Structure−Activity Relationships in Human Toll-like Receptor 2-Specific Monoacyl Lipopeptides.
Deepak B. Salunke
Department of Medicinal Chemistry, University of Kansas, Multidisciplinary Research Building, Room 320D, 2030 Becker Drive, Lawrence Kansas 66047, United States.
Vaccination has made a massive impact on human health; even antibiotics have not had such a major effect on mortality reduction and population growth. Contrary to early vaccines, which used killed whole organisms, or attenuated live vaccines, modern vaccines increasingly rely on subunit vaccines which have the distinct advantages of ease of production, quality control, and safety. However, such subunit antigens which are largely soluble proteins are poorly immunogenic, and require “adjuvant” to induce strong and long lasting immune responses. Adjuvants exert their effects through different mechanisms. Adjuvants, such as alum and emulsions (e.g. MF59®), function as delivery systems by generating depots that trap antigens at the injection site, providing slow release in order to continue the stimulation of the immune system. Several ligands for the pattern recognition receptors (PRR), are also known to induce the innate immunity, predominantly targeting the APCs and consequently influencing the adaptive immune response. Therefore, the members of nearly all of the PRR families are potential targets for adjuvant development. These include Toll-like receptors (TLRs), NOD-like receptors (NLRs), RIG-I-like receptors (RLRs) and C-type lectin receptors (CLRs). Toll-like receptor 2-agonistic lipopeptides typified by S-[2,3-bis(palmitoyloxy)-(2RS)-propyl]-Rcysteinyl-S-serine (PAM2CS) compounds are potential vaccine adjuvants. We had previously determined that at least one acyl group of optimal length (C16) and an appropriately orientated ester carbonyl group is essential for TLR2-agonistic activity. We found that these structurally simpler analogues display agonistic activities with human, but not murine, TLR2. SAR studies on the monoacyl derivatives show that the optimal acyl chain length is C16, and aryl substituents are not tolerated. A variety of alkyl and acyl substituents on the cysteine amine were examined. All N-alkyl derivatives were inactive. In contradistinction, short-chain N-acyl analogues were found to be highly active, with a clear dependence on the chain length. A cysteine N-acetyl analogue (N-Ac PAMCS) was found to be the most potent (EC50: 1 nM), followed by the N-butyryl analogue. The N-acetyl analogue is human TLR2-specific, with its potency comparable to that of PAM2CS. Based on this background and the current research work, I will briefly talk about the concept of vaccine adjuvant development and the role TLRs as bridge between innate and adaptive immunity. Our recent investigation on the development of TLR2-specific monoacyl lipopeptides published recently (J. Med. Chem. 2012, 55, 3353) will be discussed in detail.