Copyright, Purdue Research Foundation, 1996

| BMS 445 Intro | | Drug Groups | | E-mail | | Slides / Graphics |

Penicillins are among the most heavily used antibacterials in medicine. Their efficacy against susceptible organisms and large margin of safety make them nearly ideal antibacterials.

Penicillin G: Note: In penicillin G, R2 is a hydrogen atom

Classes of Penicillins

Beginning with penicillin G, (benzylpenicillin), first isolated from a Penicillium mold in 1928, this family has grown to include many natural and semisynthetic members. An excellent table containing a breakdown of the various classes of penicillins with their trade names, routes of administration, and pharmacokinetic data for humans can be found in AMA Drug Evaluations (6th edition, pages 1306-7). Names of the groups are a mixture of source, chemical structure, and primary therapeutic target. Some therapeutic indications are included in the discussion to provide one with a feeling for how the drugs are used and that some of them are preferred for certain types of infections. The list is severely abbreviated, however, and one should consult detailed reference materials for additional information on spectrum and clinical use.

Natural penicillins

Natural penicillins include penicillin G for parenteral use and penicillin V for oral use only. These drugs are primarily indicated for infections caused by gram-positive organisms including anaerobes (exept Bacteroides fragilis). Spirochetes and some gram-negative organisms are susceptible, however, the latter require higher concentrations. The Enterobacteriaceae and Pseudomonas aeruginosa are resistant. Penicillin V should not be used for serious infections because of its incomplete and variable absorption.


Aminopenicillins have a relatively broad spectrum of activity, for penicillins, which includes many gram-positive and gram-negative bacteria. Ampicillin can be used both orally (acid stable) and parenterally, whereas amoxicillin is used primarily orally. These drugs are beta-lactamase sensitive. The oral bioavailability of ampicillin (35-50%) is lower and more variable than that of amoxicillin (ca. 70-90%). These drugs are not very active against Bacteroides fragilis despite being effective against other anaerobes. Because of the beta-lactamase sensitivity, some commercial preparations of these two drugs contain inhibitors, e.g., clavulanic acid and sulbactam.

Antistaphylococcal penicillins

All antistaphylococcal penicillins have oral dose forms. Some have IV/IM and intramammary (VM) forms.

Penicillinase-resistant (antistaphylococcal) penicillins include the classical compound methicillin, widely used in senstivity testing for this group, and nafcillin. The isoxazolyl penicillins constitute an important subgroup that includes the closely related oxacillin, cloxacillin, and dicloxacillin. None of these drugs has outstanding bioavailability when given orally (maximum 50%), but all of the isoxazolyls have oral dose forms. Only oxacillin and nafcillin have a parenteral (IV/IM) dose form. This group includes the only penicillin derivatives that have significant beta-lactamase resistance. The trade name of methicillin, Staphcillin, betrays its primary therapeutic application. Resistance (stemming from changes in the penicillin binding proteins) to methicillin is growing among staphylococci and one should use vancomycin as a substitute when necessary. Penicillin G is much more effective against non-beta-lactamase producing staphylococci and is preferred over the antistaphylococcal derivatives. Members of this class are preferred for treatment of Staphylococcus aureus infections of the skin and soft tissues. Parenterally administered nafcillin should be used for serious infections and orally administered dicloxacillin (or cloxacillin) should be used for the remainder. As a rule of thumb, it is safe to assume that penicillin derivatives in the other classes are beta-lactamase sensitive.

Antipseudomonal penicillins

Only the indanyl carbenicillin has an oral dose form. Other antipseudomonal penicillins are given IV/IM.

Antipseudomonal penicillins include the carboxypenicillins (carbenicillin and ticarcillin), acylureidopenicillins (azlocillin and mezlocillin), and one piperazine penicillin derivative (piperacillin). With the exception of the indanyl derivative of carbenicillin, which is an oral dose form, all are for parenteral use (IM or IV). Except for carbencillin, the reason most of these drugs are used parenterally is that they are poorly absorbed orally rather than because they are destroyed by gastric acidity. All of these drugs are sensitive to beta-lactamase destruction. Piperacillin is more active than the others against Pseudomonas aeruginosa in vitro and has activity that corresponds to gentamicin, an aminoglycoside. Although carbenicillin and others in this group are active against many gram-positive organisms, they are generally less effective than penicillin G and ampicillin. There is not strong support for their use against infections caused by gram-positive bacteria.

Amidino penicillins

Amidino penicillins are the final group and include only one member, amidinocillin. The major accepted use of this drug is for urinary tract infections caused by gram-negative organisms. Unfortunately, it is not absorbed from the gastrointestinal tract and must be given parenterally.

