Peptide drug pipeline: stapled, cyclic & CPP conjugates

Why Protein-Protein Interactions Demand a New Peptide Toolkit

Over half a million protein-protein interaction (PPI) dysregulations have been implicated in pathological events, yet these targets have historically resisted both small-molecule and biologic intervention due to their flat, featureless, and often intracellular interfaces. Antibodies cannot cross the cell membrane to engage intracellular targets, creating a central imperative for membrane-permeable peptide modalities that can reach this vast, underexploited target space.

The convergence of PPI biology, intracellular target accessibility, and advanced peptide chemistry has catalyzed a renaissance in peptide therapeutics. Three principal advanced peptide classes—hydrocarbon-stapled peptides, cyclic/macrocyclic peptides, and cell-penetrating peptide (CPP) conjugates—have emerged as the primary structural strategies for accessing this space. The dataset underpinning this analysis spans publications from 2006 to 2023 and patents from 1993 to 2024, drawing on targeted searches across mechanism, disease application, and assignee dimensions.

The molecular targets recurrently identified across this literature include Bcl-2 family members, the p53-MDM2 axis, estrogen receptor alpha (ERα), the CTLA-4/B7-1 immune checkpoint, integrin-related receptors, and the P-glycoprotein efflux pump. Each represents a class of interaction that neither small molecules nor antibodies have been able to address efficiently—and each is now the subject of active patent and academic investigation. According to WIPO, peptide-based therapeutics represent one of the fastest-growing segments of the global biopharmaceutical patent landscape.

Stapled Peptides: Locking Helices to Unlock Intracellular Targets

Stapled peptides achieve their therapeutic effect by introducing a hydrocarbon crosslink between the side chains of two non-natural amino acids at defined i, i+4 or i, i+7 positions within an α-helix, locking the peptide into its bioactive conformation and simultaneously improving target affinity, proteolytic stability, and cell membrane permeability relative to linear counterparts. The p53-MDM2 axis and Bcl-2 family members represent the primary proof-of-concept systems for this platform, with stapled α-helical peptides recapitulating the BH3 helix of pro-apoptotic proteins and the transactivation domain of p53.

A key translational signal in this dataset comes from a 2020 review by UCB Pharma Ltd., which explicitly states that a number of stapled peptide drug candidates have recently entered clinical trials—though specific trial identifiers are not enumerated in the available abstract text. The commercial patent landscape is also active: Sutura Therapeutics Ltd. filed an Australian patent covering drug-carrying cell-penetrating molecules (DCCPMs) where the CPP component is stabilized by stapling or stitching via non-olefin-metathesis crosslinks.

“A number of stapled peptide drug candidates have recently entered clinical trials”—a signal from UCB Pharma Ltd.’s 2020 review that the platform has crossed from academic proof-of-concept into translational medicine.

Perhaps the most striking combination signal in this dataset is the stPERML-R7 construct, which integrates three functional elements into a single peptide-based ERα degrader: a helix-stabilizing hydrocarbon staple, a hepta-arginine CPP motif, and an IAP-recruiting small molecule (LCL161). Reported by the National Institute of Health Sciences, Japan in 2021, this PROTAC-type degrader demonstrates how stapling can be integrated with CPP motifs and small-molecule warheads to create multi-functional constructs targeting nuclear receptors and oncogenic transcription factors.

Macrocyclization of all-D amino acid linear peptides to confer cellular permeability on otherwise membrane-impermeant scaffolds is also highlighted in this dataset, pointing to the breadth of structural strategies being explored beyond the canonical all-L hydrocarbon staple. The dataset spans publications from multiple institutions including the University of Groningen, which provided a foundational review of the stapled peptide PPI inhibition framework.

Cyclic and Macrocyclic Peptides: From 40+ Approved Drugs to Emerging Scaffolds

Cyclic peptides represent the most clinically validated class of advanced peptide therapeutics: more than 40 cyclic peptide drugs are currently on the market, with a 2022 mini-review from the Shanghai Institute of Materia Medica documenting approvals between 2001 and 2021. Cyclization—whether head-to-tail, sidechain-to-backbone, or sidechain-to-sidechain—reduces conformational entropy, improves protease resistance, and can enhance membrane permeability relative to linear counterparts.

The FDA approval of plecanatide in 2017 for chronic idiopathic constipation is explicitly cited as a recent market entrant, underscoring that the cyclic peptide drug class continues to generate commercially successful therapeutics. Beyond conventional monocyclic scaffolds, bicyclic and tricyclic architectures are emerging as a distinct direction: a multivalent tricyclic peptide assembled from cyclic c[WR]4 and c[WR]5 monomer building blocks has been described as a nuclear-targeting molecular transporter, reported by Chapman University School of Pharmacy in 2020.

Cyclotides—plant-derived cyclic cystine-knotted peptides—receive dedicated attention in this dataset as ultra-stable scaffolds with inherent cell-penetrating properties. According to researchers at Nature-published studies on natural cyclic scaffolds, the cystine-knot motif confers exceptional resistance to thermal, chemical, and proteolytic degradation. One cyclotide is reported in clinical trials for multiple sclerosis, with confirmed oral activity in mice—a combination of properties rarely achieved in the peptide therapeutic space.

