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SYSTEMS PHARMACOLOGY

Chemotherapy of Infection: Part I

Dr Dhaya Perumal

London Metropolitan University

Dept. Health & Human Sciences

Tower Building: Room T13-10

Telephone: 020 7133 4195

d.perumal@londonmet.ac.uk

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Pathogenic (Infectious) Organisms

Are those organisms that cause diseases in human beings/ animals

Types of pathogenic organisms:1. Microbes

Bacteria, fungi and viruses 2. Parasites

Protozoa and helminthes (worms)

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Chemotherapeutic agents

Are naturally occurring or chemically synthesized substances intended to be toxic for the pathogenic organisms but innocuous to the host

Aim- To treat acute, severe, persistent or progressive infectious disease

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Factors contributing to microbial threats to health

1. Microbial adaptation and change2. Human susceptibility to infection3. Climate and weather4. Changing ecosystem5. Human demographics and behaviour6. Economic development and land use7. International travel and commerce

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Factors contributing to microbial threats to health (Contd.)

8. Technology and Industry 9. Breakdown of Public Health Measures10. Poverty and Social Inequality 11. War and famine12. Lack of Political will13. Intent to harm (e.g. weapons of mass

destruction)

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Classic Definition: Antibiotic

(from Greek, anti – against, bios – life) A natural substance, or derivative of a

natural substance, which when taken in small doses will either kill or prevent the growth of a microorganism, but will not seriously harm the person taking it

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Antibiotic-producing microorganisms

Penicillium and Cephalosporium Beta-lactam antibiotics: penicillin and cephalosporin

Actinomycetes, Streptomyces speciesTetracyclines AminoglycosidesMacrolides Chloramphenicol

Bacillus speciesPolypeptide antibiotics: polymyxin and bacitracin

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Definition (Modified):Antibiotics/Antibacterials/Antimicrobials

Any chemical compound used to kill or inhibit the growth of infectious organisms, particularly bacteria and fungi

All antibiotics share the property of selective toxicity: they are more toxic to an invading microorganism than to the animal/human host

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Choice of suitable drug Two considerations:1. Patient - history of allergy

- renal/hepatic function- susceptibilty to infection- ability to tolerate by mouth- severity of illness- ethnic origin- age- other medication- pregnancy, breast-feeding, OC

use

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Choice of suitable drug contd.

2. Known or likely causative organism- antibacterial sensitivity

Final choice depends on microbiological, pharmacological and toxicological properties

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Rational approach to selecting drug

Example: to treat UTI in a pregnant patient who has nausea. The organism found to be resistant to ampicillin but - sensitive to nitrofurantoin (can cause nausea)- gentamycin (only by injection and avoided in pregnancy)- tetracycline (dental discolouration)- trimethoprim (teratogenic) and - cefalexin

Safest in pregnancy is penicillins and cephalosporins

Therefore cefalexin indicated for this patient

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Spectrum of Activity

A.     Broad spectrum of activityAn antimicrobial drug that is effective against a large variety of microorganisms

ADVANTAGES: A high degree of efficacy against an unidentified pathogen

DISADVANTAGES:A high likelihood of the drug also destroying the friendly/helpful bacteria making up an individual’s normal microbial flora

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Spectrum of Activity (Contd.)

B. Narrow spectrum of activity

An antimicrobial drug that is effective against only a relatively small subset of bacteria

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Effects of Antimicrobials

A. Bactericidal (kill)

Interaction results in an irreversible disruption or binding cell death

B. Bacteriostatic (inhibit growth)

Interaction effect involves lower affinity binding and is reversible when the antibacterial is removed from the environment

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DIAGNOSTIC STAINING TECHNIQUE

Crystal violet

Iodine Alcohol wash

Safranin

Gram – Positive Bacteria : BLUEGram – Negative Bacteria : RED

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DIAGNOSTIC STAINING TECHNIQUE (Contd.)

