Oxazolidinone antibiotic; bacterial protein synthesis

Linezolid inhibits initiation complex formation with either the 30S or the 70S ribosomal subunits from Escherichia coli. Linezolid inhibits complex formation with Staphylococcus aureus 70S tight-couple ribosomes. Linezolid is a potent inhibitor of cell-free transcription-translation in E. coli, exhibiting 50% inhibitory concentrations (IC50s) of 1.8 mM. Linezolid is an oxazolidinone, a new class of antibacterial agents with enhanced activity against pathogens. Linezolid MICs vary slightly with the test method, laboratory, and significance attributed to thin hazes of bacterial survival, but all workers find that the susceptibility distributions are narrow and unimodal, with MIC values between 0.5 and 4 mg/L for streptococci, enterococci and staphylococci. Linezolid entails mutation of the 23S rRNA that forms the binding site. Linezolid is an oxazolidinone whose mechanism of action involves inhibition of protein synthesis at a very early stage. Linezolid is added to 7H10 agar medium (Difco) supplemented with OADC (oleic acid, albumin, dextrose, and catalase) at 50°C to 56°C by doubling the dilutions to yield a final concentration of 0.125 μg/mL to 4 μg/mL. Linezolid shows excellent in vitro activity against all the strains tested (MICs ≤ 1 μg/ml), including those resistant to SIRE.

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Initiation of translation requires the formation of a ternary complex between tRNAfMet, the 30S or the 70S subunit, and mRNA. This initiation complex can be assayed by measuring the binding of radiolabeled tRNAfMet to either the 30S or the 70S subunit. Figure 1A shows that in the presence of the initiation factors IF1, IF2, and IF3, Linezolid had 50% inhibitory concentrations (IC50s) of 110 μM (37 μg/ml) and 130 μM (44 μg/ml) for E. coli 30S and 70S initiation complex formation, respectively. The integrity of the assay was confirmed by demonstrating that kasugamycin was inhibitory to 70S initiation complex formation, with an IC50 of 154 μM (Fig. 1B). Oxazolidinone inhibition of initiation complex formation was studied further with S. aureus 70S ribosomes (Fig. 2). With a truncated mRNA, an IC50 value of 116 μM was obtained for linezolid. These reactions were performed in the absence of initiation factors and with salt-washed 70S ribosomes. Initiation factors IF1, IF2, and IF3 play important roles in the initiation of translation in bacteria. The tRNAfMet is bound by IF2 and is delivered to the 30S subunit joining IF1, IF3, and the mRNA as part of the initiation complex. Linezolid did not inhibit formation of the IF2-tRNAfMet complex when either 5 or 0.5 pmol of E. coli IF2 was used (Table 1). The role of initiation factors in the mechanism of action of linezolid was further investigated by forming E. coli 70S ribosome initiation complexes in the absence of any of the three factors. Figure 4 demonstrates that an IC50 of 152 μM was obtained for linezolid under these conditions. [1]

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The oxazolidinones are a new class of synthetic antibiotics with good activity against gram-positive pathogenic bacteria. Experiments with a susceptible Escherichia coli strain, UC6782, demonstrated that in vivo protein synthesis was inhibited by both eperezolid (formerly U-100592) and Linezolid (formerly U-100766). Both Linezolid and eperezolid were potent inhibitors of cell-free transcription-translation in E. coli, exhibiting 50% inhibitory concentrations (IC50s) of 1.8 and 2.5 microM, respectively. The ability to demonstrate inhibition of in vitro translation directed by phage MS2 RNA was greatly dependent upon the amount of RNA added to the assay. For eperezolid, 128 microg of RNA per ml produced an IC50 of 50 microM whereas a concentration of 32 microg/ml yielded an IC50 of 20 microM. Investigating lower RNA template concentrations in linezolid inhibition experiments revealed that 32 and 8 microg of MS2 phage RNA per ml produced IC50s of 24 and 15 microM, respectively. This phenomenon was shared by the translation initiation inhibitor kasugamycin but not by streptomycin. Neither oxazolidinone inhibited the formation of N-formylmethionyl-tRNA, elongation, or termination reactions of bacterial translation. The oxazolidinones appear to inhibit bacterial translation at the initiation phase of protein synthesis [2].

Linezolid is fully bioavailable following oral administration, with maximum plasma linezolid concentrations achieved between 1 and 2 hours after oral administration. The elimination half-life of linezolid is 5–7 hours, and twice-daily administration of 400–600mg provides steady-state concentrations in the therapeutic range.

