No abstract

Lamotrigine, approved for use as an antiseizure medication as well as the treatment of bipolar disorder, inhibits sodium channels in the brain to reduce repetitive neuronal firing and pathological release of glutamate. The shared homology of sodium channels and lack of selectivity associated with channel blocking agents can cause slowing of cardiac conduction and increased proarrhythmic potential. The Vaughan‐Williams classification system differentiates sodium channel blockers using biophysical properties of binding. As such, Class Ib inhibitors, including mexiletine, do not slow cardiac conduction as measured by the electrocardiogram, at therapeutically relevant exposure. Our goal was to characterize the biophysical properties of Na V 1.5 block and to support the observed clinical safety of lamotrigine. We used HEK‐293 cells stably expressing the hNa V 1.5 channel and voltage clamp electrophysiology to quantify the potency (half‐maximal inhibitory concentration) against peak and late channel current, on‐/off‐rate binding kinetics, voltage‐dependence, and tonic block of the cardiac sodium channel by lamotrigine; and compared to clinically relevant Class Ia (quinidine), Ib (mexiletine), and Ic (flecainide) inhibitors. Lamotrigine blocked peak and late Na V 1.5 current at therapeutically relevant exposure, with rapid kinetics and biophysical properties similar to the class Ib inhibitor mexiletine. However, no clinically meaningful prolongation in QRS or PR interval was observed in healthy subjects in a new analysis of a previously reported thorough QT clinical trial (SCA104648). In conclusion, the weak Na V 1.5 block and rapid kinetics do not translate into clinically relevant conduction slowing at therapeutic exposure and support the clinical safety of lamotrigine in patients suffering from epilepsy and bipolar disorder.

Human monocytes respond to a variety of stimuli in vitro by producing a number of physiologically important macromolecules including the cytokines. SK&F 86002, a dual inhibitor of the arachidonate metabolism, has been shown to inhibit LPS induced IL-1 production in human monocytes. We examined its effect on the production of other cytokines which are coordinately expressed as a result of LPS stimulation such as tumor necrosis factor alpha (TNF), alpha interferon (IFN-A), interferon beta-2 (IL-6) and granulocyte colony stimulating factor (g-CSF). The IC50 of SK&F 86002 for the TNF production was 5-8 microM, and greater than 20 microM for the other three cytokines. These IC50s were significantly higher than that previously reported for IL-1 production (1-2 microM). Taken together these data indicate that the inhibitory effect of SK&F 86002 on IL-1 production is selective and the production of cytokines in drug treated monocytes can be differentially affected.

The enzyme 20 alpha-hydroxysteroid dehydrogenase (20 alpha SDH) has previously been shown to be a specific enzyme marker of mature T cells. In vivo, the expression of 20 alpha SDH is thymus dependent, in that splenic lymphocytes from athymic mice have only low levels of activity, although the levels of enzyme activity increase gradually with age. In vitro, 20 alpha SDH can be induced in splenic lymphocytes from nu/nu mice by conditioned media from mitogen- or alloantigen-stimulated normal lymphocytes. Induction is rapid in vitro. Beginning after a lag period of approximately 6 hr, enzyme activity increases linearly for approximately 20 to 30 hr resulting in a 5- to 10-fold increase in activity. Induction is blocked by mitomycin C, suggesting a requirement for cell proliferation. The phenotype of both the precursor and the induced lymphocyte populations is Thy 1.2-, Lyt 1-, 2-, suggesting that induction of 20 alpha SDH expression is an early step in T cell differentiation. The factor responsible for 20 alpha SDH induction has been partially purified and is distinct from other known lymphokines in both its biochemical and functional properties. The term interleukin 3 is proposed for this factor.

One of the characteristic cellular immune responses associated with the regression of Moloney leukemia/sarcoma virus-induced tumors is a T cell proliferative response against the major viral envelope glycoprotein, gp70. The results described here demonstrated that associated with this proliferative response is the production of a lymphokine, Interleukin 3 (IL-3). The production of IL-3 was immunologically specific and showed the same specificity as that observed in blastogenic responses. IL-3 production was dependent upon an antigen-specific Thy-1.2+, Lyt-1+, 2- lymphocyte subpopulation but did not require the presence of an Ia+ or an adherent accessory cell. The results also suggested that IL-3 may constitute one of the blastogenic factors previously shown to be involved in the proliferative response to gp70. In particular, purified IL-3 was found to induce proliferation of both normal and immune nylon wool purified splenic lymphocytes. The phenotype of the responding lymphocyte subpopulation was Thy-1.2-, Lyt-1-, 2-, Ig-, and Ia-. Maximal IL-3 production occurred approximately 48 hr after the addition of antigen and its production was significantly blocked by mitomycin C. These characteristics were unlike those for the general production of blastogenic factor activity suggesting that IL-3 is responsible for only a minor component of the proliferative response.