Despite the many advancements in techniques and formulations for regional and local anesthesia, a key area for improvement remains the development of anesthetic agents that are selective for sensory nerves. Conventional agents such as lidocaine and bupivacaine reliably suppress pain, but they block both sensory and motor nerve conduction.1-3 This lack of selectivity creates clinical limitations: patients receiving neuraxial or peripheral nerve blocks may be unable to move affected muscles, delating labor, rehabilitation, and functional recovery.1,3 The long-standing goal has therefore been the development of sensory nerve-selective anesthetic agents that block pain while preserving motor function.2,3
The central obstacle to this goal lies in the mechanism of local anesthetics themselves. Most agents act by blocking voltage-gated sodium channels (VGSCs), which are shared by both sensory and motor axons.4 Although smaller-diameter sensory fibers are somewhat more sensitive than larger motor fibers, the difference is insufficient to prevent motor blockade at clinically effective doses.3,4 To rigorously assess whether new compounds achieve true selectivity, investigators have developed in vivo rat models that simultaneously measure somatosensory-evoked potentials (SSEPs) and motor-evoked potentials (MEPs), allowing objective separation of sensory and motor effects.3
One early strategy toward sensory nerve-selective anesthetic agents employed QX-314, a permanently charged lidocaine derivative that cannot cross neuronal membranes independently.2,3 To enable intracellular entry, investigators paired QX-314 with capsaicin, which opens TRPV1 channels expressed almost exclusively on nociceptive sensory neurons. Once these channels are activated, QX-314 has been limited by delayed onset, incomplete selectivity, and dose-dependent neurotoxicity. Its derivative, QX-OH, has not yet overcome these limitations, highlighting the need for further structural refinement of quaternary lidocaine analogs.5
A more promising approach has emerged with 2′, 6′-pipecolylxylidine (PPX), a metabolite of commonly used amino-amide anesthetics including lidocaine, bupivacaine, and ropivacaine. Researchers at Boston Children’s Hospital demonstrated that PPX can produce robust sensory blockade while preserving motor function.1,6 This selectivity appears to stem from PPX’s intermediate lipophilicity. Motor axons are insulated by thick myelin sheaths, whereas nociceptive sensory fibers are thinly myelinated or unmyelinated. PPX is sufficiently lipophilic to penetrate sensory fibers but not enough to traverse heavily myelinated motor nerves. In rat sciatic nerve models, PPX produced analgesia without measurable motor impairment.1,6
The most recent advancement in sensory-selective anesthesia research seeks to target specific sodium channel subtypes rather than all VGSC’s indiscriminately. NaV1.7 and NaV1.8 are expressed predominantly in peripheral nociceptive neurons and play a central role in pain transmission. Unlike traditional anesthetics, the NaV1.8-selective inhibitor suzetrigine (Journavx) acts upstream of central pain processing by interrupting peripheral nociceptive signaling. In January 2025, suzetrigine received FDA approval for the treatment of acute pain, marking a major milestone in the field.7 By providing effective, non-opioid analgesia, such agents offer a viable strategy to reduce opioid exposure and its associated risks, including dependence, respiratory depression, and overdose.7
By preserving motor function, sensory nerve-selective anesthetic agents can enable early mobilization, ultimately reducing complications related to immobility and accelerating functional recovery.3 Continued advancements in channel specificity, drug structure, and delivery mechanisms bring this long-standing clinical goal increasingly within reach.
References
1. Fliesler N. A safe, pain-specific anesthetic shows preclinical promise. Boston Children’s Answers. September 17, 2025.
2. Sagie I, Kohane DS. Prolonged sensory-selective nerve blockade. Proc Natl Acad Sci U S A. 2010;107(8):3740-3745. doi:10.1073/pnas.0911542107.
3. Teng Y, Zou X, Liu J, et al. An in vivo rat model for comparing selective blockade between sensory and motor nerve conduction. Sci Rep. 2025;15:12201. doi:10.1038/s41598-025-12201-5
4. Iqbal F, Thompson AJ, Riaz S, et al. Anesthetics: from modes of action to unconsciousness and neurotoxicity. J Neurophysiol. 2019;122(2):760-787. doi:10.1152/jn.00210.2019.
5. Wang Q, Zhang Y, Liu J, Zhang W. Quaternary lidocaine derivatives: past, present, and future. Drug Des Devel Ther. 2021;15:195-207.
6. Ostertag-Hill CA, Chen S, Xue T, et al. Sensory-selective peripheral and neuraxial nerve blockade with 2′,6′-pipecoloxylidide. Anesthesiology. 2025;143(5):1296-1312. doi:10.1097/ALN.0000000000005679.
7. Galchen R. The radical development of an entirely new painkiller. The New Yorker. May 26, 2025