Novel fentanyl-based dual μ/δ-opioid agonists for the treatment of acute and chronic pain

Abstract

Approximately one third of the adult U.S. population suffers from some type of on-going, chronic pain annually, and many more will have some type of acute pain associated with trauma or surgery. First-line therapies for moderate to severe pain include prescriptions for common mu opioid receptor agonists such as morphine and its various derivatives. The epidemic use, misuse and diversion of prescription opioids have highlighted just one of the adverse effects of mu opioid analgesics. Alternative approaches include novel opioids that target delta or kappa opioid receptors, or compounds that interact with two or more of the opioid receptors.

Aims: Here we report the pharmacology of a newly synthesized bifunctional opioid agonist (RV-Jim-C3) derived from combined structures of fentanyl and enkephalin in rodents. RV-Jim-C3 has high affinity binding to both mu and delta opioid receptors.

Main methods: Mice and rats were used to test RV-Jim-C3 in a tailflick test with and without opioid selective antagonist for antinociception. RV-Jim-C3 was tested for anti-inflammatory and antihypersensitivity effects in a model of formalin-induced flinching and spinal nerve ligation. To rule out motor impairment, rotarod was tested in rats.

Key findings: RV-Jim-C3 demonstrates potent-efficacious activity in several in vivo pain models including inflammatory pain, antihyperalgesia and antiallodynic with no significant motor impairment.

Significance: This is the first report of a fentanyl-based structure with delta and mu opioid receptor activity that exhibits outstanding antinociceptive efficacy in neuropathic pain, reducing the propensity of unwanted side effects driven by current therapies that are unifunctional mu opioid agonists.

Keywords: Allodynia; Chronic pain; Delta opioid; Fentanyl; Formalin flinch; Hyperalgesia; Inflammatory; Mice; Naloxone; Rat; Spinal nerve ligation; mu opioid.

Figure 1

A) RV-Jim-C3 results in a dose- and time-related antinociceptive activity after spinal administration. Warm water (52°C) tail flick test using mice to demonstrate that Rv-Jim-C3 produces a maximal effect 15 minutes after intrathecal administration with recovery to baseline latencies by 60 mins (N=10 per dose). Intrathecal vehicle had no significant effect on tail flick latencies. B) The dose response curve for RV-Jim-C3 in the tail flick test resulted in an A₅₀ of 3.9 ug/5ul (95% C.I. = 2.2–6.9 with R² value of 0.95) at the 15 minute time point.

Figure 1

A) RV-Jim-C3 results in a dose- and time-related antinociceptive activity after spinal administration. Warm water (52°C) tail flick test using mice to demonstrate that Rv-Jim-C3 produces a maximal effect 15 minutes after intrathecal administration with recovery to baseline latencies by 60 mins (N=10 per dose). Intrathecal vehicle had no significant effect on tail flick latencies. B) The dose response curve for RV-Jim-C3 in the tail flick test resulted in an A₅₀ of 3.9 ug/5ul (95% C.I. = 2.2–6.9 with R² value of 0.95) at the 15 minute time point.

Figure 2

A) Systemic naloxone blocks the antinociceptive effects of RV-Jim-C3. RV-Jim-C3 when administered by the i.t. route results in a significant increase in tailflick latencies with a maximum effect at 10ug/5ul. The RV-Jim-C3 increase in tail flick latencies was a significantly reduced by a 10 min pretreatment with 1 mg/kg naloxone. B) Demonstrates the percent antinociceptive activity of RV-Jim-C3 in the absence and presence of systemic naloxone. Naloxone alone had no effect. (n= 10, *p<0.05, **p<0.001).

Figure 3

The 5 minute intrathecal pretreatment with either Naltrindole (5ug/5ul), or CTAP (5 ug/5ul) significantly attenuated the antinociceptive activity of intrathecal RV-Jim-C3. (n= 6, *p<0.05).

Figure 4

A) Formalin (2%/20ul, i.paw) in mice results in two phases of paw flinching characterized as phase I, 0–10minutes and phase II, from 10 minutes to 40 minutes. RV-Jim-C3 (10ug/5ul) when given intrathecally 10 minutes prior to formalin resulted in a significant decrease in flinching in both the first and second phase at all time points (n=6). B) Likewise, formalin (2%/20ul, i.paw) in mice results in two phase of paw guarding. RV-Jim-C3 (10ug/5ul, i.t.) 10 mins prior to formalin significantly reduced paw guarding in both phases (n=6). Vehicle has no effect on formalin flinching or guarding (n=6).

Figure 5

The spinal administration of RV-Jim-C3 (10ug/5ul) resulted in a significant inhibition of spinal nerve ligation (SNL)-induced thermal and mechanical hypersensitivity in rats. A) Paw withdrawal latencies resulted in a significant decrease post-SNL that was significantly reversed at the 20 and 40 minute time points as well as later time points with a return to post-injury baselines at 160 minutes (n=6, *p<0.05). B) Paw withdrawal thresholds resulted in a significant decrease post-SNL that was significantly reversed at the 20 through til the 100 minute time point with a return to post-injury baselines at 140 minutes (n=6, *p<0.05). Vehicle had no effect on mechanical or thermal latencies.

Figure 6

Rats were tested for sedation and/or motor impairment using a rotating rod. Animals are trained on a rotating rod for 3 consecutive trials for a total of 120 seconds. The spinal administration of RV-Jim-C3 or vehicle did not result in any significant changes in an animals’ ability to walk on a rotating rod when compared to naive baselines.