Laith Mohammed Abbas Al-Huseini Antimicrobials Cell Wall Inhibitors (Penicillins) Laith Mohammed Abbas Al-Huseini M.B.Ch.B., M.Sc, M.Res, Ph.D Department of Pharmacology and Therapeutics

Background Some antimicrobial drugs selectively interfere with synthesis of the bacterial cell wall—a structure that mammalian cells do not possess. The cell wall is composed of a polymer called peptidoglycan that consists of glycan units joined to each other by peptide cross-links. To be maximally effective, inhibitors of cell wall synthesis require actively proliferating microorganisms. The most important members of this group of drugs are the β-lactam antibiotics (named after the β-lactam ring that is essential to their activity like: penicillins, cephalosporins, monobactams, carbapenems, and β-lactamase inhibitors), and Cell wall destructors.

Background The penicillins were the first antibiotics discovered as natural products from the mold Penicillium. In 1928, Sir Alexander Fleming, professor of bacteriology at St. Mary's Hospital in London, was culturing Staphylococcus aureus. He noticed zones of inhibition where mold spores were growing. He named the mold Penicillium rubrum. It was determined that a secretion of the mold was effective against Gram-positive bacteria.

Structure All penicillins have the basic structure of a thiazolidine ring (A) is attached to a β-lactam ring (B) that carries a secondary amino group (RNH–). Substituents (R) can be attached to the amino group. Structural integrity of the 6-aminopenicillanic acid nucleus (rings A plus B) is essential for the biologic activity of these compounds. Hydrolysis of the β-lactam ring by bacterial β-lactamases yields penicilloic acid, which lacks antibacterial activity.

Mechanism of Action All penicillins produce their bactericidal effects by inhibition of bacterial cell wall synthesis. Specifically, the cross linking of peptides on the polysaccharide chain is prevented. If cell walls are improperly made cell walls allow water to flow into the cell causing it to burst. These drugs are bactericidal and work in a time-dependent fashion.

Mechanism of Action Penicillins inactivate numerous proteins on the bacterial cell membrane. These penicillin-binding proteins (PBPs) are bacterial enzymes involved in the synthesis of the cell wall and in the maintenance of the morphologic features of the bacterium. The number of PBPs varies with the type of organism. Alterations in some of these PBPs provide the organism with resistance to the penicillins. Some PBPs catalyze formation of the cross-linkages between peptidoglycan chains. Penicillins inhibit this transpeptidase-catalyzed reaction, thus hindering the formation of cross-links essential for cell wall integrity.

Mechanism of Action Many bacteria, particularly the gram+ve cocci, produce degradative enzymes (autolysins) that participate in the normal remodeling of the bacterial cell wall. In the presence of a penicillin, the degradative action of the autolysins proceeds in the absence of cell wall synthesis. Thus, the antibacterial effect of a penicillin is the result of both inhibition of cell wall synthesis and destruction of the existing cell wall by autolysins.

Spectrum of Antibacterial Activity Determined by their ability to cross the bacterial peptidoglycan cell wall to reach the PBPs in the periplasmic space. Size, charge, and hydrophobicity of the particular β-lactam antibiotic determine the susceptibility of PBPs. Gr+ve microorganisms have cell walls that are easily traversed by penicillins, and, therefore, in the absence of resistance, they are susceptible to these drugs. Gr-ve microorganisms have an outer LPS membrane surrounding the cell wall that presents a barrier to the water-soluble penicillins. Gr-ve bacteria have proteins inserted in the LPS layer that act as water-filled channels (porins) to permit transmembrane entry.

