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

2. 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.

3. 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.

4. Mechanism of Action
All penicillin derivatives 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.

5. Mechanism of Action
It is this last step in peptidoglycan synthesis that is inhibited by the beta-lactam antibiotics.
Penicillin binds at the active site of the transpeptidase enzyme that cross-links the peptidoglycan strands.
It does this by mimicking the D-alanyl-D-alanine residues that would normally bind to this site.
Penicillin irreversibly inhibits the enzyme transpeptidase

6. Mechanism of Action
Related targets for the actions of penicillins and cephalosporins; these are collectively termed penicillin-binding proteins (PBPs)
Covalently bind to a heterologous group of proteins called penicillin-binding proteins These PBPs may number 3 to 6 in any given bacteria.
There functions are diverse: catalyze the transpeptidase reaction, maintain shape, forms septums during division, Inhibit autolytic enzymes.

7. 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.

8. Classification NARROW SPECTRUM PENICILLINS
β-lactamase sensitive (NATURAL PENICILLINS)
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)

9. Classification β-lactamase resistant (ANTISTEPHYLOCOCCAL PENICILLINS)
Acid resistant
- Cloxacillin
- Dicloxacillin
- flucloxacillin
Acid labile
- Methicillin (I.M,I.V)
- Nafcillin (I.M,I.V)

10. Classification EXTENDED SPECTRUM PENICILLINS Acid resistant
Aminopenicillins: Ampicillin, Amoxicillin, Bacampicillin, Talampicillin
Acid labile (ANTIPSEUDOMONAL PENICILLINS)
Carboxypenicillins: Carbenicillin, Ticarcillin
Ureidopenicillins: Piperacillin, Mezlocillin, Azlocillin
BETA LACTAMASE INHIBITORS
Sulbactam, Tazobactam, Clavulanic acid

11. Pharmacokinetics
Oral Administration of Penicillin G. About one-third of an orally administered dose of penicillin G is absorbed from the intestinal tract under favorable conditions.
Gastric juice at pH 2 rapidly destroys the antibiotic. The decrease in gastric acid production with aging accounts for better absorption of penicillin G from the gastrointestinal tract of older individuals.
Absorption is rapid, and maximal concentrations in blood are attained in 30 to 60 minutes. The peak value is approximately 0.5 unit/ml (0.3 mg/ml) after an oral dose of 400,000 units (about 250 mg) in an adult.
Ingestion of food may interfere with enteric absorption of all penicillins, perhaps by adsorption of the antibiotic onto food particles. Thus, oral penicillin G should be administered at least 30 minutes before a meal or 2 hours after. Despite the convenience of oral administration of penicillin G, this route should be used only in infections in which clinical experience has proven its efficacy.

12. Pharmacokinetics
Oral Administration of Penicillin V. The sole virtue of penicillin V in comparison with penicillin G is that it is more stable in an acidic medium, and therefore is better absorbed from the gastrointestinal tract.
On an equivalent oral-dose basis, penicillin V (K+ salt PEN-VEE K, V-CILLIN K, others) yields plasma concentrations two to five times greater than those provided by penicillin G. The peak concentration in the blood of an adult after an oral dose of 500 mg is nearly 3 mg/ml. Once absorbed, penicillin V is distributed in the body and excreted by the kidney in the same manner as penicillin G.

13. Pharmacokinetics Parenteral Administration of Penicillin G
After intramuscular injection, peak concentrations in plasma are reached within 15 to 30 minutes. This value declines rapidly, since the half-life of penicillin G is 30 minutes.
Repository preparations of penicillin G are employed. The two such compounds currently favored are penicillin G procaine (maintained for as long as 4 to 5 days.) and penicillin G benzathine. (duration of antimicrobial activity in the plasma is about 26 day)
Such agents release penicillin G slowly from the area in which they are injected and produce relatively low but persistent concentrations of antibiotic in the blood.
Intra-thecal administration is inadvisable particularly with benzylpenicillin as it can cause convulsions.

14. Distribution
Penicillin G is distributed widely throughout the body, but the concentrations in various fluids and tissues differ widely. Its apparent volume of distribution is about 0.35 liters/kg. Approximately 60% of the penicillin G in plasma is reversibly bound to albumin. Significant amounts appear in liver, bile, kidney, semen, joint fluid, lymph, and intestine.
While probenecid markedly decreases the tubular secretion of the penicillins, this is not the only factor responsible for the elevated plasma concentrations of the antibiotic that follow its administration. Probenecid also produces a significant decrease in the apparent volume of distribution of the penicillins.

15. Distribution
Cerebrospinal Fluid. Penicillin does not readily enter the CSF when the meninges are normal.
However, when the meninges are acutely inflamed, penicillin penetrates into the CSF more easily. Although the concentrations attained vary and are unpredictable, they are usually in the range of 5% of the value in plasma and are therapeutically effective against susceptible microorganisms.

