Method for the treatment of cardiac arrhythmias and shortening of action potential duration

The invention relates to the use of zucapsaicin (cis-8-methly-N-vanillly-6-nonenamide), the cis-isomer of capsaicin, to treat myocardial disorders, including the prevention, suppression or reversal of an abnormal cardiac rhythm, such as ventricular tachycardia. In vitro, zucapsaicin exhibits electrophysiologic properties distinct from capsaicin. In contrast to capsaicin, zucapsaicin significantly shortens the action potential duration at a dose of 10.sup.-5 M and has no effect on the amplitude of Phase 1 of the action potential in normal Purkinje cells. Zucapsaicin also prevents the induction of ventricular tachycardia of focal Purkinje origin when given intravenously after coronary occlusion in a dog model of acute myocardial infarction.

BACKGROUND 
Rapid treatment of cardiac arrhythmias, especially those occurring during 
ischemia, acute myocardial infarction or congestive heart failure, is 
vital to the patient. Arrhythmias, such as ventricular and atrial 
extrasystoles, and non-sustained ventricular tachycardia and bradycardia 
are commonly present from the onset of ischemia. These may lead to the 
most lethal of the arrhythmias, including sustained ventricular 
tachycardia and fibrillation which, if not treated promptly, lead to 
cardiopulmonary collapse and death. There is, therefore, a need for 
specific pharmacologically active compounds to treat myocardial disorders, 
including the prevention, suppression or reversal of abnormal cardiac 
rhythms, thereby preventing cardiac arrest, especially in ischemia, acute 
myocardial infarction or congestive heart failure. 
Recently, in vitro studies have suggested that capsaicin 
(trans-8-methly-N-vanillly-6-nonenamide) may have potential antiarrhythmic 
and/or antiischemic activity. Capsaicin is a naturally occurring compound 
derived from plants of the Solanaceae family, commonly known as hot red 
peppers. The cis-isomer of capsaicin, zucapsaicin or civamide 
(cis-8-methly-N-vanillly-6-nonenamide), is not a naturally occurring 
compound, but is produced by means of chemical synthesis. Capsaicin has 
been used to study the neurophysiology and pharmacology of pain, and both 
capsaicin and zucapsaicin are now known to be effective pain relievers 
that are believed to act on peripheral sensory neurons to deplete and 
prevent reaccumulation of neuropeptides, such as substance P and 
calcitonin gene-related peptide (CGRP). Zucapsaicin is now believed to be 
even more potent as a depleter of neuropeptides from sensory nerves than 
is capsaicin. 
In the nervous system, capsaicin-induced release of neuropeptides, such as 
substance P and calcitonin gene-related peptide (CGRP), is associated with 
certain electrophysiologic effects in the nerve cell membrane, including 
the opening of nonspecific cationic channels and changes in sodium, 
potassium and calcium ionic currents, that ultimately result in excitation 
of the neuron. Capsaicin also appears to affect the electro-physiologic 
properties of cells other than neurons. In vitro studies of the effect of 
capsaicin on normal, isolated rat ventricular myocytes have shown that 
10.sup.-5 M capsaicin causes prolongation of the action potential duration 
associated with inhibition of potassium ion channel conductances in the 
membrane, specifically the transient outward (I.sub.to), delayed rectifier 
(I.sub.K) and inward rectifier (I.sub.K1) currents. (Castle, N., 1992, 
Cardiovasc. Res. 26:1137). This pharmacologic activity may account for the 
observation that when isolated, perfused rat and guinea pig hearts with 
experimentally-produced regional ischemia were treated with capsaicin, the 
incidence of ischemic ventricular tachycardia and/or ischemic ventricular 
fibrillation was significantly reduced. (D'Alonzo, A. J. et al. 1995, Eur. 
J. Pharmacology 272:269). The heart is innervated by noncholinergic, 
non-adrenergic sensory neurons which contain substance P and CGRP. 
Intravenous injection of capsaicin in animals has been shown to produce 
neural stimulation and release of CGRP which is known to be a potent 
coronary vasodilator. The cardiostimulatory actions of capsaicin (positive 
chronotropic and inotropic effects) are also thought to be due, in part, 
to CGRP release from sensory nerves within the heart in some animal 
species. (Franco-Cereceda, A., et al. 1988, Acta Physiol. Scand. 132:181). 
