antibacterial agents - cephalosporins - sar (structure activity relationship )

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Patrick

An Introduction to Medicinal Chemistry 3/e

Chapter 16

ANTIBACTERIAL AGENTS

Part 2: Cephalosporins

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CEPHALOSPORINS

N

S

OAc

CO2H

O

HN H HC

O

R

N

S Me

Me

HN

CO2H

O

C H H

O

R

Penicillins Cephalosporins

Dihydrothiazine ring (system) Thiazolidine ring (system)

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1. Introduction

1. Antibacterial agents which inhibit bacterial

cell wall synthesis.

2. Discovered from a fungal colony in

Sardinian sewer water (1948).

3. Cephalosporin C identified in 1961.

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N

O

HHHN

O

S

CO2H

OC

Me

O

H2N

CO2H

7H

61

2

34

58

7-Aminoadipic side chain

2. Structure of Cephalosporin C

b-Lactam

ring

Dihydrothiazine

ring

N

O

HHH2N S

CO2H

OC

Me

O

7-Aminocephalosporinic acid (7-ACA)

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3. Properties of Cephalosporin C

Disadvantages

• Polar due to the side chain - difficult to isolate and purify

• Low potency (1/1000 the activity of Pen G)- limited to the treatment

of urinary tract infections (UTI) where it is concentrated in the

urine

• Not absorbed orally

Advantages

• Non toxic

• Lower risk of allergic reactions compared to penicillins

• More stable to acid conditions

• More stable to b-lactamases

• Ratio of activity vs. Gram -ve and Gram +ve bacteria is better

Conclusion

• Useful as a lead compound

N

O

HHHN

O

S

CO2H

OC

Me

O

H2N

CO2H

7H

61

2

34

58

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4. Biosynthesis of Cephalosporins

SH

H2N

CO2H

H

CO2H

H2NCys

ValMe

Me

R OH

O

HOC

Me

ON

O

HHHNR

O

S

CO2H

OC

Me

O

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5. SAR of Cephalosporins

Similar to penicillins • The b-lactam ring is crucial to the mechanism

• The carboxylic acid at position 4 is important to binding

• The bicyclic system is important in increasing ring strain

• Stereochemistry is important

• The acetoxy substituent is important to the mechanism

Possible modifications

• 7-Acylamino side chain

• 3-Acetoxymethyl side chain

• Substitution at C-7

N

O

HHHNR

O

S

CO2H

OC

Me

O

7 61

2

34

58

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SAR of Sephalosporins

Possible Modification Sites

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6. Mechanism of Action

N

O

HHHNR

O

S

CO2H

OC

Me

O

7

OH

Ser Enzyme

-CH3CO2-

N

O

HHHNR

O

S

CO2HO

Ser

Enzyme

• The Acetoxy group acts as a good leaving

group and aids the mechanism

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7. Variation of the 7-Acylamino Side Chain

Not possible to generate analogues by fermentation

Not possible to generate analogues by a full synthesis

Restricted to semi-synthetic procedure

• 7-ACA not available by fermentation.

• 7-ACA not available by enzymatic hydrolysis of cephalosporin C.

N

O

HHH2N S

CO2H

OC

Me

O

7-ACA

RCOCl N

O

HHHNR

O

S

CO2H

OC

Me

O

• Generated by a chemical hydrolysis

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7. Variation of the 7-Acylamino Side Chain

Generation of 7-ACA

• Need to hydrolyse a relatively un-reactive secondary amide in

the presence of a labile b-lactam ring

N

S

OAc

CO2SiMe3

O

H HHNR1

O3

7

4

Protecting group

PCl5

Imino chloride

O

HNR1

Cl

ROH

Imino ether

O

HNR1

OR

H2O

-R1CO2H

7-ACA

N

S

OAc

CO2H

O

H HH2N

R2COCl

N

S

OAc

CO2H

O

H HHNR2

O

Range of Cephalosporins

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8. First Generation Cephalosporins

Cephalothin N

O

HHHN S

CO2H

OAcOS

7

3

• First generation cephalosporin

• More active than penicillin G vs. some Gram -ve bacteria

• Less likely to cause allergic reactions

• Useful vs. penicillinase producing strains of Staphylococcus aureus

• Not active vs. Pseudonomas aeruginosa

• Poorly absorbed from GIT

• Administered by injection

• Metabolised to give a free 3-hydroxymethyl group

(deacetylation)

• Metabolite is less active

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Cephalothin - drug metabolism

