Antibiotics II: The Beta-Lactam Antibiotics

Question 1

Penicillin Timeline

Answer 1

• First discovered in 1928 by Fleming
• Isolated by Florey and Chain in 1939
• The first clinical trial in humans was carried out in 1941
• Mass production achieved in 1944
• Crystal structure determined using Xrays in 1945
• Total synthesis of penicillin V by Sheahan in 1957
• 6-APA isolated in 1959
• Clavulanic acid discovered in 1976

Question 2

Penicillin Structure and SAR essentials

Answer 2

• Amide bond essential
• Stereochemistry essential
• Free acid essential
• Lactam essential
• Bicyclic structure essential
• Acyl side chain
• Thiazolidine ring
• B-Lactam ring

Question 3

B-Lactam Mode of Action

Answer 3

• Irreversible covalent bonding to penicillin binding proteins (PBPs), enzymes that catalyse the final steps of peptidoglycan synthesis
• Bacteria usually have ~5 types of PBPs, each having different functions essential for cell viability
• The B-lactams acylate the active site serine residue of PBPs to inhibit transpeptidation
• PBPs have differing affinities for -lactams
• Optimum antibacterial effect is achieved through binding to multiple types of PBPs
• Resitance can be mediated by mutated PBPs e.g. PBP2a

Question 4

Penicillin G

Answer 4

• Essentially non-toxic, active against Gram +ve cocci and Gram –ve bacteria but not broad spectrum
• Allergies observed, not orally active, acid sensitive and destroyed by B-lactamases

Question 5

Why is Penicillin G Acid Sensitive?

Answer 5

• The B-lactam is extremely reactive, unlike tertiary amide
• Acid-catalysed ring opening relieves the ring strain imposed by fusing a 4-membered to a 5-membered ring
• Participation from the side chain opens the lactam

Question 6

Penicillin V

Answer 6

• Side chain is less likely to react with the B-lactam ring due to electronic effects
• Acid stable so can be taken orally but now sensitive to B-lactamases
• Less potent that penicillin G
• Acid stable, orally active, less potent that penicillin G

Question 7

Resistance: B-Lactamase

Answer 7

• Penicillin resistance was first observed in 1940s
• > 90% of S. aureus produced B-lactamases by 1960s
• Resistance is transferable between strains
• Gram –ve bacteria contain B-lactamase within the periplasmic space surrounding the cell
• Design lactamase-resistant derivatives by attaching bulky groups that prevent access to the enzyme active site
• Care must be taken not to prevent the drug from accessing the target transpeptidase
• Such B-lactamase resistant penicillins have little or no activity against Gram –ve bacteria

Question 8

Methicillin

Answer 8

• Resistant to B-lactamases but acid sensitive so not orally active
• Weakly active against Gram +ve bacteria but not active against Gram –ve bacteria, also MRSA

Question 9

Which has a higher bioavailability

Answer 9

Flucloxacillin has a higher bioavailability

Question 10

Clavulanic Acid

Answer 10

• Suicide inhibitor of B-lactamases, bonds covalently to the lactamase enzyme and inactivates it
• Combined with amoxycillin - Augmentin

Question 11

Cephalosporin Structure and SAR essentials

Answer 11

• Amide bond essential
• Stereochemistry essential
• Bicyclic structure essential
• Free acid essential
• Lactam essential
• Acyl side chain
• DIhydrothiazine ring
• B-lactam ring

Question 12

Cephalosporin C

Answer 12

• Active against Gram +ve and Gram –ve bacteria but very low potency and not orally active, difficult to isolate and purify
• Essentially non-toxic, low risk of allergies
• Relatively stable to acid and B-lactamase

Question 13

Cephalosporin Derivatives

Answer 13

• Since the initial isolation of cephalosporin C a number of semi-synthetic cephalosporins have been developed
• 7-ACA not readily available by biotechnology or by chemical synthesis
• Semi-synthetic derivatives developed in ‘generations’
• Typically broader spectrum than penicillins
• Effective against many resistant strains of bacteria
• Much more expensive to produce
• Many cephalosporins are reserved for hospital use only

Question 14

1st Generation Cephalosporins

Answer 14

• Generally lower activity than comparable penicillins
• Better range of activity than comparable penicillins
• Best activity is against Gram +ve cocci
• Useful against some Gram -ve infections
• Useful against S. aureus and streptococcal infections when penicillins have to be avoided
• Poorly absorbed across the gut wall (except for 3-methyl substituted cephalosporins)
• Most are administered by injection
• Resistance has appeared amongst Gram -ve bacteria (presence of more effective B-lactamases)

Question 15

1st Generation: Cephalothin

Answer 15

• More active than Pen G against some Gram -ve bacteria
• Less likely to cause allergic reactions
• Useful against penicillinase-producing strains of S. aureus
• Poorly absorbed from GIT, administered by injection
• 3-hydroxymethyl metabolite is less active

Question 16

1st Generation: Cephalexin

Answer 16

• Methyl group not good for activity but aids oral activity and molecule is absorbed through GIT
• Hydrophilic amino group compensates for loss of activity due to the 3-methyl substituent
• Greater activity against Gram –ve bacteria

Question 17

2nd Generation: Cefoxitin

Answer 17

• Broader spectrum of activity and greater resistance to lactamase enzymes than most 1st generation cephalosporins
• The 7-methoxy group may act as a steric shield
• Urethane group is stable to metabolism
• Introducing a methoxy group to the equivalent position of penicillins (position 6) eliminates activity.

Question 18

2nd Generation: Cefuroxime

Answer 18

• Resistant to esterases due to the urethane group
• Wide spectrum of activity, including organisms that have
gained resistance to penicillin
• Used clinically against respiratory infections

Question 19

3rd Generation Cephalosporins

Answer 19

• Aminothiazole ring enhances penetration of cephalosporins across the outer membrane of Gram -ve bacteria
• May also increase affinity for the transpeptidase enzyme
• Good activity against Gram -ve bacteria, variable activity
against Gram +ve cocci, variable activity vs. P. aeruginosa • Lack activity against MRSA
• Generally reserved for other troublesome infections

Question 20

Carbapenems

Answer 20

• Thienamycin, imipenem, ertapenem
• Bind to PBP2 to cause cell lysis
• Broad spectrum of activity, including anaerobes and Gram –ve nosocomial infections
• Highly resistant to B-lactamases
• Poor acid stability, much less that Penicillin G, and solutions are not stable
• Some allergic reactions observed

Stereochemistry different to that in penicillins and cephalosporins