Penicillin-G (Pn-G) or Benzyl Penicillin : Therapeutic uses, Dosage & Side Effects
Penicillin-G or Benzyl Penicillin is a narrow spectrum Penicillin antibiotic; activity is limited primarily to gram positive bacteria and few others. Penicillin G potassium is used for treating some types of infections caused by certain bacteria.
Dose : 0.5-5 MU
Route : IV, IM (Penicillin G is acid labile-destroyed by gastric acid. That’s why, It is not used orally).
Onset of action : Within few minutes
Plasma Half-life : 30 Min.
Duration of action : ?
Bioavailability : <33 % orally
Plasma protein binding : 60%
Metabolism : Liver
Pregnancy risk category : B
Chemical formula : C16H18N2O4S
IUPAC name : (2S,5R,6R)-3,3-Dimethyl-7-oxo-6-(2-phenylacetamido)-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid
Trade name : Pfizerpen
Antibacterial spectrum : Penicillin-G activity is limited primarily to gram-positive bacteria and few others.
- Cocci Streptococci (except viridans, group D or enterococci) are highly sensitive, so are many pneumococci. Staph. aureus, though originally very sensitive, has acquired resistance to such an extent that it must be counted out of PnG spectrum. Gram negative cocci-Neisseria gonorrhoeae and N. meningitidis are susceptible to PnG, though increasing number of gonococci have developed partial and others high degree resistance.
- Bacilli Gram-positive bacilli-majority of B. anthracis, Corynebacterium diphtheriae, and practically all Clostridia (tetani and others), Listeria are highly sensitive, so are spirochetes (Treponema pallidum, Leptospira, and others), but Bacteroides fragilis is largely resistant.
Actinomyces israelii is only moderately sensitive. Majority of gram-negative bacilli (except a few E. coli, Proteus), Mycobacterium tuberculosis, rickettsiae, chlamydiae, protozoa, fungi and viruses are totally insensitive to PnG.
Therapeutic uses : Penicillin G is the drug of choice for infection caused by organisms susceptible to it, unless the patient is allergic to this antibiotic. However, use has declined very much due to fear of causing anaphylaxis.
Penicillin-G is used for following purposes :
- Streptococcal infections Like pharyngitis otitis media, scarlet fever, rheumatic fever respond to ordinary doses of PnG given for 7-10 days. For subacute bacterial endocarditis (SABE) caused by Strep. viridans or faecalis high doses (10-20 Ml. i.v. daily) along with gentamicin given for 2-6 weeks is needed.Reports suggest good efficacy in tuberculosis, MAC infection in AIDS patients and in leprosy.
- Pneumococcal infections PnG is not used now for empirical therapy of pneumococcal (lobar) pneumonia and meningitis because many strains have become highly penicillin resistant. However, PnG 3-6 MU i.v. every 6 hours is the drug of choice if organism is sensitive.
- Meningococcal infections are still mostly responsive; meningitis and other infections may be treated with intravenous injection of high doses.
- Gonorrhoea PnG has become unreliable for treatment of gonorrhoea due to spread of resistant strains.
- Syphilis T. pallidum has not shown any resistance and PnG is the drug of choice. Early and latent syphilis is treated either with daily injection of 1 .2 M U of procaine penicillin for 10 days or with 1-3 weekly doses of 2.4 MU benzathine penicillin. For late syphilis, benzathine penicillin 2.4 MU weekly for 4 weeks is recommended. Cardiovascular and neurosyphilis requires 5 MU i.m. 6 hourly o f sod. PnG for 2 weeks followed by the above regimen.
- Leptospirosis PnG 1 .5 MU injected i. v. 6 hourly for 7 days is curative.
- Diphtheria Antitoxin therapy is of prime importance. Procaine penicillin 1-2 MU daily for 10 days has adjuvant value and prevents carrier state.
- Tetanus & gas gangrene Antitoxin and other measures are more important; PnG 6-12 MU I day is used to kill the causative organism and has adjuvant value.
- Penicillin G is the drug of choice for rare infections like anthrax, actinomycosis, trench mouth, rat bite fever and those caused by Listeria monocytogenes, Pasteurella multocida.
