mohammed nooraldeen al-qattan (phd) cephalosporins · •cephalosporin analogues can not be...

Cephalosporins

Mohammed Nooraldeen Al-Qattan (PhD)

Brief-HistoryBrief-History

• In early 1940s, a fungus was discovered which grows in sewer waters and produces compounds which cause death to bacteria

• In 1948, extracts of the fungus Cephalosporium acremonium(Acremonium chrysogenum) were shown to contain:- Cephalosporin P1 (a steroid with minimum antibacterial activity- Cephalosporin N (Penicillin derivative)- Cephalosporin C

• In 1961, The structure of cephalosporin C was established by X-ray crystallography.

Cephalosporin P1

Brief-History (Cont.)Brief-History (Cont.)

• The structure of penicillin N was discovered to be D-(4- amino-4-carboxybutyl) penicillanic acid

• The amino acid side chain ↑ Gram-ve (specially Salmonella spp) & ↓ Gram+ve

• The interest in Cephalosporin C was started after discovering the possible removal of α-aminoadipoyl side chain

Cephalosporin C

Adipic acid

Strucutral features of Cephalosporin CStrucutral features of Cephalosporin C

• The structure of cephalosporin C is similar to that of penicillin, however with 4-membered β-lactam ring fused to 6-membered dihydrothiazine ring.

• Cephalosporin C is less potent than penicillin G (1/1000), however it has equalactivity toward Gram+ve and Gram-ve bacteria.

• Cephalosporins have lower strainwithin the molecule therefore, more stable toward acids, β-lactamases and lessallergenic.

Biosynthesis of CephalosporinsBiosynthesis of Cephalosporins

• Cephalosporin are also biosynthesized from Cys and Val amino acids.

Synthesis of Cephalosporin analogues Synthesis of Cephalosporin analogues

• Cephalosporin analogues can not be obtained by fermentation

• Unlike penicillins, the intermediate for semi-synthesis of 7-aminocephalosporinic acid (7-ACA) can not be obtained by fermentation nor by enzymatic hydrolysis of cephalosporin C

• The chemical hydrolysis of cephalosporin C to 7-ACA is difficult due to the need to hydrolyze 2° amine in presence of reactive β-lactam ring.

• Chemical hydrolysis of Cephalosporin C involve:

1) Formation of imino chloride

2) Conversion of imino chloride by alcohol to imino ether

3) Hydrolysis of imino ether by aqueous acid to 7-ACA-

Synthesis of Cephalosporin analogues Synthesis of Cephalosporin analogues

Trimethylsilyl

Synthesis of 3-methylated cephalosporins from penicillinSynthesis of 3-methylated cephalosporins from penicillin

Synthesis of Cephalosporin analogues (Cont.)Synthesis of Cephalosporin analogues (Cont.)

• The semisynthetic Cephalosporins offer the following advantages

1) Increased acid stability

2) Improved pharmacokinetic properties (e.g. ↑ oral absorption)

3) Broaden spectrum of activity & increase activity against resisitant strains

4) Decreased allegrenicity

5) Increased tolerance after parenteral administration

Degradation pathways for Cephalosporins

Degradation pathways for Cephalosporins

• The pathway depends on structure of Cephalosporin

• 3-acetoxylmethyl group is most reactive (nucleophilic displacement, acid solvolysis)

• The lactonize products are inactive.

• The broken β-lactam products are inactive.

Degradation pathways for Cephalosporins(Cont.)

Degradation pathways for Cephalosporins(Cont.)

Degradation pathways for Cephalosporins (Cont.)Degradation pathways for Cephalosporins (Cont.)

Fast release

Slow release

Acid stability and oral bio-availability of CephalosporinsAcid stability and oral bio-availability of Cephalosporins

• The phenylglycyl substitution at C7 - NH2 group at 7-acylamino side chain ↑ stability of lactam ring- Carrier mediated transport of these dipeptide-like & zwitterionic compound

• Absence of leaving group at C3- At low pH 3-Acetoxy group (7-ACA) 3-CH2-OH (7-ADCA) lactonizationinactivation.

• Esterification of C3-carboxylic acid prodrug, acid-stable, lipophilic

7-aminocephalosporinic acid (7-ACA)7-aminodeacetyloxycephalosporanic acid (7-ADCA)

Acid stability and oral bio-availability of Cephalosporins (Cont.)Acid stability and oral bio-availability of Cephalosporins (Cont.)

CH3 group in ethanoate ester can be replaced by NH2 group which provides:

Similar valency

Similar Size

Different electronic properties feed electrons to C=O ↓hydrolysis

Spectrum of activity for CephalosporinsSpectrum of activity for Cephalosporins

• Are broad-spectrum like penicillins analogues

• More resistant to β-lactamases than Ampicillin, and is related to:

1) Source of β-lactamases.