Strategy

• Replace the acetoxy group with a metabolically stable leaving

group

N

O

HHHN S

CO2H

OAcOS

7

3

Metabolism N

O

HHHN S

CO2H

OHOS

Less active

OH is a poorer leaving group


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Cephaloridine

• The pyridine ring is stable to metabolism

• The pyridine ring is a good leaving group

(Neutralization of charge)

• Exists as a zwitterion and is soluble in water

• Poorly absorbed through the gut wall

• Administered by injection

N

O

HHHN S

CO2

NOS

7

3


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Cefalexin

• The methyl group at position 3 is not a good leaving group

• The methyl group is bad for activity but aids oral absorption -

mechanism unknown

• Cefalexin can be administered orally

• A hydrophilic amino group at the a-carbon of the side chain

helps to compensate for the loss of activity due to the methyl

group

N

O

HHHN S

CO2H

Me

O

H2N H

7

3


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Synthesis of Cephalosporins with a 3-methyl substituent


O

N

HNC

S Me

Me

CO2Me

H

R

O6 2

Toluene/

PTSA S

OH

N

CH2

Me

CO2Me

H

N

CO2Me

S

OH

CH3

CH3

N

S

CO2Me

- HS

N

HCO2Me

OH

CH3

H

CH3

N

S

CO2Me

H

3

CO2Me

Me

MeS

N

O

H2O2

H

H

OH H

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1st Generation Cephalosporins

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Summary


• Generally lower activity than comparable penicillins

• Better range of activity comparable penicillins

• Best activity is against Gram-positive cocci

• Useful against some Gram negative infections

• Useful against Staphylococcus 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 negative

bacteria (presence of more effective β-lactamases)

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9.1 Cephamycins

9. Second Generation Cephalosporins

• Isolated from a culture of Streptomyces clavuligerus

• First β-lactam to be isolated from a bacterial source

• Modifications carried out on the 7-acylamino side chain

N

O

HOMeHN S

CO2H

OO

CNH2

O

HO2C

H2N H

Cephamycin C

The parent

compound

Methoxy Group at C7

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Cefoxitin

• Broader spectrum of activity than most first generation

cephalosporins.

• Greater resistance to β-lactamase enzymes.

• The 7-methoxy group may act as a steric shield.

• The urethane group is stable to metabolism compared to

the ester.

• Introducing a methoxy group to the equivalent position

of penicillins (position 6) eliminates activity.

N

O

HOMeHN S

CO2H

OO

CNH2

O

S

7

3

9.1 Cephamycins


Cephalothin

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Cefuroxime

• Much greater stability against some b-lactamases

• Resistant to esterases due to the urethane group

• Wide spectrum of activity

• Useful against organisms that have gained resistance to penicillin

• Not active against Pseudomonas aeruginosa

• Used clinically against respiratory infections


N

O

HHHN S

CO2H

O

C

O

CNH2

O

O

NO

Me

9.2 Oximinocephalosporins

Zinnat

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Second Generation

Cephalosporins

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• 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. Pseudomonas aeruginosa

• Lack activity vs. Methicillin-resistant Staphylococcus aureus (MRSA) +ve

• Generally reserved for troublesome infections

Oximinocephalosporins

N

O

HHHN S

CO2H

R

C

O

N

S

NO

Me

H2N

CH2OCOMeH

CefotaximeCeftizoxime

N

NNCH2S

O

OH

Me

Ceftriaxone

R

Aminothiazole

ring

10. Third Generation Cephalosporins

Sodium(6R,7R)-3-[(acetyloxy)methyl]-7-[[(Z)-2-(2-aminothiazol-4-yl)-2- (methoxyimino)acetyl]amino]-8-

oxo-5-thia-1-azabicyclo[4.2.0]oct-2-ene-2-carboxylate

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• Injectable cephalosporin

• Excellent activity vs. Pseudomonas aeruginosa and other

Gram -ve bacteria

• Can cross the blood brain barrier

• Used to treat meningitis

10. Third Generation Cephalosporins

N

O

HHHN S

CO2

N

C

O

S

N

NO

H2N

Me CO2HMe

Ceftazidime


(6R,7R,Z)-7-(2-(2-aminothiazol-4-yl)-2-(2-carboxypropan-2-yloxyimino)acetamido)-8-

oxo-3-(pyridinium-1-ylmethyl)-5-thia-1-aza-bicyclo[4.2.0]oct-2-ene-2-carboxylate

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• Zwitterionic compounds

• Enhanced ability to cross the outer membrane of Gram

negative bacteria

• Good affinity for the transpeptidase enzyme

• Low affinity for some β-lactamases

• Active vs. Gram +ve cocci and a broad array of Gram -ve

bacteria

• Active vs. Pseudomonas aeruginosa

11. Fourth Generation Cephalosporins


N

O

HHHN S

CO2H

R

C

O

N

S

NO

Me

H2N

NCH2

Me

Cefipime

CefpiromeNCH2

R

(Cefrom)

1-methylpyrrolidinium-1-yl

5H,6H,7H-cyclopenta[b]pyridin-1-ium

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Cefquinome

tetrahydroquinolinium

derivative

-Resistant to β-lactamase.

