{"id":"f95a146d-4edd-463c-8aff-e94aa004e38a","arxiv_id":"2412.04894","paper_version":1,"verdict":"CONDITIONAL","confidence":"MODERATE","novelty_score":5.0,"correctness_risk":"medium","formal_verification":"none","parameter_count":4,"one_line_summary":"Polymyxin B binds electrostatically to the Curosurf bilayer surface without penetrating it, and the safe admixture window is below about 5 wt percent.","lead":"What this paper found: polymyxin B sticks to the surface of Curosurf lung surfactant and does not insert into its membrane, while a simpler model membrane is more easily penetrated. Why it matters: it identifies a dose window below 5 percent antibiotic by weight for a combined antibiotic-surfactant therapy against lung infections.","discovery_kind":"new_application","skeptic_critique":{"model":"deepseek-v4-flash","headline":"SANS comparison of PSUR dL combines vesicle-model fits for pure PSUR with paracrystal lamellar fits for PxB-PSUR; if the model change shifts dL by ~0.1 nm, the no-penetration interpretation is not supported.","rationale":"The reader's weakest-assumption analysis identifies exactly the point I consider most load-bearing: the SANS dL comparison for PSUR spans two different fitting models, and the claimed PSUR effect is small enough that a model-dependent systematic shift could mimic or mask it. This is not an accusation of error; it is a concrete cross-calibration gap that the paper does not close. The multi-technique dataset is otherwise coherent and gives partial independent support to the no-penetration claim: the Pyr4PC/Pyr10PC lateral-pressure profiles show no deep-core perturbation in either system, and DSC shows PSUR Tm unaffected while MS Tm rises. Those observations make the central claim plausible, which is why I do not recommend moving from the reader's CONDITIONAL verdict. The proposed test is quantitative and uses data already collected: if the paracrystal model reproduces the vesicle-model dL on PxB-free PSUR and on simulated ULV curves, the concern is resolved and the structural argument stands. If it does not, the authors need to either re-fit the whole dL series with one consistent model or soften the SANS-based claim to a fluorescence-plus-DSC claim. No machine-checked proof or independent code is provided, so this wet-lab conclusion cannot be verified from the text alone; the conditional status is appropriate.","tokens_in":28639,"tokens_out":5203,"duration_ms":62509,"concrete_test":"Re-fit the PxB-free PSUR ULV SANS pattern (Fig. 4A, violet) with the same paracrystal lamellar model used for PxB-PSUR mixtures, over the same q range (0.1-2.5 nm-1) and with the same instrumental resolution smearing. Then compare the resulting dL with the reported vesicle-model value 3.74 +/- 0.02 nm. In parallel, fit PxB-PSUR data at 4.8 and 11.1 wt% without constraining dL and inspect the dL versus wt% trend. As a control, generate a synthetic ULV curve from the vesicle model with a known dL (e.g., 3.74 nm) and fit it with the paracrystal model to estimate the systematic model offset. If the paracrystal-model dL for PxB-free PSUR is within 0.05 nm of the vesicle-model value and the synthetic offset is negligible, the SANS comparison is valid; if the offset exceeds about 0.1 nm, the no-penetration conclusion would need to rest on the fluorescence data alone.","verdict_should_be":"UNCHANGED","load_bearing_attack":"The paper's central structural conclusion is that PxB binds to the PSUR bilayer surface without inserting its acyl tail, supported mainly by the observation that the SANS-derived bilayer thickness dL of PSUR changes negligibly, whereas the protein-free MS shows a biphasic thinning. The comparison, however, is made across two different structural models: PxB-free PSUR ULVs are fitted with the polydisperse unilamellar vesicle model, while every PxB-containing PSUR sample is fitted with the paracrystal lamellar model (Methods, SANS; Fig. 4A). The paracrystal model introduces a lamellar repeat distance, a number-of-layers parameter NL, and its own polydispersity treatment, so dL is not automatically on the same scale as the vesicle-model dL. The observed PSUR thinning appears small, of order 0.1 nm, so a model-induced offset of even 0.05-0.1 nm would alter the conclusion. The lateral-pressure measurements at Pyr4PC/Pyr10PC provide independent