Amino Acids for Drug Delivery

Drug delivery technologies are crucial for the therapeutic application of drugs and have driven the successful use of numerous drugs in disease treatment. These technologies improve specific drug delivery, off-target effects and patient compliance. As therapeutics have moved from small molecules to nucleic acids, peptides, proteins and antibodies, drug delivery systems have continuously developed to meet new problems. And, of these, amino acids — due to their distinct biology, including excellent biocompatibility, solubleness and receptor-targeting properties — have become critical parts of drug delivery systems.

What is Drug Delivery?

Drug delivery is a means of getting the active pharmaceutical ingredients to specified parts of the body for optimal therapeutic effects. With the help of a drug delivery system, drugs can be more bioavailable, effective, less side-effect ridden, and with better therapeutic plans. The core purpose of drug delivery is to regulate release rate, location and timing of drugs with specialized systems, and maximize the efficacy and compliance of patients.

Fig. 1. Amino acids and their pharmaceutical applications (International Journal of Pharmaceutics. 2022, 613: 121375).

Drug transport in the body is intricate and can be subject to many influences that degrade drug performance. Specifically, the factors influencing drug transport can be summarized as follows: First, the physicochemical properties of the drug play a crucial role. Solubility, stability, polarity and molecular weight directly affect how effectively and where a drug will be transported in the body. For example, too low a solubility can cause bad absorption and too low a stability can make the drug useless on delivery. Second, the composition of biological membranes – the barrier that makes the drug flow possible – can dramatically influence the rate and quantity of drug flow, depending on how porous, how thick and how large they are. Third, how drugs are metabolised and discarded in the body has implications for how they get around and work. Structure and activity changes during metabolism could affect drug transport and therapeutic action, and rapid elimination can shorten the residence time of the drug and reduce the therapeutic effect. Finally, individual biological differences, such as variations in physiological structure, metabolic rate, and receptor distribution, contribute to differences in drug transport and efficacy across individuals.

Drug Delivery Systems

A drug delivery system (DDS) refers to a technological framework that comprehensively regulates the spatial, temporal, and dosage distribution of drugs within the body. Its purpose is to deliver the appropriate amount of medication to the correct location at the right time, thereby increasing drug utilization efficiency, enhancing therapeutic effects, reducing costs, and minimizing side effects. DDS is an interdisciplinary field combining medicine, engineering (materials, mechanics, electronics), and pharmacy. Its scope includes not only the drugs themselves and the carrier materials or devices used to deliver them but also technologies for the physical and chemical modification or functionalization of drugs or carriers. Traditional drug delivery methods typically involve oral administration, injections, or topical application to introduce drugs into the body. However, with advancements in modern technology, novel delivery systems such as targeted delivery, controlled-release systems, and nano-drug carriers have been extensively studied and applied in clinical treatments. An effective DDS provides the following advantages:

• Improved bioavailability: Optimizing processes such as absorption, distribution, metabolism, and excretion to enhance the effective concentration of drugs.
• Targeted delivery: Directing drugs to specific tissues or cells, increasing therapeutic efficacy, and reducing side effects in non-target regions.
• Extended drug action: Utilizing controlled-release technology to prolong the drug's half-life in the body for sustained therapeutic effects.
• Enhanced patient compliance: Reducing dosing frequency and discomfort to improve adherence to treatment plans.

Drug Delivery Methods

From a research and development perspective, current drug delivery technologies can be broadly categorized into three main approaches: drug modification, microenvironmental regulation, and drug delivery systems and devices.

• Drug Modification

Drug modification refers to the chemical or biological alteration of drug molecules. By regulating interactions between the drug and molecules, cells, or tissues within the body, as well as its interaction with specific targets, drug modification optimizes pharmacokinetic properties and therapeutic effects. This approach is applicable to improving the delivery of all types of drugs, including small molecules, peptides, proteins, nucleic acids, and cells. Common methods of drug modification include: first, covalent modification, which involves linking different molecular fragments via covalent bonds to enhance drug properties—for example, attaching hydrophilic groups such as polyethylene glycol to hydrophobic drugs to increase water solubility; second, conjugation modification, where drugs are coupled with targeting molecules like antibodies, peptides, or aptamers to enable specific recognition and binding to target cells or tissues, thereby increasing drug concentration at the target site and reducing off-target distribution; third, prodrug modification, which connects active drug molecules to inactive carriers via chemical bonds, allowing the active drug to be released through specific enzymatic or hydrolytic processes for controlled release and targeted delivery; and fourth, glycosylation modification, where sugar molecules are attached to drugs to improve solubility, stability, and immunogenicity, while also enhancing interactions with cell surfaces and increasing cellular penetration.

• Microenvironmental Regulation

Microenvironmental regulation modifies the local environment within the body to enhance drug delivery. On one hand, it can adjust the absorption environment, such as altering the pH of the gastrointestinal tract to improve drug solubility. This can be achieved using excipients and additives, as seen in Pfizer's COVID-19 drug PAXLOVID, which includes a metabolic inhibitor to reduce liver metabolism and increase drug bioavailability. On the other hand, microenvironmental regulation can address barriers to drug action. For instance, using modified pH regulators can inhibit protein hydrolysis, thereby enhancing the stability of proteins and peptides in physiological fluids.