Classification β-lactamase sensitive 1- Narrow spectrum penicillins (Natural penicillins) β-lactamase sensitive Acid resistant -Penicillin V (oral) Acid labile - Penicillin-G (benzyl penicillin) (I.M,IV) - Procaine penicillin-G (I.M, depot inj) - Benzathine penicillin-G (I.M, depot inj)

Classification 2- β-lactamase resistant (Antistaphylococcal penicillins) Acid resistant - Cloxacillin - Dicloxacillin - flucloxacillin Acid labile - Methicillin (I.M,I.V) - Nafcillin (I.M,I.V)

Classification 3- Extended-spectrum penicillins Acid resistant Aminopenicillins: Ampicillin, Amoxicillin, Bacampicillin, Talampicillin Acid labile (Antipseudomonal penicillins) Carboxypenicillins: Carbenicillin, Ticarcillin Ureidopenicillins: Piperacillin, Mezlocillin, Azlocillin

Pharmacokinetics The route of administration of a β-lactam antibiotic is determined by the stability of the drug to gastric acid and by the severity of the infection. The combination of ampicillin with sulbactam, ticarcillin with clavulanic acid, and piperacillin with tazobactam, and the antistaphylococcal penicillins (nafcillin and oxacillin) must be administered intravenously (IV) or intramuscularly (IM). Penicillin V and dicloxacillin are available only as oral preparations. Others are effective by the oral, IV, or IM routes. IV administration of penicillin G is preferred to the IM route because of irritation and local pain from intramuscular injection Depot forms: Procaine penicillin G and benzathine penicillin G are administered IM and serve as depot forms. They are slowly absorbed into the circulation and persist at low levels over a long time period.

Pharmacokinetics Absorption: Most of the penicillins are incompletely absorbed after oral administration. They reach the intestine in sufficient amounts to affect the composition of the intestinal flora. Food decreases the absorption of all the penicillinase-resistant penicillins because as gastric emptying time increases, the drugs are destroyed by stomach acid (amoxicillin being an exception). Therefore, they should be taken on an empty stomach. Benzathine and procaine penicillins are formulated to delay absorption, resulting in prolonged blood and tissue concentrations.

Distribution The β-lactam antibiotics distribute well throughout the body. All the penicillins cross the placental barrier, but none have been shown to have teratogenic effects. Penetration into bone or cerebrospinal fluid (CSF) is insufficient for therapy unless these sites are inflamed Penicillin levels in the prostate are insufficient to be effective against infections.

Excretion The primary route of excretion is through the organic acid (tubular) secretory system of the kidney as well as by glomerular filtration. Patients with impaired renal function must have dosage regimens adjusted. Nafcillin and oxacillin are exceptions to the rule. They are primarily metabolized in the liver and do not require dose adjustment for renal insufficiency. Probenecid inhibits the secretion of penicillins by competing for active tubular secretion via the organic acid transporter and, thus, can increase blood levels. Clearance values are considerably lower in neonates and infants, because of incomplete development of renal function The penicillins are also excreted in breast milk.

Unitage of Penicillin The international unit of penicillin is the specific penicillin activity contained in 0.6 mg of the crystalline sodium salt of penicillin G. One milligram of pure penicillin G sodium thus equals 1667 units; 1.0 mg of pure penicillin G potassium represents 1595 units. The dosage and the antibacterial potency of the semisynthetic penicillins are expressed in terms of weight. The minimum inhibitory concentration(MIC) of any penicillin is usually given in ug/ml Most penicillins ae dispensed as the sodium or potassium salt of the free acid.

Therapeutic Uses Penicillin G is a drug of choice for infections caused by streptococci, meningococci, some enterococci, penicillin susceptible pneumococci, non-β-lactamase-producing staphylococci, Treponema pallidum and certain other spirochetes, some Clostridium species, Actinomyces and certain other gram-positive rods, and non-β-lactamase-producing gram negative anaerobic organisms. Depending on the organism, the site, and the severity of infection, effective doses range between 4 and 24 million units per day administered intravenously in four to six divided doses. High-dose penicillin G can also be given as a continuous intravenous infusion.

Therapeutic Uses Penicillin V, the oral form of penicillin, is indicated only in minor infections because of its relatively poor bioavailability, the need for dosing four times a day. Benzathine penicillin and procaine penicillin G for intramuscular injection yield low but prolonged drug levels. A single intramuscular injection of benzathine penicillin, 1.2 million units, is effective treatment for β-hemolytic streptococcal pharyngitis; given intramuscularly once every 3–4 weeks, it prevents reinfection. Procaine penicillin G was once a commonly used treatment for uncomplicated pneumococcal pneumonia and gonorrhea.

