Fan - Antimycobacterial Therapies

Question 1

Under what circumstances are second line TB drugs used?

Answer 1

• multi drug resistant TB - resistance to isoniazid and rifampin
• extensively drug-resistant TB - MDR +. Resistanc to Fluoroquinoolones and inject isle Aminoglycosides
• first line is too toxic

Question 2

What parts of mycobacterium are unique, which allow us to target them? What do we use to target them?

Answer 2

Mycolate - inhibited by isoniazid (INH)

Arabinogalactan - inhibited by ethambutol (EMB)

Question 3

What do each of the first line TB drugs do?

Answer 3

Rifampicin - inhibits RNA synthesis by target RNA Polymerase

Pyrazinamide - inhibits protein synthesis in dormant bacteria

Isoniazid - inhibits mycolic acid synthesis

Ethambutol - inhibits cell wall synthesis by targeting arabinogalactan synthesis

Question 4

How do we initially treat TB???

Answer 4

Start With all 4 first line treatment.

• if the patient can cough up some sputum for us to to genome sequencing then we can narrow down which drugs to use.
• Otherwise, it takes 3-8 weeks to identify resistant strains.

Question 5

Mechanisms of drug resistance

Answer 5

Genetic mechanism - natural resistance due to chromosomal mutation

Biochemical mutation - over expressed drug target, decreased drug binding, increased drug removal, deficiency in pro-drug activation.

Question 6

Why are insufficient drugs so bad?

Answer 6

The create an environment that gives all of the resources to the microbes that are most resistant and then they grow.

Question 7

If bacteria are culture-positive, how do we treat them?

Answer 7

So, after the 8 weeks are up and we can figure out what we are working with, we scale back the 4 drugs and only use 2.
- use for either 6 or 9 months.

Question 8

If bacteria are culture-negative how do we treat them?

Answer 8

Again we treated for 8 weeks with the 4 drugs and then treat for 2 more months with INH and RIF

Question 9

Why do we do multi-drug therapy?

Answer 9

For INH you have 1 spontaneous resistant per 1,000,000 bacteria. In TB lesion you have about 100,000,000 bacteria. So, even before treatment you would have 100 bacteria that are resistant. That 100 though can now take all of the resources and dominate. Rifampin is better than INH in this regards because only 1/100,000,000 will have spontaneous resistance. But that leaves 1 that will take the resources and Dominate. If we combine the two then you will be highly unlikely to have any resistant microbes. This is why we start with 4 drugs usually.

Question 10

Challenges in treating. TB

Answer 10

• very slow growing Infection - bacteria can hide in macrophages and wait until immunosuppression occurs and then they break out and make moves.
• MT readily accumulate mutations that cause drug resistance
• doctor and patient compliance - long treatment (6-9 months)
• lipid-rich cell wall - provides protection from many antibiotics.
• HIV/AIDS often coexist with TB

Question 11

Isoniazid

Answer 11

Pro-drug that is activated by bacterial KatG.

• KatG oxidizes the INH to form double bonds.
• this ionized form then combines with NAD to make the active form of the drug.
• inhibits mycolic synthesis.

Question 12

Mechanism of action of isoniazid

Answer 12

Mycolic acid has a long chain and a short chain that are both synthesized by FAS-I (for the short parts) and FAS-II (for the longer part). They then come together to form the cell wall.
- INH targets the FabI step of biosynthesis. INH has a higher affinity for the NADH than does FabI, so biosynthesis of mycolic acid does not proceed.

Question 13

Adverse effects of isoniazid

Answer 13

Hepatitis (varies with age, it is safer the younger you are)

Peripheral neuropathy

Question 14

Why is peripheral neuropathy a side effect of isoniazid?

Answer 14

Because isoniazid looks a whole lot like Vitamin B6, so you would have a pyridoxine deficiency.
- give pyridoxine when you give this drug.

Question 15

Resistance to INH?

Answer 15

• mutations in KatG so INH can’t be formed
• mutations in NADH binding pocket of Fab1 that reduce affinity for INH
• mutations that increase expression of Fab1
• mutations that cause elevated NADH, which outcompeted INH-NAD for Fab1

Question 16

Rifampin

Answer 16

Inhibits RNA synthesis

Question 17

RIfampin mechanism of action

Answer 17

• prevents RNA exit from polymerase by interacting with large subunit of bacterial polymerase and directly blocking the path of the growing RNA
• blocks departure of bacterial RNA polymerase from gene promoters.
• human RNA polymerase do not bind rifampin.

Question 18

Rifampin resistance

Answer 18

Resistance arises from mutations in the gene encoding the beta-subunit of RNA Polymerase so that they don’t bind the drug.

Question 19

Adverse effects of rifampin

Answer 19

• May cause flu-like symptoms
• induces Cytochrome P450 members including CYP3A, which causes elimination of other drugs
• can cause hepatitis
• can give purple or red color urine, sweat, or tears

Question 20

What added benefits do the derivatives of rifampin have?

Answer 20

• Higher potencies
• longer half lives
• better membrane permeability
• less active in CYP3A induction so it has better compatibility with other meds

Question 21

Pyrazinamide

Answer 21

• looks similar to INH but acts differently.
• also a pro-drug
• if Pyrazinamide is used in treatment, treatment can be shortened to 6 months instead of 9.

Question 22

Mode of action of PZA?

Answer 22

Not exactly sure of the mechanism but we now that it doesn’t t affect the cell wall synthesis. It might have multiple modes, one might be inhibition of trans-translation. The other might be inhibition of pantothenate and CoA synthesis.

Question 23

Ethambutol

Answer 23

Inhibits biosynthesis of arabinogalactan. It blocks the EmbA and EmbB arabinotransferases that incorporate arabinotransferases into the arabinogalactan layer.
- inhibits rapidly growing extracellular Mtb

Question 24

Adverse effects of ethambutol

Answer 24

Optic neuritis and red/green color blindness, which is why we don’t give it to children. They can’t tell us if they can’t see.

Question 25

Treatment of TB disease in person with HIV

Answer 25

• Need to give more aggressive regiment because they are immunocomprimised
• longer treatment - 9-12 months
• there are adverse reactions with RIF antiretrovirals, which is why we have to switch to rifabutin

Question 26

Treatment o MAC infection

Answer 26

• much less susceptible than M.tuberculosis to most anti-TBs
• give azithromycin and clarithromycin
• give prophylactic therapy