In summary, the effects of capsaicin at the level of the myocardium appear 
to be a complex mixture of direct actions on cardiac myocytes, as well as 
effects mediated by sensory nerve stimulation. 
Although the in vitro studies suggest that capsaicin may be a potential 
antiarrhythmic compound, there is no evidence that capsaicin will prevent, 
suppress or reverse cardiac arrhythmias in vivo, or will do so in doses 
that are physiologically achievable and tolerable. Because zucapsaicin, 
the cis-isomer of capsaicin, is believed to be even more potent than 
capsaicin in neuropeptide depletion from peripheral sensory neurons, there 
is a need for evaluation of the electrophysiologic effects of zucapsaicin 
in the myocardium, especially in Purkinje fibers which conduct electrical 
impulses responsible for coordinated ventricular contraction. There is a 
further need for evaluation of the potential use of zucapsaicin to treat 
myocardial disorders, and to prevent, suppress or reverse heart 
arrhythmias, particularly in ischemia, acute myocardial infarction and 
congestive heart failure. 
SUMMARY OF THE INVENTION 
In accordance with the present invention, zucapsaicin was evaluated in 
vitro as to potential electrophysiologic effects in normal Purkinje tissue 
from the dog. Further, zucapsaicin was evaluated for its potential 
antiarrhythmic effects in an in vivo model of acute myocardial ischemia in 
the dog. 
Surprisingly, zucapsaicin was discovered to have in vitro 
electrophysiologic properties that are distinct from those of capsaicin. 
It was discovered that zucapsaicin significantly shortens the action 
potential duration in normal Purkinje cells at a concentration of 
10.sup.-5 M and does not prolong the action potential duration at any 
concentration evaluated; whereas, 10.sup.-6 M capsaicin significantly 
prolongs the action potential duration in normal Purkinje cells. 
Shortening of the action potential duration may be associated with the 
impediment of calcium entry into cells and with potential in vivo 
antiarrhythmic effects. Further, in contrast to capsaicin, zucapsaicin 
exhibited no significant change in the amplitude of Phase 1 of the action 
potential, which usually reflects the potassium ionic current, I.sub.to ; 
whereas capsaicin showed a diminished I.sub.to voltage, which was reversed 
upon removal of the drug. 
Zucapsaicin was also discovered to prevent the induction of ventricular 
tachycardia, in an in vivo dog model of acute myocardial infarction, when 
given intravenously at a dose of 50 .mu.g/kg, 1.5 to 2.5 hours after 
coronary artery occlusion. 
Accordingly, the present invention relates to a composition comprising an 
admixture of zucapsaicin (cis-8methly-N-vanillly-6-nonenamide), with a 
pharmaceutically acceptable vehicle, the zucapsaicin being present in an 
amount sufficient to alter an electrophysiologic property of the 
myocardium and, preferably, to prevent, suppress or reverse an abnormal 
cardiac rhythm and restore a normal cardiac rhythm in a patient to whom 
the composition is administered. The composition may be administered to 
the patient by any route, including oral, transdermal, intravenous, 
intradermal, subcutaneous, intramuscular and cerebrospinal administration, 
or combinations of these. The zucapsaicin in the composition may be 
present in the amount of between about 5 .mu.g/kg and about 500 .mu.g/kg 
body weight, preferably about 50 .mu.g/kg body weight, in a 
pharmaceutically acceptable vehicle. 
The invention further relates to a method of treating a myocardial 
disorder, including ischemia, acute myocardial infarction and congestive 
heart failure, with zucapsaicin in an effective amount to alter an 
electrophysiologic property of the myocardium and to prevent, suppress or 
reverse an abnormal cardiac rhythm. The invention also includes a method 
of treating the myocardium with an amount of zucapsaicin effective to 
alter an electrophysiologic property of the myocardium, including 
shortening of the action potential duration of myocardial cells, and to 
prevent, suppress or reverse an abnormal cardiac rhythm, including 
ventricular arrhythmias, such as ventricular tachycardia.

