Cancer Biology of Molecular Imaging
Cancer is a complex series of stepwise genetic alterations resulting in common biologic changes in the transformed cells. Distinguishing features of cancer include rapid proliferation of cells, immortality, resistance to apoptosis, resistance to suppression of proliferation, metastatic behavior, characteristic changes in metabolism, and resistance to immunologic attack. Cancer cells recruit normal host tissues to support growth of the tumor mass.
Fibrocytes and collagen producing cells provide structure for the tumor cells. Endothelial cells are recruited to form blood vessels. Tumor blood vessels have incomplete endothelium, making the vessels leaky. This allows large molecules to leak into the tumor interstitium.
The middle of the tumor mass has few, if any, lymphatic vessels. The combination of vessel leakiness and few lymphatics results in an increased interstitial pressure in the tumor, making it difficult for chemotherapy to diffuse into the tumor mass.
A common anatomic approach to measure tumor response (RECIST) employs measurements of the size of the mass on CT before and 4 weeks after therapy. Total disappearance of the lesion is required for a complete response, a 30% reduction in the sum of long dimensions defines a partial response, and >20% increase in the sum of long diameters identifies progressive disease. Adding metabolic information recorded with [18F]FDG-PET/CT and the PERCIST criteria may refine these measurements. In addition to [18F]FDG, radiopharmaceuticals are available to measure other attributes of the tumor. Depending on the radiopharmaceutical, images can provide information on tumor hypoxia, expression of integrins, or specific tumor markers that are overexpressed by the lesion, such as carbonic anhydrase, expressed by renal cell cancer, or receptors, such as somatostatin, expressed on neuroendocrine tumors.
KeywordsCancer biology Molecular imaging
Anti-CD33 monoclonal antibody labeled with 213-bismuth
v-Akt murine thymoma viral oncogene homolog 1 (Akt)
Acute myeloid leukemia
Adenosine 5´ triphosphate
B-type raf kinase
Bone scan index
Carbonic anhydrase 9
Castrate-resistant prostate cancer
Cutaneous T-cell lymphoma
Cytotoxic T lymphocyte antigen 4
Epidermal growth factor Receptor
European Organization for Research and Treatment of Cancer
Extravasation and passive retention
Extracellular signal regulated kinase
Gastrointestinal stromal tumor
Human epidermal growth factor receptor 2
Human glandular kallikrein 2
Herpes simplex virus thymidine kinase gene
- HIF-1 alpha
Hypoxia-induced growth factor alpha
Isocitrate dehydrogenase enzymes
Part of the name of a specific glioma cell line (U251 MG)
Micro ribonucleic acids
Magnetic resonance imaging
Messenger ribonucleic acid
N-acetylated alpha-linked acidic dipeptidase
Nicotinamide-adenine dinucleotide phosphate and its reduced form
National Institutes of Health
Non-small cell lung cancers
Poly ADP (adenosine diphosphate)-ribose polymerase
Poly(ADP-ribose) polymerase inhibitor
Paired box gene 8
PET response criteria in solid tumors
Positron emission tomography
Part of the name of a specific BRAFV600K kinase inhibitor (PLX4032)
Prostate-specific membrane antigen
Phosphatase and tensin homolog
Nectin 4, a tumor cell marker
Retuximab, cyclophosphamide, doxorubicin, vincristine, and prednisone (drug combination)
Response evaluation criteria in solid tumors
Small cell lung cancer
Six-transmembrane epithelial antigen of the prostate
Standardized uptake value
Standardized uptake value
Thyroid differentiation score
Vascular endothelial growth factor
This is the second edition of this chapter with emphasis on the nexus of cancer biology and molecular radio-imaging and targeted therapy. Those who have read the first edition will find much that is familiar, because the foundations for the cancer biology of molecular radiotargeting have not fundamentally changed, even though enrichments have occurred in relevant knowledge. To this end, I have done my best to include updates. The Cancer Genome Atlas is a huge source of knowledge for molecular radiotargeting and has already impacted thyroid cancer radiotherapy. Also, I have expanded the discussion of metabolomics to include glutamine imaging, provided an example of molecular imaging of an oncogene, and elaborated on androgen receptor (AR) and the integrated biochemical system that it drives (the AR axis in advanced prostate cancer). I added a more complete section on tumor immunology, setting the stage for imaging advances that will undoubtedly come in the years ahead. I have discussed the prospective role of radioantibody imaging, as I truly believe that the future will show astonishing advances based on our ability to perform optimization of immune targeting reagents. Elsewhere in this book, much information about specific applications of radiopeptide imaging and therapy, as well as organ-specific applications, is provided. This chapter does not comprehensively cover all molecular imaging but instead discusses key features of relevant cancer biology, as I see it, from the perspective of someone who has focused on oncologic applications of radiotracers for more than 40 years.
