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Lecture 5

Medicinal Chemistry 1PC 509

Prof. Dr/ Ghaneya Sayed Hassanghanmoom2@yahoo.com

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Macrolides Properties:• Isolated form Sterptomyces species.• Lead compound is Erythromycin.

Spectrum :• Nearly as penicillin G narrow spectrum [used as alternative for patients allergic to penicillins].• Drug of choice for chlamydial infections, mycoplasma, pneumonia, Corynebacterium diphtheriae & Ligionella pneumophila.

M.O.A: Specific inhibitor of bacterial protein biosynthesis by binding to the 50S ribosomal subunit interfere with translocation reactions & formation of initiation complexes inhibit protein synthesis bacteriostatic [may be cidal at doses]

Safe to human [not affect 60 S unit] selective chemotherapy.

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Resistance: due to:1) Inability of m.o. to take up the drug.2) Produce enzymes that methylate specific adenine residue to erythromycin binding site affinity of erythromycin to bind.3) Presence of plasmid-associated erythromycin esterase.

Side effects:(1) GIT problems.(2) Cholestatic jaundice: especially with Erythromycin Estolate [result from hypersensitivity reaction bile becomes granular in bile duct its flow back up into circulation].(3) Ototoxicity: transient deafness [especially at doses].(4) Erythromycin level of simultaneously administered anti-coagulants, anti-diabetetics, cyclosporine, carbamazepine…. By inhibition of microsomal enzymes.“Metabolizing microsomal enzymes’ inhibitors”(5) Not used for patients with hepatic dysfunction drug accumulation in liver.

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Chemical properties:(1) Large lactone ring [cyclic ester] with 12-17 atoms so, the name Macrolides.(2) With 2 or more sugars attached to this ring one of them is amino sugar [so, marcrolides are weakly basic].(3) Free bases are water insoluble.

Stability:In neutral medium macrolides are stable.In acidic medium hydrolysis of glycosidic bonds.In basic medium saponification of lactone ring [opening].

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Erythromycin

O

CH3

CH3

CH3

CH3

O

O

C2H5

R1

O

HO

CH3

HO

CH3O CH3

CH3OH

OR2

O O CH3

N - Me2HO

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9

10

14

Erythronolide

Desosamine

Cladinose

Erythromycin AErythromycin B

Erythromycin C

R1 R2

OH CH3

H CH3

HOH1 2

345

78

1112

13

2,4,6,8,10,12 CH33,5,6,11,12 OH1,9 Oxo13 Ethyl

1'

2' 3' 4'5'

6'

1''2''

3''4''

5''6''

Obtained by fermentation of Sterptomyces erythereus.

Consist of :► Erythronolide [aglycone part, large lactone ring, 14-membered ].► Desosamine [amino sugar]► Cladinose [neutral sugar].

Used usually as mixture of 90% Erythromycin A + 10% Erythromycin B + traces of Erythromycin C.

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To overcome its bitter taste & its irregular absorption:Result from acid destruction & adsorption of food.(1) Enteric coated & delayed-release dose dosage forms:Over come bitter taste but not solve problem of poor absorption.

(2) Erythromycin Salts:Salts formed on dimethyl amino group of Desosamine.☻HCl salt moderate solubility.☻ Glucepate & Lactopionate freely soluble in water I.V.☻ Stearate with solubility tasteless [combined with enteric coating] hydrolyzed in intestine to free erythromycin absorption.

