A fully synthetic antibiotic class

• Oxazolidinones are prepared entirely by organic synthesis — no fermentation product
• 1978: DuPont patents 5-(halomethyl)-3-aryl-2-oxazolidinones with activity against plant pathogens
• Further optimization → first agents with activity against human pathogens

Two FDA/EMA-approved agents

• Investigational agents: sutezolid, radezolid, delpazolid, contezolid
• No drug-class cross-resistance with β-lactams, vancomycin, daptomycin, or quinupristin-dalfopristin

Basic oxazolidinone scaffold

• Core five-membered 2-oxazolidinone ring (the “A-ring”)
• C5 modification of the A-ring and N-aryl B-ring are essential for antibacterial activity
• Fluorination of the B-ring further increases potency

Linezolid — structural features

• C5 acetamide side chain on the A-ring
• N-aryl B-ring with single fluorine
• Single morpholine D-substituent
• Sufficient for activity against typical Gram-positive targets

Tedizolid — engineered for breadth

• C5 hydroxymethyl group replaces acetamide → preserves activity against cfr-carrying strains
• Added pyridine (C-ring) and tetrazole (D-ring) moieties → lower MICs
• Administered as the phosphate prodrug (Sivextro)

Where do oxazolidinones bind?

Bacteriostatic — and why that’s fine

• Generally bacteriostatic against staphylococci and enterococci; modestly bactericidal against streptococci
• “Static vs cidal” distinction may be clinically overrated — systematic reviews have found no consistent outcome difference
• Don’t withhold linezolid in S. aureus bacteremia on cidal-vs-static grounds alone

Aerobic Gram-positive activity

Tedizolid is more potent in vitro — but…

• Tedizolid MICs are 2–8× lower than linezolid across most Gram-positive species
• Lower MIC ≠ better clinical outcome without head-to-head trials
• CLSI: tedizolid susceptibility can be inferred from linezolid susceptibility for S. aureus, E. faecalis, S. agalactiae, S. pyogenes, S. anginosus
• Linezolid-resistant strains may still be tedizolid-susceptible

Mycobacteria

• Excellent activity against MDR- and XDR-M. tuberculosis
  • Linezolid MIC range: 0.125–2 μg/mL
  • Tedizolid MIC range: 0.125–0.5 μg/mL
• Active against rapid growers (M. abscessus spp., M. massiliense) and slow-grower M. kansasii
• Higher MICs for M. avium / M. intracellulare

Nocardia and other higher-order bacteria

• Linezolid: virtually 100% in vitro susceptibility across Nocardia spp.
  • MIC₉₀ 1–4 μg/mL
• Tedizolid: lower MICs than linezolid for most Nocardia spp.
  • Comparable to linezolid for N. nova complex and N. brasiliensis
• Variable activity vs. actinomycetes; Corynebacterium, Listeria, Bacillus, Erysipelothrix, Rhodococcus equi, Leuconostoc, Pediococcus — case-by-case

Where oxazolidinones do NOT shine

• Aerobic Gram-negatives: limited
• Incomplete coverage of respiratory pathogens H. influenzae and Moraxella catarrhalis
• Anaerobes: decent activity against both Gram-positive and Gram-negative anaerobes
  • Clostridium spp., Bacteroides spp.

The headline numbers

• Linezolid resistance reported pre-marketing (Compassionate Use Program, VRE)
• ~25 years post-launch, resistance still <1% of clinical isolates
• Tedizolid resistance rarer than linezolid resistance over a 5-year surveillance
• Enterococci > staphylococci for resistance frequency

Risk factors for resistance

• Prior linezolid exposure
• Prolonged therapy (weeks to months)
• Horizontal spread within ICUs and long-term care facilities
• Stewardship implication: limit duration, monitor when prolonged use unavoidable

Mechanism 1 — 23S rRNA mutations

Mechanism 2 — ribosomal protein mutations

Mechanism 3 — cfr methyltransferase

Mechanism 4 — optrA and poxtA

Tedizolid vs. linezolid-resistant strains

• Tedizolid retains activity against many cfr-positive strains of S. aureus, CoNS, and enterococci
• Activity lost when cfr coexists with chromosomal ribosomal mutations
• Practical message: test before assuming susceptibility in known LZD-R isolates

Linezolid — dosing fundamentals

• Adults / adolescents: 600 mg PO or IV every 12 hours
• Uncomplicated SSTI in adults: 400 mg every 12 hours
• Pediatric (birth–11 y): 10 mg/kg every 8 hours (every 12 h for uncomplicated SSTI in children 5–11 y)
• Bioavailability ~100% — oral and IV interchangeable

