Lec9- Anti TB agents Flashcards
1
Q
Mycobacteria and disease
A
- Mycobacterium tuberculosis- the agent of tuberculosis in humans. Humans are the only reservoir for the bacterium
- Mycobacterium bovis- the causative agent of TB in cows and rarely in humans. Humans can be infected by the consumption of unpasteurised milk
- Mycobacterium avium-intracellulare- causes a TB-like disease especially prevalent is AIDS patients
- Mycobacterium leprae- the causative agent of leprosy
2
Q
Tuberculosis- facts
A
- “White plague”, “consumption”, “the captain of death”
- Mycobacterium tuberculosis (MTB) was the cause of white plague of the 17th and 18th centuries in Europe
- During this period nearly 100% of European population was infected with MTB, and 25% of all adult deaths were caused by MTB
- Egyptian mummy, 3400 BC, DNA identified
- 1/3 of world population have latent TB
- 9 million new cases per year; 2 million death per year
- 10% of all deaths from infectious disease
- Association with poverty, poor living conditions, poor nutrition, AIDS
- Contagious (WHO flights >8hr are risk)
3
Q
TB case notification and rates
A
4
Q
Most frequent countries of birth for non-UK born TB cases, UK, 2016
A
5
Q
Rate of TB by deprivation decile, England, 2016
A
- In 2016, the rate of TB was 21.5 per 100,000 in the 10% of the population living in the most deprived areas compared with only 3.4 per 100,000 in the 10% of the population living in the least deprived areas, with a clear trend of an increasing rate of TB with increasing deprivation
6
Q
Tuberculosis
A
- Primary infection (inhalation of droplets)
- Bacteria settle in lungs and grow
- Delayed-type hypersensitivity, aggregates of macrophage (tubercles)
- In individual with low resistance => acute lung infection, destruction of tissue, spread to other parts of the body, death
7
Q
Mycobacteria
A
- Coloured scanning electron micrograph (SEM) of Mycobacterium tuberculosis bacteria
- Rod-shaped, G+ bacteria
- Acid-fast (Ziehl-Neelsen stain)
- Possess unique lipid-rich cell wall
- Slow growth (doubles in 24 hrs)
- In most cases acute infection doesn’t occur remains localised and subsides
- Individuals are hypersensitised
- Mantoux and heaf tests for hypersensitivity: intradermal injection of tuberculin (protein fraction from M.tuberculosis)
- Hardening and swelling indicates previous exposure not active disease
8
Q
TB protection, diagnosis and treatment
A
- BCG vaccine (live cells of bacille Calmette-Guerin, attenuated strain of M.bovis subcultured 230 times 1908-19)
- Given on entry to secondary school (13yrs) in UK (currently in risk areas and only when Mantoux/Heaf-negative)
- Vaccination means Mantoux/Heaf tests are not useful for diagnosis
9
Q
Diagnosis and treatment
A
- Chest X-ray, Mantoux/Heaf tests (but only unvaccinated), direct sputum stain (Ziehl-Neelsen fluorescent Ab), PCR (detects DNA)
- Comination therapy (isoniazid, rifampicin, ethambutol, pyrazinamide) due to drug resistance
- Multiple drug resistance TB especially in AIDS
10
Q
Mycobacterial cell wall structure
A
- Main things to remember
- Cell wall contains mycolic acid
- Long chain fat derivatives= Hydrophobic surface= stops drug molecules going in
- Stops reactive O2 species from killing them (main mechanism of macrophages to kill organisms) allowing them to survive in macrophages
11
Q
Mycolic acids in M.tuberculosis
A
*
12
Q
Mycolic acids- consequences
A
- The high concentration of lipids in the cell wall of mycobacterium tuberculosis means that the cell wall is very waxy
- Impermeability to stains and dyes
- Resistance to many antibiotics- cant get in
- Resistance to killing by acidic and alkaline compounds
- Resistance to osmotic lysis
- Resistance to lethal oxidations and consequent survival inside macrophages
13
Q
Anti-mycobacterial agents- Agents used in TB
Bold also used in leprosy
A
-
Ribosome
- __Streptomycin
- Capreomycin
- Metabolsim- Dapsone
-
Cell Wall-
- Isoniazid
- Ethambutol
- Cycloserine
-
Chromosome
- Rifampicin
- Rifabutin
-
Unknown
- Pyrazinamide
- Clofazimine
14
Q
Anti-TB combination Therapy
A
- A two-month long treatment with four drugs; Either
- Streptomycin, Isoniazid, Rifampin, Pyrazinamide OR
- Ethambutol, Isoniazid, Rifampin and Pyrazinamide (different mechanism because the organism is probably resistant to one or multiple different drugs)
- This is then followed by four months of isoniazid and rifampin
- Patients following this regimen become non-infectious after the first few weeks
- The remaining months are necessary to eradicate the slow-growing fraction of the.
- Side effects (hepatotoxicity) sometimes force treatment termination
- Early termination encourages the growth of drug-resistance bacilli
15
Q
Streptomycin- Ribosome
A
- Streptomycin, along with kanamycin and amikacin, are aminoglycosides
- Target the 30S subunit of the ribosome
- Possibly binding at the interface between the ribosomal subunits
- The mycobacteria cell wall does not seem to hamper the entrance of these very hydrophilic antibiotics (Entry via the porins)
- V.polar molecule so shouldn’t enter the bacteria but it does (probably specific protein channels)
16
Q
Rifampin- Chromosome
A
- Semisynthetic antibiotic obtained by reacting 3-formylrifamycin with 1-amino-4-methylpiperazine
- Specifically inhibits bacterial RNA polymerase (responsible for DNA transcription) by forming a stable drug-enzyme complex
- The corresponding mammalian enzymes are not affected by rifampin
- Bacterial resistance to rifapin is caused by mutations leading to a change in the structure of the beta subunit of RNA polymerase
17
Q
Isoniazid- Cell Wall
A
- Isoniazid is first activated by KatG into an unstable intermediate which reacts with NAD to give compounds such the one shown below
- The resulting adduct then binds the NAD(H) recognition site of InhA and this leads to the antimycobacterial effect [InhA is involved in mycolic acid synthesis]
- Isoniazid is a pro-drug
20
Q
The Fluoroquinolones
A
- This class of antibiotics is indispensable for the treatment of MDR TB
- E.g. Ciprofloxacin
- Mechanism: Inhibition of the enzymes topoisomerase II (DNA gyrase) and topoisomerase IV, which are required for bacterial DNA replication, transcription, repair, strand supercoiling repair and recombination
- acts on chromosomes
21
Q
Coming soon- Teixobactin
A
- A promising therapeutic candidate discovered in a screen of uncultured [soil] bacteria
- Active against G+ bacteria and M.Tuberculosis
- Effective against drug-resistant pathogens in a number of animal models of infection
- Binds to multiple targets, none of which is a protein
- Teixobactin inhibits cell wall synthesis by binding to a highly conserved motif of lipid II (precursor of peptidoglycan) and lipid III (precursor
- No mutant MRSA or TB could be produced that were resistant to teixobactin
- The properties of this compound suggest a path towards developing antibiotics that are likely to avoid development of reisistance
- Resistance would likely to the form of an antibioitc modifying enzyme rather than modification of binding targets