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Finally, we compared the activities of Linezolid and vancomycin in pneumonia induced by IRDL-7971 in HLA-DR3 transgenic mice using a previously used dosing regimen (20). As shown in Fig. 5, Linezolid conferred significant protection over vancomycin (P = 0.0002; n = 10 to 14 mice/group) and untreated mice (P = 0.0004; n = 8 to 10 mice/group). Surprisingly, vancomycin failed to confer significant protection from lethal pneumonia (P = 0.50; n = 8 to 14 mice/group). Serum cytokine analyses revealed that compared to untreated and vancomycin-treated mice infected with IDRL-7971, linezolid-treated HLA-DR3 transgenic mice infected with IDRL-7971, had significantly lower levels of IL-2, IL-6, and the chemokine KC (Fig. 6). These results suggest that linezolid played a protective role by attenuating the production of SAg in vivo [4].

Synthesis of [32P]mRNA for ribosome binding studies. [1]
The one-step PCR procedure described by Sandhu et al. (24) was used to synthesize a 200-bp mRNA with a defined sequence. Four adjacent oligonucleotide primers with short overlaps were annealed to each other and were subjected to PCR. The primer sequences were as follows (primer sequences are 5′ to 3′): primer 1, GGGAATTCGCAGGTTTAAAAATGAAAGGTAAAGGTAAAGGTAAA; primer 2, GGTGGTGGCCTGGGCAAAGGTAAAGGT; primer 3, AAAGGT AAAAAAGGTAAAGGTAAAGGTAAAAAAGGTAAAAAAGGTAAAGG TGGTGGTTAATAAAAAAAATAAAAAG; and primer 4, CTAGAGGATCCTTTTTATTTTTTTATTAACCACCAC. Primers 1 and 2 were annealed to the sequence 5′-ACCTTTTTTACCTTTACCTTTACCTTTTTTACCTTTTTTACCTTTACCTTTACCTTATCCTTTACCTTTGCCCAGGCCAC-3′. Primer 3 was annealed to the sequence 5′-ACCTTTGCCCAGGCCACCACCTTTACCTTTTTTACCTTTTTTACCTTTACCTTCACC-3′, and primer 4 was annealed to primer 3. Extension by PCR resulted in the asymmetric synthesis of intermediates which annealed to each other, thereby priming the synthesis of a double-stranded DNA template. This template was subsequently used to produce an mRNA with the sequence 5′-GGGAAUUCGGAGGUUUAAAAAUG-(GGUAAA)33UAAUAA-3′ (the Shine-Dalgarno sequence and the AUG start codon are underlined). The coding sequence contained Gly (GGU) and Lys (AAA) codons followed by tandem stop codons. [32P]mRNA was transcribed with a Ribomax kit and either [32P]CTP or [32P]GTP. RNA was isolated by phenol extraction and chromatography through a Quick Spin G-25 column.

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Binding of labeled synthetic mRNA to ribosomes. [1]
Binding of 32P-labeled synthetic mRNA was carried out for 15 min at 24°C in duplicate 50-μl reaction mixtures containing 200 to 400 μg of 70S ribosomes, 20 mM MgCl2, 10 mM Tris-HCl (pH 7.4), 1 mM DTT, 80 mM NH4Cl, and 1 μl (16,000 dpm) of 32P-labeled mRNA. Duplicate reactions were terminated by the addition of 1 ml of ice-cold buffer containing 20 mM Tris (pH 7.4), 20 mM MgCl2, and 100 mM NH4Cl. The mRNA-ribosome complex was then trapped on Millipore HA filters, and the radioactivity was counted after the addition of scintillation fluid.

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E. coli initiation complex assay with AUG. [1]
E. coli 70S ribosomes (10 pmol) were incubated with [35S]tRNAfMet (45,000 dpm) in 20-μl reaction mixtures containing 20 mM HEPES (pH 7.6), 3 mM MgCl2, 150 mM NH4Cl, 4 mM DTT, 0.05 mM spermine, 2 mM spermidine, and 0.25 μg of the AUG trinucleotide. Duplicate reaction mixtures were incubated at 37°C for 10 min, and the reactions were stopped by the addition of 2 ml of cold buffer A. Complexes were filtered through Millipore filters (pore size, 0.45 μm) and washed with 50 ml of buffer A, and the radioactivity was counted after the addition of scintillation fluid.

Linezolid was a potent inhibitor of cell-free transcription-translation in E. coli. IC50 was 1.8 mM. linezolid MICs vary slightly owing to the different test method and laboratory. The MIC values were between 0.5 and 4 mg/L for streptococci, enterococci and staphylococci.

