Organelle-Specific Drug Delivery Systems in the Treatment of Rare Mitochondrial and Peroxisomal Disorders: A Frontier in Precision Nanotherapeutics

Rare mitochondrial and peroxisomal disorders represent a class of genetically inherited metabolic diseases characterized by defects in organelle function that critically impair cellular homeostasis and energy metabolism. Although individually rare, collectively these disorders affect thousands worldwide, with mitochondrial diseases estimated to occur in approximately 1 in 4,300 individuals (Gorman et al., 2016), while peroxisomal disorders like Zellweger spectrum disorders are reported with an incidence of 1 in 50,000 to 1 in 100,000 live births (Braverman et al., 2016). These conditions stem from mutations in either mitochondrial DNA (mtDNA) or nuclear genes encoding mitochondrial or peroxisomal proteins, leading to clinical heterogeneity that ranges from neurodegeneration and developmental delays to multi-organ failure (Wanders et al., 2020; Rahman, 2020).

Despite advancements in molecular diagnostics and disease classification, therapeutic options for these disorders remain limited and primarily supportive rather than curative. Conventional pharmacological interventions face major challenges due to poor bioavailability, lack of specificity, and limited ability to cross subcellular barriers to reach dysfunctional organelles (Vafai & Mootha, 2012). Additionally, systemic drug administration often leads to off-target effects and cumulative toxicity, further complicating long-term disease management (Wallace, 2013).

To address these limitations, organelle-specific drug delivery systems have emerged as a transformative strategy, particularly with the rise of nanotechnology-enabled precision therapeutics. These systems exploit the unique biochemical and structural properties of mitochondria and peroxisomes to facilitate targeted drug accumulation, thereby improving efficacy while minimizing systemic side effects (Ramasamy et al., 2020). For example, nanocarriers functionalized with mitochondria-targeting moieties such as triphenylphosphonium (TPP?) or peroxisomal targeting signals (PTS) can selectively deliver bioactive agents to their intended subcellular destinations (Pathak et al., 2014). This review aims to provide a comprehensive overview of organelle-specific drug delivery systems in the context of rare mitochondrial and peroxisomal disorders. It discusses the molecular pathology of these conditions, evaluates current and emerging nanocarrier platforms for targeted therapy, and explores translational advancements and challenges in the field. By bridging insights from molecular biology, materials science, and pharmacology, this review highlights the growing relevance of subcellular targeting in the era of precision nanotherapeutics.

II. Mitochondrial and Peroxisomal Disorders: Pathophysiological Landscape

A. Mitochondrial Disorders

Mitochondrial disorders are a group of clinically heterogeneous diseases caused by mutations in either mitochondrial DNA (mtDNA) or nuclear DNA that encodes mitochondrial proteins. mtDNA mutations are maternally inherited and often affect genes involved in oxidative phosphorylation, while nuclear gene mutations follow Mendelian inheritance patterns and can impact mitochondrial structure, function, or biogenesis (Gorman et al., 2016; Vafai & Mootha, 2012).

Common mitochondrial disorders include:

50. Mitochondrial Encephalomyopathy, Lactic Acidosis, and Stroke-like episodes (MELAS)
50. Leigh Syndrome, a progressive neurodegenerative disorder of infancy
50. Leber’s Hereditary Optic Neuropathy (LHON), a condition marked by acute or subacute vision loss (Rahman, 2020; Wallace, 2013)

The pathophysiology is primarily driven by impaired oxidative phosphorylation leading to inadequate ATP production, excessive generation of reactive oxygen species (ROS), and subsequent oxidative damage to proteins, lipids, and DNA (Wallace, 2013). These biochemical abnormalities often culminate in multi-system involvement, particularly affecting energy-demanding organs such as the brain, muscles, and heart.

B. Peroxisomal Disorders

Peroxisomal disorders encompass a range of genetic diseases caused by dysfunction of peroxisomes—organelles involved in lipid metabolism, hydrogen peroxide detoxification, and bile acid synthesis (Wanders et al., 2020). These disorders are broadly classified into:

50. Peroxisomal Biogenesis Disorders (PBDs), such as Zellweger Spectrum Disorders (ZSDs), which involve defective peroxisome assembly
50. Single Enzyme Deficiencies, such as X-linked Adrenoleukodystrophy (X-ALD), caused by mutations in the ABCD1 gene (Braverman et al., 2016)

ZSDs manifest with craniofacial abnormalities, hypotonia, seizures, and profound developmental delays, often resulting in early mortality. In contrast, X-ALD is characterized by the accumulation of very-long-chain fatty acids (VLCFAs) in tissues, leading to inflammatory demyelination and adrenal insufficiency (Wiesinger et al., 2013). Dysregulation of peroxisomal β-oxidation and plasmalogen biosynthesis in these conditions impairs cellular membrane integrity and results in neurodegeneration, systemic inflammation, and hepatic dysfunction (Ferdinandusse et al., 2016).

