This thesis reports the rational design and development of novel agonists and antagonists for protease activated receptor-2 (PAR2). PAR2 has been implicated in the progression of numerous inflammatory and proliferative diseases, and is upregulated in response to several inflammatory mediators. The regulation of PAR2 in disease modelshas relied on the use of knockout mice, serine proteases and, more often, PAR2- activating peptides (PAR2-APs). The latter are synthetic peptides that are capable of activating PARs in lieu of proteases, although they have much lower agonist potency. At present there are no reliable antagonists for this receptor. To determine the best approaches for furthering the current status of ligands for PAR2, Chapter 1 surveys the current ligands for all PARs, including structure-activity relationship (SAR) studies of PAR-APs through to more recently developed small organic molecule PAR1 antagonists. This review also included brief descriptions of the main assays used to characterize PAR ligands in vitro, as well as background material relevant to PAR activation, activation mechanisms, cell biology, and physiological properties anticipated for PAR ligands. Chapter 2 is an SAR study of novel libraries of peptide ligands based on modifications to the hexapeptide agonist SLIGRL-NH2. Although some modifications of the PAR2-APs SLIGKV-NH2 and SLIGRL-NH2 have been previously reported, there has been limited success in improving agonist potency. Chapter 2 reports the design, synthesis and evaluation of diverse new peptide libraries containing natural and unnatural amino acids, towards the objective of generating potent PAR2 agonists and understanding the molecular basis for their interactions with PAR2. This resulted in a better understanding of the substituent requirements at each position of the PAR2-AP for agonist activity, and the generation of a more refined pharmacophoric model for agonist activity. Compounds were exclusively assessed for PAR2 agonist function in an intracellular Ca2+ release assay, with several exhibiting improved agonist potency over previously reported PAR-APs. A desensitization experiment was also designed to distinguish PAR2 from PAR1 activation, and the proteolytic stability was investigated for one of the more potent agonists (EC50 0.32 ± 0.04 μM).Chapter 3 entails the design and development of non-peptidic agonists for PAR2 using information collected from SAR studies of PAR2-APs reported in Chapter 2. Previously reported agonists for PAR2 were peptides of relatively low potency and selectivity. Recent reports suggest that such PAR2-APs may not be selective for PAR2 over other GPCRs. In addition, being peptides they have low bioavailability and, as such, are notideal as drug candidates. In Chapter 3, new non-peptidic PAR2 agonists were designed through rational modifications to peptide agonists. These were then optimized in SAR studies that culminated in the development of a non-peptidic PAR2 agonist that is equipotent to the most potent PAR2-AP reported (EC50 0.28 ± 0.01 μM), selective for PAR2 over PAR1 and has improved serum stability over the peptide agonists. Chapter 4 investigates modification of PAR2-APs to create antagonists by restraining the conformation of these peptides using macrocyclisation. This method has been reported to improve and/or modify the activity of other peptides that act at GPCRs. The aim was to modify some of the cyclic scaffolds that are known to bind at other GPCRs for PAR2 affinity. Using information from SAR studies of the PAR2-APs to optimise their functional properties, diverse structural analogues of lead compounds were then synthesized and assessed for antagonist activity against PAR2 on human cell lines. SAR and binding information on the requirements of PAR2 inhibition led to the development of PAR2 antagonists of moderate (μM) potency. Chapter 5 examines alternative methods of modifying PAR2 agonists to produce antagonists, using information from SAR data of PAR2-APs and our new non-peptidic agonists, as well as structure-activity hints from reported PAR1 and PAR2 antagonists. Using this information, key fragments of PAR2 agonists were modified to inhibit agonist activity whilst retaining receptor affinity. Lead compounds were assessed for PAR2 activity in functional assays. Promising leads were further diversified to improve antagonist activity and PAR2 selectivity, whilst building important SAR data for PAR2inhibition. This study resulted in the most potent antagonist of PAR2 known to date (IC50 1.0 ± 0.2 μM), inhibiting both PAR2-AP and trypsin mediated activation, and this compound and its analogues provide a platform for the development of potent and selective PAR2 antagonists for possible therapeutic applications.