Encapsulation efficiency is the percentage of drug that is successfully entrapped into the micelle or nanoparticle.
Encapsulation efficiency (EE%) is calculated by (total drug added – free non-entrapped drug) divided by the total drug added. Loading capacity is the amount of drug loaded per unit weight of the nanoparticle, indicating the percentage of mass of the nanoparticle that is due to the encapsulated drug.
Loading capacity (LC%) can be calculated by the amount of total entrapped drug divided by the total nanoparticle weight. In drug delivery, yield, given as a percent, is a reflection of the amount of drug delivered per amount encapsulated.
Micelles can have limited stability in vitro and in vivo stability due to the dynamic nature of their self-assembly into drug delivery systems; this instability can lead to premature drug release.
There are two main approaches to improve micelle stability. First, the selection of block copolymers can improve micelle stability by using a core-forming hydrophobic block. Second, micelle crosslinking strategies can be used to improve stability. Since drug loading in micelles occurs during the self-assembly, crosslinking strategies occur after micelle formation and drug loading.
Micelles can be permanently crosslinked by amide bond formation, thiol-ene, click chemistry, etcamong others. More recently, reversibly micelle crosslinking has been explored to improve stability and create responsive drug carriers that will release payload at the site of action. For example, pH sensitive or redox sensitive micelles have been used for responsive delivery to tumor sites. Responsive crosslinked micelles can be created by core-crosslinked, shell-crosslinked, or intermediate layer crosslinked micelles, with reversible linkages such as pH sensitive, ketal, acetal, imine, or redox labile linkages such as disulfide.
In addition, the use of enzyme sensitive peptides for crosslinking has been explored. For all types of crosslinking, the polymers composing the micelle need chemical functional groups that facilitate crosslinking, such as end-functional polymers, or side-chain functional polymers. Ideally, the crosslinking strategy chosen should have mild and biocompatible reaction conditions and not require a catalyst, so as to avoid additional purification steps.
Yes, polymeric nanoparticles are currently under pre-clinical evaluation for oral drug delivery. In general, drug delivery of peptides, oligonucleotides, and macromolecules by oral administrations requires overcoming several challenges compared to parenteral delivery.
The stomach and intestine have pH values ranging from 1-8. At low pH, biologics can undergo oxidation or hydrolysis and lose activity. In addition, unprotected drugs can be degraded by enzymes and proteases within the GI tract. Encapsulation of the drug within a polymeric nanocarrier can protect the drug from these obstacles. Another significant obstacle is drug penetration of the mucosal barrier of the intestines.
Nanoparticle surface modification with PEG can help nanoparticles to penetrate the mucosal barrier. Mucoadhesive polymers, such as chitosan, polyacrylic acid, and block copolymers, have been used to improve intestinal absorption. In addition, using using end functionalized polymersend-group functionalized polymers, polymeric nanoparticles can be modified to target cells that facilitate transcytosis.
The ideal nanocarrier size is dependsdependent on application and desired drug loading capacity. In general, nanocarriers should be less than 400 nm to prevent mononuclear phagocyte recognition and clearance by the immune system.
Nanocarrier size influences distribution in vivo, where smaller particles have lower liver uptake. For most these applications, nanocarriers should be below 200 nm, and ideally ≤ 100 nm to benefit from the enhanced permeability and retention (EPR) effect for extravasation into tumors. At sizes of <100nm, there has been reported higher rates of endosytosis and more rapid lymphatic transport have been reported.
It is important to note, that as size decreases, loading capacity also decreases, thus more NPs may be needed for therapeutic efficacy. Besides nanocarrier size, the composition, shape, and surface charge can effect release behavior and accumulation.
The protocol published in the Polymeric Drug Delivery Techniques Guide (page 24 – PNIPAM Drug Delivery Systems) gives rise to a lipid monolayer, not a bilayer, around the PLGA core. The presence of the lipid monolayer has been shown to act as a "fence" to slow down/retard drug release from the PLGA core. However, the purpose of adding the lipid monolayer (lecithin and lipid-PEG) is not to slow down drug release; rather, it is to improve the biocompatibility of the nanoparticle in the bloodstream.
Hydrogels, implants, and carrier technologies are used for extended release of proteins and peptides. Injectable implant systems have been designed to release drugs for several weeks to months. Injectable systems can consist of a polymer-drug solution that precipitates in vivo, or a thermosensitive gel that transitions to a solid gel when injected.
Commonly used polymers for this include block co-polymers of poly(lactic-co-glycolic acid), polylactic acid, and polycaprolactone. Polymer choice selection and molecular weight, controls the drug release and duration of action for the implant. It is important to note that for proteins and peptides, the acidic degradation products of PLGA may lead to instability of the biologic.
For injectable technologies and extended release systems, a significant concern is burst release of the protein or peptide. Burst release can be addressed by changing formulations or polymer selection. For example, PLGA with carboxyl groups can extend protein release. Sustained protein and peptide delivery by encapsulating into mirco and nanoparticles has also been investigated. Particles can protect the peptide or protein from enzymatic degradation and improve half-life time, ; however, the majority of these systems have significant burst release. One strategy to overcome this is a hybrid system that embeds nanoparticles within a gel. For example, drug containing PLGA nanoparticles have been dispersed in injectable hydrogel systems to sustain drug release.
Sigma-Aldrich offers a chemically diverse selection of biodegradable and block copolymers amenable to extended release formulations.
For protein PEGylation, there are two main choices, type of chemistry for conjugation and structure of PEG. The selection of chemistry will determine the site of attachment on a protein and whether PEGylation is random or site-specific.
For example, amine-reactive PEGs are often used because proteins naturally contain available amine groups in their structure and the amines tend to be surface available. In addition, amine-based chemistries have mild reaction conditions. Amine-based PEGylation often results in multiple PEG molecules per protein. However, amine-based PEGylation is non-specific and leads to random conjugation and a heterogeneous population of product with positional isomers. N-terminal amine PEGylation can be a strategy to improve site selectivity by exploiting the lower pKa of the N-terminal amine in proteins and using PEGylation chemistries that work in lower reaction pH. Thiol-based conjugation is more selective due to the limited availability of single cysteine residues in proteins. In addition, site-specific PEGylation can be used with bioorthogonal reaction chemistries. Both thiol and site-specific PEGylation generally require genetic engineering and modification of the protein to include either free thiols or non-natural functional groups.
Besides conjugation chemistry, choice of PEG size and geometry is critical. Large PEG molecules are not easily cleared by the glomerular filtration and may cause tissue vacuolization. Branched PEGs may provide better shielding and improved circulation time.
PEGs with a variety of conjugation chemistries and structures are essential to find the best selection for your drug of interest. Sigma Aldrich offers a wide selection of functionalized PEGs amenable to a variety of conjugation strategies.