Linezolid — distribution

• Plasma protein binding: 31%
• Excellent penetration into lung (ELF, alveolar macrophages) and skin and soft tissue
• CSF: trough <0.2–7.0 μg/mL; peaks 3.1–12.5 μg/mL in meningitis; CSF/plasma ratio 0.77–1.0
• Bone and joint penetration variable — case-by-case

Linezolid — metabolism and clearance

• Metabolized by oxidation — minimal CYP450 interaction
• 65% nonrenal clearance; 30% excreted unchanged in urine
• Two inactive carboxylic-acid metabolites in feces
• Half-life ~5–7 hours in adults
• Hemodialysis removes linezolid → dose after dialysis
• CRRT also removes drug; no routine dose change recommended

Linezolid — PK/PD targets

• Driver of efficacy: fT > MIC ≈ 85% and AUC₀₋₂₄/MIC 80–120
• High inter-individual variability can compromise targets at standard dose
• At-risk populations:
  • Overexposure: renal dysfunction, older age, hepatic failure, low body weight
  • Under-exposure: augmented renal clearance, younger age, obesity
  • Both: critical illness

Linezolid — when to consider TDM

• Trough C_min 2–7 mg/L suggested for Gram-positive infection
• C_min <2 mg/L proposed for M. tuberculosis (MICs lower)
• C_min >7.5–22.1 mg/L → hematologic toxicity risk rises
• Strongest case for TDM: critically ill, prolonged therapy, renal dysfunction, drug interactions

Linezolid — drug interactions (PK)

• Rifampin → ↓ AUC ~32% (P-gp induction?)
• Levothyroxine → ↓ linezolid concentrations
• Clarithromycin → ↑ linezolid AUC >3-fold
• Amlodipine, amiodarone, omeprazole, warfarin → linezolid overexposure reports
• Linezolid likely a P-glycoprotein substrate

Tedizolid — the prodrug story

• Tedizolid phosphate → cleaved by plasma phosphatases → active tedizolid
• Bioavailability ~91% — no dose adjustment IV vs. PO
• 200 mg once daily PO or IV (adults and pediatric ≥ 12 y)
• Half-life ~12 hours supports once-daily dosing
• Oral product taken without regard to meals

Tedizolid — distribution and metabolism

• Plasma protein binding 70–90% (free fraction ≈ linezolid)
• ELF AUC : plasma AUC = 40 and AM : plasma = 20 → high lung penetration
• Hepatic metabolism → sulfate conjugate in feces; <3% unchanged in urine
• No dose adjustment for hepatic, renal, dialysis, or obesity (BMI >30)

Tedizolid — special PK/PD caveats

• Free AUC/MIC ratio best predicts efficacy in murine models
• Markedly reduced antistaphylococcal activity in granulocytopenic mice — concerning signal
• Less pronounced effect in S. pneumoniae lung models
• Product label cautions against use in neutropenic patients — sparse human data

Tedizolid — drug interactions

• Negligible CYP450 interaction
• Inhibits intestinal BCRP → ↑ serum levels of rosuvastatin, methotrexate
• Weak, reversible MAOI activity in vitro — less risk than linezolid but not zero
• Four post-marketing serotonin syndrome reports to FAERS

FDA-approved indications — linezolid

• Nosocomial pneumonia (MRSA/MSSA, S. pneumoniae)
• Community-acquired pneumonia (S. pneumoniae including bacteremic; MSSA)
• Complicated SSTI (including diabetic foot, without osteomyelitis)
• Uncomplicated SSTI (MSSA, S. pyogenes)
• VRE E. faecium infections (including bacteremia)

FDA-approved indications — tedizolid

• Acute bacterial skin and skin-structure infection (ABSSSI) in adults and pediatric patients ≥ 12 years
• Susceptible Gram-positive pathogens:
  • S. aureus (MRSA/MSSA), S. pyogenes, S. agalactiae, S. anginosus group, E. faecalis

MRSA SSTI — linezolid vs vancomycin

• Cochrane review (9 RCTs): linezolid superior to vancomycin for clinical and microbiologic cure; shorter LOS, lower cost
• Superiority observed in adults but not in <18 y
• Recent network meta-analysis: linezolid superior to vancomycin; comparable to dapto, ceftaroline, telavancin, tigecycline, tedizolid

MRSA nosocomial pneumonia — the Wunderink saga

• Pooled analysis of 2 RCTs (Wunderink 2003): linezolid + aztreonam ≈ vanc + aztreonam overall; subgroup analysis suggested superior survival in MRSA
• Subgroup analysis criticized by FDA (Powers, 2004)
• Subsequent ZEPHyR trial (Wunderink 2012): higher clinical success with linezolid, no 60-day mortality difference; questioned by under-dosed vancomycin arm

