Intrathecal injection of pH-responsive nanoparticles containing the neurokinin 1 receptor (NK1R) antagonist aprepitant was found to inhibit induced nociception in rodents, according to a study published in Nature Nanotechnology.1

Nanoparticle-mediated drug delivery preserves key properties of encapsulated molecules while enhancing delivery to target tissues. In the case of stimulus-responsive particles, this results in targeted drug delivery.

Nociception is mediated in spinal neurons via substance P and NK1R signaling in endosomes, leading to sustained cell excitation and transmission of pain signals.2 Blocking of this signal with antagonists of NK1R was found to provide better analgesia when endosomal vs cell surface receptors were targeted.2 Aprepitant, a hydrophobic NK1R antagonist, was approved by the US Food and Drug Administration in 2006 for the treatment of chemotherapy-induced and postoperative nausea and vomiting.

Synthesized pH-responsive nanoparticles — diblock copolymers with a hydrophilic shell and a hydrophobic core — assembled with aprepitant (at graded concentrations of 25, 50, and 100 nM, or at a single concentration of 100 nM), were shown to disassemble at acidic pH. Cellular uptake and subsequent trafficking of nanoparticles labeled with a fluorescent dye was assessed by confocal microscopy in HEK293 cells which have a similar endosomal trafficking and signalling of NK1R than spinal neurons. Nanoparticles incubated with HEK293 cells were shown to localize to early and late endosomes in which fluorescently labeled NK1R was expressed. This localization occurred within 30 minutes of induced endocytosis (triggered by incubation with substance P). Particle disassembly then occurred within acidified endosomes. Intrathecal injection of fluorescent nanoparticles assembled with aprepitant indicated their distribution to neurons located in lamina I, II, and III of the dorsal horn.

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Mechanical nociception was assessed with the von Frey assay. In this test, rodents are stimulated with von Frey filaments at the plantar surface. Hindpaw withdrawal upon stimulation with filament indicates a positive response. Increased von Frey threshold in response to stimulation is reflective of analgesic effect.

The effect of aprepitant encapsulated in nanoparticles and injected intrathecally was assessed in vivo in animal models of acute nociceptive pain evoked in mice with intraplantar injection of capsaicin, in models of sustained mechanical allodynia evoked in mice by intraplantar injection of complete Freund’s adjuvant (CFA), and in models of mechanical hyperalgesia (neuropathic pain) evoked in rats by surgical injury to the sural nerve. Intraplantar injection of capsaicin or CFA and injury to the sural nerve were all found to result in NK1R endocytosis in spinal neurons and to reduce the von Frey threshold.

The effect of intrathecal injection of: pH-responsive nanoparticles loaded with graded concentrations of aprepitant or 100 nM aprepitant, unloaded pH-responsive nanoparticles, non-pH-responsive nanoparticles loaded with aprepitant, or free aprepitant was examined on the animal response to the different types of induced pain.

Animals injected with nanoparticles loaded with 100 nM aprepitant had the greatest analgesic effect on capsaicin-induced acute nociceptive pain (inhibition: at 1 h, 34±3%; at 4 h, 35±2) and on CFA-induced mechanical allodynia (54±4% inhibition at 1.5 h, maintained up to 6 h). In sural nerve injury-induced mechanical hyperalgesia (lasting >50 days), mechanical hyperalgesia (observed 10 days after surgery) was inhibited by aprepitant at 300 nM (maximum at 1 h: 40±2% inhibition) and 1 μM (75±4% inhibition at 1 h, with return to baseline at 3 h), and to a greater extent, by nanoparticles loaded with aprepitant at 100, 300, and 500 nM (inhibition at 1.5 h: 80±4% with 300 nM aprepitant; maintained for 4.5 h; full inhibition at 500 nM).

“Nanoparticle encapsulation enhanced the anti-nociceptive actions of aprepitant in preclinical models of pain. These findings are consistent with the improved capacity of nanoparticle-encapsulated aprepitant to inhibit [substance P]-induced excitation of spinal neurons and to cause a sustained inhibition of endosomal signalling. Nanoparticle uptake and sustained release of aprepitant in acidic endosomes containing the activated NK1R could account for these enhanced and persistent anti-nociceptive effects,” concluded the researchers. “Further studies are necessary before nanoparticle-encapsulated analgesics can be advanced to clinical trials. They include toxicology, pharmacokinetic and pharmacodynamic studies in disease-relevant preclinical models.”


1.      Ramírez-garcía PD, Retamal JS, Shenoy P, Imlach W, Sykes M, Truong N, et al. A pH-responsive nanoparticle targets the neurokinin 1 receptor in endosomes to prevent chronic pain. Nat Nanotechnol. 2019;14(12):1150-1159.

2.      Jensen, D. D. et al. Neurokinin 1 receptor signaling in endosomes mediates sustained nociception and is a viable therapeutic target for prolonged pain relief. Sci. Transl. Med. 9, eaal3447 (2017).