Peptidic lysosomotropic prodrugs of antibiotics and antitumoral agents could be of advantage in chemotherapy, providing that free, active drug is released at, or close to, the desired site of action. Thus, aminoacyl derivatives of doxorubicin, e.g., where the drug is attached to the amino acid by a primary amino function, are sensitive to lysosomal hydrolases. We have examined whether a similar approach can be used for drugs carrying a carboxyl group such as beta-lactam antibiotics. Because the C adjacent to the carboxyl group in beta-lactams has the D configuration, we have examined and report here the synthesis and susceptibility of model peptides, namely Boc-D-Pro-L-Ala and Boc-L-Pro-L-Ala to lysosomal hydrolases. Hydrolysis of the D isomer proceeds considerably more slowly than that of the L isomer. Lysosomal carboxypeptidase(s) and/or amidases appear therefore to have a much narrower specificity than aminopeptidase(s), which will severely limit the applicability of the concept of peptidic lysosomotropic prodrugs.

Amino acids have been coupled to the carboxyl group of penicillin V and cephalothin by methods that keep the beta-lactam ring intact. Derivatives were successfully obtained with both neutral (Leu, Val, Ala, Ile, Trp, Tyr, Gly) and one acidic (Glu) amino acids. The new compounds were inactive in vitro against Staphylococcus aureus or Micrococcus luteus. Incubation in the presence of purified carboxypeptidases (A, B), soluble lysosomal fractions from liver, or cellular homogenates from liver, kidney, fibroblasts, and macrophages did not allow recovery of the antibacterial activity. Injection in mice also failed to cause liberation of microbiologically active compounds. HPLC studies confirmed that the amide linkage between the antibiotic and the amino acid was not hydrolyzed in the presence of soluble lysosomal fractions from liver. However, conversion of cephalothin and cephalothin-leucine to desacetyl derivatives was observed in the presence of soluble lysosomal fractions and extracts from liver and semipurified orange peel acetylesterase(s). It is concluded that amino acid derivatives of beta-lactam antibiotics do not offer potential chemotherapeutic use as prodrugs.