Quinolone
Bacterial DNA gyrase [2]
Bacterial topoisomerase IV [2]

Norfloxacin (MK-0366) is a synthetic antibacterial chemotherapeutic agent that is occasionally used to treat both simple and complex urinary tract infections. Broad-spectrum antibiotic norfloxacin (MK-0366) is effective against both Gram-positive and Gram-negative bacteria. It stops cell division by blocking enzymes that are required to separate bacterial DNA, such as DNA gyrase, a type II topoisomerase, and topoisomerase IV. Currently, the adult population has three approved uses—one of which is restricted—and the other is ineffective because of bacterial resistance.

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Against Gram-negative bacteria (Escherichia coli, Klebsiella pneumoniae, Pseudomonas aeruginosa, Campylobacter jejuni), Norfloxacin (MK-0366) exhibited potent concentration-dependent antibacterial activity, with MIC values ranging from 0.06 to 2 μg/mL for susceptible strains. Resistant Campylobacter strains showed MIC values > 8 μg/mL [1][2]
- Against some Gram-positive bacteria (Staphylococcus aureus, Streptococcus pyogenes), the drug displayed moderate antibacterial activity, with MIC values of 1-8 μg/mL [2]
- Norfloxacin (MK-0366) inhibited bacterial DNA replication and transcription by targeting DNA gyrase and topoisomerase IV, stabilizing enzyme-DNA cleavage complexes and preventing DNA strand religation [2]

In murine models of urinary tract infection (UTI) induced by Escherichia coli, oral administration of Norfloxacin (MK-0366) at 20-50 mg/kg/day for 3-5 days significantly reduced bacterial load in the kidneys and bladder, with cure rates > 80% [2]
- In clinical studies of women with uncomplicated acute cystitis, oral Norfloxacin (MK-0366) (400 mg twice daily for 3 days) achieved clinical cure rates of 85-95% and microbiological eradication rates of 80-90% for susceptible uropathogens [3]
- The drug showed good tissue penetration, reaching therapeutic concentrations in the urinary tract, gastrointestinal tract, and skin/soft tissues [2]

Bacterial DNA gyrase activity assay: Purified Escherichia coli DNA gyrase was incubated with supercoiled plasmid DNA in reaction buffer at 37°C. Norfloxacin (MK-0366) was added at serial concentrations (0.03-16 μg/mL), and the mixture was incubated for 60 minutes. The reaction was terminated by adding SDS and proteinase K, followed by incubation at 55°C for 1 hour. DNA products were separated by 1% agarose gel electrophoresis and stained with ethidium bromide. The inhibition of DNA gyrase-mediated supercoiling relaxation was quantified by measuring the intensity of supercoiled DNA bands [2]
- Bacterial topoisomerase IV activity assay: Isolated Staphylococcus aureus topoisomerase IV was incubated with relaxed plasmid DNA in reaction buffer. Norfloxacin (MK-0366) was added at concentrations of 0.06-32 μg/mL, and the mixture was incubated at 37°C for 45 minutes. The reaction was stopped by adding stop solution, and DNA products were analyzed by agarose gel electrophoresis to assess inhibition of DNA decatenation [2]

Bacterial growth inhibition assay: Bacterial strains (Escherichia coli, Campylobacter jejuni, Staphylococcus aureus) were cultured in Mueller-Hinton broth at 37°C with shaking. Norfloxacin (MK-0366) was added at serial concentrations (0.015-64 μg/mL), and bacterial growth was monitored by measuring optical density at 600 nm (OD600) after 24 hours. The MIC was defined as the lowest concentration inhibiting ≥90% bacterial growth [1][2]

Urinary tract infection mouse model: Female BALB/c mice were intraurethrally inoculated with a pathogenic strain of Escherichia coli to induce UTI. Norfloxacin (MK-0366) was dissolved in sterile water and administered orally via gavage at doses of 20, 30, or 50 mg/kg/day. Dosing started 24 hours post-infection and continued once daily for 3-5 days. Mice were euthanized, and kidneys and bladder tissues were collected to quantify bacterial load via colony counting [2]

