How to Store Polymyxin: Stability and Handling, Per the Research

What this page covers

The polymyxins are cyclic lipopeptide antibiotics, principally polymyxin B and polymyxin E (colistin, given clinically as the prodrug colistimethate sodium). Because they are peptides rather than conventional small molecules, storage questions — refrigerated versus frozen, dry powder versus reconstituted solution, how long a vial stays intact, what degradation looks like — recur in both laboratory and pharmacy settings. This page summarises what published work has reported about polymyxin stability and handling, and explicitly separates polymyxin-specific findings from general lyophilized-peptide chemistry. This page is for educational purposes only and is not medical advice; consult a licensed physician for any question about a medicine, a prescription product or a clinical decision. Nothing below is a protocol, a handling instruction or a recommendation.

Two tiers of evidence: compound-specific versus general

Polymyxin-specific stability work does exist, and it is worth naming precisely. A 2019 review chapter in Advances in Experimental Medicine and Biology addressed the bioanalysis and stability of polymyxins, covering how these lipopeptides are measured and the stability considerations that accompany that measurement (PMID 31364072). A 2021 study in Molecules revisited colistimethate acidic hydrolysis and modelled its temperature dependence with the Arrhenius equation, using UPLC-QToF mass spectrometry to follow the chemistry (PMID 33467022). A 2023 study in Theriogenology reported that polymyxin B neutralised endotoxin and improved the quality of chicken semen during liquid storage, an example of the compound remaining functionally active inside a stored liquid biological matrix (PMID 36577162).

Everything else commonly said about peptide storage — that dry lyophilized cake is more chemically inert than solution because water is the reactant in hydrolysis, that repeated warming and cooling of an aqueous solution gives degradation more opportunity than a single steady temperature, that moisture ingress into a hygroscopic powder is a practical risk — belongs to general lyophilized-peptide science, not to polymyxin-specific measurement. Where this page states such a principle, it is labelled as general and no study is attached to it, because no verified polymyxin paper on this page measured it.

Quick orientation table

Refrigeration: lyophilized powder versus reconstituted solution

The sharpest distinction in peptide storage is physical state. Once a polymyxin is in water, it participates in solution chemistry, and the kinetics of that chemistry are temperature-dependent — which is precisely what the 2021 colistimethate work formalised when researchers fitted acidic hydrolysis data to the Arrhenius equation and characterised the products by UPLC-QToF MS (PMID 33467022). The practical implication described in that literature is conceptual rather than prescriptive: colder solutions hydrolyse more slowly than warmer ones, and the relationship is exponential rather than linear.

For the dry state, the honest position is that none of the verified polymyxin papers on this page measured lyophilized-cake shelf life at specific refrigerated temperatures. Statements about a sealed vial of powder being more robust than an aqueous solution follow from general peptide chemistry — water is required for hydrolytic bond cleavage, so removing it slows that pathway. Approved injectable products containing polymyxin B or colistimethate carry manufacturer-set storage directions and in-use limits in their labelling, and those product-specific directions, not general peptide theory, are what regulate clinical handling. Research-use-only powders sold for laboratory work carry no such clinical labelling at all.

Shelf life and expiry: how the dates are generated

An expiry date is an inference from stability data, not an observation about a particular vial. Accelerated stability testing works by measuring degradation faster at elevated temperature and extrapolating to storage temperature, and the colistimethate study is a clean published illustration of that logic applied to a polymyxin: the study derived temperature-dependent hydrolysis kinetics using the Arrhenius framework (PMID 33467022). Because colistimethate is itself a prodrug that converts to colistin, the chemistry that defines its shelf life is also the chemistry that defines its activity, which is one reason bioanalytical method design and stability assessment are treated together in the 2019 polymyxin review (PMID 31364072).

Three clarifications follow from that literature. First, an expiry date describes the container and formulation that were tested, not powder transferred to another vessel. Second, potency loss and impurity formation are separate endpoints; a solution can lose parent compound while gaining degradants. Third, none of the verified studies here reported a numerical shelf life for lyophilized polymyxin B powder, so any specific month-count circulating informally is not supported by the papers cited on this page.

Room temperature and travel

Ambient exposure is a temperature-and-time question. The Arrhenius modelling reported for colistimethate acidic hydrolysis is the most directly relevant published anchor, because it quantified how the rate of breakdown scales with temperature rather than simply asserting that heat is bad (PMID 33467022). Separately, researchers working with polymyxin B in a stored liquid medium reported that it still neutralised endotoxin and improved semen quality across liquid storage of chicken semen, showing that functional activity can persist in a refrigerated-type liquid workflow (PMID 36577162).

General handling science, again not polymyxin-specific, adds that transport stress involves more than peak temperature: cumulative time in the warm, light exposure, vibration and condensation on cold glass returning to ambient all appear in generic peptide-handling discussions. No verified study on this page measured any of those variables for polymyxin.

Freezing and freeze–thaw

Freeze–thaw stability is a standard element of bioanalytical method validation, and stability in that analytical context is part of what the 2019 polymyxin review addressed when it treated bioanalysis and stability of polymyxins together (PMID 31364072). What the verified literature on this page does not provide is a polymyxin-specific count of tolerated freeze–thaw cycles, a recommended freezer temperature, or a comparison of frozen versus refrigerated lyophilized powder. Claims of that kind are extrapolations from generic peptide practice.

