Tumor cells circulate in the peripheral blood of all major carcinomas but not in healthy subjects or patients with nonmalignant diseases

Biomarkers in oncology drug development

Biomarker measurements have become an essential component of oncology drug development, particularly so in this era of targeted therapies. Such measurements ensure that clinical studies are testing our biological hypotheses and can help make the difficult decisions required to choose which drugs to stop developing or de-prioritise. For those drugs taken forward, biomarker measurements may also help choose the appropriate dose, schedule and patient population. In this review we discuss the intrinsic properties of biological sample based efficacy measurements and how these relate to their implementation in oncology drug development by way of points to consider and examples.

National Academy of Clinical Biochemistry Laboratory Medicine Practice Guidelines for Use of Tumor Markers in Testicular, Prostate, Colorectal, Breast, and Ovarian Cancers

Background: Updated National Academy of Clinical Biochemistry (NACB) Laboratory Medicine Practice Guidelines for the use of tumor markers in the clinic have been developed.

Methods: Published reports relevant to use of tumor markers for 5 cancer sites—testicular, prostate, colorectal, breast, and ovarian—were critically reviewed.

Results: For testicular cancer, α-fetoprotein, human chorionic gonadotropin, and lactate dehydrogenase are recommended for diagnosis/case finding, staging, prognosis determination, recurrence detection, and therapy monitoring. α-Fetoprotein is also recommended for differential diagnosis of nonseminomatous and seminomatous germ cell tumors. Prostate-specific antigen (PSA) is not recommended for prostate cancer screening, but may be used for detecting disease recurrence and monitoring therapy. Free PSA measurement data are useful for distinguishing malignant from benign prostatic disease when total PSA is <10 μg/L. In colorectal cancer, carcinoembryonic antigen is recommended (with some caveats) for prognosis determination, postoperative surveillance, and therapy monitoring in advanced disease. Fecal occult blood testing may be used for screening asymptomatic adults 50 years or older. For breast cancer, estrogen and progesterone receptors are mandatory for predicting response to hormone therapy, human epidermal growth factor receptor-2 measurement is mandatory for predicting response to trastuzumab, and urokinase plasminogen activator/plasminogen activator inhibitor 1 may be used for determining prognosis in lymph node–negative patients. CA15-3/BR27–29 or carcinoembryonic antigen may be used for therapy monitoring in advanced disease. CA125 is recommended (with transvaginal ultrasound) for early detection of ovarian cancer in women at high risk for this disease. CA125 is also recommended for differential diagnosis of suspicious pelvic masses in postmenopausal women, as well as for detection of recurrence, monitoring of therapy, and determination of prognosis in women with ovarian cancer.

Conclusions: Implementation of these recommendations should encourage optimal use of tumor markers.

Circulating tumor cells predict survival benefit from treatment in metastatic castration-resistant prostate cancer

PURPOSE

A method for enumerating circulating tumor cells (CTC) has received regulatory clearance. The primary objective of this prospective study was to establish the relationship between posttreatment CTC count and overall survival (OS) in castration-resistant prostate cancer (CRPC). Secondary objectives included determining the prognostic utility of CTC measurement before initiating therapy, and the relationship of CTC to prostate-specific antigen (PSA) changes and OS at these and other time points.

EXPERIMENTAL DESIGN

Blood was drawn from CRPC patients with progressive disease starting a new line of chemotherapy before treatment and monthly thereafter. Patients were stratified into predetermined Favorable or Unfavorable groups (<5 and > or =5 CTC/7.5mL).

RESULTS

Two hundred thirty-one of 276 enrolled patients (84%) were evaluable. Patients with Unfavorable pretreatment CTC (57%) had shorter OS (median OS, 11.5 versus 21.7 months; Cox hazard ratio, 3.3; P < 0.0001). Unfavorable posttreatment CTC counts also predicted shorter OS at 2 to 5, 6 to 8, 9 to 12, and 13 to 20 weeks (median OS, 6.7-9.5 versus 19.6-20.7 months; Cox hazard ratio, 3.6-6.5; P < 0.0001). CTC counts predicted OS better than PSA decrement algorithms at all time points; area under the receiver operator curve for CTC was 81% to 87% and 58% to 68% for 30% PSA reduction (P = 0.0218). Prognosis for patients with (a) Unfavorable baseline CTC who converted to Favorable CTC improved (6.8 to 21.3 months); (b) Favorable baseline CTC who converted to Unfavorable worsened (>26 to 9.3 months).

CONCLUSIONS

CTC are the most accurate and independent predictor of OS in CRPC. These data led to Food and Drug Administration clearance of this assay for the evaluation of CRPC.

Circulating tumor cells in metastatic breast cancer: from prognostic stratification to modification of the staging system?

BACKGROUND

The aim of the current study was to assess the prognostic value of baseline circulating tumor cells (CTCs) in a large cohort of patients with newly diagnosed metastatic breast cancer (MBC).

METHODS

This retrospective study included 185 patients with newly diagnosed MBC evaluated between 2001 and 2007. CTCs were isolated and enumerated before patients started first-line treatment using the CellSearch system. Overall survival (OS) was calculated from the date of CTC measurement, estimated by the Kaplan-Meier product limit method, and compared between groups with the log-rank test. Cox proportional hazards models were fitted to determine the association between CTC levels and OS after controlling for other prognostic factors.

RESULTS

The median age of the patients at the time of MBC diagnosis was 49 years. Fifty-six (30.3%) patients presented with de novo metastatic disease, and 129 (69.7%) presented with newly recurrent breast cancer. A total of 114 patients (61.6%) had CTC<5, and 71 (38.4%) had CTC >or= 5. The median OS was 28.3 months and 15 months (P< .0001) for patients with CTC<5 and CTC >or= 5, respectively. Superior survival among patients with CTC<5 was observed regardless of hormone receptor and HER-2/neu status, site of first metastases, or whether the patient had recurrent or de novo metastatic disease. In the multivariate model, patients with CTC >or= 5 had a hazards ratio of death of 3.64 (95% confidence interval, 2.11-6.30) compared with patients with CTC <5.

CONCLUSIONS

The results of this large retrospective study confirms that CTCs are a strong independent predictor of survival among women with either de novo or newly recurrent MBC. CTCs should be considered as a new stratification method for women with newly diagnosed MBC.