16. Excretion
Under normal conditions, penicillin G is rapidly eliminated from the body, mainly by the kidney but in small part in the bile and by other routes. Approximately 60% to 90% of an intramuscular dose of penicillin G in aqueous solution is eliminated in the urine, largely within the first hour after injection.
The half-time for elimination is about 30 minutes in normal adults (upto 10 hours in renal failure) . Approximately 10% of the drug is eliminated by glomerular filtration and 90% by tubular secretion.
Renal clearance approximates the total renal plasma flow. The maximal tubular secretory capacity for penicillin in the normal male adult is about 3 million units (1.8 g) per hour.

17. Excretion
Clearance values are considerably lower in neonates and infants, because of incomplete development of renal function; as a result, after doses proportionate to surface area, the persistence of penicillin in the blood is several times as long in premature infants as in children and adults.
The half-life of the antibiotic in children less than 1 week old is 3 hours; by 14 days of age it is 1.4 hours. After renal function is fully established in young children, the rate of renal excretion of penicillin G is considerably more rapid than in adults.

18. 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.

19. Therapeutic Uses Pneumococcal Infections Pneumococcal Meningitis
Pneumococcal Pneumonia
Streptococcal Infections
Streptococcal Pharyngitis (including Scarlet Fever)
Streptococcal Pneumonia, Arthritis, Meningitis, and Endocarditis
Staphylococcal Infections
Meningococcal Infections
Gonococcal Infections
Syphilis
Actinomycosis
Diphtheria
Anthrax
Clostridial Infections
Fusospirochetal Infections
Rat-Bite Fever
Listeria Infections
Lyme Disease
Erysipeloid
Surgical Procedures in Patients with Valvular Heart Disease

20. Therapeutic Uses
The antimicrobial activity of carbenicillin, its indanyl ester (carbenicillin indanyl), and ticarcillin is extended to include Pseudomonas, Enterobacter, and Proteus species.
Other extended-spectrum penicillins include mezlocillin and piperacillin, which have useful antimicrobial activity against Pseudomonas, Klebsiella, and certain other gram-negative microorganisms.

21. Resistance
Resistance to penicillins and other beta lactams is due to one of four general mechanisms:
Inactivation of the antibiotic by beta lactamase
Modification of target PBPs
Impaired penetration of drug to target PBPs
The presence of an efflux pump.

22. Resistance A reduction in the permeability of the outer membrane.
Thus there is a decreased ability of the drug to penetrate to the target site.
The occurrence of modified penicillin binding sites. This mechanism is responsible in methicillin resistance in Pneumococci.

23. Adverse Effects Hypersensitivity Reactions:
Hypersensitivity reactions are by far the most common adverse effects noted with the penicillins, and these agents probably are the most common cause of drug allergy.
There is no convincing evidence that any single penicillin differs from the group in its potential for causing true allergic reactions.
The basis of which is the fact that degradation products of penicillin combine with host protein and become antigenic.

24. Adverse Effects
These are cross-reactions between various types of penicillins.
In approximate order of decreasing frequency, manifestations of allergy to penicillins include maculopapular rash, urticarial rash, fever, bronchospasm, vasculitis, serum sickness, exfoliative dermatitis, Stevens-Johnson syndrome, and anaphylaxis.
The overall incidence of such reactions to the penicillins varies from 0.7% to 10% in different studies.
Very high doses of penicillin G can cause seizures in kidney failure.

25. Stevens Johnson Syndrome
Adverse Effects
Stevens Johnson Syndrome
Adverse drug reactions.
Painful Blistering of the skin and mucous membrane involvment.
In many cases preceded with flu like symptoms and high fever.
As it evolves the skin literally sloughs off.
Ocular involvement includes severe conjunctivis, iritis, palpebral edema, conjunctival and corneal blisters and erosions, and corneal perforation.

26. Adverse Effects
Other Adverse Reactions. The penicillins have minimal direct toxicity. Many persons who take various penicillin preparations by mouth experience nausea, with or without vomiting, and some have mild-to-severe diarrhea. These manifestations often are related to the dose of the drug.
Most common among the irritative responses to penicillin are pain and sterile inflammatory reactions at the sites of intramuscular injections, reactions that are related to concentration.
When penicillin is injected accidentally into the sciatic nerve, severe pain occurs and dysfunction in the area of distribution of this nerve develops and persists for weeks.

27. Adverse Effects
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)

28. Drug-drug Interactions
Penicillins bind to and inactivate aminoglycosides. This is a form of chemical antagonism. If an aminoglycoside and a penicillin are combined. they MUST NOT be administered simultaneously through the same I.V. line or through the same syringe. They will crystallize and precipitate in the line or in the vessels!
When an aminoglycoside and a penicillin are administered, the infusions should be staggered by about 1 to 2 hours.

