Antibiotics 1

Front 1

Four criteria for good antibiotics

Back 1

target is essential to the microorganism survival
The compound is non-toxic to humans
structure and function of the target is highly conserved across a variety of spp (broad spectrum)
Resistance to inhibitors not easily acquired

Front 2

1930s

Back 2

observation affinity of organic dyes for bacterial cells

Testing of dyes for antibacterial activity in vivo

Front 3

1935

Back 3

prontosil (red dye) antibacterial activity (in vivo)

Structure

Front 4

Sulfanilamide

Back 4

first synthetic antibacterial (G+’ve bacteria)

Front 5

Protonsil metabolised to

Back 5

Sulphanilamide and 1,2,4-triaminobenzene

Structure

Front 6

Why not stat with primary amine than acetylate early and then deacetylate

Back 6

Anilne primary amine will be protonated by chlorosulphonic acid thus become meta directing instead of para
The bulky acetanilide amide group is para directing because of steric hindrance. Mechanism

Front 7

Sulpohnamide structure activity relationship

Back 7

Para substituted to mirror PABA
para amine & benzene for activity
Amide less active that sulfonamide
Monosubstitution on sulfonamide may increase activity and Disubstitution abolishes activity

Front 8

Sulpohnamide pH

Back 8

lipophilicity determines their ability to get inside the bacterial cell
ionization at physiological pH to resemble PABA for better incorporated into enzyme for inhibition

Front 9

Sulpohnamide pKa and activity

Back 9

Amine pKa around 4.7 in both Sulfonamides & PABA
Carboxylic acid pKa in PABA is 6.5, while the Sulfonamide has a pKa around 10.4

Front 10

pka

Back 10

pKa is the negative base-10 logarithm of the acid dissociation constant of a solution. pKa = -log10Ka. The lower the pKa value, the stronger the acid.
Suphonamide active when ionised
pH at which acid or base is 50% ionised

Front 11

Sulphonamide active at pka

Back 11

6.4 to 7.4 Groups attached to N are electron withdrawing which spread the electron density of sulphonamides (pka 10.4) thus lowering pka on the amine in order to move their pka toward 6.4

Front 12

Sulphonamide requirements for activity

Back 12

Sulphonamide skeleton to resemble PABA
S directly attached to benzene and para to free amine (substituted - get inactivated)
Substitution of -SO3 destroys activity.
Nitrogen can only be primary or secondary Heterocyclic substituents enhance activity

Front 13

Sulphonamide MOA

Back 13

Sulphonamides are folate antagonists
Competitive inhibitor of dihydropteroate synthetase (DHPS)
Inhibits activation of folate to its active form, tetrahydrofolate.

Front 14

Why Sulphonamides do not affect humans

Back 14

Bacteria synthesize folate - form cofactors for purine, pyrimidine and amino acid synthesis
Mammals acquire folic acid from diet so they are unaffected

Front 15

Sulphonamide Resistance

Back 15

increasing the concentration of PABA
change in the enzyme structure to better distinguish between PABA and the drug.
uptake Folic acid from their surroundings
Decrease sulphonamide permeability
Destroy sulphonamide
Production of altered dihydrofolate reductase

Front 16

Sulphonamide uses

Back 16

treatment of acute, uncomplicated urinary-tract infections, particularly those caused by Escherichia coli. GIT infection
Eye lotions
Infections of mucous membranes

Front 17

Sulphonamide Adverse affects

Back 17

Crystalluria, kidney damage, Stevens-Johnson
allergic reactions, especially rashes, photosensitivity and fever.
Hemolytic anemia (G6PDH deficient pts.
neutropenia and thrombocytopenia

Front 18

Early Sulphonamide Side effect

Back 18

Earlier solutions were to increase urine flow

increase the pH of urine to around 10 using sodium bicarbonate

Front 19

Suphonamide Drug interactions

Back 19

Warfarin, phenytoin, methotrexate.

