Exploration of Bis-Cinnamido-Polyamines as Intrinsic Antimicrobial Agents and Antibiotic Enhancers

Abstract

The marine natural product ianthelliformisamine C is a bis-cinnamido substituted spermine derivative that exhibits intrinsic antimicrobial properties and can enhance the action of doxycycline towards the Gram-negative bacterium Pseudomonas aeruginosa. As part of a study to explore the structure-activity requirements of these activities, we have synthesized a set of analogues that vary in the presence/absence of methoxyl group and bromine atoms and in the polyamine chain length. Intrinsic antimicrobial activity towards Staphylococcus aureus, methicillin-resistant S. aureus (MRSA) and the fungus Cryptococcus neoformans was observed for only the longest polyamine chain examples of non-brominated analogues while all examples bearing either one or two bromine atoms were active. Weak to no activity was typically observed towards Gram-negative bacteria, with exceptions being the longest polyamine chain examples 13f, 14f and 16f against Escherichia coli (MIC 1.56, 7.2 and 5.3 µM, respectively). Many of these longer polyamine-chain analogues also exhibited cytotoxic and/or red blood cell hemolytic properties, diminishing their potential as antimicrobial lead compounds. Two of the non-toxic, non-halogenated analogues, 13b and 13d, exhibited a strong ability to enhance the action of doxycycline against P. aeruginosa, with >64-fold and >32-fold enhancement, respectively. These results suggest that any future efforts to optimize the antibiotic-enhancing properties of cinnamido-polyamines should explore a wider range of aromatic ring substituents that do not include bromine or methoxyl groups.

Keywords: antibiotics; antifungal agents; antimicrobial; indole; polyamine; potentiator; structure-activity relationships.

Figure 1

Structures of antimicrobial marine natural products squalamine (1), ent-eusynstyelamide B (2), synoxazolidinone A (3) and ianthelliformisamine C (4).

Figure 2

Structures of cinnamic acid head groups 5–8.

Scheme 1

Synthetic routes for the preparation of cinnamic acids 7 and 8. Reagents and conditions: (a) 1N NaOH, EtOH, reflux, N₂, 4 h, 87% for 7; 86% for 8; (b) NaH, triethyl phosphonoacetate, rt, N₂, 24 h, 68%.

Figure 3

Boc-protected polyamines 12a–f.

Scheme 2

General method for the synthesis of target polyamine analogues 13–16. Reagents and conditions: (a) Cinnamic acid (5) (2.2 equiv.), Boc-protected polyamine (12a–f) (1 equiv.), EDC·HCl/HOBt (2.6 equiv.), DIPEA (6 equiv.), CH₂Cl₂, rt, 20 h (yields 38–93%) or cinnamic acid (6–8) (2.2 equiv.), Boc-protected polyamine (12a–f) (1 equiv.), EDC·HCl (2.8 equiv.), DMAP (5 equiv.) and CH₂Cl₂, rt, 20 h (yields 26–88%); (b) TFA (0.2 mL) and CH₂Cl₂ (2 mL), N₂, rt, 2 h (yields 27–92%).

Figure 4

Structures of 3-phenylpropanamido-polyamines 17 and 18.