• Drug Delivery Systems and Devices

Drug delivery systems and devices achieve controlled drug release by creating barriers between the drug and the internal environment of the body. Based on the route of administration, these systems and devices can be classified into oral, transdermal, implantable, inhalable, and injectable types. The primary mechanism involves encapsulating the drug with carriers made from natural materials such as lipids, proteins, and viruses, or synthetic materials like polymers, inorganic salts, and silicone. Drug release is regulated through processes such as dissolution, diffusion, permeation, ion exchange, and degradation of the carrier material. Drug delivery carriers represent a cutting-edge direction in current delivery systems and are categorized into natural and artificial carriers. Natural carriers include extracellular vesicles (e.g., exosomes, microvesicles), viruses, and cells (e.g., red blood cells, immune cells), while artificial carriers include liposomes, polymer carriers, micelles, and other nanoparticles.

What is an Amino Acid?

Amino acids play dual roles in life processes, serving as the structural foundation of matter and maintaining biological activity. As the primary components of various functional substances such as peptides, proteins, non-ribosomal peptides (NRPs), and polyketides (PKs), amino acids exhibit diverse physicochemical properties, including crosslinking potential, chirality, and charge characteristics. Furthermore, self-assembled nanocomposites derived from amino acids have demonstrated exceptional pharmacokinetic properties and remarkable drug delivery performance. As a result, the research and development of amino acid-based materials with drug delivery capabilities (including peptides, proteins, NRPs, and PKs) has become one of the most prominent focuses in the field of drug delivery.

Amino Acids Ingredients

BOC Sciences is dedicated to providing high-quality amino acid products for industries such as pharmaceuticals, cosmetics, food, and biotechnology. With an advanced synthesis platform, we offer custom production of both natural and non-natural amino acids to meet diverse customer needs. In the drug delivery domain, BOC Sciences delivers precise amino acid synthesis services, including specialized amino acids and derivatives designed for drug modification, targeted delivery, and enhanced drug stability. Our services range from molecular design to large-scale production, supporting both new drug development and commercial manufacturing.

Amino Acid Materials in Drug Delivery

Amino acids, with their unique chemical properties and wide applicability, have emerged as essential tools for improving drug bioavailability. They not only enhance the solubility and permeability of drugs but also open new avenues for drug delivery by modulating gene expression and cellular uptake pathways through pairing with drug molecules.

• Enhancing Solubility and Absorption

Amino acids demonstrate significant potential in drug delivery, particularly in improving the solubility, dissolution rate, and bioavailability of poorly soluble drugs. Combining drugs with highly water-soluble amino acids has become a critical strategy for enhancing the pharmacokinetics of low-solubility drugs. For example, lyophilization of indomethacin with arginine (ARG) or lysine (LYS) significantly improves its dissolution rate. Similarly, conjugating insulin with basic amino acids such as L-LYS, L-HIS, and L-ARG enhances its water solubility, thereby improving the oral absorption of peptide drugs. Amino acids are also commonly used as salt-forming agents to optimize the physicochemical and biological properties of drugs. For instance, forming a highly soluble salt with L-ARG and racemic ibuprofen substantially improves the oral pharmacokinetics of ibuprofen. In ternary systems, amino acids have demonstrated notable effects, such as improving the solubility and bioavailability of drugs like naproxen, furosemide, and rifampin. Compared to cyclodextrins, amino acids also possess a distinct advantage in enhancing transmembrane drug permeability. These studies highlight the broad prospects of amino acids in drug delivery as ideal excipients for optimizing drug performance.

• Improving Drug Stability

As coformers in cocrystals and coamorphous systems, amino acids enhance drug solubility, bioavailability, and physical stability through hydrogen bonding or electrostatic interactions. Leveraging their zwitterionic properties, amino acids can form stable cocrystals with drug molecules, increasing storage stability and reducing sensitivity to humidity. Developing coamorphous systems by combining drugs with amino acids significantly lowers the risk of recrystallization and extends the stability of the drug. Specific amino acids, such as proline and lysine, are often chosen as coformers due to their unique structures and interaction capabilities. For example, proline is an effective cocrystal coformer for diclofenac potassium, markedly enhancing its stability and moisture resistance. The selection of amino acids should align with the drug's properties, such as acidity or basicity, to further optimize performance. The application of amino acids in drug delivery not only improves drug efficacy but also offers innovative strategies for developing safer and more effective drug formulations.

• Enhancing Drug Permeability

Amino acids also exhibit significant potential in improving drug permeability and bioavailability. The absorption of oral drugs depends on their solubility in the gastrointestinal tract and permeability across biological membranes, making permeability a critical parameter for evaluating drug absorption. Forming prodrugs with amino acids can substantially improve the absorption of low-permeability drugs. For instance, amino acids enhance the transmembrane capacity of certain drugs in Caco-2 cell models, increasing their oral bioavailability. Salt formation is another widely used approach to improve solubility and permeability, as seen in trimethoprim salts prepared with aspartic acid (ASP) and glutamic acid (GLU). Additionally, amino acids can form neutral complexes via ion-pairing mechanisms, enhancing drug lipophilicity and promoting passive diffusion across cell membranes. This mechanism has achieved breakthroughs in insulin delivery, where amino acids such as lysine (LYS) and histidine (HIS) enhance insulin permeability through ionizable side chains. Optimizing amino acid concentrations can further improve drug delivery efficiency, providing new directions for oral administration and other drug delivery systems.

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