The treatment of bacterial infections with antibiotics is one of the mainstays of human medicine. Unfortunately the effectiveness of antibiotics has become limited due to an increase in bacterial antibiotic resistance in the face of a decreasing efforts and success in discovery of new classes of antibiotics. Today, infectious diseases are the second leading cause of death worldwide and the largest cause of premature deaths and loss of work productivity in industrialized countries. Nosocomial bacterial infections that are resistant to therapy result in annual costs of more than $2 billion and account for more than 80,000 direct and indirect deaths in North America alone, whereas a major complication of microbial diseases, namely sepsis, accounts for 700,000 cases and 140,000 deaths in North America.
A major limitation in antibiotic development has been difficulties in finding new structures with equivalent properties to the conventional antibiotics, namely low toxicity for the host and a broad spectrum of action against bacterial pathogens. Recent novel antibiotic classes, including the oxazolidinones (linezolid), the streptogramins (synercid) and the glycolipids (daptomycin) are all active only against Gram positive pathogens. Cationic antimicrobial peptides, found in most species of life, represent a good template for a new generation of antimicrobials. They kill both Gram negative and Gram positive microorganisms rapidly and directly, do not easily select mutants, work against common clinically-resistant bacteria such as methicillin-resistant Staphylococcus aureus (MRSA) and vancomycin resistant Enterococcus (VRE), show a synergistic effect with conventional antibiotics, and can often activate host innate immunity without displaying immunogenicity (Hancock R E W. 2001. Cationic peptides: effectors in innate immunity and novel antimicrobials. Lancet Infectious Diseases 1, 156-164). Moreover, they seem to counteract some of the more harmful aspects of inflammation (e.g. sepsis, endotoxaemia), which is extremely important since rapid killing of bacteria and subsequent liberation of bacterial components such as LPS or peptidoglycan can induce fatal immune dysregulation (Jarisch-Herxheimer reaction) (Gough M, Hancock R E W, Kelly N M. 1996. Anti-endotoxic potential of cationic peptide antimicrobials. Infect. Immun. 64, 4922-4927). A need exists in the art for developing new treatments for infections to be used as broad spectrum antibiotics and/or as agents that selectively enhance aspects of innate immunity while suppressing potentially harmful inflammation.
The innate immune system is a highly effective and evolved general defense system that involves a variety of effector functions including phagocytic cells, complement, and the like, but is generally incompletely understood. Elements of innate immunity are always present at low levels and are activated very rapidly when stimulated by pathogens, acting to prevent these pathogens from causing disease. Generally speaking many known innate immune responses are “triggered” by the binding of microbial signaling molecules, like lipopolysaccharide (LPS), with pattern recognition receptors such as Toll-like receptors (TLR) on the surface of host cells. Many of the effector functions of innate immunity are grouped together in the inflammatory response. However, too severe an inflammatory response can result in responses that are harmful to the body, and, in an extreme case, sepsis and potentially death can occur; indeed sepsis occurs in approximately 780,000 patients in North America annually with 140,000 deaths. Thus, a therapeutic intervention to boost innate immunity, which is based on stimulation of TLR signaling (for example using a TLR agonist), has the potential disadvantage that it could stimulate a potentially harmful inflammatory response and/or exacerbate the natural inflammatory response to infection. A further need exists in the art for therapeutic inverterventions to boost innate immunity that are effective and have fewer undesirable side effects or adverse reactions.