MRSA pneumonia — meta-analytic verdict

• Pooled RCT data: no difference in clinical or microbiologic efficacy or all-cause mortality
• Nephrotoxicity higher with vancomycin — robust signal
• Practical message: either is acceptable; choose by toxicity profile, route, and TDM feasibility

S. aureus bacteremia — linezolid’s evolving role

• Effective for MSSA and MRSA bacteremia in selected patients
  • Persistent bacteremia, vancomycin failure, salvage scenarios
• Static vs. cidal concerns no longer hold up
• Oral step-down to linezolid as effective as exclusive parenteral therapy in propensity-matched cohorts (mostly uncomplicated bacteremia)

POET — oral step-down for left-sided IE

• 400 adults with stable left-sided IE (Strep, E. faecalis, S. aureus, CoNS)
• ≥ 10 days IV therapy → randomized to continued IV vs. oral switch
• 6-month composite (death, surgery, embolism, relapse): 9.0% PO vs. 12.1% IV — noninferior
• MRSA IE not evaluated — do not extrapolate
• Guidelines still do not list linezolid for MRSA endocarditis

Coagulase-negative staphylococci

• Case series support linezolid for bone/joint, meningitis, VP-shunt, and endocarditis with prosthetic material
• Insufficient data to recommend linezolid as routine first-line
• CoNS endocarditis: small subset in POET treated successfully with oral step-down

Vancomycin-resistant enterococci

• Multiple meta-analyses comparing linezolid vs. daptomycin for VRE bacteremia
  • Early data favored linezolid (low daptomycin doses limited interpretation)
  • More recent 22-study meta-analysis: nonsignificant ↑ mortality with daptomycin; no difference in microbiologic cure or recurrence
• VA cohort (Britt 2015): daptomycin better clinical success and 30-day mortality even at relatively low doses
• Choice should be individualized — susceptibility, exposure history, source, comorbidities

VRE endocarditis

• Both linezolid and daptomycin are recommended for enterococcal IE resistant to penicillin, aminoglycosides, vancomycin
• Evidence base limited — small numbers, treatment success and failure described
• Animal data suggest possible synergy with gentamicin, doxycycline, ceftriaxone, daptomycin; clinical use of combinations unproven

Streptococci including S. pneumoniae

• Effective for CAP caused by S. pneumoniae in open-label RCTs
• Not first-line empirical CAP — poor H. influenzae and atypical coverage
• Case reports support pneumococcal CNS infection use, usually with ceftriaxone
• Group A streptococcal necrotizing soft-tissue infection / toxic shock: considered as a clindamycin substitute (also inhibits exotoxin production)
• 2023 focused debate: linezolid is a reasonable alternative when clindamycin is contraindicated, resistant, or in short supply

Tedizolid — ESTABLISH-1 and -2

• Two non-inferiority Phase III trials in ABSSSI
• Tedizolid 200 mg × 6 days vs linezolid 600 mg BID × 10 days
• S. aureus most common pathogen; MRSA in 27–42%
• Both trials: non-inferior at 48–72 h and end of therapy
• Less GI toxicity and less thrombocytopenia with tedizolid

Tedizolid — beyond skin

• Phase III RCT (pediatric ≥ 12 y) for ABSSSI: comparable efficacy and safety
• Phase III HAP/VAP trial (Wunderink 2021): non-inferior day-28 mortality vs linezolid; failed non-inferiority for investigator-assessed clinical response — reason unclear
• Network meta-analysis: tedizolid clinical response superior to vancomycin, comparable to linezolid, dapto, ceftaroline, telavancin, teicoplanin, tigecycline

Linezolid in MDR/XDR-TB — the modern role

• Linezolid is now central to drug-resistant TB regimens
• BPaL regimen (bedaquiline + pretomanid + linezolid 1200 mg/day, 6 mo): ~90% favorable outcomes in highly drug-resistant TB
• WHO 2022: BPaL endorsed for 6–9 months in MDR/RR-TB
• BUT: >80% experienced toxicity at 1200 mg/day (neuropathy, myelosuppression)

ZeNix — getting the dose right

• 2×2 factorial: linezolid 1200 vs 600 mg/day and 26 vs 9 weeks within BPaL
• 600 mg/day × 26 weeks: favorable outcome 91% (vs 94% with 1200 mg)
• Substantially less neuropathy and myelosuppression at the lower dose
• Modeling: 600 mg/day balances efficacy and AE risk
• Bottom line: 600 mg/day × 26 weeks is the preferred dose

