Tetracycline - Medicinal Chemistry III B. Pharma 6th Semester

Tetracycline

Contents

• Tetracyclines – Introduction

• Structure and chemistry of Tetracyclines

• Mechanism of Action of Tetracyclines

• Spectrum of activity

• Structure and Activity Relationship of Tetracyclines

• Study of individual compounds

Learning Objectives

At the end of this lecture, student will be able to

• Explain the structure and chemistry of Tetracyclines

• Discuss the mode of action of Tetracyclines

• Explain about the spectrum of activity

• Compare the structure with the activity of Tetracyclines

• Differentiate the structural features of individual compounds

TETRACYCLINES

• Most important broad-spectrum antibiotics.

• Nine compounds in this family are in medicinal use.

• Tetracycline

• Rolitetracycline

• Oxytetracycline

• Chlortetracycline

• Demeclocycline

• Meclocycline

• Methacycline

• Doxycycine

• Minocycline

Introduction

• Obtained by fermentation procedures from Streptomyces spp., by chemical transformation of natural products.

• The important members of the group are derivatives of an Octahydronaphthacene, a hydrocarbon system that comprises of four annulated six – membered rings

• The group name is derived from the tetracyclic system

• The antibiotic spectra and chemical properties of these compounds are very similar but not identical.

Chemistry

• Structure and chemistry of Tetracyclines:

• Tetracyclines are amphoteric compounds, forming salts with either acids or bases. In neutral solutions, these substances exist mainly as zwitter ions

• Acid salts are formed through protonation of the enol group on C-2

Tetracycline Analog

R1

R2                     

R3

R4

Tetracycline

H

CH3

OH

H

Chlortetracycline

Cl

CH3

OH

H

Oxytetracycline

H

CH3

OH

OH

Demeclocycline

Cl

H

OH

H

Methacycline

H

CH3

-                      

OH

Meclocycline

Cl

CH3

-

OH

Doxycycline

H

H

CH3

OH

Minocycline

N(CH3)2                   

H

H

H

• ‘The unusual structural groupings in the tetracyclines produce three acidity constants in aqueous solutions of acid salts for eg.,

Tetracycline Analog

pKa1  

pKa2

pKa3

Tetracycline

3.3 

7.7 

9.5

Chlortetracycline 

3.3 

7.4 

9.3

• The groups are as shown below,

Tetracyclines have the ability to undergo epimerization at C-4 in solutions of intermediate pH. These isomers are called ‘epitetracyclines’

Under acidic conditions, an equilibrium is established in about 1day and consists of approximately equal amounts of the isomers

These ‘4-epitetracyclines’ exhibit much less activity than the ‘natural’ isomers, thus accounting for the decreased therapeutic value of aged solutions

Epitetracycline                                       Tetracycline

• Strong acids and strong bases attack tetracyclines with a hydroxyl groups on C-6, causing a loss in activity through modification of the C-ring.

• Strong acid produce dehydration through a reaction involving the 6- hydroxyl group and the 5a-hydrogen.

• The double bond thus formed between positions 5a and 6 induces a shift in the position of the double bond between C-11a and C-12 to a position between C-11 and C-11a, forming the more energetically favored resonance system of the naphthalene group found in the inactive

ANHYDROTETRACYCLINES

• Bases promote a reaction between 6-hydroxy group and the ketone group at the 11 position, causing the bond between the 11 and 11a atoms to cleave, forming the lactone ring found in the inactive isotetracycline.

• These unfavourable reactions led to the development of more stable and longer-acting compounds as 6-deoxytetracyclines, methacycline, doxycycline and minocycline.

• Stable  chelate  complexes  are  formed  by  the  tetracyclines  with many  metals  including  calcium,  magnesium  and  iron    such chelates are usually very insoluble in water causing impaired absorption of most of the tetracyclines in presence of milk, calcium, magnesium and aluminum containing antacids and iron salts.

