Engineered Opioid Receptors as Therapeutic Agents for Pain Control

描述（由申请人提供）：阿片类药物已非常成功地用于治疗中度至重度急性和慢性疼痛。不幸的是，它们的使用伴随着许多麻烦的副作用，如恶心、便秘、呼吸抑制、镇静、瘙痒、耐受性和依赖性的发展。许多方法已被用来减轻这些副作用而不减少镇痛效果，与不同的成功。值得注意的方法是设计受体选择性配体，激活三种克隆阿片受体中的一种，并共同施用药物来阻断阿片副作用。尽管这些方法和其他方法都是可行的，但它们有限成功的原因之一是克隆受体之间的氨基酸序列同源性很高。这种同源性延缓了阿片类药物的设计和开发，该药物将靶向并激活特定的阿片类受体。在目前的研究中，我们建议使用另一种方法，即设计突变阿片受体用于疼痛管理。我们的方法是基于意外发现的受体突变，u-阿片受体（MOR）中的S4.45(196)L，导致阿片生物碱拮抗剂在体外和体内激活突变受体的能力。我们假设，如果这样的突变受体可以被传递到痛觉神经元并在痛觉神经元中表达，那么拮抗剂激活突变受体应该会产生镇痛反应，而不会引起慢性拮抗剂治疗期间的耐受反应。我们的总体目标是开发这种受体突变体作为疼痛管理的治疗剂。因此，在目前的建议中，我们将(1)确定阿片样物质拮抗剂激活受体突变体的分子基础；(2)验证和完善S4.54突变的受体模型，以设计纳洛酮和纳曲酮作为完全激动剂的突变MOR；(3)开发一种双链腺相关病毒（dsAAV），用于在疼痛通路的特定部位递送突变受体，并评估拮抗剂和激动剂在引发抗感觉性反应方面的效果。我们将检查注射dsAAV后对阿片激动剂和拮抗剂的急性和慢性反应。通过建立受体模型，并通过受体诱变研究和突变小鼠系的产生来证明拮抗剂活性的结构基础，我们可以设计一种突变受体，使阿片拮抗剂的行为像完全激动剂一样。由于dsAAV已成功用于基因治疗，dsAAV的最终递送和突变受体在通常表达MOR的伤害性神经元上的表达应该会在全身给药阿片拮抗剂后产生镇痛反应，而不会产生耐受性。这种突变的阿片受体基因治疗方法可能是最终治疗慢性疼痛的新范例。公共卫生相关性：在治疗中度至重度疼痛时，吗啡仍然是首选药物。然而，药物的许多副作用，特别是在长期治疗中耐受性和依赖性的发展，减少了使用这种阿片类镇痛药进行疼痛管理的可取性。开发既能减轻疼痛又没有任何副作用的药物分子或治疗模式一直是药理学家和药物化学家的圣杯。随着一种突变的u-阿片受体（MOR）的发现，它可以被阿片拮抗剂激活而不改变激动剂的特性，我们假设这种突变受体可以开发成用于疼痛管理的治疗剂。我们已经证明了这种方法的可行性，通过“敲入”这种突变S4.54(196)A进入MOR，并产生一个小鼠系，其中阿片类拮抗剂，纳洛酮和纳曲酮产生抗伤害性反应。然而，这种突变只导致这两种阿片拮抗剂的部分激动特性。因此，在拟议的研究中，我们将利用建模和突变分析来研究这种拮抗活性的分子基础。为了将拮抗剂转化为完全激动剂，将确定额外的受体突变。我们将开发一种基因治疗载体，最初使用dsAAV2病毒，将突变受体递送到疼痛通路的各个区域，并研究使用阿片类拮抗剂作为抗伤害性药物的可行性。外源引入的突变MOR的激活，以及拮抗剂对内源性阿片受体的失活，将为开发一种不可能产生耐受性和依赖性的疼痛治疗范例提供独特的机会。

DESCRIPTION (provided by applicant): Opioids have been used very successfully for the treatment of moderate to severe acute and chronic pain. Unfortunately, their uses have been associated with many troublesome side effects such as nausea, constipation, respiratory depression, sedation, pruritus, tolerance and dependence development. Many approaches have been used to alleviate these side effects without diminishing the analgesic effects, with variable success. Notable approaches have been the design of receptor selective ligands that would activate one of the three cloned opioid receptors and co-administration of pharmaceutical agents to block the opioid side effects. Although these approaches and others are viable ones, one of the reasons for their limited success is the high amino acid sequence homology among the cloned receptors. Such homology has slowed the design and development of an opioid drug that will target and activate a specific opioid receptor. In the current studies, we propose to use an alternative approach, i.e., to engineer mutant opioid receptors for pain management. Our approach is based on an accidentally discovered receptor mutation, S4.45(196)L in the u-opioid receptor (MOR), that resulted in the ability of opioid alkaloid antagonists to activate the mutant receptor, both in vitro and in vivo. We hypothesize that, if such a mutant receptor could be delivered to and expressed in the nociceptive neurons, then activation of the mutant receptors by antagonists should produce analgesic responses without eliciting the tolerance responses during chronic treatment with the antagonists. Our overall goal is to develop such receptor mutants as therapeutic agents for pain management. Thus, in the