Recent developments in nanoparticle-based treatment for atrial fibrillation and other human cardiac arrhythmias

Lipid nanocapsules

One of the earliest novel delivery systems for amiodarone in preclinical studies was the lipid nanocapsules (LNCs). LNCs are nanoscale carriers, typically ranging from 20 to 100 nm in diameter, composed of three primary components: (1) a nonionic surfactant forming the capsule membrane; (2) an aqueous phase of sodium chloride solution; and (3) an oily core consisting of medium-chain triglycerides, such as capric and caprylic acid derivatives^[27-29]. In preclinical formulations, amiodarone was successfully encapsulated within LNCs utilizing polyethylene glycol-660 hydroxystearate as the surfactant, with soybean lecithin added to enhance capsule stability^[30].

These amiodarone-loaded LNCs demonstrated a low polydispersity index and pH-sensitive drug release, thereby addressing the significant burst release commonly observed in earlier nanoparticle systems designed for low molecular weight compounds^[31,32]. The use of biocompatible excipients and the physicochemical properties of the formulation suggest potential suitability for intravenous or intraperitoneal administration^[30]. However, these findings are currently limited to preclinical evaluations and require further investigation in clinical settings to confirm their safety and therapeutic efficacy in humans.

Liposomes

Liposomes have also been explored in preclinical studies as a novel delivery system for amiodarone. These spherical vesicles, ranging in size from approximately 30 nm to several micrometers, are composed of one or more phospholipid bilayers formed from natural, non-toxic phospholipids and cholesterol^[33]. The lipophilic character of the bilayer facilitates the encapsulation and solubilization of amphiphilic and lipophilic drugs, such as amiodarone. Additionally, liposomes are generally considered non-immunogenic following both systemic and non-systemic administration^[16,33]. In animal models, amiodarone-loaded liposomes have demonstrated improved therapeutic profiles compared to free amiodarone. In a rat model of ischemia/reperfusion (I/R) injury, intravenous administration of liposomal amiodarone at 3 mg/kg in Wistar rats significantly reduced the duration of ventricular tachycardia and ventricular fibrillation (VT/VF), as well as mortality from these arrhythmias, compared to a higher dose (10 mg/kg) of free amiodarone^[34]. Liposomal amiodarone was also associated with a reduced incidence of bradycardia and hypotension in the same I/R model^[34]. Furthermore, in Sprague-Dawley rats subjected to cardiac radiofrequency (RF) ablation, liposomal amiodarone exhibited more sustained and stable drug release compared to its free form^[35]. These properties contributed to enhanced antiarrhythmic efficacy at lower doses, potentially reducing dose-dependent side effects such as bradycardia and hypotension.

Biodistribution studies further revealed that liposomal amiodarone preferentially accumulates in the myocardium of rats following cardiac I/R injury and RF ablation, likely due to increased vascular permeability in inflamed cardiac tissues^[34,35]. This targeted accumulation underscores the potential utility of liposomal amiodarone in the treatment of arrhythmias associated with ischemic heart disease or post-catheter ablation interventions. However, despite reduced drug concentrations in the lungs and kidneys, potentially minimizing pulmonary and renal toxicity, liposomal delivery was associated with increased amiodarone accumulation in the liver and spleen. This effect is likely due to passive uptake by these macrophage-rich organs (i.e., organs with a high concentration of macrophages), raising concerns about potential hepatic and splenic toxicity with long-term use^[35].

Collectively, these findings from preclinical studies suggest that liposomal delivery of amiodarone holds promise for targeted cardiac therapy, but further investigation is required to evaluate its safety and efficacy in clinical settings.

Niosomes

Niosomes have emerged in preclinical research as a promising alternative to liposomes for drug delivery. Structurally like liposomes, niosomes are spherical vesicles comprising at least one bilayer but are formed using nonionic surfactants rather than phospholipids. This substitution imparts greater stability, resulting in improved shelf life and resistance to photodegradation compared to liposomal formulations^[36]. Moreover, the simpler preparation methods make niosomes more cost-effective for large-scale production^[37].