(3) Erythromycin Esters:Estolate: [C2’’-propionyl ester] & [N-lauryl sulfate salt] with superior oral absorption [ acid stable + intrinsic absorption + oral absorption not affected by food].Ethyl succinate: mixed double ester [ one carboxylic of succinic acid react with C2’’ of erythromycin & other carboxylic react with ethanol]. It’s insoluble in water [used as oral suspension for pediatrics as it masks bad taste of erythromycin]

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O

CH3

CH3

CH3

CH3

O

O

C2H5

OH

O

HO

CH3

HO

CH3O CH3

CH3OH

OCH3

O O CH3

N - Me2RO

6

9

10

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1. Erythromycin Base2. Erythromicine HCl3. Erythromicine Estolate4. Erythromycin Ethylsuccinate

5. Erythromycin Gluceptate6. Erythromycin Lactobionate

7. Erythromycin Stearate

H

H

H

H

H

COCH2CH3

CO(CH2)COOC2H5

HCl

Glucoheptanoic acid

Lactobiono-S-lactone

CH3(CH2)11OSO3H

-----

-----

CH3(CH2)16COOH

R Salt

Erythronolide

Desosamine

Cladinose

ESTERSALT

N.B: Erythromycin esters not considered as latent prodrugs, as they are active itself & their activity not depend on their hydrolysis. But they are partially hydrolyzed to free drugs in blood.

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Erythromycin is Unstable to Acids due to Rapid Cyclic Ketal formation inactivation + GIT cramps.

O

O

O

OH

O

HOHO

O

OCH3

O O

N - (CH3)2RO

OH

H+

O

O

HO

OH

O

OHO

O

OCH3

O O

N - (CH3)2RO

OH

H+

O

O

O

O

OHO

O

OCH3

O O

N - (CH3)2RO

OH

Active Macrolide Cyclic Ketal Inactive + causing GIT cramps

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9

12- H2O

Hemiketal

So, we make semi-synthetic analogues to prevent this ketal formation.

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Semi-synthetic analoguesClarithromycin [Klacid®]

O

CH3

CH3

CH3

CH3

O

O

C2H5

OH

O

CH3

HO

CH3

O CH3

CH3OH

OCH3

O O CH3

N - Me2HO

CH3O 6

Semi-synthetic drug from erythromycin by methylation of 6-OH etherification. 6-OH involved in ketal formation which form inactive drug + GIT cramps. Methylation of 6-OH ADVANTAGES:(1) No cyclic ketal formation [inactivation + GIT cramps].(2) absorption & better blood level.(3) lipophilicity & less frequent dose for mild infections.(4) gastric upset.

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Azithromycin [Zithromax®, Azithrocin®]

O

CH3

CH3

CH3

CH3

O

C2H5

OH

O

HO

CH3

HO

CH3

O CH3

CH3OH

OCH3

O O CH3

N - Me2HO

89

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N 9aCH3

Semi-synthetic ring-expanded analogue [15-membered] by insertion of N-methyl between C9-C10 + removal of C=O. Not form internal cyclic ketal. ADVANTAGES :(1) No cyclic ketal formation [inactivation + GIT cramps](2) More acid stable.(3) With longer duration [once daily](4) Greater G-ve activity > Erytho- & Clarithromycin.

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Chloramphenicol

O2N

H

CH2

NH

HOHOH

CO

CHCl

Cl1 2 3

D(-) threo-2,2-dicloro acetamido-1-(4-nitro phenyl) propane-1,3-diol

►Natural antibiotic; produced by fermentation of Streptomcyes venezuelae. But now produced by total synthesis [less expensive].►Used orally or I.V.

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Spectrum:1) Broad spectrum, active ≠ rickettsial infections.2) Drug of choice in Salmonella infections [typhoid fever].3) against H.influenza, -lactamase producing strains & meningococcal infections.4) With excellent activity ≠ anaerobes.5) Now mostly used as eye drops for ophthalmic infections.

M.O.A:Binds to 50 S ribosomal subunits interfere with action of peptidyl transferase block attachment of amino acids to nascent peptide chain on ribosomes inhibit protein synthesis Bacteriostatic.

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Side effects:(1) Bone marrow depression: May cause fatal blood dyscrasias [aplastic anemia, hypoplastic anemia, thrombocytopenia & granulocytopenia].