Circulating tumor cell detection predicts early metastatic relapse after neoadjuvant chemotherapy in large operable and locally advanced breast cancer in a phase II randomized trial

PURPOSE

Circulating tumor cells in blood from metastatic breast cancer patients have been reported as a surrogate marker for tumor response and shorter survival. The aim of this study was to determine whether circulating tumor cells are present in the blood of patients with large operable or locally advanced breast cancer before neoadjuvant chemotherapy and after neoadjuvant chemotherapy before surgery.

EXPERIMENTAL DESIGN

Blood samples of 7.5 mL were obtained on CellSave tubes from patients included in a phase II trial (REMAGUS 02). Circulating tumor cells were immunomagnetically separated and fluorescently stained by the CellSearch system. Blood from 20 metastatic breast cancer patients was used as a positive control.

RESULTS

From October 2004 to July 2006, preneoadjuvant chemotherapy and/or postneoadjuvant chemotherapy blood samples were obtained from 118 patients. At least 1 circulating tumor cell was detected in 22 of 97 patients with preneoadjuvant chemotherapy samples (23%; 95% confidence interval, 15-31%; median, 2 cells; range, 1-17 cells). Circulating tumor cell positivity rates were 17% in 86 postneoadjuvant chemotherapy samples and 27% in all 118 patients. Persistence of circulating tumor cells at the end of neoadjuvant chemotherapy was not correlated with treatment response. After a short median follow-up of 18 months, the presence of circulating tumor cells (P=0.017), hormone receptor negativity, and large tumor size were independent prognostic factors for shorter distant metastasis-free survival.

CONCLUSION

Circulating tumor cells can be detected by the CellSearch system at a low cutoff of 1 cell in 27% of patients receiving neoadjuvant chemotherapy. Circulating tumor cell detection was not correlated to the primary tumor response but is an independent prognostic factor for early relapse.

Detection of circulating tumor cells in patients with urothelial cancer

BACKGROUND

Approximately 50% of patients with metastatic urothelial cancer (UC) respond to chemotherapy and several months of therapy is required to assess for radiographic response. Blood-based biomarkers may identify patients in whom a specific therapy provides clinical benefit, and this study sought to characterize circulating tumor cells (CTCs) in patients with metastatic UC.

PATIENTS AND METHODS

Peripheral blood from patients with metastatic UC was evaluated for CTCs using the CellSearch system. We assessed for associations between CTC counts and the number and sites of metastatic disease.

RESULTS

CTC evaluations were carried out in 33 patients with metastatic UC. Fourteen of 33 patients (44%; 95% confidence interval 27% to 59%) had a positive assay (range 0-87 cells/7.5 ml of blood) with 10 patients (31%) having five or more CTCs. A significantly higher number of CTCs was seen in patients with two or more sites of metastases compared with those with less than one or one site of metastases (3.5 versus 0, P = 0.04).

CONCLUSIONS

CTCs, detected by antibody capture technology, are present in 44% of patients with metastatic UC. Higher numbers of CTCs are seen in patients with a greater number of metastatic sites. One-third of patients have five or more CTCs providing a potential early marker to monitor response to chemotherapy.

Circulating tumor cells and bone marrow micrometastasis

Sensitive immunocytochemical and molecular assays allow the detection of single circulating tumor cells (CTC) in the peripheral blood and disseminated tumor cells (DTC) in the bone marrow as a common and easily accessible homing organ for cells released by epithelial tumors of various origins. The results obtained thus far have provided direct evidence that tumor cell dissemination starts already early during tumor development and progression. Tumor cells are frequently detected in the blood and bone marrow of cancer patients without clinical or even histopathologic signs of metastasis. The detection of DTC and CTC yields important prognostic information and might help to tailor systemic therapies to the individual needs of a cancer patient. In the present review, we provide a critical review of (a) the current methods used for detection of CTC/DTC and (b) data on the molecular characterization of CTC/DTC with a particular emphasis on tumor dormancy, cancer stem cell theory, and novel targets for biological therapies; and we pinpoint to (c) critical issues that need to be addressed to establish CTC/DTC measurements in clinical practice.

Phosphorylated EGFR and PI3K/Akt signaling kinases are expressed in circulating tumor cells of breast cancer patients

Introduction

The phosphoinositide-3 kinase (PI3K)/Akt pathway, operating downstream of epidermal growth factor receptor (EGFR) and human epidermal growth factor receptor (HER)2, is implicated in cell migration and survival. EGFR and HER2 are expressed in circulating tumor cells, but the activation status of downstream signaling molecules has not yet been reported.

Methods

To investigate expression levels of EGFR, HER2, PI3K, and Akt in circulating tumor cells, we used peripheral blood mononuclear cells from 32 cytokeratin-19 mRNA-positive patients with early (n = 16) and metastatic (n = 16) breast cancer.

Peripheral blood mononuclear cell cytospins were double stained with cytokeratin antibody along with one of the following: EGFR, phospho-EGFR, HER2, phospho-PI3K, or phospho-Akt antibodies.

Results

EGFR and HER2 were expressed in circulating tumor cells of 38% and 50% patients with early and 44% and 63% patients with metastatic disease, respectively. Interestingly, phospho-PI3K and phospho-Akt expression levels were similar at 88% (14 out of 16) and 81% (13 out of 16), respectively, in circulating tumor cells of patients with early and metastatic disease. Phospho-EGFR was observed in circulating tumor cells of two (33%) early and six (86%) metastatic EGFR-positive patients. Immunomagnetic separation of peripheral blood mononuclear cells, using EpCAM antibody, and subsequent double-staining experiments of circulating tumor cells showed that EGFR was co-expressed with HER2, phospho-Akt and phospho-PI3K kinases, indicating activation of the corresponding survival signaling pathway.

Conclusions

Our findings demonstrate that circulating tumor cells express receptors and activated signaling kinases of the EGFR/HER2/PI3K/Akt pathway, which could be used as targets for their effective elimination.

