Investigation into the intestinal metabolism of [D-Ala1] peptide T amide: implication for oral drug delivery.

ABSTRACT

The anti-AIDS drug, [D-Ala1] Peptide T amide (D-ASTTTNYT.NH2) is an octapeptide which competitively inhibits the attachment of HIV to the receptor CD4 molecule on the T-lymphocyte. The objective of the study is to investigate the degradative process of this peptide and its effective enzyme inhibitors. The metabolites of [D-Ala1] Peptide T amide in rabbit brush-border membrane vesicles at pH 6.5 are ASTT, ASTTTN, YT and Y. The sequential time-course study of each metabolite reveals that enkephalinase (EC 3.4.24.11) plays an important role in the hydrolysis of [D-Ala1] Peptide T amide to ASTT. With the addition of an enkephalinase inhibitor, thiorphan, 85% of degradation was inhibited. Aminopeptidase is also involved in its degradative process and 25% of inhibition was observed by amastatin, an aminopeptidase inhibitor. The results show that no significant difference was observed between the in situ and chronical loop perfusion studies and enzyme activities are somewhat inhibited under acidic conditions in both methods. Approx. 90% of the parent peptide remained when rats were perfused with pH 4.0 peptide solution at a flow rate of 0.123 ml/min, while only 60% was recovered when pH 6.5 peptide solution was applied. The addition of amastatin made a quadrupled increase in the amount of parent peptide recovered. A 117-fold increment was observed when thiorphan was added. The dimensionless wall permeability of this peptide was 1.19 +/- 0.16 when pH 4.0 peptide solution was used during chronical loop perfusion study. Therefore, this study suggests that [D-Ala1] Peptide T amide could be absorbed via small intestine where enzymatic degradation s a rate-limiting step for the absorption of this peptide.