a study utilizing [15O]H2O as a comparison to [18F]fluciclatide in patients with non-small cell lung cancer and melanoma, Kenny et al. showed that the uptake of [18F]fluciclatide was not merely a function of perfusion and suggested using later imaging time points to determine sites of neovasculature .
Due to their relatively low uptake in tumor lesions (<5%ID/g), suboptimal background contrast, and synthetic complexity, the aforementioned 18F-labeled RGD peptides have shown limited utility in the clinic. This has led to investigations using 68Ga-labeled peptides in hopes of increasing the uptake of the tracer in the tumor or decreasing its uptake in nontarget tissues . For example, 68Ga-NOTA-RGD has been used in patients to identify colorectal cancer metastases prior to therapy . These studies suggest that PET scans using RGD-based peptides could aid in the selection of patients for anti-angiogenic therapy .
In another effort to improve the tumor retention of RGD-based peptides, multimeric radiopharmaceuticals have also been investigated. In light of this topic, it is important to diverge – if only briefly – to discuss a concept known as “bivalent targeting.” Bivalent targeting strategies are predicated on the use of two biological targeting moieties that are tethered together (1) to increase the probability of interactions with a single cell surface receptor or (2) to facilitate the binding of two cell surface receptors simultaneously. Homodimers (or homomultimers) utilize the same biological targeting vector to achieve greater cell surface receptor occupancy, whereas heterodimers target different receptors often in an effort to increase the types of cancer targeted by the radiopharmaceutical [95, 96]. The use of two targeting moieties may also increase the chance that the peptide will interact with the cell surface in the correct orientation for receptor binding. Because the distance between any two cell surface receptors is unknown, researchers have attempted to investigate various linking molecules that can accommodate cell surface variations . Additionally, increased linker size may decrease the metabolism rate of the radiopharmaceutical and alter its pharmacokinetic properties, ensuring a longer biological half-life and possibly resulting in greater tumor uptake.
Imaging the Expression of Bombesin Receptors
Naturally occurring BBN and a subset of the BB2-targeting peptides used to deliver PET radioisotopes to tumor sites
Peptide sequence a
Demobesin 4 [DB4]
68Ga-labeled BBN analogs have been quickly translated to the clinic. BBN(7-14) conjugated to a radiometallated NOTA chelator via an aminocaproic acid – 68Ga-NOTA-Aca-BBN(7-14) – has been investigated to image primary gliomas. Normal, healthy subjects showed rapid clearance of the 68Ga-NOTA-Aca-BBN(7-14) through the kidneys and had higher accumulation of the radioactivity in the bladder . Twelve patients with high- to low-grade gliomas also showed rapid accumulation of 68Ga-NOTA-Aca-BBN(7-14) in lesions within 30 min with very high tumor-to-background ratios (24 ± 9), and all 14 lesions in the 12 patients were identified . Zhang et al. suggested the possible utility of 68Ga-NOTA-Aca-BBN(7-14) PET not only to identify tumors but also to delineate tumor margins that are difficult to determine clinically using five-aminolevulinic acid (ALA) .
Imaging the Expression of Prostate-Specific Membrane Antigen
Prostate-specific membrane antigen (PSMA) is a transmembrane glycoprotein with a large extracellular portion and small intracellular segment. PSMA expression has been shown in normal prostate tissue, but the level of expression increases in prostate cancer, making it an enticing target for the imaging and therapy of prostate cancers. Despite the number of studies implicating PSMA expression in the diagnosis of prostate cancer, the normal function of PSMA is still unknown . The elevated levels of shed PSMA in the serum of patients require that PSMA-targeting agents must localize in cancerous tissues in high enough abundance to compensate for high background activity concentrations.
The only commercially available PSMA-targeted imaging agent is 111In-capromab pendetide (ProstaScint®), a murine monoclonal antibody that targets the intracellular portion of the antigen . Unfortunately, 111In-capromab pendetide has two major drawbacks. First, the radiotracer only targets PSMA in apoptotic or necrotic tissues, in which the intracellular portion of PSMA is available due to membrane damage. And second, because of the limited sensitivity and selectivity of the radiolabeled antibody, the clinical use of 111In-capromab requires a relatively long delay between injection and imaging in order to increase target-to-background contrast [108, 109].
Imaging the Expression of Steroid Hormone Receptors
Targeting the Hypoxic Tumor Microenvironment
Cancer cells release growth factors such as vascular endothelial growth factor to stimulate angiogenesis from neighboring vasculature. However, these new blood vessels are often poorly formed and inefficient at facilitating the transport of nutrients and oxygen. Within the tumor microenvironment, high rates of cellular proliferation and metabolism combine with this poorly defined vasculature to create local regions of hypoxia . Tumor hypoxia is often indicative of aggressive and metastatic malignant phenotypes and frequent portend complications during disease management. As a result, it follows that the ability to noninvasively delineate hypoxic from normoxic tumors could benefit clinicians during staging, prognosis, treatment planning, and treatment monitoring. The presence of tumor hypoxia is often associated with increased resistance to radiation therapy as well, as much of the damage done by radiation is mediated by reactive oxygen species. Therefore, the identification and mapping of hypoxia are also important for planning optimal radiation therapy treatment regimes.
Imaging Tumor Hypoxia with [18F]FMISO
Imaging Tumor Hypoxia with 64Cu-ATSM
Despite the utility of Cu-ATSM in some cancer models, there is variability in its hypoxia-dependent uptake between different cell lines, which could be due in part to the complexity – and particularly the pH dependence – of its intracellular trapping mechanism. As with [18F]FMISO, a series of Cu-ATSM derivatives have been developed in hopes of improving the hypoxia-selective uptake and tumor retention .
In the preceding pages, we have covered a number of extremely promising imaging agents that stand on the cusp of regulatory approval and routine clinical use. The diversity of these radiopharmaceuticals reflects the wide array of information that clinicians seek as they make patient care and treatment decisions. The case studies we have discussed have included small molecules and antibodies, short-lived radiohalogens and long-lived radiometals, and targets ranging from cell surface antigens to tumor hypoxia. Yet despite the potential of these radiopharmaceuticals, the final obstacle (regulatory approval) is often the biggest hurdle of all. In the United States, this has been particularly true – and frustrating – in the case of 68Ga-DOTATOC. In light of this, we are eager to take this opportunity to underscore the paramount importance of the standardization of procedures across clinical trials and institutions, as this is absolutely essential to developing the substantial body of evidence required for regulatory approval. Hopefully, such measures will streamline the path of these imaging agents from the laboratory to the clinic and thus expedite the positive impact that they will undoubtedly have on patient care.
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