Background

[PubMed]

Carcinoembryonic antigen (CEA) was first identified from extracts of human colon adenocarcinoma (1) and fetal gut (2). It is a β-glycoprotein, and its predominant expression on the cell surface is increased in a variety of carcinomas and in certain inflammatory states such as inflammatory bowel disease (3, 4). CEA has a molecular weight of ~180 kDa, and it can be shed and detected in the serum (5). CEA expression is observed in patients with various carcinomas of the colon, lungs, thyroid, uterus, ovaries, pancreas, and medullary thyroid (6). Radiolabeled monoclonal antibodies (mAbs) have been developed for both the diagnosis and treatment of tumors (7, 8). ^99mTc-Arcitumomab (a murine anti-CEA mAbFab’ fragment) was approved by the United States Food and Drug Administration in 1999 for whole-body planar and single-photon emission computed tomography (SPECT) imaging of CEA expression.

Single-chain variable fragments (scFvs) of antibodies with a molecular mass of 25 kDa are cleared very rapidly from the circulation, but they exhibit poor tumor retention because they have lower affinity than the parent antibody (9). On the other hand, bivalent antibody fragments possess more ideal tumor-targeting characteristics, including rapid tissue penetration, high target retention, and rapid blood clearance. The diabody fragment (a dimer of scFvs; molecular mass = 55 kDa) has been evaluated for targeting in several tumor antigen systems and has demonstrated rapid tumor localization and high-contrast imaging (9, 10). In particular, a murine anti-CEA T84.66 diabody, which retains excellent CEA-binding properties, was radiolabeled with ¹²⁴I for localization of CEA-positive tumors in mice (11). However, for clinical development, ¹⁸F offers the advantages of easy availability, a higher positron yield (nearly 100% versus 23% for ¹²⁴I), and a short half-life (109.7 min versus 4.18 days for ¹²⁴I). Therefore, a diabody labeled with ¹⁸F is more suitable for routine clinical use because of the biological targeting and clearance kinetics of diabodies. 4-[¹⁸F]Fluorobenzoyl-T84.66 diabody ([¹⁸F]FB-T84.66 diabody) is being developed as a positron emission tomography (PET) agent to image CEA-expressing tumors.

Synthesis

[PubMed]

N-Succinimidyl 4-[¹⁸F]fluorobenzoate ([¹⁸F]SFB) was synthesized in two steps from [¹⁸F]KF (Kryptofix 2.2.2./K₂CO₃) in 138 ± 8 min using a commercially available synthetic module with a radiochemical yield of 32.5 ± 5.9% (decay-corrected) (12). [¹⁸F]SFB was conjugated to T84.66 diabody to form [¹⁸F]FB-T84.66 diabody at 40°C for 30 min. [¹⁸F]FB-T84.66 diabody was isolated by gel filtration through PD-10 column with a radiochemical purity of >98%. The overall radiochemical yields were 1.40 ± 0.16% (decay-corrected). The specific activity was 1.83 ± 1.71 GBq/μmol (49.5 ± 46.2 Ci/μmol) at the end of synthesis. Total synthesis time was 35 ± 5 min starting from [¹⁸F]SFB.

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

Radioimmunoreactivity of [¹⁸F]FB-T84.66 diabody for LS 174T human colon carcinoma cell (high CEA expression) was determined to be 57.1 ± 2.0% (n = 4) (12), higher than the previously reported value of 42% for the ¹²⁴I-labeled T84.66 diabody (11). For CEA-negative C6 rat glioma cells, which are negative for CEA expression, [¹⁸F]FB-T84.66 diabody binding was 6.3 ± 1.6% (n = 4), indicating minimal nonspecific binding.

Animal Studies

Human Studies

[PubMed]

No publication is currently available.

NIH Support

R21 EB001785, R21 CA102123, P50 CA114747, U54 CA119367, R24 CA93862,

P01 CA43904, P50 CA86306, CA16042

References

1.

Gold P., Freedman S.O. Demonstration of tumor-specific antigens in human colonic carcinomata by immunological tolerance and absorption techniques. The Journal of Experimental Medicine. 1964;121:439–462. [PMC free article: PMC2137957] [PubMed: 14270243]

2.

Krupey J., Gold P., Freedman S.O. Purification and characterization of carcinoembryonic antigens of the human digestive system. Nature. 1967;215(5096):67–8. [PubMed: 6053407]

3.

Kowalsky, R.J., and S.W. Falen, Radiopharmaceuticals in nuclear pharmacy and nuclear medicine2004, American Pharmacists Association: Washington, D.C. p. 733-752.

4.

Wahl R.L., Philpott G., Parker C.W. Monoclonal antibody radioimmunodetection of human-derived colon cancer. Invest Radiol. 1983;18(1):58–62. [PubMed: 6832932]

5.

Primus F.J., Freeman J.W., Goldenberg D.M. Immunological heterogeneity of carcinoembryonic antigen: purification from meconium of an antigen related to carcinoembryonic antigen. Cancer Res. 1983;43(2):679–85. [PubMed: 6401222]

6.

Package Insert. CEA-Scan (Arcitumomab) for the preparation of Tc 99m Arcitumomab1999, Immunomedics, Inc. p. 1-15.

7.

Kenanova V., Wu A.M. Tailoring antibodies for radionuclide delivery. Expert Opin Drug Deliv. 2006;3(1):53–70. [PubMed: 16370940]

8.

Wu A.M., Senter P.D. Arming antibodies: prospects and challenges for immunoconjugates. Nat Biotechnol. 2005;23(9):1137–46. [PubMed: 16151407]

9.

Wu A.M., Chen W., Raubitschek A., Williams L.E., Neumaier M., Fischer R., Hu S.Z., Odom-Maryon T., Wong J.Y., Shively J.E. Tumor localization of anti-CEA single-chain Fvs: improved targeting by non-covalent dimers. Immunotechnology. 1996;2(1):21–36. [PubMed: 9373325]

10.

Viti F., Tarli L., Giovannoni L., Zardi L., Neri D. Increased binding affinity and valence of recombinant antibody fragments lead to improved targeting of tumoral angiogenesis. Cancer Res. 1999;59(2):347–52. [PubMed: 9927045]

11.

Sundaresan G., Yazaki P.J., Shively J.E., Finn R.D., Larson S.M., Raubitschek A.A., Williams L.E., Chatziioannou A.F., Gambhir S.S., Wu A.M. 124I-labeled engineered anti-CEA minibodies and diabodies allow high-contrast, antigen-specific small-animal PET imaging of xenografts in athymic mice. J Nucl Med. 2003;44(12):1962–9. [PMC free article: PMC4167879] [PubMed: 14660722]

12.

Cai W., Olafsen T., Zhang X., Cao Q., Gambhir S.S., Williams L.E., Wu A.M., Chen X. PET Imaging of Colorectal Cancer in Xenograft-Bearing Mice by Use of an 18F-Labeled T84.66 Anti-Carcinoembryonic Antigen Diabody. J Nucl Med. 2007;48(2):304–310. [PubMed: 17268029]