What Is dPEG®

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The term “dPEG®” is Quanta BioDesign’s trademarked acronym for “discrete poly- ethylene glycol)” or “discrete PEG”. Like traditional PEGs, our products contain a hydrophilic, water-soluble backbone consisting of repeating ethylene oxide units; however unlike traditional PEGs each of our products represents a single compound with a unique, specific, single molecular weight (MW). Our dPEG® products are synthesized from pure building blocks (e.g. triethylene glycol or tetraethylene glycol) in a series of step-wise reactions to provide specific MWs, architectures, functional groups, and organic moieties suited for a wide variety of applications. These products have now been used for the modification of therapeutic macromolecules, as linkers in antibody-drug conjugates (ADCs), for bioconjugation of biologics, for surface modification, and in diagnostics, to name a few. 1

Our single MW dPEG® product consists of linear chains of four to forty-eight ethylene oxide units as well as branched structures containing three to nine of these linear chains, thereby providing absolute homogeneity, at this time, in the range of 200 Da to 8 kDa. We produce modifiers, homofunctional crosslinkers, and heterofunctional crosslinkers as well as a number of primary antibody conjugates, biotinylation reagents, and fluorescent tags. Our products contain methoxy, alcohol and carboxylic acid end-capping, various functionalities for conjugation to amines, thiols, alcohols, carboxylic acids, ketones, aldehydes, alkynes, and azides, and a wide variety of organic moieties (i.e. lipoamide, lissamine rhodamine, ICG, etc.). In our numbering system, we name compounds as dPEG®x, where x stands for the number of oxygen atoms in the spacer unit. We do this in order to simplify the naming of the compounds. Our catalog has the exact structure of the single compound. For example, product number 10244, amino-dPEG®4-acid, has an amino group on one end, four (4) ethylene oxide units, and a carboxylic acid group on the other end. Similarly, product number 10885, Fluorescein-5(6)-amido- dPEG®12-NHS ester, has an amine reactive NHS ester on one end, 12 ethylene oxide units, and a fluorescent organic moiety on the other end. 2

Our dPEG® products convey the beneficial properties of traditional PEGs, such as increased water solubility, reduced aggregation, increased hydrodynamic volume, and reduced immunogenicity, but they do so without the complications of polydispersity and allow for the rational design of dPEG® conjugates and analysis of their structure-activity relationships.

Quanta BioDesign strives to protect our clients’ interests by patenting the processes for the production of dPEG® constructs (see US Patents #7,888,536 and 8,637,711) allowing them “freedom to operate”, as well as composition of matter patents on critical dPEG® constructs (see US Patent Pending #2013/0052130). QBD continues to innovate to meet our clients present and future needs.

What is the difference between monodisperse PEGs and discrete PEGs?

Traditionally, PEGs have been prepared by polymerization processes and the final product is actually a Gaussian distribution of sizes and molecular weights. The reported molecular weight is an average molecular weight, and the polydispersity index (PDI) gives an indication of the range of molecular weights in the sample. Polydisperse samples have a PDI > 1 and are a heterogeneous mixture of sizes and molecular weights. Typically, the PDI for PEG is less than 1.2, and although this indicates a broad distribution of molecular weights, it is relatively low when compared to other polymeric materials and most likely reflects issues in the development and application of polydisperse PEGs in therapeutics. 3

Monodisperse PEGs have a PDI = 1 and are typically separated from polymerization mixtures by various modes of chromatography to provide a single chain length and molecular weight, thereby avoiding complications resulting from polydispersity. While there are a few commercially available monodisperse PEGs with up to 28 ethylene oxide units (MW ~ 1250), the vast majority have fewer than 12 units (MW ~ 500). 4,5,6

In our patented production process, dPEG® products are prepared via standard organic chemistry methodology and not polymerization techniques, thereby providing a single molecule with a defined and specified chain length, molecular weight, and purity. 7 This process is routinely applied to provide linear dPEG® products up to 2 kDa and branched dPEG® products up to 8 kDa and is easily scaleable.

Although the terms monodisperse PEG and discrete PEG have overlap in their meanings, there are significant differences that are consequential in drug development and characterization. Describing our dPEG® products as “monodisperse” though not inappropriate has connotations that are not true with respect to our compounds. “Monodisperse” implies that:

1) The compounds are single compounds that were made from a one-pot polymerization process and then purified from a polymeric mixture, which our compounds are not.

2) There is only one compound of uniform functionality, size, and shape formed from a polymeric process, which is purely a theoretical concept.

We prefer the term “discrete” since these compounds are synthesized as single molecular weight compounds from pure starting materials in a step-wise fashion using standard organic methodology, thus the terminology associated with polymer chemistry is not completely accurate. Therefore, a monodisperse PEG is a single compound prepared via a polymerization process and separated from the mixture, while a dPEG® is a single molecule of specified length and molecular weight prepared via defined step-wise reactions.

Do your dPEG® products contain other PEG homologues?