Prophylactic uses of Penicillin G are as following :
- Rheumatic fever Low concentrations of penicillin prevent colonization by streptococci responsible for rheumatic fever. Benzathine penicillin 1 .2 MU every 4 weeks till 18 years of age or 5 years after an attack, whichever is more.
- Bacterial endocarditis Dental extractions, endoscopies, catheterization, etc. cause bacteremia which in patients with valvular defects can cause endocarditis. PnG can afford protection, but amoxicillin is preferred now.
- Agranulocytosis patients Penicillin may be used alone or in combination with an aminoglycoside antibiotic to prevent respiratory and other acute infections.
Mechanism of action : Penicillins interfere with the synthesis of bacterial cell wall.
- The penicillins interfere with the last step of bacterial cell wall synthesis (transpeptidation or cross-linkage), resulting in exposure of the osmotically less stable membrane.
- Cell lysis can then occur, either through osmotic pressure or through the activation of autolysins. These drugs are bactericidal and work in a time-dependent fashion.
Penicillins are only effective against rapidly growing organisms that synthesize a peptidoglycan cell wall. Consequently, they are inactive against organisms devoid of this structure, such as mycobacteria, protozoa, fungi, and viruses.
Preparations and dose
- Sod. penicillin-G (crystalline penicillin) injection 0.5-5 MU i.m./i.v. 6-12 hourly. It is available as dry powder in vials to be dissolved in sterile water at the time of injection.
BENZYL PEN 0.5, 1 MU inj.
Repository penicillin-G injections These are insoluble salts of PnG which must be given by deep i.m. (never i.v.)
injection. They release PnG slowly at the site of injection, which then meets the same fate as soluble PnG.
- Procaine penicillin-G inj. 0.5-1 MU i.m. 12-24 hourly as aqueous suspension. Plasma concentrations attained are lower, but are sustained for 12-24 hours; PROCAINE PENICJLLIN-G 0.5, 1 MU dry powder in vial.
- Fortified procaine penicillin-G inj. contains 3 lac U procaine penicillin and 1 lac U sod. penicillin G to provide rapid as well as sustained blood levels. FORTIFIED P.P. INJ 3+ 1 lac U vial
- Benzathine penicillin-G 0.6-2.4 MU i.m. every 2-4 weeks as aqueous suspension. It releases penicillin extremely slowly-plasma concentrations are very low but remain effective for prophylactic purposes for up to
4 weeks:
PENl DURE-LA (long acting), LONGACILLIN, PENCOM, 0.6, 1 .2, 2.4 MU as dry powder in vial.
Side effects : Penicillins are among the safest drugs, and blood levels are not monitored. Penicillin G is one of the most nontoxic antibiotics; up to 100 MU (60 g) has been injected in a day without any direct toxicity. However, adverse reactions may occur :
- Hypersensitivity: Approximately 5% percent of patients have some kind of reaction, ranging from rashes to angioedema (marked swelling of the lips, tongue, and periorbital area) and anaphylaxis. Cross-allergic reactions occur among the β-lactam antibiotics. To determine whether treatment with a β-lactam is safe when an allergy is noted, patient history regarding severity of previous reaction is essential.
- Superinfections: Diarrhea is a common problem that is caused by a disruption of the normal balance of intestinal microorganisms. It occurs to a greater extent with those agents that are incompletely absorbed and have an extended antibacterial spectrum. Pseudomembranous colitis from Clostridium difficile and other organisms may occur with penicillin use.
- Nephritis: Penicillins, particularly methicillin, have the potential to cause acute interstitial nephritis. [Note: Methicillin is therefore no longer used clinically.]
- Neurotoxicity: The penicillins are irritating to neuronal tissue, and they can provoke seizures if injected intrathecally or if very high blood levels are reached. Epileptic patients are particularly at risk due to the ability of penicillins to cause GABAergic inhibition.
- Hematologic toxicities: Decreased coagulation may be observed with high doses of piperacillin, ticarcillin, and nafcillin (and, to some extent, with penicillin-G). Cytopenias have been associated with therapy of greater than 2 weeks, and therefore, blood counts should be monitored weekly for such patients.
Drug Interactions :
Contraindications :
For detailed query or in case of uncertainty, Always consult your doctor or pharmacist.