-Its zwitterionic rapid penetration across biological membranes,

including porins.

-Has a higher affinity to target penicillin binding proteins.

-Besides zwitterionic, it is also made of β-lactam nucleus, quaternary

ammonium, and aminothiazolyl moiety

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Imidazo-pyridazin

derivative

Cefozopran

5-amino-1,2,4-

thiadiazol-3-ylidene

(-)-1-[[(6R,7R)-7-[2-(5-Amino-1,2,4-thiadiazol-3-yl)glyoxylamido]-2-

carboxy-8-oxo-5-thia-1-azabicyclo[4.2.0]oct-2-en-3-yl]methyl]-1H-

imidazo[1,2-b]pyridazin-4-ium hydroxide inner salt, 7²-(Z)-(O-

methyloxime)

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5th generation:

Ceftobiprole (Zeftera/Zevtera)

• Injectable

• Activity against methicillin-resistant Staphylococcus aureus (mrsu), penicillin-resistant Streptococcus pneumoniae (prsp), Pseudomonas aeruginosa, and Enterococci.

• It was discovered by Basilea Pharmaceutical and was developed by Johnson & Johnson Pharmaceutical Research and Development.

• It has been shown to be statistically non-inferior to the combination of vancomycin and ceftazidime for the treatment of skin and soft tissue infections.

• Ceftobiprole has been granted "fast-track" status from the FDA and is licensed for complicated skin infections.

pyrrolidinyl-3-

pyrrolidinylidene

methyl

5-amino-1,2,4-

thiadiazol-3-ylidene

Hydroxylimine

broad-

spectrum

activity

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Ceftobiprole (Zeftera/Zevtera)

5-Amino-1,2,4-

thiadiazol-3-ylidene Pyrrolidinyl-3-

pyrrolidinylidene methyl

Hydroxylimine

First of a new class of parenteral cephem antibiotics

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Structure–activity relationships for Ceftaroline (adapted

from Zhanel et al.,15 with permission).

Laudano J B J. Antimicrob. Chemother. 2011;66:iii11-iii18

© The Author 2011. Published by Oxford University Press on behalf of the British Society for Antimicrobial Chemotherapy.

All rights reserved. For Permissions, please e-mail: [email protected]

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Chemical structures for ceftaroline fosamil

(prodrug) and ceftaroline (active metabolite).

Biek D et al. J. Antimicrob. Chemother. 2010;65:iv9-iv16

© The Author 2010. Published by Oxford University Press on behalf of the British Society for Antimicrobial

Chemotherapy. All rights reserved. For Permissions, please e-mail: [email protected]

1 ©

Summary Cephalosporins contain a strained β-lactam ring fused to a

dihydrothiazine ring.

In general, first-generation cephalosporins offer advantages over

penicillins in that they have greater stability to acid conditions and

β-lactamases, and have a good ratio of activity against Gram-

positive and Gram-negative bacteria. However, they have poor

oral availability and are generally lower in activity.

Variation of the 7-acylamino side chain alters antimicrobial

activity, whereas variation of the side chain at position 3

predominantly alters the metabolic and pharmacokinetic

properties of the compound. Introduction of a methoxy

substitution at C-7 is possible.

Semisynthetic cephalosporins can be prepared from 7-

aminocephalosporanic acid (7-ACA).

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7-ACA is obtained from the chemical hydrolysis of cephalosporins.

This requires prior activation of the side chain to make it more

reactive than the β-lactam ring.

Deacetylation of cephalosporins occurs metabolically to produce

inactive metabolites. Metabolism can be blocked by replacing the

susceptible acetoxy group with metabolically stable groups.

A methyl substituent at position 3 is good for oral absorption but bad

for activity unless a hydrophilic group is present at the α-position of

the acyl side chain.

3-Methylated cephalosporins can be synthesized from penicillins.

Cephamycins are cephalosporins containing a methoxy group at

position 7.

Oximinocephalosporins have resulted in several generations of

cephalosporins with increased potency and a broader spectrum of

activity, particularly against Gram-negative bacteria.

antibacterial agents - cephalosporins - sar (structure activity relationship )

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