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Antimicrobial agents Cotrimoxazole and Penicillin
1. Antimicrobial agents:
Specific Drugs
Dr. Pravin Prasad
M.B.B.S., MD Clinical Pharmacology
Lecturer, Lumbini Medical College
20 February, 2019 (8 Falgun 2075), Wednesday
2. By the end of the class BSN 1st year
students will be able to:
Classify AMAs that act by inhibiting:
Folate synthesis
Cell wall synthesis
Explain the mechanism of action of cotrimoxazole
and penicillin
List the uses and adverse effect of cotrimoxazole
and penicillin
3. Folate synthesis inhibitors AMAs
First effective AMA to be used systematically in
man
Limited role because of:
High toxicity
Development of resistance drugs
Availability of safer drugs
Primarily bacteriostatic
Acts on both gram positive and gram negative
4. Folate synthesis inhibitors
Classification
Short acting Sulfixazole, sulfadiazine
Intermediate acting Sulfamethoxazole
Long acting Sulfamethoxypyridazine,
sulfadoxine
Special purpose drugs Sulfasalazine, silver
sulfadiazine
6. Folate synthesis inhibitors
Adverse effects:
Hypersensitivity reaction
Haemolytic anaemia
Kernicterus
•Contraindicated: Pregnancy, infant
Kidney damage (precipitated in acidic urine)
•Can be avoided by drinking plenty of water and
alkalinizing the urine
7. Folate synthesis inhibitors:
Cotrimoxazole
Combination of trimethoprim and
sulfamethoxazole
Less chance of resistance if used in combination
Individual drugs- bacteriostatic
•Combination- bactericidal
Ratio of drugs 1:5 (T:S)
8. Folate synthesis inhibitors:
Cotrimoxazole
Sequential block in the
formation of
tetrahydrofolic acid
Required for nucleic
acid synthesis
Resistance if
dihydrofolate reductase
enzyme is altered
p-aminobenzoic acid
Dihydrofolic acid
Tetrahydrofolic acid
Dihydropteroic
synthase
Dihydrofolate
reductase
S
T
13. Penicillin: Mechanism of Action
Enters gram positive
bacteria through porin
channels
Binds to
transpeptidase enzyme
(Penicillin binding
proteins, PBPs)
Gets inhibited
14. Penicillin: Mechanism of Action
Transpeptidation
reaction cannot occur
No cross linking of
polymers
Weak cell wall formed
Lysis of cell occurs
15. Penicillin: Mechanism of Resistance
Resistance develops by:
Change in porin structure
Altered structure of PBPs
Production of penicillinase (beta lactamase)
•Thus combined with beta lactamase inhibitors
like clavulanic acid, sulbactam, tazobactam