PET imaging of GRP78 expression using ⁶⁴Cu-labeled MAb159 antibody

Objectives The overexpression of GRP78 receptor on cell surface has been linked with the tumor growth, metastasis, and resistance to therapy. Based on a novel anti-GRP78 monoclonal antibody (MAb159), we developed a ⁶⁴Cu-labeled probe for positron emission tomography (PET) imaging of tumor GRP78 expression.

Methods ⁶⁴Cu-DOTA-MAb159 and ⁶⁴Cu-DOTA- IgG were synthesized. The GRP78 targeting efficacy and in vivo distribution were evaluated in mice xenografts via serial static PET imaging. ⁶⁴Cu-DOTA-hIgG was used as a negative control to validate the binding specificity of ⁶⁴Cu-DOTA-MAb159. Mab159-Cy5.5 and IgG-Cy5.5 were synthesized and injected into HT29 and PC-3 tumor bearing mice. Their distribution within the tumor tissues was investigated by confocal imaging.

Results The radiotracer was synthesized with a specific activity of 0.8 MBq/μg antibody and 42-56% radiochemical yield. In BxPC3 pancreactic cancer xenografts, ⁶⁴Cu-DOTA-MAb159 demonstrated prominent tumor accumulation, which were 4.3 ± 1.2, 15.4 ± 2.6, 18.3 ± 1.0, and 14.9 ± 0.7 at 1, 17, 48, and 72 h post injection, respectively. Tumor uptake of ⁶⁴Cu-DOTA-MAb159 was 5.71%ID/g and 4.22 ± 0.10 %ID/g at 48 h post injection in HT29 and PC-3 xenografts, respectively.⁶⁴Cu-DOTA-hIgG showed minimal tumor accumulation in all three tumor models at all tested time points. Ex vivo confocal imaging for tumors injected with MAb159-Cy5.5 or IgG-Cy5.5 confirmed GRP78 specific accumulation of MAb159-Cy5.5.

Conclusions We have demonstrated that imaging GRP78 with ⁶⁴Cu-DOTA-MAb159 is feasible. Moreover, the applications of the newly developed probe could be extended to other cancer types overexpressing GRP78. Further studies need to be done to evaluate its role in determining therapeutic efficacy and assessing treatment response.

Research Support This work was supported by the 1R01EB014354-01A1 (NIBIB), P30-CA016086-35-37 (NCI), the American Cancer Society (12199ss1-MRSG-12-034-01-CCE), Vasgene Therapeutics Inc. (NIH: CA168158-01; CA171538-01), National Natural Science Foundation of China (No. U1032002, 81071206, 81271621, 81301266).