Beyond Small Molecules

Bacteriophages, Endolysins, Antimicrobial Peptides & Monoclonal Antibodies as Antimicrobial Agents

Russell E. Lewis, Pharm.D.

2026-06-23

Bacteriophages, Endolysins,
Antimicrobial Peptides & Monoclonal Antibodies

Russell E. Lewis, Pharm.D
Associate Professor of Infectious Diseases (MEDS-10/B)

russelledward.lewis@unipd.it
https://github.com/Russlewisbo
Slides and course materials: www.idpadova.com

Learning objectives

By the end of this session, you should be able to:

• Explain the biology of obligately lytic vs. temperate phages and why it dictates therapeutic suitability
• Critically interpret phage susceptibility testing (plaque assay, EoP, phagogram) as a pharmacodynamic surrogate
• Reason through phage PK/PD, immunogenicity, and phage–antibiotic interactions at the bedside
• Appraise the disconnect between compassionate-use case reports and randomized trial data
• Place endolysins, antimicrobial peptides, and monoclonal antibodies within the same “targeted biologic” antimicrobial framework

This is pitched at fellow level. The throughline of the whole lecture is that these are biologic, narrow-spectrum, self-amplifying or non-replicating agents whose pharmacology breaks the small-molecule rules you already know. Keep returning to the PK/PD and immunology, because that is where the field is genuinely unsettled.

Why now? The post-antibiotic anxiety

• Infectious disease caused ~1/3 of US deaths in 1900;
  the antibiotic “golden era” (1950–70) reversed this
• New class discovery has stalled while resistance accelerates
• Bacterial AMR was associated with ~4.95 million deaths (1.27 million attributable) in 2019; projections approach 10 million/year by 2050
• S. aureus sepsis mortality fell from ~85% (pre-antibiotic) to ~23%, but MRSA bacteremia mortality remains high

The development of phages, lysins, AMPs and mAbs are responses to a market and scientific failure: small-molecule discovery is economically broken. Note the projection figure is contested but rhetorically dominant [1–4].

Four platforms, one logic

Targeted biologic antimicrobials at a glance

Platform	Nature	Replicates?	Spectrum
Bacteriophages	Lytic virus	Yes (self-amplifying)	Very narrow (strain)
Endolysins	Phage-derived enzyme	No	Narrow–moderate
Antimicrobial peptides	Small cationic peptide	No	Broad (often)
Monoclonal antibodies	Host-type IgG	No	Very narrow (epitope)

A Brief History

Discovery: Hankin, Twort, d’Hérelle

Félix d’Hérelle

• 1896 — Hankin describes antibacterial “substance” in the Ganges/Jumna passing through filters and killing Vibrio cholerae; later analyses doubt phages were the agent
• 1915 — Twort reports “ultramicroscopic” lytic agent
• 1917 — d’Hérelle gives unambiguous evidence of a replicating, bacteria-dependent organism; coins bacteriophage (“to devour bacteria”)
• First plaque assay and first quantification of titer described in the same lineage

d’Hérelle is the central figure: he both discovered and immediately therapeutically deployed phages. The anti-Shiga “microbe” was isolated from filtered stool of recovering shigellosis patients.[5–8]

Eclipse and survival in the East

Elivia Institute, Tbilsi, Georgia

• First published therapeutic use ~4 years after d’Hérelle’s discovery; he personally treated many patients
• Penicillin and the antibiotic era eclipsed phage therapy in the West
• Narrow host range, lack of pharma interest, and geopolitics shifted research to chemotherapeutics
• Phage therapy persisted in Eastern Europe and the former Soviet Union (Eliava Institute, Tbilisi) and is still used there

The West assumed new drug classes could be discovered to outrun resistance. That assumption failed, which is why the field looks East for a century of empirical experience — albeit poorly standardized by modern trial standards [9–11].

The modern resurgence

• AMR threat has driven renewed interest as a self-amplifying, self-limiting “drug”
• An estimated 10³¹ phage particles on Earth — a near-inexhaustible, biodiverse resource
• High-profile compassionate-use successes (Patterson/Acinetobacter, 2017) catalyzed Western academic programs
• Trajectory now hinges on translational PK/PD research and well-designed clinical trials

The 2017 Schooley/Patterson case at UCSD is the inflection point for US academic phage therapy and led to IPATH. However, enthusiasm currently outstrips controlled evidence.

[12–14]

Phage Biology

Diversity and taxonomy

• Most abundant biological entity on Earth (~10³¹ particles)
• As of 2022, 1653 phage genera recognized by the ICTV
• Only ~6300 phages examined by EM; ~14,200 sequenced — most with dsDNA genomes
• Classical morphology-based families (Siphoviridae, Myoviridae, Podoviridae) now reorganized; morphology terms persist descriptively

[13,15,16]

International Coimmittee on Taxonamy of Viruses (ICTV) abolished the Siphoviridae/Myoviridae/Podoviridae families (Caudovirales reclassification). This presentation uses these names descriptively for sheath morphology, which is still pedagogically useful.

