The Past, Present, and Future of Biomarkers: A Need for Molecular Beacons for the Clinical Management of Pancreatic Cancer
Pancreatic ductal adenocarcinoma (PDA) is one of the most lethal cancers. It is the fourth leading cause of cancer-related death in the United States [1]. Nearly 40,000 Americans are affected by this disease every year and more than half of these individuals succumb to cancer-related complications [1]. Even with cases that are identified early and undergo surgical resection, the diagnosis of PDA is associated with an overall 5-year survival rate of only 6% to 25% [2]. Although many resources and large genome-profiling studies have been completed (Fig. 1) [3,4], the clinical management of this disease has still made only modest strides in the past 2 decades.
Treatment options other than surgical resection (only 20% of patients meet the criteria for resection) include chemotherapy protocols that routinely use gemcitabine, and often include radiotherapy. Since 1997, gemcitabine has been used as the standard of care for the treatment of PDA in both the postoperative adjuvant and metastatic settings [5]. Moreover, numerous clinical trials have compared multiple drugs in combination with gemcitabine, including 5-fluorouracil (5-FU), capecitabine (Xeloda), and erlotinib (Tarceva), but all these drug combinations have failed despite costly clinical trials involving thousands of patients. Dr Roland Schmid [6], in an editorial in Gastroenterology, stated: “In fact, pancreatic cancer is the number 1 killer in phase III trials.”
For PDA, there have been 2 areas of recent promise. First, in medical oncology, therapies targeted against DNA repair pathways (eg, BRCA2 and Fanconi anemia deficiencies) have attracted attention. Second, a less targeted and selected therapy, but one that showed a modest improvement in patient outcomes in the metastatic setting, was presented at the 2010 American Society of Clinical Oncology (ASCO) meeting, with the regimen of FOLFIRINOX (5-FU/leucovorin [LV], irinotecan [I], and oxaliplatin [O]), which increased survival in the metastatic setting by roughly 4 months [7]. Other intriguing therapies that have received attention in the national landscape include paclitaxel (Abraxane), which has validated the concept of targeting the stromal portion (ie, the SPARC protein) of a tumor mass [8]. Abraxane is currently indicated for chemoresistant breast cancers, and it is debatable whether this drug will be useful (based on SPARC positivity) for a select group of patients who have pancreatic cancer in the phase III setting. Other than a half-dozen promising, theoretic strategies, most physicians (particularly oncologists) treating patients with PDA agree that they are largely focused on treating their patients in the palliative setting.
These modest improvements in patient outcomes are not caused by a lack of understanding or interest in the genetic basis of this disease [3,4]. A plethora of work was recently added to by a genome-wide survey of multiple pancreatic cancers. These data were coordinated with gene expression analyses of 24 pancreatic tumors to define 12 core signaling pathways involved in the development of PDA [4]. Although many novel targets (including membrane proteins) were newly identified, these studies primarily discovered and validated pathways and mutated genes that are disrupted in PDA [9]. An additional survey of familial pancreatic cancer genomes revealed an additional gene mutated in the Fanconi anemia DNA repair pathway: PALB2 [3]. Identifying other genes disrupted in this pathway may have important predictive biomarker implications for the treatment of PDA that harbor such PALB2 mutations and related pathway mutations (discussed later).
Current biomarkers used for the clinical management of PDA
Tumor biomarkers (prognostic and predictive) are molecules, typically proteins, that are produced by the body in response to the presence of tumor cells and that may be detected in blood, urine, other bodily secretions, or tissue samples. The definition of biomarkers can be stratified into those with (1) prognostic and (2) predictive value. Simplistically, prognostic markers can be considered as those that identify patients with different risks of outcome (eg, for recurrence of disease) and can be helpful in up-front therapeutic decision making. A predictive marker can be defined as a clinically useful marker for prediction of a response to a specific therapy or treatment regimen. Prognostic and predictive markers can be (1) a defined genetic sequence (eg, BRCA2); (2) a cutoff level of a specific protein expression (eg, SPARC expression); (3) detection of a specific level of mRNA expression (eg, epidermal growth factor receptor [EGFR] mRNA levels); (4) any identifiable pathologic feature of a tumor (eg, lymph node positivity); (5) subcellular localization of a protein in a tumor or stromal cell (eg, HuR); and/or (6) any relevant molecular signature (mRNA, DNA, protein, or any combination of these aspects of a tumor); for example, as is currently used in breast cancer. Typically, a biomarker is described in the literature as 1 gene (either a DNA mutation or polymorphism on the genetic level; or the quantitative expression on the mRNA level with the use of quantitative polymerase chain reaction [PCR] technology; or immunohistochemistry on the protein level) (Fig. 2). A marker can have both prognostic and predictive value [10].
