Design, Preparation, and Characterization of PNA-Based Hybridization Probes for Affibody-Molecule-Mediated Pretargeting

In radioimmunotherapy, the contrast between tumor and normal tissue can be improved by using a pretargeting strategy with a primary targeting agent, which is conjugated to a recognition tag, and a secondary radiolabeled molecule binding specifically to the recognition tag. The secondary molecule is injected after the targeting agent has accumulated in the tumor and is designed to have a favorable biodistribution profile, with fast clearance from blood and low uptake in normal tissues. In this study, we have designed and evaluated two complementary peptide nucleic acid (PNA)-based probes for specific and high-affinity association in vivo. An anti-HER2 Affibody-PNA chimera, Z_HER2:342-SR-HP1, was produced by a semisynthetic approach using sortase A catalyzed ligation of a recombinantly produced Affibody molecule to a PNA-based HP1-probe assembled using solid-phase chemistry. A complementary HP2 probe carrying a DOTA chelator and a tyrosine for dual radiolabeling was prepared by solid-phase synthesis. Circular dichroism (CD) spectroscopy and UV thermal melts showed that the probes can hybridize to form a structured duplex with a very high melting temperature (T_m), both in HP1:HP2 and in Z_HER2:342-SR-HP1:HP2 (T_m = 86-88 °C), and the high binding affinity between Z_HER2:342-SR-HP1 and HP2 was confirmed in a surface plasmon resonance (SPR)-based binding study. Following a moderately fast association (1.7 × 10⁵ M^-1 s^-1), the dissociation of the probes was extremely slow and <5% dissociation was observed after 17 h. The equilibrium dissociation constant (K_D) for Z_HER2:342-SR-HP1:HP2 binding to HER2 was estimated by SPR to be 212 pM, suggesting that the conjugation to PNA does not impair Affibody binding to HER2. The biodistribution profiles of ¹¹¹In- and ¹²⁵I-labeled HP2 were measured in NMRI mice, showing very fast blood clearance rates and low accumulation of radioactivity in kidneys and other organs. The measured radioactivity in blood was 0.63 ± 0.15 and 0.41 ± 0.15%ID/g for ¹²⁵I- and ¹¹¹In-HP2, respectively, at 1 h p.i., and at 4 h p.i., the kidney accumulation of radioactivity was 0.17 ± 0.04%ID/g for ¹²⁵I-HP2 and 3.83 ± 0.39%ID/g for ¹¹¹In-HP2. Taken together, the results suggest that a PNA-based system has suitable biophysical and in vivo properties and is a promising approach for pretargeting of Affibody molecules.