evidence, but the no-penetration claim is explicitly stated to be 'derived from SANS data analysis and changes in lateral pressure', so the SANS leg should be secure. Because the paper never cross-checks dL from the two models on the same sample or against a simulated standard, this is the weakest load-bearing point in the argument.","agreement_with_reader":"agree"},"referee_report":{"model":"deepseek-v4-flash","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.","tokens_in":28868,"tokens_out":9503,"duration_ms":96566,"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":[{"comment":"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.","section":"Methods, SANS; Fig. 4A/B"},{"comment":"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.","section":"Zeta potential; Fig. 1; Fig. 4B; stoichiometry paragraph"}],"minor_comments":[{"comment":"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.","section":"The effect of PxB on the lipid bilayer"},{"comment":"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.","section":"Results and discussion, SANS paragraph"},{"comment":"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.","section":"Introduction"},{"comment":"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.","section":"Materials and Methods, hydration medium"},{"comment":"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.","section":"Lateral pressure discussion"},{"comment":"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.","section":"Conclusion"}],"recommendation":"major_revision","confidential_remarks":"The paper is within the journal's scope and the experimental work is substantial. The main risk is the SANS cross-model comparison for dL; if the authors can supply the requested validation, the central no-penetration claim becomes much more secure. The free/bound PxB issue should be acknowledged even if a full binding isotherm cannot be provided. I see no grounds for questioning the integrity of the work."},"author_rebuttal":null,"desk_editor":{"model":"deepseek-v4-flash","letter":"Short version: this is a careful multi-technique study of polymyxin B binding to Curosurf and a four-lipid model system. The new quantitative data — zeta potential, DSC, SAXS/SANS, lateral pressure — hang together, and the claim that PxB orders PSUR without penetrating the bilayer is mostly supported. The weakest link is exactly what the stress-test note flags: dL for PxB-free PSUR comes from a unilamellar vesicle model, while PxB-PSUR is fitted with a paracrystal lamellar model. The observed PSUR thinning is small, of order 0.1 nm, so a model-induced offset of that size could change the interpretation. This is a fixable flaw, not a fatal one. The authors should fit the same sample with both models or use a model-independent check, and report whether dL shifts.\n\nWhat is genuinely new: for clinically used Curosurf, the paper reports bilayer thickness, lamellar repeat, transition temperature, surface charge, and lateral pressure as functions of PxB loading, plus the biphasic dL response in the model system with a minimum near electroneutrality. The charge-inversion ceiling of about 5 wt% is a concrete, useful number for formulation and animal-study design. The authors also honestly note that free versus bound PxB was not measured; the stoichiometry assumes all added drug binds, and that approximation should be stated more prominently.\n\nThe soft spots, in order: (1) the SANS model switch, which the stress-test note correctly identifies as the load-bearing point for the no-penetration conclusion; (2) the inferred protective role of PE, SM, SP-B, and SP-C — that rests on a single system comparison, reasonable but not proven; (3) the DSC claim that PxB does not affect PSUR Tm is based on data with scatter around 0.3-0.5 °C, so a small shift could be hidden — minor, because WAXS temperature scans agree. The zeta and SAXS data are internally consistent, error bars are given for most quantities, and the citation pattern is appropriate, building on the same group's earlier work and on Khondker et al.