Pretargeting using peptide nucleic acid

Pretargeting studies in animals and humans have usually involved (strept)avidin and biotin. Depending on the particular strategy, endogenous biotin can adversely influence localization when these molecules are used.

As an alternative to (strept)avidin and biotin, we have explored the use of a single-stranded peptide nucleic acid (PNA) bound to a protein administered first and followed by the complementary single-stranded PNA radiolabeled with 99mTc. Target localization of the PNA-bound protein in a mouse infection and a mouse tumor model occurred by passive diffusion while the radiolabeled complementary PNA localized by in vivo hybridization. The PNA-streptavidin was prepared by adding biotin-conjugated PNA to streptavidin; the complementary PNA, derivatized with a primary amine, was conjugated with acetyl S-protected NHS-MAG3 bifunctional chelator and radiolabeled with 99mTc.

In both the infection and tumor mouse models, increased localization of radiolabel was achieved in animals receiving both injectates compared with control animals receiving only the radiolabeled PNA. In the infection model, the infected to normal thigh radioactivity ratio was 3.5 for the study animals compared with 1.7 for control animals (P = 0.0001). In the tumor model, these values were 1.7 versus 1.2 (P = 0.003).

We conclude that PNA may be considered an alternative to (strept)avidin and biotin for pretargeting studies.