ADME investigations of unnatural peptides: distribution of a 14C-labeled beta 3-octaarginine in rats

The highly positively charged, cell-penetrating beta3-octaarginine has been prepared with a radioactive label by acetylation at the N-terminus with a doubly (14)C-labeled acetyl group ((14)CH3-(14)CO). With the radioactive compound, an ADME study (Absorption, Distribution, Metabolism, Excretion) 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 Cmax and decreased afterwards only very slowly. No metabolites were observed in the systemic circulation. QWBA Data indicated that the distribution of the acetyl-beta-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 beta-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, beta-peptides bearing proteinogenic side chains are compared with peptides consisting entirely of D-alpha-amino acid residues (the enantiomers of the 'natural' building blocks), and suggestions are made regarding a possible focus of future biomedical investigations with beta-peptides.