<p>The G protein-coupled receptors (GPCRs) are important targets in drug discovery. In several cases, the endogenous ligands that activate the GPCRs of pharmaceutical interest are peptides. Unfortunately, peptides are in general not suitable as drugs, since the peptide structure is associated with several disadvantages, such as low oral bioavailability, rapid degradation and low receptor subtype selectivity. Thus, there is a strong need for drug-like nonpeptide ligands to peptide-activated GPCRs. However, to discover nonpeptide ligands that mimic the effect of the endogenous peptide, i.e. peptidomimetics, is a tremendous challenge. In fact, morphine and the related opioids were the only known examples of peptidomimetics before 1995 and these ligands were known long before the native endogenous peptide ligands were discovered. </p><p>The main objective of the work described in this thesis was to design, synthesize and biologically evaluate selective nonpeptide agonists to the peptide-activated GPCR AT₂. The AT₂ receptor belongs to the renin–angiotensin system, where the octapeptide angiotensin II (Ang II) is the major effector peptide. Ang II mediates its effects through the two GPCRs AT₁ and AT₂. The AT₁ receptor is already an established target in the treatment of hypertension. The physiological role of the AT₂ receptor, which is up-regulated in certain pathological conditions, is not fully understood but it seems to include positive effects such as vasodilatation, tissue repair, tissue regeneration and neuronal differentiation. </p><p>In the current investigation we started from the nonpeptide and nonselective (AT₁/ AT₂) compound L-162,313. This ligand is a known AT₁ receptor agonist but its effect on the AT₂ receptor was unknown at the start of this project. We were able to show that it acts as an agonist also at the AT₂ receptor. Furthermore, stepwise synthetic modifications of L-162,313 led to the identification of the first selective nonpeptide AT₂ receptor agonist. Following the discovery of this compound several selective nonpeptide AT₂ receptor agonists were identified. It was also revealed that a minor structural alteration of one of these compounds interconverted the functional activity from agonism to antagonism. The structural requirement for agonism vs antagonism was therefore studied. The functionality switch was suggested, at least partly, to be due to the spatial relationship between the methyleneimidazole group and the isobutyl side chain of the compounds. To further investigate the bioactive conformation(s) of this series of compounds enantiomerically pure analogues with conformationally constrained isobutyl chains were prepared. This study revealed that the direction of the isobutyl side chain determine whether the compounds act as agonists or antagonists at the AT₂ receptor. Further investigations are required to fully elucidate the bioactive conformation(s) of these nonpeptide AT₂ receptor agonists.</p><p>We believe that the selective nonpeptide AT₂ receptor agonists and antagonists identified in this thesis will serve as important research tools in the continuing investigation of the physiological role of the AT₂ receptor. We also believe that these drug-like compounds might provide potential leads in drug discovery processes.</p>

The G protein-coupled receptors (GPCRs) are important targets in drug discovery. In several cases, the endogenous ligands that activate the GPCRs of pharmaceutical interest are peptides. Unfortunately, peptides are in general not suitable as drugs, since the peptide structure is associated with several disadvantages, such as low oral bioavailability, rapid degradation and low receptor subtype selectivity. Thus, there is a strong need for drug-like nonpeptide ligands to peptide-activated GPCRs. However, to discover nonpeptide ligands that mimic the effect of the endogenous peptide, i.e. peptidomimetics, is a tremendous challenge. In fact, morphine and the related opioids were the only known examples of peptidomimetics before 1995 and these ligands were known long before the native endogenous peptide ligands were discovered. The main objective of the work described in this thesis was to design, synthesize and biologically evaluate selective nonpeptide agonists to the peptide-activated GPCR AT2. The AT2 receptor belongs to the renin–angiotensin system, where the octapeptide angiotensin II (Ang II) is the major effector peptide. Ang II mediates its effects through the two GPCRs AT1 and AT2. The AT1 receptor is already an established target in the treatment of hypertension. The physiological role of the AT2 receptor, which is up-regulated in certain pathological conditions, is not fully understood but it seems to include positive effects such as vasodilatation, tissue repair, tissue regeneration and neuronal differentiation. In the current investigation we started from the nonpeptide and nonselective (AT1/ AT2) compound L-162,313. This ligand is a known AT1 receptor agonist but its effect on the AT2 receptor was unknown at the start of this project. We were able to show that it acts as an agonist also at the AT2 receptor. Furthermore, stepwise synthetic modifications of L-162,313 led to the identification of the first selective nonpeptide AT2 receptor agonist. Following the discovery of this compound several selective nonpeptide AT2 receptor agonists were identified. It was also revealed that a minor structural alteration of one of these compounds interconverted the functional activity from agonism to antagonism. The structural requirement for agonism vs antagonism was therefore studied. The functionality switch was suggested, at least partly, to be due to the spatial relationship between the methyleneimidazole group and the isobutyl side chain of the compounds. To further investigate the bioactive conformation(s) of this series of compounds enantiomerically pure analogues with conformationally constrained isobutyl chains were prepared. This study revealed that the direction of the isobutyl side chain determine whether the compounds act as agonists or antagonists at the AT2 receptor. Further investigations are required to fully elucidate the bioactive conformation(s) of these nonpeptide AT2 receptor agonists. We believe that the selective nonpeptide AT2 receptor agonists and antagonists identified in this thesis will serve as important research tools in the continuing investigation of the physiological role of the AT2 receptor. We also believe that these drug-like compounds might provide potential leads in drug discovery processes.

Many peptide receptors are of pharmaceutical interest and there is thus a need for new ligands for such receptors. Unfortunately, peptides are not suitable as orally administrated drugs since they are associated with poor absorption, rapid metabolism and low sub-receptor selectivity. One approach that should allow identification of more drug-like ligands is to use the structural information of the endogenous ligand to develop peptidomimetic compounds. The main objective of the work described in this thesis was to convert angiotensin II (Ang II, Asp-Arg-Val-Tyr-Ile-His-Pro-Phe) to small drug-like compounds with retained bioactivity at the AT2 receptor. The study was performed step-wise via incorporation of well-defined secondary structure mimetics and repeated truncation of the peptide. Five scaffolds, comprising a benzene ring as a central element, suitable as a γ-turn or dipeptide mimetics were designed and synthesized. In order to decorate the scaffolds, a method of microwave-assisted alkoxycarbonylation was developed. After incorporation of the scaffolds into Ang II-related peptides or peptide fragments, the affinities for both the AT1 and the AT2 receptor were determined. In the first series of ligands, two tyrosine-related scaffolds were introduced as γ-turn mimetics in Ang II. All five pseudopeptides exhibited good affinities for the AT2 receptor. One compound was chosen for functional studies and was shown to act as an AT2 receptor agonist. After truncation of Ang II it was shown that C-terminal pentapeptide analogs were AT2 receptor selective agonists. A series of pseudopeptides comprising tyrosine-related scaffolds, derived from the pentapeptides, displayed high AT2 receptor affinities. Two compounds had agonistic effect at the AT2 receptor. This study revealed that the N-terminal part was of less importance while a C-terminal Ile residue was a key element for enhanced AT2 receptor affinity. In the final set of compounds, the peptide was truncated to tripeptide C-terminal fragments. After replacing His-Pro by a histidine-related scaffold small drug-like peptidomimetic compounds with nanomolar affinity for the AT2 receptor were identified.