Glypican-3–Targeted 89Zr PET Imaging of Hepatocellular Carcinoma

With more than 782,000 new cases and 746,000 resultant deaths annually, hepatocellular carcinoma (HCC) is the fifth most prevalent malignancy and the second leading cause of cancer-related deaths worldwide (1,2). High-quality, accurate imaging is an essential component in the detection, staging, and treatment planning of HCC, wherein subtle findings can dramatically alter the management plan (3,4). Multiphase contrast-enhanced CT or MR imaging is the current gold standard, with HCC tumors classically demonstrating late arterial enhancement with portal venous washout, obviating tissue diagnosis (3–5). However, CT and MR scans frequently detect lesions that cannot be further defined, introducing uncertainty about the diagnosis or disease extent (4–7). Comparison of liver tumor enhancement patterns at various time points after intravenous contrast administration to distinguish benign entities from HCC is limited by lack of specificity (4,6). This problem is further exacerbated by institutional variability in protocols defining the type, dosage, and timing of intravenous contrast material (7). As scan sensitivity improves, an increasing number of such indeterminate lesions are detected. Doubt raised by these lesions triggers costly repeated imaging, or biopsy with bleeding or tumor-seeding risks (8,9), all contributing to delayed treatment of these patients. Furthermore, basing eligibility for potentially curative liver resection or transplantation on suboptimal scans leads to either early recurrences or missed treatment opportunities. Studies comparing tumor extent on preoperative imaging with liver explant pathology have demonstrated a troubling discordance (10), and most relapses after curative treatment occur within 2 y (11,12), reflecting subclinical disease missed at the time of initial radiographic diagnosis, both highlighting the inaccuracy of current imaging. Targeted molecular imaging addresses many limitations presented by conventional diagnostic approaches. 89Zr has recently emerged as a promising PET radioisotope when targeted with monoclonal antibodies (mAbs) (13,14), and conjugation of 89Zr to several preclinical and clinically available mAbs has demonstrated high spatial resolution and excellent signal-to-noise ratios (15–17). Reports of antibody-labeled 64Cu and 86Y (half-life, 12.7 and 14.7 h, respectively) have shown promise (18–20). However, the location of HCC in the liver, a primary clearance organ with relatively high background radioisotope uptake, compels use of a highly specific targeting ligand and a longer-half-life radionuclide. 89Zr possesses a far more optimal half-life (78.4 h), allowing unwanted background signal to clear so that the liver tumor can be visualized. Although under investigation, antibody-based biologic therapy is currently not part of standard HCC treatments (21); therefore, mAbs are not commercially available and novel targeting moieties must be used for PET imaging. Glypican-3 (GPC3) is a heparan sulfate proteoglycan important in regulating embryonal cell growth (22). It is an ideal target because of its overexpression in up to 80% of HCCs and absence in normal tissues, cirrhotic liver, and benign lesions (23–25). A membrane-bound proteoglycan, it is readily accessible to antibody-mediated targeting, and immunohistochemical GPC3 staining has demonstrated 97% specificity (24–26). Previous experience using our αGPC3 mAb has demonstrated its efficacy as a targeting moiety for MR imaging (27). Herein we report the development of 89Zr-αGPC3 as a PET contrast agent to identify GPC3-expressing tumors and to serve as an adjunct to multiphase CT and MR in HCC imaging. To our knowledge, this is the first report of mAb-targeted 89Zr-PET in HCC and is the proof-of-concept study for immuno-PET imaging of primary liver tumors.