When I was a medical student, I was awarded an NIH fellowship to develop a radiotracer probe, technetium sulfur colloid, which is still in use today. This effort set me on the path of nuclear medicine and targeted probe development. As part of my education, my mentor, Wil B. Nelp of the University of Washington, took me to a conference in Richland, Washington, site of the Hanford National Laboratories. Much testing on the radiobiology of radiation was conducted on this campus, which was also the site of development of bomb-grade weapon materials. At this conference, in 1966, I met Shields Warren, a pathologist who was, at that time, a chairman of the Atomic Bomb Casualty Commission, an effort jointly supported by both the Japanese and US Academy of Sciences and the source of much of the current information on human dosimetry and biologic effects. Dr. Warren wrote an inscription on my program that I will never forget: “Congratulations to a student who can start a career with today’s sciences.” To all students of molecular imaging and targeted therapy, now 50 years later, these words are more apt than ever!
Molecular imaging offers quantitative detection of the molecules and molecular-based events that are fundamental to the malignant state, in vivo in living subjects. This is nothing new to nuclear medicine and its clinical applications of the tracer principle, but the opportunities have become so vast in the last decade that new excitement extends well beyond nuclear medicine to energize the whole of oncology. This new excitement stems from the recognition that molecular imaging has the potential to expedite laboratory discovery and personalize the care of individual patients with cancer. In this chapter, we will touch on how facets of the biology of cancer can be exploited in the performance of molecular imaging. This chapter is a revision of that presented in the first edition, with key updates and additions regarding cancer immunology, cancer genomics, pharmacology, and theranostics.
The Evolving Context of Molecular Imaging Practice: The Rapid Pace of Discovery in Cancer Genomics
As we have seen, an important hallmark of cancer is genomic instability and mutation that leads to changes in the genome, some of which have a direct consequence on the development of malignant phenotype (driver mutations), but others (passenger mutations) may be alterations that make the cancer different from normal tissues but have no direct effect on tissue phenotype.
In the last 10 years, revolutionary developments in rapid genomic sequencing have led to a sea change in our understanding of the genetic events important to the process of malignant transformation and maintenance of the malignant state. The practical importance of this new understanding of the cancer genome is that it allows a path forward to implement specific therapies that suppress tumor growth. For patients with lung cancer, for example, modern clinical practice includes a rapid genomic profile that may identify an alteration in the patient’s own tumor, which will likely make it responsive to treatment with specific drugs. As an example, detection of driver mutations based on substitution of leucine for arginine (L858R mutation) in the gene for epidermal growth factor receptor (EGFR) in some lung cancers allows treatment and likely response with erlotinib-targeted therapy. This kind of precision medicine will undoubtedly lead to expanded opportunities for molecular imaging to monitor these targeted treatments and determine, as is often the case, the point in time when the cancer mutates again and escapes from drug control.
