Ferulic acid dimer as a non-opioid therapeutic for acute pain

Abstract

Purpose: Search for alternate pain medications has gained more importance in the past few years due to adverse effects associated with currently prescribed drugs including nervous system dysfunction with opioids, gastrointestinal discomfort with nonsteroidal anti-inflammatory drugs, and cardiovascular anomalies with cyclooxygenase-2 (COX-2) inhibitors. Phytomedicine has been explored for the treatment of pain, as these have been used for generations in regional communities and tend to lack major side effects in general. One such phytomedicine, incarvillateine (INCA), derived from the Chinese herb Incarvillea sinensis has its primary antinociceptive action through the adenosine receptor, a novel pain target. We hypothesized that derivatives of cinnamic acid dimers, which are structurally similar to INCA, would show potent antinociceptive action and that their effect would be mediated through adenosine receptor action.

Materials and methods: Dimers of cinnamic acid (INCA analogs) were synthesized using cavitand-mediated photodimerization (CMP) method, which utilizes a macromolecule (γ-cyclodextrin) to control excited state reactivity of photoactive compounds. Acute pain response was assessed by using formalin-induced licking behavior in hind paw of mice, and neurologic function was monitored through locomotor activity, mechanical hyperalgesia, and thermal sensitivity upon administration of test compound. For mechanistic studies, binding to adenosine receptor was determined by using computer modeling.

Results: Ferulic acid dimer (FAD), which has the same chemical functionalities on the aromatic ring as INCA, showed significant suppression of formalin-induced acute pain. Antinociceptive effect was observed primarily in the inflammatory phase, and no apparent behavioral changes related to the nervous system were noticeable. Inhibition of opioid receptor did not reverse antinociceptive response, and modeling data suggest adenosine 3 receptor binding.

Conclusion: FAD (INCA analog) shows potent nonopioid antinociceptive action mediated predominantly through A₃AR - adenosine 3 receptor action. Further characterization and selection of such INCA analogs will help us generate a new class of antinociceptives with precise chemical modifications by using CMP methodology.

Keywords: adenosine; antinociceptive; cinnamic acid; formalin; incarvillateine.

Figure 1

Structure of incarvillateine (INCA), the diphenyl dicarboxylic acid core in INCA, its isomeric structural variants, and their corresponding common names (truxillic and truxinic acids).

Figure 2

Photodimerization of trans cinnamic acid and the four possible isomeric dimers.

Figure 3

Cavitand-mediated photodimerization (CMP) method for synthesis of cinnamic acid dimers.

Figure 4

Structure of incarvillateine – and other analogs used in this study.

Figure 5

X-ray crystallography structure of ferulic acid dimer (syn H-H). Carbons (gray), hydrogens (white), and oxygens (red).

Figure 6

Effect of indomethacin, ferulic acid monomer (FAM), ferulic acid dimer (FAD), syn H-H, anti H-T, and isopropyl CAD (all drugs at 30 mg/kg, ip) on formalin-induced acute pain in left hind paw of male mice (n=5 animals per group, *p<0.05 compared to vehicle control for late-phase response).

Figure 7

Dose–response with ferulic acid dimer (FAD; 10, 20, and 30 mg/kg, ip) on formalin-induced acute pain on left hind paw in male mice (n=5 animals per group, *p<0.05 compared to vehicle control for late-phase response).

Figure 8

Effect of ferulic acid monomer (FAM, 30 mg/kg, ip) and ferulic acid dimer (FAD, 30 mg/kg, ip) on formalin-induced acute pain on left hind paw in male (A) and female (B) mice (n=5 animals per group, *p<0.05 compared to vehicle control for late-phase response).

Figure 9

Nervous system function tests with ferulic acid dimer (FAD) administration. Effect of FAD on mechanical hyperalgesia (Von Frey test) in (A) male and (B) female mice. The 50% mechanical paw withdraw threshold (threshold 50%), expressed in grams was evaluated before (Basal) and 1 h after treatment with FAD or vehicle control. Effect of FAD on thermal sensitivity (Hargreaves test) in (C) male and (D) female mice. The latency, expressed in seconds, was evaluated before (Basal) and 1 h after treatment with FAD or vehicle control. Effect of FAD on locomotor function (Rota-rod test) in (E) male and (F) female mice. The latency, expressed in seconds, was evaluated before (Basal) and 1 h after treatment with FAD or vehicle control. The falling latency was recorded and averaged (n=5 animals per group for all tests).

Figure 10

Effect of ferulic acid dimer (FAD) on opioid system. General opioid receptor blocker, naloxone (NAL, 2 mg/kg, ip) was administered 20 min before injecting FAD, followed by formalin procedure (n=5 animals per group, *p<0.05 vs vehicle late-phase control).

Figure 11

Ferulic acid dimer (FAD) (pink, tube model) with homology model of adenosine A₃AR receptor (PDB code 1OEA).