Review: Biological relevance of disseminated tumor cells in cancer patients

The prognosis of cancer patients is largely determined by the occurrence of distant metastases. In patients with primary tumors, this relapse is mainly due to clinically occult micrometastasis present in secondary organs at primary diagnosis but not detectable even with high resolution imaging procedures. Sensitive and specific immunocytochemical and molecular assays enable the detection and characterization of disseminated tumor cells (DTC) at the single cell level in bone marrow (BM) as the common homing site of DTC and circulating tumor cells (CTC) in peripheral blood. Because of the high variability of results in DTC and CTC detection, there is an urgent need for standardized methods. In this review, we will focus on BM and present currently available methods for the detection and characterization of DTC. Furthermore, we will discuss data on the biology of DTC and the clinical relevance of DTC detection. While the prognostic impact of DTC in BM has clearly been shown for primary breast cancer patients, less is known about the clinical relevance of DTC in patients with other carcinomas. Current findings suggest that DTC are capable to survive chemotherapy and persist in a dormant nonproliferating state over years. To what extent these DTC have stem cell properties is subject of ongoing investigations. Further characterization is required to understand the biology of DTC and to identify new targets for improved risk prevention and tailoring of therapy. Our review will focus on breast, colon, lung, and prostate cancer as the main tumor entities in Europe and the United States.

Clinical significance of circulating tumor cells in blood from patients with gastrointestinal cancers

BACKGROUND

Circulating tumor cells (CTCs) measured by the CellSearch system in metastatic breast cancer have been reported to correlate with shorter overall survival. The purpose of this study was to clarify the clinicopathologic characteristics of CTCs in gastrointestinal cancers.

METHODS

Pre- and postoperative CTCs from 130 gastrointestinal cancer patients and 41 healthy volunteers were measured by this system. Correlation between CTC counts and clinicopathologic variables was examined.

RESULTS

The number of CTCs in metastatic patients (n = 79) was larger than in nonmetastatic patients (n = 35) and in healthy donors (n = 41) (P < 0.001). CTC counts were larger in metastatic gastric cancer (n = 27) than in nonmetastatic gastric cancer (n = 14) (P = 0.016). Two or more CTCs was significantly correlated with advanced tumor stage in all gastrointestinal cancers (P < 0.001) and in gastric cancer (P = 0.032). Two or more CTCs had significant correlation with peritoneal dissemination of gastric or colorectal cancer (P = 0.007) and pleural dissemination of esophageal cancer (P = 0.033). The survival of patients with > or =2 CTCs was shorter than that of patients with <2 CTCs (P = 0.005). The change in CTCs tended to correlate with disease progression and chemotherapeutic effect.

CONCLUSION

This study suggests that measurement of CTCs in gastrointestinal cancer patients could be useful as a tool for judging tumor stage, predicting the presence of peritoneal or pleural dissemination and patients' survival, and monitoring response to cancer therapy.

Circulating tumour cell (CTC) counts as intermediate end points in castration-resistant prostate cancer (CRPC): a single-centre experience

BACKGROUND

The purpose of this study was to evaluate the association of circulating tumour cell (CTC) counts, before and after commencing treatment, with overall survival (OS) in patients with castration-resistant prostate cancer (CRPC).

EXPERIMENTAL DESIGN

A 7.5 ml of blood was collected before and after treatment in 119 patients with CRPC. CTCs were enumerated using the CellSearchSystem.

RESULTS

Higher CTC counts associated with baseline characteristics portending aggressive disease. Multivariate analyses indicated that a CTC >or=5 was an independent prognostic factor at all time points evaluated. Patients with baseline CTC >or=5 had shorter OS than those with <5 [median OS 19.5 versus >30 months, hazard ratio (HR) 3.25, P=0.012]; patients with CTC >50 had a poorer OS than those with CTCs 5-50 (median OS 6.3 versus 21.1 months, HR 4.1, P<0.001). Patients whose CTC counts reduced from >or=5 at baseline to <5 following treatment had a better OS compared with those who did not. CTC counts showed a similar, but earlier and independent, ability to time to disease progression to predict OS.

CONCLUSION

CTC counts predict OS and provide independent prognostic information to time to disease progression; CTC dynamics following therapy need to be evaluated as an intermediate end point of outcome in randomised phase III trials.

Enrichment with anti-cytokeratin alone or combined with anti-EpCAM antibodies significantly increases the sensitivity for circulating tumor cell detection in metastatic breast cancer patients

INTRODUCTION

Circulating tumor cells (CTCs) are detectable in most cancer patients and they can meet an existing medical need to monitor cancer patients during a course of treatment and to help determine recurrent disease. CTCs are rarely found in the blood of cancer patients and enrichment is necessary for sensitive CTC detection. Most CTC enrichment technologies are anti-EpCAM antibody based even though CTC identification criteria are cytokeratin positive (CK+), CD45 negative (CD45-) and 4'6-diamidino-2-phenylindole (nuclear stain) positive (DAPI+). However, some tumor cells express low or no EpCAM. Here we present a highly sensitive and reproducible enrichment method that is based on binding to anti-CK alone or a combination of anti-CK and anti-EpCAM antibodies.

METHODS

Blood samples from 49 patients with metastatic breast cancer were processed using the CellSearchtrade mark system (Veridex, LLC, Raritan, NJ, USA), in parallel with our CTC assay method. We used anti-CK alone or in combination with anti-EpCAM antibodies for CTC enrichment. Brightfield and fluorescence labeled anti-CK, anti-CD45 and DAPI (nuclear stain) images were used for CTC identification. The Ariol(R) system (Genetix USA Inc, San Jose, CA, USA) was used for automated cell image capture and analysis of CTCs on glass slides.

RESULTS

Our method has the capability to enrich three types of CTCs including CK+&EpCAM+, CK+&EpCAM-/low, and CK-/low&EpCAM+ cells. In the blind method comparison, our anti-CK antibody enrichment method showed a significantly higher CTC positive rate (49% vs. 29%) and a larger dynamic CTC detected range (1 to 571 vs. 1 to 270) than that of the CellSearchtrade mark system in the total of 49 breast cancer patients. Our method detected 15 to 111% more CTCs than the CellSearchtrade mark method in patients with higher CTC counts (>20 CTCs per 7.5 ml of blood). The three fluorescent and brightfield images from the Ariol(R) system reduced the number of false-positive CTC events according to the established CTC criteria.

CONCLUSION

Our data indicate that the tumor-specific intracellular CK marker could be used for efficient CTC enrichment. Enrichment with anti-CK alone or combined with anti-EpCAM antibodies significantly enhances assay sensitivity. The three fluorescent and brightfield superior images with the Ariol(R) system reduced false-positive CTC events.