As a high-level first pass, most genes that are directly linked to cancer causation fall into two categories: oncogenes, which are driver mutations for a process that leads to cancer, and suppressor genes, which must be disabled to permit expression of the malignant state. Current estimates are that about 140 such genes have been identified in the genome of tumors to date, of which 80 are tumor suppressor genes and 60 are oncogenes . Oncogenes have received more attention from the molecular imaging community because it is easier to image a molecule or molecular-related event that is additive or increased relative to the tissue of origin. Examples include oncogene mutations that lead to HER2 overexpression seen in breast cancer, c-Kit mutations seen in gastrointestinal stromal tumors, and EGFR mutations seen in brain tumors and lung cancers. Examples of suppressor genes include BRCA 1 and 2 mutations, associated with breast, ovarian, and pancreatic cancer; p53 mutations; breast, sarcoma, and brain tumors; and RB gene in retinoblastoma. See specific examples described in the section entitled “Oncogene and Non-oncogene Addiction”.
Hallmarks of Cancer
Distinguishing features of cancer cells include rapid proliferation, immortality, resistance to apoptosis, resistance to suppression of proliferation, metastatic behavior, and a variety of stress responses, which include a characteristic pattern of metabolism such as the Warburg effect and resistance to immunologic attack that allow cancer cells to better survive in hostile environments. The characteristic behaviors that make a cancer cell malignant are now generally believed to occur through a series of stepwise genetic alterations. In principle, the phenotypic alterations of each of these characteristics are associated with underlying key molecules that can be targeted with selective tracers and images.
The Tumor Mass
A general property essential to the long-term growth of cancer cells is the recruitment of a supporting framework to create the mass that characterizes clinical malignancies. Prior to recruitment of blood vessels, the tumor size is limited by the diffusion distance of oxygen in tissues, which is approximately 100–200 microns. Once a blood supply is established, the tumor mass can grow .
Imaging the Tumor Mass
With the widespread introduction of 18F-FDG-PET imaging, there is recognition that metabolic change should also be incorporated into the measurement of tumor treatment response. Initial assessment was based on the EORTC recommendation and employed standardized uptake values for FDG uptake, normalized to body weight (SUVmax) . Expert opinion offers alternative response parameters for making this assessment, such as PERCIST, which incorporates PET SUV measurements into treatment response. PERCIST uses an SUV measure, SUVpeak, which is based on lean body mass, and uses a circular region of 1.2 cm in diameter (with an approximate mass of 1 gram) centered around the hottest voxel, over which the standardized uptake value (SUV) is averaged. In order for PERCIST criteria to be applied to measure tumor response to treatment, the SULpeak must be >/SUL average liver, + 2 standard deviation SUL liver. In this way, tumor and liver SUV are compared .
Fox and colleagues have recently proposed an alternative that takes advantage of semiautomated lesion-tracking software to look at all the lesions simultaneously in advanced patients; clearly the size and metabolism of the mass in some combination appear likely to be an important measure of treatment response . It is important to keep in mind that when imaging a tumor mass, changes in a particular parameter involve all of the cells in the mass, both malignant and benign; this is true whether imaging gross anatomic measures obtained by CT or a more tumor cell-associated hallmark such as 18F-FDG uptake due to the Warburg effect.
Once cancer cells are collected into a mass, the cells must recruit blood vessels in order to grow. The blood vessel system in a tumor, called “neovasculature,” has some unique characteristics. The vessels are poorly organized, extremely permeable, and often have an incomplete endothelial barrier. Most tumors have minimal lymphatics in the tumor mass, which causes the mass to have a higher pressure than normal tissue, limiting the diffusion of many molecules (especially antibodies) into the tumor mass. The incomplete endothelial barrier of the neovasculature allows tumor cells direct entry to the host vasculature, providing a path for the tumor cells to circulate and metastasize.
Folkman was the first to recognize that inhibition of angiogenesis could inhibit tumor growth and coined the tumor “angiogenic switch,” for the moment when a small collection of tumor cells begins the vascularization process . Ferrara discovered that vascular endothelial growth factor (VEGF) played a crucial role in the development of neovasculature . In response to VEGF produced at the site of the tumor cells, stem cells are recruited from the bone marrow and migrate to the tumor mass, where they serve an essential role in the production of neovasculature.
