Cell Wall Inhibitors. Assistant Professor Naza M. Ali. Lec 3 7 Nov 2017

Transcription

1 Cell Wall Inhibitors Assistant Professor Naza M. Ali Lec 3 7 Nov 2017

2 Cell wall The cell wall is a rigid outer layer, it completely surrounds the cytoplasmic membrane, maintaining the shape of the cell and preventing the cell lysis from high osmotic pressure. Cell wall is composed of a polymer called peptidoglycan that consists of glycan units joined to each other by peptide crosslinks.

3 Cell Wall Inhibitors: Inhibitors of cell wall synthesis require actively Proliferating microorganisms mean they have little or no effect on bacteria that are not growing and dividing. I. Penicillins Penicillins are classified as beta-lactam drugs because of their unique four-membered lactam ring. All have a thiazolidine ring (A) is attached to a betalactam ring (B) that carries a secondary amino group RNH-

4 B A

5 Penicillin are among the most widely effective antibiotics The least toxic drugs, but increased resistance has limited their use. Members of this family differ from one another in the R substituent attached to the 6-aminopenicillanic acid residue The nature of this side chain affects 1.The antimicrobial spectrum, 2. Stability to stomach acid, 3. Susceptibility to bacterial degradative enzymes betalactamases.

6 Mechanism of action Interfere with last step of bacterial cell wall synthesis they are bactericidal 1. Penicillin-binding proteins: Penicillins inactivate numerous proteins on the bacterial cell membrane. These PBPs are bacterial enzymes involved in the synthesis of the cell wall and in the maintenance of the morphologic features of the bacterium. Exposure to these antibiotics can lead to morphologic changes or lysis of susceptible bacteria.

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8 2. Inhibit of transpeptidase: 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. As a result of this blockade of cell wall synthesis.

9 3. Activation of autolytic enzyme: The gram-positive cocci, produce degradative enzymes (autolysins) that participate in the normal remodeling of the bacterial cell wall.

10 Mechanism of resistance 1. Beta-Lactamase activity: this family of enzymes hydrolyzes the cyclic amide bond of the beta-lactam ring, which results in loss of bactericidal activity. 2. Decreased permeability to the drug: decreased penetration of the antibiotic through the Outer cell membrane prevents the drug from reaching the target PBPs. 3. Altered PBPs: modified PBPs have a lower affinity for beta-lactam antibiotic.

11 Antibacterial spectrum 1. Natural penicillins: are obtained from fermentations of the mold Penicillum Penicillin G (benzylpenicillin) is the cornerstone of therapy for infections caused by a number of gram-positive cocci, gram-negative cocci, gram-positive bacilli, spirochetes and anaerobic. Penicillin G is susceptible to inactivation by lactamases (penicillinases). Penicillin V has a spectrum similar to that of penicillin G is more acid-stable than penicillin G.

12 2. Antistaphylococcal penicillins: Methicillin Nafcillin Oxacillin Dicloxacillin Are resistant to staphylococcal beta lactamases

13 3. Extended-spectrum penicillins: (semisynthetic) Ampicillin, Amoxicillin Inactivated by beta lactamase have an antibacterial spectrum similar to that of penicillin G but are more effective against gram-negative bacilli. Infections of E. coli, H. influnzae, P. mirabilis, S. typhi

14 Ampicillin is the drug of choice for the gram-positive bacillus Listeria monocytogenes. In the treatment of respiratory infections Amoxicillin used prophylactically by dentists for patients with abnormal heart valves who are to undergo extensive oral surgery.

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16 4) Antipseudomonal penicillins Carbenicillin Ticarcillin Piperacillin Are effective against P. aeruginosa Suscptible to hydrolysis by Beta lactamase Formulation with bete-lactamase inhibitors extends antimicrobial spectrum of these antibiotics to include penicillinase-producing organisms have synergistic action when used with aminoglycoside. Must be administered IV or IM

17 Antipseudomonal P. aeruginosa Inactivated by beta lactamase Carbenicillin Ticarcillin Classification Spectrum Susceptible Drug Name Natural penicillins gram-positive cocci, gram-negative cocci, gram-positive bacilli, spirochetes anaerobic Susceptible to inactivation by lactamases (penicillinases) Penicillin G Penicillin V Antistaphylococcal Staphylococcal Resistant to beta lactamase Methicillin Nafcillin Oxacillin Dicloxacillin Extended-spectrum similar to penicillin G more effective gram-negative bacilli Infections of E. coli, H. influnzae, P. mirabilis, S. typhi Inactivated by beta lactamase Ampicillin Amoxicillin

18 Pharmacokinetics Route of administration is determine -by the stability of the drug to gastric acid -by the severity of the infection Oral, I.M, I.V Depot forms: procaine penicillin G, benzathine penicillin

19 Absorption They are polar molecules Absorption of most all penicillinase- resistant penicillin is decreased by food. Should administered at least 1-2 before or after a meal because gastric emptying time is lengthened, and the drugs are destroyed in the acidic environment.

20 Distribution: They distribute well All penicillin's cross placental barrier (not teratogenic) Penetration into certain sites ( bone or CSF) is insufficient for therapy unless these sites are inflamed, Penicillin levels in the prostate are insufficient to be effective against infections.

21 Enhanced penetration of penicillin into the cerebral spinal fluid (CSF) during inflammation.

22 Excretion: By tubular secretion system and glomerular filtration. Patients with impaired renal function must have dosage regimens adjusted.

23 Adverse Reaction Hypersensitivity: due to its metabolite, penicilloic acid, which reacts with proteins and serves as a hapten to cause an immune reaction. 5% of patient have maculopapular rash with marked swelling of lips, tongue & anaphylaxis Diarrhea: due to disruption of the normal balance of intestinal microorganisms, is a common problem. It occurs to a greater extent with those have extended antimicrobial spectrum.

24 Nephritis: methicillin cause nephrotoxicity. Neurotoxicity if injected intrathecally Hematologic toxicities Cation toxicity: penicillins are administered as sodium or potassium salt. Toxicities caused by large quantities of Na or K

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