Ziehl-Neelsen Stain (Acid-fast bacteria) 1. Bacteria + Carbofuchsin (bring to boil 3 times)2. HCl + Alcohol (1-2 min)3. Alkaline methylene blue (3 min)

RESULT:- Acid-fast bacilli: RED- Other bacteria: BLUE

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Spectrum of Antibacterials

1. Gram Negative Bacteria

2. Acid fast Bacteria

Aerobic – requires oxygen Anaerobic – does not require oxygen

 

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Sensitivity Test

Antibiotic sensitivity determined by size of inhibition zone

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Mode of action of antibacterials

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Bacterial cell wall structure

Gram Negative Gram positive

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Mode of action of antibacterials

A. Inhibition of cell wall synthesisB. Disruption of cell membrane functionC. Inhibition of protein synthesisD. Inhibition of nucleic acid synthesisE. Action as antimetabolites

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Major modes of action of drugs

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A. Inhibition of cell wall synthesis

Most bacteria have peptidoglycan-based cell walls (mammals do not)

Successful cell wall synthesis by these bacteria is impossible in the absence of peptidoglycan synthesis

In the absence of cell wall integrity, most bacteria are susceptible to osmotic lysis

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SYNTHESIS OF PEPTIDOGLYCAN• Bacteria increase their size following binary fission, links inpeptidoglycan break, new peptidoglycan monomers insert and the peptide cross links must be resealed

1. Bacterial enzymes, autolysins, break the glycosidic bonds between thepeptidoglycan monomers and the peptide cross-bridges that link the rows of sugars together. 2. In this way, new peptidoglycan monomers can be inserted and enable bacterial growth.

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SYNTHESIS OF PEPTIDOGLYCAN

1. Transglycosidase enzymes catalize the formation of glycosidic bonds between the N-acetylmuramic acid (NAM) and N-acetylglucosamine (NAG) of the peptidoglycan momomers and the NAG and NAM of the existing peptidoglycan 2. Finally, transpeptidase enzymes reform the peptide cross-links between the rows and layers of peptidoglycan to make the wall strong.

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Beta-lactam antibiotics: Penicillins and Cephalosporins

Stereochemically related to D-alanyl-D-alanine, which is a substrate for the last step in peptidoglycan synthesis

Block the final transpeptidation (cross-linkage of pentapeptide side chains)

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MECHANISM OF ACTION

inhibits the formation of inhibits the formation of peptidoglycan peptidoglycan cross links cross links in bacterial cell wall in bacterial cell wall

final final transpeptidationtranspeptidation step in the synthesis of the step in the synthesis of the peptidoglycanpeptidoglycan is facilitated by is facilitated by transpeptidasestranspeptidases(known as penicillin(known as penicillin--binding proteins, binding proteins, PBPsPBPs).).

TranspeptidasesTranspeptidases –– bacterial enzymes that crossbacterial enzymes that cross--link the link the peptidoglycan peptidoglycan chains to form rigid cell wallchains to form rigid cell wall

the betathe beta--lactam lactam moiety binds to moiety binds to transpeptidasetranspeptidase, , iinhibits formation of nhibits formation of peptidoglycanpeptidoglycan component of component of the cell wall, the cell wall, weakening the cell wall when weakening the cell wall when bacterium multiplies. bacterium multiplies.

Dividing cells swell Dividing cells swell osmoticallyosmotically and rupture i.e. and rupture i.e. bacteriocidalbacteriocidal against dividing cells.against dividing cells.

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Role of Penicillins in Blocking Transpeptidase Enzymesfrom Assembling the Peptide Cross-Links in Peptidoglycan

Penicillins (and cephalosporins) bind to the transpeptidase enzymes (also called penicillin-binding proteins) responsible for resealing the cell wall as newpeptidoglycan monomers are added during bacterial cell growth. This blocks the transpeptidase enzymes from cross-linking the sugar chains and results in a weak cell wall and subsequent osmotic lysis of the bacterium

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Glycopeptides: Vancomycin covalently bind to the terminal two D-alanine

residues at the free carboxyl end of the pentapeptide

Sterically hinder the elongation of the peptidoglycan backbone

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Polypeptide: Bacitracin

Blocks the dephosphorylation of the lipid carrier

Cycloserineby competitive inhibition, the drug prevents the addition of the two terminal alanines to the initial tripeptide side-chain on N-acetylmuramic acid

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B. Disruption of cell membrane function

Damage to cytoplasmic membrane – Increase permeability by disorganizing the structure or inhibiting the function of bacterial membranes

- Polymyxins- Nystatin- Amphotericin B- Imidazoles

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C. Inhibition of protein synthesis