Table 1. Representative Mitochondrial and Peroxisomal Disorders and Their Key Features

Figure 1: Flowchart of Classification and Pathogenesis inherited organelle disorders

III. Organelle-Specific Targeting Strategies

Targeting drugs directly to mitochondria or peroxisomes represents a transformative strategy in treating organelle-specific disorders. This approach enhances therapeutic efficacy, minimizes systemic toxicity, and aligns with the principles of precision medicine. Recent advances in nanotechnology have facilitated the design of delivery platforms that exploit organelle-specific physicochemical and biochemical features.

A. Mitochondrial Targeting Approaches

1. Lipophilic Cations (e.g., TPP?-Conjugated Drugs)

Triphenylphosphonium (TPP?) is a delocalized lipophilic cation that can selectively accumulate in the mitochondrial matrix, driven by the negative membrane potential of the inner mitochondrial membrane (Murphy & Smith, 2007). TPP? has been used to deliver a range of antioxidants, chemotherapeutics, and imaging agents directly to mitochondria, enhancing both bioavailability and organelle-specific action (Smith et al., 2012).

2. Mitochondria-Penetrating Peptides (MPPs)

MPPs are short peptides engineered to possess both positive charge and moderate lipophilicity, enabling them to translocate across mitochondrial membranes. They are capable of delivering cargo such as nucleic acids, proteins, and small molecules with high mitochondrial selectivity (Horton et al., 2008). Examples include Szeto-Schiller (SS) peptides and synthetic amphipathic carriers that preserve mitochondrial membrane integrity while facilitating uptake.

3. pH/Redox-Sensitive Nanoparticles

The mitochondrial microenvironment, particularly its slightly alkaline pH and redox state, can be exploited for selective drug release. Nanocarriers loaded with pH- or glutathione-responsive linkers degrade preferentially within mitochondria, ensuring that the drug payload is activated only at the target site (Yousaf et al., 2021). This strategy is especially valuable for delivering ROS-sensitive drugs or mitochondrial enzyme modulators.

B. Peroxisomal Targeting Approaches

1. Peroxisomal Targeting Signals (PTS1/PTS2)

Endogenous protein import into peroxisomes is mediated by peroxisomal targeting signals. PTS1 is a C-terminal tripeptide (Ser-Lys-Leu), while PTS2 is an N-terminal nonapeptide. These sequences can be engineered onto therapeutic proteins or enzymes to facilitate their direct import into peroxisomes via the PEX receptor system (Braverman et al., 2016; Purdue & Lazarow, 2001).

2. Ligand-Conjugated Nanoparticles

Targeting peroxisomes with ligand-conjugated nanoparticles remains an emerging field. Ligands that bind selectively to peroxisomal membrane receptors or associated enzymes may serve as anchors for nanoparticles, directing their intracellular trafficking (Weller et al., 2013). Peptides mimicking peroxisomal import signals or receptor-specific aptamers are being explored to enhance targeting specificity.

3. Enzyme-Loaded Nanocarriers

To address single-enzyme deficiencies in peroxisomal disorders, nanocarriers have been developed to encapsulate and deliver functional enzymes such as acyl-CoA oxidase or peroxisomal catalase. These platforms are often designed with endosomal escape capabilities and peroxisomal targeting motifs, ensuring the bioactive enzyme reaches the correct subcellular compartment (Reddy et al., 2011).

Table 2. Organelle-Targeting Strategies and Their Mechanistic Attributes

IV. Nanocarrier Platforms for Organelle-Specific Delivery

Nanocarrier platforms serve as precision tools for delivering therapeutic agents directly to intracellular organelles such as mitochondria and peroxisomes. Their structural diversity, biocompatibility, and functional tunability enable the incorporation of targeting moieties, responsive release mechanisms, and diagnostic functionalities—key attributes for treating rare metabolic disorders characterized by subcellular dysfunction.

A. Liposomes and Dendrimers

Liposomes are phospholipid bilayer vesicles capable of encapsulating hydrophilic and hydrophobic drugs. Through surface modification with targeting ligands like triphenylphosphonium (TPP?) or peroxisomal peptides, liposomes can achieve organelle specificity (Bozzuto & Molinari, 2015). Dendrimers, with their highly branched structure and modifiable surface groups, offer multivalency for conjugating both therapeutic and targeting agents. They have demonstrated success in mitochondrial targeting via conjugation with delocalized lipophilic cations and in delivering antioxidant enzymes for peroxisomal restoration (Tomalia et al., 2012). In rare metabolic diseases such as Zellweger spectrum disorders and mitochondrial encephalopathies, liposome and dendrimer-based carriers show promise for overcoming enzyme mislocalization and drug inefficacy (Wang et al., 2020).

