Microbiology

Penicillin G exerts a bactericidal against penicillin-susceptible microorganisms during the stage of active multiplication.  It acts through the inhibition of biosynthesis of cell wall peptidoglycan rendering the cell wall osmotically unstable. It is not active against the penicillinase-producing bacteria, which include many strains of staphylococci or against organisms resistant to beta-lactams because of alterations in the penicillin-binding proteins, such as methicillin-resistant staphylococci. Penicillin G is highly active in vitro against streptococci (groups A, B, C, G, H, L, and M) and Neisseria meningitidis.

Other organisms susceptible to penicillin G are N. gonorrhoeae, Corynebacterium diphtheriae, Bacillus anthracis, Clostridium spp., Actinomyces species, “ Spirillum minus”, Streptobacillus moniliformis, Listeria monocytogenes and Leptospira spp.;  Treponema pallidum is extremely sensitive to the bactericidal action of penicillin G. 

Some species of gram-negative bacilli were previously considered susceptible to very high intravenous doses of penicillin G (up to 80 million units/day) including some strains of Escherichia coli, Proteus mirabilis, Salmonella spp . and Shigella spp., Enterobacter aerogenes (formerly Aerobacter aerogenes) and Alcaligenes faecalis.  Penicillin G is no longer considered a drug of choice for infections caused by these organisms.

Susceptibility Test Methods

When available, the clinical microbiology laboratory should provide the results of in vitro susceptibility test results for antimicrobial drugs used in local hospitals and practice areas to the physician as periodic reports that describe the susceptibility profile of nosocomial and community-acquired pathogens.  These reports should aid the physician in selecting the most effective antimicrobial.

Dilution Techniques

Quantitative methods are used to determine antimicrobial minimum inhibitory concentrations (MICs).  These MICs provide estimates of the susceptibility of bacteria to antimicrobial compounds.  The MICs should be determined using a standardized procedure.  Standardized procedures are based on dilution method 1,2,3 (broth or agar or dilution) or equivalent using standardized inoculum and concentrations of penicillin.  The MIC values should be interpreted according to the criteria in Table 1.

Diffusion Techniques

Quantitative methods that require measurement of zone diameters also provide reproducible estimates of the susceptibility of bacteria to antimicrobial compounds.  One such standardized procedure 2,4 requires the use of standardized inoculum concentrations. This procedure uses paper disks impregnated with 10 units of penicillin to test the susceptibility of microorganisms to penicillin.  Interpretation involves correlation of the diameter obtained in the disk test with the MIC for penicillin.  Reports from the laboratory providing results of the standard single-disk susceptibility test with a 10 unit penicillin disk should be interpreted according to the following criteria in Table 1.

Table 1: Susceptibility Test Interpretive Criteria for Penicillin

a Organisms for which no values for disk susceptibility appear cannot be reliably tested with this method.
b Penicillin-resistant strains of staphylococci produce β-lactamase. An induced β-lactamase test should be performed on all S. aureus isolates for which the penicillin MIC is ≤0.12 mcg/mL or zone diameter is ≥29 mm before reporting as penicillin susceptible. Rare isolates of staphylococci that contain genes for β-lactamase production may not produce a positive induced β-lactamase test. For serious infections requiring penicillin therapy, laboratories should perform MIC tests and induced beta-lactamase testing on all subsequent isolates from the same patient. 2
c A positive N. gonorrhoeae β-lactamase test predicts one form of resistance to penicillin. Strains with chromosomally mediated resistance can be detected only by agar dilution or disk diffusion susceptibility test methods. 2 Isolates with zone diameters ≤19 mm generally produce β-lactamase. 2,4
d Susceptibility testing of penicillins for treatment of β-hemolytic streptococcal infections need not be performed routinely, because non-susceptible isolates are extremely rare in any β-hemolytic streptococcus and have not been reported from Streptococcus pyogenes. Any β-hemolytic streptococcal isolate found to be non-susceptible to penicillin should be re-identified, retested, and, if confirmed, submitted to a public health authority. 2,4
e The current absence of resistant isolates precludes defining results other than "Susceptible". Isolates yielding results suggestive of "Nonsusceptible" should be submitted to a reference laboratory for further testing.
f B. anthracis strains may contain inducible β-lactamases. In vitro penicillinase induction studies suggest that penicillin MICs may increase during therapy. However, β-lactamase testing of clinical isolates of B. anthracis is unreliable and should not be performed. 3