The bacterial ribosome and the animal ribosome differ structurally

Inhibition of some step in the complex process of protein synthesis

Attack on specific ribosomes

Tetracyclines Interferes with the attachment of t-RNA to m-RNA-

ribosome complex preventing the addition of new amino acids to the growing peptide chain

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Protein synthesis

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C. Inhibition of protein synthesis (Contd)

Chloramphenicol- Binds to the 50S portion and inhibits formation of peptide

bonds 

Macrolides, Fusidic Acid- Binds to the 50S portion and prevents the translocation of

ribosome along mRNA  

Aminoglycosides- Changes the shape of the 30S portion causing the

misreading of code on mRNA

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D. Inhibition of nucleic acid synthesis

Quinolones- Inhibit DNA gyrase activity (DNA gyrase –

topoisomerase II- is essential for DNA replication and allows supercoils to be relaxed and reformed)

 Rifampicin- Inhibit RNA synthesis by inhibiting DNA-

dependent RNA polymerase

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E. Action as antimetabolites

Inhibit the bacterial enzymes required for the synthesis of folic acid (tetrahydrofolic acid, THF)

Sulfonamides: structurally similar to para aminobenzoic acid (PABA), the substrate for the first enzyme in the THF pathway

Trimethoprim: structurally similar to dihydrofolate (DHF) and competitively inhibits the second step in THF synthesis mediated by the DHF reductase

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SIDE-EFFECTS Toxicities: inability of drug to completely

distinguish host physiology from pathogen physiology

Allergies

Normal flora disruptions

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Antibiotic Resistance

1. Evasion - The organism may enter or be present in an antimicrobial-resistant state such that all members of a population are destroyed by the antimicrobial except those that happen to be in the resistant state (e.g. endospores)

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Antibiotic Resistance Contd.)

2. organism may become mutated such that the site of action of the antimicrobial is no longer affected by it (a mutation affecting ribosome structure) typically resistant to only a single type of antibiotic

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Antibiotic Resistance (Contd.)

3.Extrachromosomal antibiotic resistance (acquired antibiotic resistance)

Is associated with resistance (R) plasmids Does not involve the mutation within a given

bacteria to antibiotic resistance but instead the acquisition of resistance plasmids from other bacteria

Involves an inactivation of the antibiotic or a prevention of entry rather than a change in the structure of the antibiotic target

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The “Super Bug” Issue MRSA (methicillin-resistant Staphylococcus aureus) Resistance developed against

- Beta-lactam antibiotics- Aminoglycosides (Streptomycin)- Macrolides- Chloramphenicol- Sulphonamides (Sulpamethoxazole + Trimethoprim)- Rifampicin- Fusidic Acid- Quinolones

Vancomycin was the last resort against it but resistance has also developed

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Limiting Antibiotic Resistance

Should be employed only when necessary (now often used indiscriminately and to excess)

High concentrations of drug should be maintained over long periods (i.e. taking all of one’s pills over prescribed duration of treatment)

Two antibiotics administered simultaneously may be capable of synergism when necessary

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Combinations of antimicrobial agents

Necessary when:

- Treating a life-threatening infection

- Preventing the emergence of resistance

- Treating a mixed infection

- Enhancing antibacterial activity

- Using lower concentrations of a toxic drug

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Common Uses of Antibiotics a. Gastro-intestinal system (Invasive salmonellosis,

Typhoid fever, Biliary tract, Peritonitis)b. Cardiovascular system (Endocarditis)c. Respiratory system (Chronic bronchitis,

Pneumonia)d. Central nervous system (Meningitis caused by

Meningococci, Pneumococci, Haemophilus influenzae, Listeria)

e. Urinary tract (Acute pyelonephritis or prostatitis, Lower urinary tract infection)

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Common Uses of Antibiotics (Contd.)

f. Genital system (Syphilis, Gonorrhoea, Uncomplicated genital chlamydial infection, Urethritis or pelvic inflammatory disease)

g. Blood (Septicaemia, Meningococcal septicaemia)

h. Musculoskeletal system (Septic arthritis, Osteomyelitis)

i. Eye, ear, nose and oropharynx (Conjunctivitis, Sinusitis, Otitis media, Throat, Dental infection)

j. Skin (Acne, Cellulitis, animal/insect bite)

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Common antibacterials Beta-lactam Antibiotics A.      Penicillins1. Natural penicillins

