Prodrugs of peptides. 9. Bioreversible N-alpha-hydroxyalkylation of the peptide bond to effect protection against carboxypeptidase or other proteolytic enzymes

Abstract

Various N-alpha-hydroxyalkyl derivatives of N-acyl amino acids and di- and tripeptides were prepared by hydrolysis or aminolysis of N-acyl 5-oxazolidinones. The stability of these derivatives was studied in aqueous solution as a function of pH. The compounds were all degraded quantitatively to their parent N-acylated amino acid or peptide and aldehyde but with vastly different rates. At pH 7.4 and 37 degrees C the half-lives of decomposition ranged from 4 min to 1500 hr. The structural factors influencing the stability included both steric and polar effects within the acyl and N-alpha-hydroxyalkyl moieties as well as within the amino acid attached to the N-alpha-hydroxyalkylated N-acyl amino acid. Whereas the N-benzyloxycarbonyl (Z) derivatives of the dipeptides Gly-L-Leu and Gly-L-Ala were readily hydrolyzed by carboxypeptidase A, the N-hydroxymethylated compounds, i.e., Z-Gly(CH2OH)-Leu and Z-Gly(CH2OH)-Ala, were resistant to cleavage by the enzyme as revealed by their similar rates of decomposition in the presence or absence of the enzyme at pH 7.4 and 37 degrees C. The results suggest that N-alpha-hydroxyalkylation of a peptide bond protects not only this bond but also an adjacent peptide bond against proteolytic cleavage. Since the N-alpha-hydroxyalkyl derivatives are readily bioreversible, undergoing spontaneous hydrolysis at physiological pH, this prodrug approach promises to overcome the enzymatic barrier to absorption of various peptides.