ANTI-BACTERIAL CHEMOTHERAPY ..(1+2)

1. ANTI-BACTERIAL CHEMOTHERAPY..(1+2) By Dr Israa Omar

3. A. ANTIMICROBIAL AGENTS THAT INTERFERE WITH THE SYNTHESIS OR ACTION OF FOLATE

4. sulfonamides • Examples include sulfadiazine , sulfadimidine , sulfamethoxazole (intermediate acting) • Sulfametopyrazine (long acting) • Sulfasalazine (poorly absorbed in the gastrointestinal tract) • Sulfamethoxazole (in combination with trimethoprim as co-trimoxazole).

5. Mechanism of action • Sulfonamide is a structural analogue of p-aminobenzoic acid (PABA), which is an essential precursor in the synthesis of folic acid in bacteria. • folate is required for the synthesis of the precursors of DNA and RNA both in bacteria and in mammals, but whereas bacteria need to synthesis folic acid, mammals can obtain it from dietary sources. • Sulfonamides compete with PABA for the enzyme dihydropteroatesynthetase, and the effect of the sulfonamide may be overcome by adding excess PABA.

6. Mechanism of action • The action of a sulfonamide is to inhibit growth of the bacteria, not to kill them; that is to say, it is bacteriostatic rather than bactericidal. • The action is impaired in the presence of pus or products of tissue breakdown, because these contain thymidine and purines, which bacteria utilize directly, bypassing the requirement for folic acid. • Resistance to the drugs, which is common, is plasmid-mediated and results from the synthesis of a bacterial enzyme insensitive to the drug.

7. Pharmacokinetics • Most sulfonamides are readily absorbed in the gastrointestinal tract and reach maximum concentrations in the plasma in 4-6 hours. • They are usually not given topically because of the risk of sensitization or allergic reactions. • The drugs pass into inflammatory exudates and cross both placental and blood-brain barriers. • They are metabolized mainly in the liver, the major product being an acetylated derivative that lacks antibacterial action.

8. Side effects • Mild to moderate side effects include nausea and vomiting, headache and mental depression. • Cyanosis caused by methaemoglobinaemia (Fe3+HB) may occur but is a lot less alarming than it looks. • Serious adverse effects include hepatitis, hypersensitivity reactions (rashes, fever, anaphylactic reactions), bone marrow depression • Crystalluria; this effect results from the precipitation of acetylated metabolites in the urine.

9. Clinical uses of sulfonamides • Combined with trimethoprim(Trimethoprim/sulfamethoxazole‘co-trimoxazole’) for Pneumocystis carinii. • Combined with pyrimethamine (Sulfadoxine/pyrimethamine)for drug-resistant malaria, and for toxoplasmosis. • In inflammatory bowel disease:sulfasalazine (sulfapyridine-aminosalicylatecombination. • For infected burns (silver sulfadiazine given topically). • For some sexually transmitted infections (e.g. trachoma, chlamydia, chancroid). • For respiratory infections: use now confined to a few special problems (e.g. infection with Nocardia). • For acute urinary tract infection (now seldom used).

10. TRIMETHOPRIM • Trimethoprim is chemically related to the antimalarial drug pyrimethamine , both being folate antagonists. • Structurally, it resembles the pteridine moiety of folate and the similarity is close enough to fool the bacterial dihydrofolate reductase, which is many times more sensitive to trimethoprim than the equivalent enzyme in humans .

12. Pharmacokinetic aspects • Trimethoprim is given orally. • It is fully absorbed from the gastrointestinal tract and widely distributed throughout the tissues and body fluids. • It reaches high concentrations in the lungs and kidneys, and fairly high concentrations in the cerebrospinal fluid (CSF). • When given with sulfamethoxazole, about half the dose of each is excreted within 24 hours. • Because trimethoprim is a weak base, its elimination by the kidney increases with decreasing urinary pH.

13. Antimicrobial agents that interfere with the synthesis or action of folate • Sulfonamides are bacteriostatic; they act by interfering with folate synthesis and thus with nucleotide synthesis. • Unwanted effects include crystalluria and hypersensitivities. • Trimethoprim is bacteriostatic. It acts by antagonizing folate. • Co-trimoxazole is a mixture of trimethoprim with sulfamethoxazole, which affects bacterial nucleotide synthesis at two points in the pathway

14. Clinical uses of trimethoprim/co-trimoxazole • For urinary tract and respiratory infections: trimethoprim, used on its own, is usually preferred. • For infection with Pneumocystis carinii, which causes pneumonia in patients with AIDS: co-trimoxazole is used in high dose.