Therapeutic Uses Penicillins Resistant to Staphylococcal Beta Lactamase (Methicillin, nafcillin, cloxacillin and dicloxacillin ) These semisynthetic penicillins are indicated for infections caused by β-lactamase-producing staphylococci. cloxacillin and dicloxacillin is suitable for treatment of mild to moderate localized staphylococcal infections. They are relatively acid-stable and have reasonable bioavailability. However, food interferes with absorption, and the drugs should be administered 1 hour before or after meals. Methicillin, the first antistaphylococcal penicillin to be developed, is no longer used clinically due to high rates of adverse effects. Nafcillin given by intermittent intravenous infusion and considered the drug of choice for serious systemic staphylococcal infections.

Therapeutic Uses Extended-Spectrum Penicillins (Aminopenicillins, carboxypenicillins, and Ureidopenicillins) These drugs have greater activity than penicillin against gram-negative bacteria because of their enhanced ability to penetrate their outer membrane. Like penicillin G, they are inactivated by many β lactamases. The aminopenicillins (ampicillin and amoxicillin) have very similar spectrums of activity, but amoxicillin is better absorbed orally. Amoxicillin, 250–500 mg three times daily, is equivalent to the same amount of ampicillin given four times daily. Given orally to treat urinary tract infections, sinusitis, otitis, and lower respiratory tract infections. Ampicillin, at dosages of 4–12 g/d IV, is useful for treating serious infections caused by susceptible organisms, including anaerobes, enterococci, L monocytogenes, and β-lactamase-negative strains of gram-negative cocci and bacilli such as E coli, and Salmonella sp. Non-β-lactamase-producing strains of H influenzae

Therapeutic Uses Carbenicillin, the first antipseudomonal carboxypenicillin, is no longer used in the USA, because there are more active, better tolerated alternatives. Ticarcillin has similar activity to carbenicillin. It is less active than ampicillin against enterococci. The ureidopenicillins, piperacillin, mezlocillin, and azlocillin, are also active against selected gram-negative bacilli, such as Klebsiella pneumoniae.

Resistance Natural resistance to the penicillins occurs in organisms that either lack a peptidoglycan cell wall (for example, Mycoplasma pneumoniae) or have cell walls that are impermeable to the drugs. Acquired resistance to the penicillins by plasmid-mediated β-lactamases has become a significant clinical problem. β-lactamases enzymes hydrolyzes the cyclic amide bond of the β-lactam ring, which results in loss of bactericidal activity. Other mechanism involves decreased penetration of the antibiotic through the outer cell membrane of the bacteria prevents the drug from reaching the target PBPs. Additionally, modified PBPs have a lower affinity for β-lactam antibiotics, requiring clinically unattainable concentrations of the drug to effect inhibition of bacterial growth. This explains MRSA resistance to most commercially available β-lactams.

Adverse Effects The penicillins are generally well tolerated, and, unfortunately, this may encourage inappropriate use. Hypersensitivity Reactions: The antigenic determinants are degradation products of penicillins, particularly penicilloic acid and products of alkaline hydrolysis bound to host protein. 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. A history of a penicillin reaction is not reliable; many people claim such a history, but only a small number of these will have a serious reaction when given penicillin. Penicillin skin testing may also be used to evaluate Type I hypersensitivity. If skin testing is negative, most patients can safely receive penicillin.

Adverse Effects 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. Convulsions and encephalopathy can occur, especially at higher doses and especially if administered intrathecally (NOT advised). Interstitial nephritis (Methicillin) Coomb's positive hemolytic anemia Neutropenia (especially the b-lactamase -resistant penicillins) Decreased platelet aggregation (carbenicillin and ticarcillin) Hypernatremia and hypokalemia (carbenicillin)