Abstract
(18)F-FAC (2'-deoxy-2'-(18)F-fluoro-β-d-arabinofuranosylcytosine) has close structural similarity to gemcitabine and thus offers the potential to image drug delivery to tumors. We compared tumor (18)F-FAC PET images with (14)C-gemcitabine levels, established ex vivo, in 3 mouse models of pancreatic cancer. We further modified tumor gemcitabine levels with injectable PEGylated recombinant human hyaluronidase (PEGPH20) to test whether changes in gemcitabine would be tracked by (18)F-FAC. Methods:(18)F-FAC was synthesized as described previously. Three patient-derived xenograft (PDX) models were grown in the flanks of NSG mice. Mice were given PEGPH20 or vehicle intravenously 24 h before coinjection of (18)F-FAC and (14)C-gemcitabine. Animals were euthanized and imaged 1 h after tracer administration. Tumor and muscle uptake of both (18)F-FAC and (14)C-gemcitabine was obtained ex vivo. The efficacy of PEPGPH20 was validated through staining with hyaluronic acid binding protein. Additionally, an organoid culture, initiated from a KPC (Pdx-1 Cre LSL-Kras(G12D) LSL-p53(R172H)) tumor, was used to generate orthotopically growing tumors in C57BL/6J mice, and these tumors were then serially transplanted. Animals were injected with PEGPH20 and (14)C-gemcitabine as described above to validate increased drug uptake by ex vivo assay. PET/MR images were obtained using a PET insert on a 7-T MR scanner. Animals were imaged immediately before injection with PEGPH20 and again 24 h later. Results: Tumor-to-muscle ratios of (14)C-gemcitabine and (18)F-FAC correlated well across all PDX models and treatments (R(2) = 0.78). There was a significant increase in the tumor PET signal in PEGPH20-treated PDX animals, and this signal was matched in ex vivo counts for 2 of 3 models. In KPC-derived tumors, PEGPH20 raised (14)C-gemcitabine levels (tumor-to-muscle ratio of 1.9 vs. 2.4, control vs. treated, P = 0.013). PET/MR (18)F-FAC images showed a 12% increase in tumor (18)F-FAC uptake after PEGPH20 treatment (P = 0.023). PEGPH20-treated animals uniformly displayed clear reductions in hyaluronic acid staining. Conclusion:(18)F-FAC PET was shown to be a good surrogate for gemcitabine uptake and, when combined with MR, to successfully determine drug uptake in tumors growing in the pancreas. PEGPH20 had moderate effects on tumor uptake of gemcitabine.