What are the different types of drugs available for miRNA?

Introduction to miRNA
MicroRNAs (miRNAs) are a class of endogenous, small non-coding RNA molecules typically ranging from 19 to 25 nucleotides in length that play fundamental roles in the regulation of gene expression at the post-transcriptional level. They have emerged as critical regulators in numerous biological processes and disease states, making them attractive targets for therapeutic interventions. Over the past two decades, pioneering studies have revealed that aberrations in miRNA expression can contribute to a variety of pathological conditions, ranging from cancer to cardiovascular and neurological disorders. Consequently, there has been intensive research into developing drugs that either restore the normal function of miRNAs or inhibit their pathological actions.

Definition and Function of miRNA
miRNAs are short RNA molecules encoded within the genome. They are transcribed primarily by RNA polymerase II as long primary transcripts (pri-miRNAs) and subsequently processed in a multi-step biogenesis pathway that involves the microprocessor complex (with Drosha and DGCR8) in the nucleus and Dicer in the cytoplasm. The mature miRNAs are then incorporated into the RNA-induced silencing complex (RISC), where they guide the complex to target messenger RNAs (mRNAs) through partial complementarity, particularly within a defined “seed region” at nucleotides 2–8. This binding leads to translational repression and/or mRNA degradation, thereby modulating the expression levels of a wide array of genes.

Role of miRNA in Gene Regulation
The regulatory functions of miRNAs are extensive. They fine-tune gene expression in a context-dependent fashion, affecting various processes such as cell differentiation, proliferation, apoptosis, immune responses, and metabolic homeostasis. miRNAs function as master regulators because they can target multiple mRNAs typically involved in the same cellular pathway and, conversely, a single mRNA may be regulated by several different miRNAs. This multifaceted regulation is fundamental not only under normal physiological conditions but also in disease states. Over- or under-expression of specific miRNA species has been associated with pathological conditions. For instance, overexpressed oncogenic miRNAs (oncomiRs) can downregulate tumor suppressor genes, while decreased levels of tumor suppressor miRNAs can lead to the unchecked expression of oncogenes, ultimately promoting disease progression.

Types of miRNA-targeting Drugs
Multiple strategies have been developed over time to therapeutically target miRNAs. Generally, these strategies fall into three categories: miRNA inhibitors, miRNA mimics, and small molecule modulators. These drug types are designed to either inhibit a pathogenic miRNA, restore the function of miRNAs that are under-expressed, or modulate the miRNA pathways through direct chemical interactions.

miRNA Inhibitors
miRNA inhibitors, often referred to as anti-miRs or antagomirs, are designed to specifically bind to and inactivate endogenous miRNAs that are implicated in disease. They are typically single-stranded chemically modified oligonucleotides that are complementary to the target miRNA sequence. Common chemical modifications include 2′-O-methyl (2′-OMe), 2′-O-methoxyethyl (2′-MOE), phosphorothioate backbones, and locked nucleic acids (LNA). These modifications increase the stability of the oligonucleotides in biological fluids, reduce degradation by nucleases, and improve binding affinity, thus enhancing their therapeutic potential.
A good example is the inhibition of miR-21, an oncogenic miRNA implicated in several types of cancers. Chemical inhibitors of miR-21 have been shown to interfere with its binding to target mRNAs, thereby restoring normal gene expression patterns in tumor cells. Several patents have described methods for targeting miRNAs by designing inhibitors that sequester specific miRNA molecules, consequently preventing them from repressing their target mRNAs. In clinical settings, antagomiRs designed to silence aberrant miRNAs have progressed into clinical trials, with miravirsen—an inhibitor targeting miR-122 for hepatitis C infection—as a notable example.

