Mechanism of drug action (antibacterial, antifungal, antiprotozoans)
An antimicrobial is naturally occurring microbial products that kills or inhibits the growth of microorganisms. The microbial agent may be a chemical compounds and physical agents. These agents interfere with the growth and reproduction of causative organisms like bacteria, fungi, parasites, virus etc.
Ideal characteristics of antibiotics
- Selective toxicity (it should kill or inhibit the pathogen)
- It must be microbicidal.
- It should be water soluble and stable.
- It must not allow susceptible organism to become resistant.
- It should complement the host immune system.
- It should be rapidly metabolized, readily delivered to the site of action.
- Reasonable price.
- High therapeutic index
Classification of antimicrobial agents
Mechanism of drug action (antibacterial)
There are three ways of classifying antibacterial agents:
1) according to whether they are bactericidal or bacteriostatic.
2) by Target site.
3) by chemical structure.
Bacteriostatic or bactericidal
- Some antibacterial agents kill bacteria(bactericidal) while others only inhibits their growth (bacteriostatic). Thus, the bactericidal process is irreversible, while bacteriostatic is reversible.
- Bacteriostatic agents are successful in the treatment of infections because they prevent the bacterial population from increasing and host defense mechanism can consequently cope with the static populations.
- As a means of classification the distinction between bacteriostatic and bactericidal agents is blurred because some agents are capable of killing some species but are only bacteriostatic for others.
By Target site
There are five main target sites for antibacterial actions –
- Cell wall synthesis
- Protein synthesis
- Nucleic acid synthesis
- Metabolic pathways
- Cell membrane functions
- These target differs to a greater or lesser degree from those in the host(human) cells and so allow inhibitions of the bacterial cell without inhibitions of the mammalian cell targets.
- Each target site encompasses a multitude of synthetic reactions, each of which may be specifically inhibited by an antibacterial agent.
- A range of chemically diverse molecules may inhibit different reactions at the same target site(eg. Protein synthesis inhibitors).
Inhibitions of cell wall synthesis
- Peptidoglycan is a vital component of the bacterial cell wall
- It is a compound unique to bacteria and therefore provides an optimum target for selective toxicity.
- Synthesis of peptidoglycan precursors starts in the cytoplasm, wall subunits are then transported across the cytoplasmic membrane and finally inserted into the growing peptidoglycan molecule.
- So, different stages are potential targets for inhibition.
- The antibacterial that inhibit cell wall synthesis are varied in chemical structure.
- The most important agents are:-
- beta lactams and
- These are active only against gram positive organisms.
- It contain a beta – lactam ring and inhibit cell wall synthesis by binding to penicillin- binding protein.
- The different groups within the family are distinguished by the structure of the ring attached to the beta- lactam ring in penicillin this is a five membered ring in cephalosporins a six membered rings and by the side chains attached to these rings.
- Pbps are membrane proteins capable of binding to penicillin and are responsible for the final stages of cross- linking of the bacterial cell wall structure.
- Inhibition of one or more of these essential enzyme results in an accumulation of precursor cell wall unit, leading to activation of the cells autolytic system and cell lysis.
- Administered tramuscularly or intravenously, some orally active agents are also there.
- Glycopeptides are large molecules and act at an earlier stage than beta lactams.
- It includes vancomycin and teicoplanin which faces difficulty to penitrate into gram – ve cells.
- Teicoplanin is a natural complex of five different but closely related molecules.
- Glycopeptides are bactericidal.
- It interfere with cell wall synthesis by binding to terminal D- alanine-D-alanine at the end of pentapeptide chains that are part of the growing bacterial cell wall structure.
- This binding inhibits the transglycosylation reaction and prevents incorporation of new subunits into the growing cell wall.
Inhibitors of protein synthesis
- Protein synthesis proceeds in an essentially similar manner in prokaryotic and eukaryotic cells.
- It is possible to exploit the differences (70s&80s ribosome) to achieve selective toxicity.
- The process of translation of the mRNA chain into its corresponding peptide chain is complex and a range of antibacterial agents acts as inhibitors (mechanism of action is not yet known).
- Aminoglycosides are a family of related molecule with bactericidal activity.
- It contains either streptidine or 2-deoxystreptamine.
- The original structure have been modified chemically by changing the side chains to produce molecule such as amikacin and netilmicin which are active against organism that have developed resistance earlier to aminoglycosides.