B. Polymeric Nanoparticles

Polymeric nanoparticles fabricated from biocompatible and biodegradable polymers like poly(lactic-co-glycolic acid) (PLGA) and PEG-PLGA copolymers are widely utilized in controlled drug delivery. These nanoparticles can be engineered to release drugs in response to pH, redox potential, or enzymatic activity, matching the microenvironment of target organelles (Danhier et al., 2012). Their versatility allows incorporation of mitochondrial or peroxisomal targeting sequences and the delivery of nucleic acids, enzyme replacements, and small molecules for sustained therapeutic effects in disorders like MELAS or X-linked adrenoleukodystrophy (Xu et al., 2016).

C. Inorganic Nanoparticles

Gold nanoparticles (AuNPs) and mesoporous silica nanoparticles (MSNs) offer unique optical and structural properties, respectively, suitable for theranostic applications. Their surfaces can be functionalized with organelle-specific ligands and imaging agents, enabling simultaneous drug delivery and diagnostic tracking (Wang et al., 2014). AuNPs conjugated with mitochondrial targeting sequences have been employed to deliver apoptosis modulators in neurodegenerative models, while MSNs loaded with enzymes or gene-editing tools are under investigation for correcting metabolic defects in peroxisomal disorders (Chen et al., 2013).

D. Hybrid and Biomimetic Nanocarriers

Hybrid nanocarriers combine organic and inorganic elements for enhanced functionality. Biomimetic nanocarriers, such as cell membrane-coated nanoparticles and exosome-based systems, mimic native cellular interactions and evade immune clearance (Fang et al., 2018). Exosomes, being naturally secreted vesicles, possess intrinsic targeting capabilities and have shown potential in organelle-specific delivery by surface engineering with peptides or aptamers (El Andaloussi et al., 2013). In preclinical studies, exosome-loaded antioxidant enzymes and cell membrane-camouflaged nanoparticles have successfully targeted dysfunctional mitochondria and peroxisomes, indicating translational potential for rare genetic diseases (Jiang et al., 2020).

Table 3. Nanocarrier Platforms for Organelle-Specific Therapeutics

V. Organelle-Targeted Therapeutics: Preclinical and Clinical Advances

Organelle-specific therapeutics are progressing from bench to bedside with promising preclinical data and emerging clinical trials targeting mitochondrial and peroxisomal dysfunctions. However, challenges remain in translation due to biological complexity, delivery limitations, and regulatory hurdles.

A. Experimental Models for Evaluation

Robust and representative disease models are essential to assess the efficacy and organelle-targeting precision of nanotherapeutics. In vitro models include immortalized cell lines and CRISPR-engineered cells harboring mitochondrial or peroxisomal mutations. Patient-derived organoids offer 3D structural fidelity and genotype-specific drug response prediction (Lancaster & Knoblich, 2014). In vivo models such as transgenic mice and zebrafish are commonly used for evaluating biodistribution, pharmacokinetics, and therapeutic index in systemic disorders like Leigh syndrome or X-linked adrenoleukodystrophy (Meyer et al., 2017).

B. Notable Preclinical Studies

Several organelle-targeted therapeutics have demonstrated efficacy in preclinical models:

50. Mitochondrial-targeted antioxidants: Molecules like MitoQ and SkQ1, conjugated with triphenylphosphonium (TPP?), accumulate selectively in mitochondria and neutralize ROS. In animal models of mitochondrial encephalomyopathy, these agents improved mitochondrial function, reduced lipid peroxidation, and ameliorated cognitive decline (Smith & Murphy, 2010; Skulachev et al., 2009).
50. Gene and nucleic acid delivery: Targeted delivery of mtDNA, mRNA, or CRISPR-Cas9 systems into mitochondria or peroxisomes has been explored using liposome–peptide hybrids and viral vectors. In peroxisomal disorders, antisense oligonucleotides and siRNA-loaded nanoparticles have shown restoration of enzyme expression and metabolic correction (Baker et al., 2017).

These findings support the translational potential of subcellular delivery platforms in rare metabolic diseases.

C. Clinical Trials and Translational Gaps

Some organelle-targeted compounds have entered clinical evaluation, especially in the context of mitochondrial diseases:

50. MitoQ has undergone Phase II trials for Parkinson's disease and vascular dysfunction, with modest efficacy and good safety profiles (Rossman et al., 2018).
50. Elamipretide (SS-31), a peptide-based mitochondrial protectant, has advanced to clinical trials for primary mitochondrial myopathy and Barth syndrome, showing improved exercise tolerance and cardiac function (Karaa et al., 2018).

However, the translation of peroxisome-targeted therapies remains limited due to:

50. Poor understanding of trafficking mechanisms,
50. Lack of standardized biomarkers,
50. Delivery inefficiencies and off-target effects,
50. Regulatory complexities in defining pharmacokinetics at the organelle level (Van Veldhoven & Baes, 2013).

Bridging these gaps necessitates integrative approaches combining omics, high-throughput screening, and regulatory innovations.

Table 4. Organelle-Targeted Therapeutics: Preclinical and Clinical Progress