WO1994022436A1

WO1994022436A1 - A method for treating abnormal cardiac contraction

Info

Publication number: WO1994022436A1
Application number: PCT/US1994/003355
Authority: WO
Inventors: George Cooper, Iv
Original assignee: George Cooper, Iv
Priority date: 1993-03-31
Filing date: 1994-03-31
Publication date: 1994-10-13
Also published as: AU6525894A

Abstract

Cardiac contractile dysfunction is specific to stress loading. Microtubules which are adjacent to the myofilaments polymerize in response to stress loading, and account for contractile dysfunction in pressure overloading cardiac hypertrophy. Microtubular depolymerization reverses the contractile abnormality of the pressure hypertrophied cardiocyte.

Description

A METHOD FOR TREATING ABNORMAL CARDIAC CONTRACTION BACKGROUND OF THE INVENTION

Cardiac hypertrophy is the generic response to a wide spectrum of physiological and pathological deviations from normal homeostasis that have as their common theme increased hemodynamic loading of the heart. This compensatory growth process proceeds until the load stimulus is abated via a re-normalization of stress per unit of myocardial mass. Cardiac hypertrophy fails to be functionally compensatory, however, either when the load increase exceeds the inherent growth capacity of the terminally differentiated cardiac muscle cell, or cardiocyte, to re- normalize stress or when the intrinsic contractile performance per unit mass of hypertrophied myocardium is less than that of normal myocardium. Thus cardiac compensation for an increased load may be imperfect because of either quantitative and qualitative defects of hypertrophied myocardium.

Hemodynamic overloads causing cardiac hypertrophy consist of either volume overloading, wherein an increased blood volume is pumped during each cardiac cycle against a normal impedance, or pressure overloading, wherein a normal blood volume is pumped during each cardiac cycle against an increased impedance. The inventor has observed that for an equivalent degree and duration of hypertrophy, volume overloading results in entirely normal cardiac contraction and energetics, while pressure overloading results in distinctly abnormal cardiac contraction and energies, a result consonant with clinical experience with the intact right ventricle. On the level of isolated right ventricular cardiocytes, the same model of right ventricular pressure overload that the contractile defect seen in isolated tissue is duplicated when characterized as sarcomere shortening in the muscle cell. Thus it is the nature of the inducing stress rather than hypertrophy itself that causes the qualitative defects of myocardium hypertrophying in response to a pressure overload, and the contractile defect, at least, resides in the cardiocyte.

Cardiocyte structure, composition, and function each respond dynamically to the full potential spectrum of imposed loads, with deviations either below or above normal loading causing rapid but reversible changes in each of these properties. The cardiocyte itself is competent to respond directly to load in terms of RNA and protein synthesis rates. However, there are neither qualitative nor quantitative differences between hypertrophied cardiocytes of pressure versus volume overloaded right ventricles when defined in terms of standard ultrastruc- ture, yet the contractile defect is expressed quite clearly in the pressure overloaded cell.

The microtubular component of the cytoskeleton is an intracellular structure, which, in excess, is responsible for the contractile abnormalities of cardiocytes hypertrophying in response to a pressure overload. Therefore, microtubules are increased in the pressure hypertrophied cardiocyte, and any contractile abnormality which this might cause is fully reversed when the microtubules are depolymerized by either chemical or physical agents. Further, at the level of the isolated cell, it is appropriate to extend such a treatment stratagem to the pressure hypertrophied heart exhibiting contractile dysfunction in vivo, especially in the case of clinical disease states. There is a persistent increase in polymerized tubulin in cardiocytes that hypertrophy in response to a stress but not a strain overload. The contractile defect exhibited by these cells is fully reversed when the microtubules are depolymerized by either chemical (e.g. colchicine) or physical (e.g. hypother- mia) agents. Thus, this fully reversible cytoskeletal alteration accounts for the entirety of the contractile abnormality observed on the cellular level in the pressure overloaded right ventricle. This is equally true for cardiocytes isolated from the pressure hypertrophied, dysfunctional left ventricle.