On the patent side, Chugai Pharmaceutical Co., Ltd. filed an active EP patent on novel peptide-compound cyclization methods aimed at discovering drugs for “tough targets,” while Yissum Research and Development Company of the Hebrew University of Jerusalem holds an active EP patent (2024) on N-methylated cyclic RGD hexapeptides with intestinal permeability properties for integrin-related diseases. N-methylation is explicitly linked to improved intestinal permeability approaching passive transcellular markers—a critical property for any oral peptide drug candidate. Standards bodies such as ISO have increasingly engaged with peptide characterization methodologies as the field has matured.

Cell-Penetrating Peptide Conjugates: Delivering the Previously Undeliverable

Cell-penetrating peptide (CPP) conjugates address the fundamental membrane impermeability of otherwise potent therapeutic payloads—including small molecules, proteins, and oligonucleotides—by covalently or non-covalently linking them to short (7–30 amino acid) cationic or amphipathic peptides that translocate across cell membranes via endocytic and direct translocation pathways. The ConjuPepDB database, cited by Semmelweis University (2020), covers more than 1,600 peptide–drug conjugates, evidencing the substantial scale of this field.

Two broad conjugate classes are identified in this dataset. First, CPP–drug conjugates (PDCs), where the CPP directs cytotoxic or therapeutic payloads to target cells: a study from the University of Cologne describes PDCs incorporating sC18 CPP variants conjugated to doxorubicin via thiol–Michael addition, identifying sC18ΔE as an optimized vector with superior cellular uptake. Second, CPP–oligonucleotide conjugates for intracellular nucleic acid delivery: the Florey Institute of Neuroscience and Mental Health at the University of Melbourne describes CPP–ASO conjugates using native chemical ligation (NCL) as a high-efficiency, native-bond-forming conjugation strategy, using ApoE(133–150) as the CPP scaffold.

The patent landscape reflects growing commercial interest. Ruprecht-Karls-Universität Heidelberg holds an active EP patent (2019) on protein–multivalent CPP conjugates comprising at least two CPP units covalently attached to a protein, for diagnostic and therapeutic applications. Sutura Therapeutics Ltd. (UK) filed an Australian patent covering drug-carrying cell-penetrating molecules where the CPP is stabilized by stapling or stitching. Two Israeli patents from Aventisub LLC cover foundational membrane-penetrating peptide delivery systems for intracellular delivery in in vivo, ex vivo, and in vitro settings. Regulatory agencies including the FDA have issued guidance on peptide conjugate characterization as the modality has advanced toward clinical stages.

Antimicrobial applications extend the CPP conjugate paradigm beyond oncology: BicycleTx Limited co-authored 2022 literature on CPP–antibiotic vector-cargo conjugates, linking membrane-active peptide translocation mechanisms to conjugate antibiotic design. Phylogica Pty Ltd. reports successful in vivo delivery of an oligonucleotide therapeutic fused to a Phylomer CPP—representing IND-enabling-level in vivo data for the CPP platform approach.

Convergent Strategies and the Next Design Paradigm

The most significant translational signal in this dataset is not any single modality in isolation but the convergence of stapling, CPP motifs, and small-molecule warheads into unified constructs—a design paradigm exemplified by the stPERML-R7 ERα degrader and pointing toward a new generation of multi-functional peptide therapeutics. Three principal convergent strategies emerge from the evidence.

Stapled peptide + CPP + small molecule (PROTAC-type degraders)

The stPERML-R7 construct integrates a helix-stabilizing hydrocarbon staple, a hepta-arginine CPP motif, and an IAP-recruiting small molecule (LCL161) into a single peptide-based ERα degrader. Reported by the National Institute of Health Sciences, Japan (2021), this construct demonstrates how stapling can be integrated with CPP motifs and small-molecule warheads to create multi-functional degraders. Increased α-helicity was confirmed while retaining ERα binding affinity—a design paradigm that may extend to other nuclear receptor and oncogenic transcription factor targets.

Cyclic peptide → peptidomimetic small molecule pipeline

PRISM BioLab Co., Ltd. (Japan) describes a two-step rational design strategy for the CTLA-4/B7-1 checkpoint axis: identify inhibitory cyclic peptides via screening, then translate the pharmacophore into orally available small-molecule peptidomimetics with improved oral availability and cell permeability. This scaffold-hopping pipeline concept directly addresses the oral bioavailability limitation of macrocyclic peptides, potentially bridging the gap between peptide-derived pharmacophores and small-molecule drug candidates.

CPP + ASO via native chemical ligation

The NCL approach to CPP–ASO conjugation, described by the Florey Institute / University of Melbourne, is highlighted as a high-efficiency, native-bond-forming strategy for modular trifunctional conjugate synthesis. The platform enables precise attachment of CPP, oligonucleotide, and additional functional elements—a modular architecture suited to the diversity of intracellular nucleic acid targets now emerging from the genomics and RNA biology fields. Databases such as those maintained by the NIH National Center for Biotechnology Information increasingly index CPP–ASO conjugate sequences, reflecting the field’s maturation.

The assignee landscape reflects a clear division: academic institutions dominate literature output, with the University of Tokyo, University of Queensland, University of Groningen, and Florey Institute / University of Melbourne among the most active research nodes. Commercial patent activity is concentrated in Japan (Chugai Pharmaceutical, PRISM BioLab), Israel (Yissum / Hebrew University), the UK (Sutura Therapeutics, BicycleTx Limited), and Germany (Ruprecht-Karls-Universität Heidelberg). This geographic distribution suggests that the peptide drug pipeline is genuinely international, with no single country dominating the innovation landscape. Patent databases maintained by the EPO confirm the sustained filing activity in peptide cyclization and CPP delivery system technologies across these jurisdictions.