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Preparation of E. coli 70S ribosomes.[1]
Ribosomes were prepared by the method of Rheinberger et al. Fifty grams (wet weight) of frozen MRE600 cells was mixed with an equal weight of alumina, and the cells were lysed at 0°C by grinding with a mortar and pestle. Fifty milliliters of buffer A containing 1 μg of DNase per ml was added and the suspension was stirred for 20 min. The alumina, unbroken cells, and cellular debris were removed by two centrifugations at 10,000 × g for 10 min. The supernatant was centrifuged again for 30 min at 30,000 × g, and the upper two-thirds of the resulting supernatant was centrifuged again at 30,000 × g for 16 h (S30 extract). The ribosome pellet was suspended in buffer B and centrifuged at 10,000 × g for 10 min, and the clear supernatant was centrifuged at 105,000 × g for 4 h. The pelleted ribosomes were washed twice more in buffer C while maintaining the ribosomes at 5 to 10 mg/ml (14.4 A260 units = 1 mg/ml), suspended in buffer A at 80 to 100 mg of ribosomes per ml, and stored at −80°C.

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Preparation of E. coli ribosomal subunits.[1]
Ribosomal subunits were prepared as described by Staehelin and Maglott, with the following modifications. The S30 extract was prepared as described above by using MRE600 mid-logarithmic-phase cells, and the 30S subunits were stored in liquid nitrogen.

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Preparation of S. aureus ribosomes.[1]
S. aureus cells (50 g [wet weight]) were resuspended in 100 ml of lysis buffer (50 mM Tris-HCl [pH 8.0], 100 mM NaCl, 2 mg of lysostaphin per ml, 10,000 U of DNase I) and incubated for 1 h in a 37°C water bath. β-Mercaptoethanol was added to a final concentration of 5 mM, and the lysed cells were centrifuged at 10,000 × g for 10 min to remove unbroken cells and cell fragments. The supernatant was centrifuged at 30,000 × g, and the resulting supernatant was centrifuged at 100,000 × g for 16 h to pellet the ribosomes. The ribosome pellet was resuspended in buffer B and was again centrifuged at 100,000 × g for 16 h. The pellet was resuspended in buffer A, applied to linear 5 to 40% (wt/vol) sucrose gradients prepared in buffer A, and centrifuged for 16 h in a Beckman SW28 rotor. Gradients were fractionated; and the 70S ribosomes were pooled, pelleted at 300,000 × g for 5 h, and resuspended in buffer A before they were stored at −80°C.

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Initiation factor assays.[1]
Initiation factors were assayed as described by Hershey et al. Complexes between IF2 and tRNAfMet were formed in the reaction mixtures (final volume, 65 μl) containing 190 mM Tris-HCl (pH 7.4), 19 mM MgCl2, 3.8 mM DTT, 1.9 mM GTP, 540 mM NH4Cl, 5 pmol of IF2, and 2 μl of [35S]tRNAfMet (10,000 dpm). Duplicate reaction mixtures were incubated for 10 min at 37°C, and the reactions were stopped by the addition of 1 ml ice-cold buffer A containing 1% glutaraldehyde. Complexes were trapped on Millipore HA filters (pore size, 0.45 μm), washed with 50 ml of buffer A containing 1% glutaraldehyde, and counted after the addition of liquid scintillation fluid.

