MicroSPECT/CT Imaging of Cell-Line and Patient-Derived EGFR-Positive Tumor Xenografts in Mice with Panitumumab Fab Modified with Hexahistidine Peptides To Enable Labeling with Tc(I) Tricarbonyl Complex.

We aimed to investigate the feasibility of conjugating synthetic hexahistidine peptides (His) peptides to panitumumab Fab (PmFab) to enable labeling with [Tc(HO)(CO)] complex and study these radioimmunoconjugates for imaging EGFR-overexpressing tumor xenografts in mice by microSPECT/CT. Fab were reacted with a 10-fold excess of sulfo-SMCC to introduce maleimide functional groups for reaction with the terminal thiol on peptides [CGYGGHHHHHH] that harbored the His motif. Modification of Fab with His peptides was assessed by SDS-PAGE/Western blot, and the number of His peptides introduced was quantified by a radiometric assay incorporating I-labeled peptides into the conjugation reaction. Radiolabeling was achieved by incubation of PmFab-His in PBS, pH 7.0, with [Tc(HO)(CO)] in a 1.4 MBq/μg ratio. The complex was prepared by adding [TcO] to an Isolink kit (Paul Scherrer Institute). Immunoreactivity was assessed in a direct (saturation) binding assay using MDA-MB-468 human triple-negative breast cancer (TNBC) cells. Tumor and normal tissue uptake and imaging properties of Tc-PmFab-His (70 μg; 35-40 MBq) injected i.v. (tail vein) were compared to irrelevant Tc-Fab 3913 in NOD/SCID mice engrafted subcutaneously (s.c.) with EGFR-overexpressing MDA-MB-468 or PANC-1 human pancreatic ductal carcinoma (PDCa) cell-line derived xenografts (CLX) at 4 and 24 h post injection (p.i.). In addition, tumor imaging studies were performed with Tc-PmFab-His in mice with patient-derived tumor xenografts (PDX) of TNBC, PDCa, and head and neck squamous cell carcinoma (HNSCC). Biodistribution studies in nontumor bearing Balb/c mice were performed to project the radiation absorbed doses for imaging studies in humans with Tc-PmFab-His. PmFab was derivatized with 0.80 ± 0.03 His peptides. Western blot and SDS-PAGE confirmed the presence of His peptides. Tc-PmFab-His was labeled to high radiochemical purity (≥95%), and the for binding to EGFR on MDA-MB-468 cells was 5.5 ± 0.4 × 10 mol/L. Tumor uptake of Tc-PmFab-His at 24 h p.i. was significantly ( < 0.05) higher than irrelevant Tc-Fab 3913 in mice with MDA-MB-468 tumors (14.9 ± 3.1%ID/g vs 3.0 ± 0.9%ID/g) and in mice with PANC-1 tumors (5.6 ± 0.6 vs 0.5 ± 0.1%ID/g). In mice implanted orthotopically in the pancreas with the same PDCa PDX, tumor uptake at 24 h p.i. was 4.2 ± 0.2%ID/g. Locoregional metastases of these PDCa tumors in the peritoneum exhibited slightly and significantly lower uptake than the primary tumors (3.1 ± 0.3 vs 4.2 ± 0.3%ID/g; = 0.02). In mice implanted with different TNBC or HNSCC PDX, tumor uptake at 24 h p.i. was variable and ranged from 3.7 to 11.4%ID/g and 3.8-14.5%ID/g, respectively. MicroSPECT/CT visualized all CLX and PDX tumor xenografts at 4 and 24 h p.i. Dosimetry estimates revealed that in humans, the whole body dose from administration of 740-1110 MBq of Tc-PmFab-His would be 2-3 mSv, which is less than for a Tc-medronate bone scan (4 mSv).