Bioconjugation in the Drug Delivery System

Bioconjugation technology represents a cornerstone in modern drug delivery systems, offering precise control over drug localization, release kinetics, and therapeutic outcomes. This technology leverages the specificity of biomolecular interactions to ensure drugs reach their intended targets effectively. With ongoing advancements and interdisciplinary collaborations, bioconjugation is poised to revolutionize therapeutic strategies, paving the way for safer, more effective treatments across a spectrum of diseases and conditions.

Drug Delivery System

A drug delivery system (DDS) refers to the methods, formulations, technologies, and systems used to transport a pharmaceutical compound to achieve its therapeutic effect in humans or animals. The goal of a drug delivery system is to optimize the pharmacokinetics, pharmacodynamics, and bio-distribution of a drug, enhancing its efficacy and safety profile. Drug delivery systems can range from simple oral formulations to complex nanotechnology-based systems.

Impact Factors of the Drug Delivery System

The influencing factors of drug transport mainly in the following aspects:

Physical and Chemical Properties: The factors affecting drug transport are mainly in the following aspects: first, the physicochemical properties of drugs. The solubility, stability, polarity, and molecular weight of a drug directly determine its transport efficiency and distribution range in vivo. For example, low solubility may lead to poor absorption in vivo, while poor stability may lead to drug inactivation during transport.

Characteristics of Biofilm: Biofilms are the main barriers to drug transport, and their permeability, area, and thickness have significant effects on the rate and amount of drug transport.

Metabolism and Excretion: Drug after a series of metabolisms in the body, its structure and activity may change, thus affecting the transshipment and curative effect. At the same time, the elimination of the drug also influences its distribution in the body, the discharge rate could lead to a drug in the body to stay short, difficult to achieve the result of continuous treatment.

Biological Individual Differences: There are differences in physiological structure, metabolic rate, and drug receptor distribution among different individuals, which lead to different drug transport and effects within different individuals.

Types of Bioconjugates in Drug Delivery System

In drug delivery systems, bioconjugation allows for the attachment of drugs to various carriers such as nanoparticles, liposomes, or hydrogels, enhancing their stability, targeting ability, and pharmacokinetic profile. For different applications and purposes, researchers employ specific chemical reactions or biological methods to covalently attach therapeutic molecules to carrier molecules and produce various bioconjugates. Here are some common use types of bioconjugates in drug delivery systems:

Lipid Conjugates

Covalent conjugation to lipid molecules has been used to enhance the delivery of siRNAs and antagomir ASOs. Cholesterol siRNAs (conjugated to the 3' terminus of the passenger strand) have been utilized for hepatic gene silencing (for example, Apolipoprotein B, Apob) and, more recently, to silence myostatin (Mstn) in murine skeletal muscle (a target organ in which it has historically been particularly challenging to achieve effective RNAi) after systemic delivery. Other lipid derivatives have also been exploited to enhance siRNA delivery. For example, siRNAs conjugated to α-tocopherol (vitamin E) were reported to induce potent silencing of Apob in the mouse liver. In this case, the lipid moiety was conjugated to the 5' terminus of the passenger strand of a Dicer substrate siRNA 27/29mer duplex. Upon cellular entry, the siRNA is cleaved by Dicer so as to generate the mature 19 + 2mer active RNAi trigger and to simultaneously cleave off the α-tocopherol. Similarly, siRNAs conjugated to long-chain (>C18) fatty acids via a trans-4-hydroxyprolinol linker attached to the 3' end of the passenger strand were capable of inducing comparable levels of Apob silencing to cholesterol-conjugated siRNAs.

Lipid–siRNA conjugate wherein cholesterol is conjugated to the 3' terminus of the passenger strand. (Roberts T C., et al., 2020)

GalNAc Conjugates

GalNAc is a carbohydrate that binds to the liver-highly expressed sialoglycoprotein receptor 1 (ASGR1, ASPGR) with high affinity (Kd = 2.5nM) and promotes uptake of PO ASOs and siRNAs into hepatocytes via endocytosis. Galnac-conjugated ASOs are preferentially delivered to hepatocytes in vivo, whereas unconjugated ASOs are mainly detected in nonparenchymal hepatocytes. Despite the presence of other structural variants, the triantennary GalNAc structure is commonly used as the conjugated moiety. GalNAc conjugation enhances ASO potency ~ 7-fold in mice, liver-specific, and approximately 30-fold in human patients. Therefore, GalNAc coupling is currently being developed for delivering one of the main strategies of experimental oligonucleotide drugs, because it has high potential liver silence, relative to the small size of nanoparticle composites, the definite chemical composition and the synthesis of low cost.