Agents that target VEGF and its receptor include the antibody, bevacizumab, and the peptides sorafenib and sunitinib. Bevacizumab binds to VEGF while the peptides interfere with the receptor. Both show strong antitumor activity against some highly vascular tumors, which exert their antitumor effects by inhibiting the VEGF receptor expressed on tumor cells. Bevacizumab has been radiolabeled with zirconium-89 and localizes in breast cancer . In patients treated with these agents, serial imaging with FDG has shown a marked decrease in FDG uptake, reflecting the effectiveness of anti-VEGF therapy on tumor size and metabolism. The response of tumors to therapy such as single high-dose fraction radiation has been linked by some to endothelial damage, whereby radiation induces catastrophic apoptosis in endothelial cells, which results in irreversible damage to tumors.
In contrast to normal vessels, the leaky neovasculature can be exploited to image the tumor mass. The “leaky” incomplete endothelium allows passage and retention of large molecules and nanoparticles into tumors. This process is called the “extravasation and passive retention (EPR) effect ” and is thought to occur because the larger molecules and nanoparticles enter into the perivascular space through relatively large pores, but have difficulty getting back out into the vasculature, much like a lobster has trouble finding its way out of a lobster trap. The continuous expression of angiogenesis factors by the tumor causes a concomitant increased expression of integrin receptors, such as αvβ3 integrin or other members of the integrin family, to provide a homing signal for the cells mobilized to create the new blood vessels for the tumor. Agents binding to this integrin have been synthesized and localized in sites of neovascularity, such as tumors  and healing infarcts [9, 10]. Other endothelial sites for imaging include the antigens PSMA, which can be imaged with the antibody J591. In fact, in a review by Schliemann and Neri, nine vascular targets were described for which antibodies have been developed .
Solid tumors tend to outgrow their blood supply, causing cancer cells to become hypoxic . The reduced cellular oxygen content that accompanies hypoxia can be imaged using a class of compounds called nitroimidazoles . F-18 misonidazole has been used most often, although numerous alternatives are being developed. The principal localization is passive diffusion into the tumor. Once in the cell, nitroimidazoles undergo a single-electron reduction. In the presence of oxygen, the molecule is immediately reoxidized. As a result, the molecule diffuses back out of normoxic tissue. In the presence of hypoxia, the reoxidation cannot occur, trapping the molecule within the hypoxic but viable tissue. Hypoxia is a key property of tumors that reduces the effectiveness of radiation therapy. Well-oxygenated tissue is about three times more sensitive to radiation damage than hypoxic tissue. Some investigators suggest using F-18 misonidazole to image hypoxic regions within tumors to allow a boost of radiation to the hypoxic region without damaging normal tissues.
Cellular Constituents and Cell-Cell Synergism of the Tumor Mass
The tumor mass contains a mixture of tumor cells and cells recruited from the adjacent normal tissues of the host, such as fibroblasts, collagen fibrils, and vascular endothelium. The amount of stroma is highly tumor dependent, with lesions such as lymphoma having relatively little stroma, and others such as those in the pancreas having relatively few tumor cells and abundant stroma. In addition, tissue macrophages and other immune cells are represented in the mass. Typically, about 3% of cellular mass is represented by endothelial cells. But recent evidence suggests that single-dose fraction radiation’s major effect occurs by selectively damaging the vascular endothelium, the most sensitive cell in the tumor .
The mass cannot grow without the active synergism of each cellular component. There is also evidence that cells interact with one another in more subtle ways. Small molecules, large molecular complexes, and entire pieces of membrane are exchanged between tumor cells and supporting stroma. For example, P glycoprotein, an ABC transporter protein responsible for multidrug resistance, was shown to transfer from resistant to sensitive cells, as a functional entity, so that sensitive cells could acquire sufficient resistance to resist otherwise lethal levels of anticancer drugs .
