Changes in the antibacterial activity of melanin-bound drugs.

Affinity to melanin and changes of antibacterial activity of melanin-bound drugs were examined in 11 drugs, all of which showed an affinity for melanin. The highest melanin-binding ratio was seen in aminoglycosides. Among these, sisomicin sulfate (SISO) had the highest binding ratio (95.5%). The melanin-binding ratios seen in sulbenicillin sodium (SBPC), cephalosporins and fluoroquinolones were relatively low during the early phase of the reaction, but increased with time. The highest ratio seen in cefazolin sodium was 60.1%. Melanin-bound aminoglycosides showed a reduction in their antibacterial activity with both Bacillus subtilis and Escherichia coli. The reduction rate in the antibacterial activity of melanin-bound SISO against B. subtilis was 50.0% and that against E. coli was 43.0%. No changes in antibacterial activity were seen in the other 6 drugs bound to melanin.