Quality Control

Standardized susceptibility test procedures require the use of laboratory control microorganisms to monitor and ensure the accuracy and precision of the supplies and reagents used in the assay, and the techniques of the individuals performing the test.  Standard penicillin powder should provide MIC values provided below.  For the diffusion technique, the 10 unit penicillin disk should provide the following zone diameters with the quality control strains:

Table 2.  In Vitro Susceptibility Test Quality Control Ranges for Penicillin

a Using agar dilution method only. No criteria for broth microdilution are available. 2

Therapy

Penicillin G Potassium for Injection, USP is indicated in the treatment of serious infections caused by susceptible strains of the designated microorganisms in the conditions listed below.  Appropriate culture and susceptibility tests should be done before treatment in order to isolate and identify organisms causing infection and to determine their susceptibility to penicillin G.

Therapy with Penicillin G Potassium for Injection, USP may be initiated before results of such tests are known when there is reason to believe the infection may involve any of the organisms listed below; however, once these results become available, appropriate therapy should be continued.

To reduce the development of drug-resistant bacteria and maintain effectiveness of Penicillin G Potassium for Injection, USP and other antibacterial drugs, Penicillin G Potassium for Injection, USP should be used only to treat or prevent infections that are proven or strongly suspected to be caused by susceptible bacteria.  When culture and susceptibility information are available, they should be considered in selecting or modifying antibacterial therapy.  In the absence of such data, local epidemiology and susceptibility patterns may contribute to the empiric selection of therapy.

General

Penicillin should be used with caution in individuals with histories of significant allergies and/or asthma (see WARNINGS).  Whenever allergic reactions occur, penicillin should be withdrawn unless, in the opinion of the physician, the condition being treated is life-threatening and amenable only to penicillin therapy.  Penicillin G Potassium, USP by the intravenous route in high doses (above 10 million units) should be administered slowly because of the potential adverse effects of electrolyte imbalance from the potassium content of the penicillin.  Penicillin G Potassium for Injection, USP contains 1.68 mEq potassium and 0.3 mEq of sodium per million units.  The use of antibiotics may promote overgrowth of nonsusceptible organisms, including fungi.  Indwelling intravenous catheters encourage superinfections.  Should superinfection occur, appropriate measures should be taken.  When indicated, incision and drainage or other surgical procedures should be performed in conjunction with antibiotic therapy.

Prescribing Penicillin G Potassium for Injection in the absence of proven or strongly suspected bacterial infection or a prophylactic indication is unlikely to provide benefit to the patient and increases the risk of the development of drug-resistant bacteria.

Information for Patients

Patients should be counseled that antibacterial drugs including Penicillin G Potassium for Injection should only be used to treat bacterial infections.  They do not treat viral infections (e.g., the common cold).  When Penicillin G Potassium for Injection is prescribed to treat a bacterial infection, patients should be told that although it is common to feel better early in the course of therapy, the medication should be taken exactly as directed.  Skipping doses or not completing the full course of therapy may (1) decrease the effectiveness of the immediate treatment and (2) increase the likelihood that bacteria will develop resistance and will not be treatable by Penicillin G Potassium for Injection or other antibacterial drugs in the future.

Diarrhea is a common problem caused by antibiotics which usually ends when the antibiotic is discontinued.  Sometimes after starting treatment with antibiotics, patients can develop watery and bloody stools (with or without stomach cramps and fever) even as late as two or more months after having taken the last dose of the antibiotic.  If this occurs, patients should contact their physician as soon as possible.