Monitoring serial changes in circulating human breast cancer cells in murine xenograft models

Circulating tumor cells (CTC) are emerging as a powerful prognostic and predictive biomarker in several types of cancer, including breast, colon, and prostate. Studies of CTC in metastasis and further development of CTC as a biomarker in cancer have been limited by the inability to repetitively monitor CTC in mouse models of cancer. We have validated a method to enumerate CTC in blood samples obtained from living mice using a modified version of an in vitro diagnostic system for quantifying CTC in patients. Different routes of blood collection were tested to identify a method to reproducibly recover CTC from tumor-bearing mice without interference from contaminating normal murine epithelial cells. CTC are present in blood samples from mice bearing orthotopic xenografts of several different breast cancer cell lines and primary breast cancer cells from patient biopsies. We also show that this technology can be used for serial monitoring of CTC in mouse xenograft models of human breast cancer. These results establish a new method for studying CTC in mouse models of epithelial cancer, providing the foundation for studies of molecular regulation of CTC in cancer and CTC as biomarker for therapeutic efficacy.

Detection of mutations in EGFR in circulating lung-cancer cells

BACKGROUND

The use of tyrosine kinase inhibitors to target the epidermal growth factor receptor gene (EGFR) in patients with non-small-cell lung cancer is effective but limited by the emergence of drug-resistance mutations. Molecular characterization of circulating tumor cells may provide a strategy for noninvasive serial monitoring of tumor genotypes during treatment.

METHODS

We captured highly purified circulating tumor cells from the blood of patients with non-small-cell lung cancer using a microfluidic device containing microposts coated with antibodies against epithelial cells. We performed EGFR mutational analysis on DNA recovered from circulating tumor cells using allele-specific polymerase-chain-reaction amplification and compared the results with those from concurrently isolated free plasma DNA and from the original tumor-biopsy specimens.

RESULTS

We isolated circulating tumor cells from 27 patients with metastatic non-small-cell lung cancer (median number, 74 cells per milliliter). We identified the expected EGFR activating mutation in circulating tumor cells from 11 of 12 patients (92%) and in matched free plasma DNA from 4 of 12 patients (33%) (P=0.009). We detected the T790M mutation, which confers drug resistance, in circulating tumor cells collected from patients with EGFR mutations who had received tyrosine kinase inhibitors. When T790M was detectable in pretreatment tumor-biopsy specimens, the presence of the mutation correlated with reduced progression-free survival (7.7 months vs. 16.5 months, P<0.001). Serial analysis of circulating tumor cells showed that a reduction in the number of captured cells was associated with a radiographic tumor response; an increase in the number of cells was associated with tumor progression, with the emergence of additional EGFR mutations in some cases.

CONCLUSIONS

Molecular analysis of circulating tumor cells from the blood of patients with lung cancer offers the possibility of monitoring changes in epithelial tumor genotypes during the course of treatment.

Relationship of circulating tumor cells to tumor response, progression-free survival, and overall survival in patients with metastatic colorectal cancer

PURPOSE

As treatment options expand for metastatic colorectal cancer (mCRC), a blood marker with a prognostic and predictive role could guide treatment. We tested the hypothesis that circulating tumor cells (CTCs) could predict clinical outcome in patients with mCRC.

PATIENTS AND METHODS

In a prospective multicenter study, CTCs were enumerated in the peripheral blood of 430 patients with mCRC at baseline and after starting first-, second-, or third-line therapy. CTCs were measured using an immunomagnetic separation technique.

RESULTS

Patients were stratified into unfavorable and favorable prognostic groups based on CTC levels of three or more or less than three CTCs/7.5 mL, respectively. Patients with unfavorable compared with favorable baseline CTCs had shorter median progression-free survival (PFS; 4.5 v 7.9 months; P = .0002) and overall survival (OS; 9.4 v 18.5 months; P < .0001). Differences persisted at 1 to 2, 3 to 5, 6 to 12, and 13 to 20 weeks after therapy. Conversion of baseline unfavorable CTCs to favorable at 3 to 5 weeks was associated with significantly longer PFS and OS compared with patients with unfavorable CTCs at both time points (PFS, 6.2 v 1.6 months; P = .02; OS, 11.0 v 3.7 months; P = .0002). Among nonprogressing patients, favorable compared with unfavorable CTCs within 1 month of imaging was associated with longer survival (18.8 v 7.1 months; P < .0001). Baseline and follow-up CTC levels remained strong predictors of PFS and OS after adjustment for clinically significant factors.

CONCLUSION

The number of CTCs before and during treatment is an independent predictor of PFS and OS in patients with metastatic colorectal cancer. CTCs provide prognostic information in addition to that of imaging studies.

Progress in the development of prognostic and predictive markers for gastrointestinal malignancies

Gastrointestinal cancers remain a significant cause of morbidity and mortality. While increasing therapeutic options have improved outcomes for many patients, they have also complicated treatment decision-making. Unfortunately, most patients with advanced gastrointestinal malignancies die from their disease. Prognostic and predictive markers could improve treatment significantly by identifying patients who may or may not require a given therapy, and determining those most likely to benefit from a therapy. Candidates for such markers include blood antigens and circulating tumor cells, tumor enzyme and gene expression, and pharmacodynamic endpoints. In this review, we summarize reported and ongoing research to define and validate prognostic and predictive markers in gastrointestinal malignancies, with an emphasis on colorectal cancer and brief overview of pancreatic and neuroendocrine tumors.

Circulating tumor cell number and prognosis in progressive castration-resistant prostate cancer

PURPOSE

The development of tumor-specific markers to select targeted therapies and to assess clinical outcome remains a significant area of unmet need. We evaluated the association of baseline circulating tumor cell (CTC) number with clinical characteristics and survival in patients with castrate metastatic disease considered for different hormonal and cytotoxic therapies.

EXPERIMENTAL DESIGN

CTC were isolated by immunomagnetic capture from 7.5-mL samples of blood from 120 patients with progressive clinical castrate metastatic disease. We estimated the probability of survival over time by the Kaplan-Meier method. The concordance probability estimate was used to gauge the discriminatory strength of the informative prognostic factors.

RESULTS

Sixty-nine (57%) patients had five or more CTC whereas 30 (25%) had two cells or less. Higher CTC numbers were observed in patients with bone metastases relative to those with soft tissue disease and in patients who had received prior cytotoxic chemotherapy relative to those who had not. CTC counts were modestly correlated to measurements of tumor burden such as prostate-specific antigen and bone scan index, reflecting the percentage of boney skeleton involved with tumor. Baseline CTC number was strongly associated with survival, without a threshold effect, which increased further when baseline prostate-specific antigen and albumin were included.