Immune Cell Cancer Immunotherapy
The cellular component of the tumor mass includes a variety of tumor immune cells, including T cells, macrophages, and dendritic cells, a group of specialized immune cells that are involved as the first line of defense in alerting the immune system to foreign antigens. In particular, activated T cells, especially CD8 expressing cells within the cancer mass, can play a role in cancer therapy. In the last few years, treatment of cancers with immune therapy has led to significant responses—even complete responses—in advanced cancers, especially advanced melanoma and non-small cell lung cancers. The results are so exciting that in 2013, Science magazine selected cancer immunotherapy as the most significant breakthrough of the year . Effective treatments are based on growing knowledge of the role the immune system plays in the delicate balance of immune modulation, a series of checks and balances on the immune response. One of the newly recognized hallmarks of cancer is the ability of cancer cells to suppress the body’s immune response, so much so that the tumor mass is most frequently a sanctuary for proliferation of cancer cells within the tumor mass. Specialized antibody therapies can neutralize key molecules that modulate immune response, and for some tumors, this treatment (“immune checkpoint blockade”) can be part of a potent defense against even the most advanced tumors.
Immune checkpoint blockade. Human T cells are a critical component of the activity of the human immune system as part of body’s primary defense against invasion by foreign tissue and aberrant normal tissues such as cancer. In general, the native proteins in the body do not provoke an immune response in T cells, but among the millions and millions of T cells in the human body, it is thought that there may be a few that can even be immunized against normal macromolecules of normal tissues. But also, a plethora of antigens can be recognized as foreign, including those that may be only slightly different from the native molecules. These differences could arise due to mutation in the gene from which they arise, or from mistakes in the formation of molecules that lead to a slightly altered structure.
It turns out that cancers, and some cancers in particular (i.e., melanoma, non-small cell lung cancer), have a large number of these slightly altered antigens (often called neoantigens) that can create immunized T cells. These genetic changes may be either driver mutations or passenger mutations, but in any event represent altered features of the tissue, which may be recognized by the immune system. This immunity can be enhanced by therapies that block the action of immune suppressor molecule CTLA4 (such as the antibody Ipilimumab), and the result may be long-term anti-cancer response.
It is important to remember that the immune checkpoint inhibitor antibody is not the agent that actually kills the tumor; it is the body’s own immune cells which do the killing. An additional fact is that the immune cells must be in direct contact with the tumor in order to kill it, and so the immune cells must be able to enter the tumor, if “immune checkpoint” therapy is to be effective.
The tumor mass is an immunologic sanctuary, in that neo-antigens and oncoproteins manufactured by the malignant cell should be more immunogenic than they are in real life. In an exciting new development with profound clinical implications, so-called immune checkpoint blockade antibodies such as ipilimumab (anti-CTLA4) and nivolumab (anti-PD-1) target molecules on tumor-specific T cells that can suppress this antitumor immunity. Both ipilimumab and nivolumab, either alone or in combination, have been introduced into clinical trials and have induced long-term remissions in a significant portion (20–30%) of very advanced and hitherto intractable solid tumors, such as melanoma and lung cancer .
The immunologic invasion that follows immune checkpoint treatment, predominantly T cells, can be intense, enough to cause false positives on cross-sectional volumetric PET-FDG imaging. Similar findings were observed after ipilimumab and nivolumab therapy and in this case include increase in size of tumor mass because of immune cell infiltration and false positives on PET. For this reason, modified response criteria have been suggested for monitoring the response profile of solid tumors after these powerful immunologic treatments .
Imaging the Immune T Cells
An indium-111-labeled anti-CD5 antibody, T101, was used to image malignant T-cell lymphocytes as part of a study in mycosis fungoides (cutaneous T-cell lymphoma). Images of the pelvis are shown, at 2 h and 24 h after injection. More than 95% of the radiolabeled T-101 cleared from the blood within 2 h, binding to T65 antigen on T cells, followed by rapid localization, in liver spleen and bone marrow with subsequent gradual clearing of these tissues and concomitant localization to lymph nodes in tumor-involved regions, such as bulky inguinal nodes. Subsequently, using a kinetic model, we estimated the rate of localization of malignant T cells to lymph node in this patient. We calculated that about 3.3 × 107 cells/g/h localized to malignant lymph nodes in this patient and that a plateau occurred at 72 h, when influx was equal to output. At this point, the ratio of cell concentration between lymph node and blood was about 434. Similar studies will likely be performed with improved T-cell labeling methodologies in adoptive tumor cell therapy in man and are likely to give important information about the effectiveness of treatment in individual patients.