No, as described above, our dPEG® products are prepared using standard organic chemistry techniques for synthesis and purification. The starting materials are of high purity and are used in a series of multi-step reactions to afford the final dPEG® of a specified size, molecular weight, architecture, and functionality that is characterized in the same manner as small molecules. Thus, the only PEG in our dPEG® products is the one described in the name and number and the purity that we report is not based on average molecular weight, but on a specific molecular weight.

Conversely, traditional PEGs prepared via polymerization contain other homologues, and PEG used in the past typically had a PDI of 1.2. 3 Years of research and development have driven this value lower and these days the accepted standard for PEG reagents with molecular weights less than 30 kDa is 1.05, which can be a mixture of 30 products or more. For larger PEGs, acceptable PDIs can range as high as 1.1, and this can be a mixture of over 100 different polymers. 8 As the requirements for the approval of new PEG conjugates become more stringent the trend towards a narrower range of molecular weights is expected to continue. For instance, the first two PEG conjugates brought to market in the early 1990s, Adagen® and Oncaspar®, needed only demonstrate reproducibility of conjugation, but by the time Pegasys® and Peg-Intron® were introduced ten years later the characterization of each isomer was required. 9 Although the conjugation sites of these therapeutics were characterized, they still employed polydisperse PEGs, which gives a population of drug conjugates that undoubtedly have different biological properties. As further research demonstrates the impact the PEG size and shape have on both the chemistry and biology, it can be expected that monodisperse and discrete PEGs will become increasingly important. 8,9 Recently, for instance, the FDA approved an IND for a US company only when they switched from a polydisperse PEG to a dPEG® product to ensure reproducibility of the manufacturing process. The drug is currently in Phase I/II trials for three different indications. 13

Why don’t you have a dPEG® product in the xxxx kDa size range? Or, do you have high molecular weight dPEG® products?

Many of our customers come to us thinking that they need a large PEG in order to improve water solubility, eliminate aggregation, reduce non-specific binding, or impart reduced antigenicity/immunogenicity to their target. The prevailing thought in the bioconjugation community has been that “larger is better”, however this is not always the case and is dependent on the specific application. Some case studies are illustrative.

During the development of Pegasys®, the 19 kDa IFN-a2A was conjugated to both a 5 kDa PEG and a branched 40 kDa PEG. While the 5 kDa conjugate showed little improvement over unmodified IFN-a2A, the 40 kDa conjugate showed substantially modified PK parameters resulting in dramatically improved clinical efficacy. Despite the fact that the mono-PEGylated conjugate only retained 7% of the parent’s in-vitro activity, the improved in-vivo profile provided a blockbuster drug and first-line treatment for hepatitis C. 10

In the case of Pegasys® a large 40 kDa polydisperse PEG was required to elicit the desired response, however Peg-Intron®, a mono-PEGylated IFN-a2B for the treatment of hepatitis C, was able to achieve the desired effect with a linear 12 kDa polydisperse PEG. In this case the conjugate retains 28% of the unmodified IFN-a2B in-vitro activity, and although the increase in systemic exposure is modest when compared to the 40 kDa conjugate, the balance between PK and PD provided another blockbuster drug for the treatment of hepatitis C. 9,10

An example of an even smaller polydisperse PEG conveying the desired properties can be seen with Somavert®, a PEGylated form of the 22 kDa human growth hormone for the treatment of acromegaly. In this case conjugation of four to six 5 kDa polydisperse PEGs resulted in a 28-fold decrease in binding affinity, but this was offset by a 400-fold increase in serum half-life, providing a second-line treatment for acromegaly. 9,10

These examples demonstrate the successes of 40 kDa, 12 kDa, and 5 kDa polydisperse PEGs and illustrate that PEG size alone is not as important as the proper balance of PK and PD in order to achieve the desired clinical effect, as well as reproducibility of manufacturing processes. In fact, while a 5 kDa PEG failed with Pegasys® it provided the desired profile with Somavert®. Even though there still are a number of therapeutics in various stages of development that use polydisperse PEG in the 2-5 kDa range, 10 there are more and more companies focusing their efforts on dPEG® constructs. These intermediate molecular weight range products fall within Quanta BioDesign’s ability to produce discrete molecules, thereby giving researchers the tools to balance the PK and PD of their therapeutics by studying unique conjugates rather than the mix of products found in polydisperse PEGs, each of which may have differing physicochemical and biological properties.

