Tuberculosis

Question 1

What is mycobacterium tuberculosis?

Answer 1

slow-growing aerobic bacterium, relatively resistant to most antibiotics
can be dormant (granulomas)
an acid fast bacteria - after staining with dye, cannot be decolorized by acid wash
made up of lipid rich cell wall containing mycolic acids and is impermeable to many drug
causes both latent and active infections

Question 2

What is the mycobacterium composed of?

Answer 2

mycolic acid rich, arabinogalactan, peptidoglycan, lipid bilayer

Question 3

What is the treatment of active Tb infections?

Answer 3

RIPE - combo of rifampin, isoniazid (INH), pyrazinamide, and ethambutol
alternative: rifapentine, INH, pyrazinamide, and moxifloxacin
use a combo of drugs because different ones are needed to combat dividing and dormant forms and to help with resistance

Question 4

Isoniazid

Answer 4

specific for M. tb
bactericidal
only active against growing M. tb
pro-drug: activated by M. tb KatG protein

Question 5

What is the MOA of isoniazid?

Answer 5

activation by KatG –> forms adducts with NAD+ and NADP+ –> inhibits enzymes that use NAD+ and NADP+
activated isoniazid inhibits InhA (component of FAS II) and KasA, inhibiting mycolic acid synthesis –> defective cell wall

Question 6

What is InhA?

Answer 6

a component of FAS II that catalyzes the NADH-dependent reduction of fatty acids bound to acyl carrier protein
isoniazid inhibits this

Question 7

What is the synthesis of mycolic acid?

Answer 7

FAS I makes 20 carbon precursors (linked to CoA) –> FAS II makes 56 carbon chains (linked to CoA) –> addition of side chain and cyclopropyl groups –> transport out of cell –> linkage to arabinogalactan

Question 8

Isoniazid resistance

Answer 8

over-expression of InhA (low level of resistance)
mutations in KatG (higher levels of resistance)

Question 9

What is the metabolism of isoniazid?

Answer 9

acetylation by liver N-acetyltransferase (NAT2)
rate determined genetically - slow or rapid metabolizers

Question 10

What is the toxicity of isoniazid?

Answer 10

hepatitis is major concern - loss of appetite, N/V, jaundice
peripheral neuropathy

Question 11

What is the mechanism of isoniazid toxicity?

Answer 11

acetylisoniazid can be converted to acetylhydrazine: CYP2E1 converts acetylhydrazine to hepatotoxic metabolites
NAT2 can acetylate acetylhyrazine to nontoxic diacetylhydrazine
rapid acetylators will rapidly remove acetylhyrazine; slow acetylators or induction of CYP2E1 will lead to more toxic metabolites

Question 12

Which drug induces CYP2E1?

Answer 12

rifampin induces CYP2E1 –> potentiates isoniazid hepatotoxicity

Question 13

How do you reverse peripheral neuropathy caused by isoniazid toxicity?

Answer 13

reversed by administering pyridoxine (vitamin B6)
isoniazid resembles pridoxine: isoniazid competitively inhibits pyridoxine phosphokinase (prevents conversion to its active form); INH metabolites directly inactivate pyridoxine species

Question 14

Pyrazinamide

Answer 14

sterilizing agent against residual intracellular bacteria
structurally similar to nicotinamide
pro-drug: requires conversion to pyrazinoic acid by pncA

Question 15

What is the activity of pyrazinamide dependent on?

Answer 15

pH!
inactive at neutral pH
activated by low pH - pH<5.5

Question 16

What is the MOA of pyrazinamide?

Answer 16

inhibition of panD leading to inhibition of coenzyme A synthesis (CoA used for a lot of different processes)
reduces accumulation of CoA precursors after panD step, increases levels of free fatty acids

Question 17

What does panD do?

Answer 17

panD converts L-aspartate to alanine
pyrazinoic acid (POA) binds to panD, but pyrazinamide (PZA) does not
POA does not bind to mutant panD

Question 18

What is the binding affinity of panD?

Answer 18

panD binding affinity is low
binding leads to degradation of panD (doesn’t completely inhibit panD, leads to loss of panD)
accumulation in granuloma offsets low binding affinity

Question 19

Pyrazinamide resistance

Answer 19

primarily due to mutations in pncA

Question 20

What is the toxicity of pyrazinamide?