DETAILED DESCRIPTION OF THE INVENTION 
Zucapsaicin was evaluated in vitro as to potential electrophysiologic 
effects in normal Purkinje tissue from the dog. The details and results of 
the evaluation are provided in greater detail in the Examples below. In 
summary, the in vitro evaluation shows that zucapsaicin has 
electrophysiologic properties that, surprisingly, are distinct from those 
of capsaicin. In accordance with the invention, zucapsaicin significantly 
shortens the action potential duration in normal Purkinje cells at an in 
vitro concentration of 10.sup.-5 M and does not prolong the action 
potential duration at any concentration evaluated; whereas, 10.sup.-6 M 
capsaicin significantly prolongs the action potential duration in normal 
Purkinje cells. (This effect of capsaicin is similar to that shown in 
previous investigations, as described above.) Shortening of the action 
potential duration is believed to be associated with a blockade of the 
membrane calcium ionic current, I.sub.Ca or the ATP-dependent potassium 
current, I.sub.KATP. Because damaged, ischemic cells are usually 
overloaded with calcium, these regions of myocardial ischemic tissue 
become prone to arrhythmias, especially ventricular tachycardia associated 
with delayed after-depolarizations (DAD). Therefore, compounds which 
impede calcium entry into the cell may prevent both DAD and ventricular 
tachycardia. The shortening of the action potential duration exhibited by 
zucapsaicin in vitro is reflective of a potential antiarrhythmic effect in 
vivo, in particular an inhibitory effect on ventricular tachycardia 
associated with DAD due to blocking of calcium entry into ischemic cells. 
Further, in contrast to capsaicin, zucapsaicin exhibits no significant 
change in the amplitude of Phase 1 of the action potential, which usually 
reflects the potassium ionic current, I.sub.to ; whereas capsaicin shows a 
diminished I.sub.to voltage, which is reversed upon removal of the drug. 
(This effect of capsaicin is similar to that shown in previous 
investigations, as described above.) Therefore, the results show that 
surprising and significant electrophysiologic differences exist between 
effects of zucapsaicin and capsaicin on normal myocardial cells, such as 
Purkinje cells. 
Zucapsaicin was also discovered to prevent the induction of ventricular 
tachycardia, in an in vivo dog model of acute myocardial infarction, when 
given intravenously at a dose of 50 .mu.g/kg body weight, 1.5 to 2.5 hours 
after coronary occlusion. 
In accordance with the invention, and as described more particularly in the 
Examples which follow, in vivo administration of a composition comprising 
an admixture of an effective amount of zucapsaicin with a pharmaceutically 
acceptable vehicle, alters an electrophysiologic property of the 
myocardium, including Purkinje cells and, preferably, prevents, suppresses 
or reverses abnormal cardiac rhythms, especially ventricular arrhythmias 
such as ventricular tachycardia. The zucapsaicin may be administered to 
the patient by any route, including oral, transdermal, intravenous, 
intradermal, subcutaneous, intramuscular and cerebrospinal administration, 
or combinations of these. The zucapsaicin may be administered in the 
amount of between about 5 .mu.g/kg and about 500 .mu.g/kg body weight, 
preferably about 50 .mu.g/kg body weight, in a pharmaceutically acceptable 
vehicle. A 50 .mu.g/kg intravenous dose of zucapsaicin is calculated to be 
approximately equivalent to an in vitro concentration of 10.sup.-5 M or 
less, assuming a homogeneous distribution of the drug throughout the body. 
Zucapsaicin may be administered at any time during or after the onset of 
symptoms of ischemia, acute myocardial infarction or congestive heart 
failure. For example, in a model of acute myocardial infarction in a dog 
described in Example 2, zucapsaicin, given at between about 1.5 and 2.5 
hours after coronary occlusion, was effective to prevent or suppress 
ventricular tachycardia for at least three hours after coronary occlusion. 
Preferably, administration of an effective dose of zucapsaicin restores a 
normal cardiac rhythm in the patient. 
Any pharmaceutically acceptable vehicle may be utilized in the composition. 
For example, zucapsaicin in a suitable vehicle may be an appropriate 
dilution of a stock solution (VEH-X) comprising (w/w) 0.3% zucapsaicin, 
1.166% dehydrated ethanol, 0.5% sodium chloride, 0.336% potassium 
phosphate, 8% polysorbate 20, 0.1% disodium ethlyenediaminetetracetate 
(EDTA), 0.0734% disodium phosphate, 0.02% benzalkonium chloride 50%, 
0.0116% butlyated hydroxytoluene (BHT) and 89.493% water. 