CONCLUSIONS

Baseline CTC was predictive of survival, with no threshold effect. The shedding of cells into the circulation represents an intrinsic property of the tumor, distinct from extent of disease, and provides unique information relative to prognosis.

Liver regeneration and tumor stimulation--a review of cytokine and angiogenic factors

Liver resection for metastatic (colorectal carcinoma) tumors is often followed by a significant incidence of tumor recurrence. Cellular and molecular changes resulting from hepatectomy and the subsequent liver regeneration process may influence the kinetics of tumor growth and contribute to recurrence. Clinical and experimental evidence suggests that factors involved in liver regeneration may also stimulate the growth of occult tumors and the reactivation of dormant micrometastases. An understanding of the underlying changes may enable alternative strategies to minimize tumor recurrence and improve patient survival after hepatectomy.

Circulating tumor cells in peripheral blood samples from patients with increased serum prostate specific antigen: initial results in early prostate cancer

PURPOSE

We evaluated the hypothesis that circulating tumor cells as determined using the CellSearch System would correlate with tumor volume, pathological stage and Gleason score in men with localized prostate cancer.

MATERIALS AND METHODS

Samples of blood (30 ml) were drawn from 97 men with localized prostate cancer before radical prostatectomy, on postoperative days 2 to 3 and at 6 weeks. A control group consisted of 25 men with an increased prostate specific antigen and no tumor detected on extended prostate biopsy. Samples were analyzed for circulating tumor cells using the CellSearch System.

RESULTS

Circulating tumor cells were detected in 21% of patients with cancer and 20% of controls (p = 0.946). At 6 weeks after prostatectomy circulating tumor cells were detected in 16% and 11% (p = 0.51) of the men positive and negative for circulating tumor cells at baseline, respectively. Of the 20 patients with cancer who had circulating tumor cells at baseline 18 showed no circulating tumor cells after surgery. Circulating tumor cell values did not correlate with tumor volume, pathological stage or Gleason score. Only 3.1% of the men with cancer and 8% of the control group had 3 or more circulating tumor cells per 22.5 ml blood at baseline.

CONCLUSIONS

In metastatic breast, prostate and other cancers more than 5 circulating tumor cells are often detected using the CellSearch System, and may correlate with prognosis. However, in the setting of localized prostate cancer the number of detectable circulating tumor cells was low, with findings comparable to those in men who were biopsy negative for cancer. We found no correlation between the number of circulating tumor cells and known prognostic factors in this population.

Detection of circulating tumor cells in patients with pancreatic cancer: a preliminary result

BACKGROUND/PURPOSE

It has been reported that circulating tumor cells (CTCs) can be used to predict survival in metastatic breast cancer. In this preliminary study, we examined the level of CTCs in pancreatic cancer (PC) patients to elucidate whether we could predict survival in PC.

METHODS

The eligible subjects, at Tokyo Medical University Hospital, were 26 patients with PC, 11 with chronic pancreatitis, and 10 healthy volunteers. Three PC patients underwent surgery, 18 patients (who were stage IV) were treated with gemcitabine (GEM), and 5 patients received best supportive care (BSC).

RESULTS

The CTC count was 1/7.5 ml blood or higher (defined as positive) in 11 of the 26 patients (42%; mean, 16.9/7.5 ml blood; range, 1-105/7.5 ml blood). Gemcitabine was administered to 6 of the 11 CTC-positive patients (3.8 courses on average). The treatment was continued for more than three courses in 2 patients, in both of whom the CTC count was only 1/7.5 ml blood. Operation was performed in 1 of the 11 CTC-positive patients. The remaining 4 patients of the 11 CTC-positive patients received only BSC. CTC was negative in 15 patients with PC (stage II, 1; stage III, 1; stage IVa, 7; and stage IVb, 6) and in the subjects with benign conditions. The median survival times (MSTs) of the CTC-positive and-negative patients were 110.5 and 375.8 days (P < 0.001). When the analysis was limited to the 14 stage-IVb patients, the MSTs of the CTC-positive and-negative patients were 52.5 and 308.3 days (P < 0.01).

CONCLUSIONS

The present study demonstrated that the detection of CTCs in peripheral blood may be useful to predict prognosis in patients with PC.

Dormant tumor cells as a therapeutic target?

Tumor dormancy is characterised by the persistence of residual tumor cells for long periods. Recurrence from minimal residual disease is a major cause of cancer death. Thus, understanding how cancer cells become and remain dormant, may lead to new strategies to prevent relapse. Evidence has emerged that a balance exists between host and dormant tumor cells. Cross-talk between tumor cells and their micro-environment, angiogenesis, and anti-tumor immune response participate in the control of dormant tumor cells. Tumor cells have several mechanisms of maintaining equilibrium, and immune escape, including expression of immuno-regulatory molecules (e.g., increased expression of B7.1 and B7-H1); epigenetic modifications (e.g., silencing of the SOCS1 gene, de-regulating the JAK/STAT pathway); and autocrine loops. These new findings offer new opportunities to design specific treatments, to modify the balance in favor of the host immune response.

A simple two-gene prognostic model for adenocarcinoma of the lung

OBJECTIVE

We hypothesized that clinical outcome of resected early-stage adenocarcinoma of the lung can be predicted by the expression of a few critically important genes as measured by quantitative real-time reverse-transcriptase polymerase chain reaction in formalin-fixed paraffin-embedded primary tumors.

METHODS

Twenty-two prognostic genes for the metastatic phenotype were identified through complementary DNA microarray analysis of 4 cancer cell lines and bioinformatics analysis. Expression levels of a subset of these genes (n = 13) were measured by real-time time reverse-transcriptase polymerase chain reaction in formalin-fixed paraffin-embedded primary adenocarcinoma from patients whose disease recurred within 2 years (n = 9) and in patients who did not have a recurrence (n = 11). Receiver operating characteristic curves were analyzed to establish prognostic values of single genes. The most informative gene was combined with the remaining genes to determine whether there was a particular pair(s) that yielded high diagnostic accuracy. A small validation study was performed.

RESULTS

Receiver operating characteristic curve analysis of the single genes revealed that high expression of CK19 was associated with nonrecurrence (area under the curve = 0.859, confidence interval = 0.651-0.970). The CK19/EpCAM2 gene ratio had the most reproducible prognostic accuracy, followed by the CK19/P-cadherin ratio. A Kaplan-Meier survival analysis generated from the CK19/EpCAM2 ratio resulted in highly significant curves as a function of marker positivity (P = .0007; hazard ratio = 10.7). Significance declined but was maintained in the validation study.

