In vivo demonstration of an active tumor pretargeting approach with peptide nucleic acid bioconjugates as complementary system

Aim
The ability of early-stage diagnosis of tumor malignancies and personalized treatment ultimately relies on the availability of highly tumor-affine compounds with purposeful pharmacological profile. Although monoclonal antibodies (mAbs) specifically bind to tumor-associated epitopes, the conventional concept of directly radiolabeled tumor-specific mAb has several drawbacks most prominently the prolonged radiation exposure of non-cancerous tissue. However, the concept of tumor pretargeting allows for the rational use of long-circulating high-affinity mAbs for non-invasive cancer radioimmunodetection and therapy. Our work describes a successful tumor pretargeting utilizing an EGFR-specific mAb and peptide nucleic acid (PNA) derivatives as the complementary system for specific radionuclide delivery to pretargeted tumor tissue.

Methods
After chemical synthesis, purification and detailed characterization of the individual components including antibody-PNA conjugates and different PNA oligomers, biodistribution studies were carried out using healthy Wistar rats. Finally, the pretargeting approach was evaluated in murine A431 tumor xenografts by single photon emission computed tomography.

Results
After optimizing the pharmacokinetic properties of PNA oligomers and investigating their hybridization properties, we elaborated a versatile conjugation protocol based on coupling a cysteine-functionalized PNA oligomer to a maleimido-functionalized mAb. The in vivo studies demonstrated a rapid and efficient accumulation of activity at the tumor site with a tumor-to-muscle ratio of > 8 and clearly distinguishable tumor visualization.

Conclusion
This successful tumor pretargeting study has demonstrated the high potential of this concept by applying radiolabeled complementary PNA strands as an alternative in vivo recognition and radionuclide transporting system. The next step involves the translation of these results to the application of therapeutic relevant radionuclides.