In preclinical studies, niosomes have been employed to enhance the bioavailability of antiarrhythmic agents. Dronedarone - a non-iodinated benzofuran analog of amiodarone - was successfully encapsulated in niosomes composed of Span® 80 and cholesterol (1:1), with Gellan gum used as a mucoadhesive agent (i.e., able to stick to mucosal surfaces). When administered intranasally to male New Zealand rabbits, this formulation resulted in significantly increased maximum plasma concentration (C_max), the area under the concentration-time curve from 0 to 72 h (AUC_0-72), and total drug exposure (AUC_0-∞) compared to an orally administered dronedarone suspension^[38].

Similarly, in a preclinical model involving male Wistar rats, the calcium channel blocker diltiazem - another drug used in atrial fibrillation management - was encapsulated in niosomes formulated with Span® 60 and cholesterol (1:1). Intranasal administration of this formulation led to enhanced pharmacokinetic parameters, including increased mean residence time (MRT), elimination half-life (t₁/₂), and AUC, along with a reduced elimination rate constant (K_e), compared to an aqueous diltiazem solution in phosphate-buffered saline^[39].

Niosomes have also shown potential for improving the oral bioavailability of drugs. Although the precise mechanisms remain under investigation, previous studies have demonstrated that incorporating bile salts into vesicular systems can increase their resistance to bile salt-induced disruption in the gastrointestinal (GI) tract^[40]. This approach has been applied in a preclinical context to enhance the oral delivery of carvedilol, a third-generation β-blocker used in the prevention and treatment of atrial fibrillation. In male Wistar rats, oral administration of carvedilol-loaded niosomes with bile salts significantly increased intestinal absorption and systemic bioavailability - as indicated by higher AUC_0-∞ - compared to carvedilol suspended in 0.5% sodium carboxymethyl cellulose^[41].

These preclinical findings underscore the versatility and promise of niosomal drug delivery systems for improving the pharmacokinetics of antiarrhythmic agents. However, further studies, including clinical trials, are necessary to validate their translational potential.

Exosomes

Exosomes are another class of nanoscale vesicles currently being investigated in preclinical studies for drug delivery applications. These endogenous, spherical vesicles range from approximately 40 to 200 nm in diameter and can transport a diverse array of signaling biomolecules, including proteins, nucleic acids, and non-coding RNAs such as microRNAs (miR)^[42].

In preclinical experiments, exosomes derived from adipose tissue-derived mesenchymal stem cells (AMSCs) and loaded with miR-320d demonstrated anti-apoptotic effects when applied to isolated cardiomyocytes obtained from a murine model of AF^[43]. Similarly, bone marrow mesenchymal stem cell (BMSC)-derived exosomes loaded with miR-148a showed protective effects in HL-1 mouse cardiomyocyte cells subjected to electrical stimulation to induce an AF-like phenotype. Treatment with these engineered exosomes significantly inhibited apoptosis in the stimulated cardiomyocytes^[44].

While these findings highlight the potential of exosome-based therapies for AF, it is important to note that all current evidence is limited to preclinical models. Significant challenges remain before clinical translation can be realized, including the development of reproducible, high-purity isolation techniques and efficient, scalable methods for exosome loading^[45].

Solid lipid nanoparticles

In contrast to the aqueous or semi-aqueous core vesicles discussed so far, solid lipid nanoparticles (SLNs) represent a distinct class of drug delivery systems characterized by a solid lipid core surrounded by a monolayer of surfactant^[46]. This rigid lipid matrix enhances the encapsulation efficiency and stability of hydrophobic drugs relative to liposomes and polymeric nanoparticles^[47].

Carvedilol, a third-generation β-blocker commonly used in the management of atrial fibrillation, exhibits poor oral bioavailability (~ 25%) due to extensive first-pass hepatic metabolism^[48]. In preclinical studies, encapsulating carvedilol in SLNs significantly improved its cellular uptake. Specifically, SLN-encapsulated carvedilol demonstrated superior absorption in Caco-2 cells (a human colorectal adenocarcinoma cell line commonly used to model intestinal epithelium) compared to carvedilol physically mixed with SLNs or carvedilol alone, suggesting the potential for overcoming its limited oral bioavailability^[48].