Anatomy of a tailed phage

Tail fibers are the receptor-binding proteins — remember this when we get to resistance (receptor loss) and engineering (tail-fiber swapping to expand host range) [16,17]

Lytic vs. lysogenic life cycles

Temperate phages can mediate lysogenic conversion (e.g., Shiga toxin, cholera toxin, diphtheria toxin are all prophage-encoded). Hence sequence and confirm lytic lifestyle before clinical use [18–20].

Adsorption: not just Brownian luck

• Encounters are largely Brownian/stochastic — phages are non-motile

• Reversible binding precedes irreversible binding and injection

• Phages can “walk” or “roll” across the surface to find an injection site

• Mucus-associated subdiffusive motion (Ig-like capsid domains binding mucin) raises encounter frequency at mucosal surfaces

  
  
  

The Barr “bacteriophage adherence to mucus” (BAM) model reframes phages as a mucosal innate-immune layer and has implications for inhaled/oral delivery[17,21,22].

Spectrum & Susceptibility Testing

Why we must test every isolate

• Narrow host range means no assumed susceptibility — test against each clinical isolate
• Best suited to monomicrobial infection; polymicrobial sites (diabetic foot, intra-abdominal) are harder
• If multiple strains of one species are present, test each strain
• This is the operational bottleneck that makes phage therapy a personalized intervention

Contrast with antibiotic susceptibility: with phages there is no species-level breakpoint you can extrapolate. Every isolate is its own experiment. This is the logistical reality that limits scalability today [23].

The plaque assay and efficiency of plaquing

• Lawn of clinical isolate + serial phage titers → zones of clearance (plaques)
• EoP = titer producing plaques on the clinical isolate ÷ titer on the propagation host
• A pharmacodynamic surrogate analogous to MIC; allows ranking of candidate phages
• An EoP ≥ 0.1 appears associated with better outcomes (thresholds still provisional)

EoP is the closest thing to a “susceptibility breakpoint” but it is not standardized across labs, and plaque morphology, multiplicity of infection, and propagation host all influence it. Treat thresholds skeptically [23].

Spot tests and phagograms

• Dilutional spot test: spot decreasing titers on a lawn; compare candidate phages in one assay
• Phagogram: co-incubate fixed phage + bacteria, track optical density/respiration vs. growth control over ~24 h
• Time course mirrors broth microdilution kinetics used for antibiotic MICs
• Higher-throughput readouts (e.g., OmniLog respirometry) enable rapid screening

Phagograms are the most “PD-like” assay because they capture suppression over time, including regrowth from resistant subpopulations — which a single plaque count misses [23,24].

Testing for combinations — before the bedside

• Phage therapy is usually given with antibiotics — confirm no antagonism, ideally synergy
• For biofilm indications, test anti-biofilm activity against the clinical isolate in vitro first
• Combination/cocktail design should be deliberate, not ad hoc
• In vitro testing for synergy/antagonism between selected antibiotics and phages is a recommended step

This sets up the later “therapeutic phage interactions” section. The key practice point: you screen not just phage-vs-bug but phage-vs-antibiotic-vs-bug [23,25,26].

Bacterial Resistance to Phages

Resistance is the default, not the exception

• ~10²⁵ phage infections per second in the ocean; 20–40% of marine bacteria killed daily
• Enormous selective pressure has produced layered anti-phage defenses
• Spontaneous phage-resistant mutants arise at ~10⁻⁵ (range 10⁻⁹–10⁻²)
• Clinically, the question is not whether resistance emerges but whether it carries a fitness cost we can exploit

Reframe resistance as a feature, not only a bug: because phage receptors are often virulence factors, resistance frequently comes at a cost. This is the conceptual bridge to phage steering later [27–30].

Receptor modification — and its trade-off

• Most common mechanism: alteration or loss of the phage-binding receptor
• Receptors are often virulence factors or efflux/porin components, so resistance can shift antibiotic susceptibility too
• Phage-resistant mutants are frequently less virulent
• The most common resistance route may yield bacteria that are easier to treat — a therapeutic silver lining

[29–32]

The Chan 2016 OMKO1 / Pseudomonas example: resistance via the OprM efflux porin restores sensitivity to ceftazidime/ciprofloxacin. This is the canonical “evolutionary trade-off” story and underpins phage steering.

Abortive infection and CRISPR-Cas

• Abortive infection (Abi): infected cell commits altruistic suicide (DNA/membrane degradation) before progeny mature — kin selection
• CRISPR-Cas: adaptive immunity in ~40% of sequenced bacteria; spacers guide Cas nucleases to phage DNA
• Other layers: restriction-modification, CBASS/cyclic-nucleotide signaling, superinfection exclusion
• Pre-clinical screening of each clinical isolate sidesteps most defenses at the patient level

The practical reassurance is that you screen the actual isolate, so you only deploy phages that already work against it. The anti-phage “pan-immune system” is an explosively active research area [29,33,34].