- Aminoglycosides act by binding to specific proteins in the 30s ribosomal subunits where they interfere with the binding of formylmethionyl transfer RNA to the ribosome.thereby preventing the formation of initiation complexes from which protein synthesis proceeds.
- In addition aminoglycosides causes misreading of mRNA codons and tend to break apart functional polysomes into non-functional monosomes.
- It must be given intravenously or intramuscularly for systematic treatment.
- Gentamicin and the newer aminoglycosides are used to treat serious gram-ve infections.
- It are bacteriostatic compounds and are a family of large cyclic structures that have several sites of possible chemical substitutions.
- It inhibit protein synthesis by binding to the small ribosomal subunits in a manner that prevents amino acyl transfer RNA. From entering the acceptor sites on the ribosome.
- They are usually administered orally.
- They are active against a wide variety of bacteria but their use is restricted due to widespread resistance.
- It should be avoided in pregnancy and in children under 8 years of age.
- It contains a nitrobenzene nucleus and prevent peptide bonds synthesis with a bacteriostatic result.
- It has affinity for the large ribisomal subunits(50s) where it blocks the action of peptidyl transferase, thereby preventing peptide bond synthesis.
- Drug had inhibitory activity on humans mitochondrial ribosomes which may account for some of the dose- dependent toxicity to bone marrow.
- It is well absorbed when given orally
- It can be given intravenously or topical preparations are also available.
- It is metabolized in the liver by conjugation with glucuronic acid to yield a microbiologically inactive form that is excreted by the kidneys.
Inhibitors of nucleic acid synthesis
- These are bactericidal synthetic agents that interfere with replication of the bacterial chromosome.
- The antibacterial activity of quinolones is due to their ability to inhibit the activity of bacterial DNA gyrase and topoisomerase.
- During replication of the bacterial chromisome DNA gyrase produces and removes supercoils in DNA ahead of the replication fork to maintain the proper tension required for efficient DNA duplication.
- Topoisomerase acts to remove supercoils and to separate newly formed DNA strands after replication.
- These enzyme thus act in correct to ensure that the DNA molecule has the proper confirmation for efficient replication and packaging within the cell.
- Quinolones are able to interfere with these essential enzyme in bacteria while not affecting their counter parts in mammalian cells.
- Rifamycins is clinically the most important and blocks the synthesis of mRNA.
- It is a large molecule with complex structure and are bactericidal in activity.
- It binds to DNA dependent RNA polymerase and blocks the synthesis of mRNA.
- Administered orally.
- Metabolized in liver and excreted in bile
Inhibitors of cytoplasmic membrane
- The cytoplasmic membrane that encompass all kinds of living cells perform a variety of vital functions.
- The structure of these membrane in bacterial cell differs from that in mammalian cells and allow the application of some selectively toxic molecules but there are few in number compared with those acting at other target sites.
- Lipopeptidea are a new class of membrane active antibiotics
- Daeptomycin is a lipopeptide antibiotic with bactericidal activity against a wide variety of gram positive bacteria including vancomycin, resistant E. faecalic and E. faecium.
- The compound acts in a calcium dependent matter to insert and depolarize the bacterial cytoplasmic membrane.
- This action leads to a number of consequences including the inability to synthesize ATP and interference with uptake of nutrients.
- It acts on the membrane of gram Negative bacteria except proteue spp.
- They are bactericidal cyclic polypeptides that disrupt the structure of cell membranes.
- The free amino group of polymyxins acts as cationic detergents disrupting the phospholipids structure of the cell membrane.
- Primarily used topically in ointment but have also been used in PST for gut decontamination, wound irrigation and as a bladder washout.
By chemical structures
- Classification based on chemical structure alone is not of practical use, because there is such diversity.however a combination of target site and chemical structure provides a useful working classification to organize antibacterial agents into specific families.
Resistance to antibacterial agents
- Some bacteria are born resistant,others have resistance thrust upon them.in other words some species are innately resistant to some families of antibiotics either because they lack a susceptible target or because they are impermeable to the antibacterial agent.
The genetics of resistance
- Chromosomal mutation may result in resistance to a class of antimicrobial agents.
- Genes on transmissible plasmids may result in resistance to different classes of antimicrobial agents(multiple resistance).
- Resistance may be acquired from transposons and other mobile elements.
- Cassettes of resistance genes may be organized into genetic elements called integrons
- Compared with antibacterials, the number of suitable antifungal drugs is very limited.
- Selective toxicity is much more difficult to achieve in the eukaryotic fungal cells than in the prokaryot for bacteria.
