Therapeutic potential of neuropeptide FF receptor antagonists in the modulation of opioid analgesia and chronic pain – PAINFF

Chronic pain is a major health problem and a socio-economical burden. Despite sustained research efforts and the increasing diversity of available therapeutic approaches, most medications fall short in alleviating chronic pain syndromes. As a matter of fact, opiates and their derivatives, which target the mu-opioid receptor, still represent the most effective means of treating severe pain across a wide range of conditions. Despite their undeniable benefit for treating severe acute pain, opioids still lack effectiveness for long-term therapy. Indeed, repeated administration of opioids exacerbates adverse effects, such as pain hypersensitivity also known as opioid-induced hyperalgesia (OIH), which in turn impairs the analgesic efficacy (tolerance) and triggers the need for dose-escalation. In addition to pain hypersensitivity, risks of respiratory depression, addiction potential and constipation considerably affect the patient’s quality of life and can lead to fatal consequences. This is particularly true in the USA where almost half a million Americans have died from drug overdose (essentially opiates) since 2000. Altogether, these data point to the urgent need to develop novel analgesics with reduced side effects. In the last years, we have been working on the hypothesis that side effects induced by chronic administration of opiates, including OIH and analgesic tolerance result from the imbalance of a homeostatic equilibrium between opioid anti-nociceptive systems and anti-opioid pro-nociceptive systems. From this hypothesis, we have developed antagonists that display similar activity on two anti-opioid receptors, GPCRs NPFF1R and NPFF2R, and we have shown that these antagonists when co-administered with opiates very efficiently block the development of OIH and analgesic tolerance. So NPFFR antagonists may be regarded as potential therapeutic agents for improving the efficacy of opiates in pain treatment. Very recently, we have patented NPFF1R selective antagonists as well as dual opioid agonist/NPFF receptor antagonists for pain treatment. Although these compound display very promising activity, several questions remain almost completely unanswered concerning their mechanism of action including (i) What is the specific role of each NPFF1R and NPFF2R subtypes in the modulation of opioid analgesia? (ii) Are these receptors involved in the development of hyperalgesia induced by chronic inflammatory or neuropathic pain syndromes? (iii) Where are expressed these receptor subtypes and their endogenous ligands? (iv) How activation of these receptors contributes to sensitization of spinal pain projecting neurons in different pain models and what is their relationship with opioid receptors? This project will consist of 3 work packages that should greatly help us to tackle the different issues stated above. WP1 will be dedicated to resynthesis of NPFF1R antagonists we already identified as well as to the development and characterization of highly selective NPFF2R antagonists on the basis of our preliminary results. In WP2, we will use these antagonists and conditional knockout mice for NPFF1R and NPFF2R to study the respective role of these receptors in the modulation of nociception, opioid analgesia and chronic pain, as well as the mechanism of action of these receptors and their endogenous ligands. Finally in WP3, based on our preliminary data, we will study at the molecular level the functional relationship between mu-opioid receptor and NPFF1R in the modulation of nociception and opioid analgesia. Collectively, data obtained during this project should help us to better understand how endogenous anti-nociceptive (opioid) and pro-nociceptive (NPFF) systems are interconnected to generate a pain homeostatic equilibrium and will greatly contribute to push forward NPFFR antagonists to preclinical and clinical trials.