2. Penicillinase-resistant penicillin

3. Amino Penicillin

4. Antipseudomonal Penicillin

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1. Natural Penicillins

a. Penicillin G (Benzyl)

b. Penicillin G sodium/potassium

c. Penicillin G procaine

d. Penicillin G benzathine

e. Penicillin V (Phenoxymethyl-)

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2. Penicillinase-Resistant Penicillins

a. Cloxacillin Orbenin

b. Dicloxacillin Diclocil

c. Methicillin Pyopen

d. Nafcillin

e. Oxacillin

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3. Amino Penicillins (broad spectrum)

a. Amoxacillin (Amoxil)

b. Ampicillin (Penbritin)

c. Bacampicillin (Penglobe)

d. Pivmecillinam

e. Pivampicillin

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4. Antipseudomonal Penicillins

a. Carbenicillin indanyl sodium (Geopen)

b. Mezlocillin

c. Piperacillin (Pipracil)

d. Ticarcillin (Timentin)

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Beta-lactamase

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Beta-lactamase inhibitors 1. Fixed Combination Only

- Clavulanic Acid

- Tazobactam

2. Free combination possible

- Sulbactam

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B. Cephalosporins

a. Ist generation

b. 2nd generation - with Haemophilus influenzae and

Bacteroides fragilis activity

c. 3rd generation - with Pseudomonas aeruginosa activity

d. 4th generation

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a) Ist generation Parenteral form - Cephalothin (Keflin)

- Cefazolin (Cefamezin)

- Cefaprin (Lopitrex)

Oral form - Cefadroxil (Duracef)

- Cephalexin (Keflex, Ceporex)

- Cephradine (Velosef)

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b). 2nd generation Parenteral form - Cefmetazole (Cefmetazon)

- Cefonicid (Monocid)- Cefoperazone (Cefobid)- Cefoxitin (Mefoxin)

Oral form - Cefaclor (Ceclor)- Cefamandole (Mandol)- Cefetamet pivoxil

(Globocef)- Cefprozil (Procef)- Cefuroxime axetil (Zinnat)- Loracarbef (Lorabid)

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c). 3rd generation Parenteral form - Cefotaxime (Claforan)

- Ceftazidime (Fortum)

- Ceftriaxone (Rocephin)

Oral form - Cefixime

- Cefpodoxime Proxetil (Banan)

- Ceftibuten (Cedax)

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d). 4th generation Parenteral Form - Cefepime (Maxipime)

- Cefpirome (Cefrom)

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Tetracyclines Cause dental staining and hypoplasia; Absorption

is affected by milk, iron preparations, antacids (Ca2+, Mg2+, Fe2+ ions);

Drug of choice for chlamydia, rickettsia, brucella and spirochaete;

E.g. Tetracyclines - Doxycycline (Vibramycin), Minocycline (Minocin), Oxytetracycline (Terramycin), Tetracycline (Achromycin)

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QuinolonesFor complicated urinary tract infections (Not

recommended for those aged < 12 years)

a. Nalidixic Acid - Nalidixic Acid (Wintomylon), Pipemidic Acid (Urotractin)

b. Fluoroquinolones - Ciprofloxacin (Ciproxin), Levofloxacin (Cravit), Lomefloxacin (Maxaquin), Moxifloxacin (Avelox), Norfloxacin (Lexinor), Ofloxacin (Tarivid)

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Extended spectrum Pefloxacin (Peflacine) Sparfloxacin (Zaglam) Trovafloxacin (Trovan)

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Macrolides

1. Azithromycin2. Clarithomycin3. Roxithromycin4. Erythromycin

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Sulphonamides

1. Sulfadiazine

2. Sulfamethoxazole

3. Sulfisoxazole

4. Trimethoprim + Sulfamethoxazole

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Aminoglycosides Usually in parenteral form Can cause serious ototoxicity and nephrotoxicity

1. Amikacin (Amikin)2. Gentamicin (Garamycin)3. Kanamycin (Kanamycin)4. Netilmycin (Netromycin)5. Streptomycin 6. Tobramycin (Nebcin)

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Chloramphenicol

Reserved for typhoid fever Causes serious blood disorder

Lincosamides Clindamycin (Dalacin-C) –

Pseudomembranous colitis (antibiotic associated)

Lincomycin (Lincocin)