Intramuscular Therapy

Care should be taken to avoid intravenous or accidental intra-arterial administration, or injection into or near major peripheral nerves or blood vessels, since such injections may produce neurovascular damage.  Particular care should be taken with IV administration because of the possibility of thrombophlebitis.

Laboratory Tests

Periodic assessment of organ system function, including frequent evaluation of electrolyte balance, hepatic, renal and hematopoietic systems, and cardiac and vascular status should be performed during prolonged therapy with high doses of intravenous penicillin G (see ADVERSE REACTIONS).  If any impairment of function is suspected or known to exist, a reduction in the total dosage should be considered (see DOSAGE AND ADMINISTRATION).  In suspected staphylococcal infections, proper laboratory studies, including susceptibility tests should be performed.  All infections due to Group A beta-hemolytic streptococci should be treated for at least 10 days.

Patients being treated for gonococcal infection should have a serologic test for syphilis before receiving penicillin.  All cases of penicillin treated syphilis should receive adequate follow-up including clinical and serological examinations.  The recommended follow-up varies with the stage of syphilis being treated.

Drug Interactions

Bacteriostatic antibacterials ( i.e., chloramphenicol, erythromycins, sulfonamides or tetracyclines) may antagonize the bactericidal effect of penicillin, and concurrent use of these drugs should be avoided.  This has been documented in vitro; however, the clinical significance of this interaction is not well documented.

Penicillin blood levels may be prolonged by concurrent administration of probenecid which blocks the renal tubular secretion of penicillins.  Other drugs may compete with penicillin G for renal tubular secretion and thus prolong the serum half-life of penicillin.  These drugs include: aspirin, phenylbutazone, sulfonamides, indomethacin, thiazide diuretics, furosemide and ethacrynic acid.

Drug/Laboratory Test Interactions

After treatment with penicillin G, a false-positive reaction for glucose in the urine may occur with Benedict’s solution, Fehling’s solution or Clinitest ® tablet, but not with the enzyme-based tests, such as Clinistix ®and Tes-Tape ®.  Penicillin G has been associated with pseudoproteinuria by certain test methods.

Carcinogenesis, Mutagenesis, Impairment of Fertility

No long-term animal studies have been conducted with this drug.

Teratogenic Effects
Pregnancy Category B

Reproduction studies performed in the mouse, rat, and rabbit have revealed no evidence of impaired fertility or harm to the fetus due to penicillin G.  Human experience with the penicillins during pregnancy has not shown any positive evidence of adverse effects on the fetus.  There are, however, no adequate and well controlled studies in pregnant women showing conclusively that harmful effects of these drugs on the fetus can be excluded.  Because animal reproduction studies are not always predictive of human response, this drug should be used during pregnancy only if clearly needed.

Nursing Mothers

Penicillins are excreted in human milk.  Caution should be exercised when penicillins are administered to a nursing woman.

Pediatric Use

Incompletely developed renal function in newborns may delay elimination of penicillin; therefore, appropriate reductions in the dosage and frequency of administration should be made in these patients.  All newborns treated with penicillins should be monitored closely for clinical and laboratory evidence of toxic or adverse effects (see PRECAUTIONS).

Pediatric doses are generally determined on a weight basis and should be calculated for each patient individually.  Recommended guidelines for pediatric dosages are presented in DOSAGE AND ADMINISTRATION.

Geriatric Use

Clinical studies of Penicillin G Injection did not include sufficient numbers of subjects aged 65 and over to determine whether they respond differently from younger subjects.  Other reported clinical experience has not identified differences in responses between the elderly and younger patients.  In general, dose selection for an elderly patient should be cautious, usually starting at the low end of the dosing range, reflecting the greater frequency of decreased hepatic, renal, or cardiac function, and of concomitant disease or other drug therapy.

This drug is known to be substantially excreted by the kidney, and the risk of toxic reactions to this drug may be greater in patients with impaired renal function.  Because elderly patients are more likely to have decreased renal function, care should be taken in dose selection, and it may be useful to monitor renal function.