Absorption, Distribution and Excretion
Linezolid is extensively absorbed following oral administration and has an absolute bioavailability of approximately 100%. Maximum plasma concentrations are reached within approximately 1 to 2 hours after dosing (Tmax) and range from 8.1-12.9 mcg/mL after single doses and 11.0-21.2 mcg/mL after multiple dosing. The absorption of orally administered linezolid is not significantly affected by co-administration with food and it may therefore be given without regard to the timing of meals.
Urinary excretion is the primary means by which linezolid and its metabolic products are excreted. Following the administration of a radiolabeled dose of linezolid under steady-state conditions, approximately 84% of radioactivity was recovered in the urine, of which approximately 30% is unchanged parent drug, 40% is the hydroxyethyl glycine metabolite, and 10% is the aminoethoxyacetic acid metabolite. Fecal elimination is comparatively minor, with no parent drug observed in feces and only 6% and 3% of an administered dose found in the feces as the hydroxyethyl glycine metabolite and the aminoethoxyacetic acid metabolite, respectively.
At steady-state, the volume of distribution of linezolid in healthy adults is approximately 40-50 liters.
Total clearance of linezolid is estimated to be 100-200 mL/min, the majority of which appears to be non-renal. Mean renal clearance is approximately 40 mL/min, which suggests net tubular reabsorption, while non-renal clearance is estimated to account for roughly 65% of total clearance, or 70-150 mL/min on average. Variability in linezolid clearance is high, particularly for non-renal clearance.
Distributed to well-perfused tissues; volume of distribution slightly lower in women than men. VolD (steady state) - 40 to 50 L.
AUC is lower for pediatric patients compared with adults and a wider variability of linezolid AUC cross all pediatric age groups as compared with adults. Most pre-term neonates less than 7 days of age (gestational age less than 34 weeks) have larger AUC values than many full-term neonates and older infants.
Linezolid was rapidly absorbed after p.o. dosing with an p.o. bioavailability of > 95% in rat and dog, and > 70% in mouse. Twenty-eight-day i.v./p.o. toxicokinetic studies in rat (20-200 mg kg(-1) day(-1)) and dog (10-80 mg kg(-1) day(-1)) revealed neither a meaningful increase in clearance nor accumulation upon multiple dosing. Linezolid had limited protein binding (<35%) and was very well distributed to most extravascular sites, with a volume of distribution at steady-state (V(ss)) approximately equal to total body water. Linezolid circulated mainly as parent drug and was excreted mainly as parent drug and two inactive carboxylic acids, PNU-142586 and PNU-142300. Minor secondary metabolites were also characterized. In all species, the clearance rate was determined by metabolism. Radioactivity recovery was essentially complete within 24-48 hr. Renal excretion of parent drug and metabolites was a major elimination route. Parent drug underwent renal tubular reabsorption, significantly slowing parent drug excretion and allowing a slow metabolic process to become rate-limiting in overall clearance. It is concluded that ADME data were relatively consistent across species and supported the rat and dog as the principal non-clinical safety species.
In two randomized, double-blind, placebo-controlled, dose-escalating trials, subjects were exposed either to oral (375, 500 or 625 mg) or intravenous (500 or 625 mg) linezolid or placebo twice daily. Serial blood and urine samples were obtained after the first- and multiple-dose administrations for up to 18 days. Non-compartmental pharmacokinetic analyses were used to describe the disposition of linezolid. Plasma linezolid concentrations and area under the concentration-time curves (AUC) increased proportionally with dose irrespective of the route of administration. Plasma linezolid concentrations remained above the MIC90 for susceptible target pathogens (4.0 mg/L) for the majority of the 12 hr dosing interval. Mean clearance, half-life and volume of distribution were similar irrespective of dose for both the oral and intravenous routes. Linezolid was well tolerated and the frequency of drug-related adverse events was similar between the linezolid and placebo groups. Oral and intravenous linezolid exhibit linear pharmacokinetics, with concentrations remaining above the target MIC90 /minimal inhibitory concentration/ for most of the dosing interval. These results support a twice-daily schedule for linezolid and demonstrate the feasibility of converting from intravenous to oral dosing without a dose adjustment.
For more Absorption, Distribution and Excretion (Complete) data for LINEZOLID (16 total), please visit the HSDB record page.

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Metabolism / Metabolites
Linezolid is primarily metabolized to two inactive metabolites: an aminoethoxyacetic acid metabolite (PNU-142300) and a hydroxyethyl glycine metabolite (PNU-142586), both of which are the result of morpholine ring oxidation. The hydroxyethyl glycine metabolite - the most abundant of the two metabolites - is likely generated via non-enzymatic processes, though further detail has not been elucidated. While the specific enzymes responsible for the biotransformation of linezolid are unclear, it does not appear to be subject to metabolism via the CYP450 enzyme system, nor does it meaningfully inhibit or induce these enzymes. Linezolid is, however, a reversible and non-selective inhibitor of monoamine oxidase enzymes.
In vitro studies have not shown that linezolid is metabolized by human cytochrome p450 enzymes. Linezolid does not inhibit the cytochrome p450 enzymes.
Linezolid is primarily metabolized via oxidation of the morpholine ring. Two inactive metabolites are formed: the aminoethoxyacetic acid metabolite and the hydroxyethyl glycine metabolite. The hydroxyethyl glycine metabolite is formed via a non-enzymatic chemical oxidation mechanism in vitro.
The drug is metabolized principally via oxidation to 2 inactive metabolites; an aminoethoxyacetic acid metabolite and a hydroxyethyl glycine metabolite. Linezolid is not metabolized to any measurable extent by the cytochrome p450 (CYP) enzyme system. Linezolid does not inhibit CYP isoenzymes 1A2, 2C9, 2C19, 2D6, 2E1, or 3A4 and is not an enzyme inducer, suggesting that the drug is unlikely to alter the pharmacokinetics of drugs metabolized by these enzymes.
In vitro studies were conducted to identify the hepatic enzyme(s) responsible for the oxidative metabolism of linezolid. In human liver microsomes, linezolid was oxidized to a single metabolite, hydroxylinezolid (M1). Formation of M1 was determined to be dependent upon microsomal protein and NADPH. Over a concentration range of 2 to 700 uM, the rate of M1 formation conformed to first-order (nonsaturable) kinetics. Application of conventional in vitro techniques were unable to identify the molecular origin of M1 based on the following experiments: a) inhibitor/substrates for various cytochrome P-450 (CYP) enzymes were unable to inhibit M1 formation; b) formation of M1 did not correlate (r(2) < 0.23) with any of the measured catalytic activities across a population of human livers (n = 14); c) M1 formation was not detectable in incubations using microsomes prepared from a baculovirus insect cell line expressing CYPs 1A1, 1A2, 2A6, 2B6, 2C8, 2C9, 2C19, 2D6, 2E1, 3A4, 3A5, and 4A11. In addition, results obtained from an in vitro P-450 inhibition screen revealed that linezolid was devoid of any inhibitory activity toward the following CYP enzymes (CYP1A2, CYP2C9, CYP2C19, CYP2D6, CYP2E1, and CYP3A4). Additional in vitro studies excluded the possibility of flavin-containing monooxygenase and monoamine oxidase as potential enzymes responsible for metabolite formation. However, metabolite formation was found to be optimal under basic (pH 9.0) conditions, which suggests the potential involvement of either an uncharacterized P-450 enzyme or an alternative microsomal mediated oxidative pathway.
Linezolid is primarily metabolized by oxidation of the morpholine ring, which results in two inactive ring-opened carboxylic acid metabolites: the aminoethoxyacetic acid metabolite (A), and the hydroxyethyl glycine metabolite (B). Formation of metabolite B is mediated by a non-enzymatic chemical oxidation mechanism in vitro. Linezolid is not an inducer of cytochrome P450 (CYP) in rats, and it has been demonstrated from in vitro studies that linezolid is not detectably metabolized by human cytochrome P450 and it does not inhibit the activities of clinically significant human CYP isoforms (1A2, 2C9, 2C19, 2D6, 2E1, 3A4).
Linezolid is rapidly and extensively absorbed after oral dosing. Maximum plasma concentrations are reached approximately 1 to 2 hours after dosing, and the absolute bioavailability is approximately 100%. Linezolid is primarily metabolized by oxidation of the morpholine ring, which results in two inactive ring-opened carboxylic acid metabolites: the aminoethoxyacetic acid metabolite (A), and the hydroxyethyl glycine metabolite (A308). Half Life: 4.5-5.5 hours.

