ADME Investigations of Unnatural Peptides: Distribution of a 14 C‐Labeled β 3 ‐Octaarginine in Rats

The highly positively charged, cell‐penetrating β   3 ‐octaarginine has been prepared with a radioactive label by acetylation at the N‐terminus with a doubly 14 C‐labeled acetyl group ( 14 CH 3 14 CO). With the radioactive compound, an ADME study ( A bsorption, D istribution, M etabolism, E xcretion) was performed in male rats following an intravenous or oral dose of 1 mg/kg. Sampling was carried out after periods ranging from 5 min to 4 d or 7 d for blood/excretia and quantitative whole‐body autoradioluminography (QWBA), respectively. After p.o. dosing, no systemic exposure to peptide‐related radioactivity was observed, and the dose was completely excreted in the feces within 24 h suggesting the absence of relevant absorption; less than 3% of the i.v. dose was excreted from the animals within 4 d. Blood levels, after i.v. dosing, dropped within 4 d to less than 2% of C max and decreased afterwards only very slowly. No metabolites were observed in the systemic circulation. QWBA Data indicated that the distribution of the acetyl‐ β ‐octaarginine‐related radioactivity in the organs and tissues shifted over time. Notably, after 7 d, the highest concentration was measured in the lymph nodes, and the largest amount was found in the liver. A comparison with the results of two previous ADME investigations of β ‐peptides ( cf. Table 1 ) reveals that the distribution of the compounds within the animals is structure‐dependent, and that there is a full range from oral availability with rather rapid excretion (of a tetrapeptide) to essentially complete lack of both oral absorption and excretion after i.v. administration (of a highly charged octapeptide). A discussion is presented about the in vivo stability and ‘drug‐ability’ of peptides. In general, β ‐peptides bearing proteinogenic side chains are compared with peptides consisting entirely of D ‐ α ‐amino acid residues (the enantiomers of the ‘natural’ building blocks), and suggestions are made regarding a possible focus of future biomedical investigations with β ‐peptides.