CONCLUSIONS

This preliminary study provides evidence that the CK19/EpCAM2 and/or CK19/P-cadherin ratio(s) may be a simple and accurate prognostic indicator of clinical outcome in early-stage adenocarcinoma of the lung. If further validation studies from large patient cohorts confirm the results, adjuvant therapy could be targeted to this high-risk group.

Isolation of rare circulating tumour cells in cancer patients by microchip technology

Viable tumour-derived epithelial cells (circulating tumour cells or CTCs) have been identified in peripheral blood from cancer patients and are probably the origin of intractable metastatic disease. Although extremely rare, CTCs represent a potential alternative to invasive biopsies as a source of tumour tissue for the detection, characterization and monitoring of non-haematologic cancers. The ability to identify, isolate, propagate and molecularly characterize CTC subpopulations could further the discovery of cancer stem cell biomarkers and expand the understanding of the biology of metastasis. Current strategies for isolating CTCs are limited to complex analytic approaches that generate very low yield and purity. Here we describe the development of a unique microfluidic platform (the 'CTC-chip') capable of efficient and selective separation of viable CTCs from peripheral whole blood samples, mediated by the interaction of target CTCs with antibody (EpCAM)-coated microposts under precisely controlled laminar flow conditions, and without requisite pre-labelling or processing of samples. The CTC-chip successfully identified CTCs in the peripheral blood of patients with metastatic lung, prostate, pancreatic, breast and colon cancer in 115 of 116 (99%) samples, with a range of 5-1,281 CTCs per ml and approximately 50% purity. In addition, CTCs were isolated in 7/7 patients with early-stage prostate cancer. Given the high sensitivity and specificity of the CTC-chip, we tested its potential utility in monitoring response to anti-cancer therapy. In a small cohort of patients with metastatic cancer undergoing systemic treatment, temporal changes in CTC numbers correlated reasonably well with the clinical course of disease as measured by standard radiographic methods. Thus, the CTC-chip provides a new and effective tool for accurate identification and measurement of CTCs in patients with cancer. It has broad implications in advancing both cancer biology research and clinical cancer management, including the detection, diagnosis and monitoring of cancer.

Circulating tumor cells in colorectal cancer: correlation with clinical and pathological variables

BACKGROUND

The CellSearch System is a technique to detect circulating tumor cells (CTCs) in patients with cancer. Few data have been published concerning the role of CTCs detection by this method in colorectal cancer. The aim of this study was to correlate the presence of CTCs with the commonest clinical and morphological variables.

PATIENTS AND METHODS

Blood samples were collected from 97 patients and 30 healthy volunteers. Quantification of CTCs in 7.5 ml of blood was carried out with the CellSearch System. The results were expressed as number of CTCs/7.5 ml and the cut-off of >or=2 CTCs/7.5 ml was chosen to define the test as positive.

RESULTS

Positive CTCs were detected in 34 of 94 patients (36.2%). Correlation was not found among positive CTCs and location of primary tumor, increased carcinoembryonic antigen level, increased lactate dehydrogenase level or grade of differentiation. Only stage correlated with positive CTCs (20.7% in stage II, 24.1% in stage III and 60.7% in stage IV, P = 0.005).

CONCLUSIONS

CTCs detection by CellSearch is a highly reproducible method that correlates with stage but not with other clinical and morphological variables in patients with colorectal cancer. Colon cancer tumor cells are detectable in all stages. Further studies are warranted.

Phase I dose escalation study of the anti insulin-like growth factor-I receptor monoclonal antibody CP-751,871 in patients with refractory solid tumors

PURPOSE

This phase I study was undertaken to define the maximum tolerated dose, safety, and pharmacokinetic profile of CP-751,871.

EXPERIMENTAL DESIGN

Using a rapid dose escalation design, patients with advanced nonhematologic malignancies were treated with CP-751,871 in four dose escalation cohorts. CP-751,871 was administered i.v. on day 1 of each 21-day cycle. Pharmacokinetic evaluation was done in all treatment cohorts during cycles 1 and 4.

RESULTS

Twenty-four patients received 110 cycles at four dose levels. The maximum tolerated dose exceeded the maximal feasible dose of 20 mg/kg and, thus, was not identified. Treatment-related toxicities were generally mild. The most common adverse events were hyperglycemia, anorexia, nausea, elevated aspartate aminotransferase, elevated gamma-glutamyltransferase, diarrhea, hyperuracemia, and fatigue. At 20 mg/kg, 10 of 15 patients experienced stability of disease. Two of these patients experienced long-term stability. There were no objective responses. Pharmacokinetic analysis revealed a dose-dependent increase in CP-751,871 exposure and approximately 2-fold accumulation on repeated dosing in 21-day cycles. Plasma concentrations of CP-751,871 attained were several log-fold greater than the biologically active concentration. Treatment with CP-751,871 increased serum insulin and human growth hormone levels, with modest increases in serum glucose levels.

CONCLUSIONS

CP-751,871 has a favorable safety profile and was well tolerated when given in continuous cycles. At the maximal feasible dose of 20 mg/kg, there was a moderate accumulation in plasma exposure, and most of the treated patients experienced stability of disease.

The significance of circulating epithelial cells in Breast Cancer patients by a novel negative selection method

PURPOSE

A negative selection method for the enumeration and characterization of circulating epithelial/cancer cells (CCC) in Breast Cancer (BC) patients is described. This manual procedure yields reproducible results of high sensitivity and selectivity suitable for research laboratories.

PATIENTS AND METHODS

We conducted a prospective blood sampling study in 105 women with stage 1-4 BC attending clinics at the University of Maryland Greenebaum Cancer Center to define the prevalence of CCC utilizing our sensitive double gradient centrifugation and magnetic cell sorting CCC detection and enumeration method. CCC were isolated and enumerated from 15 to 20 ml of venous blood drawn before the start of systemic therapy and periodically thereafter for up to 24 months. One or more CCC/sample was considered a positive result.

RESULTS

We analyzed 487 samples for the presence of CCC; the median number of samples/patient was 4 (range 1-8). CCC were detected in 56% of patients, 19%-stage 1; 43%-stage 2; 46%-stage 3; 83%-stage 4. The probability of being positive for the presence of CCC is significantly associated with the stage of cancer (P < 0.0001). The frequency of CCC positive patients and samples increased with the advancing stage of disease. Presence of more than 10 CCC/sample was associated with the decreased survival and increased probability of having metastatic disease P = 0.001.

