Indications

Cephalexin is prescribed for the treatment of otitis media, genitourinary, bone, respiratory, and skin structure infections.

Mechanism of Action

The mechanism of action of Cephalexin resembles that of penicillin where it inhibits synthesis of the bacterial cell wall, its absence influences death as a result of bacterial lysis. Cell lysis is further mediated by autolytic enzymes particular to the bacterial cell wall, which includes autolysis. Research indicates that there is a probability that Cephalexin impedes the functionality of an autolysin inhibitor.

Pharmacodynamics

Cephalexin is a 1st generation cephalosporin antibiotic that is widely prescribed for the treatment of external infections that may arise from complications associated with lacerations or minor wounds. The drug is effective in fighting a majority of gram-positive bacteria. Cephalexin illustrates vitro activity that opposes methicillin-susceptible Staphylococcus aureus, a notable pathogen in osteoarticular infections. However, pharmacodynamics and pharmacokinetics are inadequately defined in children.

Interactions

Cephalexin may minimize the impact of typhoid and BCG vaccines. Notably, these three drugs should not be administered at the same time. Patients are also advised to take the drug on an empty stomach at least 1-2 hours after meals.

Dosing Information

A standard dose of Cephalexin should be administered orally in 250mg every 6 hours. Alternatively, in 12 hours, a dose of 500mg should be administered to the patient during the 7-14days treatment period. In instances where the infections are severe, higher doses up to 4g should be administered in 2-4 equal does every day. For pediatric patients, the appropriate daily dose of Cephalexin is 25-40mg/kg administered in equal doses for a period of 7-14days. Severe infections may necessitate 50-100mg/kg administered in equal doses. The treatment of Otitis media necessitates 75-100mg/kg in equal doses of Cephalexin. For patients with renal impairments, the dosage requirements may be adjusted accordingly for both pediatric and adult patients.

Elimination

Cephalexin undergoes tubular secretion and glomerular filtration before it is eliminated in urine. Studies indicate that about 90% of Cephalexin is eliminated in its unaltered form in urine within 8 hours.

Side effects

Allergic reactions to Cephalexin may result in respiratory issues, swelling of the tongue, lips, face, or the throat, and hives. Nonetheless, one may need to consult a doctor if they experience watery diarrhea or intense stomach pains, unusual bleeding or easy bruising, minimal or no urination, hallucinations, confusion or agitation, and severe skin reaction. Common side effects associated with Cephalexin include vaginal discharge or itching, skin rash, fever, nausea, vomiting, joint pain, headache, feeling of exhaustion, dizziness, or diarrhea.

Safety Precautions

A patient should inform the pharmacist or doctor of any allergic reactions to Cephalexin, associated cephalosporin antibiotics, and additional ingredients or medications. The patient should also indicate any nutritional supplements, vitamins, herbal products or medications they are taking or they are planning to take. It is important to notify the doctor of a preexisting liver, kidney or gastrointestinal disease, especially colitis, if one is either pregnant or planning on getting pregnant, and if they conceive while taking Cephalexin. If Cephalexin is prescribed to a patient with no clear indications of a bacterial infection, there are minimal chances that the drug will benefit the patient. Instead, it will increase the patient’s chances of developing drug-resistant bacteria. Using Cephalexin over extended periods of time may induce the overgrowth of non-susceptible organisms. Doctors should examine their patients for superinfections during therapy for the implementation of appropriate treatment measures.

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%.

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

Oral absorption: >90% Cmax 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.

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

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.

Brand Name(s) in US

Keflex and generic forms

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.

Brand name

Keflex (Panixine (Ranbaxy).