15. Cell Wall Inhibitors

17. Cell Wall Inhibitors • b-Lactams • Penicillins • Cephalosporins • Monobactams (Aztreonam) • Carbapenems (Imipenem) • Non b-Lactams • Vancomycin (glycopeptide) • Cycloserine • Fosfomycin • Bacitracin (cyclic peptide)

18. S CH3 R1 CH3 S R1 N COOH O N R2 O COOH Penicillins Cephalosporins β β Beta Lactam Ring

19. R1 R2 R1 N N R2 O COOH O Carbapenems Monobactams β β Beta Lactam Ring

20. Cell coverings of bacteria • Inner: Cell / plasma membrane • Outer: Cell wall • Peptidoglycan layer consists of glycan chains • Glycan chains: linear strands of two alternating amino sugarsN-acetylglucosamine (Nag) N-acetylmuramic acid (Nam) • Cross linking of these strands by peptide chains

21. Gram-neg & gram-pos Cell Walls

22. β-LACTAM ANTIBIOTICS 1. Penicillinsand cephalosporins • Penicillins and cephalosporins • Part of group of drugs called β –lactams • Have shared chemical structure called β-lactam ring • Competitively inhibits function of penicillin-binding proteins • Inhibits peptide bridge formation between glycan molecules • This causes the cell wall to develop weak points at the growth sites and become fragile.

23. Dye Binds to Exposed Cell Wall Peptidoglycan, Gram positive (Rt.)

24. Mechanism of action of PENICILLIN • All β-lactam antibiotics interfere with the synthesis of the bacterial cell wall peptidoglycan. • After attachment to penicillin-binding proteins on bacteria (there may be seven or more types in different organisms), they inhibit the transpeptidation enzyme that cross-links the peptide chains attached to the backbone of the peptidoglycan.

26. Mechanism of action

28. Mechanism of action • The final bactericidal event is the inactivation of an inhibitor of autolyticenzymes{in other word promotion OR stimulation of autolytic enzyme} in the cell wall, leading to lysis of the bacterium. • Some organisms, referred to as 'tolerant', have defective autolytic enzymes and are inhibited but not lysed in the presence of the drug.

29. Types of penicillin and their antimicrobial activity • The first penicillins were the naturally occurring benzylpenicillin and its congeners, including phenoxymethylpenicillin. • Benzylpenicillinis active against a wide range of organisms and is the drug of first choice for many infections. • Its main drawbacks are poor absorption in the gastrointestinal tract (which means it must be given by injection) and its susceptibility to bacterial β-lactamases

30. Various semisynthetic penicillins have been prepared by adding different side-chains to the penicillin nucleus. • β-lactamase-resistant penicillins (e.g. flucloxacillin) • Broad-spectrum penicillins (e.g. ampicillin, pivampicillin and amoxicillin) • Ticarcilinwith antipseudomonal activity against P. aeruginosa. • Amoxicillin is sometimes combined with β-lactamase inhibitor clavulanic acid as co-amoxiclav

31. Pharmacokinetic aspects • When given orally, different penicillins are absorbed to differing degrees depending on their stability in acid and their adsorption to foodstuffs in the gut. • Penicillins can be given IM or IV , but intrathecal administration is inadvisable, particularly in the case of benzylpenicillin, as it can cause convulsions. • The penicillins are widely distributed in body fluids, passing into joints; into pleural and pericardial cavities; into bile, saliva and milk; and across the placenta.

32. Pharmacokinetic aspects • Being lipid-insoluble, they do not enter mammalian{check it} cells and do not, therefore, cross the blood-brain barrier unless the meninges are inflamed, in which case they readily reach therapeutically effective concentrations in the CSF as well. • Elimination of most penicillins occurs rapidly and is mainly renal, 90% being through tubular secretion. • The relatively short plasma half-life is a potential problem in the clinical use of benzylpenicillin

33. Unwanted effects • Penicillins are relatively free from direct toxic effects (other than their pro-convulsanteffect when given intrathecally). • The main unwanted effects are • Hypersensitivity reactions caused by the degradation products of penicillin, which combine with host protein and become antigenic. • Skin rashes and fever are common; • A delayed type of serum sickness occurs infrequently.

34. Unwanted effects • Much more serious is acute anaphylactic shock, which although fortunately very rare, may in some cases be fatal. • When given orally, penicillins, particularly the broad-spectrum type, alter the bacterial flora in the gut. This can be associated with gastrointestinal disturbances and in some cases with superinfectionby other, penicillin-insensitive, micro-organisms.

