Antimicrobial Drugs Penicillins 7 December 2018 Antimicrobial Drugs Penicillins Pharmacist Omar Abdulrahman Abdulqader B. SC. Pharmacy M. Sc. Pharmaceutical Chemistry

Antimicrobial Drugs Chemicals used to treat microbial infection. Antimicrobial drugs include: Antibacterial drugs Antifungal drugs Antiprotozoal drugs Antihelminthic drugs

Introduction The development of antibiotics over the past eight decades has been one of medicinal chemistry’s greatest success stories. However, on a cautionary note, the pathogens are fighting back and we humans are locked in a never-ending race with these microscopic adversaries. While deaths from bacterial infections have declined markedly in the developed world, deaths from bacterial infections are still relatively common in the developing world.

The World Health Organisation (WHO) has estimated that tuberculosis (TB) was responsible for around 2 million deaths in 2002, and that in 2000 1.9 million children died of respiratory infections, with 70% of the deaths occurring in Africa and Asia. In the developed world, multiple-resistant Staphylococcus aureus (MRSA) is a growing problem, with most new infections acquired in hospitals. Deaths from MRSA infections are becoming more common among elderly or immunocompromised patients, and this has attracted widespread publicity.

Bacterial pathogens Bacteria are single-cell microorganisms that were first observed by Anton van Leeuwenhoek in the 1670s, using the microscope, which he had developed. In comparison with plant and animal cells, they are relatively simple in structure. Bacterial cells lack clearly defined nuclei and organelles which animal cells possess. The bacterial cell also has a quite distinct biochemistry; possessing enzymes, which enable it to synthesize essential vitamins which animal cells, can obtain directly from food.

Bacterial cells have cell membranes and cell walls, whereas animal cells have only membranes. The cell wall is crucial to the bacterial cell’s survival, enabling them to colonise a very wide range of environments and osmotic pressures. The cell wall prevents the uncontrolled flow of water into the cell, and provides protection against a myriad of hostile environmental factors such as heat, cold, acidity, alkalinity, salinity and radiation.

Bacteria can be characterised by a staining technique, which allows them to be defined as Gram positive or Gram negative. The staining technique involves the addition of a purple dye followed by washing with acetone. Bacteria with a thick cell wall (20-40nm) absorb the dye and are stained purple and are classed as Gram positive. Bacteria which possess a thin cell wall (<10nm) absorb only a small amount of the dye, which is washed out by acetone.

They are stained pink by a second dye and are defined as Gram negative. These latter also possess an outer membrane, made up of liposaccharides, which is similar to the cell wall. Such differences in cell walls play a key role in the targeting of both types of bacteria by antibacterial agents.

Mechanisms of Antibacterial action There are five principle mechanisms by which antibiotics act: Inhibition of cell wall synthesis: This results in the construction of faulty cell walls, which are unable to control the flow of water and nutrients in/out of cell, lysis and cell death results. Targeting of plasma membrane: The membrane becomes permeable, resulting in cell death.

Antimetabolites Selectively target bacterial-enzyme catalysis, impeding bacterial growth. Inhibition of protein synthesis: Selectively block synthesis of essential proteins and enzymes. Inhibition of nucleic acid functions: Selectively target transcription and replication, which impede cell division.

A substance is classified as an antibiotic if the following conditions are met: It is a product of metabolism (although it may be duplicated or even have been anticipated by chemical synthesis). It is a synthetic product produced as a structural analog of a naturally occurring antibiotic. It antagonizes the growth or survival of one or more species of microorganisms. It is effective in low concentrations

The β-Lactam antibiotics Penicillins

1- β lactam ring 2- Thiazolidine ring

Penicillin G Active against Gram +ve bacilli and some Gram –ve cocci. Non toxic. Limited range of activity. Inactive orally, must be injected. Sensitive to β- lactamases. Same patients are allergic. Inactive against Stphylococci.

SAR Amide & carboxylic acid are involved in binding. Carboxylic acid binds as the carboxylate ion. Mechanism of action involves the β - lactam ring. Activity related to β - lactam ring strain (subject to stability factors).

Bicyclic system increases β - lactam ring strain. No much variation in structure modification is possible. Variations are limited to side chain (R).

