Chapter 8 Antibiotics Wei-Min Chen, Prof. Department of Medicinal Chemistry, JNU
Topics in This Class • Antibiotics • Definition, Classification, Development, and Action mechanism of antibiotics. • β–Lactam antibiotics • Penicillins • Benzylpenicillin sodium, Amoxicillin, Oxacillin sodium • Semi-synthetic penicillins Key issues: β–Lactam Antibiotics and Penicillins Tough issues: Actionmechanisms of antibiotics and Resistance
HISTORICAL BACKGROUND • Since 1940s, antibiotics have transformed medicine; most infectious diseases can be effectively controlled by the appropriate use of the correct drugs. • plague(瘟疫) • typhus(伤寒) • cholera(霍乱) • tuberculosis(肺结核)
1928 Alexander Fleming, Inhibition of S. aureus colonies by mold Penicillium notatum; Discovered "miracle drug" Penicillin
Some Terms • Antibacterial Agents (Antimicrobial agents) The drugs having bacteriostatic and bactericidal activities for pathogenic microorganism，used to treat pathogenic bacterial infections . • “bacteriostatic ’’ agents They can inhibit bacterial cell growth only，but do not actively kill the cells. • “bactericidal’’ agents They can kill bacterial cells，but are nontoxic to human cells .
Classification of Antimicrobial agents • Antibiotics: (to be modeled after natural products) • Synthetic antibacterial agents (Chapter 9) • Sulfonamides,（Antibacterial synergists） • Quinolone antibacterial agents • Nitrofurantoin antibacterial agents • Oxazolidiones • Antifungal agents • Taberculostatics
Definition of Antibiotics • Antibiotic (Literal definition): Against (anti-) life (-biotic); • Antibiotic (Old definition): Chemical substance produced by various species of microorganisms that is capable, in low concentrations, of inhibiting the growth of or killing other microorganisms (Proposed by Waksman in 1942)
Definition of Antibiotics • Antibiotic (New definition): Substance produced by a microorganism or a similar product produced wholly (synthetic) or partially (semi-synthetic) by chemical synthesis and in low concentrations inhibits the growth of or kills microorganisms
If a substance is classified as an antibiotic • It is a product of metabolism (although it may be duplicated or even have been anticipated by chemical synthesis). • It is a synthetic product produced as a structural analogue of a naturally occurring antibiotic. • It antagonizes the growth or survival of one or more species of microorganisms. • It is effective in low concentrations.
Classification of Antibiotics (According their structures) 1.β-Lactam antibiotics (β –内酰胺类) 2.Tetracyclines antibiotics (四环素类) 3. Aminoglycoside antibiotics (氨基糖甙类) 4. Macrolides antibiotics (大环内酯类) 5. Others (Chloramphenicol, 氯霉素)
Action Mechanisms of Antibiotic • Inhibition of Cell Wall Synthesis (most common mechanism) →(β –Lactam) • Inhibition of Protein Synthesis (Translation) (second largest class) →(Aminoglycosides, Tetracyclines, Erythromycins, Chloramphenicol) • Alteration of Cell Membranes →(Amphotericin B) • Inhibition of Nucleic Acid Synthesis or action on DNA and/or RNA →(Actinomycin, Rifampin)
Selective toxicity • Cell wall synthesis inhibitors: Human cell don't have cell walls • Protein synthesis inhibitors: • Target 30S or 50S ribosomal structures that bacteria have, that we don't have • Target bacterial protein synthesis that is occurring at a much faster rate than in mammalian cells • Actively taken up into bacteria cells-not ours
1.β –Lactam Antibiotics Penicillins Cephalosporins Monobactams Carbapenems β-Lactamase Inhibitors
1.β–Lactam Antibiotics • These all share a common β-lactam ring. The ring is very strained and the bond between the carbonyl and the nitrogen in the β-lactam ring is very labile and hence makes the molecule reactive.
The R-group substitute of the penicillin nucleus can be changed to give the molecule different antibacterial properties. The two naturally occurring penicillins from Penicillium notatum are Penicillin G, [Benzyl penicillin, R = C6H6] and Penicillin V, [Phenoxymethyl penicillin, R = CH2O(C6H6)]
The β-lactam structure is derived from two covalently bonded amino acid residues;cysteine and valine. This forms via a tripeptide intermediate where the third amino acid is replaced by the variable R-group.
Mechanism of Action D-Alanyl-D-Alanin Penicillins
The Cell Wall Synthesis transpeptidase
Mechanism of Action 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. The labile β-lactam ring in penicillin reacts with a serine residue in the transpeptidase. This reaction is irreversible and so the growth of the bacterial cell wall is inhibited.
Penicillin G and V are only active against Gram Positive bacterial cells, which have an exposed layer of peptidoglycan around the outside of the cell wall, as shown below. Gram Negative bacteria have a more complicated composition, which Penicillin G and V can not destroy .
All β–lactam antibiotics owe their activity to their ability to act as irreversible inhibitors of the D-alanyl-D-alanine carboxypeptidase / Transpeptidase. These enzymes are collectively known asPenicllin-Binding Poteins, PBPs
glycopeptide PBPs peptidoglycan strands -Lactam Cell Wall
Clinical use • β-lactam antibiotics are indicated for the prophylaxis and treatment of bacterial infections caused by susceptible organisms. At first, β-lactam antibiotics were mainly active only against Gram-positive bacteria, yet the recent development of broad-spectrum β-lactam antibiotics active against various Gram-negative organisms has increased their usefulness.
Benzylpenicillin Sodium • Chemical name： （2S，5R，6R）-3，3-dimethyl-6-(2-benzylacetamido)-7-oxo-4-thia-1-azabicyclo[3.2.0]heptane-2-carboxylic acid • Penicillin G
Penicillins nucleus consists of the β–lactam ring fused through a nitrogen atom and the adjacent tetrahedral carbon atom to a 5-member thiazolidine ring. Both two rings are strained. In addition, the carbonyl group in benzylpenicillin can’t form conjugate structure with the unshared electron pair of nitrogen, so it’s not a normal amide. it can be easily attacked by nucleophilic agent.
Resistance: Penicillin G is a typical example of compounds that inhibit the action of DD-transpeptidases, but it is also a substrate for β–lactamases.
The disadvantages of Penicillins (Penicillin G) • Narrower in antibacterial spectrum, active for G-P only, poor active for G-N. • Can not be oral administrated , just for injection. • Allergic action (frequency : 0.7–10%). • Susceptible to acid and Penicillinase. • Produces Penicllins-resistant bacteria.
Oxacillin Sodium • （2S，5R，6R）-3，3-Dimethyl-6-（5-methyl-3-phenyl-4-isooxazoleformamide）-7-oxo-4-thia-1-azabicyclo[3. 2.0]heptane-2-carboxylic acid sodium monohydrate • Oxacillin was designed and discovered according to bioisosterism from the Methicillin as lead compound.
Amoxicillin • (2S,5R,6R)-3,3-Dimethyl-6-[(R)-(-)-2-amino-2-(4-hydroxyphenyl)acetamido]-7-oxo-4-thia-1-azabicyclo[3.2.0.]heptane-2-carboxylic acid trihydrate
SAR of Penicillins The presence of a carboxy group is a requirement for PBP recognition. When esterition of it, it behaves a pro-drug The bioavailability will be raisen.
SAR of Penicillins Three chiral centers are requirement for Penicillins bioactivity
SAR of Penicillins Side chain can be replaced with different R group to obtain different compounds With broad antibacterial spectrum