In vitro and in situ inhibition of the sodium channel blocker saxitoxin by monoclonal antibodies

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Abstract
The sodium channel blocker saxitoxin (STX) was conjugated to keyhole limpet hemocyanin (KLH) and used to immunize BALB/c mice. Anti‐STX antibodies were detected in serum by an enzyme‐linked immunosorbent assay (ELISA) within a week or two after the first immunization. Spleens from immunized mice were fused with NS‐1 myeloma cells and approximately 7000 resultant hybrids were screened by ELISA for reactivity to STX. Two stable hybrids were isolated, subcloned, and characterized. These hybrids, termed S1A5 and S3E.2, secreted specific anti‐STX antibodies that did not recognize the closely related toxin tetrodotoxin (TDT), as determined by competition ELISA. The S1A5 monoclonal antibody (mAb) was of the lgMk class and S3E.2 of the IgG‐1k subclass with affinity constants (Ka values) of approximately 106 M−1. The protective ability of these antibodies was tested by a competitive displacement assay for [3H]STX binding on rat brain membranes. Purified S3E.2 strongly displaced [3H]STX binding, whereas S1A5 weakly inhibited [3H]STX binding to membranes. One nano‐mole of S3E.2 or S1A5 was able to bind 0.03 nmol or 0.005 nmol, respectively, of STX. The S3E.2 mAb offered partial protection against STX‐induced reduction of peripheral nerve action potential in rat tibial nerve when administered in situ at concentrations 10‐ to 30‐fold greater than STX. The S1A5 mAb, despite its ability to inhibit STX binding in vitro, was completely ineffectual in situ. These antibodies, particularly S3E.2, thus represent potentially useful reagents for neurobiologic research, detection of toxin contamination, and diagnosis of poisoning, and may provide protection against the toxicity of STX in vivo.