第三型抗心律不整藥物對於hERG鉀離子通道抑制和 促進作用之生物物理機制與溫度依賴性

Abstract

hERG K+ 鉀離子通道對於控制心臟動作電位的持續時間相當重要。Amiodarone (AMD）是一種廣泛使用的第三型抗心律不整藥物，可抑制hERG電流；但具備相對較少的心律失常副作用之產生。我們使用注射的非洲爪蟾卵母細胞(Xenopus oocyte)與雙電極電壓鉗制技術(two-electrode voltage clamp techniques)來表現hERG 通道於卵母細胞上，並記錄電位刺激改變產生之hERG電流以及AMD 對此通道蛋白的電流修飾作用。我們發現AMD 會結合在hERG 通道蛋白的去活化狀態，並且其解離常數(Kd)值為1.4μM。並且AMD 在22 ℃下會減緩通道蛋白活化速率(activation rate)以及增強不活化速率(inactivation rate)來抑制輕度和強去極化刺激下產生之hERG 電流。在37 ℃時hERG 通道蛋白活化速率(activation rate)會顯著的加速，但不活化速率(inactivation rate)卻減慢。因此，AMD 在輕度和強烈去極化電位刺激（例如，0.3 sec pulse）下分別以弱的和強的去極化刺激（例如-60mV 和+30mV）則各別會減弱(-60mV)與增強(+30mV)15-20％的抑制作用。同時，AMD 可以劑量依賴性地(dose-dependently)抑制hERG 回返性尾巴電流(resurgent tail current)，而不會顯著改變22 ℃ 和37 ℃時尾巴電流衰減的速率，顯示藥物的抑制作用是透過促進hERG 通過靜默路徑(silent route)來進行不活化狀態的恢復。最重要的是，AMD 在37 ℃之下對於prepulse 後之去極化後刺激(afterdepolarization)所引發的hERG 電流並不會產生抑制作用而是明顯的促進作用，但在22 ℃時此促進作用的產生並沒有那麼明顯。綜合以上，AMD 是一種有效的hERG 通道門閥開關修飾(gating modifier)藥物，能夠延長心臟動作電位的高原期（而不會增加去極化後的電流抑制）。但是，在體溫過低或其它減緩hERG 通道活化速 (activation rate)的情況下，應謹慎使用AMD。 另一方面，Ibutilide（IBT）和dofetilide（DOF）有效抑制hERG K+電流以延長心臟動作電位，但會有快速性心律失常相關副作用的潛在風險。在藥理上，尚不清楚IBT 和DOF 是主要作為hERG 通道的孔洞阻斷劑或門閥開關修飾劑。在臨床上，考慮到嚴重的副作用（例如Torsade de Pointes, TdP）的風險增加，一般不建議將其它第三型抗心律不整藥物與IBT 或DOF 合併使用。但是，有臨床報告顯示，合併使用IBT 和AMD 可降低TdP 之發生率，而不是增加。我們發現IBT 和DOF 不會阻塞開啟的hERG 通道，但是劑量依賴性地增強了不活化以抑制hERG 電流。將IBT 或DOF 與AMD 合併使用時，在去極化電流和復極化hERG 回返性尾巴電流具有加成抑制的作用，但在去極化後刺激階段則不然。事實上，合併使用時，甚至可以促進去極化後刺激階段的hERG 電流。因此，IBT 或DOF 與AMD的合併使用，能更有效地延長心臟動作電位，以減慢心律，而不會使早發性去極化後刺激和致命的快速性心律失常的風險有顯著地增加。

hERG K+ channel is important for controlling the duration of cardiac action potentials. Amiodarone (AMD), a widely prescribed class III antiarrhythmic, could inhibit hERG currents with relatively few tachyarrhythmic adverse events. We use injected Xenopus oocyte with two-electrode voltage clamp techniques to characterize the action of AMD on hERG channels. We found that AMD binds to the resting hERG channel with an apparent dissociation constant of ~1.4 μM, and inhibits hERG currents at mild and strong depolarization pulses by slowing activation and enhancing inactivation, respectively, at 22 oC. The activation kinetics of hERG channel activation are much faster but inactivation kinetics are slower at 37 oC. AMD accordingly has a 15-20% weaker and stronger inhibitory effect at mild and strong depolarization (e.g. -60 mV and +30 mV, 0.3 sec-pulse), respectively. In the meanwhile, the resurgent hERG tail currents are dose-dependently inhibited by AMD without altering the kinetics of current decay at both 22 oC and 37 oC, indicating facilitation of recovery from inactivation via the silent route. Most importantly, AMD no longer inhibits but enhances hERG currents at a mild pulse shortly after a prepulse at 37 oC, but not so much at 22 oC. We conclude that AMD is an effective hERG channel gating modifier capable of lengthening the plateau phase of cardiac action potential (without increasing the chance of afterdepolarization). AMD, however, should be used with caution in hypothermia or the other scenarios which slow hERG channel activation. On the other hand, ibutilide (IBT) and dofetilide (DOF) effectively inhibits hERG K+ currents to lengthen the cardiac action potential, but is associated with a potential risk of tachyarrhythmia. Pharmacologically, it is unclear whether IBT and DOF acts chiefly as a pore blocker or a gating modifier of hERG channels. Clinically, concomitant use of the other type III anti-arrhythmics with IBT or DOF is in general not recommended in view of the increased risk of serious side effect such as Torsade de Pointes (TdP). However, there are clinical reports on a decreased rather than increased incidence of TdP with concomitant application of IBT and AMD. We found that IBT and DOF do not block the open hERG channel pore, but dose-dependently enhance the inactivation to inhibit hERG currents. Concomitant use of IBT or DOF with AMD has an additive inhibitory effect on hERG currents during the depolarization and the repolarization resurgent tail, but not in the afterdepolarization phases. In fact, the hERG currents during the afterdepolarization phase could even be enhanced. Combination of IBT or DOF with AMD may therefore more effectively lengthen cardiac action potentials to slow the heart rate, without as much increase of the risk of early afterdepolarization and deadly tachyarrhythmias.