The earliest event upon microbial invasion is the recognition of the pathogen at the plasma membrane. Recognition of microbial pathogens is evolutionary conserved in all classes of plants and animals, mediated by a range of pattern recognition receptors (PRRs) of the innate immune system. The innate immune response is distinguished as the first line of defence that precedes the highly specialized adaptive immune response, which confers long lasting immunity and immunological memory (Janeway and Medzhitov 2002). PRRs sense highly conserved molecular structures across a wide range of pathogens. Such structures exclusively from bacteria, fungi, parasites and viruses with the capacity to stimulate the innate immune system are referred to as pathogen associated molecular patterns (PAMPs). Examples of PAMPs include lipid-based bacterial cell wall components such as lipoproteins and lipopolysaccharides, microbial protein components such as flagellin, and foreign nucleic acids such as single stranded or double stranded DNA and RNA. The innate immune system includes cells and responses that defend the host from infection by pathogens in a non-specific manner. Cells of the innate immune system produce proinflammatory cytokines and chemokines that are involved in clearing the pathogens and also help shape the downstream adaptive immune response. A number of therapeutic strategies have been taken to exploit the human innate immune system as a tool to recognize and eliminate cancer cells, which is the premise of cancer immunotherapy.
The signaling PRRs include large families of membrane bound Toll like receptors (TLRs) and cytosolic PRRs. TLRs are the best-studied class of receptors that are essential players in the detection of a range of lipid-, protein-, nucleic acid-based PAMPs (Beutler 2009; Kawai and Akira 2011). Cytosolic DNA and RNA sensors on the other hand, have more recently been identified to play major roles in the recognition of nucleic acid PAMPs and in triggering innate immune responses (Keating et al. 2011). Several approaches have been attempted to design small molecules to activate TLRs to mount an innate immune response for use in treating infections, immune disorders, cancer, and as vaccine adjuvants. Local in vivo delivery methods of TLR ligands include topically applied antivirals or anti-tumor agents, intramuscular injections, and intranasal and mucosal administration of immune adjuvants (Hemmi et al. 2002; Ambach et al., 2004). However, the promising therapeutic potential of TLR agonists has been limited by drug delivery issues and by unwanted immune side effects. In terms of drug delivery, challenges remains to systemically target and efficiently release TLR agonists. Furthermore TLR ligands as anti-cancer agents face the challenge of the tumor microenvironment, which tends to suppress anti-cancer agent potential by preventing the agent to penetrate. This invention provides novel compounds that are TLR7 and/or TLR8 agonists conjugated to lipids which become cationic at physiological pH. They can aid cellular uptake, and when complexed to DNA, they are able to concomitantly introduce therapeutic genes into cells and trigger a strong innate immune response by stimulating multiple pathways of the innate immune system.