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REVIEW 2 major objections 6 minor 13 references

Polymyxin B-Enriched Exogenous Lung Surfactant: Thermodynamics and Structure

T0 review · 2 major / 6 minor · reviewed 2026-08-11 · deepseek-v4-flash

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Pith's one-line read Polymyxin B can be mixed with Curosurf at low loadings without disrupting the surfactant's lipid structure, because the drug binds flat to the bilayer surface rather than inserting into the core.

desk verdict A careful multi-technique study of polymyxin B with Curosurf; the main claim leans on a SANS model switch that needs a cross-check, but the dataset is solid and worth refereeing. read the letter →

arxiv 2412.04894 v1 pith:AHKM37EI submitted 2024-12-06 physics.bio-ph cond-mat.soft

Nina Královič-Kanjaková , Ali Asi Shirazi , Lukáš Hubčík , Mária Klacsová , Atoosa Keshavarzi , Juan Carlos Martínez , Sophie Combet (LLB) , José Teixeira
show 1 more author
Daniela Uhríková
This is my paper · ORCID
classification physics.bio-phcond-mat.soft
keywords polymyxinBpulmonarysurfactantCurosurfsmall-angleneutronscatteringzetapotentialbilayerthicknessphasetransitiondrugdelivery
verification ladder T0 review T1 audit T2 compute T3 formal

The pith

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The reading

This paper asks whether the antibiotic polymyxin B can be mixed into Curosurf, the clinical lung surfactant used in neonatal intensive care, without wrecking the lipid architecture that makes the surfactant work. Its answer is yes, at low loading. The authors argue that cationic PxB molecules bind electrostatically to the negatively charged bilayer surface of Curosurf, lie flat on it, and do not push their hydrophobic tail into the membrane core, so the ordered lamellar stacking is preserved and even strengthened. In a protein-free four-lipid model, the same drug penetrates more deeply, thins the bilayer, and raises the chain-melting temperature, showing that the minor protein and lipid components of Curosurf shield its bilayer. The safe window ends around 5 to 7 weight percent PxB, where the surface charge inverts and the vesicles stop aggregating.

What carries the argument

The load-bearing entity is the amphiphilic polymyxin B molecule, a cyclic decapeptide with five cationic Dab residues in its ring and a short branched acyl tail. Its large polar area of about 6.5 square nanometers lets the ring lie across roughly ten lipid headgroups, making electrostatic screening of the negative surfactant surface geometrically plausible. The argument is carried by comparing bilayer thicknesses and stacking from two SANS models, the polydisperse unilamellar-vesicle model for drug-free extruded vesicles and the paracrystal lamellar model for PxB-containing mixtures, the latter also returning the average number of stacked bilayers; by zeta potential to locate the charge-inversion point; and by pyrene excimer probes that report lateral pressure near the interface and in the hydrophobic core.

What would settle it

Fit the PxB-free extruded Curosurf SANS data with the same paracrystal lamellar model used for the PxB mixtures; if the model choice alone shifts the bilayer thickness by more than the reported few-hundredths of a nanometer, the surface-only interpretation is not settled.

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Extended reading notes

Core claim

The central claim of the authors is that in the clinical surfactant Curosurf, polymyxin B acts as a surface-bound, SP-B-like agent: its cyclic peptide head with five cationic Dab residues docks onto negative lipid headgroups, screens the surface charge, and promotes stacking of unilamellar vesicles into ordered multilamellar assemblies, while the drug's short lipophilic tail remains outside the hydrophobic core. The supporting evidence is convergent: zeta potential reaches zero at 5 to 7 weight percent PxB; the small-angle X-ray repeat distance drops from 8.86 nm toward 6.35 nm; SANS fits show growth of a small population of oligolamellar vesicles peaking near 5 weight percent; excimer fluorescence at depth C10 shows no lateral-pressure change; and DSC shows no shift in the gel-to-fluid transition temperature of Curosurf. In the protein-free model system, by contrast, the tail inserts, bilayer thickness thins biphasically with a minimum near electroneutrality, and the transition temperature rises by roughly 2 degrees Celsius. The authors conclude with a quantitative warning: keep PxB below 5 weight percent of the surfactant mass.

Load-bearing premise

The claim that PxB does not enter the Curosurf bilayer depends on comparing bilayer thicknesses from two different SANS fitting models, and if switching models shifts the thickness systematically, the small thinning seen with PxB could be a fitting artifact rather than evidence of surface-only binding.