Triantennary N-acetylgalactosamine (GalNAc) moiety conjugated to an ASO. (Roberts T C., et al., 2020)

Antibody and Aptamer Conjugates

Despite the abundance of technologies to deliver nucleic acids to hepatocytes, strategies that target other tissue-specific cell-surface receptors are still needed. Antibodies have been used as delivery vehicles for other classes of drugs, although their utility for oligonucleotide delivery is still in the early stages of development. Therapeutic oligonucleotides and nucleic acid aptamers have also been explored in order to enhance the combination of siRNA and ASO delivery to specific target cells. Aptamers can be considered to be chemical antibodies, they with high affinity with their target protein, but compared with antibody has many advantages, because of their simple manufacture and low cost (that is, through chemical synthesis), smaller volume, and lower immunogenicity.

Aptamer–siRNA conjugate. In vitro transcription can be used to generate a chimeric aptamer–passenger strand as a single molecule. (Roberts T C., et al., 2020)

Peptide Conjugates

Peptides are an attractive source of ligands that may confer tissue/cell-targeting, cell-penetrating (that is, CPPs) or endosomolytic properties onto therapeutic oligonucleotide conjugates. CPPs (also known as protein transduction domains) are short (typically <30 amino acids) amphipathic or cationic peptide fragments that are typically derived from naturally occurring protein translocation motifs (as in the case of HIV-TAT (transactivator of transcription protein), Penetratin 1 (homeodomain of the Drosophila Antennapedia protein) and Transportan (a chimeric peptide consisting of part of the galanin neuropeptide fused to the wasp venom, mastoparan)) or are based on polymers of basic amino acids (that is, arginine and lysine).

Peptide–ASO conjugate (Roberts T C., et al., 2020)

Nanocarriers

Advances in nanotechnology and materials science offer advantages and potential solutions to the challenges of oligonucleotide drug delivery, particularly the requirements for crossing biological barriers and transmembrane intracellular delivery. Key advantages of nanoparticle delivery systems include customizing optimized nanoparticle biophysical (e.g., size, shape, and chemical/material composition) and biological (e.g., ligand functionalization for targeting) properties, allowing for highly customized delivery platforms.

Nanocarriers preparation methods. (Wang S., et al., 2023)

Advantages of Bioconjugation in Drug Delivery

Bioconjugation technology offers several distinct advantages in drug delivery systems:

Enhanced Targeting: By conjugating drugs with targeting ligands such as antibodies or peptides, bioconjugation facilitates precise delivery to specific cells or tissues, improving therapeutic efficacy and reducing off-target effects.

Improved Pharmacokinetics: Conjugating drugs to carriers like nanoparticles or liposomes enhance their stability in biological fluids, prolongs circulation time, and facilitates controlled release at the target site, optimizing drug bioavailability.

Reduced Toxicity: Targeted delivery minimizes exposure of healthy tissues to therapeutic agents, thereby lowering systemic toxicity and improving patient tolerance to treatment.

Versatility: Bioconjugation can be tailored to accommodate various types of drugs and carriers, making it suitable for a wide range of therapeutic applications from small molecules to biologics.

Customizability: The flexibility of bioconjugation allows for the design of multifunctional drug delivery systems capable of combining diagnostic and therapeutic functionalities in one platform.

Applications in Drug Delivery Systems

Bioconjugation technology finds extensive applications across various drug delivery strategies:

Targeted Drug Delivery

Bioconjugation significantly advances targeted drug delivery by improving the accuracy and effectiveness of treatments. This method involves chemically attaching a drug molecule to a targeting agent, such as an antibody, peptide, or ligand, which specifically binds to receptors or biomarkers on diseased cells. This precise targeting ensures the drug is delivered directly to the desired site, minimizing off-target effects and reducing overall toxicity. As a result, bioconjugation enhances therapeutic outcomes, allows for lower drug dosages, and increases patient safety. This approach is crucial in cancer therapy, where targeted delivery aims to deliver cytotoxic drugs directly to tumor cells. Therefore, bioconjugation is a crucial innovation in developing advanced and personalized drug delivery systems.