Turning now to the drawing figures, examples will be utilized to describe 
the invention. 
EXAMPLE 1 
To study the electrophysiologic properties of zucapsaicin, dose response 
curves were performed in isolated Purkinje fibers from the dog, utilizing 
standard intracellular microelectrode techniques. Measurements were made 
of the action potential duration (reflective of ionic currents, including 
calcium current, I.sub.Ca), action potential amplitude (an index of sodium 
current, I.sub.Na), and resting membrane potential (an index of potassium 
current, I.sub.K1. 
To isolate Purkinje fibers, anesthetized dogs underwent thoracotomy and 
rapid removal of the still beating heart. The heart was placed in a cold, 
oxygenated, electrolyte buffer. Purkinje fibers connecting muscle 
trabeculae were visually observed and were removed by a small scissors 
while the tissue was kept under the buffer. The free-running fibers were 
pinned to a tissue bath and superfused with buffer. After stabilization, 
the fibers were paced at a rate of 100 beats per minute and control 
electrophysiologic readings were taken. Incremental concentrations of 
zucapsaicin or capsaicin in a suitable dilution of vehicle (VEH-X), or 
vehicle alone, were then added to the isolated Purkinje fibers in the 
constant buffer superfusion. After an equilibration period at each 
concentration level, electrophysiologic readings were made. Following 
exposure to the highest concentration, the preparations were washed with 
buffer and electrophysiologic readings were again recorded to determine 
whether the effects of the intervention were reversible. 
Measurements of action potential duration were made at the 90% 
repolarization level after the plateau region of the action potential (APD 
90), utilizing standard intracellular recording techniques with electrodes 
filled with 3mM KC1 and having a measured tip resistance of 12-45 ohms. As 
illustrated in FIG. 1, the vehicle (VEH) alone did not statistically (.+-. 
standard error of the mean, SEM) alter the action potential duration of 
normal Purkinje fibers. As expected, capsaicin (CAP), the trans-isomer, 
significantly prolonged the action potential duration by about 20 
milliseconds at a dose of 10.sup.-6 M (p&lt;0.05). In contrast, zucapsaicin 
or civamide (CIV), the cis-isomer, significantly shortened the action 
potential duration at a dose of 10.sup.-5 M (p&lt;0.05) and did not prolong 
the action potential duration at any dose utilized. Similar results were 
achieved when the action potential duration was measured at the 50% 
repolarization level (APD 50) (not shown). 
Further standard electrophysiologic studies showed that neither zucapsaicin 
nor capsaicin at doses ranging from 10.sup.-8 M to 10.sup.-5 M had an 
effect on resting membrane potential (I.sub.K1) or action potential 
amplitude (I.sub.Na) in normal Purkinje fibers. However, capsaicin did 
show an expected diminished amplitude of Phase 1 of the action potential 
(reflective of the potassium current, I.sub.to), which was reversed when 
the drug was washed from the cell preparation. In contrast, zucapsaicin 
did not alter the amplitude of Phase 1 of the action potential. 
The results of the above studies show that surprising and significant 
electrophysiologic differences exist between the effects of the 
trans-isomer capsaicin and the cis-isomer zucapsaicin in normal Purkinje 
fibers. Zucapsaicin does not show any effect on potassium ionic currents, 
but capsaicin inhibits potassium ionic currents, particularly I.sub.to. 
Zucapsaicin shows statistically significant shortening of the action 
potential duration at a dose of 10.sup.-5 M which is physiologically 
achievable in vivo; whereas capsaicin at 10.sup.-6 M lengthens the action 
potential duration. 
EXAMPLE 2 
In order to investigate the antiarrhythmic potential of zucapsaicin in 
vivo, studies were performed using 13 dogs undergoing induced acute 
myocardial infarction. Coronary artery occlusion was produced in the 
anaesthetized dogs by tying off the anterior descending coronary artery. 
Coronary occlusion was maintained for one hour in order to allow a stable 
ischemic zone to develop. 
