Molecular targeting of papillary thyroid carcinoma with fluorescently labeled ratiometric activatable cell penetrating peptides in a transgenic murine model.

BACKGROUND AND OBJECTIVES: Molecularly targeted fluorescent molecules may help detect tumors that are unseen by traditional white-light surgical techniques. We sought to evaluate a fluorescent ratiometric activatable cell penetrating peptide (RACPP) for tumor detection in a transgenic model of PTC.
METHODS: Thirteen BRAFV600E mice with PTC were studied-seven injected intravenously with RACPP, four controls with saline. Total thyroidectomy was performed with microscopic white-light visualization. Fluorescent imaging of post-thyroidectomy fields was performed, and tissue with increased signal was removed and evaluated for PTC. Final samples were analyzed by a pathologist blinded to conditions. Vocal cord function was evaluated postoperatively with video laryngoscopy.
RESULTS: The average in situ ratiometric (Cy5/Cy7) thyroid tumor-to-background contrast ratio was 2.27 +/- 0.91. Fluorescence-guided clean-up following thyroidectomy identified additional tumor in 2 of 7 RACPP animals (smallest dimension 1.2 mm), and decreased the number of animals with residual tumor from 4 to 3. All retained tumor foci on final pathology were smaller than 0.76 mm. Intact vocal abduction was present in all of the RACPP animals.
CONCLUSIONS: RACPPs successfully targeted PTC in a transgenic thyroidectomy model, and allowed for residual tumor detection that reduced positive margins beyond what was possible with white-light surgery alone.