2) Mainly due to cephem ring system rather than C7-acyl group

3) Acyl group of β-lactamases resistant penicillin ↓ S. aureus & Gram+ve bacteria for

4) ↑ by steric and electronic effect of C7-acyl-Cα-alkoximino group

5) ↑ by 7α-OCH3 group

6) ↑ in Cephalothin and Cefoxitin

7) ↓ in Cephaloridine and Cefazolin

• Less active against non- β-lactamase Gram+ve producing bacteria than Ampicillin.

• More active against Klebsiella sp.

Transient inhibition of β-lactamases

Transient inhibition of β-lactamases

This property only available for cephalosporins having leaving group at C3 (e.g. Cephalothin and Cefoxitin)

Imin-ene conjugate

SAR for CephalosporinsSAR for Cephalosporins

β-lactam ring strain is

essential

β-lactam ring strain is

essential

Carboxylic groupIonizableCarboxylic groupIonizable

Bicyclic system strain is less than penicillinBicyclic system strain is less than penicillin

Acetyloxy group is good leaving group ↓ stability of β-lactam ring

Affects pharmacokineticsIf Replaced with CH3 ↑absorption &

↓activity (unless accompanied with Cα-hydrophilic group)

Acetyloxy group is good leaving group ↓ stability of β-lactam ring

Affects pharmacokineticsIf Replaced with CH3 ↑absorption &

↓activity (unless accompanied with Cα-hydrophilic group)

Stereochemistryis essential

Stereochemistryis essential

C7-OCH3Β-lactamase stable

C7-OCH3Β-lactamase stable

O=OxacephemC=CarbacephemO=Oxacephem

C=Carbacephem

eneSaturation inactiveeneSaturation inactive

Acylamido side chain is essentialAcylamido side

chain is essential

Lipophilic Ar sub ↑Gram+ve &

↓Gram-ve

Lipophilic Ar sub ↑Gram+ve &

↓Gram-ve

SAR for Cephalosporins (Cont.)SAR for Cephalosporins (Cont.)

Affects pharmacokinetics & pharmacodynamics1) Benzoyl ester ↑Gram+ve & ↓Gram-ve2) Pyridine, imidazole (no ester) Azide ion (N=N=N) ↓Gram-ve3) Aromatic thiol (no ester)↑Gram-ve4) CH3 and Cl (no ester) orally active 5) N-Methyl-Thiotetrazole (no ester) blood coagul. Prob.

Affects pharmacokinetics & pharmacodynamics1) Benzoyl ester ↑Gram+ve & ↓Gram-ve2) Pyridine, imidazole (no ester) Azide ion (N=N=N) ↓Gram-ve3) Aromatic thiol (no ester)↑Gram-ve4) CH3 and Cl (no ester) orally active 5) N-Methyl-Thiotetrazole (no ester) blood coagul. Prob.

Substitutions affects pharmacodynamics α & α1-NH2 1) basic protonated at

acidic conditions acid stable

2) β-lactamase stable3) ↑ vs Gram+ve4) ↓ vs Gram-ve5) L-isomer > D-isomer

Substitutions affects pharmacodynamics α & α1-NH2 1) basic protonated at

acidic conditions acid stable

2) β-lactamase stable3) ↑ vs Gram+ve4) ↓ vs Gram-ve5) L-isomer > D-isomer

Ar= thiophene, tetrazole, furan, pyridine, andaminothiazoles

Ar= thiophene, tetrazole, furan, pyridine, andaminothiazoles

1) Oxidation of ring to sulfoxide (S=O) or sulfone (O=S=O) ↓ activtiy.

2) Replacement of S with O Oxacepam↑ acylating power ↑ activity.

3) Replacement of S with -C= ↑stability ↑ t0.5.

1) Oxidation of ring to sulfoxide (S=O) or sulfone (O=S=O) ↓ activtiy.

2) Replacement of S with O Oxacepam↑ acylating power ↑ activity.

3) Replacement of S with -C= ↑stability ↑ t0.5.

Converted to ester prodrug ↑ acid-stability ↑ oral bioav.

Converted to ester prodrug ↑ acid-stability ↑ oral bioav.

SAR for Cephalosporins (Cont.)SAR for Cephalosporins (Cont.)

• A good leaving group is required at position 3 of dihydrozolidine ring in order to initiate β-lactam ring breakage transpeptidase inhibition.