35. Clinical uses of the penicillins • Bacterial meningitis (e.g. By N. Meningitidis, S.Pneumoniae): benzylpenicillin, high doses IV • Bone and jointinfections (e.g. With staph. Aureus): flucloxacillin • Skin and soft tissueinfections (e.g. With strep. Pyogenes or staph. Aureus): benzylpenicillin, flucloxacillin; animal bites: co-amoxiclav • Pharyngitis (from strep. Pyogenes): phenoxylmethylpenicillin • Otitis media (organisms commonly include strep. Pyogenes, haemophilusinfluenzae): amoxicillin

36. Bronchitis (mixed infections common): amoxicillin • Pneumonia: amoxicillin • Urinary tract infections (e.g. with Escherichia coli): amoxicillin • Gonorrhea: amoxicillin (plus probenecid) • Syphilis: procaine benzylpenicillin • Endocarditis (e.g. with Strep. viridans or Enterococcus faecalis) • Serious infections with Pseudomonas aeruginosa: ticarcillin, piperacillin. • EOL

37. Lecture No:02CEPHALOSPORINS ANDCEPHAMYCINS • They are usually classified in arbitrary in terms of chronological order in which they were produced: • First generation : cephalexin, cefadroxil • Second generation: cefuroxime, cephamandole and cefoxitin • Third generation: cefotaxime, ceftazidine, cefiximeand ceftrixone, more active against g-ve • Fourth generation: cefepime highly resistant to beta lactamase enzyme

38. Mechanism of action • The mechanism of action of these agents is similar to that of the penicillins: interference with bacterial peptidoglycan synthesis after binding to the β-lactam-binding proteins.

39. Resistance • Resistance to this group of drugs has increased because of plasmid-encoded or chromosomal β-lactamase. • Nearly all Gram-negative bacteria have a chromosomal gene coding for a β-lactamase that is more active in hydrolyzing cephalosporins than penicillins, and in several organisms a single mutation can result in high-level constitutive production of this enzyme.

40. Pharmacokinetic aspects • Some cephalosporins may be given orally , but most are given parenterally, IM or IV. • After absorption, they are widely distributed in the body and some, such as cefotaxime, cefuroxime and ceftriaxone, cross the blood-brain barrier. • Excretion is mostly via the kidney, largely by tubular secretion, but 40% of ceftriaxone is eliminated in the bile

41. Adverse effects • Hypersensitivity reactions, very similar to those seen with penicillin, may occur, and there may be some cross-sensitivity; about 10% of penicillin-sensitive individuals will have allergic reactions to cephalosporins. • Nephrotoxicity has been reported (especially with cefradine), as has drug-induced alcohol intolerance. • Diarrhea can occur with oral cephalosporins

42. Clinical uses of the cephalosporins • Septicaemia(e.g. cefuroxime, cefotaxime) • Pneumonia caused by susceptible organisms • Meningitis (e.g. ceftriaxone, cefotaxime) • Biliary tract infection • Urinary tract infection (especially in pregnancy or in patients unresponsive to other drugs) • Sinusitis (e.g. cefadroxil).

43. OTHER β-LACTAM ANTIBIOTICS CARBAPENEMS AND MONOBACTAMS • Carbapenemsand monobactamswere developed to deal with β-lactamase-producing Gram-negative organisms resistant to penicillins

44. 2. Carbapenem • Imipenem, an example of a carbapenem, acts in the same way as the other β-lactams . • It has a very broad spectrum of antimicrobial activity, being active against many aerobic and anaerobic Gram-positive and Gram-negative organisms. • Imipenemwas originally resistant to all β-lactamases, but some organisms now have chromosomal genes that code for imipenem-hydrolysing β-lactamases. • It is sometimes given together with cilastatin, which inhibits its inactivation by renal enzymes..

45. Meropenem is similar but is not metabolized by the kidney. • Ertapenem has a broad spectrum of antibacterial actions but is licensed only for a limited range of indications

46. Unwanted effects • Unwanted effects are generally similar to those seen with other β-lactams, nausea and vomiting being the most frequently seen. • Neurotoxicity can occur with high plasma concentrations

47. 3. MONOBACTAMS • The main monobactam is aztreonam, a simple monocyclic β-lactam with a complex substituent which is resistant to most β-lactamases. • It is given parenterally and has a plasma half-life of 2 hours. • Aztreonam has an unusual spectrum of activity and is effective only against Gram-negative aerobic rods such as pseudomonads, N.meningitidisand H.influenzae. • It has no action against Gram-positive organisms or anaerobes

48. Unwanted effects • Unwanted effects are, in general, similar to those of other β-lactam antibiotics, but this agent does not necessarily cross-react immunologically with penicillin and its products, and so does not usually cause allergic reactions in penicillin-sensitive individuals.

49. Non b-Lactams

50. 1. Vancomycin • Non Beta-Lactam • Teicoplaninis similar but longer lasting • Inhibits transglycosylaseenzyme from binding to D-ala-D-ala; stops elongation of peptidoglycan chain; Bactericidal • IV only; Renal elimination • Used to treat Serious infections (sepsis) due to Staph, Methicillin-resistant strains, Alt. to β-lactams in endocarditis caused by Staphylococci and Streptococci • C.difficileenterocolitis is treated orally with vancomycin, metronidazole is preferred