There are several important findings from this study. The primary conclusion is that the PAM4-immunoassay is able to identify approximately two-thirds of stage-1 PDAC patients, and does so with high discriminatory power with respect to benign pancreatic disease. To the best of our knowledge, there are only a few reports that describe the use of a noninvasive biomarker assay to detect stage-1 disease, and the majority of these discuss the performance characteristics of CA19-9.28–30
The sensitivity reported for CA19-9 in stage-1 PDAC ranges from 40%-64%, with our results showing a detection rate of 58%. However, the specificity reported for CA19-9 in the literature14,31,32
is considerably lower than reported for the PAM4-antigen, as is also true for the paired study described herein, particularly with respect to discrimination of PDAC and CP. Positive likelihood ratios were significantly higher for the PAM4-immunoassay.
The data suggest that the PAM4-antigen is not expressed by pancreatic tumors originating from non-epithelial tissues. However, PAM4-antigen is expressed and released by biliary and periampullary adenocarcinomas. Detection of these latter cancers, although rare (approximately 3500 new cases/year altogether in the U.S), is likely to prove of clinical value, with follow-up imaging studies providing confirmation of tumor mass and location. That these latter cancers express the PAM4-antigen and are detectable by the PAM4-immunoassay was not unexpected, considering that these tissues are derived from closely-related structures in early embryonic development. Indeed, many of the reported biomarkers for PDAC are reactive with these tumors as well. The limited expression of PAM4 in the control colon cancer group confirms our prior serum assay and immunohistochemical studies indicating that PAM4 has limited elevation with other gastrointestinal and non-gastrointestinal cancers.17–20
Also of importance is the ability of the PAM4-immunoassay to discriminate PDAC (and adjacent carcinomas) from benign, non-malignant disease of the pancreas, and in particular, the discrimination of PDAC and CP. In a prior study,20
we reported a discordance between the PAM4-antigen levels in the serum of patients with CP (N=29 with 38% FP) and immunohistochemical data on a separate group of patients with CP, where only 2 of 19 specimens had evidence of weak, focal expression of PAM4-antigen localized to ADM. In a followup immunohistochemical study of an additional 32 surgically resected specimens from CP patients, PAM4 labeling was observed in 6 of 32 (19%); however, PAM4 reactivity was restricted to the PanIN precursor lesions associated with CP, and was not observed in non-neoplastic inflamed pancreatic tissue.21
Taken together, the data from immunohistochemical labeling of over 50 specimens of chronic pancreatitis demonstrate that mAb-PAM4 does not react with the inflamed, non-neoplastic tissues. Thus, the question of the biological and clinical significance of a positive serum PAM4-antigen level in this patient group is of great importance. Are these assay results false positives or do they provide evidence of incipient PDAC (and/or precursor lesions)? Unfortunately, clinical followup for this patient group was unavailable. However, at the very least, the data suggest the need to undertake a prospective investigation to identify PAM4-positive CP patients, and/or others considered to be at high-risk for PDAC, for whom followup surveillance could be performed for potential discovery of early PDAC.
Based on the limitations of the CA19-9 assay (the CA19-9 assay cannot be performed in Lewis antigen-negative patients and is altered by bilirubin levels), we determined that a combined PAM4 and CA19-9 biomarker assay would provide a superior detection and diagnostic tool than either assay alone. Overall sensitivity was improved without loss of specificity. Thus, a positive result provides a rationale for proceeding to diagnostic imaging for confirmation of disease. Further, this combined biomarker assay could play a role in monitoring of treatment and perhaps prove useful for earlier detection of recurrent disease.
Due to the asymptomatic nature of PDAC, and its relatively low incidence, both medical and economic concerns have argued against screening of the general population at large.33–35
However, with recent studies providing the ability to identify specific patient groups at high risk for PDAC, a rationale exists for longitudinal surveillance as a means to improve early detection specifically in these risk-settings. Individuals with a family history of PDAC,2,36,37
those patients with long-standing CP,38,39
patients with new-onset diabetes who meet certain other conditions,40,41
or those with FAMMM syndrome,42
etc., could be evaluated on a long-term basis for the detection of early malignant changes by use of the combined PAM4/CA19-9 assay procedure. Imaging has played an important role in these surveillance programs, the most significant being endoscopic ultrasound (EUS) scanning of the pancreas. EUS offers the additional ability to obtain fine-needle aspirates and/or fluids (pancreatic juice and cyst fluids) that can be examined for specific morphologic and biomarker changes representative of malignancy. Several reports have identified increased levels of CEACAMs and CA19-9 antigen,43–45
or have observed mutations in KRAS
and other genes potentially indicative of cancer from these endoscopically-retrieved materials.15,46,47
Likewise, it is of interest to evaluate these fluids for the PAM4-antigen. However, it is obvious that use of a serum-based biomarker to detect early-stage PDAC would provide a clinically more valuable and cost-effective tool for monitoring patients at high-risk for this disease.
In summary, a blinded analysis of over 600 patient specimens demonstrated that the PAM4-assay could detect two thirds of patients with early PDAC, and did so with high specificity by discriminating PDAC from benign diseases of the pancreas. Further, inclusion of a second biomarker, CA19-9, significantly enhanced overall, positive identification of PDAC patients without loss of specificity. Although we have examined the CA19-9 assay in combination with the PAM4-assay, we appreciate that other biomarkers may prove of equal or greater value in combination with PAM4. Thus, our results provide a basis for future studies of biomarker combinations with PAM4 for surveillance of patients at high risk for PDAC, and as a potential means for monitoring patients with more advanced disease. Finally, the ability of the PAM4-immunoassay to identify a significant number of patients with biliary and periampullary adenocarcinomas, although relatively rare, may provide an additional means for improving the management of patients with these cancers.