Prior to occluding the artery, 20 multipolar plunge needle electrodes were 
placed in the ischemic risk zone to record bipolar electrograms through 
the left ventricular wall of the myocardium. Three-dimensional maps of 
induced ventricular tachycardia were constructed from multiplexed signals 
for up to 14 seconds of data. The signals were digitized at 3 kHz and 
filtered from 3 to 1300 Hz, allowing recording of Purkinje activity on 
endocardial electrograms which recorded Purkinje activity underlying the 
area of myocardial infarction. 
After stabilization of the ischemic zone, ventricular tachycardia was 
induced by as many as five early extrastimuli, which were programmed after 
a series of eight regular stimuli at a heart rate of 200 beats per minute. 
The ventricular tachycardia produced by this method is thought to be due 
either to "reentry" mechanisms or to delayed after-depolarizations (DAD). 
Reentry is believed to be a source of abnormal cardiac rhythms, distinct 
from DAD, and is also found in ischemia. The ventricular tachycardia was 
monomorphic with a cycle length varying from 110 to 150 milliseconds, and 
originated from a focus in the endocardium. 
After ventricular tachycardia was induced two separate times to show 
reproducibility, the test dogs were given 50 .mu.g/kg body weight of 
zucapsaicin in a vehicle (a suitable dilution of the stock solution VEH-X) 
intravenously at between about 1.5 and 2.5 hours after the coronary 
occlusion. Attempts to induce ventricular tachycardia were repeated. In 
control dogs not given the drug, ventricular tachycardia induction was 
reproducible for at least three hours after occlusion. However, in 
zucapsaicin-treated dogs, ventricular tachycardia with a focal Purkinje 
origin (n=6) were not inducible again for at least three hours after 
occlusion. Therefore, zucapsaicin prevented or suppressed the induction of 
ventricular tachycardia in ischemic myocardium when focal Purkinje tissue 
was the origin of ventricular tachycardia. The effect of zucapsaicin on 
induction of epicardial reentrant ventricular tachycardia was also 
studied. It was found that epicardial reentrant ventricular tachycardia 
was also prevented by zucapsaicin but required a higher dose (about 200 
.mu.g/kg). 
As illustrated in FIG. 2, treatment with intravenous zucapsaicin (CIV) at 
50 .mu.g/kg body weight also produced shortening of the refractory period 
(ERP) in normal myocardium from 138.+-.3 to 132.+-.4 milliseconds 
(p&lt;0.01), consistent with the shortening of the action potential duration 
demonstrated by zucapsaicin in vitro (see EXAMPLE 1). Zucapsaicin also 
decreased mean arterial pressure (MAP) from 76.+-.7 to 66.+-.7 mmHg 
(p&lt;0.05). 
Further, as illustrated in FIG. 3, zucapsaicin treatment did not 
significantly alter activation times of drive (S1) or premature (S2) 
stimuli in normal (NZ) or ischemic (IZ) zones. These results are 
consistent with the absence of significant effects by zucapsaicin at this 
dose on I.sub.Na, in vitro. In dogs which did not have inducible 
ventricular tachycardia after coronary occlusion (n=3), treatment with 
zucapsaicin intravenously up to 500 .mu.g/kg body weight did not provoke 
ventricular tachycardia. 
As shown in FIG. 4, in one animal with endocardial focal ventricular 
tachycardia induced with four extra stimuli S2-S5 (open arrows on V5R), 
Purkinje activity was recorded (filled arrows on F-EN) prior to the QRS 
(V5R, vertical line) and prior to any endocardial (EN, E: east, W: west, 
O: overlying) or epicardial activity occurring within the risk zone. These 
and other experimental data by applicants suggest that Purkinje tissue 
underlying an area of myocardial ischemia may be the source of ventricular 
tachycardia. (Martins, J., 1995, J. Invest. Med. 43:431A). Since Purkinje 
cells are more likely to show DAD than myocytes, the data of FIG. 4 are 
consistent with DAD-mediated ventricular tachycardia originating in 
Purkinje cells. Therefore, in this model, zucapsaicin is also effective to 
prevent or suppress the induction of ventricular tachycardia originating 
in Purkinje tissue. 
While the invention has been described herein with reference to the 
preferred embodiments, it is to be understood that it is not intended to 
limit the invention to the specific forms disclosed. On the contrary, it 
is intended to cover all modifications and alternative forms falling 
within the spirit and scope of the invention.