SUMMARY OF THE INVENTION

Cellular and ventricular contractile function are normal in right ventricular (RV) volume overload (VO) and abnormal in RV pressure overload (PO) that is, contractile dysfunction is specific to stress loading. Microtubules (MTs) , which are adjacent to the myofilaments, polymerize in response to stress loading. Cardiac muscle cells,or cardiocytes, are enzymatically isolated from each ventricle with RVPO (pulmonary artery band) or RWO (atrial septal defect) . MT depolymerization by colchicine (10^~6M for 1 hr) normalizes contractility in RVPO cells but had little effect on either hypertrophied RWO or normal LV cells. Cooling cells to 0°C for 1 hr, which also depolymerizes MTs without affecting intermediate filaments, has the same normalizing effect on contractile dysfunction as did colchicine. Cytochalasin D, which depolymerizes microfilaments, is without effect on contractile function. In contrast, taxol (10^"5M for 3 hr) or 50% D₂0, which stabilize or polymerize MTs, decrease sarcomere motion in normal cells to an extent comparable to that seen in untreated RVPO cells. Thus microtubules, a load responsive intracellular structure, account for the contractile dysfunction of PO cardiac hypertrophy. Further, microtubular depolymerization completely reverses the contractile abnormality of the pressure hypertrophied cardiocyte.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In the face of a hemodynamic overload, the need for the heart to provide adequate systemic blood flow is accomplished via an increase in the mass of the cardiac pump. This process, cardiac hypertrophy, is thus the basic compensatory mechanism for a variety of human disease states in which the active stress per unit mass of the myocardium of an entire cardiac ventricle is increased (hypertension or valvular stenosis) or the active stress per unit mass of remaining myocardium is increased after part of the myocardium is lost (myocardial infarction) . Cardiac hypertrophy is fully compensatory, when contractile function of the enlarged myocardium remains normal. However, this compensa¬ tion fails when the contractile function per unit mass of enlarged myocardium becomes abnormal. This decompensation of contractile function is the major underlying etiology of the congestive heart failure state, an entity which in patients is currently a leading cause of death and debility. In pressure overload cardiac hypertrophy there is a contractile defect at the levels of isolated cardiac tissue and muscle cells from that tissue. Thus the contractile defect observed in the intact organism can be attributed to a cellular defect. Identification of that cellular defect provides the opportunity for the development of a specific therapy for the contractile dysfunction seen in some forms of cardiac hypertrophy. An excess of microtubules is a cellular defect in pressure hypertrophied myocardium. When the microtubules are removed via depolymeri¬ zation, the contractile defect is, pari passu, entirely removed. What is true for the pressure hypertrophied right ventricle is equally true for the pressure hypertrophied left ventricle. That is, when left ventricle is pressure overloaded, left ventricular hypertrophy ensues, and this is accompanied by contractile dysfunction as the degree of left ventricular hypertrophy increases, a situation quite similar to that seen in patients with pressure overload left ventricular hypertrophy. When the cardiac muscle cells are isolated from left ventricles, decreased cellular contractile function is observed. This is associated with increased microtubules. Microtubular depolymerization returns the cellular contractile function to normal.

In practice, an agent which will depolymerize micro- tubules is introduced into hypertrophied cardiocytes. Preferred depolymerizing agents are those which will bind to the tubular monermers and prevent the formation of microtubule polymers. The depolymerizing agent could be colchicine.

Introduction of the agent may be localized by known means of introducing chemicals or drugs into cardiocytes. Introduction of the agent may be systemic by known means such as intravenous, intramuscular or oral means.

It is acknowledged that an agent such as colchicine, which will depolymerize microtubules, has high systemic toxicity and will not permit in vivo use when introduced systemically.

Depolymerization of microtubules may be achieved by cooling cardiocytes by exposure to 0°C for 1 hour. The cold depolymerized microtubules will achieve normal contractile function, as seen when microtubules are depolymerized chemically, such as by colchicine. When the cardiocytes are studied at 37°C (TO) , then studied again at 37°C after an intervening hour at 0°C (TI) , and then studied again after a further hour of 37°C (T2) , the initially abnormal contractile function of right ventricular cardiocytes are normalized at TI and remain the same as that for left ventricular cardiocytes at T2; both right ventricular and left ventricular cardiocytes show a modest decrement and contractile function duringmicrotubular repolymerizationbetween TI and T2 similar in degree to the increment and contractile function seen for normal cardiocytes during microtubular depolymerization. Of particular interest is the fact that when cardiocyte microtubules which polymerized under a stress load in vivo polymerized under zero load in vi tro the initial contractile abnormality was not recapitulated.

Claims

WHAT IS CLAIMED IS:

1. A method for treating abnormal cardiac contraction, comprising the steps of introducing a depolymerizing agent into microtubules of hypertrophied cardiocytes wherein the micro- tubules are depolymerized so as to restore normal cardiac contractile function.

2. A method for treating abnormal cardiac contraction as described in claim 1, wherein said depolymerizing agent binds to tubular monermers to prevent said microtubules from forming polymers.

3. A method for treating abnormal cardiac contraction as described in claim 2, wherein said depolymerizing agent is colchicine.

4. A method for treating abnormal cardiac contraction as described in claim 1, wherein said depolymerizing agent is introduced locally into said cardiocytes.

5. A method for treating abnormal cardiac contraction as described in claim 2, wherein said depolymerizing agent is introduced locally into said cardiocytes.

6. A method for treating abnormal cardiac contraction as described in claim 1 wherein the depolymerizing agent is introduced systemically.

7. A method for treating abnormal cardiac contraction as described in claim 2 wherein the depolymerizing agent is introduced systemically.

8. A method for treating abnormal cardiac contraction comprising the steps of first reducing the temperature of microtubules of hypertrophied cardiocytes from normal body temperature to a temperature which will cause said microtubes to depolymerize and subsequently restoring the cardiocytes to normal body temperature.