Absorption, Distribution and Excretion
Norfloxacin is primarily eliminated through metabolism, bile excretion, and renal excretion. Its renal clearance is as high as approximately 275 mL/min, indicating that it undergoes glomerular filtration and tubular secretion during renal excretion. Norfloxacin can cross the placenta and is distributed in umbilical cord blood and amniotic fluid. It is currently unknown whether the drug is distributed into breast milk. Following a single oral dose of 200 mg norfloxacin in lactating women, the drug was not detected in breast milk, but whether higher doses will result in distribution into breast milk remains to be determined. Some other quinolone drugs (such as ciprofloxacin, levofloxacin, and ofloxacin) are excreted into breast milk. In adults, after a twice-daily oral administration of 400 mg norfloxacin, prostate tissue samples collected 1–4 hours after the second dose showed drug concentrations ranging from 0.24–4.65 μg/g; serum concentrations ranged from 0.42–5.3 μg/mL. Norfloxacin binds to serum proteins at a rate of 10-15%. The bile concentration of norfloxacin may be up to 10 times higher than the simultaneous serum concentration. In patients undergoing cholecystectomy, following a single oral dose of 400 mg norfloxacin preoperatively, the drug concentrations in gallbladder bile collected approximately 3.5-6 hours after administration were 0.6-15.6 μg/mL, in gallbladder tissue 0.4-7.5 μg/mL, and in serum 0.4-1.8 μg/mL. Information on the distribution of norfloxacin is limited. In adults, after oral administration, norfloxacin is distributed in the renal parenchyma, gallbladder, liver, prostate tissue, testes, semen, uterus, fallopian tubes, cervix and vaginal tissue, vesicular fluid, tonsils, maxillary sinus mucosa, sputum, and bile. For more complete data on the absorption, distribution, and excretion of norfloxacin (9 metabolites), please visit the HSDB records page. Metabolism/Metabolites Metabolized by the liver and kidneys Norfloxacin is excreted via renal and non-renal pathways. The drug is partially metabolized, with the piperazine group undergoing modification to generate six metabolites, named M-1, M-2, M-3, M-4(1), M-4(2), and M-5. Although some metabolites possess microbial activity, their activity is lower than that of the parent drug. Studies have shown that norfloxacin undergoes first-pass metabolism in the liver, but further research is needed to fully elucidate the drug's metabolic pathway. Pefloxacin, N-demethylated, is a known human metabolite of pefloxacin. Biological half-life 3-4 hours Serum norfloxacin concentrations are higher and the half-life is prolonged in patients with impaired renal function. In adult patients with impaired renal function, the mean half-lives of norfloxacin were 4.4, 6.6, and 7.6 hours when creatinine clearance was 30-80, 10-29, and less than 10 mL/min/1.73 m², respectively. Limited data suggest that hepatic impairment has little effect on the half-life of the drug. The effective plasma or serum half-life of norfloxacin in adults with normal renal function is 2.3-4 hours. The mean effective half-life of the drug in elderly patients aged 65-75 years with normal renal function is 4 hours. Absorption: Oral norfloxacin (MK-0366) is well absorbed, with an oral bioavailability of approximately 80-90%. Peak plasma concentration (Cmax) of 1.5-2.5 μg/mL is reached within 1-2 hours after a 400 mg dose [2]. Distribution: The drug is widely distributed throughout the body tissues and fluids, with high concentrations in the kidneys, bladder, urine, and gastrointestinal tract. The plasma protein binding rate is approximately 10-15% [2]
- Metabolism: Norfloxacin (MK-0366) is minimally metabolized by the liver, with over 80% of the drug excreted unchanged [2]
- Excretion: It is mainly excreted via the kidneys, with 60-70% of the administered dose excreted in the urine within 24 hours. The plasma elimination half-life is approximately 3-4 hours [2]

