Heart failure affects more than 64 million people globally [1], with its prevalence ranging from 0.12 to 6.70% [2]. It is a complication of other diseases such as diabetes mellitus [3], ischemic heart disease, or long-standing hypertension [4], or primarily due to genetic and idiopathic conditions [5]. These conditions can give rise to the development of cardiomyopathy. Myocardial hypertrophy is the most common cause of heart failure [6], characterized by enlargement of the heart [3].

Myocardial hypertrophy occurs following remodeling of the heart, triggered by various stimuli. Remodeling causes myocardial structural, molecular, and cellular changes, leading to alterations in cardiac function, size, and shape [7]. Many pathological events, such as neurohormonal activation, cardiac volume overload, and pressure overload are thought to be centrally involved in the development of cardiac hypertrophy [7]. Deficiencies in micronutrients such as copper, zinc, and selenium may also play a role in the development of heart failure [8]. Abnormality in copper homeostasis manifested by a deficiency in myocardial copper may contribute to the pathogenesis of cardiac hypertrophy [9]. Patients with ischemic heart disease tend to have a lower cardiac copper content [10], possibly due to increased myocardial copper efflux [11] and reduced activity in certain copper-dependent enzymes [10]. Animals fed a diet deficient in copper had significantly heavier hearts [12], owing to alterations in heart biochemical properties and morphology [13]. Similar observations were also noted in pressure-overloadinduced hypertrophied hearts in rats ^[14][15][14,15].

The exact mechanism of how copper deficiency induces cardiac hypertrophy is not well understood but may be partly attributable to dietary and hereditary etiologies. Genetic polymorphism in copper-transporting ATPase has been postulated to cause pathological cardiac changes, as observed in Menkes’ disease, an X-linked genetic defect affecting energy-dependent copper transporters [16]. Copper plays an important role in many cellular processes such as antioxidant activity and mitochondrial respiration [17]. Dietary supplementation of copper reversed cardiac hypertrophy in rats [18].

Cardiac hypertrophy and heart failure are managed pharmacologically using angiotensin-converting enzyme inhibitors, angiotensin receptor blockers, calcium channel blockers [19], β-blockers, or angiotensin receptor–neprilysin inhibitors ^[20][21][22][20,21,22]. Due to the discovery of copper-deficiency-induced cardiac hypertrophy, clinical trials have been conducted to investigate the effects of trientine, a copper chelator, on cardiac hypertrophy or hypertrophic cardiomyopathy ^[23][24][23,24].

Trientine, also known as triethylenetetramine (TETA) dihydrochloride (Figure 1), is an orphan drug [25]. It is an organic compound with a molecular formula of C₆H₁₈N₄, which was originally approved in 1985 by the U.S. Food and Drug Administration as second-line therapy for patients with Wilson’s disease who cannot tolerate penicillamine ^[26][27][26,27]. Wilson’s disease is a genetic condition that arises from copper accumulation in the body, particularly the liver. This could lead to liver cirrhosis and degenerative neurological conditions which could be fatal [28]. Trientine chelates hepatic excess copper and increases urinary copper excretion, leading to hepatic improvement in the majority of patients with the disease. Other copper modulators are dimercaprol and zinc salts (acetate, sulfate, and gluconate) [27].

The role of trientine in cardiomyopathy has gained much interest, although its mechanistic action is poorly understood.

