Drugs acting on bacterial protein biosynthesis Proteins 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

Aminoglycosides They 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 structure Most 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.

Aminoglycosides Chemical structure Highly 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 uses Parenterally 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 penicillins Mainly 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 action Aminoglycosides 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 synthesis Bacterial 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 resistance Bacteria has two major resistant mechanisms to aminoglycosides: 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. Inactivating aminoglycosides by changing the chemical structure, especially the essential functional groups for activity.

Inactivating enzymes Nine 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 I Ring III AAC APH Kanamycin A

Inactivating enzymes New 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.

No OH group Gentamicin Amikacin No OH group Tobramycin

SAR of aminoglycosides Ring II Ring I Ring III 1΄ 2΄ 3΄ 4΄ 5΄ 6΄ 2 1 3 4 5 6 1΄΄ 2΄΄ 3΄΄ 4΄΄ 5΄΄ 6΄΄ 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.

SAR of aminoglycosides Ring II Ring I Ring III 1΄ 2΄ 3΄ 4΄ 5΄ 6΄ 2 1 3 4 5 6 1΄΄ 2΄΄ 3΄΄ 4΄΄ 5΄΄ 6΄΄ 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.

Streptomycin Active 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.

Gentamicin Isolated 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