• The acetyloxy group is a good leaving group

Classification of cephalosporinsClassification of cephalosporins

• Cephalosporins are classified into 1st, 2nd, 3rd and 4th generations based on:- time of discovery- spectrum of activity

• 1st 4th Generation is associated with:

- Activity against Gram+ve Gram-ve- ↑ resistance to β-lactamases

• Individuals of each generation is differed by pharmacokinetic properties (e.g. half-life and plasma protein binding) with minimal differences in structures

1st-Generation Cephalosporins (General concept)1st-Generation Cephalosporins (General concept)

• Derived from 7-aminocephalosporinic acid (7-ACA)

• Lower activity but broader spectrum than penicillin.

• Activity against Gram+ve > Gram-ve

• Active against S. aureus and streptococcal infections

• Inactive against P. aeruginosa

• More acid-stable than penicillins

• Poor absorption through gut due to being zwitterions (i.e. usually as inj.)

• Sensitive to β-lactamases specially for Gram-ve bacteria

• Bulky groups at acylamido side chain ↓ binding to β-lactamases and transpeptidases

• The C3-acetyloxy leaving group is lost by metabolism ↓ activity.

1st-Generation Cephalosporins (Parenteral)1st-Generation Cephalosporins (Parenteral)

1. Cephaloglycin:

• The father of 1st generation cephalosporins.

• It is 7-ACA contains D-Phenylglycine

7-[(D--Amino--phenyl)acetamido]- 3-acetoxymethyl-3-cephem-4-carboxylic acid


2. Cephalothin• Bulky groups at acylamido side chain ↓ binding to β-lactamases and

transpeptidases

• Metabolism of acetyloxy group –OH group laconization↓ activtiy.

Poorly absorbed from GITUsed for MRSA

ThiopheneThiophene


3. Cephaloridine

• C3-Methylpyridine ring good leaving group (resist esterase) ↑ activity

• Zwitterion↓ absorption Inj. only Good leaving group +

Resistant to estrases

1st-Generation Cephalosporins (Oral)1st-Generation Cephalosporins (Oral)

7-(D-α-amino- α -phenylacetamido)-3-methylcephemcarboxylic acid

Oral inactivation

Acid hydrolysis

Solvolysis or biotrans of C3-methylacetoxy

4. Cefalexin

• C3-Methyl bad leaving group↓ activity↑ oral bioav. not cleaved by estrases.

• C7-acylamino group with Cα-amine electron withdrawing ↑ acid stability+ C3-Methyl = activity

• ↓ protein binding, ↑ renal excr Rx UTI

CH3 ↑absorption &↓activity

(unless accompanied with Cα-hydrophilic group)

1st-Generation Cephalosporins (Parenteral/Oral)1st-Generation Cephalosporins (Parenteral/Oral)

5. Cephradine

• It is partially hydrogenated cephalexin and has similar spectrum and kinetics

• Acid-stable and absorbed almost completely

• The only cephalosporin available as oral and parenteral


6. Cefadroxil: • It is similar to cephalexin by having C3-CH3 , however with para-hydroxyl group at

phenyl ring (4-Hydroxy-D-phenylglycine). Activity of D-isomer > L-isomer

• It has high oral stability and bioavialbility (80% of administered dose reach the blood)

• It has low urinary excretion prolonged duration of action single daily dose


7. Cefazolin• ↓ resistance to β-lactamases, ↑Gram-ve bacilli, ↓ Gram+ve cocci

• C3-(5-methyl-2-thio-1,3,4-thiadiazole) (thiol-containing heterocycle)

• C7- tetrazolylacetamido.

• ↑ Protein bound (75%)

• Thrombophlebitis after iv inj

7-(α-tetrazolylacetamido)-3-(5-methyl-1,3,4-thiadiazole 2-thiomethyl)

cephemcarboxylic acid

2nd-Generation Cephalosporins (General concept)2nd-Generation Cephalosporins (General concept)

• Derived from Cephalomycin C

• Resistant to β-lactamase due to C7-methoxy group

• Acid-stable due to C3-urethrane group

2nd-Generation Cephalosporins (parenteral)2nd-Generation Cephalosporins (parenteral)

1. Cephamycin C• The first compound was Cephamycin C which was isolated from Streptomyces

clavuligerus bacteria.

• C7-α-methoxy substitution ↑stability toward β-lactamases

• C3- Urethrane or carbamate (O-C(=O)-NH2) ↑stability toward acids

• Cephamycin C has not been used clinically, however, used as lead compound.

