Peptidomimetics derived from the bioactive hexapeptide angiotensin IV (Ang IV, Val1-Tyr2-Ile3-His4-Pro5-Phe6) have been designed and synthesized. These peptidomimetics are aimed at inhibiting the insulin-regulated amino peptidase (IRAP), also known as the AT4 receptor. This membrane-bound zinc-metallopeptidase is currently under investigation regarding its potential as a target for cognitive enhancers. The work presented herein was based on stepwise replacement of the amino acid residues in Ang IV by natural and unnatural amino acids, non-peptidic building blocks, and also on the introduction of conformational constraints. Initially, we focused on the introduction of secondary structure mimetics and backbone mimetics. The C-terminal tripeptide His-Pro-Phe was successfully replaced by a γ-turn mimetic scaffold, 2-(aminomethyl)phenylacetic acid (AMPA), which was coupled via an amide bond to the carboxyl terminus of Val-Tyr-Ile. Substitution of Val-Tyr-Ile, Val-Tyr, Tyr-Ile and Tyr, respectively, by 4-hydroxydiphenylmethane scaffolds comprising a 1,3,5-substituted benzene ring as a central moiety unfortunately rendered peptidomimetics that were less potent than Ang IV. The subsequent approach involved the introduction of conformational constraints into Val-Tyr-Ile-AMPA by replacing Val and Ile by amino acid residues appropriate for disulfide cyclization or ring-closing metathesis. Chemically diverse structures encompassing an N-terminal 13- or 14-membered macrocyclic tripeptide and a C-terminal non-peptidic moiety were developed. Tyr2 and AMPA were modified to acquire further knowledge about the structure-activity relationships and, in addition, to improve the metabolic stability and reduce the polarity. Several of the compounds displayed a high capacity to inhibit IRAP and exhibited Ki values in the low nanomolar range. Hence, the new compounds were more than ten times more potent than the parent peptide Ang IV. Enhanced selectivity over the closely related aminopeptidase N (AP-N) was achieved, as well as improved stability against proteolysis by metallopeptidases present in the assays. However, additional investigations are required to elucidate the bioactive conformation(s) of the relatively flexible N-terminal macrocycles. The compounds presented in this thesis have provided important information on structure-activity relationships regarding the interaction of Ang IV-related pseudopeptides and peptidomimetics with IRAP. The best compounds in the series constitute important starting points for further discovery of Ang IV peptidomimetics suitable as tools in the investigation of IRAP and other potential targets for Ang IV. The literature presents strong support for the hypothesis that drug-like IRAP inhibitors would serve as a new type of future cognitive enhancers with potential use in the treatment of cognitive disorders, e.g. Alzheimer’s disease.

在本實驗室之前的研究中顯示在發炎狀態的大白鼠上以intrathecal (i.t.) 方式給予angiotensin IV (Ang IV) ，Ang IV 是insulin-regulated aminopeptidase (IRAP) inhibitor，可以減少腳掌發炎之大白鼠的疼痛過敏化。然而，這個由Ang IV所產生之效果背後的機制還未被完全釐清。 在這次的實驗中，我們利用由carrageenan引起腳掌腫脹之大鼠進行plantar test來研究Ang IV所產生之抗疼痛過敏化其中可能機制。由於有文獻指出，在發炎狀態的大白鼠上以intrathecal (i.t.) 方式給予oxytocin可產生一劑量相關性的抗疼痛過敏化作用，因此我們推測在大白鼠的脊髓層次上由oxytocin以及Ang IV所個別誘導的抗疼痛過敏化之間可能有關聯性。利用 i.t.單獨給予atosiban (selective oxytocin receptor antagonist) 可以觀察到一個較強烈的疼痛過敏化現象，然而合併給予atosiban，可使Ang IV所產生的抗疼痛過敏化完全被阻斷掉。因此我們推測oxytocin在以intrathecal (i.t.) 方式給予Ang IV而阻斷IRAP的活性，進而產生抗疼痛過敏化作用的過程中是一個主要的IRAP受質。 當我們在carrageenan誘導疼痛過敏化的大白鼠上單獨給予低劑量之oxytocin並沒有產生統計上顯著之抗疼痛過敏作用。然而合併給予oxytocin和Ang IV後，則可以有觀察到Ang IV 能增加並且延長oxytocin所引起的抗疼痛過敏化作用。 就內生性oxytocin來看，電刺激paraventricular hypothalamic nucleus (PVN)已被證實可以增加內生性的oxytocin分泌到脊髓。而這樣的一個神經路徑被發現和疼痛的調節具有很密切的關聯性。從我們的結果中，我們發現在carrageenan誘導疼痛過敏化的大白鼠上，i.t.給予Ang IV可以延長PVN電刺激產生的抗疼痛過敏化作用，由此推測 Ang IV可能可以保護內生性oxytocin不被分解或失去活性。 除了oxytocin之外，IRAP在體外的實驗中被證實可以分解數種具生理活性之peptides 的N-terminal amino acid，其中包括vasopressin, bradykinin 以及 enkephalin等。在這些物質中，bradykinin已被證實在週邊發炎的過程中具有重要的調控效果，若IRAP在週邊組織扮演了一個分解促發炎物質的角色，利用Ang IV也許可以去阻斷IRAP之活性進而增加這些促發炎物質。因此，我們在由carrageenan誘導疼痛過敏化的動物模式上利用腳掌局部注射 Ang IV來探討在發炎部位Ang IV可能的作用。我們猜測Ang IV 可能可以調控不同物質(例如：bradykinin)而在發炎部位產生局部之作用， 但結果顯示在疼痛過敏化以及腫脹程度上，Ang IV並不具有調控週邊組織發炎過程的能力。所以推測Ang IV在週邊發炎位置可能並沒有扮演非常重要的角色。 總而言之，本實驗結果證實了Ang IV以及 oxytocin在發炎大白鼠的脊髓層次上所個別引起之抗疼痛過敏化作用之間具有相關性，其中的機制可能是Ang IV在發炎大白鼠的脊髓層次上藉由抑制IRAP降解oxytocin的活性，進而產生抗疼痛過敏化的效果。 / In our previous study, we showed that intrathecal (i.t.) administration of angiotensin IV (Ang IV), an insulin-regulated aminopeptidase (IRAP) inhibitor, could attenuate hyperalgesia in rats with inflammation. However, the underlying mechanism(s) for this effect of Ang IV was not clarified yet. Using the plantar test in rats with carrageenan-induced paw inflammation, we attempted to investigate the possible mechanism(s) of Ang IV in the present study. Because it has been reported that i.t. administration of oxytocin produced a dose-dependent anti-hyperalgesia effect in rats with inflammation, we speculate that there is a possible correlation between the oxytocin-induced and angiotensin IV-induced anti-hyperalgesia at the spinal level in rats. Using i.t. co-administered atosiban (a selective oxytocin receptor antagonist), the anti-hyperalgesia effect of Ang IV was completely abolished, although a severer hyperalgesia was observed in rats receiving atosiban alone. This indicates that oxytocin could be the major substrate of IRAP responsible for the anti-hyperalgesia caused by intrathecal administration of Ang IV, which blocked the activity of IRAP. Using i.t. administration of oxytocin in rats with carrageenan-induced hyperalgesia, the anti-hyperalgesia effect of oxytocin was potent and significant. When Ang IV was co-administered with the low dose of oxytocin, a significant enhancing effect of Ang IV on anti-hyperalgesia of oxytocin was observed. In view of the endogenous oxytocin, electrical stimulation of the paraventricular hypothalamic nucleus (PVN) has been proved to cause the increase of oxytocin release at the spinal cord. This neural pathway has been found to be highly related to the modulation of pain. In our results, we found that i.t. administration of Ang IV could prolong the anti-hyperalgesia induced by PVN stimulation. This suggests a possible protective effect of Ang IV on endogenous oxytocin degradation/dysfunctioning. In addition to oxytocin, it was well known that IRAP is able to cleave the N-terminal amino acid from several bioactive peptides in vitro, including vasopressin, bradykinin and enkephalin. Among these substrates, bradykinin has been demonstrated to be an important mediator in peripheral inflammation. It is a pro-inflammatory substance that can be enhanced by Ang IV, if the peripheral IRAP plays a role in its degradation. Therefore, we examined the possible local effect of intraplantarly injected Ang IV on the carrageenan-induced hyperalgesia in the same model. However, our results showed no effect of local Ang IV on hyperalgesia and paw edema, indicating that Ang IV may not be able to regulate the peripheral inflammatory process. Overall, the present study verified the possible interaction between the oxytocin-induced and angiotensin IV-induced anti-hyperalgesia at the spinal level in rats with inflammation. It suggests that Ang IV may act though the inhibition of the activity of IRAP to reduce the degradation of oxytocin, thereby lead to anti-hyperalgesia in rats with inflammation.

The need for alternative cognitive enhancers has risen due to the fact that clinical trial results of the drugs currently approved for treating these disorders have not been satisfactory. IRAP has become a possible drug target for treating cognitive impairment brought about by Alzheimer’s disease, head trauma or cerebral ischemia, among others. This came after the revelation that Angiotensin IV enhances memory and learning. Angiotensin IV, the endogenous ligand of IRAP has been structurally modified with the aim of producing potent IRAP inhibitors. However, the peptidic nature of these inhibitors restricts their use; they are not likely to cross the blood brain barrier. Other strategies for generating IRAP inhibitors have been through structure-based design and receptor based virtual screening. These drug-like molecules have exhibited positive results in animal studies. IRAP inhibitors have been identified via a HTS of 10500 low-molecular weight compounds to give the hit based on a spirooxindole dihydroquinazolinone scaffold, with an IC50 value of 1.5 µM. In this project, some analogues to this hit compound have successfully been synthesized using a known method, whereas others have been synthesized after additional method development. The application of the developed method was found to be limited, because poor yield was obtained when a compound with an electron withdrawing substituent on the aniline was synthesized. As a result of this, modification of this method may be required or new methods may have to be developed to synthesize these types of analogues. Inhibition capability of 5 new spirooxindole dihydroquinazolinones was tested through a biochemical assay. Compound 6e emerged as the most potent inhibitor in the series, with an IC50 value of 0.2 µM. This compound will now serve as a lead compound and should be used as a starting point for future optimization in order to generate more potent IRAP inhibitors.