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The impact of new delivery technologies in designing peptide therapies.
The emerging field of peptide and protein therapeutics is responsible for a new therapeutic era. Peptides are attractive therapeutic molecules due to their high specificity and potency. Peptides biodegrade into nontoxic or low toxicity metabolites, with minimal potential for drug–drug interactions and low immunogenicity compared to larger proteins. These advantages are reflected in a regulatory approval rate of more than 20% probability, which is double that of small molecules (1). The average number of new candidates entering clinical evaluation every year has steadily increased from 1.2 per year in the 1970s to 4.6 per year in the 1980s, 9.7 per year in the 1990s, and 16.8 per year in the 2000s (2).
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Peptides were not favored as drug candidates because of their physicochemical characteristics and the necessity for expensive and complicated manufacturing processes. Peptides often have short half-lives (of less than 20 minutes), thereby making chronic administration problematic and costly. Two major technological advances contributed to the industrial acceptability of peptide-drug candidates:
Today, the most important drawback in translating peptides into clinically useful therapies is the lack of adequate oral bioavailability. As the preferred route of administration for medicines is the oral route, and given the lack of patient compliance with therapeutics that require chronic self-intravenous administration, the pharmaceutical industry originally opted to focus its efforts on the development of oral alternatives for peptide-based drugs.
Aikaterini Lalatsa, PhD
Due to the increasing cost of R&D and the decreasing number of approved drugs, new alternative approaches are needed to boost the productivity of the pharmaceutical industry (3). Parenteral administration of peptides is usually painful, and requires sterile manufacturing or aseptic processing of thermally unstable biomacromolecules. Technologies that enable the delivery of biologicals across mucosal barriers such as the gastrointestinal tract (GIT), the nasal mucosa, and the blood-brain barrier (BBB), therefore, offer potential for the development of effective and safe noninvasive biologicals, and can enhance the commercial success of peptide therapeutics.
The major challenge in peptide delivery stems from their low physicochemical and proteolytic stability as well as poor permeation across biological barriers in the absence of a specific transport system, which is due to their hydrophilicity, charge, and high molecular weight (> 500 Da). Peptides routinely violate the majority or all of Lipinski's predictors for good absorption and bioavailability (4).
Following parenteral administration, the peptide drug is subjected to extensive degradation in the bloodstream, often resulting in a short plasma half-life. In addition, the peptide drug is also subjected to metabolism by liver enzymes and clearance by the kidneys (5). Linear peptides possess high conformation flexibility that can result in peptide denaturation and poor targeting to the tissue of interest, which can further result in poor shelf stability.
Traditional drug development of peptides and proteins has relied on parenteral injection of liquid formulations as the fastest and often least expensive route to commercialization. The key drivers for selecting a peptide delivery method for commercial development include patient convenience and compliance, requirement for local or topical delivery, systemic toxicity or other safety issues, as well as market competition. The latter driving force, combined with research efforts, has led to the development of controlled-release technologies for peptide delivery by parenteral routes (e.g., intramuscular or subcutaneous) and prompted the development of technologies for noninvasive peptide delivery. The oral, nasal, and pulmonary approaches are the focus of the pharmaceutical industry while transdermal and ocular technologies are researched because these routes are preferred for achieving local levels able to elicit therapeutic benefit.
Factors that determine the selection and development of an appropriate delivery system and route of administration are the therapeutic dose and release profile required, the duration of treatment, the disease conditions, and target patient population (intravenous injections or infusions for hospitalized patients, and higher patient compliance systems for out-patients). Additional factors include the impact of processing conditions on stability and bioactivity of peptides and proteins to avoid increase in immunogenicity or loss of efficacy, and finally, the bioavailability by means of the particular route and delivery system chosen (6).
Implants, capable of releasing peptides in a controlled manner for a desired length of time, are clinically important systems for prolonged release of proteolytic labile peptides. However, zero-order release kinetics usually achieved with these systems (i.e., ability to deliver a drug at a rate that is independent of time with the concentration of drug within a pharmaceutical dosage form) are not always the best delivery regimes compared with pulsatile systems because down-regulation of receptors can occur.
As an alternative to repeated injections or infusion pumps, depot-delivery systems provide continuous peptide delivery after a single administration, usually with a frequency of once-monthly or three-monthly for chronic conditions. Depot-delivery systems can be divided into four major groups: implants, microspheres, nano-particles, and injectable solutions such as in situ forming gels. As implants necessitate the use of large gauge needles (i.e., 16 gauge) or surgical procedures for administration, they are less patient-preferred (6).
Microspheres followed by in situ forming gels systems have resulted in the majority of approved peptide therapeutics and are prepared from degradable polymers such as polyanhydrides, polyesters usually from poly(lactic-co-glycolic acid), lipids such as Depofoam (Pacira) (7) and Fluid-Crystal (Camurus) (8), or even by the self-assembly of the actual endogenous peptide (e.g., lanreotide acetate [Somatuline Autogel, Ipsen]) (9) and their derivatives (usually with polyethylene glycol, poly(orthoesters), sucrose acetate isobutyrate), collagen, hyaluronic acid, and chitosan (10, 11). Nanoparticulate parenteral delivery, although still in preclinical stage, is showing promise particularly for delivery of peptides across notoriously impermeable barriers, such as the BBB (12, 13), where neuropeptides can prove significant therapies for neurological disorders (e.g., pain, depression, and neurodegenerative disorders).