In their minimum essential core guidelines, the IU Medical School curriculum committee, Infection Topic Committee report of September 1987, listed the following beta-lactams as being particularly important: pencillin G, ampicillin, amoxicillin, ampicillin, ampicillin plus clavulanic acid, and piperacillin. They did not list any antistaphylococcal pencillin derivatives, but this is probably an oversight.

Beta-Lactamase Inhibitors

Protective action of clavulanic acid. Mice infected with penicillinase producing organisms and treated with penicillin G [Pen], clavulanic acid [Clav] or a combination of the two.

Beta-lactamase interference with the action of the penicillins has led to the development of inhibitors that can be used concurrently. FDA has approved commercial combination dose forms of clavulanic acid or sulbactam with ampicillin or amoxicillin. The benefit of such combinations is evident from the results of experiments like the one depicted in the accompanying figure. Note that with either drug alone, all mice died and that with the combination, all lived.

Clavulanic acid and sulbactam are ultimately irreversible inhibitors of many beta-lactamases. Clavulanic acid inhibits many plasmid-mediated and some chromosomally mediated beta-lactamases (Richmond classes II through V). (USPDI11th1991). Because it can penetrate the bacterial cell wall to bind to penicillin-binding protein 2, synergy with other beta-lactams is possible, but it has essentially no antibacterial activity by itself.

Sulbactam inhibits many bacterial beta-lactamases, thereby protecting many penicillins and cephalosporins from destruction. It has no antibacterial activity of its own.

Because these drugs do not necessarily have the same pharmacokinetic characteristics as ampicillin or amoxicillin with which they may be combined, one should be attentive to problems that might arise in unusual cases.


Only sample information on pharmacokinetics is given here. Detailed tables are available in USPDI (updated annually) and other references.


Comparison of plasma concentration-time profiles of penicillin G dose forms. Note that decreasing water solubility of the salts from sodium to procaine and finally to benzathine produces profiles with progressivelylower peaks and longer durations.All drugs given by IM injection.

Absorption of penicillin derivatives varies widely depending on the drug, the route, and the dose form. Penicillin V, ampicillin, amoxicillin, and indanyl carbenicillin are available in oral dose forms. Others are not because they are either destroyed by gastric acidity or they are too poorly absorbed from the intestine. Because of variable absorption of the oral dose forms, they should not be used in serious infections.

Because the penicillin derivatives have extremely short elimination half-lives, parenteral preparations with slowed absorption have been prepared. An example of the impact of dose form on ampicillin absorption contrasting ampicillin trihydrate and ampicillin sodium was given in the lecture on Rules of Antimicrobial Therapy. A stylized example of plasma penicillin concentrations following intramuscular injection of the sodium, procaine, and benzathine salts of penicillin G are shown in the accompanying figure. The highly water soluble sodium salt is rapidly absorbed and eliminated. The Procaine salt has intermediate water solubility leading to slowed absorption, a lower peak than with the sodium salt, and longer maintenance of effective concentrations. Depending on the dose and the target organism, effective concentrations may be maintained for 12 to 24 hr. Benzathine penicillin G is very poorly water soluble, is absorbed very slowly, and produces concentrations adequate to inhibit very sensitive organisms for five days or more.


Distribution of the penicillins is generally regarded as good to most extracellular body fluids and bone. Penetration into cells, the eye, and CNS is poor. Inflammation of the meninges combined with massive doses of penicillin G (sometimes as much as 10-fold over normal) frequently results in therapeutic concentrations in the CNS. Penetration into phagocytic cells is not good.

Probenecid is a competitive inhibitor of organic anion transport in renal tubule and choroid plexus. An accompanying figure depicts its effect on serum and CSF penicillin G concentrations of rabbits given intramuscular injections of aqueous crystalline penicillin G. Probenecid increased CSF penicillin concentration because the drug was removed more slowly by the inhibited organic acid pumping mechanism in the choroid plexus. Note that at this dose there was also a detectable early increase in serum pericillin concentration. This means of increasing CSF penicillin concentration has been used experimentally in the treatment of CNS infections caused by resistant Neiserria gonorrhea.

Effect of probenecid on serum and CSF penicillin G concentrations. Rabbits were given an injection of aqueous crystalline penicillin G (100,000 U/Kg, i.m.) with or without simultaneous probenecid (25 mg/Kg, i.m.,). Open symbols are controls. Closed symbols are in the presence of probenecid. Squares represent CSF and circles represent serum penicillin G concentration. Tight & White, Antimicrob Agents Chemother 17:229, 1980.

Protein binding is low to moderate for all penicillin derivatives except the isoxazolyl derivatives, e.g., cloxacillin and dicloxacillin. For these, as much as 97% may be be bound. The primary concern with such high binding is the potential it provides for interacting with other drugs that are also highly protein bound.