\n\nWho is this for: formulation scientists and researchers working on surfactant-drug combinations. It is a careful experimental contribution, not a breakthrough. I would send it to peer review with a request to address the model-consistency question. It may need revision, but the core dataset deserves referee time.","headline":"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.","tokens_in":29537,"tokens_out":1499,"would_cite":true,"duration_ms":16778,"reading_group":"maybe","serious_thinker":"yes","would_accept_peer_review":true},"rs_alignment":null,"lean_confirmation":null,"pith_extraction":{"msc":[],"pacs":[],"model":"deepseek-v4-flash","headline":"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.","keywords":["polymyxin B","pulmonary surfactant","Curosurf","small-angle neutron scattering","zeta potential","bilayer thickness","phase transition","drug delivery"],"falsifier":"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.","tokens_in":28394,"feed_emoji":"🫁","tokens_out":9845,"duration_ms":97176,"temperature":0.7,"pith_summary":"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.","feed_headline":"Polymyxin B rides on lung surfactant's surface, not inside it","feed_subtitle":"Curosurf keeps its ordered lamellae below ~5 wt% polymyxin B; above that, the surface charge flips.","key_machinery":"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.","core_discovery":"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.","pith_inferences":[],"forward_implications":["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."],"supporting_citations":[{"why":"It supplies the membrane-charge and lipid-packing framework used to explain why PxB penetrates the protein-free model but not Curosurf.","marker":"[14]"},{"why":"It reports the biophysical and physiological effects of 2% PxB in Curosurf, the clinical baseline this study's structural results are meant to support.","marker":"[29]"},{"why":"It provides the prior finding that PxB reverses LPS-induced swelling of Curosurf, which motivates the stabilizing role claimed here.","marker":"[37]"},{"why":"It proposes PxB as a functional analogue of surfactant protein SP-B, underpinning the surface-bound mechanism.","marker":"[38]"},{"why":"It provides the paracrystal lamellar SANS model used to extract bilayer thickness and stack number from PxB-containing vesicles.","marker":"[47]"},{"why":"It establishes the excimer fluorescence method for lateral pressure at defined depths in the bilayer.","marker":"[54]"},{"why":"It characterizes the composition and thermotropic structure of porcine and bovine exogenous surfactants, the comparison standard for bilayer thickness and transition temperature.","marker":"[59]"},{"why":"It reports fluid-state bilayer thickness for bovine lung surfactant, used as a cross-check on the Curosurf values.","marker":"[82]"},{"why":"It demonstrates the suitability of the paracrystal lamellar model for lipid-additive oligolamellar vesicles in prior work.","marker":"[91]"},{"why":"It gives molecular structures and volumes for phosphatidylglycerol lipids needed to compute the model system's scattering length density.","marker":"[48]"}],"fun_headline_variants":["Polymyxin B docks on Curosurf surface, not in core","Curosurf keeps polymyxin B outside, below 5 wt%","Surface charge flips if polymyxin B exceeds 5%","Polymyxin B strengthens Curosurf lamellae from outside","For Curosurf, polymyxin B binds electrostatically, stays outside"],"cache_read_input_tokens":3200,"weakest_assumption_plain":"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.","fun_headline_variants_meta":{"raw":{"variants":["Polymyxin B docks on Curosurf surface, not in core","Curosurf keeps polymyxin B outside, below 5 wt%","Surface charge flips if polymyxin B exceeds 5%","Polymyxin B strengthens Curosurf lamellae from outside","For Curosurf, polymyxin B binds electrostatically, stays outside"]},"model":"deepseek-v4-flash","effort":"low","cost_usd":0.000214,"raw_usage":{"total_tokens":1556,"prompt_tokens":1207,"completion_tokens":349,"prompt_tokens_details":{"cached_tokens":384},"prompt_cache_hit_tokens":384,"prompt_cache_miss_tokens":823,"completion_tokens_details":{"reasoning_tokens":246}},"tokens_in":823,"tokens_out":349,"duration_ms":6967,"temperature":1.0,"reasoning_tokens":246,"cache_read_input_tokens":384,"cache_creation_input_tokens":0},"cache_creation_input_tokens":0},"created_at":"2026-08-11T21:10:07.780370+00:00","model_set":{"reader":"deepseek-v4-flash"},"falsifier":"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.","supporting_citations":[],"review_version":1}