Mitigating resistance: cocktails and cycling

• Cocktails: combine phages using different receptors to raise the barrier to resistance
• Sequential therapy: rotate phages of differing antigenicity (also mitigates neutralizing antibody)
• Cocktails must be designed for no inter-phage antagonism (receptor competition, CRISPR upregulation)
• “Phage training” — experimental coevolution to pre-adapt phages — delays resistance

Note the tension: cocktails raise the resistance barrier but complicate manufacturing, regulatory filing, and interaction testing. Precision-designed cocktails beat “throw everything in” cocktails [26,35,36].

Manufacturing, Logistics & Regulation

From sewage to sterile vial

• Discovery sources: wastewater, environmental water, anywhere bacteria are abundant
• For ESKAPE pathogens a matching phage is often found quickly; for rare isolates it may take months or never
• Candidate phages are sequenced and characterized: confirm obligately lytic; exclude toxin/resistance/integrase genes
• Amplified on bacterial hosts to reach therapeutic titers
  

Because phages are grown on bacteria, the manufacturing problem is fundamentally a contaminant problem — you are purifying a virus away from a lysed bacterial culture [23,37,38].

Timeline to produce bacteriophage therapy

Endotoxin and the contaminant problem

• Bacterial lysis releases endotoxin — FDA limit 5 EU/kg/dose
• Removal: ultrafiltration, affinity and ion-exchange chromatography
• Other risks: prophages, pathogenicity islands, toxins, mobile genetic elements from the propagation host
• Mitigation: amplify on a clean surrogate host (e.g., S. carnosus or S. aureus RN4220) rather than the clinical isolate

The RN4220/S. carnosus trick is elegant: propagate on a genetically “clean” relative so you don’t co-purify the clinical isolate’s toxins and MGEs. Sterility testing for residual live bacteria/fungi is also mandatory [37,39].

Stability, storage, and formulation

• Frozen at −80 °C with glycerol cryoprotectant; or 4–6 °C; or lyophilized for ambient storage

• Shelf life can be years with proper storage

• Diluted for use in Plasma-Lyte, normal saline, or other fluids — compatibility is phage-specific

• Always confirm retained activity in the chosen diluent before administration

Practical pearl: a phage stable in saline may be inactivated in another buffer. There is no universal formulation — each preparation needs its own compatibility check [40,41].

Regulatory pathway (EU vs. US)

• Phage therapy is not FDA-approved; use is via clinical trial or compassionate use after conventional failure
• No EU-wide authorised human phage medicinal product under EU law.
• Per-case emergency IND (eIND) or expanded-access IND required
• Emergent eIND can be granted rapidly once a phage is identified; non-emergent ~4 weeks
• Europe: magistral preparations (Belgium) and named-patient frameworks offer alternative routes

The regulatory mismatch is central: a personalized, evolving, biologic product does not fit a framework built for fixed small molecules. Belgium’s magistral pathway is the most-cited pragmatic workaround [23,42,43].

Pharmacokinetics & Pharmacodynamics

Phages break small-molecule PK rules

• Large, proteinaceous, self-replicating, and interactive with host immunity
• Concentration at the target can increase where bacteria are present (auto-dosing) then fall as bacteria clear
• “Dose” is entangled with bacterial density, burst size, and adsorption rate
• Routes: oral, intravenous, inhaled, and direct application

This is the conceptual heart of phage PK: the drug amplifies itself at the site of infection and is cleared when its substrate (bacteria) disappears. Classic Cmax/AUC thinking only partly applies [23,44].

Oral and the GI gauntlet

• Proposed absorption via transcytosis across epithelial tight junctions — limited by phage size
• Vulnerable to gastric pH <6, mucins, and immune clearance
• Mitigation: co-administer with an alkaline/antacid buffer
• Systemic absorption after oral dosing is inconsistent, though phages are recoverable in blood/urine/tissue

Oral works best for enteric targets; expect poor and erratic systemic levels. The Nguyen transcytosis paper provides a mechanism but the magnitude is small [45,46].

Intravenous: fast in, fast out

• Phages detectable in circulation soon after IV dosing, then cleared within 8–12 h by the mononuclear phagocyte system

• Distributes to synovium, heart, muscle, marrow, kidney, bladder — dose-dependent

• Site concentrations are typically several-fold lower than the input titer

• IV most strongly stimulates neutralizing antibody and complement

The reticuloendothelial/MPS clearance is the dominant elimination route and the reason repeat or continuous dosing is often used. It also primes the humoral response that can neutralize later doses [46].

