Active tumor pretargeting with peptide nucleic acid bioconjugates as complementary system

Aim
Highly tumor-affine compounds with purposeful pharmacological profile are absolutely necessary for early diagnosis of tumor malignancies and their personalized treatment. In this regard, monoclonal antibodies (mAbs) are particularly valuable as these molecules bind to tumor-associated epitopes with high specificity and affinity. The conventional concept of directly radiolabeled tumor-specific mAbs for radioimmunodetection (RID) and -therapy (RIT) has certainly several drawbacks, most prominently the prolonged radiation exposure of healthy tissues and organs. Tumor pretargeting, however, allows the rational use of long-circulating high-affinity mAbs for non-invasive cancer RID and RIT. The work presented here describes 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 tissues.

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, a versatile conjugation protocol based on coupling a cysteine-functionalized PNA oligomer to a maleimido-functionalized mAb was elaborated. 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 delivery system. The next step involves the translation of these results to the application of therapeutic relevant radionuclides.