Editorial extensions

If this is right

  • Curosurf can accommodate up to about 5 weight percent PxB without losing its ordered lamellar structure, so the galenic formulation needed for rapid alveolar spreading is preserved.
  • Below that loading, PxB acts like SP-B, promoting vesicle-to-vesicle contact and converting unilamellar vesicles into a small population of oligolamellar stacks.
  • Above 5 to 7 weight percent, the zeta potential inverts, vesicle aggregation reverses, and the surfactant surface becomes positively charged, which should be avoided in any combined formulation.
  • PxB's hydrophobic tail does not enter the hydrophobic core of Curosurf bilayers, so the chain-melting temperature and lateral pressure deep in the bilayer remain unchanged.
  • The protein-free four-lipid model is a good mimic of Curosurf thermodynamics but is less resistant to PxB penetration, so it cannot be used to predict penetration into the clinical surfactant.
  • If the surface-only mechanism is correct, a single PxB-Curosurf formulation below roughly 5 weight percent could serve as both antibiotic delivery and surfactant replacement, since the structural window matches the doses already tested in animal infection models.
  • The protective role assigned to the minor components of Curosurf suggests that synthetic or semi-synthetic surfactant formulations could be deliberately enriched with PE, SM, or SP-B analogues to widen the PxB compatibility window.
  • A direct molecular test would be neutron contrast-variation SANS with a deuterium-labelled PxB tail: the label should remain at the headgroup region for Curosurf and appear inside the chain region for the model system.
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Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, and a circularity audit.

Referee Report

2 major / 6 minor

Summary. Polymyxin B (PxB) is a cationic lipopeptide antibiotic considered for co-administration with exogenous lung surfactant. The authors characterize the interaction of PxB with the clinical surfactant Curosurf (PSUR) and with a protein-free four-phospholipid model (MS) using zeta potential, DSC, SAXS/WAXS, SANS, and pyrene excimer fluorescence. They report that PxB binds electrostatically to both systems, screens the negative surface charge, and induces aggregation of unilamellar vesicles into oligolamellar stacks, with charge inversion above about 5–7 wt% PxB. PxB leaves the gel-fluid transition temperature of PSUR essentially unchanged but increases Tm of MS. SANS-derived bilayer thickness dL is nearly constant for PSUR and shows a biphasic decrease for MS with a minimum near electroneutrality. Lateral pressure changes at the C4 level are absent for PSUR but present for MS. The authors conclude that PxB lies on the PSUR surface without inserting its acyl tail, that minor PSUR components protect against penetration, and that PxB-enriched Curosurf is viable below about 5 wt% PxB.

Significance. If the conclusions hold, this study provides a biophysical basis for a concrete safe loading threshold for PxB in Curosurf, which is relevant to combination therapy for Gram-negative pneumonia. The main strengths are the integrated use of several complementary techniques, the parallel comparison with a defined model system, the batch-to-batch reproducibility of the PSUR data, and the consistent reporting of uncertainties. The paper does not rely on circular derivations; all central quantities are measured or fitted to scattering data. However, the SANS-based no-penetration claim depends on a cross-model comparison that is not yet validated, and the distinction between total and adsorbed PxB is not addressed.

