Structure-activity studies on novel polymyxin derivatives that carry only three positive charges

Polymyxin B and colistin are pentacationic lipopeptides that possess a cyclic heptapeptide portion, a linear tripeptide portion, and a fatty acyl tail. They are used, in spite of nephrotoxicity, to treat infections caused by extremely multiresistant Gram-negative bacteria. We have recently developed novel derivatives, that carry three cationic charges only. Some of them, including NAB739, are directly antibacterial whereas others, including NAB7061, lack the direct activity but sensitize bacteria to other antibiotics. NAB739 and NAB7061 differ from the old polymyxins in their renal handling and have reduced affinity to kidney brush border membrane. To further study the structure-activity relationships, we here synthesized eight additional derivatives and tested their antibacterial activity. NAB751 carries methylheptanoyl as the fatty acyl instead of octanoyl in NAB739 and was as active as NAB739, whereas NAB750 with dodecanoyl was less active. NAB781 and NAB782 with the linear peptide portion Ser-DSer and Ser-Ser-DSer, respectively, were less active than NAB739 that carries Thr-DSer. NAB771 with Thr at position 8 in the cyclic portion (instead of Dab in NAB7061) and Thr-Dab as the linear peptide portion (instead of Thr-Abu in NAB7061), resembled NAB7061 in its activity. However, replacement of two Dab residues in the cyclic portion with Thr greatly decreased the activity, even though the loss of the cationic charges was compensated by introducing two Dab residues in the linear portion. These findings reveal that subtle structural modifications have a major effect on the antibacterial activity and that it is possible to design numerous tricationic polymyxin derivatives that are antibacterial.