Mechanism of action Penicillin inhibit a bacterial enzyme called the transpeptidase enzyme which is involved in the synthesis of the bacterial cell wall. The β - lactam ring is involved in the mechanism of inhibition. Penicillin becomes covalently linked to the enzyme’s active site leading to irreversible inhibition.

Problems with Penicillin G Sensitivity to stomach acids. Sensitivity to β - lactamases enzymes which hydrolyse the β - lactam ring. Limited range of activity.

Acid sensitivity

Better acid stability & orally active, but sensitive to β - lactamases. Slightly less active than penicillin G with allergy problems with some patients.

Sensitivity to β - lactamases β - lactamase enzyme is the enzyme that inactivate penicillins by opening β - lactam rings, that allow bacteria to be resistant to penicillin. β - lactamase is transferable between bacterial strains (bacteria can acquire resistance). 80% of staphylococcus aureus infections in hospitals were resistant to penicillin and other antibacterial agents by 1960.

Mechanism of action for lactamases is identical to the mechanism of inhibition for the target enzyme, but product is removed efficiently from the lactamase active site.

Block access of penicillin to active site of enzyme by introducing bulky groups to the side chain to act as steric shields. Size of the shield is crucial to inhibit reaction of penicillins with β - lactamases but not with the target enzyme (transpeptidase).

Methicillin : Methoxy groups block access to ß- lactamases but not to transpeptidases. Active against some penicillin G resistant strains (staphylococcus). Acid sensitive & must be injected. Lower activity compared to penicillin G (reduced access to transpeptidase). Poorer range of activity. Poor activity against some streptococci. Inactive against Gram –ve bacteria.

Oxacillin Orally active & acid resistant. Resistant to ß- lactamases. Active against staphylococcus aureus. Less active than other penicillins. Inactive against Gram –ve bacteria Nature of R & R’ influences absorption & plasma protein binding. Cloxacillin better absorbed than Oxacillin. Flucloxacillin less bound to plasma protein, leading to higher levels of free drug.

Range of activity Factors: Cell wall may have a coat preventing access to the cell. Excess transpeptidase enzyme may be present. Resistant transpeptidase enzyme (modified structure). Presence of β - lactamases. Transfer of β - lactamases between strains. Efflux mechanism

Strategy: The number of factors involved make a single strategy impossible. Use trail & error by varying R groups on the side chain Successful in producing broad spectrum antibiotics. Results demonstrate general rules for broad spectrum activity.

Results of varying R in Penicillin G R= hydrophobic, results in high activity against Gram +ve bacteria & poor activity against Gram –ve bacteria. Increasing hydrophobicity has little effect on Gram +ve activity but lowers Gram –ve activity. Increasing hydrophilic character has little effect on Gram +ve activity but increases Gram –ve activity. Hydrophilic groups at α position (eg. NH2, OH, COOH) increases activity against Gram –ve bacteria.

Aminopenicillins Have NH2 at the α position. Ampicillin & Amoxycillin

Properties: active against Gram +ve & Gram –ve bacteria which don’t produce ß- lactamases. Acid resistant & orally active, Non toxic. Sensitive to ß- lactamases Increased polarity due to extra amino group. Poor absorption through the gut wall. Disruption of gut flora lead to diarrhoea. Inactive against Pseudomonas aeruginosa.

Prodrugs of Ampicillin Properties: Increased cell membrane permeability. Polar carboxylic acid group is masked by the ester Ester is metabolised in the body by esterases to give the free drug

β - lactamase inhibitors Clavulanic acid Weak, unimportant antibacterial activity. Powerful irreversible inhibitor of β - lactamases.

Carboxypenicillins Containing COOH at α position Carbenicillin: Active over a wider range of Gram –ve bacteria than ampicillin Active against Pseudomonas aeruginosa. Resistant to most β - lactamases. Less active against Gram +ve bacteria. Acid sensitive & must be injected.

Prodrug of carbenicillin Acid resistance & taken orally

Ticarcillin: Administered by injection. Identical antibacterial spectrum to carbenicillin. Smaller doses required compared to carbenicillin. More effective against Pseudomonas aeruginosa fewer side effects Can be administerd with clavulanic acid.