Prodrug derivatives of thyrotropin-releasing hormone and other peptides

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

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.

Prodrugs of peptides. 6. Bioreversible derivatives of thyrotropin-releasing hormone (TRH) with increased lipophilicity and resistance to cleavage by the TRH-specific serum enzyme

Bioreversible derivatization of TRH (pGlu-His-Pro-NH2) to protect the tripeptide against rapid enzymatic inactivation in the systemic circulation and to improve the lipophilicity of this highly hydrophilic peptide was performed by N-acylation of the imidazole group of the histidine residue with various chloroformates. Whereas TRH was rapidly hydrolyzed at its pGlu-His bond in human plasma by a TRH-specific pyroglutamyl aminopeptidase serum enzyme, the N-alkoxycarbonyl derivatives were resistant to cleavage by the enzyme. On the other hand, these derivatives are readily bioreversible as the parent TRH is formed quantitatively from the derivatives by spontaneous hydrolysis or by plasma esterase-catalyzed hydrolysis. In addition to protecting the parent TRH against rapid inactivation in the circulation and hence potentially prolonging the duration of action of TRH in vivo, the N-alkoxycarbonyl prodrug derivatives were much more lipophilic than TRH as assessed by octanol-buffer partitioning. This property may enhance prodrug penetration of the blood-brain barrier and various other biomembranes compared to the parent peptide.

Kinetics and pattern of degradation of thyrotropin-releasing hormone (TRH) in human plasma

The kinetics and mechanism of degradation of the tripeptide TRH (pGlu-His-Pro-NH2) and its various primary and secondary degradation products (TRH-OH, His-Pro-NH2, and His-Pro) have been determined in human plasma at 37 degrees C. The rates of degradation of both TRH and TRH-OH (pGlu-His-Pro) showed mixed zero-order and first-order kinetics. At low substrate concentrations first-order kinetics occurred, with TRH and TRH-OH degradation half-lives of 9.4 and 27 min, respectively. The initial step in the plasma-catalyzed degradation of TRH is due to hydrolysis of the pGlu-His bond by the TRH-specific pyroglutamyl aminopeptidase serum enzyme, resulting in the exclusive formation of histidyl-proline amide (His-Pro-NH2). Using specific HPLC methods the major degradation route (67%) of this dipeptide in human plasma was hydrolysis of the peptide bond to yield His and Pro-NH2, whereas deamidation to yield His-Pro accounted for 29% of the total degradation. A minor pathway (less than or equal to 4%) was spontaneous cyclization to yield cyclo(His-Pro). Both His-Pro-NH2 and His-Pro degraded by first-order kinetics and faster than TRH, with half-lives of 5.3 and 2.2 min, respectively, in 80% plasma.