What is PX13-R?

PX13-R (development code LY-3437943) is a synthetic 39-amino-acid peptide engineered as a single-molecule triple agonist at the glucagon-like peptide-1 (GLP-1), glucose-dependent insulinotropic polypeptide (GIP) and glucagon receptors. It builds on the design lineage of PX12-T by adding a third receptor arm — glucagon — which in preclinical models contributes to increased energy expenditure through hepatic and adipose signaling.

PX1 Research supplies lyophilized PX13-R as a reference compound for in-vitro receptor characterization, pharmacological screening and analytical method development. Every vial ships with a batch-specific Certificate of Analysis (COA) that confirms identity by liquid chromatography-mass spectrometry (LC-MS) and quantifies purity by reversed-phase HPLC. PX13-R is offered strictly for research use only and is not for human or veterinary use.

Because the molecule integrates three incretin pathways, PX13-R is one of the most studied triple agonists in current metabolic research and appears frequently in the peer-reviewed literature under both its development code and generic name.

Mechanism of action

GLP-1 receptor agonism in preclinical systems potentiates glucose-dependent insulin release, slows gastric emptying and modulates central appetite circuits. GIP agonism further amplifies incretin-mediated insulin secretion and, in adipose models, has been associated with improved lipid handling. The novel glucagon component of PX13-R activates hepatic glucagon receptors, which in the literature is linked to increased basal metabolic rate and lipolysis.

The three receptor arms are balanced by design: unlike simple co-agonists, PX13-R's pharmacology at each receptor is tuned so that the net metabolic effect in animal studies exceeds what dual GLP-1/GIP agonists achieve alone. In cell-based reporter assays, PX13-R shows sub-nanomolar potency at GLP-1 and GIP receptors with moderate glucagon activity — a profile that has driven its selection as a lead investigational tool compound.

For laboratory characterization, PX13-R is commonly assayed against reference agonists in cAMP accumulation, β-arrestin recruitment, and internalization panels across the three cognate receptors.

Research history

PX13-R entered the peer-reviewed record with Coskun and colleagues (2022), describing its design, receptor pharmacology and preclinical metabolic profile. Subsequent phase 1 and phase 2 clinical publications in 2023 and 2024 reported dose-ranging data in metabolic research populations, generating substantial interest in the triple-agonist class.

Since 2023, PX13-R has appeared in dozens of independent laboratory reports covering receptor binding, downstream signaling, comparative pharmacology against PX12-T and PX1-S, and analytical method development for LC-MS quantification of the intact peptide.

The compound is often referenced alongside PX12-T (dual GLP-1/GIP) and survodutide (GLP-1/glucagon) to define the emerging incretin polyagonist class.

Laboratory handling and storage

PX13-R ships as a lyophilized (freeze-dried) white powder in a sealed, tamper-evident vial. Before opening, store between −20°C and −80°C protected from light; the peptide is stable in this state through the labeled expiration.

Purity and Certificate of Analysis (COA)

Every PX1 PX13-R batch is USA-manufactured and released against a documented specification of ≥99% purity by reversed-phase HPLC. Identity is confirmed on the same batch by high-resolution mass spectrometry against the theoretical monoisotopic mass of the 39-mer sequence.

The batch-specific COA — published above on this product page — includes HPLC chromatogram, integrated purity percentage, LC-MS identity confirmation, endotoxin result, and lot number. The vial label carries the same lot number so the compound in your hand traces directly back to the report you can read online.

Testing methods

The HPLC gradient is optimized to resolve PX13-R from process-related impurities including deamidation and oxidation variants. Mass spectrometry is performed on a high-resolution instrument capable of confirming intact mass within a few parts-per-million of the theoretical value.

PX13-R vs PX12-T vs PX1-S

The incretin field is usually described as three generations of receptor coverage. PX1-S is a single-receptor GLP-1 agonist. PX12-T adds GIP for dual coverage. PX13-R adds a third arm — glucagon — producing the triple GLP-1 / GIP / glucagon profile that defines the current frontier of the class. Comparative work in the published literature generally frames the three molecules along this axis rather than treating them as interchangeable.

