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Opioid receptor signaling, analgesic and side effects induced by a computationally designed

Opioid receptor signaling, analgesic and side effects induced by a computationally designed pH-dependent agonist


Novel pain killers without adverse effects are urgently needed. Opioids induce central and intestinal side effects such as respiratory depression, sedation, addiction, and constipation. They have recently shown that a newly designed agonist with a reduced acid dissociation constant (pKa) abolished pain by selectively activating peripheral μ-opioid receptors (MOR) in inflamed (acidic) tissues without eliciting side effects. Here, they extended this concept in that pKa reduction to 7.22 was achieved by placing a fluorine atom at the ethylidene bridge in the parental molecule fentanyl. The new compound (FF3) showed pH-sensitive MOR affinity, [35S]-GTPγS binding, and G protein dissociation by fluorescence resonance energy transfer. It produced injury-restricted analgesia in rat models of inflammatory, postoperative, abdominal, and neuropathic pain. At high dosages, FF3 induced sedation, motor disturbance, reward, constipation, and respiratory depression. These results support their hypothesis that a ligand’s pKa should be close to the pH of injured tissue to obtain analgesia without side effects.



FF3 preferentially binds to and activates MOR at low pH in HEK293 cells. (A) Chemical structures of fentanyl (Fen) (left) and (±)-N-[1-(2-fluoro-2-phenylethyl)piperidine-4-yl]-N-phenyl propionamide (FF3) (right). The blue circle highlights the acidic nitrogen atom subjected to pH-dependent protonation in whose vicinity electrons may be withdrawn to reduce the pKa value. Green circles denote CH2-groups where a single hydrogen may be replaced by a fluorine. (B) Displacement of bound [³H]-DAMGO by FF3 at pH 6.5 and 7.4. (C) IC50 calculated from B) at pH 6.5 and 7.4 (***P < 0.001, unpaired t-test, n = 6). (D,E) EC50 of fentanyl- (D) and FF3- (E) induced [35S]-GTPγS-binding at pH 6.5 and 7.4 (**P < 0.01, unpaired t-test, n = 5–6). (F) Gαi-mediated FRET responses (ΔFRET between Gαi-mTqΔ6 and cpVenus-Gγ2) to fentanyl and FF3. FF3 at pH 7.4 did not induce significant ΔFRET % compared to vehicle. Curves are derived by nonlinear regression fits constrained to each maximum effect. (G) Time course of FRET responses to fentanyl and FF3 (100 μM) at pH 6.5 differ from those of vehicle-treated cells (*P < 0.05, **P < 0.01 vs. vehicle-treated cells at the corresponding pH, one-way RM-ANOVA and Dunnett’s test, n = 10). (H) At pH 7.4, only fentanyl (100 µM) induced significant FRET responses compared to vehicle (*P < 0.05 vs. vehicle-treated cells at the corresponding pH, one-way RM-ANOVA and Dunnett’s test, n = 10). Data are means ± SEM.


Opioid receptor agonists are the most powerful drugs to treat severe acute and cancer-related pain. However, major problems have emerged due to their epidemic misuse and adverse effects. These side effects comprise sedation, respiratory depression, addiction, nausea, and constipation, and are mediated by central or intestinal opioid receptors. The analgesic effects result from the activation of both central and peripheral opioid receptors via inhibitory G-proteins, which dissociate into Gαi and Gβγ subunits.


Systemic FF3 dose-dependently reduces pain selectively in inflamed tissue in the CFA model. Effects after intravenous (i.v.) injection of FF3 in rats with unilateral hindpaw inflammation on mechanical (PPT (A) and PWT (B)) and heat (PWL (C)) thresholds in inflamed (left panels) and noninflamed (right panels) hindpaws were measured before (0) and 15–60 min after injection, on day 4 after CFA (*P < 0.05, **P < 0.01, ***P < 0.001 vs. vehicle, two-way RM-ANOVA and Bonferroni’s multiple comparison test, n = 9, means ± SEM).


Further downstream signaling leads to suppression of adenylyl cyclases and the modulation of ion channels, resulting in an overall decrease in neuronal excitability. Many experimental and clinical studies revealed that a substantial proportion of opioid analgesia is mediated by the activation of opioid receptors on peripheral sensory neurons. Numerous pain syndromes (e.g. arthritis, neuropathy, postoperative pain, cancer) are accompanied by injury-induced tissue acidosis and upregulation of such peripheral opioid receptors. Therefore, the potential of peripheral opioid receptors as drug targets is increasingly recognized. In contrast to previous pharmacokinetic concepts , they recently developed a new pharmacodynamics-based design for peripherally-acting opioids lacking central or intestinal side effects.