Elimination of the penicillins (and most beta-lactams) is via renal pathways (80 to 90%). Some reach therapeutic concentrations in bile, for example, ampicillin, but are ultimately eliminated in the urine. Biotransformation is not a significant factor with these drugs.

Elimination half-life of the penicillins is short, ranging from 0.4 to 1.5 hr in human patients with normal renal function. Impaired renal function can slow the rate considerably, e.g., ampicillin can change from 1 to 1.3 hr in normals to 10 to 15 hr in patients with impaired function. Penicillin G elimination half-life changes from 0.5 to 0.7 hr in normals to 2.5 to 10 hr in renally impaired patients.

Renal elimination occurs by two mechanisms: glomerular filtration and secretion by the organic anion transport system. Elimination by glomerular filtration is quantitatively less important than secretion. The high secretion rate combined with the fact that such a high proportion of the volume containing the drug is circulated through the kidney (due to the small volume of distribution and the largely extracellular distribution of the drug) is responsible for the short half-life. Protein binding significantly affects the rate of elimination by glomerular filtration (decreases with increasing binding), but has litte influence on secretion rate.

There are two consequences of the high rate of renal secretion of pencillins. Both stem from the fact that it is carrier mediated and has a transport maximum, i.e., follows Michaelis-Menton kinetics. Thus, at the very high doses obtained sometimes in therapy, the elimination half-life may slow and increasing proportion of the drug may be eliminated by glomerular filtration or other routes. The elimination rate is dose dependent in the upper dose ranges. The second consequence is that other organic anions may compete with the penicillin for the transport system to slow the rate of penicillin secretion. Probenecid is the drug that was used for this purpose in the past. Before the advent of newer, slowly absorbing dose forms and the dramatic decrease in cost of the older penicillins, probenecid was used clinically to slow elimination and prolong the action of penicillins. However, except for certain experimental uses as in the case of increasing CSF concentrations, probenecid is no longer used. Cost and ease of control favor using more drug and altering the dose forms.

Adverse Effects

Study Questions

  1. 1. What is the potential importance of clavulanic acid in penicillin therapy?
  2. Is it proper to use penicillin G topically? Why not?
  3. What is the difference in indications and routes for the various salt forms (e.g., sodium, potassium, procaine, benzathine, trihydrate as appropriate for each drug) of penicillin G and ampicillin? Think about maximum achievable plasma concentrations and reasons for needing to vary absorption rates.
  4. You should know the major family groups of the penicillins? (traditional, aminobenzyl, antipseudomonas, antistaphylococcal). What is the relative importance of each? How do the new derivatives, e.g. those in the antipseudomonas group, compare to the classical penicillin G with respect to activity on sensitive gram positive organisms?
  5. What is the importance of noting whether a derivative of a drug is acid resistant? Name two acid resistant penicillins, one each from the natural penicillins and one from the aminopenicillins.
  6. You should know that methicillin is the classical anti- staphylococcal pencillin derivative. Yet staphylococci can become resistant to it. What is the basis of this resistance and what does it predict for the potential effectiveness of other beta-lactam antimicrobials against the resistant organism?
  7. Do penicillins distribute widely throughout the body, i.e. achieve therapeutic concentrations?
  8. What is the basis of the use of probenecid to increase the concentration of penicillin (and, presumably certain other beta-lactams) in the CSF? Would you expect probenecid to have the same effect on cephalosporins if biotransformation were not a factor?
  9. What is the major route of excretion of the penicillins? Why was probenecid used in conjunction with penicillin in earlier days? This is an example of the kind of mechanism pharmacologists love to talk about.
  10. What is the only significant adverse reaction to penicillin per se? Why is the specificity (selectivity) of the penicillins so high? Note that penicillins applied directly to the brain can cause seizures, but this is not a typical use.
  11. What effect might the rapid iv injection of massive doses of potassium penicillin have on cardiac function. Note that 200,000 units of penicillin G potassium (1.6 units/ug, MW = 372), a modest dose for a 10 kg dog or child, contains 0.34 mEq of potassium. Verify this with your own calculation. At this dose, the potassium is not important unless there is renal failure, but consider a 10 to 20 fold increase in dose as may be used in some cases. Also, consider the effect of a rapid iv injection when a very high concentration of potassium may reach the heart. After im or slow iv administration the cardiac effect of the potassium would probably be insignificant if renal function is adequate.

| BMS 445 Intro | | Drug Groups | | E-mail | | Slides / Graphics |

Gordon L. Coppoc, DVM, PhD
Professor of Veterinary Pharmacology
Head, Department of Basic Medical Sciences
School of Veterinary Medicine
Purdue University
West Lafayette, IN 47907-1246
Tel: 317-494-8633Fax: 317-494-0781

Last modified 10:27 PM on 3/27/96 GLC