Pharm ID ethambutol, isoniazid, pyrazinamide, rifamycins

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Pharm ID ethambutol, isoniazid, pyrazinamide, rifamycins
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2013-06-17 19:36:33
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ethambutol isoniazid pyrazinamide rifamycins
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ethambutol, isoniazid, pyrazinamide, rifamycins
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  1. Ethambutol:
    • First line drug for treatment of TB
    • Used in conjunction with other anti-TB drugs
  2. ethambutol MOA:
    • Ethambutol is bacteriostatic.
    • The mycobacterial cell wall is composed of a peptidoglycan-arabinogalactan-mycolic acid structure.
    • Ethambutol blocks formation of the cell wall by blocking arabinosyl transferases, an enzyme responsible for forming the cell wall.
    • Resistance could result from changes in arabinsoyl transferase so that ethambutol may not work.
  3. ethambutol kinetics:
    Cleared by kidneys; thus dosage adjustments required in renal impairment
  4. ethambutol SE:
    • Optic neuritis (may lead to decreased vision &   inability to discriminate red/green)
    • Peripheral neuropathy
    • Rash
    • Fever
  5. cautions of ethambutol:
    less than 5 years old
  6. Isoniazid:
    • First line drug for treatment of TB
    • Capable of killing intracellular & extracellular   M. tuberculosis
    • Used in conjunction with other anti-TB drugs
  7. Isoniazid MOA:
    • Isoniazid inhibits mycobacterial cell wall synthesis.
    • Isoniazid is a pro-drug activated by katG, a mycobacterial enzyme.*
    • Activated isoniazid binds with nicotinamide adenine dinucleotide to form an INH-NAD complex.
    • The INH-NAD complex inhibits mycolic acid synthesis by blocking another enzyme, InhA.  This leads to increased levels of fatty acids and decreased mycolic acid,** resulting in cell death.
  8. Isoniazid kinetics:
    • Metabolized in liver via acetylation
    • Two kinds of acetylating enzymes; thus some   patients metabolize INH faster than others
    • Isoniazid is a hepatic enzyme inducer
    • Clinical significance? Decrease levels of hepatically cleared drugs
  9. Isoniazid SE:
    • Hepatitis
    • Peripheral neuropathy  (tingling, pain & loss of sensation, possibly due to drug induced B6 deficiency).  Prevention? Give B 6 prophylactaly
    • Rash
    • Fever
  10. Pyrazinamide:
    • An anti-tuberculosis drug used in conjunction with other anti-TB drugs
    • An analog of nicotinamide
  11. Pyrazinamide MOA:
    • Similar to isoniazid, pyrazinamide targets an enzyme needed to make mycolic acid.
    • Pyrazinamide is a pro-drug.  It must be converted to pyrazinoic acid by pyrazinamidase, a mycobacterial enzyme.
    • The probable mechanism is via inhibition of fatty acid synthase I, an enzyme thought to be involved in cell wall synthesis.
    • Pyrazinamide is active only if the mycobacterium is in an acidic environment. 
    • Mycobacterium are found in an acidic phagosome. 
    • So, pyrazinimide works against intracellular mycobacterium.
  12. Isoniazid SE:
    • Hepatic injury
    • N&V
    • Arthralgia
    • Increased uric acid, predisposing patient to gout
    • Anorexia
  13. Rifamycins:
    • Anti-TB drugs, used in conjunction with other anti-TB drugs
    • Also has activity against other types of   bacteria: Good Gram +; Moderate Gram -; and N. meningititis
  14. Rifamycin drugs:
    • Rifampin (Rifadin)·    Available parenteral
    • Rifabutin (Mycobutin)
    • Rifapentine (Priftin)
  15. Rifamycins MOA:
    • Rifampin is very lipophilic.
    • Mycobacterial cell contain lipids and thus rifampin can easily enter.  Also, the biofilms secreted by some bacteria, including S. aureus, are also lipids. Rifampin retains its effectiveness in these bacteria.
    • Rifampin inhibits bacterial RNA synthesis by binding bacterial RNA polymerase.
    • This prevents initiation of RNA synthesis.
    • Without RNA, protein synthesis cannot occur.
    • Bacteriocidal but high rates of resistance.*
  16. Rifamycin kinetics:
    • Eliminated in bile
    • Potent inducers of the CYP enzymes: has lots of rxns!
    • Inducer of P-glycoprotein transporter
    • Widely distributed, including into CSF
  17. indication for Rifampin (in combination):
    • Osteomyelitis
    • TB
    • Endocarditis
    • MRSA
    • Meningitis
    • Prophylaxis for exposure to N. meningitidis
    • Orthopedic infections due to Staph
  18. Rifampin SE:
    • Discoloration of secretions (red-orange)!!! On test
    • Jaundice
    • Elevated liver enzymes
    • Hepatitis
    • Rash
    • Acute interstital nephritis         RARE        
    • Thrombocytopenia                   RARE
  19. Rifampin DI:
    • There are a lot becuz it potent induce of CYP enzymes and inducer of P- glycoprotein transporter
    •       -Rifampin and warfarin: combo may decrease INH
  20. new WEEKLY regimen for latent tuberculosis:
    • Patients with a recent positive TB test usually get isoniazid(INH) DAILY for 9 months to prevent an active infection. 
    • A new alternative is INH plus rifapentine (Priftin)once a WEEK for only 12 weeks. This works as well as daily INH and causes less hepatoxicity...BUT stopping drugs early can lead to resistant TB.   
    • That's why CDC wants a health professional to actually watch each dose get swallowed...called "directly observed therapy."   
    • This new regimen costs $250 for the drugs plus extra fordirectly observed therapy...compared to $20 for 9 months of INH.    
    • Continue to use daily INH for most patients. Add pyridoxine(vitamin B6) to prevent B6 deficiency and neuropathy.    
    • Go to INH plus rifapentine if there's concern about adherence to INH for 9 months AND if directly observed therapy is feasible.  
    • Currently there's a shortage of rifapentine. Ask the pharmacy to make sure the patient can get it for 12 weeks before starting.    
    • Watch for interactions between rifapentine and hormonal contraceptives, warfarin, phenytoin, antiretrovirals, and others. Rifapentine is an enzyme inducer likerifampin.

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