Process Mass Intensity (PMI): A Holistic Analysis of Current Peptide Manufacturing Processes Informs Sustainability in Peptide Synthesis

Small molecule therapeutics represent the majority of the FDA-approved drugs. Yet, many attractive targets are poorly tractable by small molecules, generating a need for new therapeutic modalities. Due to their biocompatibility profile and structural versatility, peptide-based therapeutics are a possible solution. Additionally, in the past two decades, advances in peptide design, delivery, formulation, and devices have occurred, making therapeutic peptides an attractive modality. However, peptide manufacturing is often limited to solid-phase peptide synthesis (SPPS), liquid phase peptide synthesis (LPPS), and to a lesser extent hybrid SPPS/LPPS, with SPPS emerging as a predominant platform technology for peptide synthesis. SPPS involves the use of excess solvents and reagents which negatively impact the environment, thus highlighting the need for newer technologies to reduce the environmental footprint. Herein, fourteen American Chemical Society Green Chemistry Institute Pharmaceutical Roundtable (ACS GCIPR) member companies with peptide-based therapeutics in their portfolio have compiled Process Mass Intensity (PMI) metrics to help inform the sustainability efforts in peptide synthesis. This includes PMI assessment on 40 synthetic peptide processes at various development stages in pharma, classified according to the development phase. This is the most comprehensive assessment of synthetic peptide environmental metrics to date. The synthetic peptide manufacturing process was divided into stages (synthesis, purification, isolation) to determine their respective PMI. On average, solid-phase peptide synthesis (SPPS) (PMI ≈ 13,000) does not compare favorably with other modalities such as small molecules (PMI median 168-308) and biopharmaceuticals (PMI ≈ 8300). Thus, the high PMI for peptide synthesis warrants more environmentally friendly processes in peptide manufacturing.

Ultrasensitive and Label-Free Detection of Copper Ions by GHK-Modified Asymmetric Nanochannels

Artificial solid-state nanochannels have garnered considerable attention as promising nanofluidic tools for ion/molecular detection, DNA sequencing, and biomimicry. Recently, nanofluidic devices have emerged as cost-effective detection tools for heavy metal ions by modifying stimuli-responsive materials. In this work, high-purity glycyl-l-histidyl-l-lysine (GHK) peptide is synthesized by using 7-diphenylphosphonooxycoumarin-4-methanol (DPCM) as a protecting group and auxiliary carrier by homogeneous synthesis of photocleavable groups. Subsequently, we developed a GHK-modified asymmetric nanochannel nanofluidic diode by covalently attaching the GHK peptide to the inner surface of the nanochannels. This modification facilitated specific recognition and ultra-trace level detection of Cu2+ ions, achieving a detection limit of 10-15 M. Due to the robust complexing ability between Cu2+ and GHK peptide, the GHK-modified asymmetric nanochannels can form GHK-Cu complexes on the inner surface of nanochannels when Cu2+ passes through the nanochannels. This results in changes of current-potential (I-V) properties, which facilitated Cu2+ detection. Theoretical calculations confirmed the high affinity of the GHK peptide for Cu2+, thereby ensuring excellent Cu2+ selectivity. To evaluate the applicability of our system for detecting Cu2+ in real-world scenarios, we analyzed the concentration of Cu2+ in tap water. The GHK-Cu complexes could be dissociated by adding EDTA to the solution, enabling the regeneration and reuse of this ultrasensitive and label-free Cu2+ detection system using GHK-modified asymmetric multi-nanochannels. We anticipate that the GHK-modified asymmetric nanochannels will find future applications in the label-free detection of Cu2+ in domestic water.

