Preclinical Evaluation of an Engineered Single-Chain Fragment Variable-Fragment Crystallizable Targeting Human CD44.

Glycoprotein CD44 and alternative splice variants are overexpressed in many cancers and cancer stem cells. Binding of hyaluronic acid to CD44 activates cell signaling pathways, inducing cell proliferation, cell survival, and invasion. As such, CD44 is regarded as an excellent target for cancer therapy when this interaction can be blocked. In this study, we developed a CD44-specific antibody fragment and evaluated it for imaging CD44-positive cancers using PET. A human single-chain fragment variable (scFv) was generated by phage display, using the extracellular domain of recombinant human CD44. The specificity and affinity of the scFv-CD44 were evaluated using recombinant and tumor cell-expressed CD44. Epitope mapping of the putative CD44 binding site was performed via overlapping peptide microarray. The scFv-CD44 was reformatted into a bivalent scFv-Fc-CD44, based on human IgG-fragment crystallizable (Fc). The scFv-Fc-CD44 was radiolabeled with Cu and Zr. The purified reagents were injected into athymic nude mice bearing CD44-positive human tumors (MDA-MB-231, breast cancer, triple-negative). Biodistribution studies were performed at different times after injection of [Cu]Cu-NOTA-scFv-Fc-CD44 or [Zr]Zr-DFO-scFv-Fc-CD44. PET/CT imaging was conducted with [Zr]Zr-DFO-scFv-Fc-CD44 on days 1 and 7 after injection and compared with a scFv-Fc control antibody construct targeting glycophorin A. Epitope mapping of the scFv binding site revealed a linear epitope within the extracellular domain of human CD44, capable of blocking binding to native hyaluronic acid. Switching from a monovalent scFv to a bivalent scFv-Fc format improved its binding affinity toward native CD44 on human breast cancer cells by nearly 200-fold. In vivo biodistribution data showed the highest tumor uptake and tumor-to-blood ratios for [Zr]Zr-DFO-scFv-Fc-CD44 between days 5 and 7. PET imaging confirmed excellent tumor specificity for [Zr]Zr-DFO-scFv-Fc-CD44 when compared with the control scFv-Fc. We developed a CD44-specific scFv-Fc construct that binds with nanomolar affinity to human CD44. When radiolabeled with Cu or Zr, it demonstrated specific uptake in CD44-expressing MDA-MB-231 tumors. The high tumor uptake (∼56% injected dose/g) warrants clinical investigation of [Zr]Zr-DFO-scFv-Fc-CD44 as a versatile PET imaging agent for patients with CD44-positive tumors.