Biodegradable microspheres. 17. Lysosomal degradation of primaquine-peptide spacer arms

The pharmacological activity of drugs bound to lysosomotropic drug carriers will depend on the rate of release of the drugs from the drug-carrier complex. We have now studied the enzymatic release of primaquine (PQ) from two groups of peptide-PQ derivatives by their incubation with rat liver lysosomal fraction. The derivatives have the general structure NH2-X-Leu-Ala-Y-PQ and are intended to be coupled via their free alpha-amino group to starch microparticles. In the first group, Y was varied, being Leu, Tyr, Lys, or Asp, while X was Ala. In the second one, X was varied, being Ala, Tyr, Lys, or Asp, while Y was Leu. Thus, a systematic study of the significance of the varying amino acid composition of the tetrapeptides, which can serve as spacer arms in the microparticle-drug complexes, for the lysosomal release of PQ was possible. In addition, some epsilon-aminocaproic acid-PQ derivatives, which lack a free alpha-amino group, were incubated. This was done to study the importance of enzymes, other than aminopeptidases, during lysosomal degradation of these derivatives. The pattern and rate of degradation of all PQ derivatives was followed by HPLC analysis. The results obtained show that endopeptidases, as well as mono- and diaminopeptidases, degrade the derivatives. PQ cannot be cleaved directly from the derivatives by any carboxypeptidase-like enzyme. Asp peptides are digested slowly in the lysosomal fraction. The temporal aspects of reactions were quantitated using a kinetic model, in which first-order rate constants of all the steps of each peptide degradation sequence were estimated simultaneously.