Side effects associated with opioid analgesics, such as nausea, drowsiness, respiratory depression, and potential for addiction, are motivating the design and development of new therapies for acute, subacute and chronic pain. Voltage-gated Na+ ion channels are integral membrane proteins responsible for the transmission of signals along electrically conducting cells. Ten mammalian genes have been sequenced, which encode ten distinct channel isoforms (NaV1.1-1.9 and NaX), each having unique gating properties, and cellular and tissue distribution patterns. Recent studies have correlated a hereditary loss-of-function mutation in one human Na+ channel isoform - NaV1.7 - with a rare genetic disorder known as Congenital Insensitivity to Pain (CIP). Individuals with CIP have reduced sensitivity to normally painful stimuli without significant deficits to sensory or cognitive function. A compellin body of evidence indicates that selective inhibition of NaV1.7 in normal humans could recapitulate the phenotype of CIP. The high homology of human NaV proteins, coupled with challenges associated with high-throughput screening against multiple ion channel targets, have thwarted most efforts to develop selective antagonists for individual NaV subtypes. Recent findings indicate that a two amino acid variation in the pore region of hNaV1.7 is responsible for reduced potency of a family of naturally-occurring sodium channel antagonists, the guanidinium toxins (GTxs), against this isoform. This variation is present in all known hNaV1.7 splice variants, but is not found in any other human NaV isoform. Computational modeling studies and protein mutagenesis experiments indicate that it may be possible to restore potency of the GTxs against hNaV1.7 while destabilizing GTx binding to the other eight hNaV isoforms by judicious synthetic modifications. A plan is in place to prepare analogues of the GTxs designed to bind with high affinity to hNaV1.7, and to test these compounds by whole-cell electrophysiology to measure potency and isoform-selectivity. Success of this program will provide 1) an improved understanding of the NaV pore structure and GTx binding pose, 2) a novel tool to validate NaV1.7 as a target for pain treatment, and 3) a high-affinity, isoform-selective sodium channel antagonist as a lead compound for optimization toward a next-generation pain therapeutic.
Existing paradigms for the treatment of moderate-to-severe pain rely heavily on prescription narcotics, such as morphine, oxycodone and fentanyl, which affect the central nervous system and are associated with a range of side effects including nausea, drowsiness, respiratory depression and potential for addiction. We aim to develop a next-generation pain therapy that specifically targets pain signals without affecting cognition. Such a therapy is expected to show improved efficacy and significantly fewer/milder side effects than existing drugs.