Direct and inhaled administration

• Direct (intra-articular, intravesical, intra-operative, topical) bypasses the MPS → higher local, lower systemic titers

• Used for PJI, CIED, osteomyelitis, UTI in case reports

• Controlled-release (hydrogels, microencapsulation) under study

• Inhaled phages for respiratory infection; lung-specific clearance/barriers still being defined

Direct administration is favored for biofilm/device infections precisely because it bypasses MPS clearance and maximizes local titer at a site where chance encounters with sessile bacteria are limiting [23,46,47].

Dosing: empirical, with rare rationale

• Regimens range from single doses to multiple daily dosing to continuous infusion — mostly empirical
• One rational example: twice-daily IV based on detectable phage DNA up to 12 h post-dose
• Multi-route administration is often combined to overcome single-route barriers
• ARLG Task Force: use the highest safe, tolerated dose with endotoxin below limits; repeat dosing to maximize site levels

The honest message: dosing is not yet evidence-based. The Fabijan twice-daily regimen is the rare example where PK data actually informed the interval [23,48].

ARLG ideal PK/PD properties

A favorable phage product should combine:

• High microbial susceptibility (low EoP threshold met)

• High local concentration at the site of infection

• High adsorption rate (infectivity)

• Large burst size (progeny per infected cell)

• Short latent period (replication time within the cell)

These five parameters are the ARLG’s attempt to define a phage “PD profile.” Standardized methods to measure them in vivo do not yet exist — a key research gap the task force itself flags [23].

Immunogenicity

The neutralizing antibody response

• Phages are live viruses → IgM within weeks, followed by IgG
• Neutralizing antibodies can sharply reduce phage titers and correlate with clinical failure [49]
• Cocktails do not solve this — antibodies form against all component phages, with cross-reactivity
• Not universal: some prolonged courses do not generate neutralizing titers

The Dedrick 2021 Nature Medicine case is the cautionary tale: a pulmonary M. abscessus infection where rising neutralizing antibody limited efficacy. This is arguably the most underappreciated PD problem in the field [35,50,51].

Route matters for immunogenicity

• IV elicits the most robust humoral response; oral and direct elicit less
• Direct administration → lower neutralizing titers, rising with systemic absorption
• Autoimmune phenomena are theoretical and not described with phage therapy to date
• Sequential therapy with antigenically distinct phages can outpace antibody — but narrow inventories limit cycling

Practical implication: for a device or joint infection, direct intra-articular delivery is doubly attractive — it bypasses MPS clearance and minimizes the neutralizing antibody response [35,52].

Phage–Antibiotic
& Phage–Phage Interactions

Synergy, antagonism, or neutrality

• Phages + antibiotics can be synergistic, antagonistic, or neutral
• General rule: cell-wall agents → synergy; protein-synthesis inhibitors → potential antagonism
• “PAS” (phage–antibiotic synergy): sub-MIC β-lactams/quinolones can stimulate virulent phage production
• But responses are phage- and antibiotic-specific — general rules are only a starting point

PAS mechanism: filamentation from cell-wall stress increases phage burst size. The Gu Liu 2020 mBio paper shows synergy depends on mechanism and stoichiometry — so empirical testing beats dogma [53,54].

When combinations backfire

• Colistin + LPS-binding phages: colistin destabilizes LPS, the very receptor the phage needs → antagonism
• Burst size, pH, viscosity, and fluid dynamics all modulate interactions
• Scoring systems have been proposed but in vitro testing of the actual pair is essential
• Bottom line: confirm the specific phage–antibiotic combination empirically before use

The colistin example is the cleanest teaching case of mechanistic antagonism: the antibiotic and phage compete for / destroy the same molecular target [53,55].

Phage–phage interactions in cocktails

• Phages can interfere with one another — reduced activity of each
• Drivers: competition for shared receptors and CRISPR-Cas upregulation
• Interactions are unpredictable → evaluate combinations individually
• Argues for precision-designed cocktails over ad hoc mixtures

Same theme as resistance mitigation: a well-designed cocktail uses non-competing receptors and confirmed non-antagonism. “More phages” is not automatically “more killing” [26].

Safety

A reassuring overall profile

• Generally favorable safety across preclinical, case-report, and phase I/II data
• FDA has granted GRAS status for certain phage applications
• Phages are part of the human gut virome — ubiquitous baseline exposure
• ARLG nonetheless recommends interval monitoring of renal/liver function and CBC during therapy

Generally Recognized As Safe (GRAS)

The ecological argument for safety is strong — we ingest billions of phages daily — but it should not substitute for monitoring, especially given endotoxin and cytokine concerns with rapid lysis [[26]; @Liu2021; [56]; [57]; [23]].