CONCLUSIONS

Increasing number of CCC/sample correlates with the adverse outcome and poorer survival (P < 0.0001). Our CCC test based on the negative selection procedure may provide valuable prognostic information.

Variation of circulating tumor cell levels during treatment of metastatic breast cancer: prognostic and therapeutic implications

BACKGROUND

This study aimed to evaluate the prognostic significance of circulating tumor cells (CTCs) detection in advanced breast cancer patients.

PATIENTS AND METHODS

We tested 80 patients for CTC levels before starting a new treatment and after 4, 8 weeks, at the first clinical evaluation and every 2 months thereafter. CTCs were detected using the CellSearch System.

RESULTS

Forty-nine patients had >or=5 CTCs at baseline. At the multivariate analysis, baseline number of CTCs was significantly associated with progression-free survival [hazard ratio (HR) 2.5; 95% confidence interval (CI) 1.2-5.4]. The risk of progression for patients with CTCs >or=5 at last available blood draw was five times the risk of patients with 0-4 CTCs at the same time point (HR 5.3; 95% CI 2.8-10.4). Patients with rising or persistent >or=5 CTCs at last available blood draw showed a statistically significant higher risk of progression with respect to patients with <5 CTCs at both blood draws (HR 6.4; 95% CI 2.8-14.6).

CONCLUSION

CTCs basal value is a predictive indicator of prognosis and changes in CTC levels during therapy may indicate a clinical response. Testing CTC levels during targeted treatments might substitute other measurement parameters for response evaluation.

Potential applications for circulating tumor cells expressing the insulin-like growth factor-I receptor

PURPOSE

To detect insulin-like growth factor-IR (IGF-IR) on circulating tumor cells (CTC) as a biomarker in the clinical development of a monoclonal human antibody, CP-751,871, targeting IGF-IR.

EXPERIMENTAL DESIGN

An automated sample preparation and analysis system for enumerating CTCs (CellTracks) was adapted for detecting IGF-IR-positive CTCs with a diagnostic antibody targeting a different IGF-IR epitope to CP-751,871. This assay was used in three phase I trials of CP-751,871 as a single agent or with chemotherapy and was validated using cell lines and blood samples from healthy volunteers and patients with metastatic carcinoma.

RESULTS

There was no interference between the analytic and therapeutic antibodies. Eighty patients were enrolled on phase I studies of CP-751,871, with 47 (59%) patients having CTCs detected during the study. Before treatment, 26 patients (33%) had CTCs, with 23 having detectable IGF-IR-positive CTCs. CP-751,871 alone, and CP-751,871 with cytotoxic chemotherapy, decreased CTCs and IGF-IR-positive CTCs; these increased toward the end of the 21-day cycle in some patients, falling again with retreatment. CTCs were commonest in advanced hormone refractory prostate cancer (11 of 20). Detectable IGF-IR expression on CTCs before treatment with CP-751,871 and docetaxel was associated with a higher frequency of prostate-specific antigen decline by >50% (6 of 10 versus 2 of 8 patients). A relationship was observed between sustained decreases in CTC counts and prostate-specific antigen declines by >50%.

CONCLUSIONS

IGF-IR expression is detectable by immunofluorescence on CTCs. These data support the further evaluation of CTCs in pharmacodynamic studies and patient selection, particularly in advanced prostate cancer.

Cellular image analysis and imaging by flow cytometry

Imaging flow cytometry combines the statistical power and fluorescence sensitivity of standard flow cytometry with the spatial resolution and quantitative morphology of digital microscopy. The technique is a good fit for clinical applications by providing a convenient means for imaging and analyzing cells directly in bodily fluids. Examples are provided of the discrimination of cancerous from normal mammary epithelial cells and the high-throughput quantitation of fluorescence in situ hybridization (FISH) probes in human peripheral blood mononuclear cells. The FISH application will be enhanced further by the integration of extended depth-of-field imaging technology with the current optical system.

Detección y cuantificación de células tumorales en sangre periférica en pacientes con cáncer de colon

BACKGROUND AND OBJECTIVE

The aim of this study is the detection and quantification of circulating tumor cells (CTC) in patients diagnosed with colon cancer and to establish whether they are related to the main clinicopathologic variables for this type of carcinoma.

PATIENTS AND METHOD

Twenty-five colon cancer patients and 30 healthy volunteers were analysed. The quantification was performed using the CellSpotter Analyzer (Veridex LLC), that allows immunomagnetic isolation and immunospecific labelling of the cells for their enumeration.

RESULTS

72% of the colon cancer patients showed CTC and the mean number of cells found was 5 CTC/7.5 ml of peripheral blood. 52% of the samples contained 2 or more cells. Considering 2 cells as the cut-off point, a significant relationship with lactate dehydrogenase was found.

CONCLUSIONS

This new technology which allows isolation and quantification of CTC in peripheral blood has proven to be valid for the detection of epithelial cells in colon cancer patients in every tumor stage. The results shown in this work confirm that cytokeratin 8, 18 and 19 are detected in CTC in this tumor type and will allow us to develop a protocol for the study of the relationship of quantification of theses cells and the clinical parameters involved in colon cancer.

Detecting carcinoma cells in peripheral blood of patients with hepatocellular carcinoma by immunomagnetic beads and rt-PCR

BACKGROUND

Increasing the sensitivity and specificity of detecting circulating carcinoma cells of patients with hepatocellular carcinoma (HCC) is very important for monitoring recurrence.

GOAL

To establish a novel method of detecting circulating carcinoma cells.

STUDY

For method development, 3 sets of controls using HCC cell line HepG2 cells were used. (A): Serial dilutions of HepG2 cells were directly used to extract total RNA for nested reverse transcription-polymerase chain reaction (RT-PCR). (B): Five milliliter of healthy blood was spiked with a serial dilution of HepG2 cells and was used for Ficoll density gradient centrifugation to recover cells. The cells were used to extract total RNA for RT-PCR. (C): After cell recovery with the same procedure as B, the cells were sorted sequentially by CD45 and Ber-EP4 immunomagnetic beads and used for RNA extraction and RT-PCR. For clinical samples, 44 patients with HCC and 7 healthy subjects were included. The alpha-fetoprotein mRNA was amplified using nested RT-PCR technique.

