Limitations of Peptide Drugs
But, enthusiasm for peptide therapeutics was subsequently tempered by certain limitations of native peptides, such as short plasma half-life and negligible oral bioavailability. The short half-life of many peptide hormones is explained by the presence of numerous peptidases and excretory mechanisms that inactivate and clear peptides. This lability allows the body to rapidly modulate hormone levels to maintain homeostasis but is nonetheless inconvenient for many therapeutic development projects.
Another obstacle for peptide drug development is oral bioavailability: digestive enzymes designed to break down amide bonds of ingested proteins are effective at cleaving the same bonds in peptide hormones, and the high polarity and molecular weight of peptides severely limit intestinal permeability. As oral delivery is often viewed as attractive for supporting patient compliance, the need for injection made peptides a less appealing option for indications that required chronic, outpatient therapy. (In September 2019, FDA approved the first oral peptide drug Semaglutide tablets.)
Modification and Performance
Conjugation has emerged as a popular mechanism to alter or enhance the properties of peptide and protein drug candidates. Chemical modification of the peptide using polyethylene glycol (PEG) can improve multiple physiochemical and pharmacokinetic performance with minimal increase in manufacturing cost. PEG is a highly investigated polymer that is used in covalent modification of biopolymers as proteins and peptides. It is incorporated into the manufacturing process of the bulk API in a technique known as PEGylation. The effects of PEGylation on peptide pharmacokinetics include avoidance of reticuloendothelial (RES) clearance, mitigation of immunogenicity, and reduction of enzymatic proteolysis and of losses by renal filtration, with potentially beneficial changes in biodistribution. These effects can dramatically increase the half-life of a peptide in vivo, with potential collateral improvement in bioavailability but without adversely affecting binding and activity of the peptide ligand.
Peginesatide is a synthetic peptide, attached to polyethylene glycol ("PEGylated"). It was approved by the U.S. Food and Drug Administration for the treatment of anemia associated with chronic kidney disease (CKD) in adult patients on dialysis. But, On June 16, 2014, Affymax and Takeda issued a press release stating that Takeda will work with the FDA to withdraw the peginesatide New Drug Application.
| Peptide name | Conjugated moiety | Rationale for conjugation | Current status |
| Peginesatide | PEG | Half-life extension | Approved, then withdrawn |
PEG's most common form is a linear or branched polyether with terminal hydroxyl groups synthesized by anionic ring-opening polymerization - HO-(CH2CH2O)n-CH2CH2 -OH.
Monofunctional methoxy-PEG (mPEG) is preferred for peptide modification - CH3O-(CH2CH2O)n-CH2CH2 -OH, as it can be derivatized with a number of linkage moieties, yielding methoxyPEG-amines, -maleimides, or -carboxylic acids (Fig. 2).
Four general factors affect the performance of PEGylated peptides:
1. Molecular weight and structure - whereas PEGs of <1,000 Da can be broken down into subunits that can have some toxicity, PEGs of >1,000 Da have not demonstrated any toxicity in vivo. PEGs of up to 40-50,000 Da have been used in clinical and approved pharmaceutical applications.
2. Number of PEG chains - two or more lower-weight chains can be added to increase the total molecular weight of the PEG complex
3. Site of attachment - for each peptide, the location of the PEGylation sites has to be carefully engineered experimentally to retain the highest possible binding efficiency and activity of the peptide ligand.
4. PEGylation chemistry - the type of linkage for attaching PEG to the peptide as well as the purity of raw materials, intermediates and final product.
The latter is the most important factor determining the yield of the PEGylation process and the scalability of the manufacturing protocol. Peptide and linker have to be very pure and very stable during the conjugation reaction to yield a pure conjugate with high efficiency.
Biochempeg offers PEGylation as a cost-effective modification of peptides which has the potential to improve bioavailability compared to the unmodified molecule.
Related Article:
Types of PEGylation of Peptides