Mechanistically the glucagon arm is what differentiates PX13-R in preclinical models. GLP-1 and GIP signaling act largely on insulin secretion, gastric handling and central appetite circuitry, whereas hepatic glucagon receptor activation is associated in the literature with increased energy expenditure and lipolytic signaling. The design challenge is balance: unopposed glucagon agonism raises hepatic glucose output, so the molecule's relative potency at each of the three receptors is tuned rather than maximized.

For laboratory purposes the three compounds are frequently assayed side by side in the same cAMP accumulation and β-arrestin recruitment panels, which is why researchers commonly stock reference material for all three. PX13-R is a 39-amino-acid peptide with a fatty-acid moiety supporting albumin binding; PX12-T is a 39-mer GIP-based sequence; PX1-S is a 31-mer GLP-1 analog. The sequences are distinct enough that a purity method optimized for one will not automatically resolve the impurity profile of another.

Analytical characterization

Laboratories that work with PX13-R typically characterize incoming material on three axes before it enters a study: identity, purity and content. Identity is established by high-resolution mass spectrometry against the theoretical monoisotopic mass (≈4,731 Da for the 39-residue sequence), usually supported by MS/MS fragmentation that walks the backbone and confirms the sequence rather than just the total mass. A matching intact mass alone can be satisfied by a scrambled or partially epimerized sequence, which is why fragmentation data is the stronger identity evidence.

Purity is quantified by reversed-phase HPLC with UV detection, integrating every resolved peak in the chromatogram and expressing the main peak as a percentage of total area. The gradient matters more than the headline number: a shallow, well-optimized gradient resolves closely eluting process impurities such as deamidation products, oxidation variants, truncated sequences and acetate adducts, while an aggressive gradient can co-elute them under the main peak and inflate the reported purity. PX1 publishes the chromatogram itself, not only the integrated figure, so the resolution behind the number is auditable.

Content — how much peptide is actually in the vial once counter-ions and residual water are subtracted — is the axis most often skipped by low-cost suppliers. Net peptide content is a function of the labeled mass, the water content measured by Karl Fischer titration, and the counter-ion (typically trifluoroacetate or acetate) load. A vial that is 99% pure by HPLC can still under-deliver on content if it carries a high salt and moisture fraction, which is why the COA reports both.

Documented research applications

Published PX13-R work clusters into four areas. Receptor pharmacology studies characterize potency and efficacy at each of the three cognate receptors using reporter cell lines, defining the balanced-agonist profile that distinguishes it from co-agonists. Signaling-bias studies examine the ratio of G-protein to β-arrestin engagement, which is increasingly used to explain differences in receptor internalization and sustained signaling across the incretin class.

A second cluster covers metabolic phenotyping in preclinical models — energy expenditure, substrate utilization and hepatic lipid handling — where the glucagon arm's contribution is isolated by comparison against dual agonists. A third covers analytical chemistry: LC-MS/MS method development for quantifying intact PX13-R in complex biological matrices, an area with active method-validation literature because the fatty-acid modification complicates extraction.

The fourth and fastest-growing cluster is comparative and translational review work positioning PX13-R within the polyagonist landscape alongside PX12-T, survodutide and cagrilintide combinations. For laboratories running that comparison, reference material with a documented and consistent purity specification across lots is the prerequisite — batch-to-batch variance is otherwise indistinguishable from a real pharmacological difference.

Sourcing, provenance and what separates lab-grade material

The research-peptide market is unusually wide in quality. The same nominal PX13-R listing can represent USA-manufactured material released against a documented specification, or repackaged bulk of unknown origin with a generic certificate that was never generated from the lot in the vial. The distinction is invisible from the product photo and only becomes visible in the paperwork.

The practical test is traceability: the lot number printed on the vial label should appear on the certificate of analysis, and that certificate should show the actual chromatogram and mass spectrum for that lot rather than a representative example. A COA without a lot number, without instrument traces, or dated years before the vial was filled is a document, not evidence. PX1 publishes the batch-specific report directly on the product page so the chain from manufacturing to vial is checkable before purchase.

Beyond the certificate, consistent lab-grade supply depends on synthesis and release happening under one controlled process: domestic solid-phase manufacturing, preparative HPLC purification, lyophilization under validated cycle parameters, and third-party confirmation of purity and endotoxin. PX13-R sold by PX1 Research is produced and released on that pathway and is supplied strictly for laboratory research use — not for human or veterinary use.