The Metastatic Process
Imaging the Cancer Cell
Increasing [18F]-FDG uptake and adverse outcome (From , with permission)
Tumor type [Ref]
Breast cancer 
Thyroid cancer 
Survival; 131I uptake
Esophageal cancer 
Head and neck cancer
Prostate cancer 
The initial observation of Warburg was that proliferating tumor cells consume glucose at a higher rate compared to normal cells and that they secrete most of the glucose-derived carbon in the form of lactate rather than oxidizing glucose completely. Warburg’s initial hypothesis included the view that mitochondria must be defective in cancer cells as a basis for the shift to aerobic glycolysis. Recent experiments indicate that in most cancer cells at least, there is no problem with oxidative metabolism in mitochondria, and indeed the conversion of pyruvate to lactate based on lactic dehydrogenase enzyme is an important promoter of growth and tumorigenicity .
There is growing awareness that the activation of aerobic glycolysis is just one of the metabolic programs that are crucial to the rapidly proliferating tumor cell. In addition, there is lipid biosynthesis and glutamine-dependent anaplerosis, a term used to describe replacement of intermediates within the Krebs cycle. The purpose of these metabolic flux pathways appears to be threefold: (1) to provide energy for the biochemical processes of growth and metabolism required by proliferating cells; (2) to provide the carbon backbone for key macromolecules such as nucleic acids, lipids, and proteins; and (3) to help control the redox balance within the tumor cell. Signal transduction molecules and gene expression have now been identified that are the natural regulators of these fluxes and include the signal transduction pathways, especially PI3K\AKT\mTor axis, hypoxia-induced growth factor alpha (HIF-1 alpha), and Myc, as some of the more important cellular modulators of metabolism .
Thus, a more complete picture is emerging with regard to the altered metabolism of cancer cells. Among the drivers that stimulate the altered metabolism, the tumor microenvironment tends to be hypoxic with increased concentrations of HIF-1 alpha and oncogenic signaling, through a variety of known oncogenes such as Ras or Myc. In addition, suppressor proteins like p53 may play a role. The p53 suppressor protein stimulates the gene encoding synthesis of cytochrome c oxidase protein, so when p53 is disabled by mutation, there is direct interference with the mitochondrial respiratory chain. The altered metabolism confers certain advantages, such as increased biosynthesis of nucleotides, proteins, phospholipids, and fatty acids; increased glycolysis also directly inhibits apoptosis by neutralizing reactive oxygen species through the production of NADPH. In addition, there are liabilities with this approach, because toxic metabolites such as lactate and noncanonical nucleotide species tend to accumulate. Finally, a high energetic demand requires that alternative substrate such as fatty acids may need to be converted to ATP in order to meet the energy requirements of the cell .
Recently, glutamine has been recognized as an essential nutrient for cell growth and viability of some cancers, and in fact, tumor cells can become “addicted” to glutamine. In this case, the tumor cells fuel their growth by a combination of glucose and glutamine. Whereas PI3K and AKT are the important signal transduction molecules which control glucose uptake, recent studies suggest that MYC oncogene may be crucial for the control of glutamine uptake. Glutamine plays a key role in maintenance of Krebs cycle intermediates, as well as providing energy in the form of ATP . Thus in the situation of active glutaminolysis, glucose and glutamine are the source of energy, and most of the other substrates which are taken up contribute to macromolecular synthesis.