(2) Gray syndrome: form of CVS collapse;Occur when given for premature & newborn infants [with undeveloped glucuronidation & undeveloped renal function] accumulation of chloramphenicol poor feeding, breathing, CVS collapse [gray baby] & death.So, not recommended in 1st two weeks of life.

(3) Inhibition of hepatic mixed function oxidases: block metabolism of drugs e.g. warfarin, tolbutamide , phenytoin their concentration.

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Metabolism:• Mainly excreted in urine.• Major metabolite is C7-glucuronide, Minor obtained by deamination, dehalogenation & reduction of aromatic nitro group [all metabolites are inactive].

Resistance:Occur due to:(11 dose.(2) Irregular use.(3) Inappropriate use.(4) Production of enzymes chloramphenicol acetyl transferase

acetylation of 1-OH & 3-OH 1-acetoxy & 1,3-diacetoxy derivatives inactive.

(5) Inability of the drug to penetrate m.o.

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SAR:

C C

H

OH

CH2

H

NH

O2N OH

R

Replacement of phenyl group with: - aromatic or alicyclic moities --> inactive Cpds - Nitrothienyl derivative has activity << chloramphenicol.

(*) two chiral centres is essential for activity & only D- (-) (1R,2R) threo has marked activity.

* *

The amine propanediol side chain is the most specific part: - 1ry alcohol & 2ry alcohol ----> essential for activity - Me subistitution for H on C2 -----> No activity. - If R= CH2N3 (azidamphenicol ) OR R= COCHBr2 -----> Both give active Cpds.

- NO2 if replaced with strong electronwithdrawing gps ----> active cpds e.g. Acetyl gp ( cetophenicol ) & Methyl sulphonyl ( thiamphenicol) - if replaced by CN , CONH2 , halogen , OH & SO2R----> inactive cpds - NO2 shifting from P-position ----> reduce activiry.

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N.B. The following are active compounds:

CH2OH

NHC

1

2OH

C

O

CH(Cl)2

3

O

H3C CH2OH

NHS

1

2OH

C

O

CH(Cl)2

3

O

H3C

O

CH2OH

NH1

2OH

C

O

CH(Br)2

3O2NCH2

OH

NHO2N

1

2OH

CH2

3

N3

CH2OH

NH1

2OH

C

O

CH(Cl)2

3S

O2N

Cetophenicaol Thiamphenicol

Azidamphenicol

Less active than chloramphenicol

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Prodrugs of chloramphenicol: [C3-esters]

O2N

H

CH2

NH

HOHO

CO

CHCl

Cl1 2 3CO (CH2)14 CH3

O2N

H

CH2

NH

HOHO

CO

CHCl

Cl1 2 3CO CH2 CH2 COOH

Chloramphenicol Palmitate

Chloramphenicol Hemisuccinate

Insoluble in water.To overcome bitter taste.Used for pediatric oral suspensions.

Water soluble used for injection to overcome poor water solubility.

Esters are inactive hydrolyzed in vivo releasing active form.

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Stability:• Very stable in solid form.• In solution hydrolytic & light-induced reactions; depending on pH, heat & light:

[1] Acid-catalyzed hydrolysis of amide:

O2N

H

CH2

NH2

HOHOH1 2 3

CO

CHCl

ClHO

dichloro acetic acid1-(p-nitrophenyl)-2-amino propan-1,3-diol

O2N

H

CH2

NH

HOHOH1 2 3

CO

CHCl

Cl+

[2] Alkaline hydrolysis of -chloro ,-dihydroxy derivative.