General principles described for lyophilized peptides include the observation that freezing an aqueous solution concentrates solutes in the unfrozen phase and can shift local pH, and that aliquoting exists to avoid repeated cycling of a single container. These are process concepts rather than measured polymyxin outcomes.

Degradation signs and how degradation is actually detected

Visual cues — a collapsed or discoloured cake, clumping that suggests moisture ingress, cloudiness or particulates after reconstitution — are the crude first-pass signals discussed generally for lyophilized peptides. They are insensitive: the colistimethate work needed UPLC-QToF mass spectrometry to resolve hydrolysis chemistry that no eye could see (PMID 33467022). Chromatographic and mass-spectrometric methods are the actual instruments of stability assessment for this class, which is why method development and stability were reviewed alongside one another in 2019 (PMID 31364072).

Matrix and formulation change what is measured

Polymyxin behaviour is not a property of the molecule alone. Researchers reported that lysophosphatidylcholine potentiated the antibacterial activity of polymyxin B, showing that co-solutes in the surrounding medium can alter measured potency (PMID 32988824). Formulation format matters too: a 2020 study described bioinspired adhesive antibacterial microneedles for transdermal drug delivery (PMID 32490376), and a 2025 study described a tetrahedral framework nucleic acid-based antibiotic delivery system aimed at overcoming biofilm barriers in chronic infections (PMID 40597263). A solid microneedle array, a nucleic-acid carrier and a lyophilized vial are three different storage problems, and stability data from one does not transfer to another.

Why the underlying chemistry is studied at all

Polymyxins act at the bacterial outer membrane, and structural work continues to map that interface: a 2020 Nature paper reported the structure of the essential inner membrane lipopolysaccharide–PbgA complex (PMID 32788728), while a 2025 structural-biology paper presented cryo-EM analysis of the bacterial membrane proteins ArnA and AcrB (PMID 40927951). Related peptide-engineering work reported fluorinated multi-mechanism antimicrobial peptides developed for improved potency against multidrug-resistant bacterial infections (PMID 42468316). These papers concern mechanism and design rather than storage, and none of them reported storage conditions for polymyxin.

Degradation, Impurities and Toxicity Signals: What Studies Report

No verified study on this page linked a specific storage condition to a specific adverse outcome in humans; that connection is not established in the literature summarised here. What the literature does contain is toxicology of the intact drug. A 2025 study in the European Journal of Pharmacology investigated how finerenone alleviated polymyxin B-induced kidney injury, combining network pharmacology, transcriptomics and molecular docking (PMID 40812665). Endotoxin biology is also relevant to why polymyxin B appears in laboratory workflows: a 2024 study reported caspase-11-mediated inflammasome activation in macrophages during systemic A. actinomycetemcomitans infection exacerbating arthritis (PMID 39143049), and critical-care work on polymyxin-related interventions has been presented in symposium proceedings (PMID 27885969). None of those papers evaluated degraded or improperly stored material.

What the evidence does not settle

• No verified paper here reported a numerical shelf life for lyophilized polymyxin B or colistimethate powder at any given temperature.
• No verified paper here reported a tolerated number of freeze–thaw cycles for polymyxin solutions.
• The Arrhenius modelling published for colistimethate addressed acidic hydrolysis conditions specifically (PMID 33467022), which is not the same as neutral-pH or dry-state storage.
• Functional activity reported during liquid storage in one biological matrix (PMID 36577162) does not generalise to other diluents or containers.

Related reading on PeptideU

The polymyxin overview page in the Learn library covers mechanism, pharmacology and regulatory status, and the general guide to storing peptides covers lyophilization, reconstitution chemistry and the handling principles that apply across the peptide class rather than to one compound.

References

• Bioanalysis and Stability of Polymyxins (Advances in Experimental Medicine and Biology, 2019)
• Colistimethate Acidic Hydrolysis Revisited: Arrhenius Equation Modeling Using UPLC-QToF MS (Molecules, 2021)
• Polymyxin B neutralizes endotoxin and improves the quality of chicken semen during liquid storage (Theriogenology, 2023)
• Lysophosphatidylcholine Potentiates Antibacterial Activity of Polymyxin B (Antimicrobial Agents and Chemotherapy, 2020)
• Bioinspired Adhesive and Antibacterial Microneedles for Versatile Transdermal Drug Delivery (Research, 2020)
• A novel tetrahedral framework nucleic acid-based antibiotic delivery system: overcoming biofilm barriers to combat chronic infections (Journal of Nanobiotechnology, 2025)
• Structure of the essential inner membrane lipopolysaccharide-PbgA complex (Nature, 2020)
• Off-target structural insights: ArnA and AcrB in bacterial membrane-protein cryo-EM analysis (Acta Crystallographica Section D, 2025)
• Fluorinated multi-mechanism antimicrobial peptides for improved potency against multidrug-resistant bacterial infections (Drug Resistance Updates, 2026)
• The multi-target intervention reveals the mechanism by which Finerenone alleviates Polymyxin B-induced kidney injury (European Journal of Pharmacology, 2025)
• Caspase-11 mediated inflammasome activation in macrophages by systemic infection of A. actinomycetemcomitans exacerbates arthritis (International Journal of Oral Science, 2024)
• 36th International Symposium on Intensive Care and Emergency Medicine (Critical Care, 2016)