Additional metabolic enzymes that may have oncogenic function include fumarate hydratase (associated with leiomyosarcoma) and succinate dehydrogenase (paraganglioma). When these enzymes are defective, accumulation of fumarate and succinate occurs. The excess of these metabolites causes overexpression of Hif-1 alpha, which promotes the vascularization of tumor as well as increased glycolysis, and may explain the tumor development. Recently, 70% of gliomas have been found to have mutations in isocitrate dehydrogenase enzymes (IDH1 and IDH2), suggesting a role for metabolic enzymes as oncogenes or tumor suppressors . Data recently reported suggests that this mutation in IDH1 is not a loss of function but instead leads to the ability to produce 2-hydroxygutarate, a metabolite whose excess has been associated with human malignancy in patients with inborn errors of 2-HG metabolism .
Correlation of increased FDG uptake with poor prognosis. Molecular imaging of radiotracers has been greatly enhanced by the introduction of relatively simple quantitative concepts, such as standardized uptake value (SUV). The SUV is a relative uptake in tissue (commonly tumor), corrected for body size and amount of injected activity, and may be expressed as maximum SUV in a region, average SUV, corrected for body weight, lean body mass . (As a mass correction, lean body mass is preferred, since there is the least correlation of this parameter with tumor uptake and blood SUV. Tumor response based on SUV measures has recently been proposed in the “PERCIST” algorithm, and the advantages of PET-FDG imaging based on SUV measures for monitoring treatment response has been proposed . The SUV in various forms is available as a standard measure on most PET cameras in use over the past 10 years. Therefore it is no surprise that correlations with prognosis and SUV have been estimated for a variety of tumors. Table 2 is a list of tumor types which have shown a strong inverse correlation of SUV with prognosis.
18F-FLT Imaging of Proliferation After Chemotherapy
The patient in Fig. 15 was imaged after two cycles of RCHOP chemotherapy. The upper row is the baseline study and therapy. Abnormal proliferation is an important component of the malignant state, and a tracer such as FLT has potential for major benefit in evaluating tumors. FLT is taken up by nucleoside transporters on the membrane of tumor cells, is phosphorylated by thymidine kinase, and used in the exogenous pathway under the enzymatic control of thymidine kinase, but is not incorporated into DNA. The phosphorylated form is retained in tumor.
Apoptosis is an important process in the life cycle of normal cells. Programmed cell death allows the cell to undergo an energy-dependent well-orchestrated process of involution. Apoptosis can be induced by both intrinsic and extrinsic pathways, and one of the hallmarks of cancer is resistance to the apoptotic process . Overcoming this resistance through therapy may contribute to accelerated treatment response in some patients. One of the earliest events in the process of apoptosis is a reversal of the normal cell membrane lipid structure, with exposure of phosphatidylserine on the outer leaflet of the cell membrane. In normal cells, phosphatidylserine is confined to the inner leaflet of the cell membrane. The externalization of this phospholipid allows the involuting cell to bind selectively to clotting factors and proteins. A physiologic protein, annexin V, has an affinity of >10−8 for membrane-bound phosphatidylserine. This protein has been labeled with technetium-99 m and was found to localize at sites of apoptosis both in experimental animals and in patients undergoing chemo- or radiotherapy. Apoptosis also occurs in non-neoplastic conditions, such as immune-induced inflammation and acute myocardial infarction. An alternative to annexin as a marker of apoptosis has been developed by comparing many different factors that bind to apoptotic cells.
Oncogene and Non-oncogene Addiction
Examples of Imaging the Action Driver Mutations in Cancer Cells Indirectly Through Effects on Metabolism
The mutations that drive the cancer cell into the malignant state can be thought of in three categories: gain-of-function mutations, translocations, and amplifications. Treatments and imaging are focused on these driver mutations, and in particular, the success of kinase inhibitors has validated this concept.