- Most drugs have one or more limitations such as side effect, poor penetration to certain tissue, problem of solubility, stability and absorption of existing drugs.
- Drug resistance is also increasing.
Classification of antifungal agents
- Antifungals can be classified on the basis of the target site and chemical structure.
- This reveals the major difference between antibacterial and antifungal agents with the major antifungal acting on the synthesis or function of the intercellular membranes.
- Exceptions are flucytosine and griseofulvin, which interfere with DNA synthesis.
- There are currently no inhibitors of fungal protein synthesis that do not also inhibit the equivalent mammalian pathway.
1) azole compounds and echinocandians (cell membrane synthesis)
- Azole antifungals act by inhibiting lanosterol C14- demethylase, an important enzyme in sterol biosynthesis
- Resistance to the azole is becoming more widespread and threatens to compromise this group of compounds.
- Newer azole compounds include posaconazole and voriconazole.
- The more recent echinocandin antifungals offer new therapeutic options against infections such as Aspergillus, candida and pneumocystis.
2) polyenes inhibit cell membrane function
- Amphotericin B and Nystatin act by binding to sterols in cell membranes, resulting in leakage of cellular contents and cell death.
- Amphotericin is used to treat the serious systemic fungal infections despite it’s serious toxic side effects; lipid formulations have lower toxicity.
- Nystatin is used only in topical formulations.
3) flucytosine and griseofulvin inhibit NA synthesis
- Flucytosine(5-flurocytosine) is deaminated to 5- flurouracil, which inhibit DNA synthesis
- Flucytosine is active only on yeasts( Candida spp. And cryptococcus).
- Resistance emerges rapidly to flucytosine so used in combination with amphotericin B.
- Griseofulvin appears to inhibit nucleic acid synthesis and to have antimitotic activity, by inhibiting microtubule assembly.
- It may also have effects on cell wall synthesis by inhibiting chitin synthesis.
- In the host, griseofulvin binds specifically to newly formed keratin and is active in vivo only against dermatophyte fungi.
4) other topical antifungal agents
- A variety of agents such as:-
- Whitfield’s ointment(mixture of benzoic and salicylic acids)
- These are available as creams for the topical treatment of superficial mycoses.
Fungi develop resistance to antifungal agents
- As compared with bacteria antifungal resistance is less studied.
- There is evidence that many similar mechanism as bacteria operate in resistance to antifungals(in case of azole compounds in Aspergillus Candida and Cryptococcus.
- Enzyme modifications
- Target modifications
- Reduced permeability
- Active efflux pumps
- Failure to activate antifungal agents.
- Very large number of different parasites are capable of infecting humans.
- The complexities of their life cycles and the differences between them in their metabolic pathways leads the drug inactive against protozoa like helminths.
- Protozoa and helminths are eukaryotes there- fore metabolically more similar to humans than are bacteria.
- Although some antibacterials do have antiprotozoan activity but in general, antibacterial are in effective against parasites.
- The major challenge is to identify targets where there are sufficient differences between host and parasites to facilitate safe drug activity.
Some if the target site includes
- Unique drug uptake(chloroquine, mefloquine,primaquine in malaria.)
- Differences in folic acid metabolism(pyrimethamine in malaria, sulfonamides in toxoplasmosis, trimethoprim in cyclosporiasis)
- Polyamine uptake(pentamidine in leshmaniasis)
- Intracellular calcium levels(praziquantel against flukes and tapeworms)
- Unique trypanothione- dependent reduction mechanism(fluromethylornithine against trypanosomes)
- Unique neurotransmitters(piperazine, ivermectin,pyrantel against nematodes)
- Cytoskeletal proteins(tubulin)-benzimidazoles against nematodes.
- Oxidative phosphorylation(niclosamide against tapeworms)
- Drug resistance is the significant problem in the treatment of parasitic infections, particularly with malaria.
- Falciparum malaria and p. Vivax is resistant to antimalarial agents like chloroquine.
- Alternative to chloquine is sulfadoxine/pyrimethamine..but they are now resistant to antifoliate compounds.
- Drug combinations are now advocated for the treatment of falciparum malaria to reduce the chance of developing drug resistance after monotherapy, as happened with chloroquine and Artemisinin combination therapy(ACT) will replace quinine.
- Protozoa makes use of enzyme and target modifications to develop resistance, but active efflux pumps have been described in resistance of P. falciparum to chloroquine, mefloquine and Artemisinin.
- Resistance to benzimidazole anthelmintic involve target modifications arising from mutation in cuticular tubulin.