Background

[PubMed]

Mucins (MUC) are a distinct class of transmembrane glycoproteins (designated as MUC1, MUC3A, MUC3B, MUC4, etc.) that are expressed primarily by the glandular and ductal cells of the epithelium in the human body and are known to modulate the tumorigenicity, progression, and pathogenesis of different cancers (1). The structure, physiological function, and process of promoting the development of malignancies by MUCs, particularly MUC1 and MUC4, are discussed in detail by Bafna et al. (1). MUC1 has been shown to be characteristically overexpressed by pancreatic adenocarcinoma (PAC) cells and is not detectable in normal pancreatic ducts or other normal tissues, or during an incident of pancreatitis (2). Because it is detectable only in the serum of patients suffering from PAC, it was suggested recently that the MUC1 antigen can be used as a biomarker for the early detection this malignancy (3, 4).

The majority of individuals suffering from PAC do not survive for more than 1 year after diagnosis, and <1% of these patients live beyond 5 years (5). Although surgical resection of the cancer is a possible treatment for this disease, only 10%–25% of the patients are considered suitable for this treatment because, by the time that the neoplasm is detected, the malignancy has metastasized and the tumor load in the patient is too high to warrant surgery (5). Patients with nonresectable PAC are treated either with gemcitabine or radiotherapy; however, these treatments are not curative and only prolong survival and improve the quality of life of the patient (5). The detection of this invasive cancer at an early stage would facilitate proper staging of the disease, which could be followed either by surgical resection of the tumor or the initiation of a vigorous treatment regimen that could possibly improve patient prognosis (6).

More than a decade ago, a ¹³¹I-labeled murine monoclonal antibody, PAM4 ([¹³¹I]-mPAM4), directed against a mucin (later identified to be MUC1 (5)) was reported to detect human xenograft PAC tumors in mice with high specificity and, compared to controls, significantly extended the survival time of the animals when used for radiotherapy of the cancer (7-9). In a preliminary study, [¹³¹I]-PAM4 was demonstrated to detect the primary and the metastasized pancreatic carcinoma tumors in humans, and there was a low nonspecific uptake of the tracer in the liver, spleen, and bone marrow of the patients (10). In another study, a chimeric form of PAM4 (cPAM4) was labeled with ¹²⁵I and ¹¹¹In to obtain [¹²⁵I]-cPAM4 and [¹¹¹In]-cPAM4, respectively, and the biodistribution of these labeled mAbs was investigated in mice bearing human xenograft PAC tumors (11). Recently, Gulec et al. studied the biodistribution of ¹¹¹In-labeled humanized PAM4 ( [¹¹¹In]-hPAM4) in a phase I clinical trial (2).

Synthesis

[PubMed]

The mPAM4 (8), cPAM4 (11), and hPAM4 (2) used in the different investigations were obtained from a commercial source.

mPAM4 was labeled with ¹³¹I using the chloramine-T method, and the specific activity (SA) of the labeled antibody was reported to be 259–444 kBq/μg (7–12 μCi/μg) (7). The [¹³¹I]-mPAM4 preparation contained <3% free ¹³¹I and no aggregates as determined with thin-layer chromatography (TLC). Gold et al. labeled mPAM4 with ¹³¹I using the Iodo-Gen procedure (5). The labeling efficiency of this method was reported to be 90%; the SA of the labeled mAb was 370–555 MBq/mg (10–15 mCi/mg), and the final preparation contained <5% unbound radioactivity as determined with high-performance liquid chromatography (HPLC) (5).

cPAM4 was labeled with ¹²⁵I using the chloramine-T procedure as described above (11). The labeled mAb had a SA of 365.6–455 kBq/μg (9.88–12.3 μCi/μg) and contained <5% unbound ¹²⁵I and <8% aggregates.

cPAM4 was labeled with ¹¹¹In after conjugation of the mAb to 1,4,7,10-tetraazacyclododecane-N,N’,N’’,N’’’-tetraacetic acid (DOTA) (11), and the DOTA:cPAM4 ratio of the conjugate was reported in another publication to be 3.3 (12). The SA of [¹¹¹In]-cPAM4 was reported to vary from 200 kBq/μg (5.41 μCi/μg) to 48.1–133.2 MBq/mg (1.3–3.6 mCi/mg) and contained <1.7% unbound radionuclides and <1% aggregates (5, 11, 12).