miRNA Mimics
miRNA mimics are synthetic, double-stranded oligonucleotides that replicate the function of endogenous miRNAs. These agents are applied to restore or augment the level of a miRNA that is downregulated in a disease state. The mimics are designed so that one strand (the guide strand) is identical to the mature miRNA and is loaded into the RISC complex, allowing it to suppress the expression of its target genes effectively. miRNA mimics have been applied in various disease contexts, such as reintroducing tumor-suppressive miRNAs in cancer or modulating gene expression in cardiovascular diseases.
For instance, MRX34 is a miR-34 mimic that was developed for cancer treatment. Although its clinical trial was halted due to immune-related adverse events, MRX34 provided proof-of-concept that miRNA replacement therapy using synthetic mimics is feasible. Similarly, remlarsen (MRG-201), which is a miR-29 mimic, is in clinical trials for treating fibrotic diseases since the miR-29 family plays a crucial role in modulating extracellular matrix production. These examples underscore the potential of miRNA mimics to restore normal regulatory functions in pathogenic conditions.

Small Molecule Modulators
Small molecule modulators represent another class of miRNA-targeting drugs. Unlike oligonucleotide-based approaches (inhibitors and mimics), small molecules can bind directly to RNA structures or interact with the proteins involved in miRNA biogenesis and processing. This approach leverages the chemical properties of small molecules to modulate the function of miRNAs, either by enhancing or inhibiting their expression levels.
Small molecules can affect miRNA pathways through various mechanisms. Certain small molecules are designed to bind to the precursor forms of miRNAs, thereby interfering with enzymes like Dicer or Drosha and ultimately altering miRNA maturation. For example, streptomycin, conventionally used as an antibiotic, has been implicated in inhibiting the processing of miR-21 by binding to its precursor structure, leading to a reduction in miR-21 levels and exhibiting potential anticancer properties. Additionally, there are compounds targeting other miRNAs involved in different diseases. Small molecule modulators do not necessarily require high complementarity like oligonucleotide-based drugs; rather, they are designed to recognize specific structural motifs or binding pockets in RNA molecules.
Recent studies have utilized both experimental high-throughput screens and computational approaches to identify small molecule modulators that can modulate miRNA functions. These strategies have led to the discovery of small molecules that can downregulate or upregulate miRNAs, expanding the therapeutic arsenal to address diseases by targeting miRNA dysregulation. Patents in this area describe methods for identifying small molecules that form complexes with RNA molecules or proteins involved in miRNA processing, which further attests to their potential as miRNA modulators.

Mechanisms of Action
The different types of miRNA-targeting drugs work through distinct mechanisms. Understanding these mechanisms is essential for appreciating how these drugs modulate gene expression and influence disease pathology.

How miRNA Inhibitors Work
miRNA inhibitors primarily function through Watson–Crick base pairing with the target miRNA. Upon binding, the formation of a stable duplex prevents the miRNA from associating with its target mRNAs, thereby lifting the repression on gene expression. The chemically modified backbone of these inhibitors protects them from degradation, ensuring they remain active in physiological conditions. By inhibiting an overexpressed oncogenic miRNA, for example, the inhibitor can restore the expression of tumor suppressor genes that were being repressed. This mechanism is particularly important in diseases such as cancer, where the overactivity of oncomiRs like miR-21 leads to abnormal cell proliferation and survival. Moreover, the inhibitors may sometimes promote the degradation of the miRNA itself or sequester it in non-functional complexes, enhancing the therapeutic effect.

Mechanism of miRNA Mimics
miRNA mimics are designed to replicate the function of naturally occurring miRNAs. Once introduced into the cell, the mimic is incorporated into the RISC complex, where its guide strand directs the complex to complementary sequences in target mRNAs. This results in the post-transcriptional repression of gene expression through either mRNA degradation or translational inhibition. The specificity of miRNA mimics is determined by their sequence, especially within the seed region, which guides the RISC to accurately bind and downregulate the intended target mRNAs. This mechanism is harnessed to restore normal regulatory processes in conditions where certain miRNAs are underexpressed. By boosting the level of a tumor-suppressive miRNA or a miRNA involved in maintaining cellular homeostasis, miRNA mimics can counteract the effects of disease-associated gene dysregulation.

