Drugs acting on bacterial protein biosynthesisProteins are very essential for most of the bacterial

metabolic functions as well as for cell integrity.Bacterial cell uses ribosomes to synthesize proteins.Targeting protein biosynthesis will produce

bactericidal agents in most of the cases.Why targeting the bacterial protein synthesis will be

selective:Different diffusion rates between bacterial and

mammalian cells.Structural differences between bacterial and

mammalian ribosomes

AminoglycosidesThey have amino sugar linked together with glycosidic

bonds.The first one discovered was streptomycin; obtained

from streptomyces bacteria in 1939.Most of them are natural products such as kanamycin,

neomycin, gentamicin, netilmicin and tobramycin.Some are semisynthetics such as amikacin which is

synthesized from kanamycin A.They are highly polar compounds, because of that

they are not orally bioavailable, either given parenterally , topically or for local GIT infections.

Aminoglycosides Chemical structureMost of them have three rings, some of which are amino sugar other are simple sugar molecules, either pentose or hexose.Some have four sugar molecules such as neomycin and paromomycin.Generally the pentose do not have the amino substituents, only the hexoses have.

O

O

O

OH

OHCH2OH

H3CHN

OH

CHO

O

NHH2N

HN

OH

HO OH

HN

HN

NH2

Streptidine

Streptose

N-methylGlucosamine

Streptomycin

Aminoglycosides Chemical structureHighly polar chemical structure.Strongly basic compounds…. Form polycationic

species at physiological pH.Normally given as sulfate salts to improve their

water solubility.Most of them, especially the sulfate salts are

highly water soluble which means:Well distributed compounds.Do not reach CNS (why?).Do not reach bone, fatty or connective tissues

(poorly vascularized tissues).

Aminoglycosides clinical usesParenterally for systemic serious infections caused by

gram –ve bacilli such as Pseudomonas, acinetobacter and enterobacter.

Less active on gram +ve and gram –ve cocci.Not active on anaerobic bacteria (because

aminoglycosides need oxygen dependent transport system to get inside the cell).

Some are widely used for skin and eye infections for local antibacterial actions such as neomycin and Gentamicin.

Some are used for GIT infections such as paromomycin and tobramycin.

Have good activity against Ps. aeruginosa especially in combination with penicillins.

Combination of aminoglycosides and penicillinsMainly used for Ps. Aeruginosa infections.The commonly used combinations:

Carbenicillin + Gentamicin.Penicillin G + streptomycin for Enterococci

infections (endocarditis).Here penicillins will destroy the integrity of

the cell wall that will help the aminoglycosides to reach ribosomes and inhibit protein biosynthesis.

Mechanism of actionAminoglycosides directly inhibit protein synthesis

by interfering with the translation process, this done by direct binding to the ribosomal 30S subunit.Inhibit the initiation process.Can cause misreading of the codons which may results

in protein mutation (mainly the deoxystreptamine containing aminoglycosides).

All aminoglycosides are bacteriostatic (at lower dose)and bactericidal at higher dose.

Spectinomycin is a bacteriostatic even at higher dose.

Bacteria protein synthesis

Bacteria protein synthesisBacterial protein synthesis can be divided into four phases:

Initiation: where a functionally competent ribosome is assembled in the correct place on an mRNA ready to commence protein synthesis.

  Elongation: whereby the correct amino acid is brought to the ribosome,

is joined to the nascent polypeptide chain, and the entire assembly moves one position along the mRNA.

  Termination: which happens when a stop codon is reached, there is no

amino acid to be incorporated and the newly-synthesized polypeptide is released from the ribosome.

  Disassembly: whereby a special factor binds to the ribosome so that it

can release the mRNA and tRNA that is still bound to it and so that it can be recycled in another round of protein synthesis.

Mechanism of bacterial resistanceBacteria has two major resistant mechanisms

to aminoglycosides:1. Affecting aminoglycosides uptake by the cell:

mainly adapted by Ps. aeruginosa by making some sort of mutation in the energy dependant transport system required to get such compounds inside the cell.

2. Inactivating aminoglycosides by changing the chemical structure, especially the essential functional groups for activity.

Inactivating enzymesNine different types of aminoacetyl

transferase (AAC): responsible for acetylating the amino groups in the structure from ring I and II.

Aminoglycoside Phosphotransferase (APH): phosphorylate the hydroxyl groups from ring I and III.

Ring II

Ring IRing III

AAC

AAC

APH

APH

Kanamycin A

Inactivating enzymesNew derivatives have been made to overcome

the enzymatic inactivation by these enzymes.Removal of the functional group susceptible

to attacking by the inactivating enzymes can lead to more active agents against the resistant strains Gentamicin and tobramycin lack the 3΄ OH in

ring I which make them resistant to APH.Amikacin has the 3-NH2 being acylated (by L-

hydroxyaminobuteroyl amide (L-HABA)), that makes it more resistant to AAC at this site.

Amikacin

Gentamicin

Tobramycin

No OH group

No OH group

SAR of aminoglycosides

Ring I is important for broad spectrum in activity, at the same time it is the major site for enzymatic inactivation by AAC and APH.

Amino group at 6΄ and 2΄ are important. Phosphorylation of 3΄ OH will reduce the binding to

ribosomes 30S subunit.

Ring II

Ring IRing III 1΄

2΄ 3΄ 4΄

5΄ 6΄

21 3

4561΄΄2΄΄3΄΄

4΄΄5΄΄

6΄΄

SAR of aminoglycosides

Ring II can accept few structural modifications and retaining the same activity: example is the acetylation of the amino group at position-1 (amikacin).

Ring II can be ribose, streptose (five memeberd ring) or streptamine (six memeberd ring).

Ring III less sensitive to structural changes but the amino group at position 3΄΄ can be 1° or 2° for maximum activity.

Ring II

Ring IRing III 1΄

2΄ 3΄ 4΄

5΄ 6΄

21 3

4561΄΄2΄΄3΄΄

4΄΄5΄΄

6΄΄

StreptomycinActive against Gram –ve more than Gram +ve bacteria, especially on mycobacterium

bacteria.Bacteria rapidly developed resistance to

streptomycin, because of that it is not recommended as monotherapy and should be given in combination.

Can not be given orally (why?).Will not be absorbed.Could be destroyed in GIT acidity.

At lower dose it only induce misreading of the mRNA.

Paromomycin

It has four sugar rings in its structure.Has a broad spectrum of action.Has poor oral availability.Mainly used for GIT infections caused by

salmonella, and shigella or that caused by protozoa specially in amaebiasis.

GentamicinIsolated from the gram +ve bacteria;

Micromonospora.Mainly active on Gram –ve bacteria.Has high activity against Ps. aeruginosa.Mainly used topically for eye, ear and skin

infections due to :Poor oral bioavailability (why?).Serious systemic toxicity (otto toxicity and

nephrotoxicity).Given IV injections for serious systemic infections

caused by Ps. aeruginosa

Spectinomycin

It is aminocyclitol antibiotic.The structure contains the unstable

hemiacetal, so it should be freshly prepared and used directly.

Is a bacteriostatic agents, mainly act on the translation process.

It has poor oral absorption, it should be given parenterally mainly as deep IM injections.

Used in gonorrhea.

Other agents acting on bacterial protein synthesis