29. Classification of the Penicillins and
Summary of Their Pharmacological Properties
1. Penicillin G and its close congener penicillin V are highly active against sensitive strains of gram-positive cocci, but they are readily hydrolyzed by penicillinase. Thus, they are ineffective against most strains of Staphylococcus aureus.
2. The penicillinase-resistant penicillins (methicillin, nafcillin, oxacillin, cloxacillin, and dicloxacillin) have less potent antimicrobial activity against microorganisms that are sensitive to penicillin G, but they are effective against penicillinase-producing Staph. aureus.
3. Ampicillin, amoxicillin, bacampicillin, and others comprise a group of penicillins whose antimicrobial activity is extended to include such gram-negative microorganisms as Haemophilus influenzae, E. coli, and Proteus mirabilis. Unfortunately, these drugs and the others listed below are hydrolyzed readily by broad-spectrum b-lactamases that are found with increasing frequency in clinical isolates of these gram-negative bacteria.

30. Cephalosporins
Cephalosporins are similar to Penicillins, but more stable to many bacterial β lactamases and therefore have a broader spectrum of activity.
The 1st source of Cephalosporins, Cephalosporium acremonium (Fungus), was isolated in 1948 by Giuseppe from the sea near a sewer outlet of the Sardinian Coast.
The nucleus of Cephalosporins, 7-aminocephalosporanic acid bears a close resemblance to 6-aminopencillanic acid.
The core of the basic cephalosporin molecule consists of a two ring system which includes a β-lactam ring condensed with dihydrothiazine ring.

32. Mechanism of Action
Cephalosporins exert bactericidal effect in manner similar to that of Penicillins.
Binding to specific PBPS
Inhibition of cell wall synthesis by inhibiting transpeptidation of Peptidoglycan
Activation of Autolytic enzymes Autolysins or Murein Hydrolases

33. Classification
Cephalosporins can be classified into four major groups or generations, depending mainly on the spectrum of antimicrobial activity.
Recently, Fifth generation cephalosporins were developed in the lab to specifically target against resistant strains of bacteria particularly Methicillin Resistance Staphlococcus Aureus (MRSA).

34. 1st Generation Cephalosporins
The agents included in this group have good activity against gram-positive cocci, such as pneumococci, streptococci and staphylococci but not active against methicillin resistant strains of staphylococci, and relatively modest activity against gram-negative microorganisms (E.Coli and Klebsiella pnumoniae).
Include: Cefazolin (IV/IM), Cefadroxil (PO),
Cephalexin (PO), Cephalothin (IV/IM),
Cephapirin (IV/IM), Cephradine (PO)

35. 2nd Generation Cephalosporins
These compounds show modest activity against gram-positive bacteria (less active than 1st generation drugs) and display greater activity against gram-negative microorganisms including Haemophilus influenza, some Enterobacter aerogenes and Neisseria Species.
In comparison to 1st generation, they have some what increased activity against gram-negative bacteria but this activity is much less than the activity of 3rd generation compounds.
Include: Cefaclor (PO), Cefamandole (IV/IM), Cefonicid (IM/IV), Cefuroxime (IV/IM/PO), Cefprozil (PO), Loracarbef (PO), Ceforanide (IM/IV)

36. 3rd Generation Cephalosporins
Though greatly inferior to 1st generation cephalosporins in regard to their activity against gram-positive cocci, the 3rd generation cephalosporins exhibit much more activity against gram-negative bacilli, most other enteric organisms and β- lactamase producing strains of Haemophilus and Neisseria.
Drugs of this group have superiority over the other two generation in having ability to reach CNS (cross BBB).
They include: Cefoperazone (IV/IM), Cefotaxime (IV/IM), Ceftriaxone (IV/IM), Cefixime (PO), Ceftazidime (IV/IM), Moxalactam (IM/IV)

37. 4th Generation Cephalosporins
They have an extended spectrum of activity as compared to the 3rd generation and have increased stability from hydrolysis by plasmid and chromosomally mediated β- lactamases.
Aerobic gram-negative bacilli resistant to 3rd generation cephalosporins can be succesfully treated with 4th generation drugs.
Drugs included in this class are: Cefepime (IV), Cefpirome (IV), Cefozopran (IV)

38. 5th Generation Cephalosporins
These 5th generation cephalosporins are active against Methicillin resistant staphylococci.
Agents under this class include: Ceftaroline Fosamil (IV), Ceftobiprole (IV)

39. Resistance to Cephalosporins
Resistance to cephalosporins can be due to following mechanisms:
Poor penetration of drug into bacteria
Lack of specific PBPS for a particular agent
Degradation of the drug by β-lactamases
Failure of activation of Autolytic enzymes in the bacterial cell wall.