Mechanisms of adverse events

• Endotoxin/bacterial debris release from rapid lysis → proinflammatory cytokine response
• Contaminants: bacterial DNA, enterotoxins, exotoxins, lipoteichoic acid → hypersensitivity/cytokine release
• Manufacturing residues: cesium chloride, polyethylene glycol
• Net effect on the microbiome is narrow vs. antibiotics but understudied

Lysis-associated cytokine release is the mechanistic worry analogous to a Jarisch-Herxheimer reaction — relevant in high-burden bacteremia where you lyse a large bacterial load quickly [58].

Adverse events in practice

• Preclinical: generally well tolerated; occasional cytokine (IL-1β/IL-6) or IgG/IgM rises without clinical change
• Case reports: mostly none or transient — transient hypotension, fevers/chills, wheeze, flushing, nausea
• Self-limited IL-6/IL-8 cytokine storm resolving in ~1 day reported
• Transient transaminitis with IV/intra-articular dosing; reversible, outcomes still favorable

The transaminitis signal (Doub & Wilson) is worth flagging for fellows managing real cases — monitor LFTs, especially with pre-existing liver disease, but it has generally been reversible and not outcome-limiting [58–60].

MDR Infections & Phage Steering

Phages against drug-resistant bacteria

• Resistance to antibiotics does not confer resistance to phages — different targets (surface receptors)
• Most case reports use phages with antibiotics, not as monotherapy
• Narrower microbiome impact than broad-spectrum antibiotics — less collateral resistance selection
• Efficacy in MDR infection will only be settled by prospective trials

The decoupling of antibiotic and phage resistance is the core rationale for MDR use. But remember the honest caveat: combination use makes attribution of benefit to the phage difficult in uncontrolled cases [12,14,59].

Phage steering: weaponizing the trade-off

• Phage steering drives bacteria toward a more susceptible phenotype [61]
• P. aeruginosa: selection for phage resistance via efflux/porin restores antibiotic susceptibility [31]
• Can even re-sensitize to drugs not normally used (e.g., tetracyclines for Pseudomonas) [61]
• Klebsiella and Acinetobacter: phage resistance entails loss of capsule/MDR and re-sensitization [32,62]

[31,32,61,62]

Steering reframes the therapeutic goal: you may not need the phage to clear the infection outright — you need it to bend the population back into the reach of a cheap antibiotic. This is one of the most intellectually exciting ideas in the field.

Biofilm Infections

Why phages suit biofilms

• Most bacteria live in sessile biofilm states, not planktonic [25]
• Phages encode depolymerases and endolysins that degrade the extracellular polymeric matrix [63]
• Confined biofilm environment can increase chance phage–bacteria encounters [25]
• Can infect metabolically reduced cells, degrading biofilm piecemeal [63]

Depolymerases are a distinctive phage advantage over antibiotics for device/biofilm infection — they actively dismantle the matrix rather than merely failing to penetrate it.

[25,63]

Delivery determines biofilm success

• Direct administration reduces biofilm; systemic administration often does not reach device biofilm
• Murine PJI: IP phage + vancomycin synergized against planktonic bacteria but not the implant biofilm
• Phages are non-motile → maximize chance encounters by delivering directly to the biofilm
• Clinical case series support phages as a powerful adjuvant in recalcitrant biofilm infection

The Morris study is the clearest demonstration that route is everything for biofilm/device infection — systemic dosing failed to reach the implant despite synergy in tissue. This justifies intra-operative and intra-articular delivery for PJI [25,64,65].

Clinical Trials: The Reality Check

Case reports vs. controlled trials

• Numerous case reports show promise across syndromes and pathogens
• M. abscessus: favorable response in 11/20 compassionate-use patients
• Since 2000, 13 English-language trials; 6 assessed efficacy
• Controlled trials have not reproduced the dramatic case-report outcomes

Publication bias inflates the apparent success of case reports. The trials are where enthusiasm meets reality, and so far reality has been sobering [14,49,66].

Early efficacy signals

• Wright 2009 (chronic P. aeruginosa otitis): single 6-phage cocktail, PST-confirmed → improved clinical scores and lower P. aeruginosa counts vs. placebo
• The first modern randomized, double-blind, placebo-controlled phage trial
• Small (n≈24) and single-center, but a genuine positive efficacy signal

Wright remains the most cited “positive” RCT — note that it confirmed susceptibility (PST) before enrollment, a design feature later “negative” trials often lacked [67].

The instructive failures

• Rose 2014 (burn wounds): fixed (non-personalized) cocktail, no prospective PST; no bacterial-load difference; spray ran off the wound
• Sarker 2016 (pediatric E. coli diarrhea, Bangladesh): oral T4-like coliphages, no PST; no clinical efficacy, no intestinal amplification
• Recurring culprits: fixed cocktails, no susceptibility testing, inadequate coverage, delivery failure

The pattern is diagnostic: the negative trials violated the very principles (personalization, PST, adequate local delivery) that the case reports followed. The failures are arguably about trial design, not phage biology [66,68,69].