major comments (2)
  1. [Methods, SANS; Fig. 4A/B] The conclusion that the PxB hydrophobic tail does not penetrate the PSUR bilayer rests in part on the negligible change in dL extracted from SANS (Fig. 4B). However, the dL values for PxB-free PSUR are obtained with the polydisperse unilamellar vesicle model, whereas every PxB-containing PSUR sample is fitted with the paracrystal lamellar model (Methods, SANS; Fig. 4A). These two models define the thickness parameter differently and involve different parameterizations (the paracrystal model adds a lamellar repeat distance, a number-of-layers parameter NL, and its own polydispersity treatment), so a systematic offset of the order of 0.1 nm between the two models cannot be excluded. Since the reported PSUR thinning is of that order, the no-penetration interpretation is not secure on the SANS leg alone. Please provide a cross-check, for example by fitting the PxB-free ULV data with the paracrystal model with NL=1, by fitting the PxB-containing data with the vesicle model in a q-range where the OLV contribution is negligible, or by demonstrating on simulated data that the two models yield the same dL for the same physical bilayer.
  2. [Zeta potential; Fig. 1; Fig. 4B; stoichiometry paragraph] The manuscript repeatedly refers to “the adsorbed amount of PxB” (e.g., in the Abstract and in the discussion of Fig. 4B), but only the total concentration of PxB added to the dispersion is measured; the free PxB concentration in solution is not determined. The electroneutrality-based stoichiometry of 1 PxB per 30–21 lipid molecules assumes complete binding of the added PxB. Please either quantify the free/bound partition (for example by separating the vesicles and assaying the supernatant) or revise the text to refer explicitly to the total PxB/lipid ratio, and soften the stoichiometric inference accordingly.
minor comments (6)
  1. [The effect of PxB on the lipid bilayer] The text says “Fig. 3B summarizes the effect of PxB on the thickness of the lipid bilayer dL,” but Fig. 3B shows the repeat distance d; the dL data are displayed in Fig. 4B. Please correct the cross-reference.
  2. [Results and discussion, SANS paragraph] The Kratky-Porod plot is described as being in “Fig. 3A, inset,” but the plot appears in the inset of Fig. 4A. Please correct the cross-reference.
  3. [Introduction] The net charge of SP-C is given as “+360”; this is likely a typo for a small positive value (a few charges). Please verify and correct.
  4. [Materials and Methods, hydration medium] The resistivity of water is given as “18.2 MW.cm”; this should be in units of MΩ·cm (or MΩ cm). Please correct the notation.
  5. [Lateral pressure discussion] The statement that the Pyr10PC probe supports the no-penetration claim should be qualified, because in MS, where tail insertion is inferred, Pyr10PC is also unchanged; the probe likely resides deeper than the short PxB tail reaches, so it is mainly the Pyr4PC response that differentiates the two systems.
  6. [Conclusion] The attribution of PSUR resistance to PxB penetration to specific minor components (PEs, SMs, SP-B/SP-C) is a plausible hypothesis but is not directly tested in this study, since these components are not varied experimentally. Please frame this statement as an interpretation consistent with the data and literature rather than as an established result.

Circularity Check

0 steps flagged · score 2.0 of 10

No significant circularity: the structural conclusions rest on direct scattering, calorimetric, and fluorescence measurements, and the SANS model-switch is a methodological caveat rather than a constructional equivalence.

full rationale

The paper's load-bearing claims are all directly measured or fitted quantities: Tm from DSC and WAXS, repeat distance d from SAXS Bragg peaks, bilayer thickness dL and layer number NL from SANS fits, zeta potentials, and excimer fluorescence lateral-pressure ratios. None of these is defined in terms of another claim, and the no-penetration conclusion for PSUR is an interpretation of a negligible dL change together with unchanged Pyr10PC lateral pressure, not a quantity that is set equal to an input. The SANS comparison across the unilamellar-vesicle model (for PxB-free PSUR) and the paracrystal lamellar model (for PxB-containing PSUR) is a legitimate robustness concern about fitting-systematics, but it is not an equivalence by construction: the paracrystal fit also returns an independent aggregation signature, and the Kratky-Porod plot independently shows nonlinearity at intermediate PxB loadings. The only same-group citations appear in methodological validation sentences, e.g. 'Our previous studies confirmed the suitability of the paracrystal lamellar model for lipid - additive oligolamellar vesicles (OLV)91,95' and 'The model was previously tested...91,92'. These citations support the choice of standard SasView models whose original forms are externally published, and they do not supply the central structural result. The paper also benchmarks its Tm and dL values against independent literature values. Accordingly, no derivation step reduces to its own input; the score of 2 reflects only these minor, non-load-bearing self-citations, not actual circularity.

Assumptions & free parameters 4 free parameters · 5 assumptions · 0 invented entities

The central claim rests mostly on standard experimental calibrations and domain-specific modeling assumptions. No new particles, forces, dimensions, or conserved quantities are introduced; PxB and Curosurf are existing clinical substances. The main burden is the SANS model choice, the excimer probe assumption, the Smoluchowski zeta model, and the unverified assumption that all added PxB binds to the vesicles.