Small Molecules and miRNA Interaction
Small molecule modulators target miRNA function from a different angle. Rather than mimicking or directly binding to the miRNA sequence, these compounds interact with RNA structures or the protein machinery involved in miRNA processing. For instance, small molecules may bind to the hairpin structure of a miRNA precursor (pre-miRNA) and interfere with the action of Dicer or Drosha, thus impeding the maturation process of the miRNA. This inhibition of miRNA biogenesis can lead to a decrease in mature miRNA levels, effectively silencing the miRNA’s downstream effects. Additionally, some small molecules may act indirectly by modulating the stability of miRNAs or influencing ancillary protein factors that determine miRNA function and localization. This modality offers the advantage of potentially higher specificity for certain structural motifs over a broad range of RNA sequences and can be optimized through medicinal chemistry and computational modeling. The mechanism of action of small molecule modulators also provides an avenue for combination therapies where a drug might simultaneously modulate miRNA activity and other related cellular pathways.

Therapeutic Applications
The diverse types of miRNA-targeting drugs have been explored in several therapeutic areas. By directly modulating miRNA expression, these drugs influence the underlying molecular pathways that contribute to disease progression. Here we review three critical areas where miRNA interventions have shown promise: cancer, cardiovascular diseases, and neurological disorders.

Cancer Treatment
Cancer is one of the most well-studied contexts for miRNA-based therapy. Dysregulation of miRNAs—such as the overexpression of oncogenic miRNAs (e.g., miR-21, miR-155) or the downregulation of tumor-suppressive miRNAs (e.g., miR-34, let-7)—plays a key role in tumor initiation, progression, metastasis, and resistance to therapy.
- miRNA Inhibitors in Cancer: Inhibition of overexpressed oncomiRs using antagomiRs can reinstate the expression of tumor suppressor genes. For example, miR-21 inhibitors have been explored to reverse the aggressive phenotype of several tumors. Additionally, patents and research articles have detailed methods to target miRNAs to overcome resistance to chemotherapy, thereby improving treatment outcomes.
- miRNA Mimics in Cancer: Restoring the expression of tumor suppressive miRNAs using mimics has been another major strategy. The miR-34 mimic, MRX34, is a notable example—even though its clinical trial encountered immune-related challenges, it demonstrated that synthetic miRNA mimics can downregulate multiple oncogenic pathways simultaneously. Furthermore, miRNA mimics are used to restore normal gene expression networks, combatting abnormal cell proliferation and metastasis.
- Small Molecule Modulators in Cancer: Small molecules such as streptomycin have been repurposed to target the precursors of specific miRNAs, thereby modulating their maturation and reducing the levels of oncogenic miRNAs like miR-21. Computational-based screening and high-throughput methods have also identified compounds that bind RNA secondary structures, offering additional therapeutic options.
Collectively, these strategies provide a multipronged approach to cancer therapy by addressing both the upregulated oncogenic pathways and the loss of tumor suppressive signals.

Cardiovascular Diseases
Cardiovascular diseases (CVDs) have been another area where miRNA-targeting drugs have shown significant promise. Given that miRNAs play crucial roles in regulating vascular function, cardiac myocyte survival, and remodeling processes, targeted modulation can potentially ameliorate heart failure, myocardial infarction, and other CVDs.
- miRNA Inhibitors: In the cardiovascular context, inhibitors targeting miRNAs like miR-208 (which is involved in cardiac hypertrophy and remodeling) have been evaluated. Inhibition of miR-208 has been suggested to block pathological cardiac remodeling, thereby preserving heart function.
- miRNA Mimics: miRNA mimics that restore the levels of protective miRNAs have been explored in pre-clinical models. For instance, restoring specific miRNAs that attenuate fibrosis or promote cardiomyocyte survival can lead to improved outcomes following myocardial infarction.
- Small Molecule Modulators: Some small molecules have been investigated for their capacity to modulate the expression of miRNAs involved in cardiovascular regulation. By targeting miRNA biogenesis or maturation, these modulators can normalize dysregulated miRNA profiles that contribute to adverse cardiac remodeling.
These approaches highlight the utility of miRNA-based interventions in modulating intricate molecular pathways that drive cardiac pathology, potentially offering improved clinical outcomes in patients with cardiovascular diseases.