Study design considerations

Receptor-pharmacology work with PX13-R almost always runs as a three-receptor matrix rather than a single assay. Separate reporter lines expressing human GLP-1R, GIPR and GCGR are each challenged with the compound across a full concentration range, and native GLP-1, GIP and glucagon serve as the reference agonists that anchor each curve. Reporting potency at one receptor in isolation is uninformative for a polyagonist, because the defining property of the molecule is the ratio between the three.

Comparator selection is the second design decision. A dual agonist such as PX12-T isolates the contribution of the glucagon arm; a GLP-1-only agonist such as PX1-S isolates the combined contribution of the GIP and glucagon arms. Running all three in the same plate on the same day removes the largest source of between-experiment variance, which is why laboratories in this area order the reference set together rather than piecemeal.

For analytical method development, the acylation drives most of the practical difficulty. Extraction from plasma or buffer containing albumin must account for the fatty-acid moiety's protein binding, and recovery is concentration-dependent in a way that unmodified peptides are not. Method validation therefore needs recovery data across the full working range rather than at a single mid-range point.

Concentration verification deserves a dedicated step. Because acylated peptides adsorb readily to labware, the concentration in the assay well can differ measurably from the nominal concentration calculated from the vial mass. Quantifying the working stock by amino acid analysis or by UV absorbance against a characterized standard turns a nominal concentration into a measured one, and it is the single most effective way to make results reproducible across sites.

Finally, lot planning matters for multi-month programmes. Reserving a single lot for the duration of a study removes lot-to-lot analytical variance from the dataset entirely; where that is impossible, running a bridging comparison between the outgoing and incoming lot in one shared assay preserves the ability to interpret the whole series.

Common research questions about PX13-R

What makes PX13-R a triple agonist rather than a co-agonist? A co-agonist study uses two separate molecules dosed together, so the exposure profile of each is independent and can drift apart over time. PX13-R is a single sequence that engages GLP-1, GIP and glucagon receptors itself, meaning the relative receptor engagement is fixed by the molecule's design and cannot decouple. That structural difference is the reason polyagonist pharmacology is treated as its own subfield rather than as combination dosing.

Is PX13-R the same as GLP-3? No. "GLP-3" is a market shorthand that emerged to describe the next step after GLP-1 and dual agonists; there is no receptor called GLP-3 and no endogenous ligand by that name. The scientific literature refers exclusively to PX13-R or to its development code LY-3437943. PX1 lists the compound as PX13-R for continuity with how customers search, but the monograph, COA and analytical documentation all use the canonical name.

What purity should PX13-R reference material meet? For receptor pharmacology and analytical method development, ≥98% by reversed-phase HPLC is the working floor and ≥99% is the practical standard, because impurities in a fatty-acylated peptide are often closely related structures — des-acyl forms and positional isomers — that may retain partial receptor activity and therefore contaminate the pharmacology rather than merely diluting it. Purity alone is not sufficient; net peptide content and water content determine how much material is actually in the vial.

How should a laboratory verify a PX13-R lot on receipt? Three checks answer most questions. Confirm the lot number on the vial matches the certificate. Confirm the certificate carries the actual chromatogram and mass spectrum rather than a summary table. Confirm the reported mass matches the theoretical monoisotopic mass for the 39-residue acylated sequence within instrument tolerance. A lot that fails any of the three should be treated as uncharacterized regardless of the headline purity figure.

Where PX13-R sits in the PX1 catalog

Researchers who stock PX13-R most often stock the rest of the incretin reference set with it: PX12-T as the dual GIP/GLP-1 comparator, PX1-S as the GLP-1-only anchor, and Cagrilintide where amylin-pathway interaction is part of the question. Buying the set from one release process keeps analytical variance common across the comparison instead of confounded with the pharmacology.

Outside the incretin class, PX13-R work frequently sits alongside metabolic tool compounds — SLU-PP-332 for ERR agonism, 5-Amino-1MQ for NNMT inhibition and MOTS-c for mitochondrial retrograde signaling — where the shared endpoints are energy expenditure and substrate handling rather than receptor occupancy.