The biodistribution and tumor targeting of [¹¹¹In]-hPAM4 was investigated in patients suffering from pancreatic cancer in a phase I clinical trial (2). Although the [¹¹¹In]-hPAM4 preparation used in the clinical investigation was reported to contain <10% free radionuclide as determined with instant TLC, the SA of the labeled antibody was not reported.

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

No publication is currently available.

Animal Studies

Human Studies

[PubMed]

Two patients with histologically confirmed metastatic pancreatic cancer were injected with [¹³¹I]-mPAM4; one patient received 270 MBq (7.3 mCi) [¹³¹I]-mPAM4 in a protein dose of 0.6 mg, and the other patient received 315 MBq (8.5 mCi) [¹³¹I]-mPAM4 in a protein dose of 0.6 mg (5). Blood samples taken from the two patients showed that the clearance of radioactivity from this tissue was biexponential. The circulating half-life of the tracer in the early component of the curve was 1.9 h in the patient with the smaller dose and 4.6 h in the patient with the larger dose; in the late component the half-life values were 28.6 h and 39.2 h, respectively. HPLC analysis of the plasma obtained from these patients at 24 h p.i. showed the presence of 66%–78% ¹³¹I in the small molecular weight fraction, indicating that mPAM4 was catabolized in the blood within 24 h p.i. Scintigraphy was performed on a patient injected with [¹³¹I]-mPAM4 (the patient had previously undergone chemotherapy and radiotherapy for the treatment of PAC), and clear uptake of the tracer was observed in tumors on the left hepatic lobe and in the mid-abdomen area of the patient (5). Similarly, tumors were also detected in posterior and anterior planar images acquired from a second patient injected with the radioiodinated mAb (5). From this study, the investigators concluded that mPAM4 specifically targeted pancreatic cancer tumors in the patients.

In a phase I clinical study to evaluate the efficacy of [⁹⁰Y]-hPAM4 for the treatment of PAC, 21 patients (4 patients with stage III, locally advanced disease; 17 patients with stage IV metastatic disease) were administered a slow intravenous injection of [¹¹¹In]-hPAM4 to investigate the biodistribution of the tracer (a surrogate for the ⁹⁰Y-labeled homolog) in the individuals (2). The patients received 3–5 mCi of the label, and the total protein content of each dose, including the unlabeled hPAM4, was 10 mg (for 6 patients) and 100 mg (for 16 patients). Starting at 4 h p.i., repeat whole-body planar scintigraphic images were acquired from the patients over 7 days. Pancreatic tumors were clearly visible at 24 h p.i. (only a representative image was presented (2)), and the visibility was progressively prominent on the subsequent images. From a review of the various images obtained from this study, the investigators concluded there was no qualitative difference was apparent in the biodistribution of [¹¹¹In]-hPAM4 at the two doses (10 and 100 mg total hPAM4), the humanized mAb showed a normal biodistribution in the patients suffering from PAC and was suitable for the scintigraphic visualization of PAC tumors in humans (2).

Supplemental Information

[Disclaimers]

No information is currently available.

NIH Support

Studies described in this chapter were supported in part by grants CA39841, CA43218, and CA54425 from the National Institutes of Health.

References

1.

Bafna S., Kaur S., Batra S.K. Membrane-bound mucins: the mechanistic basis for alterations in the growth and survival of cancer cells. Oncogene. 2010;29(20):2893–904. [PMC free article: PMC2879972] [PubMed: 20348949]

2.

Gulec S.A., Cohen S.J., Pennington K.L., Zuckier L.S., Hauke R.J., Horne H., Wegener W.A., Teoh N., Gold D.V., Sharkey R.M., Goldenberg D.M. Treatment of Advanced Pancreatic Carcinoma with 90Y-Clivatuzumab Tetraxetan: A Phase I Single-Dose Escalation Trial. Clin Cancer Res. 2011;17(12):4091–100. [PMC free article: PMC4048986] [PubMed: 21527562]

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4.

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7.

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8.

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9.

Gold D.V., Cardillo T., Vardi Y., Blumenthal R. Radioimmunotherapy of experimental pancreatic cancer with 131I-labeled monoclonal antibody PAM4. Int J Cancer. 1997;71(4):660–7. [PubMed: 9178823]

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