Glypican-3-Targeting F(ab ')2 for Zr-89 PET of Hepatocellular Carcinoma

Hepatocellular carcinoma (HCC) is an increasingly lethal malignancy for which management is critically dependent on accurate imaging. Glypican-3 (GPC3) is a cell surface receptor overexpressed in most HCCs and provides a unique target for molecular diagnostics. The use of monoclonal antibodies (mAbs) that target GPC3 (alpha GPC3) in PET imaging has shown promise but comes with inherent limitations associated with mAbs such as long circulation times. This study used Zr-89-conjugated F(ab')2 fragments directed against GPC3 (Zr-89-alpha GPC3-F(ab')2) to evaluate the feasibility of the fragments as a diagnostic immuno-PET imaging probe. Methods: Immobilized ficin was used to digest alpha GPC3, creating alpha GPC3-F(ab')2 fragments subsequently conjugated to Zr-89. In vivo biodistribution and PET studies were performed on GPC3-expressing HepG2 and GPC3-nonexpressing RH7777 orthotopic xenografts. Results: Reliable alpha GPC3-F(ab')2 production via immobilized ficin digestion was verified by high-performance liquid chromatography and sodium dodecyl sulfate polyacrylamide gel electrophoresis. Zr-89-alpha GPC3-F(ab')2 demonstrated F(ab')2-dependent, antigen-specific cell binding. HepG2 tumor uptake was higher than any other tissue, peaking at 100 +/- 21 percentage injected dose per gram (%ID/g) 24 h after injection, a value 33- to 38-fold higher than GPC3-nonexpressing RH7777 tumors. The blood half-life of the Zr-89-alpha GPC3-F(ab')2 conjugate was approximately 11 h, compared with approximately 115 h for historic mAb controls. This shorter half-life enabled clear tumor visualization on PET 4 h after administration, with a resultant peak tumor-to-liver contrast ratio of 23.3. Blocking antigen-expressing tumors with an excess of nonradiolabeled aGPC3 resulted in decreased tumor uptake similar to native liver. The kidneys exhibited high tissue uptake, peaking at 24 h with 83 +/- 12 %ID/g. HepG2 tumors ranging from 1.5 to 7 mm were clearly visible on PET, whereas larger RH7777 tumors displayed signal lower than background liver tissue. Conclusion: This study demonstrates the feasibility of using Zr-89-aGPC3-F(ab')2 for intrahepatic tumor localization with small-animal PET. Faster blood clearance and lower background liver uptake enable excellent signal-to-noise ratios at early time points. Increased renal uptake is similar to that as has been seen with clinical radioactive peptide imaging. Zr-89-alpha GPC3-F(ab')2 addresses some of the shortcomings of whole-antibody immuno-PET probes. Further optimization is warranted to maximize probe sensitivity and specificity in the process of clinical translation.