The Novel Activity of Carbamazepine as an Activation Modulator Extends from Na1.7 Mutations to the Na1.8-S242T Mutant Channel from a Patient with Painful Diabetic Neuropathy.

Neuropathic pain in patients carrying sodium channel gain-of-function mutations is generally refractory to pharmacotherapy. However, we have shown that pretreatment of cells with clinically achievable concentration of carbamazepine (CBZ; 30 M) depolarizes the voltage dependence of activation in some Na1.7 mutations such as S241T, a novel CBZ mode of action of this drug. CBZ reduces the excitability of dorsal root ganglion (DRG) neurons expressing Na1.7-S241T mutant channels, and individuals carrying the S241T mutation respond to treatment with CBZ. Whether the novel activation-modulating activity of CBZ is specific to Na1.7, and whether this pharmacogenomic approach can be extended to other sodium channel subtypes, are not known. We report here the novel Na1.8-S242T mutation, which corresponds to the Na1.7-S241T mutation, in a patient with neuropathic pain and diabetic peripheral neuropathy. Voltage-clamp recordings demonstrated hyperpolarized and accelerated activation of Na1.8-S242T. Current-clamp recordings showed that Na1.8-S242T channels render DRG neurons hyperexcitable. Structural modeling shows that despite a substantial difference in the primary amino acid sequence of Na1.7 and Na1.8, the S242 (Na1.8) and S241 (Na1.7) residues have similar position and orientation in the domain I S4-S5 linker of the channel. Pretreatment with a clinically achievable concentration of CBZ corrected the voltage dependence of activation of Na1.8-S242T channels and reduced DRG neuron excitability as predicted from our pharmacogenomic model. These findings extend the novel activation modulation mode of action of CBZ to a second sodium channel subtype, Na1.8.