Answer 20

joint pain (arthralgia) most common
hepatitis most dangerous - from hydroxylated POA
pyrazinamide most common cause of hepatitis in 4 drug treatment

Question 21

Ethambutol

Answer 21

bacteriostatic inhibitor of M. tb (doesn’t kill bacterium)
resistance due to over-expression of or mutations in arabinosyl transferase

Question 22

What is the MOA of ethambutol?

Answer 22

inhibits mycobacterial arabinosyl transferases, which are involved in the polymerization of arabinogalactan –> results in build up of arabinan
inhibits formation of arabinogalactan and liparabinomannan (LAM)

Question 23

What is ethambutol synergistic with?

Answer 23

rifampin - increases penetration into cell

Question 24

What is the toxicity of ethambutol?

Answer 24

optic neuritis

Question 25

Rifampin

Answer 25

reduced length of therapy from 18 to 9 mo most effective 1st line agent! - high sterilizing activity, rapidly renders patients non-infectious penetrates most tissues and phagocytic cells active against growing and stationary cells with low metabolic activity can kill M. tb inaccessible to many other drugs

Question 26

What is the MOA of rifampin?

Answer 26

bactericidal; most effective when cell division is occurring binds to RNA polymerase deep within the DNA/RNA channel - binds far from active site --> blocks the path of the elongating RNA

Question 27

What is rifapentine?

Answer 27

derivative of rifampin contains a cyclopentyl ring that makes it more lipophilic --> longer half life

Question 28

What are the adverse effects of rifampin?

Answer 28

colors urine, tears, and sweat orange potent inducer of CYP450s increase in CYP enzymes increases elimination of other drugs metabolized by these enzymes

Question 29

Fluoroquinolones can replace what?

Answer 29

ethambutol - shorter treatment period of 4 months moxifloxacin, gatifloxacin, and levofloxacin

Question 30

What is the MOA of fluoroquinolones?

Answer 30

traps gyrase on DNA as ternary complex --> prevents resolution of supercoiled DNA --> disrupts DNA replication bactericidal

Question 31

Which FQ is preferred?

Answer 31

moxifloxacin - better PK better penetration from plasma to tissues better at reducing and/or killing M. tb in lesions

Question 32

What drugs are associated with hepatitis?

Answer 32

isoniazid, pyrazinamide, and rifampin

Question 33

What drugs are associated with eye damage?

Answer 33

ethambutol - loss of visual acuity and red-green color blindness

Question 34

What drug is associated with discolored urine?

Answer 34

rifampin

Question 35

What is the BPaL regimen?

Answer 35

combo therapy: bedaquiline, pretomanid, and linezolid (inhibits protein synthesis) treats drug-resistant tuberculosis (XDR-TB) and treatment-intolerant or non-responsive multidrug-resistant TB (MDR-TB)

Question 36

Bedaquiline

Answer 36

oral administration bactericidal against actively growing and dormant bacilli - has sterilizing activity

Question 37

What is the MOA of bedaquiline?

Answer 37

inhibits ATP synthase resistance: mutations in atpE (target enzyme)

Question 38

What is bedaquiline metabolized by?

Answer 38

CYP3A4 - metabolite is less active

Question 39

What are the nitroimidazoles?

Answer 39

pretomanid and delamanid N-O2 group important for activity of the compound

Question 40

What is the MOA of pretomanid?

Answer 40

prodrug activated by M. tb deazaflavin-dependent nitroreductase (Ddn); inhibition of mycolic acid synthesis

Question 41

What happens with pretomanid in aerobic conditions?

Answer 41

forms reactive intermediate metabolite that inhibits mycolic acid production

Question 42

What happens with pretomanid in anaerobic conditions?

Answer 42

generates reactive nitrogen species such as NO; direct poisoning of the respiratory complex --> ATP depletion increases killing by innate immune system

Question 43

What are second line agents?

Answer 43

streptomycin - protein synthesis inhibitor ethionamide para-aminosalicyclic acid - folate synthesis antagonist cycloserine - cell wall synthesis inhibitor, analog of D-alanine capreomycin - peptide protein synthesis inhibitor