Additional preclinical investigations have evaluated the intranasal delivery of carvedilol-loaded SLNs. In an ex vivo study using sheep nasal mucosa, SLN formulations achieved enhanced mucosal penetration compared to carvedilol solution alone. Furthermore, in vivo administration of carvedilol-loaded SLNs incorporated into an in situ gelling system resulted in significantly higher absolute bioavailability following intranasal delivery in rabbits compared to conventional oral administration^[49].

These preclinical findings underscore the potential of SLNs as a promising platform for improving the bioavailability of poorly absorbed cardiovascular drugs. However, further studies are needed to optimize formulation parameters and assess safety and efficacy in clinical contexts.

Cyclic nanoparticles

Cyclodextrins are cyclic oligosaccharide-based nanoparticles composed of α-1,4-linked α-D-glucopyranose units^[50]. These molecules possess a hydrophobic, bucket-shaped cavity capable of forming supramolecular host-guest inclusion complexes with lipophilic compounds, such as amiodarone^[51]. In drug delivery applications, cyclodextrins offer several advantages, including enhanced solubility of hydrophobic drugs, improved encapsulation efficiency, protection from chemical degradation, and superior drug stability compared to liposomal systems^[52].

Recent preclinical studies have explored the use of cyclodextrin nanoparticles (CDNPs) for targeted antiarrhythmic drug delivery. Macrophages, which exhibit high phagocytic activity toward CDNPs, are abundant in the atrioventricular nodal (AVN) region of the heart, making this cell population an attractive target for localized drug delivery within the cardiac conduction system^[53,54]. In a murine model, intravenous administration of amiodarone-loaded CDNPs in C57BL/6 mice resulted in selective accumulation of nanoparticles in the myocardium, particularly within the AVN, 24 h post-injection^[55]. Additional accumulation was observed in macrophage-rich organs such as the liver and spleen, whereas a relatively lower distribution was detected in the lungs and skeletal muscle^[55]. Correspondingly, tissue analysis confirmed elevated amiodarone concentrations in the heart, liver, and spleen, suggesting the potential for reduced pulmonary toxicity but raising the possibility of hepatic and splenic adverse effects.

Although these findings underscore the potential of CDNPs for site-specific delivery of amiodarone and reduction of off-target effects, it is important to note that the current evidence remains limited to preclinical models. Demonstration of therapeutic efficacy - specifically, arrhythmia prevention or suppression such as in atrial fibrillation - has yet to be established. Rigorous evaluation in disease-relevant models and eventual clinical studies will be essential to determine the translational applicability of this approach.

Polymeric nanoparticles

Polymeric nanoparticles represent a versatile class of nanocarriers for drug delivery, typically classified as nanocapsules - comprising a polymeric shell surrounding an oily core - or nanospheres, in which the drug is uniformly dispersed within a polymeric matrix^[56]. These systems can either adsorb therapeutic agents onto their surface or encapsulate them within the core or matrix, offering several pharmacokinetic advantages, including enhanced bioavailability, sustained release, and protection from premature degradation^[57,58].

Polymeric nanoparticles may be synthesized from either synthetic or natural biodegradable polymers. Synthetic polymers can be chemically modified to tailor drug release profiles, improve targeting specificity, and reduce systemic toxicity^[59,60]. Among these, poly(lactic-co-glycolic acid) (PLGA) is a well-characterized biodegradable polymer with demonstrated biocompatibility and controlled degradation properties. In preclinical studies, oral administration of PLGA-encapsulated amiodarone in Sprague-Dawley rats resulted in significantly reduced drug-related toxicities. Animals receiving the PLGA formulation exhibited lower serum levels of hepatic enzymes (alanine aminotransferase and aspartate aminotransferase), attenuated pulmonary capillary congestion and inflammatory infiltration, and preserved thyroid function (normal T4 and thyroid stimulating hormone levels) compared to those administered with free amiodarone^[61].

Natural polymers offer distinct advantages due to their inherent biocompatibility, low immunogenicity, and additional properties such as antimicrobial activity^[62]. Chitosan, a cationic natural polymer, has garnered significant attention for drug delivery applications due to its mucoadhesive properties, capacity to enhance epithelial permeability, and affinity for negatively charged cellular membranes^[63]. In a preclinical formulation, amiodarone was first modified with cyclodextrin to enhance its solubility and then encapsulated into chitosan-based nanoparticles. This system demonstrated sustained release of amiodarone over 14 days, suggesting strong potential for use in controlled-release antiarrhythmic therapy^[64].