The limitations and usefulness of the available conventional markers
Unlike other tumor types, such as prostate cancer (eg, prostate-specific antigen), no simple blood test has been developed to detect the presence of a premalignant or malignant pancreatic cell (see Fig. 1). The most commonly used serum markers for pancreatic cancer are cancer antigen 19-9 (CA19-9), carcinoembryonic antigen (CEA), and rarely cancer antigen 125 (CA-125). The sensitivity and specificity of serum markers depend on the determination of cutoff values derived from different studies. For example, 1 study used different cutoff values of CA19-9 (37, 100, 200, and 300 U/mL) yielding different sensitivities (68%, 41%, 24%, and 15%, respectively) and specificities (70%, 86%, 96%, and 100%, respectively) for the same marker [11]. This study shows the lack of specificity and reproducibility of these tests as well as the subjectivity of using a desired cutoff value. With that stated, these markers are currently the most widely used clinically, and thus this article briefly describes the background and importance of each of these markers in the context of PDA.
CA19-9
The carbohydrate antigen CA19-9 is a tumor-associated antigen released into the serum of patients with gastrointestinal (GI) cancers [12]. CA19-9 was originally identified and developed in the late 1970s and early 1980s, when researchers were in search of a better and more specific diagnostic tool than CEA levels for GI cancers [13–15]. CA19-9 is a monosialoganglioside first isolated from a colorectal cell line, and since its discovery it has rapidly become the most widely used biomarker for pancreatic cancer [13,14]. As a biomarker in patients with PDA, CA19-9 is primarily used clinically to follow the response to therapy (both before and after surgical resection and chemoradiotherapy). CA19-9 is not specific for PDA, because it increased in 21% to 42% of cases of gastric cancer and 20% to 40% of cases of colon cancer. Overall, CA19-9 is increased in 71% to 93% of pancreatic cancer, and it has proved useful to differentiate benign from malignant pancreatic disease. However, most clinicians would gladly eliminate CA19-9 for a more reliable biomarker.
As a prognostic marker, it has been shown that CA19-9 can correlate with tumor resectability and pathologic features such as tumor stage [11]. More recently, in patients who had undergone pancreaticoduodenectomy for right-sided PDA, it was shown that postoperative levels of CA19-9 (at a cutoff value of 120U/ml) can stratify for both overall and disease-free survival, but does not correlate with margin status or lymph node positivity. In this study, Berger and colleagues [14] performed a prospective analysis of postoperative CA19-9 levels in patients treated with pancreatic resection and adjuvant chemoradiotherapy. This study confirmed the prognostic value of postresection CA19-9 levels in predicting outcome in patients with PDA.
Despite its widespread use, CA19-9 falls short as an informative marker for several reasons. First, a high percentage of patients with pancreatic cancer do not have any detectable CA19-9 in their sera, which rules out the use of this marker for use in early diagnosis and the further treatment of this cohort of patients. Second, CA19-9 does not accurately and reproducibly correlate with the clinical manifestations of this disease. For instance, in a pancreatic cancer registry in Japan, only 48% of patients with PDA smaller than 2 cm had increased CA19-9 [15]. Third, it has been observed that patients who are Lewis blood type negative (Le ab−) do not express the CA19-9 antigen, and, thus, a Lewis ab− blood type needs to be considered in patients with pancreatic cancer who lack an increased serum CA19-9 level (estimated to be 5%–10% of the population) [16,17]. Fourth, CA19-9 is also increased in inflammatory lesions of the pancreas, such as pancreatitis, which may interfere with the absolute diagnosis of PDA based on CA19-9 alone [18].
The variability in CA19-9 serum levels described earlier is not only detected in the patient population but can be variable throughout the course of treatment of an individual. However, this lack of reproducible quantitative measurement of this marker gives patients a false sense of understanding of their disease course. In many clinical instances, such as post-pancreatic resection, patients rely too heavily on their CA19-9 value as a marker for curable or silent disease. These numbers can give a false sense of hope to patients who get excited about a dramatic drop in their CA19-9 levels, only to be faced with the finding of a high CA19-9 value on follow-up visits. Therefore, CA19-9 cannot be used as a stand-alone marker, and the use of this marker should be assessed in conjunction with other markers (eg, CEA; see later discussion), imaging, and patient symptoms regarding clinical decision making. The primary use of CA19-9 as a tumor marker is to vaguely monitor a patient’s response to pancreatic cancer treatment and/or cancer progression [19].
CEA
CEA is a 180-kDa glycoprotein that was first discovered more than 45 years ago [20]. As a biomarker, CEA is primarily used in decision making in the course of treatment of colorectal cancers, but is also used in other cancers such as stomach, breast, and pancreatic cancers [21]. Similar to a CA19-9 blood test, a CEA blood test measures the specific levels of CEA protein in a patient’s serum. As with CA19-9, the specific link to pancreatic cancer is limited because of the lack of specificity (ie, many tumor types produce CEA) and because some patients never produce CEA at all. Thus, this marker cannot be used as a reliable early detection marker or a relevant biomarker throughout a patient’s disease course [22]. Distinct from levels in the serum, CEA levels in pancreatic cyst fluids may have some value in detecting pancreatic mucinous, premalignant, or malignant cysts [23].