PhagoBurn and the delivery problem

• Jault 2019 (PhagoBurn): RCT of a P. aeruginosa phage cocktail vs. standard care in burn wounds

• Phages slower to reduce bacterial burden than standard-of-care antiseptic

• Manufacturing reduced titer over storage → under-dosing; some isolates resistant at low residual titer

• A landmark in showing how CMC/manufacturing can sink a phage trial

PhagoBurn is the cautionary CMC tale: the delivered titer dropped far below intended because of stability problems, so the trial tested an underpowered dose. Manufacturing rigor is a clinical-efficacy issue, not just a regulatory one [70].

Intravesical phages and the honest null

• Leitner 2021: intravesical phages vs. antibiotics vs. placebo for UTI before TURP

• Phages were not superior to placebo and not non-inferior to antibiotics

• Well-designed, multi-arm, double-blind — a rigorous neutral result

• Reinforces that adjunctive, personalized, well-delivered phages remain the most defensible use today

Leitner is methodologically the strongest “negative” trial. The field’s current consensus position: phages are best viewed as a personalized adjuvant for recalcitrant/MDR/biofilm infection, pending better trials (e.g., ongoing CYPHY, and NIH-funded adaptive designs) [23,71].

Selected phage clinical trials

Efficacy-focused phage trials [67–71]

Study (year)	Indication	Design	Efficacy signal
Wright (2009)	Chronic P. aeruginosa otitis	RCT, DB, PC	Positive
Rose (2014)	Burn wound colonization	Self-controlled	Null
Sarker (2016)	Pediatric diarrhea	RCT, PC	Null
Jault / PhagoBurn (2019)	Burn wound P. aeruginosa	RCT	Inferior (under-dosed)
Leitner (2021)	UTI pre-TURP	RCT, DB, 3-arm	Null

One positive among the rigorous efficacy trials — and that one confirmed susceptibility first. The take-home is a design lesson as much as a biology lesson [66,67,70,71].

Endolysins

Enzybiotics: lysis from the outside

• Phage-derived enzymes that hydrolyze peptidoglycan → osmotic lysis
• Three classes: glycosidases, amidases, endopeptidases
• Gram-positive endolysins: N-terminal catalytic domain + C-terminal cell-wall-binding domain
• Gram-negative endolysins: globular, no discrete binding domain

“Enzybiotics” is the umbrella term. Unlike phages, these are defined chemical entities with conventional PK — a regulatory advantage. The trade-off: no self-replication and no in-situ amplification [72].

The Gram-negative barrier

• Exogenous endolysins reach peptidoglycan readily in Gram-positives (thick, exposed PG)
• Gram-negative outer membrane blocks access to PG
• Workarounds: EDTA/weak acids to permeabilize, or engineered “Artilysins” fusing endolysin to a membrane-translocating peptide
• Receptor-binding-protein fusions can enable Gram-negative killing

The OM barrier is why endolysins are, for now, mostly a Gram-positive story. Artilysins (Briers/Lavigne) are the leading engineering strategy to break into Gram-negatives [72–74].

Resistance, PK, and immunogenicity

• Resistance less likely than to phages — peptidoglycan bonds are highly conserved and hard to alter
• More conventional PK than phages (a defined protein, not a replicating particle)
• Can still elicit anti-protein antibodies, but routine exposure tempers immunogenicity
• Broader host range than phages, but narrower than most antibiotics

The conserved-target argument for a high resistance barrier is the main selling point. Whether anti-drug antibodies limit repeat dosing in humans is still being worked out [72,75].

Endolysins in the clinic

• SAL200 (staphylococcal, IV up to 10 mg/kg): phase I — only mild fatigue/myalgia
• Staphefekt (topical): improved symptoms in atopic dermatitis (placebo-controlled)
• Exebacase (CF-301): synergy with anti-staphylococcal antibiotics in vitro
• Exebacase phase III for S. aureus bacteremia/endocarditis stopped for futility at interim

Exebacase is the most advanced endolysin program and its phase III futility (DISRUPT) is the field’s reality check, mirroring the phage trial story: promising biology, hard-to-prove clinical benefit. Gram-positive endolysins for bone/joint infection and bacteremia remain the most likely near-term wins [76–79].

Antimicrobial Peptides

AMPs: ancient innate effectors

• Small (<100 aa), usually cationic, amphipathic (hydrophobic + hydrophilic domains)
• Primary action: cytoplasmic membrane disruption; also inhibit nucleic-acid/protein/cell-wall synthesis
• Broad spectrum and immunomodulatory properties
• Phage-encoded AMPs: lytic factors (non-enzymatic) and tail-complex proteins

AMPs differ from endolysins in being non-enzymatic and membrane-targeting. Protein E of φX174 (inhibits MraY translocase) is the classic phage-encoded “protein antibiotic” example[80–82].