• Soluble alkalinizers such as sodium bicarbonate also decreases the GI absorption of the tetracyclines.

• Deposition   of   these   antibiotics   in   teeth   cause   a   yellow discoloration that darkens (photochemical reaction) over time.

• Tetracyclines are distributed into the milk of lactating mothers and will cross the placental barrier into the fetus.

• Tetracyclines has affinity for calcium causing them to be incorporated into newly forming bones and teeth as tetracycline – calcium orthophosphate complexes.

• The possible effects of these agents on the bones and teeth of the child should be considered before their use during pregnancy or in children under 8 years of age.

• The stereochemistry of the tetracyclines is very complex. Carbon atoms 4, 4a, 5, 5a, 6 and 12a are potentially chiral depending on the substitution. Oxytetracycline and doxycycline each with a 5α- hydroxyl substituent has six asymmetric centers. Others that lack chirality at C-5 have only five asymmetric centers.

• Conjugated systems exist in the structure from C-10 through C-12 and from C-1 to C-3 (in one canonical form)

Mechanism of Action

• Tetracyclines are specific inhibitors of bacterial protein synthesis.

• They bind to the 30S ribosomal subunit and thereby prevent the binding of aminoacyl tRNA to the mRNA – ribosome complex.

• Both the bindings- of aminoacyl tRNA and the binding of tetracyclines at the binding site- require magnesium ions

• Tetracyclines remove essential metal ions (like Mg) as chelated compounds.

• Tetracyclines also bind to mammalian ribosomes but with lower affinities and do not achieve sufficient intracellular, concentrations to interfere with the protein synthesis

• The selective toxicity of the tetracyclines towards bacteria depends strongly on the self-destructive capacity of bacterial cells to concentrate these agents in the cell.

• Tetracyclines enter the bacterial cells by two processes: a. Passive diffusion and b. active transport.

Spectrum of Activity

• Tetracyclines have the broadest spectrum of activity of any known anti-bacterial agents

• They are active against a wide range of Gram +ve and Gram-Ve bacteria, Spirochetes, Mycoplasma, Rickettsiae and Chlamydiae.

• They  are  bacteriostatic  and  so  they  are  not  effective  in  the treatment of life threatening infections such as septicemia, endocarditis and meningitis.

• Parenteral tetracyclines may cause severe liver damage, especially when given in excessive dosage to pregnant women or to patients with impaired renal function.

STRUCTURE – ACTIVITY RELATIONSHIPS

• All derivatives containing fewer than four rings are inactive.

• Many of the structural features present in this molecule must remain unmodified for derivatives to retain activity.

• The substituents at carbon atoms 1, 2, 3, 4, 10, 11, 11a and 12 representing the hydrophilic faces of the molecule cannot be modified drastically, without deleterious effects on the anti- microbial properties of the resulting derivatives.

A-ring:

• A-ring substituents can be modified only slightly without drastic loss of anti-bacterial potency.

• The enolized tricarbonyl methane system at C-1 and C-3 must be intact for good activity.

  Replacement  of  the  amide  at  C-2  with  other  functions  (eg., aldehyde or nitrile) reduces or abolishes activity.

• Mono-alkyation of the amide nitrogen reduces activity proportionately to the size of the alkyl group

• Aminoalkylation of the amide nitrogen by mannich reaction, yields derivatives that are more water soluble than the parent tetracycline and are hydrolyzed to it in-vivo (eg., Rolitetracycline)

• The dimethylamino group at the 4th position must have the α- orientation.

• 4-epitetracyclines are very much less active than the natural isomers.

• Removal of the 4-dimethylamino group reduces activity even further.

• Activity is largely retained in the primary and N-methyl secondary amines but rapidly diminishes with higher alkylamines.

Ring B

• A cis-A/B ring fusion with a β-hydroxy group at C-12a is essential.

• Esters of the C-12a hydroxyl group are inactive, with the exception of the formyl ester, which readily hydrolysis in aqueous solutions.