free parameters (4)
  • dL_PSUR (bilayer thickness from SANS) = 3.72 +/- 0.02 nm for PxB-free PSUR; values with PxB in Fig. 4B
    Central to the claim that PxB does not change PSUR bilayer thickness; obtained by fitting SANS curves with vesicle and paracrystal lamellar models.
  • dL_MS (bilayer thickness from SANS) = 3.65 +/- 0.05 nm for PxB-free MS; biphasic values with PxB in Fig. 4B
    Supports the biphasic thinning and insertion interpretation in the protein-free model system; obtained from SANS model fitting.
  • NL (number of lamellae in paracrystal model) = 1.00 to 1.14 depending on PxB content
    Used to infer PxB-induced oligolamellar vesicle formation and the reversal back to unilamellar vesicles at high PxB content.
  • repeat-distance polydispersity in SANS model = 12 to 15 percent Gaussian width
    Parameter of the paracrystal lamellar model used for PxB-containing mixtures; affects the dL and repeat-distance estimates.
assumptions (5)
  • domain assumption SasView built-in vesicle model and paracrystal lamellar model give unbiased bilayer thickness dL for extruded ULVs and PxB-aggregated OLV stacks.
    Methods, SANS section. If the model switch biases dL, the negligible PSUR thinning conclusion is compromised.
  • domain assumption The excimer ratio IE/IM is a valid lateral-pressure reporter at the C4 and C10 positions and does not perturb the membrane.
    Methods, lateral pressure section. The no-penetration conclusion relies on unchanged Pyr10PC pressure.
  • domain assumption The Smoluchowski equation applies to zeta potential of PSUR and MS vesicles in 150 mM NaCl.
    Methods, zeta potential section. The electroneutrality threshold interpretation relies on zeta values.
  • domain assumption All added PxB is associated with vesicles, with no significant free PxB concentration.
    Results section, where electroneutrality is converted into a stoichiometry of 1 PxB per 30 to 21 lipids; no binding isotherm or supernatant measurement is provided.
  • domain assumption The four-lipid model system is an adequate comparator to infer the protective role of PSUR minor components.
    Conclusion attributes PSUR resistance to PEs, sphingomyelins, SP-B, and SP-C based on comparing two systems without varying these components individually.

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Pith. "Pith review of Polymyxin B-Enriched Exogenous Lung Surfactant: Thermodynamics and Structure." pith.science (2026). https://pith.science/paper/AHKM37EI

@misc{pith2026241204894,
  author       = {Pith},
  title        = {Pith review of: Polymyxin B-Enriched Exogenous Lung Surfactant: Thermodynamics and Structure},
  year         = {2026},
  howpublished = {\url{https://pith.science/paper/AHKM37EI}},
  note         = {Machine review of arXiv:2412.04894}
}
abstract

The use of exogenous pulmonary surfactant (EPS) to deliver other relevant drugs to the lung is a promising strategy for combined therapy. We evaluated the interaction of polymyxin B (PxB) with clinically used EPS, the poractant alfa Curosurf (PSUR). The effect of PxB on the protein-free model system (MS) composed of four phospholipids (diC16:0PC/16:0-18:1PC/16:0-18:2PC/16:0-18:1PG) was examined in parallel to distinguish the specificity of the composition of PSUR. We used several experimental techniques (differential scanning calorimetry, small-and wide-angle X-ray scattering, small angle neutron scattering, fluorescence spectroscopy, and electrophoretic light scattering) to characterize the binding of PxB to both EPS. Electrostatic interactions PxB -EPS are dominant. The results obtained support the concept of cationic PxB molecules lying on the surface of the PSUR bilayer, strengthening the multilamellar structure of the PSUR as derived from SAXS and SANS. A protein-free MS mimics natural EPS well but was found to be less resistant to penetration of PxB into the lipid bilayer. PxB does not affect the gel-to-fluid phase transition temperature Tm of PSUR, while Tm increased by ~ +2 $^\circ$C in MS. The decrease of the thickness of the lipid bilayer (dL) of PSUR upon PxB binding is negligible. The hydrophobic tail of the PxB molecule does not penetrate the bilayer as derived from SANS data analysis and changes in lateral pressure monitored by excimer fluorescence at two depths of the hydrophobic region of the bilayer. Changes in dL of protein-free MS show a biphasic dependence on the adsorbed amount of PxB with a minimum close to the point of electroneutrality of the mixture. Our results do not discourage the concept of a combined treatment with PxBenriched Curosurf. However, the amount of PxB must be carefully assessed (less than 5 wt% relative to the mass of the surfactant) to avoid inversion of the surface charge of the membrane.

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