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Biological Half-Life
The elimination half-life is estimated to be between 5 and 7 hours.
... A significant although weak correlation between age and total body clearance was observed. The mean (+ or - SD) values for elimination half-life, total clearance and apparent volume of distribution were 3.0 + or - 1.1 hr, 0.34 + or - 0.15 liter/h/kg and 0.73 + or - 0.18 liter/kg, respectively. ...
The following are elimination half live values of linezolid doses in adults: 400 mg tablet (single dose) - 5.2 hours; 400 mg tablet every 12 hours - 4.69 hours; 600 mg tablet (single dose) - 4.26 hours; 600 mg tablet every 12 hours - 5.4 hours; 600 mg oral suspension (single dose) - 4.6 hours; 600 mg intravenous injection (single dose) - 4.4 hours; 600 mg intravenous injection every 12 hours - 4.8 hours;. Pediatrics ranging in age from greater than 7 days of age to 11 years of age have a shorter half-life compared with adults.

Toxicity Summary
Linezolid targets the large 39S subunit of the mitochondrial ribosome thereby deactivation mitochondrial protein synthesis. As a result Linezolid is cytotoxic to the most metabolically active cells or tissues including the heart, liver, thymus and bone-marrow. (A7823). The likely target of Linezolid is the 16S rRNA molecule in the mitochondrial ribosome, which is analogous to the 23S rRNA in bacterial ribosomes.

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Hepatotoxicity
Therapy with linezolid has been associated with mild and transient elevations in serum aminotransferase and alkaline phosphatase levels in 1% to 10% of patients, although similar rates of elevations occur in patients with infections treated with comparable agents, and enzyme elevations were not found in normal volunteers given linezolid for short periods. On the other hand, ALT elevations during therapy have been higher with higher doses of linezolid, but in all instances the elevations occurred without symptoms and resolved with discontinuation of the drug.
Although the agent has been available for a limited time and its use has been restricted, several instances of clinically apparent liver disease with jaundice have been reported with linezolid therapy. A case of a hypersensitivity response with rash, eosinophilia and renal insufficiency (DRESS syndrome) with mild serum enzyme elevations has been reported. More frequently, linezolid has been linked to cases of lactic acidosis, generally arising after 1 to 8 weeks of therapy and sometimes associated with evidence of liver injury and jaundice. Lactic acidosis is usually due to injury and dysfunction of hepatic mitochondria, with resulting microvesicular steatosis and disturbed hepatic function (not necessarily accompanied by jaundice or even ALT or alkaline phosphatase elevations). Other serious side effects associated with mitochondrial damage due to linezolid therapy include peripheral and optic neuropathy, pancreatitis, serotonin syndrome and renal injury. Risk factors for developing lactic acidosis from linezolid include higher doses, longer courses of therapy and underlying chronic liver or renal disease. The mitochondrial injury is believed to be due to the inhibition of mitochondrial ribosomal function that matches the known effect of linezolid on bacterial ribosomal function. Lactic acidosis occurs after 1 to 8 weeks of treatment and can be severe, although it often resolves rapidly with discontinuation. In contrast, the optic and peripheral neuropathy due to linezolid resolves more slowly and can be permanent. Lactic acidosis can be fatal and hepatic dysfunction and jaundice have been mentioned in severe cases of lactic acidosis attributed to linezolid.
Likelihood score: A (well established cause of clinically apparent liver injury usually in association with lactic acidosis).