Super silyl-based stable protecting groups for both the C- and N-terminals of peptides: applied as effective hydrophobic tags in liquid-phase peptide synthesis

Tag-assisted liquid-phase peptide synthesis (LPPS) is one of the important processes in peptide synthesis in pharmaceutical discovery. Simple silyl groups have positive effects when incorporated in the tags due to their hydrophobic properties. Super silyl groups contain several simple silyl groups and play an important role in modern aldol reactions. In view of the unique structural architecture and hydrophobic properties of the super silyl groups, herein, two new types of stable super silyl-based groups (tris(trihexylsilyl)silyl group and propargyl super silyl group) were developed as hydrophobic tags to increase the solubility in organic solvents and the reactivity of peptides during LPPS. The tris(trihexylsilyl)silyl group can be installed at the C-terminal of the peptides in ester form and N-terminal in carbamate form for peptide synthesis and it is compatible with hydrogenation conditions (Cbz chemistry) and Fmoc-deprotection conditions (Fmoc chemistry). The propargyl super silyl group is acid-resistant, which is compatible with Boc chemistry. Both tags are complementary to each other. The preparation of these tags requires less steps than previously reported tags. Nelipepimut-S was synthesized successfully with different strategies using these two types of super silyl tags.

A Field Guide to Optimizing Peptoid Synthesis

N-Substituted glycines (peptoids) are a class of peptidomimetic molecules used as materials for health, environmental, and drug delivery applications. Automated solid-phase synthesis is the most widely used approach for preparing polypeptoids, with a range of published protocols and modifications for selected synthetic targets. Simultaneously, emerging solution-phase syntheses are being leveraged to overcome limitations in solid-phase synthesis and access high-molecular weight polypeptoids. This Perspective aims to outline strategies for the optimization of both solid- and solution-phase synthesis, provide technical considerations for robotic synthesizers, and offer an outlook on advances in synthetic methodologies. The solid-phase synthesis sections explore steps for protocol optimization, accessing complex side chains, and adaptation to robotic synthesizers; the sections on solution-phase synthesis cover the selection of initiators, side chain compatibility, and strategies for controlling polymerization efficiency and scale. This text acts as a "field guide" for researchers aiming to leverage the flexibility and adaptability of peptoids in their research.

Liquid-Phase Peptide Synthesis (LPPS): A Third Wave for the Preparation of Peptides

Since the last century, peptides have gained wide acceptance as drugs, with almost 100 already in the market and a large number in the pipeline. In this context, peptide synthesis has grown massively as a stringent field for pharmaceuticals around the globe. Three methodologies, namely, classical solution peptide synthesis (CSPS), solid-phase peptide synthesis (SPPS), and liquid-phase peptide synthesis (LPPS), have made significant contributions to the field. This review provides a comprehensive and integrated vision of LPPS as the third wave for peptide synthesis. LPPS combines the advantages of CSPS and SPPS, where peptide elongation is carried out in solution and the growing peptide chain is supported on a soluble tag, which confers characteristic properties. LPPS protocols allow the large-scale production of peptides and reduce the use of excess reagents and solvents, thus meeting the principles of green chemistry. In this review, tags associated with LPPS are broadly discussed under the following headings: polydisperse polyethylene glycol (PEG), membrane-enhanced peptide synthesis (MEPS), fluorous technology, ionic liquids (ILs), PolyCarbon, hydrophobic polymers, and group-assisted purification (GAP). It also highlights the signature accomplishments of LPPS tags and the limitations of the same.

Silicon-based hydrophobic tags applied in liquid-phase peptide synthesis: protected DRGN-1 and poly alanine chain synthesis

Two types of silicon-based hydrophobic tags, including a siloxy group containing tag and an arylsilyl group containing tag, were developed for applying them in tag-assisted liquid-phase peptide synthesis (Tag LPPS) to synthesize long peptides.

Mercapto-functionalized polyhedral oligomeric silsesquioxane as a soluble support for the synthesis of peptide thioesters

The revival of peptide-based drugs has led to the increasing demand for the development of large-scale synthesis of these complex molecules. To meet this demand, the use of mercapto-functionalized polyhedral oligomeric silsesquioxane (POSS-SH) as a soluble support for the synthesis of a model pentapeptide POSS-thioester is reported. The synthetic process provided a total yield of 62% for the pentapeptide POSS-thioester and the 1H NMR spectra validated the high purity of the products. The successful synthesis of the pentapeptide POSS-thioester with high yield and purity provides a promising way to the scale-up chemical synthesis of peptide thioesters, peptides, peptide amides, cyclic peptides, and even proteins.