Although these findings are currently limited to preclinical models, they collectively underscore the potential of polymeric nanoparticle systems to improve the safety and efficacy of amiodarone and other antiarrhythmic drugs. Further investigation is needed to evaluate their clinical translatability and therapeutic benefit in human subjects.

Magnetic nanoparticles

Magnetic nanoparticles (MNPs) have garnered significant interest in preclinical research for their capacity to enable the targeted delivery of therapeutic agents under the guidance of an external magnetic field^[71]. While MNP-based strategies have been explored in a variety of disease models, including oncology and neurodegenerative disorders, their application in the context of GP ablation for AF remains entirely preclinical at this stage^[72,73].

One of the earliest MNP-based formulations for GP modulation featured a magnetite core surrounded by a thermoresponsive hydrogel matrix capable of delivering a neurotoxic payload^[74]. In a preclinical canine study, MNPs loaded with the neurotoxin N-isopropyl acrylamide monomer (NIPA-M) were infused into the circumflex coronary artery of adult mongrel dogs. Within two hours, electrophysiological measurements indicated functional suppression of the inferior right GP (IRGP), with histological analysis confirming the accumulation of the nanoparticles within the IRGP^[74]. In addition to serving as a neurotoxic payload, N-isopropyl acrylamide (NIPA) can be integrated into the hydrogel matrix itself to exploit its thermoresponsive properties. In another preclinical study, MNPs coated with NIPA were administered via the circumflex coronary artery in beagle dogs and similarly suppressed IRGP activity^[75].

A distinct MNP formulation employed a PLGA matrix shell with a magnetite core and calcium chloride as the active agent^[76]. Although calcium is a physiological ion, excessive intracellular calcium in neurons can induce neuroapoptosis, thereby achieving a form of chemical GP ablation^[77]. In a preclinical canine model, calcium chloride-loaded MNPs were infused into the left circumflex artery (LCX) under an externally applied magnetic gradient, leading to marked suppression of both IRGP and superior left GP (SLGP) activity^[76].

To further enhance neurotoxicity, L-glutamate was added to a similar MNP formulation to potentiate calcium influx through NMDA receptor activation. This enhanced formulation was tested in a preclinical model of postoperative atrial fibrillation (POAF), created via sequential survival thoracotomies in mongrel dogs^[66]. Targeted injection of the MNPs into the left atrial GPs suppressed AF inducibility and reversed POAF-associated shortening of the atrial effective refractory period. Notably, this treatment preserved circadian heart rate variability and exercise-induced chronotropic response, suggesting modulation rather than complete ablation of cardiac autonomic function^[66].

These findings underscore the preclinical promise of MNP-based strategies for selective neuromodulation of cardiac autonomic inputs in AF. However, translation to clinical application will require substantial validation, including long-term safety profiling, precise magnetic targeting techniques, and robust demonstration of efficacy in larger and more heterogeneous animal models.

Non-magnetic nanoparticles

Non-magnetic nanoparticle formulations have also been investigated in preclinical studies as a potential strategy for GP ablation. Botulinum toxin has previously demonstrated efficacy in suppressing AF inducibility and delaying its progression when directly injected into atrial GPs, although these findings remain limited to animal models^[78,79]. Building on this concept, a preclinical study recently evaluated a chitosan-based nanoparticle formulation encapsulating botulinum toxin for neuromodulation. In this study, epicardial administration of the botulinum toxin-loaded chitosan nanoparticles significantly reduced the incidence of cardiac arrhythmias - specifically ventricular fibrillation - in rat models^[80].

While these findings are promising, particularly about localized and sustained neurotoxin delivery, further preclinical investigations using validated animal models of AF are essential to determine whether this formulation can effectively modulate atrial GPs to treat or prevent atrial fibrillation specifically. Translation to clinical application will also require a comprehensive evaluation of safety, specificity of GP targeting, and long-term outcomes.