Hepatotoxicity
As with other fluoroquinolones, the incidence of serum enzyme elevations during norfloxacin treatment is low (1% to 3%). These abnormalities are usually mild, asymptomatic, and transient, and resolve with continued treatment. Norfloxacin has also been associated with rare but occasionally severe cases of acute liver injury, which can even be fatal. Although the number of cases is small, the clinical presentation is consistent, with a short incubation period (1 day to 3 weeks) and sudden onset of hepatocellular damage. Serum enzyme elevations can be hepatocellular or cholestatic, with shorter-onset cases often being more hepatocellular. ALT levels are significantly elevated, sometimes accompanied by a rapid prothrombin time and signs of liver failure. The disease may develop within days of discontinuation of the drug. Many (but not all) cases present with allergic reactions, including fever, rash, and eosinophilia. Autoantibodies are usually absent. Cholestatic and mixed-type liver injury have also been reported, especially in cases of delayed liver injury recognition. These characteristics are typical of all fluoroquinolone-related hepatotoxicity, and this type of injury is considered specific to this class of drugs.
Probability Score: C (Possibly a rare cause of clinically significant liver damage).

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Effects during Pregnancy and Lactation
◉ Overview of Use During Lactation
There is currently no information on the clinical use of norfloxacin during lactation; however, the concentration in breast milk appears to be very low. Fluoroquinolones such as norfloxacin are traditionally not used to treat infants due to concerns about adverse effects on developing joints. However, recent studies suggest the risk is minimal. Furthermore, calcium in breast milk may inhibit the absorption of small amounts of fluoroquinolones in breast milk, but there is currently insufficient data to confirm or refute this claim. Norfloxacin has the lowest serum and milk concentrations and oral bioavailability of all fluoroquinolones, therefore the risk to the infant should be minimal. Breastfeeding women can use norfloxacin, but monitoring for potential impacts on the infant's gut microbiota, such as diarrhea or candidiasis (thrush, diaper rash), is necessary.
◉ Effects on Breastfed Infants
As of the revision date, no relevant published information was found.
◉ Effects on Lactation and Breast Milk
No published information found as of the revision date.

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Protein Binding
10% and 15% (serum protein binding)

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Drug Interactions
In patients receiving theophylline therapy, concomitant use of certain quinolones (e.g., ciprofloxacin, norfloxacin) can lead to increased plasma theophylline concentrations and decreased drug clearance, potentially increasing the risk of theophylline-related adverse reactions. Reports on the effects of norfloxacin on theophylline pharmacokinetics are inconsistent, and more research and experience are needed to assess their interaction; however, the risk of norfloxacin causing significant alterations in theophylline pharmacokinetics appears to be lower than that of some other quinolones (e.g., ciprofloxacin). In a small number of subjects, concomitant use of norfloxacin with extended-release theophylline formulations resulted in only a slight increase in serum theophylline concentrations, a smaller increase compared to some other quinolone derivatives. In other studies, concomitant use of norfloxacin in patients already on stable theophylline intake resulted in a maximum increase of 18% in plasma theophylline concentration and a decrease of 5-28% in theophylline clearance. Some clinicians believe that the interaction between norfloxacin and theophylline is likely clinically insignificant in most patients. However, there are reports of theophylline-related adverse reactions in patients receiving norfloxacin who also take theophylline. Therefore, some clinicians recommend caution when using norfloxacin in patients taking theophylline. The manufacturer of norfloxacin notes that plasma theophylline concentrations should be monitored and theophylline dosage adjusted as needed. Some quinolones (such as ciprofloxacin) have also been reported to alter the pharmacokinetics of caffeine; therefore, the potential for enhanced or prolonged caffeine effects when used concomitantly with quinolones should be considered. Concomitant use of sucralfate may interfere with the oral absorption of norfloxacin, leading to decreased serum and urinary quinolone concentrations; therefore, some clinicians do not recommend concomitant use of ofloxacin with sucralfate. If ofloxacin and sucralfate must be used concurrently, manufacturers and some clinicians recommend that norfloxacin be taken at least 2 hours before or after sucralfate. Concomitant use of probenecid significantly reduces the urinary excretion of norfloxacin, possibly because probenecid blocks the renal tubular secretion of the anti-infective drug; however, the serum concentration and half-life of norfloxacin are usually unaffected. In vitro studies have shown that chloramphenicol, rifampin, or tetracycline can inhibit the bactericidal activity of norfloxacin. One in vitro study found that norfloxacin, when used in combination with chloramphenicol or tetracycline, had an antagonistic effect on all tested Salmonella isolates. An in vitro study using Pseudomonas aeruginosa strains found that norfloxacin, when used in combination with chloramphenicol or tetracycline, had an antagonistic effect on all tested Salmonella isolates. For aminoglycoside- and carbenicillin-resistant Pseudomonas aeruginosa, imipenem and norfloxacin exhibited synergistic or partially synergistic antibacterial activity against approximately one-third of the strains, but no activity against approximately two-thirds; no antagonistic effect was observed. In vitro studies (including Gram-positive and Gram-negative bacteria) showed that norfloxacin, when used in combination with β-lactam antibiotics (e.g., ampicillin, cefotaxime, cefoxitin), produced neither synergistic nor antagonistic effects. For more complete data on norfloxacin interactions (16 in total), please visit the HSDB record page.