• Alkylation at C-11a leads to inactive compounds- therefore an enolizable β-diketone is important at C-11 and C-12.

• Epimerization at C-5a causes loss of antibacterial potency.

• Dehydrogenation to form a double bond between C-5a and C-11a markedly decreases antibacterial activity

• Substituents at positions 5, 5a, 6, 7, 8 and 9 are hydrophobic faces of the molecule – can be modified resulting in retention and sometimes improvement of antibiotic activity.

• 5-OH group in oxytetracycline and doxycycline influence pharmacokinetic properties but does not change anti-microbial activity.

Ring C

• Aromitization of ring C decreases activity (Anhydrotetracyclines)

• Neither the 6α-methyl nor the 6β-hydroxyl group is essential for antibacterial activity.

• In fact doxycycline and methacycline are more active in-vitro than the parent oxytetracycline.

• 6-epidoxycycline is much less active than doxycycline.

Advantages of 6-deoxytetracyclines

• They cannot form inactive anhydrotetracyclines in acidic conditions as they cannot dehydrate at C-5a & C-6.

• They are more stable in base because they do not undergo β-ketone cleavage followed by Lactonization to form isotetracyclines.

• Absorbed more completely following oral administration, have higher fractions of protein binding, higher volumes of distribution and lower renal clearance.

Ring D

• Acid stable 6-deoxytetracyclines & 6-demethyl-6-deoxytetracyclines are used to prepare a variety of mono- and di- substituted derivatives by electrophilic substitution reactions at C-7 and C-9 of D-ring

• Introduction of strong electron-withdrawing groups especially at C-7 (eg., chloro and nitro –Chlortetracycline) enhances activity.

• Strong electron donating groups (eg., dimethyl amino-Minocycline) enhance the activity.

• C-8 cannot be directly substituted by electrophilic aromatic substitution                                                                                                        

Tetracycline

• Isolated from fermentation of Streptomyces species

• Chem.name: 4-Dimethyl amino-1,4,4a,5,5a,6,11,12a-octahydro-3,6,10,12,12a-pentahydroxy-6-methyl-1,11-dioxo-2-naphthacene carboxamide.


Use:-

• Used as ointments for topical and ophthalmic administration

• A topical solution is used for the management of acne vulgaris..

Chlortetracycline

• Isolated from S. aureofaciens.

• Valuable antibiotic with broad spectrum activities.

• Chem.  Name:-  7-chloro-4-Dimethyl  amino-  1,4,4a,5,5a,6,11,12a- octahydro-3,6,10,12,12a-pentahydroxy-6-methyl-1,11-dioxo-2- naphthacene carboxamide.

Use:-

• Used as ointments for topical and ophthalmic administration

Oxytetracycline

• Isolated from S.rimosus.

• Absorbed well orally

• Also used for parenteral administration (intravenously & intramuscularly)

Doxycycline

Doxycycline (monohydrate):- Α-6-deoxy-5-oxytetracycline

• 6α-methyl epimer is more than 3 times as active as its β-epimer.

• Unlike other tetracyclines, it does not accumulate in patients with impaired renal function. Therefore, it is preferred for UREMIC patients with infections outside the urinary tract.

• Very well absorbed from the GIT- hence smaller dose maybe used

Minocycline

• Chem. Name:- 7-dimethylamino-6-dimethyl-6-demethyl-6-deoxy tetracycline.

• Most potent tetracycline.

• Well absorbed orally

• Has very long serum half-life due to slow urinary excretion & moderate protein binding

• Active against Gram +ve bacteria especially Staphylococci and Streptococci

• Useful alternative for the treatment of less serious tissue infections

• Recommended for the treatment of chronic bronchitis and other upper respiratory tract infections

• Used for the treatment of urinary tract infections

• It is effective in the eradication of N. meningitides in asymptomatic carriers.

 

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