Cephalexin

Absorption

Well absorbed from the upper gastrointestinal tract with nearly 100% oral bioavailability. Cephalexin is not absorbed in the stomach but is absorbed in the upper intestine.
Patients taking 250mg of cephalexin reach a maximum plasma concentration of 7.7mcg/mL and patients taking 500mg reach 12.3mcg/mL.

Toxicity

Symptoms of overdose include blood in the urine, diarrhea, nausea, upper abdominal pain, and vomiting. An overdose is generally managed through supportive treatment as diuresis, dialysis, hemodialysis, and charcoal hemoperfusion are not well studied in this case.
The oral median lethal dose of cephalexin in rats is >5000 mg/kg. The oral LD50 in a monkey is >1g/kg and the lowest dose causing a toxic effect in humans is 14mg/kg.
Cephalexin has not been shown to be harmful in pregnancy and is not associated with teratogeniticy. Cephalexin is present in breast milk, though infants may be exposed to <1% of the dose given to the mother. The effects of breast milk exposure to cephalexin have not been established and so caution must be exercised and the risk and benefit of cephalexin use in breastfeeding must be weighed.
Cephalexin has not been studied for carcinogenicity or mutagenicity. Cephalexin has no affect on fertility in rats.

Description

Use of the ampicillin-type side chain conveys oral activity to cephalexin. Whereas it no longer has an activating side chain at C-3 and, as a consequence, is somewhat less potent, it does not undergo metabolic deactivation and, thus, maintains potency. It is rapidly and completely absorbed from the GI tract and has become quite popular. Somewhat puzzling is the fact that the use of the ampicillin side chain in the cephalosporins does not result in a comparable shift in antimicrobial spectrum. Cephalexin, like the other first-generation cephalosporins is active against many Gram-positive aerobic cocci but is limited against Gram-negative bacteria. It is a widely used drug, particularly against Gram-negative bacteria causing urinary tract infections, Gram-positive infections (Staphyl ococcus aureus, Streptococcus pneumoni ae and Streptococcus pyogenes) of soft tissues, pharyngitis, and minor wounds.

Chemical properties

White cryst. powder

Originator

Ceporex,Glaxo,UK,1970

The Uses of Cephalexin

Antibacterial.

Background

Cephalexin is the first of the first generation cephalosporins. This antibiotic contains a beta lactam and a dihydrothiazide. Cephalexin is used to treat a number of susceptible bacterial infections through inhibition of cell wall synthesis. Cephalexin was approved by the FDA on 4 January 1971.

What are the applications of Application

Cephalexin is a semi-synthetic, cephalosporin antibiotic

Indications

Cephalexin is indicated for the treatment of certain infections caused by susceptible bacteria. These infections include respiratory tract infections, otitis media, skin and skin structure infections, bone infections, and genitourinary tract infections.

Definition

ChEBI: A semisynthetic first-generation cephalosporin antibiotic having methyl and beta-(2R)-2-amino-2-phenylacetamido groups at the 3- and 7- of the cephem skeleton, respectively. It is effective against both Gram-negative and G am-positive organisms, and is used for treatment of infections of the skin, respiratory tract and urinary tract.

Manufacturing Process

To a 1 liter flask containing dimethylformamide at 0°C, was added 24.8 g sodium N-(2-methoxycarbonyl-1-methylvinyl)-D-α-phenylglycine (prepared from sodium D-α-phenylglycine and methyl acetoacetate). The mixture was cooled to -40°C and methyl chloroformate (7.5 ml) and dimethylbenzylamine (0.26 ml) added. After stirring for 25 minutes, p-nitrobenzyl 7- aminodesacetoxycephalosporanate (32.8 g) in the form of its hydrochloride salt was added, followed by triethylamine (12.1 ml) and dimethylformamide (140 ml) over a period of 20 minutes. The reaction mixture was stirred for 2 hours at -25°C to -35°C, then warmed to 0°C and water (32 ml) added. To the resultant solution, hydrochloric acid (54 ml) was added followed by zinc (21.8 g) in portions over a period of 5 minutes, the temperature being maintained at 5°C to 10°C. Further hydrochloric acid (35 ml) was added and the solution stirred at 15°C to 20°C for 7 hours.
The pH was adjusted to 3.3 with triethylamine and semicarbazidehydrochloride (9.5 g) added. The mixture was brought back to pH 3 with further triethylamine, then stirred for 30 minutes at pH 3. The resultant mixture was adjusted slowly over 4 hours to pH 6.8 by addition of triethylamine, seeding being carried out when pH 4.5 was reached. The precipitated cephalexin was filtered off, washed with dimethylformamide (200 ml) and the cephalexin recovered, yield 75%.

brand name

Keflex (Panixine (Ranbaxy).

Therapeutic Function

Antibiotic

Antimicrobial activity

It is resistant to staphylococcal β-lactamase. Gram-positive rods and fastidious Gram-negative bacilli, such as Bordetella spp. and H. influenzae, are relatively resistant. It is active against a range of enterobacteria, but it is degraded by many enterobacterial β-lactamases. Citrobacter, Edwardsiella, Enterobacter, Hafnia, Providencia and Serratia spp. are all resistant. Gram-negative anaerobes other than B. fragilis are susceptible. Because of its mode of action it is only slowly bactericidal to Gram-negative bacilli.