In the areas of bioconjugation, ADCs, diagnostics, and surface modification the focus is not on large PEG size as much as on hydrophilic linkers of defined and consistent lengths. Both hetero and homobifunctional dPEG® linkers with as few as four ethylene oxide units have been shown to impart beneficial properties to the resulting conjugates, and the ability to employ varying dPEG® lengths has allowed the optimization of the desired properties. 3

For example, researchers at Immunogen used hydrophilic likers for the construction of antibody-drug conjugates. They found that dPEG® linkers with as few as four ethylene oxide units were able to provide antibody-maytansinoid conjugates that doubled typical drug-antibody ratios (DARs), were much more potent than lower DARs, did not aggregate, and retained antibody affinity. 11

In another example, researchers at the NIH synthesized a series of molecular beacons for video imaging of protease expression in a matrix metallo-protease-over-expressing tumor-bearing mouse model. In order to achieve true, real-time imaging and superior signal-to-noise ratios a probe must have the proper balance between in-vivo stability and sensitivity. The researchers conjugated a series of dPEG® products (n = 4, 12, 24, 48) to their probe to study this. While no significant in-vitro differences were observed, the dPEG®12 conjugate showed significant in-vivo enhancements including onset of activation, signal-to-noise ratio, and tumor selectivity. This suggests that targeting of specific proteases can be tested and optimized by conjugating low molecular weight dPEG® products to various probes.

While the historical scientific literature demonstrates the utility of high molecular weight PEGs, we do not offer comparable linear dPEG® products (beyond dPEG®48). Negative chemical and physical properties are observed with the larger linear PEGs. We have chosen to circumvent these intrinsic properties of large linear PEGs by synthesizing branched dPEG® constructs. These dPEG® products use shorter chains (4-24 ethylene oxide units), but can contain 3 to 9 branches assembled into a variety of architectures providing our customers with high molecular weight dPEG® products (>8 kD) as discrete single compounds.

What are our Capabilities?

Quanta BioDesign has been making dPEG® products for 12 years, during which time we have greatly expanded our capabilities and product lines. Presently we have capabilities to make branched and linear dPEG® products up to dPEG®48 in gram to multi-kilogram quantities. While we do not operate a cGMP facility we have cGMP manufacturing partners to support both commercial cGMP and non-cGMP production.

The market has hitherto been limited to smaller monodisperse PEGs (~1200 D or smaller) or very large conventional polydisperse PEGs (2,000-3,400 D up to 50,000 D or higher). Even these have not been sufficiently exploited, mostly due to a lack of commercial availability stemming from synthetic and purification challenges, limited functionality of the PEGs, and irreproducibility.

Quanta BioDesign offers a broad range of highly pure dPEG® modifiers, linkers, and spacers ranging in molecular weights from 200 to about 2,300 Daltons for linear compounds and up to 8,000 Daltons for branched compounds. Our dPEG® products are prepared via a robust and highly reproducible process and can incorporate a large range of functionalities. For a complete list of our product offering check our online catalog. We also offer custom syntheses to develop molecules tailored to specific needs, so if you do not see what you are looking for please contact us and allow us to leverage our experience in the field to assist you.

References

  1. Greg T. Hermanson, Bioconjugate Techniques, 2nd Edition, Elsevier Inc., Burlington, MA 01803 (ISBN-13: 978-0-12-370501-3; ISBN-10: 0-12-370501-0) Academic Press. 1996. pp 711-739.
  2. Selective and Specific Preparation of Discrete PEG Compounds. Davis, P.D.; Crapps, E.C. (Quanta BioDesign, Ltd). U.S. Patent 7888536, February 15, 2011.
  3. PEGylation of therapeutic proteins. Jevsevar, S; Kunstejl, M; Porekar, V.G. Biotechnology Journal. 2010, 5(1) pp. 113-128. DOI: 10.1002/biot.200900218.
  4. PEGylation, successful approach to drug delivery. Veronese, F.M.; Pasut, G. Drug Discovery Today. 2005, 10(21) pp. 1451-1458. DOI: 10.1016/S1359-6446(05)03575-0.
  5. The Pharmacology of PEGylation; Balancing PD with PK to Generate Novel Therapeutics. Fishburn, C.S. J. of Pharmaceutical Sciences. 2008, 97(10), pp.4167-4183. DOI: 10.1002/jps.21278.
  6. Synthesis and Evaluation of Hydrophilic Linkers for Antibody–Maytansinoid Conjugates. Zhao, R.Y.; Wilhelm, S.D.; Audette, C.; Jones, G.; Leece, B.A.; Lazar, A.C.; Goldmacher, V.S.; Singh, R.; Kovtun, Y.; Widdison, W.C.; Lambert, J.M.; Chari, R.V.J.J. Medicinal Chemistry. 2011, 54 (10) pp. 3606–3623. DOI: 10.1021/jm2002958.
  7. Zhu, L.; Xie, J.: Swierczewska, M.; Zhang, F.; Quan, Q.; Fang, X.; Kim, K.; Lee, S.; Chen, X.; Real-Time Video Imaging of Protease Expression In Vivo. Theranostics 2011, 1, 18-27. DOI:10.7150/thno/v01p0018.
  8. BioVectra Products; Products page. (accessed July 10, 2011).
  9. Creative PEGWorks; Custom Synthesis of Functional Monodispersed PEG Compunds. (accessed July 10, 2011).
  10. Polypure; Technology, monodispersity. (accessed July 10, 2012).
  11. Pokora, A.; Personal communication, 2011.