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Non-human toxicity values
Mouse intramuscular LD50: 470 mg/kg
Mouse intravenous LD50: 222 mg/kg
Mouse oral LD50: 4 g/kg
Rat intravenous LD50: 245 mg/kg
Gastrointestinal toxicity: Common side effects include nausea (incidence 5-10%), diarrhea (3-7%) and abdominal discomfort (2-5%), which are usually mild and reversible. [2][3]
-Central nervous system (CNS) toxicity: Rare adverse reactions include dizziness (1-3%), headache (2-4%) and insomnia (1-2%); seizures are extremely rare (<0.1%) [2]
-Tendon toxicity: There are case reports of tendinitis or tendon rupture, mainly in elderly patients or patients receiving corticosteroid therapy. [2]

[1]. Fluoroquinolone-resistant Campylobacter species and the withdrawal of fluoroquinolones from use in poultry: a public health success story. Clin Infect Dis. 2007 Apr 1;44(7):977-80. Epub 2007 Feb 14.

[2]. Norfloxacin: more than 20 years of clinical use, the results and place among fluoroquinolones in modern chemotherapy for infections. Antibiot Khimioter. 2003;48(9):28-36.

[3]. Quinolones for uncomplicated acute cystitis in women. Cochrane Database Syst Rev. 2006 Jul 19;(3):CD003597.

Norfloxacin is a quinoline monocarboxylic acid compound with broad-spectrum antibacterial activity against most Gram-negative and Gram-positive bacteria. Norfloxacin has bactericidal activity by binding to an enzyme called DNA gyrase, blocking bacterial DNA replication. It is both an antibacterial drug and a DNA synthesis inhibitor, as well as an exogenous substance and environmental pollutant. It is a quinoline monocarboxylic acid compound, belonging to the N-arylpiperazine, quinolone, quinolone antibiotic, and fluoroquinolone antibiotic class. A synthetic fluoroquinolone drug (fluoroquinolone), it has broad-spectrum antibacterial activity against most Gram-negative and Gram-positive bacteria. Norfloxacin inhibits bacterial DNA gyrase. Norfloxacin is a quinolone antibacterial drug. Norfloxacin is a first-generation fluoroquinolone drug, commonly used to treat urinary tract infections and prostatitis. Norfloxacin has been associated with rare cases of acute hepatocellular injury. Norfloxacin has been reported to be effective against Bacillus subtilis and Corynebacterium rhizogenes, and relevant data are available. Norfloxacin is a synthetic broad-spectrum fluoroquinolone drug with antibacterial activity. Norfloxacin inhibits the activity of DNA gyrase, thereby blocking bacterial DNA replication. Norfloxacin concentrates in the renal tubules and bladder, exhibiting bactericidal activity against a variety of aerobic Gram-positive and Gram-negative bacteria. A synthetic fluoroquinolone drug (fluoroquinolone) with broad-spectrum antibacterial activity against most Gram-negative and Gram-positive bacteria. Norfloxacin inhibits bacterial DNA gyrase. See also: Norfloxacin hydrochloride (its active ingredient). Indications: For the treatment of urinary tract infections.
FDA Label