Neurological Disorders
miRNAs are extensively involved in neural development, synaptic plasticity, and the regulation of neuronal survival. Aberrant miRNA expression has been implicated in various neurodegenerative conditions and psychiatric disorders, such as Alzheimer’s disease, depression, and Parkinson’s disease.
- miRNA Inhibitors in Neurology: In neurological applications, inhibitors are being employed to suppress miRNAs that contribute to pathological neuronal loss or aberrant signaling. The downregulation of specific miRNAs that exacerbate neuroinflammation or synaptic dysregulation is a key strategy to curb neurodegenerative processes.
- miRNA Mimics in Neurology: Conversely, miRNA mimics have been designed to restore the function of miRNAs that protect neuronal integrity. For example, overexpression of certain miRNAs through mimics can support neurogenesis, improve neural connectivity, and reduce apoptosis in diseased neurons.
- Small Molecule Modulators: Although less advanced than in oncology or cardiovascular medicine, small molecule modulators are also being explored for their ability to modulate miRNA expression in the brain. Given the challenge of crossing the blood–brain barrier, these molecules are designed to be both potent and bioavailable, potentially offering a means to correct miRNA dysregulation in neurodegenerative diseases.
The diverse mechanisms by which miRNA-targeting drugs work in neurological disorders underline the possibility of fine-tuning cellular responses, thereby halting or even reversing the progression of disease states.

Challenges and Future Directions
Despite the promising advances in miRNA-targeting drugs, significant challenges and opportunities remain on the road to clinical application. The specificity, delivery, and off-target effects of these drugs are crucial factors influencing their efficacy and safety.

Current Limitations
Several limitations continue to restrict the full therapeutic realization of miRNA-targeting drugs, regardless of the strategy employed:
1. Delivery Issues: One of the major challenges is the efficient delivery of miRNA drugs to target tissues. Oligonucleotide-based agents (both inhibitors and mimics) are inherently unstable in biological fluids and can be rapidly degraded by nucleases. Although chemical modifications (such as LNA, 2′-OMe, and phosphorothioate backbones) have improved stability, delivering these agents to specific tissues (such as the heart, brain, or tumor microenvironment) remains difficult.
2. Off-target Effects: Due to the inherent nature of miRNA targeting – where a single miRNA can regulate multiple genes – there is a risk of off-target effects leading to unintended gene silencing or activation. This pleiotropy can lead to adverse side effects, limiting the therapeutic window. Careful selection of miRNA targets and stringent design of oligonucleotides are essential to minimize these risks.
3. Immune Response and Toxicity: Synthetic miRNA mimics and inhibitors can trigger immune responses, particularly when delivered in high doses or without appropriate modifications. The immune-related adverse effects observed in some clinical trials (e.g., MRX34) underscore the importance of optimizing the safety profiles of these agents.
4. Pharmacokinetics and Bioavailability: Achieving favorable pharmacokinetic profiles is challenging, as these molecules must reach therapeutic concentrations in target tissues while avoiding rapid clearance from the bloodstream. Innovative delivery systems such as nanoparticles, liposomes, and exosomes are being investigated to overcome these obstacles.
5. Complexity of miRNA Networks: The interconnected network of miRNA regulation means that modulating one miRNA may inadvertently alter the expression of multiple genes and pathways. This complexity complicates the prediction of therapeutic outcomes and potential side effects, necessitating a comprehensive understanding of miRNA regulatory networks before clinical application.

