Local anesthetic anchoring to cardiac sodium channels. Implications into tissue-selective drug targeting

Local anesthetics inhibit Na+ channels in a variety of tissues, leading to potentially serious side effects when used clinically. We have created a series of novel local anesthetics by connecting benzocaine (BZ) to the sulfhydryl-reactive group methanethiosulfonate (MTS) via variable-length polyethylether linkers (L) (MTS-LX-BZ [X represents 0, 3, 6, or 9]). The application of MTS-LX-BZ agents modified native rat cardiac as well as heterologously expressed human heart (hH1) and rat skeletal muscle (rSkM1) Na+ channels in a manner resembling that of free BZ. Like BZ, the effects of MTS-LX-BZ on rSkM1 channels were completely reversible. In contrast, MTS-LX-BZ modification of heart and mutant rSkM1 channels, containing a pore cysteine at the equivalent location as cardiac Na+ channels (ie, Y401C), persisted after drug washout unless treated with DTT, which suggests anchoring to the pore via a disulfide bond. Anchored MTS-LX-BZ competitively reduced the affinity of cardiac Na+ channels for lidocaine but had minimal effects on mutant channels with disrupted local anesthetic modification properties. These results establish that anchored MTS-LX-BZ compounds interact with the local anesthetic binding site (LABS). Variation in the linker length altered the potency of channel modification by the anchored drugs, thus providing information on the spatial relationship between the anchoring site and the LABS. Our observations demonstrate that local anesthetics can be anchored to the extracellular pore cysteine in cardiac Na+ channels and dynamically interact with the intracellular LABS. These results suggest that nonselective agents, such as local anesthetics, might be made more selective by linking these agents to target-specific anchors.