2. Role of Trientine in Cardiac Copper Regulation

Copper is required for the normal function and structure of the heart. It is involved in various cellular metabolic activities such as iron and zinc uptake, as well as oxyradical scavenging [17]. It enters cells via plasma membrane copper transporter-1 (CTR-1) and copper transporter-2 (CTR-2) [29]. Once inside the cells, the ion is delivered to its targets by various chaperones [17]. However, in excess, it may be detrimental due to the generation of reactive oxygen species (ROS). Consequently, its uptake, distribution, and elimination are tightly governed to maintain cellular homeostasis ^[29][30][29,30]. The serum copper level is higher in patients with cardiomyopathy compared with that of normal individuals [31]. It is postulated that increased efflux of copper from the myocardial cells leads to a high level of serum copper [32]. To confirm this, many animal model experiments using various cardiomyopathy models such as pressure-overload-induced cardiac hypertrophy [14] or diabetic cardiomyopathy [33] have been conducted. Findings from the studies confirmed the depletion of myocardial copper content measured in left ventricles ^[14][15][33][14,15,33]. The depleted content could not be replenished even in the presence of higher circulatory copper levels [11]—a phenomenon which remains elusive. Copper exists in two valencies: Cu(I) and Cu(II). Cu(I) is mainly present intracellularly and contributes 95% of total body copper, and the remainder exists as Cu(II) in the extracellular space [34]. Its transport intracellularly is governed by various chaperones which will be discussed in the subsequent subtopics. Trientine selectively forms a complex with Cu(II), not Cu(I) [35], and serves as a copper chaperone to transport the ion to other copper-binding molecules in copper-depleted cardiomyocytes [14]. However, the mechanism is still unclear. Cu(II)-trientine enters cells as an intact complex via active transport [11]. CTR-1 expression is the major copper transporter in the heart. However, trientine does not affect myocardial CTR-1 expression [14]. Even in diabetic myocardium, it does not restore the diminished CTR-1 protein expression [34]. In cardiomyocytes transfected with CTR-1 gene silencing using siRNA, trientine-facilitated copper uptake into cardiomyocytes was unaffected, observed by increased copper accumulation in the cells. This was different from the cells that were exposed to Cu(II) chloride only, which demonstrated a decline in cellular copper content [11], confirming that the influx of copper into cells by trientine is CTR-1-independent. Studies investigating the effects of trientine on CTR-2 reported inconsistent findings (Table 1). Trientine exhibited elevated CTR-2 expression in a few studies ^[15][34][15,34] but no effect in another study [14]. This suggests that CTR-2 may partly contribute to copper transportation into cardiomyocytes. CTR-2 expression in the heart is determined by the cellular copper status, which is reduced in copper deficiency [36]. Therefore, trientine may upregulate CTR-2 expression indirectly due to the accumulated copper in cardiomyocytes. Another transporter that may be involved in copper transport is chloride channel CLC17 [37], and the effects of trientine on the transporter should be investigated. More studies need to be conducted to better understand the mechanisms of copper uptake by the drug. Further studies in animals demonstrated that trientine restored myocardial copper content in cardiomyopathy models ^[14][34][35][14,34,35] (Table 1). It was previously revealed that trientine at a low dose (21.9 mg/kg, orally twice daily for 6 weeks) replenished deprived copper in the heart, but a relatively high dose (87.6 mg/kg, orally twice daily), an equivalent dose commonly used for the treatment of Wilson’s disease, failed to reload the loss of myocardial copper content in rats with pressure-overload-induced cardiac hypertrophy. Moreover, the high dose of trientine decreased copper content in the heart of normal rats, whereas the same phenomenon was not observed with the low dose [14]. The findings suggest that trientine at a relatively high dose of more than 175 mg/kg/day promotes the removal of copper from the heart. The protective effect of trientine on the myocardial copper content agreed with other animal studies that adopted a trientine dosage of approximately 100 mg/kg/day or less ^{[15][33][34][35][38][39]}[15,33,34,35,38,39]. Administration of trientine exhibited protection against left ventricular hypertrophy in diabetic patients [40]. Clinical trials reported that serum copper was unaltered by trientine therapy after six [23] or twelve months [40] in patients with hypertrophic cardiomyopathy. Yet, the 24 h urinary copper level was significantly elevated in the patients, indicating increased excretion of excess copper (Table 1). However, a small significant rise in serum ceruloplasmin, the main copper-bearing protein in the blood, occurred in the patients, indicating increased cellular uptake of copper [23]. Despite the effects of trientine on myocardial copper content, it has no significant effects on plasma copper levels in rats, even though it reduces renal copper content and facilitates urinary excretion of excessive copper in the blood ^[14][35][39][14,35,39]. In other words, trientine therapy is not likely to cause systemic copper deficiency. A similar insignificant effect on plasma copper was also observed in patients with left ventricular hypertrophy who were receiving trientine. The effectiveness of the therapy was monitored by a decrease in left ventricular mass [40]. Regardless of its beneficial effects on diabetes-induced copper depletion in the heart ^[34][39][34,39], trientine has no significant effect on blood glucose levels in diabetic rats ^{[25][34][39][41][42]}[25,34,39,41,42]. How it offers cardioprotection without affecting blood glucose is not understood. More mechanistic studies in experimental animals should be conducted to elucidate the mechanism.