Future Research Opportunities
Despite the challenges, the field of miRNA therapeutics offers several promising directions for future research:
1. Advanced Delivery Systems: Future research will likely focus on developing novel and more precise delivery vehicles. Nanoparticle-based carriers, viral vectors with reduced immunogenicity, and exosome-mediated delivery are promising areas that may significantly enhance the stability, targeting, and bioavailability of miRNA drugs.
2. Optimized Chemical Modifications: Continued exploration of chemical modifications to improve the stability and specificity of oligonucleotide-based drugs is needed. Novel modifications that reduce off-target effects while increasing binding affinity to the target miRNA will be essential for maximizing therapeutic benefits.
3. Computational Drug Design and Screening: The integration of computational approaches with high-throughput screening has already begun to identify small molecules that modulate miRNA function. Future research will further harness machine learning and deep learning methods to predict RNA-small molecule interactions more accurately, thereby expediting the discovery of potent modulators.
4. Combination Therapies: Given the complexity of miRNA networks in disease, combination therapies that target multiple miRNAs or combine miRNA-based drugs with conventional chemotherapeutics could be a promising avenue. For example, combining miRNA inhibitors with cytostatic drugs has shown synergistic effects in reducing cancer cell viability and overcoming drug resistance.
5. Personalized Medicine: Since specific miRNA expression profiles can serve as diagnostic and prognostic biomarkers, future therapeutic strategies could be tailored to the individual patient’s miRNA signature. This personalized approach would maximize efficacy while reducing adverse effects.
6. Improved Understanding of miRNA Biology: Further basic research to elucidate the nuances of miRNA biogenesis, function, and regulation will provide deeper insights into the optimal targets for therapy. This includes understanding the structural features of miRNA precursors that allow for specific small molecule interactions and clarifying the mechanisms leading to immune activation by miRNA drugs.

Conclusion
In summary, the landscape of miRNA-targeting drugs is vast and multifaceted, with three principal types emerging as therapeutic strategies. miRNA inhibitors (antagomiRs) work by binding to and neutralizing pathogenic miRNAs, thus preventing them from downregulating crucial target genes. miRNA mimics aim to replenish tumor suppressor or otherwise beneficial miRNAs that are underexpressed in disease states, restoring normal gene regulatory networks. Small molecule modulators represent another innovative approach by interacting with the miRNA biogenesis machinery or directly binding to RNA structures to modulate miRNA expression levels.

Each type of drug acts through distinct mechanisms: inhibitors sequester miRNAs away from the RISC complex, mimics are incorporated into RISC to suppress target mRNAs, and small molecules alter miRNA processing or stabilize specific RNA conformations to indirectly modulate gene expression. The therapeutic applications of these drugs have been extensively explored in cancer, cardiovascular diseases, and neurological disorders. In cancer, targeting oncogenic miRNAs or restoring tumor suppressor miRNAs has shown significant promise despite challenges such as off-target effects and delivery issues. In cardiovascular diseases, modulators targeting miRNAs involved in cardiac remodeling and fibrosis have advanced preclinical studies, while in neurological disorders, both inhibitors and mimics are being evaluated to restore neuronal function and mitigate neurodegeneration.

However, challenges related to delivery, off-target activity, immune response, and the complexity of miRNA regulatory networks still remain. Future research will benefit from advanced delivery systems (such as nanoparticles and exosomes), novel chemical modifications, computational approaches for small molecule design, and the development of combination therapies. Ultimately, a deeper understanding of miRNA biology, together with innovative drug design and personalized therapeutic strategies, will pave the way for more effective RNA-based treatments.

In conclusion, miRNA-targeting drugs offer a promising and versatile approach to treating a myriad of diseases by directly modulating gene expression. While there remain significant hurdles in terms of specificity, stability, and delivery, the ongoing advancements in oligonucleotide chemistry, nanotechnology, and computational drug discovery indicate that the clinical application of miRNA-based therapeutics is on the horizon. The multidisciplinary and innovative nature of this field is expected to drive future breakthroughs, ultimately expanding the therapeutic arsenal against complex diseases such as cancer, cardiovascular disorders, and neurological illnesses. This general-specific-general perspective underscores that while the foundational mechanisms have been well-established, future success lies in integrating detailed molecular insights with technological innovations to achieve safe, effective, and personalized miRNA therapies.