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Mechanism of Action
The bactericidal action of norfloxacin stems from its inhibition of topoisomerase II (DNA gyrase) and topoisomerase IV, both of which are essential for bacterial DNA replication, transcription, repair, and recombination. Norfloxacin is a broad-spectrum antibiotic effective against a wide range of Gram-positive and Gram-negative bacteria. The fluorine atom at position 6 enhances its antibacterial activity against Gram-negative bacteria, while the piperazine group at position 7 confers its activity against Pseudomonas.
Norfloxacin generally has a bactericidal effect. Like other fluoroquinolone anti-infectives, norfloxacin inhibits DNA synthesis in susceptible bacteria by inhibiting type II DNA topoisomerases (DNA gyrase, topoisomerase IV).
Fluoroquinolones prolong the QT interval by blocking voltage-gated potassium channels, particularly the fast component of the delayed rectifier potassium current I (Kr) expressed by HERG (human ether-a-go-go related gene). Based on existing case reports and clinical studies, moxifloxacin carries the highest risk of QT interval prolongation among all available quinolones; therefore, it should be used with caution in patients with predisposing factors for torsades de pointes (TdP).

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Therapeutic Uses
Antibacterial; Enzyme Inhibitor; Nucleic Acid Synthesis Inhibitor
Oral norfloxacin is used to treat prostatitis caused by Escherichia coli. (Included on US product label)
Oral norfloxacin is used to treat complicated urinary tract infections in adults caused by susceptible Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Pseudomonas aeruginosa, Serratia marcescens, or Enterococcus faecalis. /Included on US product label/
Oral norfloxacin is used to treat uncomplicated urinary tract infections (UTIs) in adults caused by susceptible bacteria (including cystitis), such as Citrobacter freundii, Enterobacter aerogenes, Enterobacter cloacae, Escherichia coli, Klebsiella pneumoniae, Proteus mirabilis, Proteus vulgaris, Pseudomonas aeruginosa, etc. This medication is also used to treat uncomplicated urinary tract infections in adults caused by susceptible bacteria such as Staphylococcus aureus, Staphylococcus epidermidis, Staphylococcus saprophyticus, Streptococcus agalactiae (Group B Streptococcus), or Enterococcus faecalis. /Included on US product label/
For more complete data on the therapeutic uses of norfloxacin (10 types), please visit the HSDB record page.

