Background

[PubMed]

Prostate stem-cell antigen (PSCA) is a cell-surface protein (123 amino acids) that is expressed in normal prostate tissue and overexpressed in prostate cancer tissues (1). PSCA expression is detected in >80% of patients with local disease, including high expression in bone metastases (2). Elevated levels of PSCA are correlated with increased tumor stage, grade, and androgen independence (3). PSCA overexpression is also observed in bladder and pancreatic cancers (4).

The anti-PSCA murine monoclonal antibody (mAb) 1G8 has been shown to have anti-tumor activity (5). A humanized version of the 1G8 mAb (hu1G8) has been radiolabeled as ¹²⁴I-hu1G8 for tumor imaging in mice (6). However, it took ~1 week to observe an enhanced target/background ratio with positron emission tomography (PET) because of the slow kinetics of the mAb. A hu1G8 minibody fragment (2B3), a dimer of scFvs-C_H3 with a linker composed of 18 amino acids (molecular weight, ~80 kDa), has been developed and labeled as ¹²⁴I-2B3 minibody for tumor-targeting studies (7). To create a stable, rapidly clearing antibody fragment, the parental 2B3 diabody (p2B3-Db) (molecular weight, 55 kDa) was mutated back to the original mouse residues to produce a high-affinity, back-mutated diabody with a linker of 8 amino acids (bm2B3-Db8) (8). ¹²⁴I-p2B3-Db8 and ¹²⁴I-bm2B3-Db8 were evaluated for tumor imaging.

Synthesis

[PubMed]

The p2B3-Db8 and bm2B3-Db8 diabodies were labeled with [¹²⁴I]sodium iodide by the Iodogen method (8). ¹²⁴I-p2B3-Db8 and ¹²⁴I-bm2B3-Db8 were purified with gel filtration, and each labeled diabody exhibited a specific activity of ~5.6 MBq/nmol (0.15 mCi/nmol).

In Vitro Studies: Testing in Cells and Tissues

[PubMed]

With the use of PSCA competition assays, binding affinity (K_d) values were found to be 5.4 nM for p2B3-Db8, 1.9 nM for bm2B3-Db8, and 46 nM for 2B3 minibody (8).

Animal Studies

Human Studies

[PubMed]

No publication is currently available.

NIH Support

CA92131, CA86306, P01 CA43904, CA16042

References

1.

Reiter R.E., Gu Z., Watabe T., Thomas G., Szigeti K., Davis E., Wahl M., Nisitani S., Yamashiro J., Le Beau M.M., Loda M., Witte O.N. Prostate stem cell antigen: a cell surface marker overexpressed in prostate cancer. Proc Natl Acad Sci U S A. 1998;95(4):1735–40. [PMC free article: PMC19171] [PubMed: 9465086]

2.

Lam J.S., Yamashiro J., Shintaku I.P., Vessella R.L., Jenkins R.B., Horvath S., Said J.W., Reiter R.E. Prostate stem cell antigen is overexpressed in prostate cancer metastases. Clin Cancer Res. 2005;11(7):2591–6. [PubMed: 15814638]

3.

Gu Z., Thomas G., Yamashiro J., Shintaku I.P., Dorey F., Raitano A., Witte O.N., Said J.W., Loda M., Reiter R.E. Prostate stem cell antigen (PSCA) expression increases with high gleason score, advanced stage and bone metastasis in prostate cancer. Oncogene. 2000;19(10):1288–96. [PubMed: 10713670]

4.

Amara N., Palapattu G.S., Schrage M., Gu Z., Thomas G.V., Dorey F., Said J., Reiter R.E. Prostate stem cell antigen is overexpressed in human transitional cell carcinoma. Cancer Res. 2001;61(12):4660–5. [PubMed: 11406532]

5.

Gu Z., Yamashiro J., Kono E., Reiter R.E. Anti-prostate stem cell antigen monoclonal antibody 1G8 induces cell death in vitro and inhibits tumor growth in vivo via a Fc-independent mechanism. Cancer Res. 2005;65(20):9495–500. [PubMed: 16230414]

6.

Olafsen T., Gu Z., Sherman M.A., Leyton J.V., Witkosky M.E., Shively J.E., Raubitschek A.A., Morrison S.L., Wu A.M., Reiter R.E. Targeting, imaging, and therapy using a humanized antiprostate stem cell antigen (PSCA) antibody. J Immunother. 2007;30(4):396–405. [PubMed: 17457214]

7.

Leyton J.V., Olafsen T., Lepin E.J., Hahm S., Bauer K.B., Reiter R.E., Wu A.M. Humanized radioiodinated minibody for imaging of prostate stem cell antigen-expressing tumors. Clin Cancer Res. 2008;14(22):7488–96. [PMC free article: PMC2720761] [PubMed: 19010866]

8.

Leyton J.V., Olafsen T., Sherman M.A., Bauer K.B., Aghajanian P., Reiter R.E., Wu A.M. Engineered humanized diabodies for microPET imaging of prostate stem cell antigen-expressing tumors. Protein Eng Des Sel. 2009;22(3):209–16. [PMC free article: PMC2644405] [PubMed: 18957406]