current proposal, we will (1) determine the molecular bases for the activation of receptor mutants by opioid antagonists; (2) validate and refine the receptor model for S4.54 mutation so as to engineer a mutant MOR in which naloxone and naltrexone behave like full agonists; and (3) develop a double stranded adenoassociated virus (dsAAV) for the delivery of the mutant receptor at specific sites of the pain pathway and evaluate the antagonist and agonist efficacies in eliciting antinociceptive responses. We will examine both the acute and chronic responses to opioid agonists and antagonists after dsAAV injection. By developing the receptor model and demonstrating the structural bases for the antagonist activities via receptor mutagenesis studies and generation of mutant mouse lines, we could engineer a mutant receptor in which opioid antagonists behave like full agonists. Since dsAAV has been used successfully in gene therapy, the eventual delivery by dsAAV and expression of the mutant receptor at the nociceptive neurons that normally express MOR should result in analgesic responses after systemic administration of opioid antagonists without tolerance development. Such a mutant opioid receptor gene therapy approach could be a new paradigm for the eventual treatment of chronic pain. PUBLIC HEALTH RELEVANCE: In the treatment of moderate to severe pain, morphine remains the drug of choice. However, the many side-effects of the drugs, notably tolerance and dependence development in prolonged treatment, have reduced the desirability in the use of this opioid analgesic for pain management. It has been the Holy Grail of pharmacologists and pharmaceutical chemists to develop drug molecules or treatment paradigms that elicit the pain relief effects without any side effects. With the discovery of a mutant u-opioid receptor (MOR) that could be activated by opioid antagonists without altering the agonists' properties, we hypothesize that such a mutant receptor could be developed into therapeutic agents for the purpose of pain management. We have demonstrated the feasibility of such approach by "knocking-in" this mutation, S4.54(196)A, into MOR, and generating a mouse line in which opioid antagonists, naloxone and naltrexone produced antinociceptive responses. However, this mutation only resulted in partial agonistic properties observed with these two opioid antagonists. Therefore, in the proposed studies, we will investigate the molecular bases for such antagonistic activities using modeling and mutational analysis. Additional receptor mutations will be identified in order to convert the antagonists into full agonists. We will develop a gene therapy vehicle, initially using dsAAV2 virus, to deliver the mutant receptors into various regions of the pain pathway and to examine the feasibility of using opioid antagonists as antinociceptive agents. The activation of the exogenously introduced mutant MOR, and the inactivation of the endogenous opioid receptors by the antagonists, will provide a unique opportunity to develop a pain treatment paradigm without possible development of tolerance and dependence.