Front 20

nalidixic acid

Back 20

First quinolone Not technically a quinolone
Discovered as a by-product of chloroquine synthesis
G –ve bacteria
Commonly used for UTI

Front 21

Quinolone MOA

Back 21

Inhibition of bacterial topoisomerase IV (G+) and topoisomerase II (G- DNA gyrase) (Not present in humans) leading to inhibition of replication and transcription

Front 22

Quinolone Resistance

Back 22

– DNA Gyrase mutations
– Cellular membrane efflux mechanisms.
– Decreased number of porins in target cells

Front 23

Quinolone distribution

Back 23

Wide distribution - CSF, saliva, bone, cartilage

Front 24

Quinolone SAR

Back 24

Ketone (reduction inactivates) & -COOH DNA binding
C2 enzyme substrate complexation (substitution inactivate

Front 25

Quinolone F at C 6

Back 25

Increase lipophilicity and bacterial cell wall penetration and increased DNA gyrase binding because F is lipophilic

Front 26

Quinolone F at C 8 (Halogen)

Back 26

Increase drug absorption and half life
Unfortunately increase drug induced photo sensitivity via singlet oxygen and radical induction (Lomefloxacin) Methoxy at 8 reduce photo sensitivity (Gatifloxacin)

Front 27

Quinolone Substitution at C 7

Back 27

Increase spectrum of activity against G -ve
Piperazine (ciprofloxacin) increase binding to GABA leading to CNS S/E
Adding bulky groups at position 1 reduce GABA binding (Sparfloxacin)

Front 28

Quinolone Substitution at 1 (N)

Back 28

Broadens spectrum of activity against atypical bacteria ( cyclopropyl ) Mycoplasma, clamedia and legionella species e.g Ofloxacin
Cyclopropyl substituent considered most potent substituent here

Front 29

Quinolone C3 asymmetric C isomer

Back 29

S (-) isomer (levofloxacin) is twice active as Ofloxacin and 8-128 fold potent than R (+) isomer due to increased binding to DNA gyrase

Front 30

Position 3 and 4

Back 30

considered essential for DNA binding

Very few bioisosteric replacements are successful at these positions

Front 31

Position 7

Back 31

interacts directly with DNA gyrase and topoisomerase IV
Optimum substituents contain 5 or 6 membered heterocyclic rings e.g. Piperazinyl moeity
Methylation of piperazine increases log P

Front 32

1928 Fleming discovered

Back 32

antibacterial activity of fungus (Penicillium sp.)

Front 33

1938 Florey and Chain

Back 33

isolated penicillin

Front 34

1945 Dorothy Hodgkin

Back 34

established structure by X-ray crystallography

Front 35

1957 Sheehan

Back 35

completed first total synthesis

Front 36

1958 Beechams

Back 36

isolated 6-aminopenicillinic acid (6-APA, precursor to penicillins)

Front 37

1976 Beechams

Back 37

discovered clavulanic acid – reversal of penicillin resistance

Front 38

Penicillin

Back 38

dipeptide analogs of D-ala-D-ala
inhibition of PBPs 1a & 1b and PBP3 that function as transpeptidases in the biosynthesis of the peptidoglycan

Front 39

PBP

Back 39

Penicillin Binding Protein

Front 40

Penicillin Mechanism of Action

Back 40

Inhibition is result of acylation of the enzyme by attack of serine on the β-lactam ring
β-Lactams acylate the hydroxyl group on the serine residue of PBP active site in an irreversible manner.

Front 41

Why Beta lactum ring is very reactive

Back 41

The amide of the β-lactam ring is unusually reactive due to ring strain and a conformational arrangement which does not allow the lone pair of the nitrogen to interact with the double bond of the carbonyl.
Bicyclic system increases b-lactam ring strain

Front 42

Penicillin Structure Activity Relationship

Back 42

Amide and carboxylic acid are involved in binding
Carboxylic acid binds as the carboxylate ion
Variations are limited to the side chain (R)
β-lactam ring is an essential portion of pharmacophore its hydrolysis deactivates
Bicyclic system increases b-lactam ring strain

Front 43

Beta lactum hydrolysis due to

Back 43

β-lactamase, Alcohols and amines PRODUCING corresponding esters and amides.

Front 44

Penicillin Spectrum of activity

Back 44

Hydrophobic side chains favours activity against G+ bacteria
Increased hydrophobic character decreased G- activity, no effect on G+
Hydrophilic groups have no effect or reduction of activity on G+ but increased activity against G- bacteria

Front 45

Penicilin best activity

Back 45

NH2, OH, COOH alpha to carbonyl

Front 46

Penicilin hydrolysis reduced by

Back 46

Addition of EWGs - Reduced nucleophilic character of carbonyl e.g phenoxymethyl penicillin
Steric shields can be added to protect penicillins from b-lactamases

Front 47

Interaction of nitrogen’s lone pair with the carbonyl group is not possible Structure

Back 47

the bond angles are 90o instead of 120o, thus 30o ring strain prevents the overlap of the nitrogen lone
pair with the adjacent carbonyl system. Ring strain prevents the normal amide resonance from taking place in b-lactam rings

Front 48

penicillins and cephalosporins is very susceptible to ring-opening under strongly
acidic and basic conditions

Back 48

b-lactam carbonyl is much more
electrophilic than a normal amide - susceptible to nucleophilic attack. Nitrogen is readily protonated under acidic conditions, making the b-lactam ring sensitive to strong acids.