Regio- and Diastereoselective Vicinal Aminobromination of Electron Deficient Olefins via Phosphorus-Based GAP Protocol

Chemical synthesis based on Group-Assisted Purification chemistry (GAP) has been prolifically used as a powerful, greener and ecofriendly tool so far. Herein, we report hypervalent iodine (III) mediated regio- and diastereoselective aminobromination of electron-deficient olefins using group-assisted purification (GAP) method. By simply mixing the GAP auxiliary-anchored substrates with TsNH2-NBS as nitrogen/bromine sources and PhI(OAc)2 as a catalyst, a series of vicinal bromoamines with multifunctionalities were obtained in moderate to excellent yields (53-94%). The vicinal bromoamines were obtained without column chromatography and/or recrystallization simply by washing the crude mixtures with cosolvents and thus avoiding wastage of silica, solvents, time, and labor. The GAP auxiliary is recyclable and reusable.

Greening the synthesis of peptide therapeutics: an industrial perspective

Solid-phase peptide synthesis (SPPS) is generally the method of choice for the chemical synthesis of peptides, allowing routine synthesis of virtually any type of peptide sequence, including complex or cyclic peptide products. Importantly, SPPS can be automated and is scalable, which has led to its widespread adoption in the pharmaceutical industry, and a variety of marketed peptide-based drugs are now manufactured using this approach. However, SPPS-based synthetic strategies suffer from a negative environmental footprint mainly due to extensive solvent use. Moreover, most of the solvents used in peptide chemistry are classified as problematic by environmental agencies around the world and will soon need to be replaced, which in recent years has spurred a movement in academia and industry to make peptide synthesis greener. These efforts have been centred around solvent substitution, recycling and reduction, as well as exploring alternative synthetic methods. In this review, we focus on methods pertaining to solvent substitution and reduction with large-scale industrial production in mind, and further outline emerging technologies for peptide synthesis. Specifically, the technical requirements for large-scale manufacturing of peptide therapeutics are addressed.

Greener liquid-phase synthesis and the ACE inhibitory structure-activity relationship of an anti-SARS octapeptide

A high-efficiency strategy for resin-free and large scale liquid phase synthesis of the anti-SARS octapeptide AVLQSGFR is described. Herein, tri(4'-diphenylphosphonyloxylbenzoyl phenyl)phosphate (TDPBP) derivatives were designed as C-terminal supports to aid octapeptide intermediate purification without the need for chromatographic separation. Furthermore, the ACE inhibitory structure-activity relationship (SAR) of the anti-SARS octapeptide and its alanine-scanning analogues was systematically studied by in vitro assay and 3D-QSAR via molecular docking. This paper provides a new strategy for the development of peptide-based drugs. Simultaneously, a study on the ACE inhibition and structure-activity relationship of the anti-SARS octapeptide also lays a foundation for further understanding how the anti-SARS octapeptide acts as an ACE inhibitor.

Synthesis of Tri(4-formylphenyl) Phosphonate Derivatives as Recyclable Triple-Equivalent Supports of Peptide Synthesis

To seek the novel application of organophosphorus compounds, the designed tri(4-formylphenyl) phosphonate (TFP) derivatives were successfully synthesized herein, which were used as C-terminal protecting groups of amino acid or greener triple-equivalent supports in liquid-phase peptide synthesis (LPPS). Through the support-aided precipitation effect of TFP derivatives, the peptide intermediates during peptide synthesis were separated and collected via rapid precipitation and facile filtration without chromatographic purification. Furthermore, the TFP derivative support can be directly recycled for reuse without further regeneration after being sheared from the target peptide.

Liquid-Phase Total Synthesis of Plecanatide Aided by Diphenylphosphinyloxyl Diphenyl Ketone (DDK) Derivatives

Plecanatide is an oral guanylate cyclase-C agonist for the treatment of gastrointestinal disorders. The large-scale supply of plecanatide is restrained primarily by its industrial manufacture. Herein we developed diphenylphosphinyloxyl diphenyl ketone (DDK) derivatives as greener supports with unique precipitation-inducing properties to aid the liquid-phase total synthesis of plecanatide without the use of chromatography. Plecanatide could be obtained in high yield, and the ultimately sheared DDK derivative residue could be directly recycled or regenerated for reuse.