O2N

H

CH2

NH

HOHOH1 2 3

CO

CHCl

ClO2N

H

CH2

NH

HOHOH1 2 3

CO

CHOH

OH

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Synthesis of Chloramphenicol:

Br2

P-nitro acetophenone

1. Hexamine (CH2)6N4

.HCLCO2NO

CH3 CO2NO

CH2 Br CO2NO

CH2 NH2

CH

O2N CH

CH2OAl isopropoxideCO2NO

CH2 NH CO

CH3

NH2

CH2 OH

CO2NO

CHNH

CH2 OH

CO

CH3

OH

CH

O2N CHNH

CH2 OH

CO

CH3OH

CCl CHO Cl

Cl Cholramphenicol

2. HClAcetic anhydride

H+

DL-threo

CH

O2N CHNH2

CH2 OH

OH

D(-)-threo

D-(+) camphorsulfonate

dichloroacetyl Cl

NaHCO3

fractional crystallization

Ac-OH

no reduction for NO2Erythro + Threo

Resolution

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Mechanism of action:1- -lactam antibiotics: Inhibitors of cell wall synthesis.

2- Tetracyclines: Bind to 30 ribosomal subunit of bacteria interfere with protein biosynthesis leading to bacteriostatic.

3- Aminoglycosides: Bind to 30 S ribosomal subunit of m.o. through specific receptor protein inhibit initiation of protein synthesis. 4- Macrolides: Bind to 50 S ribosomal subunit inhibit protein synthesis.

5- Chloramphenicol: Bind to 50 S ribosomal subunits inhibit protein synthesis.

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Cyclic peptides antibioticsThe usual physiologically significant peptides are linear. Several bacterial species, however, produces antibiotic mixtures of cyclic peptides some with uncommon amino acids and some with common amino acids but with the D absolute stereochemistry. These cyclic substances often have a pendant fatty acid chain.

Mode of action: (Disruption of bacterial membrane function)

These drugs are usually water soluble and are highly lethal to susceptible bacteria as they attach themselves to the bacterial membrane and interfere with their semi permeability so that essential metabolites leak out and undesirable substances pass in.

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Side effects: They are highly toxic in humans so their use is reserved for serious situations where there are few alternatives or to topical uses.Resistance: Bacteria are rarely able to develop significant resistance to this group of antibiotics.Stability: They are generally unstable to solutions should be protected from heat, light and extremes of pH.

Cyclic peptides antibiotics: They are:1-Vancomycin 2- Teicoplanin3-Quinupristin 4-Dalfopristin (=Streptogramins).5-Capromycin6-Bacitracin7-Polmyxin B8-Colistin

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BacitracinIt is a mixture of similar peptides produced by fermentation of Bacillus subtilis. Uses:It is active mainly against gram (+) microorganisms and is used topically or intramuscularly against staphylococci resistant to other agents. It is also used orally for enter pathogenic diarrhea and, especially, against Clostridium difficile. It can also be used for preoperative bowel sanitization. It is neuro and nephrotoxic restricted in uses. Zn2+ enhances its activity.

Mode of action:1- Inhibition of peptidoglycan biosynthesis. 2- Disruption of plasma membrane function.

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It is produced by fermentation of Bacillus polymyxa .Uses:It is active against gram (-) microorganisms. It is used I.M or I.V to treat serious urinary tract infections, meningitis and septicemia caused by Pseudomonas aeruginosa. It is also used orally to treat enter pathogenic E. coli and Shigella sp. diarrheas. It is neuro and nephrotoxic especially when given parentrally (restricted in uses).

Mode of action:It binds to phosphate groups in bacterial cytoplasmic membranes and disrupts their integrity.

Polymyxin B

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ColistinColistin sulphate is a cyclic polypeptide drug produced by Bacillus polymyxa var. colistinus.Uses:It is bactericidal mainly to gram (-) bacteria. It is neuro and nephrotoxic especially when given parentrally (restricted in uses). It is used orally against diarrhea due to E. coli and shigella species.Mode of action:Colistin causes destruction of the integrity of cytoplasmic membrane. Colistin is polycationic and has both hydrophilic and lipophilic moieties. These interact with the bacterial cytoplasmic membrane, changing its permeability. This effect is bactericidal