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Effects During Pregnancy and Lactation
◉ Summary of Use during Lactation
Linezolid is excreted into breastmilk in concentrations likely to be effective against staphylococcal strains found in mastitis. Limited data indicate that the maximum dose an infant would receive through breastmilk would be only 6 to 9% of the standard infant dose and that resulting infant serum levels are trivial. If the mother requires linezolid, it is not a reason to discontinue breastfeeding. Monitor the infant for possible effects on the gastrointestinal tract, such as diarrhea and vomiting.
◉ Effects in Breastfed Infants
Relevant published information was not found as of the revision date.
◉ Effects on Lactation and Breastmilk
Relevant published information was not found as of the revision date.

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Protein Binding
Plasma protein binding of linezolid is approximately 31% - primarily to serum albumin - and is concentration-dependent.

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Interactions
Following coadministration with linezolid, minimal but statistically significant increases were observed in pseudoephedrine and phenylpropanolamine plasma concentrations; a minimal but statistically significant decrease was observed in dextrorphan (the primary metabolite of dextromethorphan) plasma concentrations. Increased blood pressure (BP) was observed following the coadministration of linezolid with either pseudoephedrine or phenylpropanolamine; no significant effects were observed with dextromethorphan. None of these coadministered drugs had a significant effect on linezolid pharmacokinetics. Minimal numbers of adverse events were reported. Potentiation of sympathomimetic activity by linezolid was judged not to be clinically significant, but patients sensitive to the effects of increased BP due to predisposing factors should be treated cautiously. No restrictions are indicated for the coadministration of dextromethorphan and linezolid.
To report 2 cases of serotonin toxicity (ST) associated with concomitant use of linezolid and serotonergic drugs and review previously published case reports. Case 1. A 38-year-old white female with cystic fibrosis treated with venlafaxine 300 mg/day for one year was prescribed linezolid 600 mg intravenously every 12 hours for treatment of methicillin-resistant Staphylococcus aureus (MRSA) pulmonary infection. She displayed symptoms of ST 8 days after the introduction of linezolid. The venlafaxine dosage was decreased to 150 mg/day, and symptoms gradually abated over 36 hours. Case 2. A 37-year-old male with multiple myeloma received citalopram 40 mg/day and trazodone 150 mg/day for anxiety-related disorders. Linezolid treatment with 600 mg orally twice daily was instituted for MRSA cellulitis. The following day, the patient developed anxiety, panic attacks, tremors, tachycardia, and hypertension that persisted throughout linezolid treatment. Symptoms finally waned 5 days after linezolid treatment was stopped.
Potential pharmacologic interaction (serotonin syndrome). Although serotonin syndrome was not reported during clinical trials with linezolid, there have been a limited number of postmarketing case reports of the syndrome in patients who received linezolid concurrently with or shortly after discontinuation of certain selective serotonin-reuptake inhibitors (SSRIs) (e.g., citalopram, paroxetine, sertraline). Clinicians should consider the possibility if signs and symptoms of serotonin syndrome (e.g., hyperpyrexia, cognitive dysfunction) occur in patients receiving such concomitant therapy. Some clinicians suggest that linezolid be used with caution in patients receiving SSRIs, and some suggest that SSRI therapy should be discontinued before linezolid is initiated and not reinitiated until 2 weeks after linezolid therapy is completed.
Toxicity resulting from excessive intra-synaptic serotonin, historically referred to as serotonin syndrome, is now understood to be an intra-synaptic serotonin concentration-related phenomenon. Recent research more clearly delineates serotonin toxicity as a discreet toxidrome characterized by clonus, hyper-reflexia, hyperthermia and agitation. Serotonergic side-effects occur with serotonergic drugs, and overdoses of serotonin re-uptake inhibitors (SRIs) frequently produce marked serotonergic side-effects, and in 15% of cases, moderate serotonergic toxicity, but not to a severe degree, which produces hyperthermia and risk of death. It is only combinations of serotonergic drugs acting by different mechanisms that are capable of raising intra-synaptic serotonin to a level that is life threatening. The combination that most commonly does this is a monoamine oxidase inhibitor (MAOI) drug combined with any SRI. There are a number of lesser-known drugs that are MAOIs, such as linezolid and moclobemide; and some opioid analgesics have serotonergic activity. These properties when combined can precipitate life threatening serotonin toxicity. Possibly preventable deaths are still occurring. Knowledge of the properties of these drugs will therefore help to ensure that problems can be avoided in most clinical situations, and treated appropriately (with 5-HT(2A) antagonists for severe cases) if they occur.
For more Interactions (Complete) data for LINEZOLID (12 total), please visit the HSDB record page.