40. Therapeutic Uses
Cephalosporins are widely used antibiotics. Unfortunately, overuse of these agents in situations where drugs with less broad spectrum activity would be more appropriate has led to the emergence of wide array of cephalosporin resistant bacteria.
Cephalosporins are effective as both Prophylactically & Therapeutically.
Alternative to Penicillins
Respiratory tract infections caused by Klebsiella, Enterobacter, Proteus, Providencia, and Haemophilus species.
Gonorrhoea
Typhoid fever
Meningitis

41. Adverse Effects Allergic Reactions Nephrotoxicity Diarrhoea
Disulfiram like Reaction
Bleeding Disorder
Superinfections

42. Other β-Lactam Antibiotics
β-Lactamase Inhibitors
Clavulanic Acid
Salbactum
Tazobactum
Monobactams
Aztreonam
Carbapenems
Doripenem
Imipenem
Ertapenem
Meropenem

43. β-Lactamase Inhibitors
β-lactamase inhibitors are used in conjunction with a β-Lactam antibiotic to extend its spectrum of activity.
Although β-lactamase inhibitors have little antibiotic activity of their own, they instead inhibit the activity of β-lactamases, (a family of enzymes that break the beta-lactam ring) that allows penicillin-like antibiotics to work, thereby conferring bacterial resistance.

44. Hence β-lactamase inhibitors are often given in combination with penicillins to tackle the problem of the resistance caused by the presence of β-lactamases from bacterial cells. 
An example is Co-Amoxiclav [Augmentin], which is a combination of amoxicillin and clavulanic acid. Salbactum usually combined with Ampicillin (Unasyn) and Tazobactum with Piperacillin (Zosyn).

45. Monobactams Monobactams are drugs with a monocyclic β- lactam ring.
They are resistant to β-lactamases and active against gram-negative rods.
They have no activity against gram-positive bacteria or anaerobes.
Penicillin-allergic patients tolerate aztreonam without reaction.

46. Carbapenems
Imipenem has good activity against gram- negative, gram-positive and anaerobic organisms.
Imipenem is inactivated by dehydropeptidases in renal tubules.
It is administered with an inhibitor of renal dehydropeptidase, cilastatin, for clinical use.
Meropenem and ertapenem are not degraded by renal dehydropeptidase.
IM ertapenem is irritating, so it is formulated with 1% lidocaine.
A carbapenem is used for P. aureginosa resistant to other drugs and mixed aerobic and anaerobic infections.

47. Cell Wall Destructors Vancomycin Daptomycin Fosfomycin Polymyxin
Cycloserine

48. Vancomycin
Vancomycin is active against gram-positive bacteria, particularly staphylococci.
Machanism: Inhibits cell wall synthesis by binding to the D-Ala-D-Ala terminus of peptidoglycan pentapeptide, which as a result inhibits transglycosylase, preventing peptidoglycan elongation and cross-linking.
β-lactamase producing staphylococci and those resistant to nafcillin and methicillin are killed by vancomycin.
Vancomycin is poorly absorbed from the GI tract.

49. It is used orally only for antibiotic-associated enterocolitis caused by C. difficile.
Metronidazole is preferred as initial therapy and vancomycin is reserved for refractory cases.
Parenteral vancomycin is used in sepsis caused by methicillin-resistant staphylococci.
Vancomycin is irritating to tissue, resulting in phlebitis at the site of injection.
A common reaction is "red man" or "red neck" syndrome.
This infusion-related flushing is caused by release of histamine.
It can be largely prevented by prolonging the infusion period to 1-2 hours or increasing the dosing interval.

50. Daptomycin
It is similar to vancomycin but is active against vancomycin-resistant enterococci and S. aureus.
It appears to bind to and depolarize the cell membrane, causing potassium efflux and rapid cell death.
It can cause myopathy, so creatine kinase levels should be monitored regularly while individual undergo daptomycin therapy.

51. Fosfomycin
It inhibits bacterial cell wall biogenesis by inactivating the cytoplasmic enzyme, enolpyruvate transferase.
Fosfomycin is active against both gram-positive and gram-negative organisms.
Fosfomycin is used for treatment of uncomplicated urinary tract infections.

52. Polymyxins
Polymyxins antibiotic primarily used for resistant Gram-negative infections.
Alters bacterial outer membrane permeability by binding to a lipopolysaccharide layer resulting in disruption of membrane integrity.
Polymyxin B is applied topically to treat infections such as those of the eye, ear, and skin. Polymyxin E, also known as colistin, is used frequently for diarrhea in children. 
Because polymyxins also react with the membranes of human cells, they can cause kidney damage and neurotoxicity. The availability of better antibiotics limits the use of polymixins.

53. Cycloserine
Cycloserine is used only to treat tuberculosis resistant to first-line agents.
Cycloserine causes serious CNS toxicity with headaches, tremors, acute psychosis, and convulsions.