Promise vs. the historical wall

• Attractive for biofilm and MDR infection; some can cross the Gram-negative OM [75]

• Historically limited by toxicity, poor PK, and weak in vivo activity [83]

• Engineered peptides (eCAPs) that mimic endogenous AMPs are an emerging fix [83]

• PLG0206: engineered AMP, phase I IV single-dose — linear PK, well tolerated; phase II in acute PJI [84]

The AMP story is the oldest and most disappointing of the three phage-derived platforms clinically, but engineering (eCAPs, PLG0206/WLBU2) is reviving it, notably for local PJI use where systemic toxicity is less limiting [83,84].

Three platforms compared

Phage-derived antibacterial platforms

Feature	Phages	Endolysins	AMPs
Self-replicating	Yes	No	No
Spectrum	Strain-narrow	Narrow–moderate	Often broad
Gram-negative use	Yes	Hard (needs engineering)	Some natively
Resistance barrier	Low–moderate	High	Variable
PK predictability	Low	Higher	Variable

Use this to consolidate before pivoting to monoclonal antibodies — a non-phage biologic that shares the “narrow, targeted, manufactured” theme but adds host-immune mechanisms.

Monoclonal Antibodies as Antimicrobials

From serum therapy to monoclonals

• Antibody-based anti-infectives predate antibiotics: serum therapy for diphtheria/pneumococcus (1890s–1930s)

• Displaced by antibiotics, now revived for the same reason as phages — AMR and unmet niches

• A monoclonal antibody (mAb) is a defined, host-type IgG targeting a single epitope

• Like phages: a narrow-spectrum, manufactured biologic — but it recruits host immunity rather than lysing directly

The historical symmetry is the hook: both phage therapy and antibody therapy were pre-antibiotic ideas eclipsed by small molecules and revived by resistance. mAbs are the more clinically mature of the two biologic platforms [85,86].

Mechanisms of anti-infective mAbs

• Toxin neutralization — bind and inactivate a toxin (no direct bactericidal effect)
• Neutralization of attachment/entry — block a viral or bacterial receptor interaction
• Opsonophagocytosis & ADCC — Fc-mediated recruitment of phagocytes/NK cells
• Complement activation (CDC) — classical pathway via Fc

Most successful anti-infective mAbs are neutralizers (toxin or virus), not direct bacterial killers. Effector-function-dependent bacterial killing has been much harder to translate [85].

Immunologic rationale and engineering

• Format evolution: murine → chimeric → humanized → fully human to cut immunogenicity
• Fc engineering tunes effector function (ADCC/CDC) up or down
• Half-life extension (e.g., YTE mutations) enables single-dose, season-long prophylaxis (nirsevimab)
• Specificity is the double-edged sword: exquisite targeting, but no breadth and vulnerability to antigenic change

YTE (M252Y/S254T/T256E) increases FcRn binding and extends half-life to ~70 days — the technical reason a single nirsevimab dose covers an RSV season. This pharmacology contrast with phages (self-amplifying, short-lived) is worth emphasizing [85,87].

Bezlotoxumab — anti-toxin, not antibacterial

• Human mAb against C. difficile toxin B; does not kill the organism
• MODIFY I/II (adjunct to standard antibiotics): recurrent CDI reduced ~17% vs. ~28% with placebo
• FDA-approved 2016 for prevention of CDI recurrence in high-risk adults
• Caveat: heart-failure exacerbation signal in patients with CHF

Bezlotoxumab is the cleanest example of the toxin-neutralization paradigm: it prevents recurrence by mopping up toxin B while antibiotics handle the vegetative organism. The CHF safety signal is a board-relevant caveat [88].

Ibalizumab — a mAb as antiretroviral

• Humanized mAb binding CD4 domain 2; blocks HIV-1 entry post-attachment (a post-attachment inhibitor)

• First-in-class, FDA-approved 2018 for multidrug-resistant HIV-1, IV every 2 weeks

• TMB-301: ≥1 log₁₀ viral-load drop in 83% at day 7; ~43% achieved <50 copies/mL by week 25

• Does not share resistance pathways with small-molecule antiretrovirals

Ibalizumab shows mAbs can serve as direct antivirals in a salvage niche, with a non-overlapping resistance mechanism — conceptually parallel to using phages where antibiotic resistance doesn’t transfer [89].

Anti-anthrax mAbs — neutralizing protective antigen

• Both target protective antigen (PA) to block anthrax toxin assembly/entry [90]
• Raxibacumab — FDA-approved 2012 for inhalational anthrax (treatment + prophylaxis) [90]
• Obiltoxaximab — FDA-approved 2016, same indication [91]
• Approved via the FDA Animal Rule (efficacy from animal models; human safety only)

The Animal Rule is the key teaching point — you cannot ethically/feasibly run efficacy RCTs for anthrax, so approval rests on animal efficacy plus human PK/safety. These are stockpiled medical-countermeasure mAbs [90,91].