[1]. Antimicrob Agents Chemother.1998 Dec;42(12):3251-5.

[2]. Antimicrob Agents Chemother.1997 Oct;41(10):2132-6.

[3]. Drugs. 2000 Apr;59(4):815-27; discussion 828.

[4]. Antimicrob Agents Chemother . 2012 Oct;56(10):5401-5.

Therapeutic Uses
Antibacterial
Intravenous and oral linezolid is indicated in the treatment of nosocomial pneumonia caused by methicillin-susceptible and methicillin resistant Staphylococcus aureus or penicillin-susceptible strains of Streptococcus pneumonia. /Included in US product labeling/
Intravenous and oral linezolid is indicated in the treatment of vancomycin-resistant Enterococcus faecium infections. /Included in US product labeling/
Oral linezolid is indicated in the treatment of uncomplicated skin and soft tissue infections caused by methicillin-susceptible strains of Staphylococcus aureus or Streptococcus pyogenes. /Included in US product labeling/
For more Therapeutic Uses (Complete) data for LINEZOLID (16 total), please visit the HSDB record page.

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Drug Warnings
Myelosuppression (including anemia, leukopenia, pancytopenia, and thrombocytopenia) has been reported in patients receiving linezolid. In cases where the outcome is known, when linezolid was discontinued, the affected hematologic parameters have risen toward pretreatment levels. Complete blood counts should be monitored weekly in patients who receive linezolid, particularly in those who receive linezolid for longer than two weeks, those with pre-existing myelosuppression, those receiving concomitant drugs that produce bone marrow suppression, or those with a chronic infection who have received previous or concomitant antibiotic therapy. Discontinuation of therapy with linezolid should be considered in patients who develop or have worsening myelosuppression.
Lactic acidosis has been reported with the use of Zyvox. In reported cases, patients experienced repeated episodes of nausea and vomiting. Patients who develop recurrent nausea or vomiting, unexplained acidosis, or a low bicarbonate level while receiving zyvox should receive immediate medical evaluation.
Spontaneous reports of serotonin syndrome associated with the co-administration of Zyvox and serotonergic agents, including antidepressants such as selective serotonin reuptake inhibitors (SSRIs), have been reported. Where administration of Zyvox and concomitant serotonergic agents is clinically appropriate, patients should be closely observed for signs and symptoms of serotonin syndrome such as cognitive dysfunction, hyperpyrexia, hyperreflexia and incoordination. If signs or symptoms occur physicians should consider discontinuation of either one or both agents.
Peripheral and optic neuropathy have been reported in patients treated with Zyvox, primarily those patients treated for longer than the maximum recommended duration of 28 days. In cases of optic neuropathy that progressed to loss of vision, patients were treated for extended periods beyond the maximum recommended duration. Visual blurring has been reported in some patients treated with Zyvox for less than 28 days. If patients experience symptoms of visual impairment, such as changes in visual acuity, changes in color vision, blurred vision, or visual field defect, prompt ophthalmic evaluation is recommended. Visual function should be monitored in all patients taking Zyvox for extended periods (> or = 3 months) and in all patients reporting new visual symptoms regardless of length of therapy with Zyvox. If peripheral or optic neuropathy occurs, the continued use of Zyvox in these patients should be weighed against the potential risks.
For more Drug Warnings (Complete) data for LINEZOLID (18 total), please visit the HSDB record page.

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Pharmacodynamics
Linezolid is an oxazolidinone antibacterial agent effective against most strains of aerobic Gram-positive bacteria and mycobacteria. It appears to be bacteriostatic against both staphylococci and enterococci and bactericidal against most isolates of streptococci. Linezolid has shown some _in vitro_ activity against Gram-negative and anaerobic bacteria but is not considered efficacious against these organisms. Linezolid is a reversible and non-selective inhibitor of monoamine oxidase (MAO) enzymes and can therefore contribute to the development of serotonin syndrome when administered alongside serotonergic agents such as selective serotonin re-uptake inhibitors (SSRIs) or tricyclic antidepressants (TCAs). Linezolid should not be used for the treatment of catheter-related bloodstream infections or catheter-site infections, as the risk of therapy appears to outweigh its benefits under these circumstances.