Front 49

Structural features essential for activity in penicillins

Back 49

The bicyclic ring system containing the b-lactam is crucial, as are the cis relationship
between the two hydrogens at positions 5 and 6, a free 3-carboxylate and a 6-amide.Changing one or more of these results in a loss of activity

Front 50

Synthesis of penicillin analogues

Back 50

fermentation process, in which different carboxylic acid derivatives were added to yield penicillins containing different 6-amido derivatives
relies on the isolation of 6-aminopenicillanic acid (6-
APA) from fermentation media. Use acyl chlorides.

Front 51

Penicillin analogue synthesis by Fermentation limitations

Back 51

not all carboxylic acids were biologically acceptable and thus only a limited number
of analogues could be prepared

Front 52

Penicilinase resistance mechanisms

Back 52

•Decreased cellular uptake of pen
•Lower binding affinity of pen to PBPa
•Production of β-lactamases by bacteria

Front 53

Overcome Penicillin resistance

Back 53

Use of steric shields - introducing bulky groups to the side chain

Front 54

How does introduction of bulky group affect activity of penicillin?

Back 54

The additional hydrophilic groups make penetration of the gut wall difficult - infections of GI by H. pylori
administered intravenously or as a prodrug.
aminopenicillins increased activity against Gram (-) bacteria by way of the import porins

Front 55

Effect Metal ions on penicillin activity

Back 55

mercury, zinc, and copper, catalyse the
degradation of penicillin's
Use plastic containers

Front 56

Effect of substituents on aromatic ring attached to the side chain amide (Structures)

Back 56

If ring has methoxy groups at para and ortho B-lactamase stability results e.g. Methicillin IV coz its acid sensitive
If one of the methoxy groups is replace by H B-sensitivity occurs
If methoxy is put between ring and penicillin B-sensitivity occurs (Phenoxymethyl penicillin)

Front 57

Beta lactum sensitive penicillins

Back 57

Benzyl penicillin Gram positive) Benz-CH2-
Phenoxymethyl penicillin Benz-O-CH2-
Ampicillin (Gram +ve and -ve) Benz-CH2(NH2)-
Amoxycillin HO-Benz-CH2(NH2)-

Front 58

Ampicillin

Back 58

Benz-CH2(NH2)- Gram -ve eff greater penetration of gram -ve bacteria due NH2 protonation thus polar but not affect gram +ve effect
NH2 get protonated thus EWD and acid stable B-lactum sensitive.

Front 59

Amoxycillin

Back 59

HO-Benz-CH2(NH2)- The -OH adjust isoelectric point to more acidic value thus enhanced intestine absorption by peptide transporter transporter than Ampicllin thus less drug induced diarrhoea coz less GI flora disturbance.

Front 60

Clavulanic acid

Back 60

Combined with amoxy to prevent B-lactamases
2 rings S replaced by O and 2 -CH3 replced by
=CH-CH2OH

Front 61

Benzyl penicillin

Back 61

Benz-CH2- More Gram +ve because benzene ring is hydrophobic. IM injection prolonged sustained release. More acid sensitive

Front 62

Phenoxymethyl penicillin (Pen V)

Back 62

Benz-O-CH2- More acid stable than Benzyl penicillin because of the electronegative oxygen atom in C7 amide chain inhibiting Beta lactam hydrolysis thus used orally. Has the same spectrum as benzyl penicillin (Gram -ve)

Front 63

Methicillin

Back 63

2(H3C-O meta-ortho )-Benz- Acid sensitive but Beta lactamase stable. Narrow spectrum - Beta lactamase producing staphylococcus. Administered IV
Resistance - reduced uptake and altered PBP e.g. PBP2. Use Vancomycin, cotri or rifampicin

Front 64

Nafacillin

Back 64

2 benzene rings fused at meta-ortho side and -CH2CH3 at other meta side of ring
Same spectrum as Methicin but more acid stable and also beta lactamase stable.

Front 65

Penicillin Pro-drugs mechanism

Back 65

Ester groups attached to enhance absorption of the dipolar penicillin. The ester is hydrolysed after absorption. Ester attached to the -COOH below the penicillin molecule e.g. Talampicillin

Front 66

Gram -ve bacteria resistance

Back 66

They have a lipopolysaccharide outer membrane which prevent access to the cell wall
Penicillins only cross at porins in the outer membrane.