BRAF Inhibition in Advanced Melanoma with V600E Mutation
Inhibition of MEK, a Downstream Signal Transduction Molecule in the RAS/BRAF/ERK Pathway, Can Reverse 131I Resistance in Patients with Thyroid Cancer
Imaging the Driver Oncogene in Castrate-Resistant Prostate Cancer: The Androgen Receptor Axis
Dihydrotestosterone, a metabolite of testosterone, is the most abundant androgen at the tissue level in vivo and binds to AR in vivo. We have extensively studied a radiolabeled analog, 18F-FDHT, to explore AR expression and in vivo binding in patients with the lethal form of prostate cancer, namely, castrate-resistant prostate cancer (CRPC). Toxicity, patient dosimetry, conditions of use, and optimized analysis were assayed, and FDHT continues to be used in a variety of clinical trials that are ongoing, e.g., 00–095 in metastatic CRPC [51, 52, 53, 54].
We have adapted this approach to the study of AR expressing primary and metastatic prostate cancers. We directly image AR under a short-time medical castration to testosterone levels < 50 pg/ml. We have also used glycolysis marker FDG, a clinical standard tracer, within 24–48 h as a marker of active tumor metabolism, and have demonstrated high patient acceptance of a two-day sequence of imaging, requiring about 1 h of scanning each day. In addition, we have analyzed imaging biopsies to explore the presence of the downstream androgen effector proteins, particularly HK2, PSMA, and STEAP [55, 56, 57] for consideration of later AR-axis monitoring using radiolabeled antibody approaches as a possible pharmacodynamic approach of AR-axis inhibition in later drug trials.
Androgen Receptor (AR) as Oncogenic Driver in Prostate Cancer
Imaging of Carbonic Anhydrase 9 as a Pathognomonic Indication of Clear Cell Renal Cancer
Many other molecular imaging approaches are available to characterize one or more aspects of a neoplasm, including F-18 estradiol for breast cancers, amino acid incorporation (FACBC, glutamate), 68Ga-PSMA for prostate imaging, and 68Ga-DOTATATE for neuroendocrine imaging. Some of these have been approved, and others will likely be available within the next 5 years. However, due to the lengthy process of regulatory approval, the major radiopharmaceutical for evaluation of the tumor patient will remain FDG. With increased understanding of changes in glucose transporter expression by the tumor cell, it is likely that more sophisticated prognostic indices than SUVmax may be developed. It is also likely that genomic analysis of circulating tumor cells may be used as a gatekeeper, to determine if a more selective MI probe PET/CT study is likely to provide clinically useful information.
The development of radiotracer probes for molecular imaging continues to proceed at a very rapid pace. Moreover, drug companies are beginning to recognize the important role that molecular imaging can play in facilitating drug development through an improved understanding of the biology of the cancer process. It is highly likely that very soon we will see ever more remarkable radiotracers, probably in combination with MRI agents and optical agents, because of the ease with which fusion imaging may be obtained in order to exploit the combined information. The field of nanotechnology offers many opportunities for probe development, including serving as a carrier for highly selective radiotracers such as complementary probes like DNA-binding or RNA-binding proteins, as well as radiolabeled drugs. Theranostic agents combine the potential for diagnosis and therapy, so that agents are not only intended to visualize tumors but in effect guide therapeutic delivery. This is already beginning in the exciting field of image-guided interventional radiology, but will extend to tracers with both diagnostic and therapeutic potential. For example, it is easy to contemplate the radiolabeled drug carried in nanoparticles so that the quantification of the payload delivered will serve as a sensor that provides, through quantitative imaging, the total amount of drug that is both taken up and retained within the tissues. Such theranostic approaches will improve selection of patients for a given type of therapy and may allow for tailoring treatment to the individual patient.
Radioantibody and Radiopeptide Imaging and Therapy (Theranostics) Will Drive Our Field for the Next Decade and Beyond
Commonly used radioantibodies at MSKCC
A33 antigen (colorectal) (see Fig. 20)
Carbonic anhydrase 9 (hypoxia, renal cell) 
GD2 (neuroblastoma, melanoma) 
B7H3 (melanoma, prostate, pediatric cancers, sarcoma) 
CD33 (AML, CML) 
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