With strong growth in biologics, large molecules, and biopharmaceutical therapeutics in recent years, the pharmaceutical and biotech industries are increasingly turning toward peptides and proteins in their search for drug discovery targets. While both offer significant therapeutic potential, there are fundamental differences between the two types of molecule.

Definitions: Peptides are short polymers formed from the linking of (usually ≤100) amino acids. They comprise some of the most basic components of human biological processes, including enzymes and hormones. The link between one amino acid residue and the next is known as a peptide bond or an amide bond — formed when a carboxyl group reacts with an amine group of an adjacent residue — giving this class of chemicals its name.

Proteins, by contrast, are longer chains of (>100) amino acids similarly linked by peptide bonds. They play a critical role in biochemical reactions within cells. Proteins are ubiquitous in cellular chemistry and structure and are crucial for carrying out most biological functions of living organisms. Scientists follow various conventions to determine the distinction between peptides and proteins. Generally speaking, however, peptide chains are short and proteins are long.

Applications and Markets

Driving the therapeutic implementation of proteins and peptides is the Human Genome Project, which led to the initial sequencing of DNA to identify ~20,000–25,000 genes of the human genome from both a physical and functional standpoint. Developments in manufacturing — including transgenic, recombinant (rDNA), and synthetic methods — have been essential as protein and peptide drugs move into the mainstream. These molecules (especially antibodies) are attractive therapeutics because of their high specificity and potency and low incidence of toxicity.

A recent report by market and technology research firm Frost and Sullivan indicated that >40 approved peptide-based drugs are in use today, and ~800 are being developed to treat allergies and cancer as well as Alzheimer's, Huntington's, and Parkinson's diseases (1). The market for protein-based drugs is also promising. BCC Research indicated in an October 2008 study that the global market for protein therapeutics was worth US$86.8 billion in 2007 and an estimated $95.2 billion in 2008 (2). This is expected to reach $160.1 billion in 2013 for a compound annual growth rate (CAGR) of 10.9%.

A great deal of protein/peptide research is driven by their unique requirements especially with regard to drug delivery. Many life science companies are embracing new approaches to provide formulations that are stable, have effective bioavailability, and enable sound manufacturing. Parenteral, nasal, and controlled-release delivery technologies have evolved to better deliver these medicines. Likewise, strides are being made in such areas as oral, transdermal, pulsatile, and on-demand delivery of peptides and proteins.

Peptides typically offer low toxicity and high specificity, and they demonstrate fewer toxicology issues than other small-molecule drugs. In many cases, those attributes lead to the development of therapies that would be otherwise difficult to commercialize. Protein drugs have received enormous attention from pharmaceutical companies because of their bioreactivity, specificity, safety, and overall success rate. Some improvements are yet to be made, especially with respect to costly production and formulation and delivery methods. Advances in protein drug delivery are expanding many drug markets and increasing patient compliance.

Manufacturing Techniques

Peptides are manufactured through three distinct techniques: solid phase synthesis, solution phase synthesis, and, and a combination of both. Each has unique applications, and their implementation can greatly affect the cost and scalability of pharmaceuticals that incorporate their respective peptides. Liquid- or solution-based peptide synthesis is the older method, but most laboratories use solid-phase synthesis (Figure 1) today. The former method is better for shorter peptide chains and still useful in large-scale production (>100 kg).

Solid-phase synthesis allows for an innate mixing of natural peptides that are difficult to express in bacteria. It can incorporate amino acids that do not occur naturally and modify peptide/protein backbones. In this method, amino acids build peptides by attaching to polymer beads suspended in a solution. They remain attached to those beads until cleaved by a reagent such as trifluoroacetic acid, which immobilizes a peptide during synthesis so it can be captured by filtration. Liquid-phase reagents and by-products are simply flushed away. The benefits of solid-phase synthesis include faster production and easy scale-up because it is a relatively simple process. It is also more suitable to longer amino acid sequences than solution-phase synthesis.

There are two different solid-phase methods: tert-butoxycarbonyl (t-Boc) and 9H-f luoren-9-ylmethoxycarbonyl (Fmoc). T-Boc is the original method, which uses an acidic condition to remove Boc from a growing peptide chain. This requires using small quantities of hydrofluoric acid, which is generally regarded as safe for regulatory purposes, and specialized equipment. The T-Boc method is preferred for complex syntheses and when synthesizing unnatural peptides.

Fmoc (Figure 1) was pioneered later and makes cleaving peptides uncomplicated. It is also easier to hydrolyze a peptide from the Fmoc resin using a weaker acid, which eliminates the need for specialized equipment. Both methods are valuable, and each suits specific applications. However, Fmoc is more widely used because it eliminates the need for hydrofluoric acid.