RSV — palivizumab to nirsevimab

• Palivizumab: anti-RSV F protein; IMpact-RSV cut hospitalization ~55% in high-risk infants; monthly dosing [92]
• Nirsevimab: YTE half-life-extended anti-F mAb; single dose per season [87]
• MELODY: ~75% reduction in medically attended RSV LRTI [87]
• HARMONIE: ~83% reduction in RSV hospitalization in a pragmatic European trial [93]

RSV prophylaxis is the great mAb success story: the leap from monthly palivizumab (high-risk only) to single-dose nirsevimab (all infants) is driven by Fc/YTE half-life engineering. This is the platform working as intended [[92]; [87]; [Drysdale202?].

Anti-SARS-CoV-2 mAbs — the escape lesson

• Sotrovimab (COMET-ICE): ~79% reduction in hospitalization/death in early high-risk COVID-19
• Casirivimab/imdevimab, bamlanivimab, tixagevimab/cilgavimab also deployed
• Authorizations withdrawn as Omicron sublineages escaped neutralization
• The defining cautionary tale: monoclonal specificity collapses against a rapidly evolving target

The COVID mAbs are the clearest illustration of the central weakness — a single-epitope agent is one mutation away from obsolescence. Directly analogous to phage resistance via receptor change; the answer in both fields is combinations/cocktails and breadth [94].

The antibacterial mAb graveyard

• Suvratoxumab (anti-S. aureus α-toxin), SAATELLITE: lower VAP incidence numerically but did not meet its primary endpoint

• Gremubamab / MEDI3902 (bispecific anti-P. aeruginosa PcrV + Psl), EVADE: did not reduce P. aeruginosa pneumonia

• Despite strong preclinical data, no antibacterial mAb is FDA-approved for treating bacterial infection

• Successes remain toxin neutralization (bezlotoxumab) and prophylaxis

  
  
  

This is the honest counterweight: against whole bacteria, mAbs have repeatedly failed in phase II/III. The wins are toxin-neutralizing (bezlotoxumab) and antiviral/prophylactic — not bactericidal [94].

Why antibacterial mAbs keep failing

• Redundancy: bacteria deploy many virulence factors — neutralizing one is rarely enough
• Single-epitope fragility: antigenic variation defeats narrow targeting (cf. COVID escape)
• Timing & access: mAbs work best as prophylaxis, less as rescue in established sepsis/biofilm
• No self-amplification: unlike phages, a fixed dose cannot “follow” the bacterial burden

Tie this directly back to phages: the very property phages have (self-amplification at the site, with depolymerase-mediated biofilm access) is what fixed-dose mAbs lack. Conversely mAbs have the mature manufacturing/regulatory path phages lack [85,95].

Synthesis & Future

Phages vs. mAbs: two targeted biologics

Targeted biologic antimicrobials

Dimension	Bacteriophages	Monoclonal antibodies
Mechanism	Direct lysis + amplification	Neutralization / Fc effector
Self-amplifying	Yes	No
Spectrum	Strain-level	Epitope-level
Main use today	Established MDR/biofilm (adjuvant)	Prophylaxis / toxin neutralization
Resistance/escape	Receptor change	Antigenic variation
Manufacturing/regulatory	Immature, personalized	Mature, standardized

The unifying thesis: both are narrow, biologic, manufactured anti-infectives revived by AMR. Their weaknesses are mirror images — phages are biologically powerful but operationally immature; mAbs are operationally mature but biologically limited against whole bacteria [23,85].

Where the field is heading

• Engineered phages: tail-fiber swapping to broaden host range; synthetic/CRISPR-armed phages [96]
• Phage steering & PAS integrated into rational antibiotic-sparing regimens [53,61]
• Standardized PK/PD and susceptibility methods; adaptive, personalized trial designs [23]
• Biologic combinations: phages + endolysins + antibiotics; mAb cocktails to counter escape [97]

The future is combinatorial and personalized. Regulatory innovation (Belgium magistral model; FDA adaptive frameworks) may matter as much as biology for whether phages reach routine practice [23,61,96,97].

Take-home messages

• Use obligately lytic, susceptibility-confirmed phages; screen every isolate
• Phage PK/PD is non-classical — self-amplification, MPS clearance, neutralizing antibody, route-dependent delivery
• Case reports outrun controlled evidence; design failures (no PST, fixed cocktails, under-dosing) explain most negative trials
• Endolysins, AMPs, and mAbs complete a spectrum of targeted biologics — mAbs win at prophylaxis/toxin neutralization, phages at established MDR/biofilm infection

Close by returning to the opening table: four platforms, one logic. The clinical art is matching the platform to the niche its pharmacology actually fits [23,66,88].

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