Nocardia — linezolid’s place

• Near-universal susceptibility, oral bioavailability, CNS penetration → attractive option
• Combination induction therapy for disseminated, cutaneous, or moderate-severe pulmonary disease
• Often with TMP-SMX + ceftriaxone; alternative when amikacin or TMP-SMX toxicity limits use
• 1-year survival ~85% — comparable to other regimens
• Main toxicity = thrombocytopenia

Oral vs. intravenous

• Bioavailability ≈ 100% (linezolid) and ~91% (tedizolid) → oral substitutes for IV when clinically appropriate
• Use oral when:
  • Patient clinically stable and reliable
  • GI absorption intact
  • Indication has supporting peer-reviewed evidence
• Strong stewardship argument — earlier discharge, lower cost

Overall tolerability

• Both agents generally well tolerated
• Common AEs (more frequent with linezolid):
  • Headache, nausea, vomiting, diarrhea
• Serious AEs are uncommon but dose- and duration-dependent

Hematologic toxicity — the headline AE

• Reversible myelosuppression: thrombocytopenia >> anemia, pancytopenia, pure red cell aplasia
• Phase III data: thrombocytopenia in 2.4% of adults (range 0.3–10%)
• Risk factors: prolonged duration, renal insufficiency, hepatic impairment, baseline marrow suppression
• Mechanism: drug-induced immune-mediated and progenitor suppression

Hematologic toxicity — monitoring

• Weekly CBC if therapy ≥ 2 weeks (per label)
• TDM useful: trough >7.5–22.1 mg/L associated with hematologic toxicity
• Tedizolid: thrombocytopenia 2.1% (6 d) vs 3.8% linezolid (10 d) in pooled ABSSSI data
• Long-term tedizolid (median 28 d): 6/81 thrombocytopenia — uncommon, often clinically tolerable

Monoamine oxidase inhibition

• Linezolid: reversible, nonselective MAO inhibitor
• Serotonin syndrome with concurrent serotonergic agents
  • Highest-risk co-medications in FAERS: citalopram, escitalopram, methadone
• Overall a rare event — risk/benefit reasonable in many patients
• Tedizolid: weak in vitro MAOI; only 4 FAERS reports — risk likely lower but not zero

Tyramine and adrenergic interactions

• Modest ↑ systolic BP with concurrent tyramine → label dietary cautions
• Blood pressure monitoring recommended with concurrent adrenergic agents or hypertension
• Practical: review SSRI/SNRI/MAOI co-medications before starting linezolid

Mitochondrial toxicity — unifying concept

• Disruption of mitochondrial protein synthesis explains:
  • Peripheral and optic neuropathy
  • Lactic acidosis
  • Possibly rhabdomyolysis and drug-induced liver injury
• Tedizolid is theoretically a more potent mitochondrial inhibitor — but lower free exposure and a recovery window each dosing interval may reduce in vivo risk

Neuropathy

• Peripheral neuropathy: distal dysesthesias, often poorly reversible — prolonged therapy
• Optic neuropathy: gradual visual blurring; visual loss can be permanent if drug not stopped — reversible if caught early
• Phase III data: peripheral/optic neuropathy similar between tedizolid (6 d) and linezolid (10 d) — short courses
• Watch for it in prolonged TB or osteomyelitis courses — French MDR-TB cohort: high neurologic AE burden

Lactic acidosis

• Rare but including fatal cases
• Most often during prolonged therapy — but can develop within the first week
• Prompt recognition and drug discontinuation are critical
• Risk increased: age, renal insufficiency, drug interactions causing overexposure
• Possible genetic predisposition (mitochondrial ribosome polymorphisms)
• Tedizolid lactic acidosis reported to FAERS — causality less clear

Miscellaneous and rare AEs

• Increased mortality when used as monotherapy for catheter-related infections with Gram-negative or mixed flora
• Hyponatremia / SIADH, PRES, seizures
• Tooth and tongue discoloration, black hairy tongue
• Hypoglycemia in diabetic patients on insulin or oral hypoglycemics
• Oral suspension contains phenylalanine — caution in PKU

Investigational oxazolidinones

• Radezolid — broader Gram-positive activity, including LZD-R strains
• Sutezolid — structurally similar to linezolid; potent anti-mycobacterial activity
• Delpazolid — Gram-positive activity similar to linezolid; superior in vitro vs mycobacteria
• Contezolid (MRX-I) — activity vs MRSA, VRE, mycobacteria; approved in China for ABSSSI