Systemic FF3 dose-dependently reduces pain selectively in inflamed tissue in the CFA model. Effects after intravenous (i.v.) injection of FF3 in rats with unilateral hindpaw inflammation on mechanical (PPT (A) and PWT (B)) and heat (PWL (C)) thresholds in inflamed (left panels) and noninflamed (right panels) hindpaws were measured before (0) and 15–60 min after injection, on day 4 after CFA (*P < 0.05, **P < 0.01, ***P < 0.001 vs. vehicle, two-way RM-ANOVA and Bonferroni’s multiple comparison test, n = 9, means ± SEM).



This strategy is based on computational simulations of pathological receptor conformations and the finding that the protonation state of a ligand is crucial for its activity at opioid receptors. They hypothesized that the ligand’s pKa should be reduced to values close to the acidic pH of injured tissue. This was achieved by fluorination of the piperidine ring in the μ-opioid receptor (MOR) agonist fentanyl, leading to the compound (±)-N-(3-fluoro-1-phenethylpiperidine-4-yl)-N-phenyl propionamide (NFEPP). In addition, their original computer simulations suggested hydrogen/fluorine exchange in fentanyl′s ethylidene bridge5. Accordingly, the compound (±)-N-[1-(2-fluoro-2-phenylethyl)piperidine-4-yl]-N-phenyl propionamide (FF3) was synthesized by a contractor (ASCA GmbH Berlin) and tested in vitro and in vivo in the present study.


Systemic FF3 dose-dependently reduces CCI-induced pain via peripheral opioid receptors. Effects after intravenous (i.v.) injection of FF3 in rats with CCI on mechanical (PPT (A) and PWT (B)) and heat (PWL (C)) thresholds in injured (left panels) and noninjured (right panels) hindlimbs before (0) and 15–60 min after injection, at 14 days following CCI (*P < 0.05, **P < 0.01, ***P < 0.001 vs. vehicle, two-way RM-ANOVA and Bonferroni’s multiple comparison test, n = 9, means ± SEM). (D) Contribution of peripheral opioid receptors to the effects of FF3. NLXM (50 μg) was injected at the site of nerve injury before FF3 (12 μg/kg, i.v.). Effects on PPT were measured 15 min after injection in injured (left panels) and noninjured (right panels) hindlimbs (**P < 0.01 vs. FF3+ vehicle at 15 min, t-test; p< 0.001 vs. baseline, paired t-test, n = 9, means ± SEM).


Systemic FF3 induces central and intestinal side effects at high doses. (A) Effects of subcutaneous (s.c.) fentanyl, morphine, and FF3 presented as the area under the curve (AUC) of the distance (in cm) travelled during 30 min after drug injection (*P < 0.05, **P < 0.01, ***P < 0.001 vs. vehicle, Kruskal-Wallis one-way ANOVA and Dunn’s multiple comparison test; vehicle, fentanyl and FF3: n = 12, morphine: n = 10, means ± SEM). (B) Effects of s.c. fentanyl, morphine, and FF3 presented as the AUC of the time (s) spent on accelerating Rota-Rod at 2, 30 and 60 min after drug injection (**P < 0.01, ***P < 0.001 vs. vehicle, one-way ANOVA and Bonferroni’s multiple comparison test, n = 10). (C) Number of defecations in 1 h after s.c. fentanyl, morphine, and FF3 injection (*P < 0.05, **P < 0.01, ***P < 0.001 vs. vehicle, Kruskal-Wallis one-way ANOVA and Dunn’s multiple comparison test; vehicle, fentanyl and FF3: n = 12; morphine: n = 10, means ± SEM). (D) Effects of fentanyl, morphine, and FF3 on conditional place preference (*P < 0.05, **P < 0.01, ***P < 0.001 vs. vehicle, one-way ANOVA and Bonferroni’ multiple comparison test; vehicle, fentanyl and FF3: n = 12, morphine: n = 10, means ± SEM).



Effects of systemic FF3 on respiration and heart rate. Effects of subcutaneous (s.c.) morphine and FF3 at 5–60 min after injection compared to vehicle on heart rate (beats per min, bpm) (A), oxygen saturation (%) (B) and respiratory rate (breaths per min, brpm) (C) (#P < 0.05, ##P < 0.001, ###P < 0.001 vs. vehicle, two-way RM ANOVA and Bonferroni’s multiple comparison test; vehicle: n = 8; morphine and FF3: n = 10, means ± SEM).


Spahn, V., Del Vecchio, G., Rodriguez-Gaztelumendi, A. et al. Opioid receptor signaling, analgesic and side effects induced by a computationally designed pH-dependent agonist. Sci Rep8, 8965 (2018). https://doi.org/10.1038/s41598-018-27313-4