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Drug Warnings
Warning: Fluoroquinolones (including norfloxacin) are associated with an increased risk of tendinitis and tendon rupture in all age groups. The risk is further increased in older patients (typically over 60 years of age), patients taking corticosteroids, and patients who have received kidney, heart, or lung transplants.
Warning: Fluoroquinolones (including norfloxacin) may worsen muscle weakness symptoms in patients with myasthenia gravis. Norfloxacin should be avoided in patients with a known history of myasthenia gravis.
Severe and even fatal hypersensitivity reactions (anaphylactic reactions) have been reported in patients receiving quinolone antibiotics (including norfloxacin), with some patients experiencing anaphylactic reactions after the first dose. Some reactions are accompanied by cardiovascular failure, loss of consciousness, tingling, pharyngeal or facial edema, dyspnea, urticaria, and pruritus. Only a small number of patients have a history of hypersensitivity reactions. If a norfloxacin anaphylactic reaction occurs, the drug should be discontinued immediately. Severe acute hypersensitivity reactions require immediate emergency treatment with epinephrine. Oxygen, intravenous fluids, antihistamines, corticosteroids, vasopressors, and airway management, including endotracheal intubation, should be administered as needed.
Other serious and even fatal events have been rarely reported in patients receiving quinolone antibiotics (including norfloxacin), some of which are caused by hypersensitivity reactions, while others are of unknown cause. These events can be severe and often occur after multiple doses. Clinical manifestations may include one or more of the following: fever, rash, or severe skin reactions (e.g., toxic epidermal necrolysis, Stevens-Johnson syndrome); vasculitis; arthralgia; myalgia; serum sickness; allergic pneumonia; interstitial nephritis; acute renal insufficiency or failure; hepatitis; jaundice; acute liver necrosis or failure; anemia, including hemolytic anemia and aplastic anemia; thrombocytopenia, including thrombotic thrombocytopenic purpura; leukopenia; agranulocytosis; pancytopenia; and/or other hematologic abnormalities. For more complete data on drug warnings for norfloxacin (24 in total), please visit the HSDB record page. Pharmacodynamics: Norfloxacin is a quinolone/fluoroquinolone antibiotic. Norfloxacin has bactericidal activity; its mechanism of action is to block bacterial DNA replication by binding to an enzyme called DNA gyrase, thereby unwinding the DNA double helix and preventing it from replicating into two DNA double helices. It is worth noting that the drug has an affinity for bacterial DNA gyrase that is 100 times higher than that for mammalian DNA gyrase. Norfloxacin (MK-0366) is a first-generation broad-spectrum fluoroquinolone antibiotic used to treat bacterial infections [2] - Mechanism of action: It exerts its antibacterial effect by dual targeting of bacterial DNA gyrase and topoisomerase IV, blocking DNA replication/transcription, ultimately leading to bacterial cell death [2] - Clinical indications: It is approved for the treatment of uncomplicated urinary tract infections, complicated urinary tract infections, traveler's diarrhea and gastrointestinal infections caused by susceptible pathogens [2][3] - Resistance background: Overuse in poultry has led to increased resistance of Campylobacter to fluoroquinolones, prompting a halt to the use of fluoroquinolones in poultry to maintain their clinical efficacy [1] Therapeutic advantages: High urinary excretion and high concentration make it particularly effective against urinary tract infections [2][3]

C16H18FN3O3

319.3308

319.133

C, 60.18; H, 5.68; F, 5.95; N, 13.16; O, 15.03

70458-96-7

68077-27-0；118803-81-9

4539

White to light yellow solid powder

1.3±0.1 g/cm3

555.8±50.0 °C at 760 mmHg

220°C

289.9±30.1 °C

0.0±1.6 mmHg at 25°C

1.595

0.82

2

7

3

23

519

0

FC1C([H])=C2C(C(C(=O)O[H])=C([H])N(C([H])([H])C([H])([H])[H])C2=C([H])C=1N1C([H])([H])C([H])([H])N([H])C([H])([H])C1([H])[H])=O

OGJPXUAPXNRGGI-UHFFFAOYSA-N

InChI=1S/C16H18FN3O3/c1-2-19-9-11(16(22)23)15(21)10-7-12(17)14(8-13(10)19)20-5-3-18-4-6-20/h7-9,18H,2-6H2,1H3,(H,22,23)

1-ethyl-6-fluoro-4-oxo-7-piperazin-1-ylquinoline-3-carboxylic acid

MK-0366; Norfloxacin; AM-715; MK 0366; MK0366; AM715; MK-366; AM 715; MK366; MK 366; Noroxin; Chibroxin

2934.99.9001

Powder      -20°C    3 years

                     4°C     2 years

In solvent   -80°C    6 months

                  -20°C    1 month

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

DMSO : 3~5 mg/mL (9.4~15.7 mM)
Water : <1 mg/mL
Ethanol : <1 mg/mL

Solubility in Formulation 1: ≥ 0.5 mg/mL (1.57 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 5.0 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: ≥ 0.5 mg/mL (1.57 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 5.0 mg/mL clear DMSO stock solution to 900 μL of corn oil and mix evenly.

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