Pharmacokinetics

Cephalexin (also called Cefalexin) is a first generation cephalosporin antibiotic. It is one of the most widely prescribed antibiotics, often used for the treatment of superficial infections that result as complications of minor wounds or lacerations. It is effective against most gram-positive bacteria through its inihibition of the cross linking reaction between N-acetyl muramicacid and N-acetylglucosamine in the cell wall, leading to cell lysis.

Pharmacokinetics

Oral absorption: >90%
C_max 500 mg oral: c. 10–20 mg/L after 1 h
Plasma half-life: 0.5–1 h
Volume of distribution: 15 L
Plasma protein binding: 10–15%
Absorption and distribution
It is almost completely absorbed when given by mouth, the peak concentration being delayed by food. Intramuscular preparations are not available: injection is painful and produces delayed peak plasma concentrations considerably lower than those obtained by oral administration.
In synovial fluid, levels of 6–38 mg/L have been described after a 4 g oral dose, but penetration into the CSF is poor. Useful levels are achieved in bone (9–44 mg/kg after 1 g orally) and in purulent sputum. Concentrations of 10–20 mg/L have been found in breast milk. Concentrations in cord blood following a maternal oral dose of 0.25 g were minimal.
Metabolism and excretion
It is not metabolized. Almost all the dose is recoverable from the urine within the first 6 h, producing urinary concentrations exceeding 1 g/L. The involvement of tubular secretion is indicated by the increased plasma peak concentration and reduced urinary excretion produced by probenecid. Renal clearance is around 200 mL/min and is depressed in renal failure, although a therapeutic concentration is still obtained in the urine. It is removed by peritoneal and hemodialysis. Some is excreted in the bile, in which therapeutic concentrations may be achieved.

Clinical Use

As for group 2 cephalosporins . It should not be used in infections in which H. influenzae is, or is likely to be, implicated. It should not be used as an alternative to penicillin in syphilis.

Clinical Use

Cephalexin, 7α-(D-amino-α-phenylacetamido)-3-methylcephemcarboxylicacid (Keflex, Keforal), was designed purposelyas an orally active, semisynthetic cephalosporin. Theoral inactivation of cephalosporins has been attributed to twocauses: instability of the β-lactam ring to acid hydrolysis(cephalothin and cephaloridine) and solvolysis or microbialtransformation of the 3-methylacetoxy group (cephalothin,cephaloglycin). The α-amino group of cephalexin renders itacid stable, and reduction of the 3-acetoxymethyl to a methylgroup circumvents reaction at that site.
Cephalexin occurs as a white crystalline monohydrate. Itis freely soluble in water, resistant to acid, and absorbed wellorally. Food does not interfere with its absorption. Becauseof minimal protein binding and nearly exclusive renal excretion,cephalexin is recommended particularly for the treatmentof urinary tract infections. It is also sometimes used forupper respiratory tract infections. Its spectrum of activity isvery similar to those of cephalothin and cephaloridine.Cephalexin is somewhat less potent than these two agentsafter parenteral administration and, therefore, is inferior tothem for the treatment of serious systemic infections.

Side Effects

Nausea, vomiting and abdominal discomfort are relatively common. Pseudomembranous colitis has been described and overgrowth of Candida with vaginitis may be troublesome. Otherwise, mild hypersensitivity reactions and biochemical changes common to cephalosporins occur. Very rare neurological disturbances have been described, particularly in patients in whom very high plasma levels have been achieved. There are rare reports of Stevens–Johnson syndrome and toxic epidermal necrolysis.

Safety Profile

Poison by intraperitoneal route.Moderately toxic by ingestion and other routes. An experimental teratogen. Other experimental reproductiveeffects. Human systemic effects by ingestion: nausea,vomiting, and diarrhea. When heated to decomposition itemits

Synthesis

Cephalexin is synthesized from cephalophenylglycine (32.1.2.9), which is synthesized by reacting 7-aminocephalosporanic acid with a mixed anhydride synthesized by reacting N-carbobenzoxyphenylglycine and isobutyl chloroformate in the presence of triethylamine. Removing the N-carbobenzoxy protective group from the resulting product (32.1.2.8) using hydrogen and a palladium on carbon catalyst gives cephalophenylglycine (32.1.2.9) in the form of an internal salt. Reducing this product with hydrogen using a palladium on barium sulfate catalyst results in the deacetoxylation at the third position of 7-aminocephalosporanic acid, making the desired cephalexin (32.1.2.10).

Veterinary Drugs and Treatments

There are no approved cephalexin products for veterinary use in the USA. However, it has been used clinically in dogs, cats, horses, rabbits, ferrets, and birds, particularly for susceptible Staphylococcal infections.

Drug interactions

Potentially hazardous interactions with other drugs
Anticoagulants: effects of coumarins may be enhanced.

Metabolism

Cephalexin is not metabolized in the body.

Metabolism

Cefalexin is not metabolised. About 80% or more of a dose is excreted unchanged in the urine in the first 6 hours by glomerular filtration and tubular secretion. Probenecid delays urinary excretion. Therapeutically effective concentrations may be found in the bile and some may be excreted by this route.