A warning of caution is that serum CEA levels can be increased (falsely positive for cancer) in patients who have certain infections, pancreatitis, and cirrhosis of the liver. In addition, individuals who smoke cigarettes can have increased CEA levels compared with nonsmokers [24–26], in the absence of malignancy. Therefore, the realistic clinical usefulness of CEA may lie in its ability to be included as part of a multipanel biomarker panel for early detection screening purposes.
CA-125 and final thoughts on conventional markers
CA-125, or carbohydrate antigen 125, was first identified by immunizing ovarian cancer cells to mice [24]. CA-125 was later characterized and labeled as the human gene encoded by the MUC16 gene [27]. MUC16 was thus identified as a new member of the mucin family [27]. CA-125 is an antigen present on 80% of nonmucinous ovarian carcinomas [27] and has therefore been investigated for possible use as a biomarker in patients with ovarian carcinoma. Subsequently, CA-125 has been found to be increased in other cancers including pancreatic cancer and other gynecologic and nongynecologic conditions [23]. CA19-9 can be more useful in patients who do not present with jaundice, whereas CA-125 provides a limited contribution in jaundiced patients [25]. In current clinical practice, CA-125 is not sufficiently accurate to be useful in patients with pancreatic cancer.
The use of these markers (CA19-9, CEA, and to some extent CA-125) has already helped guide and monitor the treatment of many patients with PDA. However, taking into account the high, but still limited, sensitivity and specificity of the CA19-9 and CEA tests, their results in differential diagnosis of distinct pancreatic tumors should be interpreted cautiously, and in reference to imaging results (eg, ultrasonography, computed tomography, and magnetic resonance imaging) [26]. Together, these markers may be useful in the future of therapy for patients with PDA. Realistically, 1 or all these markers may be used in the future in a multiplexed panel with novel, presently undiscovered markers for this disease [28]. The molecular understanding of the dysregulation of these markers (eg, hypomethylation of a specific gene or overexpression of a transcriptional factor that regulates a specific gene) [29] in PDA cells may provide a useful insight into the molecular cause of this disease.
Novel markers (including early detection) of pancreatic cancer
The prognosis of PDA is poor, not only because of its aggressive biologic behavior, but because its commonly missed or late clinical diagnosis often prevents initiation of established curative therapies such as surgery. Therefore, one of the field’s major goals is to find molecular markers, specific and sensitive enough to make an early and correct diagnosis of early stage PDA and/or pancreatic cancer precursor lesions, before the tumor becomes unresectable and while patients are still clinically asymptomatic (see Fig. 1) [30]. Although the molecular markers (CA19-9 and CEA) described earlier have been used for the early diagnosis of PDA, these markers fail in many circumstances to differentiate benign from malignant processes, and also fail to differentiate locally confined resectable disease from widespread metastatic disease.
Currently, several notable and useful molecular early detection markers are genetic based and only have clinical significance for the familial form of PDA (familial pancreatic cancer [FPC]). The genome-maintenance and DNA repair genes, which include BRCA2 (see Fig. 2A), and the Fanconi anemia genes (FANCC, FANCG, and PALB2/FANCN) have received considerable attention in the past decade because of (1) the high frequency of inherited mutations in this pathway found in patients with FPC [3,31], and (2) the recent work on platinum-based and PARP inhibitor–based therapies [32], which have shown that tumors defective in this pathway should be exquisitely sensitive to these agents [3,33–35]. Thus, as a biomarker, a DNA sequence (detection of a germline mutation in these genes from constitutional DNA) in individuals composing a family that has a high presence of pancreatic and other related cancers (ovarian and breast) can be useful as a diagnostic and a predictive marker [33]. Myriad Genetics (Salt Lake City, UT, USA) offers a genetic survey of these genes based on a simple blood test to aid physicians and families in their decision making. The results may be used both for preventive surgical options such as prophylactic resection, or treatment options such as targeted chemotherapy (if the disease is present in family members who test positive for a germline mutation in one of the genes mentioned earlier).
Canto and colleagues [36,37] have been leaders in this field of screening high-risk families, trying to explore the importance of invasive screening tools (such as endoscopic ultrasound) to monitor high-risk individuals identified with a family history of pancreatic cancer and a positive genetic test for a BRCA2 or related gene mutation [31,36,38]. However, the debate continues as to the best overall strategy [37]. Further, a recent publication warns that individuals who carry a germline BRCA2 mutation may not harbor a loss of the corresponding allele or a second hit in the pancreatic cancer cells [39]. Therefore, if the 2-hit hypothesis does not apply here, the theoretic therapeutic window (ie, an Achilles heel of the tumor) for platinum-based or PARP inhibitor–based therapies may be negated [39], along with the predictive value of the BRCA2
Related posts:
Glucose Elevations and Outcome in Critically Injured Trauma Patients
Geographic Variation in Health Care and the Affluence-Poverty Nexus
Recent Advances in the Diagnosis and Treatment of Gastrointestinal Carcinoids
Immunotherapy for Metastatic Solid Cancers
Endovascular Approaches to Arteriovenous Fistula
Varicose Vein: Current Management
