Abstract. Peptide drugs inhibiting harmful protein-protein interactions are attracting increasing attention in treatment of cardiovascular diseases. However, reaching intracellular targets still is a challenging task for peptide drugs, faced by promising developments in cell-penetrating peptides. We developed a cardiac-targeted peptide and present a radiolabeling approach to study a timely and successful uptake using PET/CT imaging in mice.

Methods. The TAT-protein transduction domain residues 48-59 (PTDGRKKRRQRRRPQ) of human immunodeficiency virus-1 covalently bound to a hydrophobic octapeptide (TAT-heart8P) and coupled to NODAGA (NODAGA-TAT-heart8P) was radiolabeled with ⁶⁸Ga and evaluated in extensive in vivo, in vitro and tissue stability studies. Cellular uptake of TAT-heart8P was examined in adult mouse cardiomyocytes. Basic pharmacokinetics of TAT-heart8P were evaluated in dogs and rats. C57BL/6J male mice were used for [⁶⁸Ga]-NODAGA-TAT-heart8P biodistribution, tissue uptake, and saturation studies. 

Results. [⁶⁸Ga]-NODAGA-TAT-heart8P showed high purity and in vitro stability as well as high hydrophilicity and very low binding to plasma proteins. TAT-heart8P showed high cellular uptake and internalization in isolated adult mouse cardiomyocytes. Basic pharmacokinetic data suggested high organ distribution and liver uptake. Organ uptake of [⁶⁸Ga]-NODAGA-TAT-heart8P in vivo occurred rapidly after injection. Initial crucial cardiac uptake was 4.3 ± 0.7 %IA/g 10 min post injection. Saturation with the unlabeled peptide injected before significantly diminished cardiac uptake to 2.3 ± 0.4 %IA/g (10 min post injection), indicating saturable uptake. Furthermore, a surprisingly higher hepatic than renal accumulation of hydrophilic compound was observed.