Phenytoin Reduces Activity of Cardiac Ryanodine Receptor 2; A Potential Mechanism for Its Cardioprotective Action

Abstract

Phenytoin is a hydantoin derivative that is used clinically for the treatment of epilepsy and has been reported to have antiarrhythmic actions on the heart. In a failing heart, the elevated diastolic Ca 21 leak from the sarcoplasmic reticulum can be normalized by the cardiac ryanodine receptor 2 (RyR2) inhibitor, dantrolene, without inhibiting Ca 21 release during systole or affecting Ca 21 release in normal healthy hearts. Unfortunately, dantrolene is hepatotoxic and unsuitable for chronic long-term administration. Because phenytoin and dantrolene belong to the hydantoin class of compounds, we test the hypothesis that dantrolene and phenytoin have similar inhibitory effects on RyR2 using a single-channel recording of RyR2 activity in artificial lipid bilayers. Phenytoin produced a reversible inhibition of RyR2 channels from sheep and human failing hearts. It followed a hyperbolic dose response with maximal inhibition of ∼50%, Hill coefficient ∼1, and IC 50 ranging from 10 to 20 mM. It caused inhibition at diastolic cytoplasmic [Ca 21 ] but not at Ca 21 levels in the dyadic cleft during systole. Notably, phenytoin inhibits RyR2 from failing human heart but not from healthy heart, indicating that phenytoin may selectively target defective RyR2 channels in humans. We conclude that phenytoin could effectively inhibit RyR2-mediated release of Ca 21 in a manner paralleling that of dantrolene. Moreover, the IC 50 of phenytoin in RyR2 is at least threefold lower than for other ion channels and clinically used serum levels, pointing to phenytoin as a more human-safe alternative to dantrolene for therapies against heart failure and cardiac arrythmias. SIGNIFICANCE STATEMENTWe show that phenytoin, a Na channel blocker used clinically for treatment of epilepsy, is a diastolic inhibitor of cardiac calcium release channels [cardiac ryanodine receptor 2 (RyR2)] at doses threefold lower than its current therapeutic levels. Phenytoin inhibits RyR2 from failing human heart and not from healthy heart, indicating that phenytoin may selectively target defective RyR2 channels in humans and pointing to phenytoin as a more human-safe alternative to dantrolene for therapies against heart failure and cardiac arrhythmias.

Understanding How Phosphorylation and Redox Modifications Regulate Cardiac Ryanodine Receptor Type 2 Activity to Produce an Arrhythmogenic Phenotype in Advanced Heart Failure

“…Some drugs have already been shown to reduce diastolic RyR2 Ca 2+ leak. Notably, dantrolene reduces diastolic Ca 2+ leak in failing canine, 123 rabbit, 124 and human heart 125 but not corresponding species (canine, rabbit, and human) nonfailing heart. 124,125 125 It is important that therapies do not depress systolic function, especially in the compromised failing heart.…”

“…Notably, dantrolene reduces diastolic Ca 2+ leak in failing canine, 123 rabbit, 124 and human heart 125 but not corresponding species (canine, rabbit, and human) nonfailing heart. 124,125 125 It is important that therapies do not depress systolic function, especially in the compromised failing heart. Interestingly, phenytoin and dantrolene do not compete for the same binding site on RyR2.…”

“…Interestingly, phenytoin and dantrolene do not compete for the same binding site on RyR2. 125 Furthermore, inhibition by dantrolene but not phenytoin was dependent on the presence of calmodulin. 125 These studies suggest phenytoin and dantrolene can stabilize the remodeled RyR2 by restoring the "zipped" configuration of the N-terminal and central domains to maintain the closed state of RyR2 during diastole.…”

Targeting Ca2 + Handling Proteins for the Treatment of Heart Failure and Arrhythmias

“…Despite this, the 'SHO-IN' clinical trial seeks to determine the safety and efficacy of dantrolene on patients with chronic HF [Japanese Registry of Clinical Trials identifier 61180059] (Kobayashi et al, 2020). Recent work has shown that phenytoin, a member of the same class of hydantoin compounds, shares the same RyR2 inhibitory effects as dantrolene (Ashna et al, 2020). In vitro application of phenytoin to RyR2 channels isolated from failing human hearts shows that phenytoin reversibly inhibits RyR2 which subsequently inhibits diastolic cytoplasmic, but not systolic, Ca 2+ release (Ashna et al, 2020).…”

“…Recent work has shown that phenytoin, a member of the same class of hydantoin compounds, shares the same RyR2 inhibitory effects as dantrolene (Ashna et al, 2020). In vitro application of phenytoin to RyR2 channels isolated from failing human hearts shows that phenytoin reversibly inhibits RyR2 which subsequently inhibits diastolic cytoplasmic, but not systolic, Ca 2+ release (Ashna et al, 2020). Interestingly, phenytoin is currently used clinically as a Na + channel blocker to treat epilepsy, therefore there is scope to determine the potential therapeutic use of phenytoin in HF (Figure 4).…”