Advents in injection devices enable self-administration by patients using a small-diameter needle and syringe, such as in the case of insulin. Prefilled syringes, auto-injectors, syringe injectors, pen devices, and needleless injectors contain cartridges loaded with the peptide. With the exception of needleless injectors, no further pharmacokinetic studies are required because these systems result in similar pharmacology and toxicology with equivalent bioavailability (14).
Oral peptide delivery
There are currently only two oral peptide formulations available on the market—desmopressin acetate (DDAVP, Sanofi-Aventis) approved for the treatment of diabetes insipidus, and cyclosporine (Neoral, Novartis) as an immunosuppressant (15). Both are cyclic peptides whose structural features protect them from intestinal proteolytic degradation. In the case of desmopressin, substitution of the last L-arginine by a D-arginine, and deamination of the first amino acid results in an oral bioavailability enhancement of 0.08–0.16% for DDAVP (16). A self-emulsifying delivery system, which forms a cyclosporine microemulsion in the aqueous environment of the GIT results in a bioavailability of 40% for Neoral (17).
The major challenge is enhancing the oral bioavailability of peptides from less than 1% (which is common for peptides) to at least 10–20%, and if possible, to 30–50% (18). The enhanced potency of peptides necessitates only minute amounts to bind to receptors. Whereas for efficacy, the low oral bioavailability requires larger doses to be administered, thereby, increasing develop-ment costs and the costs of therapies, especially if the peptide is larger than 50 amino acids and cannot be easily synthesized using solid-phase peptide synthesis. In such cases, cost constraints on healthcare providers limit their development for life-threatening and unmet diseases (19).
Chemical modification and formulation strategies
Strategies to enhance peptide oral bioavailability can be divided in chemical modification or formulation strategies. Chemical modification can involve substitution of natural amino acids with D-amino acids (20), cyclization (21), engineering peptidomimetics by replacing labile bonds with stable constructs (22), introduction of steric bulk (N-alkylation), or formation of a prodrug (13) to increase lipophilicity or decrease hydrogen bonding to enhance permeability across epithelial cells.
Formulation strategies for enhancing absorption across the GIT or improving peptide stability include co-administration of enzyme inhibitors (23, 24) or absorption enhancers (e.g., low molecular weight surfactants, bile salts, and cyclodextrins), altering the gastrointestinal retention time using mucoadhesive polymers such as chitosans (12, 25), and encapsulating or conjugating the peptide to a suitable lipidic carrier (26) or micro/nanoparticle systems (12, 13). Despite the numerous oral peptide delivery technologies, few have progressed beyond proof of concept to human clinical trials, with most of them designed to enable oral delivery of insulin fuelled by the broad existing market (see Table I). Although the hurdle to commercial development was predicted to be safety, it appears to be study design and ensuring efficacy in humans (11).
Table I: Oral peptide nanomedicines in clinical development.
Nanoparticulate technologies are receiving interest for their ability to enable oral peptide delivery to the brain. The pharmaceutical industry, driven by the medical and clinical success of intravenously administered biologics, is increasingly accepting more complex brain and peptide drug-delivery systems to enter niche treatment markets and address the growing need for brain therapeutics. The translation of a technology for oral peptide delivery to the brain can provide an answer to a therapeutic field with unmet needs.
For oral to brain peptide delivery, the focus has been on delivering endogenous opioid peptides and their analogs for the treatment of neuropathic and chronic pain. The first reported strategy able to deliver peptides orally involved a leucine-enkephalin synthetic analogue (dalargin) encapsulated in polybutylcyanoacrylate nanoparticles overcoated with polysorbate 80 (32), and in some cases, overcoated with polysorbate 80 and polyethylene glycol (20 kDa) (33). However, the technology has not yet progressed into Phase I studies.
On the other hand, Nanomerics has announced that its nanotechnology-enabled peptide pill (METDoloron) involving the molecular-envelope technology (MET) will be moving into Phase I clinical trials within the next two years (34). The technology is based on an engineered amphiphilic chitosan polymer (i.e., quaternary ammonium palmitoyl glycol chitosan) tailored to form nanoscale polymeric aggregates that are able to package or specifically interact (covalently and noncovalently) with peptides (13).
Preclinical studies showed successful delivery of leucine-enkephalin across the BBB with significantly higher pharmaco-kinetic amounts (i.e., a 67% increase in plasma levels [AUC0–24] and a 57% increase in brain maximum concentration [Cmax]). Moreover, significant enhancement of pharmacodynamic activity in a pain animal model was observed (13). Combining the molecular-envelope technology with a prodrug lipidization strategy of leucine-enkephalin potentiated the oral antinociceptive effect, leading to analgesia lasting more than eight hours after oral administration, accompanied with significant enhancements in brain bioavailability (13).
The commercialization of peptides as oral therapies is still deemed risky by the biopharmaceutical industry. However, the reward of niche treatment market areas will fuel the development of a peptide pill enabled by nanotechnology either alone, or combined with chemical modification (lipidization, cyclization) or other formulation strategies (controlled-release polymer coating, permeation enhancers, protease inhibitors).
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Aikaterini Lalatsa, PhD, is a lecturer in pharmaceutics and drug delivery at the School of Pharmacy, University of Hertfordshire, College Lane Campus, Hatfield, Hertfordshire, AL10 9AB, United Kingdom.