Streptomyces clavuligerus

(actinobacteria)Cefoxitin

Cefotetan

2nd-Generation Cephalosporins (parenteral)2nd-Generation Cephalosporins (parenteral)

2. Cefoxitin• Semi-synthesized from Cephamycin C.

• C7-α-2-thiophene

• Resistant to β-lactamases

• 7-Methoxyl substitution stabilizes, to some extent, the -lactam to alkaline hydrolysis.

• Used mainly for anerobic/aerobic infections.

• Activity Gram+ve < Gram-ve.

3. Cephaclor (similar to 1st Gen cephalexin)• It is similar to cephalexin but having C3-Cl

• Have similar spectrum and pharmacokinetics

7-[(D--amino--phenyl)acetamido]-3-chloro-3-cephem-4-carboxylic acid.

2nd-Generation Cephalosporins (Parenteral/Oral)2nd-Generation Cephalosporins (Parenteral/Oral)

4. Cefuroxime (beginning of 2nd/3rd generation)• C7-α-iminomethoxy (=N-OCH3) + Furan Resistance to β-lactamases.

↑ activity Gram-ve & ↓ Gram+ve

• C3-Urethrane resistance to acid/estrases

Stability vs estrases

And acids

Stability vs estrases

And acids

Stability vsβ-lactamases

Stability vsβ-lactamases

Cefuroxime axetil

- Acid-stable

- Lipophilic (oral)

- Prodrug- Hydrolyzed at (e.g. antacids, histamine H2-antagonists)

1-acetyoxyethyl ester

3rd-Generation Cephalosporins (General concept)3rd-Generation Cephalosporins (General concept)

• Methoxyimino (=N-OCH3) group Resistance to β-lactamases.↑ activity Gram-ve & ↓ Gram+ve

• Aminothiazole enhance penetration to Gram-ve.

• Substitutions at C3 affect pharmacokinetic properties

• Nomenclature (xim, xon, im) C7-oxyimino group

3rd-Generation Cephalosporins (Parenteral)3rd-Generation Cephalosporins (Parenteral)

Aminothiazole

1. Cefotaxime (Claforan)• First member, resistant to β-lactamase

• Active against Gram+ve, Gram-ve, aerobic and anaerobic bacteria

• Low activity against MRSA

• Can reach CSF treatment of meningitis.

• Hydrolysis of C3-acetoxy group 90% loss of activity


Aminothiazole

2. Ceftizoxime• Similar activity to Cefotaxime but with different pharmacokinetic

• The deactivation by hydrolysis is omitted due to removal of C3-acetoxy group


3. Ceftriaxone • Similar activity compared to Cefotaxime

• Thiotriazinedione is acidic

• ↑Pr. Binding + ↓Renal excr. Long t0.5

• a

Ceftriaxone has↑Pr. Binding↓Renal excr.

Long t0.5

Aminothiazole

thiotriazinone

3rd-Generation Cephalosporins (Oral)3rd-Generation Cephalosporins (Oral)

1. Cefixime• First orally active of the 3rd generation.

• It is not ester prodrug

• Oral bioavailability almost 50%

7-[2-(Amino-4-thiazolyl)-2-(carboxymethoxyimino) acetamido]-3-ethinyl-3- cephem-4-carboxylic acid.

2-(Amino-4-thiazolyl)

4th-Generation Cephalosporins4th-Generation Cephalosporins

• Are similar to 3rd –Generation Cephalosporins but are zwitterionic

• Oximinocephalosporins + C7-aminothiazole ring + C3-ionized +ve group

• ↑activity vs Gram-ve, ↑activity vs transpeptidase, ↓ sensitivity to β-lactamases

• Active against P. aerugenosa

Pyrrolidine

2,3-Cyclopentenopyridine

5th-Generation Cephalosporins5th-Generation Cephalosporins

• Active vs Methicillin-Resistant Staphylococcus aureus (MRSA)

• Active vs Multi-Drug Resistant Streptococcus pneumonia (MDRSP)

• Have C3-2-thio-1,3-thiazole ring is important for activity vs MRSA

Resistance to CephalosporinsResistance to Cephalosporins

• Resistance depends on

• Ability to reach transpeptidases

• Affinity to transpeptidases (all Ceph have high affinity except Cephamycins, Ceftazidime)

• Stability toward β-lactamases (all Ceph. are stable except Cephaloridine)

• MRSA with mutated transpeptidase are less susceptible to Penicillins and Cephalosporins.

NO

N

O

O

N

S

O

N

S

ON

O

NO

N

S

O

N

O

NO

O

1. Penam 2. Carbapenam 3. Oxapenam

4. Penem 5. Carbapenem 6. Monobactam

7. Cephem 8. Carbacephem 9. Oxacephem

mohammed nooraldeen al-qattan (phd) cephalosporins · •cephalosporin analogues can not be...

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