Nanoparticle Drug Delivery

Nanoparticle drug delivery systems, enhanced through bioconjugation, revolutionize treatment approaches by combining drugs with nanoparticles like liposomes, dendrimers, or polymeric nanoparticles. These bioconjugated nanoparticles offer improved drug stability, prolonged circulation, and targeted delivery to specific disease sites. For example, in cancer therapy, bioconjugated liposomal nanoparticles can deliver chemotherapy drugs directly to tumor cells while sparing healthy tissues, reducing adverse effects. Similarly, in infectious disease treatment, dendrimer-based nanoparticles can be bioconjugated to deliver antimicrobial agents precisely to infected cells, enhancing therapeutic efficacy. By optimizing drug delivery with bioconjugated nanoparticles, these systems advance medical treatments by maximizing effectiveness and minimizing unintended impacts on the body.

Controlled Drug Delivery

Bioconjugation offers a sophisticated means to enhance therapeutic precision and effectiveness. By chemically linking drugs to biocompatible carriers such as polymers, liposomes, or hydrogels, bioconjugation allows for the controlled and sustained release of therapeutic agents over a specified period. This method ensures that the drug is released at the optimal rate and concentration, maintaining therapeutic levels in the bloodstream or target tissue while minimizing fluctuations that could lead to side effects or reduced efficacy. Additionally, bioconjugation systems can be engineered to respond to specific physiological triggers, such as pH changes or enzymatic activity, for on-demand drug release. This level of control significantly improves patient compliance, reduces dosing frequency, and enhances overall treatment outcomes, making bioconjugation a cornerstone technology in the advancement of controlled drug delivery systems.

Liposome Drug Delivery

Liposomes, lipid-based vesicles, are widely used as drug carriers due to their biocompatibility and ability to encapsulate both hydrophobic and hydrophilic drugs. Bioconjugation enhances liposomal stability and facilitates targeted delivery, making them effective in delivering chemotherapy agents and vaccines. By tailoring liposomes through bioconjugation, researchers can also engineer controlled-release mechanisms, optimizing drug delivery schedules and improving patient outcomes. This innovative technique underscores bioconjugation's pivotal role in advancing targeted and personalized medicine strategies.

Hydrogel Drug Delivery

Hydrogel drug delivery systems exploit the properties of hydrophilic polymer networks to achieve localized and sustained drug release. Bioconjugation plays a crucial role by facilitating the incorporation of drugs into hydrogel matrices through chemical linkage or encapsulation, enabling controlled release over extended periods at specific sites. This approach is particularly advantageous in wound healing and tissue engineering applications, where bioconjugated hydrogels can deliver therapeutic agents directly to targeted tissues, promoting regeneration while minimizing systemic side effects. Stimuli-responsive designs further enhance versatility by enabling tailored release profiles in response to environmental cues, advancing the potential for precision medicine and therapeutic efficacy in biomedical contexts.

RNA Delivery

Bioconjugation plays a pivotal role in advancing the applications of RNA-based therapeutics, particularly in gene silencing and protein expression modulation. Small interfering RNA (siRNA) can be conjugated with lipid nanoparticles or polymer carriers to achieve targeted delivery to specific cells, where it can effectively silence disease-related genes by interfering with their expression. This approach is critical in treating conditions such as genetic disorders and viral infections. Conversely, messenger RNA (mRNA) can be bioconjugated to enhance its stability and delivery efficiency, enabling it to encode therapeutic proteins within target cells. This capability is transformative for developing vaccines and protein replacement therapies. By harnessing bioconjugation techniques, researchers can optimize the delivery of RNA-based therapeutics, thereby improving treatment outcomes while minimizing systemic side effects.

Future Perspectives

Looking ahead, bioconjugation technology in drug delivery systems is poised for significant advancements:

Technological Innovations: Continued research into novel conjugation methods and carrier materials will enhance the efficiency and versatility of drug delivery systems, catering to diverse therapeutic needs.

Emerging Market Demands: Increasing demand for personalized medicine and targeted therapies will drive the development of bioconjugation strategies that allow for precise customization of drug delivery systems based on individual patient profiles.

Interdisciplinary Collaborations: Collaborations between biochemists, material scientists, and clinicians will foster interdisciplinary approaches to address complex challenges in drug delivery, leading to synergistic innovations and clinical translations.