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Linezolid is an oxazolidinone antibacterial agent that acts by inhibiting the initiation of bacterial protein synthesis. Cross-resistance between linezolid and other inhibitors of protein synthesis has not been demonstrated. Linezolid has a wide spectrum of activity against gram-positive organisms including methicillin-resistant staphylococci, penicillin-resistant pneumococci and vancomycin-resistant Enterococcus faecalis and E. faecium. Anerobes such as Clostridium spp., Peptostreptococcus spp. and Prevotella spp. are also susceptible to linezolid. Linezolid is bacteriostatic against most susceptible organisms but displays bactericidal activity against some strains of pneumococci, Bacteroides fragilis and C. perfringens. In clinical trials involving hospitalised patients with skin/soft tissue infections (predominantly S. aureus), intravenous/oral linezolid (up to 1250 mg mg/day) produced clinical success in >83% of individuals. In patients with community-acquired pneumonia, success rates were >94%. Preliminary clinical data also indicate that twice daily intravenous/oral linezolid 600 mg is as effective as intravenous vancomycin 1 g in the treatment of patients with hospital-acquired pneumonia and in those with infections caused by methicillin-resistant staphylococci. Moreover, linezolid 600 mg twice daily produced >85% clinical/microbiological cure in vancomycin-resistant enterococcal infections. Linezolid is generally well tolerated and gastrointestinal disturbances are the most commonly occurring adverse events. No clinical evidence of adverse reactions as a result of monoamine oxidase inhibition has been reported.[3]

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Superantigens (SAg), the potent activators of the immune system, are important determinants of Staphylococcus aureus virulence and pathogenicity. Superior response to SAg in human leukocyte antigen (HLA)-DR3 transgenic mice rendered them more susceptible than C57BL/6 mice to pneumonia caused by SAg-producing strains of S. aureus. Linezolid, a bacterial protein synthesis inhibitor, was superior to vancomycin in inhibiting SAg production by S. aureus in vitro and conferred greater protection from pneumonia caused by SAg-producing staphylococci.[4]

C16H20FN3O4

337.35

337.143

C, 56.97; H, 5.98; F, 5.63; N, 12.46; O, 18.97

165800-03-3

Linezolid-d3;1127120-38-0

441401

White to off-white solid powder

1.3±0.1 g/cm3

585.5±50.0 °C at 760 mmHg

176-1780C

307.9±30.1 °C

0.0±1.6 mmHg at 25°C

1.554

0.3

1

6

4

24

472

1

FC1C([H])=C(C([H])=C([H])C=1N1C([H])([H])C([H])([H])OC([H])([H])C1([H])[H])N1C(=O)O[C@@]([H])(C([H])([H])N([H])C(C([H])([H])[H])=O)C1([H])[H]

TYZROVQLWOKYKF-ZDUSSCGKSA-N

InChI=1S/C16H20FN3O4/c1-11(21)18-9-13-10-20(16(22)24-13)12-2-3-15(14(17)8-12)19-4-6-23-7-5-19/h2-3,8,13H,4-7,9-10H2,1H3,(H,18,21)/t13-/m0/s1

N-[[(5S)-3-(3-fluoro-4-morpholin-4-ylphenyl)-2-oxo-1,3-oxazolidin-5-yl]methyl]acetamide

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

Note: This product requires protection from light (avoid light exposure) during transportation and storage.

Room temperature (This product is stable at ambient temperature for a few days during ordinary shipping and time spent in Customs)

DMSO : 68~100 mg/mL ( 201.57~296.43 mM )
Ethanol : ~10 mg/mL

Solubility in Formulation 1: ≥ 2.5 mg/mL (7.41 mM) (saturation unknown) in 10% DMSO + 40% PEG300 + 5% Tween80 + 45% Saline (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 400 μL PEG300 and mix evenly; then add 50 μL Tween-80 to the above solution and mix evenly; then add 450 μL normal saline to adjust the volume to 1 mL.
Preparation of saline: Dissolve 0.9 g of sodium chloride in 100 mL ddH₂ O to obtain a clear solution.

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Solubility in Formulation 2: ≥ 2.5 mg/mL (7.41 mM) (saturation unknown) in 10% DMSO + 90% (20% SBE-β-CD in Saline) (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 900 μL of 20% SBE-β-CD physiological saline solution and mix evenly.
Preparation of 20% SBE-β-CD in Saline (4°C，1 week): Dissolve 2 g SBE-β-CD in 10 mL saline to obtain a clear solution.

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Solubility in Formulation 3: ≥ 2.5 mg/mL (7.41 mM) (saturation unknown) in 10% DMSO + 90% Corn Oil (add these co-solvents sequentially from left to right, and one by one), clear solution.
For example, if 1 mL of working solution is to be prepared, you can add 100 μL of 25.0 mg/mL clear DMSO stock solution to 900 μL of corn oil and mix evenly.

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Solubility in Formulation 4: 30% PEG400 + 0.5% Tween 80+ 5% propylene glycol: 30mg/ml (88.93mM)

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 (Please use freshly prepared in vivo formulations for optimal results.)