RESULTS

The spiking experiments using HepG2 cells showed that 10 cells in 5 mL blood could be detected by method C and an excellent dose-response to the number of spiked cells. Whereas, method B lacked any dose-response and would yield high false-positive rates. In clinical samples, the improved method led to a positive detection rate of 52.9%, 76.9%, and 92.9% in Child-Plug class A, B, and C, respectively. There was significant difference between class A and class C (P<0.05). The total positive detection rate was 72.7%.

CONCLUSIONS

Combining negative and positive immunomagnetic beads with RT-PCR technique may improve the sensitivity and specificity of detecting circulating HCC cells.

Rare-Event Analysis in Flow Cytometry

Technical aspects of rare-event detection are discussed in this article in a practical context, with two real-life examples. A growing number of flow cytometry-based assays depend on rare-event detection for basic science and clinical applications.

Circulating tumor cells in peripheral blood caused by surgical manipulation of non-small-cell lung cancer: pilot study using an immunocytology method

OBJECTIVE

Reverse transcriptase-polymerase chain reaction assay results have indicated that tumor cells sometimes appear during surgery for primary non-small-cell lung cancer. In this study, we attempted to determine whether cancer cells can be detected during and after surgery using an immunocytology method.

METHODS

Nine patients undergoing a lobectomy for non-small-cell lung cancer were studied. The presence of circulating tumor cells was determined by the detection of magnified EpCAM antibodies. The criteria used to identify circulating tumor cells were a round-to-oval morphology with a visible nucleus (4'-6'-diamidino-2-phenylindole (DAPI)-positive), which were positive for cytokeratin and negative for CD45.

RESULTS

One patient showed evidence of circulating tumor cells at thoracotomy, and 3 patients did so after surgery. Ten days after the operation, the circulating tumor cells had disappeared in all these cases. The median follow-up period was 14 months, and there was no cancer recurrence in any of the patients.

CONCLUSION

Using this technique, tumor cells were detected in the peripheral blood of patients before and after lobectomy procedures. It could be argued that this method can provide useful information about patients undergoing lung cancer treatment.

In vivo quantitation of rare circulating tumor cells by multiphoton intravital flow cytometry

Quantitation of circulating tumor cells (CTCs) constitutes an emerging tool for the diagnosis and staging of cancer, assessment of response to therapy, and evaluation of residual disease after surgery. Unfortunately, no existing technology has the sensitivity to measure the low numbers of tumor cells (<1 CTC per ml of whole blood) that characterize minimal levels of disease. We present a method, intravital flow cytometry, that noninvasively counts rare CTCs in vivo as they flow through the peripheral vasculature. The method involves i.v. injection of a tumor-specific fluorescent ligand followed by multiphoton fluorescence imaging of superficial blood vessels to quantitate the flowing CTCs. Studies in mice with metastatic tumors demonstrate that CTCs can be quantitated weeks before metastatic disease is detected by other means. Analysis of whole blood samples from cancer patients further establishes that human CTCs can be selectively labeled and quantitated when present at approximately 2 CTCs per ml, opening opportunities for earlier assessment of metastatic disease.

Circulating tumor cell analysis in patients with progressive castration-resistant prostate cancer

PURPOSE

To better direct targeted therapies to the patients with tumors that express the target, there is an urgent need for blood-based assays that provide expression information on a consistent basis in real time with minimal patient discomfort. We aimed to use immunomagnetic-capture technology to isolate and analyze circulating tumor cells (CTC) from small volumes of peripheral blood of patients with advanced prostate cancer.

EXPERIMENTAL DESIGN

Blood was collected from 63 patients with metastatic prostate cancer. CTCs were isolated by the Cell Search system, which uses antibodies to epithelial cell adhesion marker and immunomagnetic capture. CTCs were defined as nucleated cells positive for cytokeratins and negative for CD45. Captured cells were analyzed by immunofluorescence, Papanicolau staining, and fluorescence in situ hybridization.

RESULTS

Most patients (65%) had 5 or more CTCs per 7.5 mL blood sample. Cell counts were consistent between laboratories (c = 0.99) and did not change significantly over 72 or 96 h of storage before processing (c = 0.99). Their identity as prostate cancer cells was confirmed by conventional cytologic analysis. Molecular profiling, including analysis of epidermal growth factor receptor (EGFR) expression, chromosome ploidy, and androgen receptor (AR) gene amplification, was possible for all prostate cancer patients with >or=5 CTCs.

CONCLUSIONS

The analysis of cancer-related alterations at the DNA and protein level from CTCs is feasible in a hospital-based clinical laboratory. The alterations observed in EGFR and AR suggest that the methodology may have a role in clinical decision making.

Dormant Tumor Cells Develop Cross-Resistance to Apoptosis Induced by CTLs or Imatinib Mesylate via Methylation of Suppressor of Cytokine Signaling 1

In the BCR/ABL DA1-3b mouse model of acute myelogenous leukemia, dormant tumor cells may persist in the host in a state of equilibrium with the CD8(+) CTL-mediated immune response by actively inhibiting T cells. Dormant tumor cells also show a progressive decrease of suppressor of cytokine signaling 1 (SOCS1) gene expression and a deregulation of the Janus-activated kinase/signal transducers and activators of transcription (JAK/STAT) pathway due to methylation of the SOCS1 gene. Dormant tumor cells were more resistant to apoptosis induced by specific CTLs, but resistance decreased when SOCS1 expression was restored via demethylation or gene transfer. AG490 JAK2 inhibitor decreased the resistance of dormant tumor cells to CTLs, but MG132 proteasome inhibitor was effective only in SOCS1-transfected cells. Thus, SOCS1 regulation of the JAK/STAT pathways contributes to the resistance of tumor cells to CTL-mediated killing. Resistance of dormant tumor cells to apoptosis was also observed when induced by irradiation, cytarabine, or imatinib mesylate, but was reduced by SOCS1 gene transfer. This cross-resistance to apoptosis was induced by interleukin 3 (IL-3) overproduction by dormant tumor cells and was reversed with an anti-IL-3 antibody. Thus, tumor cells that remain dormant for long periods in the host in spite of a specific CTL immune response may deregulate their JAK/STAT pathways and develop cross-resistance to various treatments through an IL-3 autocrine loop. These data suggest possible new therapeutic targets to eradicate dormant tumor cells.