N-Caffeoylphenalkylamide derivatives as bacterial efflux pump inhibitors Serge Michalet, a, * Gilbert Cartier, a Bruno David, b Anne-Marie Mariotte, a Marie-Genevie `ve Dijoux-franca, a Glenn W. Kaatz, c Michael Stavri d and Simon Gibbons d a DPM UMR 5063 CNRS-UJF, Equipe de Pharmacognosie, UFR de Pharmacie de Grenoble, Domaine de La Merci, 38706 La Tronche cedex, France b Centre de Recherche des Substances Naturelles, UMS CNRS/IRPF No. 2597, 3, rue des Satellites, 31432 Toulouse, France c Department of Internal Medicine, Division of Infectious Diseases, School of Medicine, Wayne State University and the John D. Dingell Department of Veteran Affairs Medical Center, Detroit, MI 48201, USA d Centre for Pharmacognosy and Phytotherapy, School of Pharmacy, University of London, 29/39 Brunswick Square, WC1N 1AX London, UK Received 16 October 2006; revised 15 December 2006; accepted 16 December 2006 Available online 22 December 2006 Abstract—As part of an ongoing project to identify plant natural products as efflux pump inhibitors (EPIs), bioassay-guided frac- tionation of the methanolic extract of Mirabilis jalapa Linn. (Nyctaginaceae) led to the isolation of an active polyphenolic amide: N-trans-feruloyl 4 0 -O-methyldopamine. This compound showed moderate activity as an EPI against multidrug-resistant (MDR) Staphylococcus aureus overexpressing the multidrug efflux transporter NorA, causing an 8-fold reduction of norfloxacin MIC at 292 lM (100 lg/mL). This prompted us to synthesize derivatives in order to provide structure–activity relationships and to access more potent inhibitors. Among the synthetic compounds, some were more active than the natural compound and N-trans-3,4-O- dimethylcaffeoyl tryptamine showed potentiation of norfloxacin in MDR S. aureus comparable to that of the standard reserpine. Ó 2007 Elsevier Ltd. All rights reserved. In recent years bacterial resistance to antibiotics has be- come a serious problem of public health that concerns almost all antibacterial agents. Among the mechanisms involved (target modification, enzymatic inactivation or reduction of accumulation within the cell), active ef- flux has received particular attention since it has been recognised as one of the most important causes of intrin- sic antibiotic resistance in bacteria. 1 One species is particularly resistant to treatment, nota- bly Staphylococcus aureus. This pathogen has evolved from the MRSA phenotype (methicillin-resistant S. aureus) to the VRSA phenotype (vancomycin-resis- tant S. aureus), whereas vancomycin was the drug of last-resort for treatment of MRSA. 2 Resistance has also been reported for newer agents such as linezolid and daptomycin. 3,4 Of particular concern is the presence of multidrug resis- tance (MDR) efflux pumps that extrude a wide range of structurally unrelated compounds. The NorA protein of S. aureus is a drug/proton antiporter belonging to the major facilitator family (MFS) of transporters and is responsible for the efflux of substances such as norflox- acin, ciprofloxacin, ethidium bromide and acriflavin. 5,6 It has been recognised as one of the major efflux pumps protecting S. aureus from antibiotics. 7 One of the strategies employed to overcome bacterial resistance is the use of EPIs that could restore antibiotic activity in resistant strains. This approach is promising as it would be a way to improve the efficacy and/or ex- tend the clinical utility of existing antibiotics. 8 The com- bination of a resistance inhibitor with an antibiotic has already proven its efficacy with the clavulanic acid/ amoxicillin association. Furthermore, this combination can reduce the in vitro frequency of emergence of resis- tant mutant strains. 9 At present this combination thera- py is taken into account by a number of pharmaceutical companies and several strategies are being developed by different groups. 10 0960-894X/$ - see front matter Ó 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.bmcl.2006.12.059 Keywords: EPI; Staphylococcus aureus; N-Hydroxycinnamic acid der- ivatives; Mirabilis jalapa. * Corresponding author. Tel.: +33 4 76 51 86 88; fax: +33 4 76 63 71 65; e-mail: sergemichalet@yahoo.fr Bioorganic & Medicinal Chemistry Letters 17 (2007) 1755–1758