Penicillin G Injection contains 6.8 mg (0.3 mEq) of sodium per million units.  At the usual recommended doses, patients would receive between 6.8 and 163.2 mg/day (0.3 and 7.2 mEq) of sodium.  The geriatric population may respond with a blunted natriuresis to salt loading.  This may be clinically important with regard to such diseases as congestive heart failure.

Renal Impairment

Penicillin G is relatively nontoxic, and dosage adjustments are generally required only in cases of severe renal impairment.  The recommended dosage regimens are as follows:

Creatinine clearance less than 10 mL/min/1.73 m 2; administer a full loading dose (see recommended dosages in the tables above) followed by one-half of the loading dose every 8 to 10 hours.

Uremic patients with a creatinine clearance greater than 10 mL/min/1.73 m 2; administer a full loading dose (see recommended dosages in the tables above) followed by one-half of the loading dose every 4 to 5 hours.  Additional dosage modifications should be made in patients with hepatic disease and renal impairment.

For most acute infections, treatment should be continued for at least 48 to 72 hours after the patient becomes asymptomatic.  Antibiotic therapy for Group A β-hemolytic streptococcal infections should be maintained for at least 10 days to reduce the risk of rheumatic fever.  Parenteral drug products should be inspected visually for particulate matter and discoloration prior to administration whenever solution and container permit.

Reconstitution

The following table shows the amount of solvent required for solution of various concentrations:

When the required volume of solvent is greater than the capacity of the vial, the penicillin can be dissolved by first injecting only a portion of the solvent into the vial, then withdrawing the resultant solution and combining it with the remainder of the solvent in a larger sterile container.

Buffered Penicillin G Potassium for Injection is highly water soluble.  It may be dissolved in small amounts of Water for Injection, or Sterile Isotonic Sodium Chloride Solution for Parenteral Use.  All solutions should be stored in a refrigerator at 2º to 8ºC (36º to 46ºF).  When refrigerated, penicillin solutions may be stored for seven days without loss of potency.  DISCARD UNUSED SOLUTION AFTER 7 DAYS.

Buffered Penicillin G Potassium for Injection may be given intramuscularly or by continuous intravenous drip for dosages of 500,000, 1,000,000, or 5,000,000 units.  It is also suitable for intrapleural, intra-articular, and other local instillations.

THE 20,000,000 UNIT DOSAGE MAY BE ADMINISTERED BY INTRAVENOUS INFUSION ONLY.

(1) Intramuscular Injection: Keep total volume of injection small.  The intramuscular route is the preferred route of administration.  Solutions containing up to 100,000 units of penicillin per mL of diluent may be used with a minimum of discomfort.  Greater concentration of penicillin G per mL is physically possible and may be employed where therapy demands.  When large dosages are required, it may be advisable to administer aqueous solutions of penicillin by means of continuous intravenous drip.

(2) Continuous Intravenous Drip: Determine the volume of fluid and rate of its administration required by the patient in a 24 hour period in the usual manner for fluid therapy, and add the appropriate daily dosage of penicillin to this fluid.  For example, if an adult patient requires 2 liters of fluid in 24 hours and a daily dosage of 10 million units of penicillin, add 5 million units to 1 liter and adjust the rate of flow so the liter will be infused in 12 hours.

(3) Intrapleural or Other Local Infusion: If fluid is aspirated, give infusion in a volume equal to 1/4 or 1/2 the amount of fluid aspirated, otherwise, prepare as for intramuscular injection.

(4) Intrathecal Use: The intrathecal use of penicillin in meningitis must be highly individualized.  It should be employed only with full consideration of the possible irritating effects of penicillin when used by this route.  The preferred route of therapy in bacterial meningitides is intravenous, supplemented by intramuscular injection.

Parenteral drug products should be inspected visually for particulate matter and discoloration prior to administration, whenever solution and container permit. Sterile solution may be left in the refrigerator for one week without significant loss of potency.