Antimycobacterials Drugs

1. PHAR 532
LECTURE 10
ANTIMYCOBACTERIAL DRUGS
Antimicrobial agents and
Immunization

2. Mycobacteria
Mycobacteria: rod-shaped, aerobic bacteria that do not
form spores. Called “acid-fast” bacilli.
Mycobacterium tuberculosis causes tuberculosis and is a
very important pathogen of humans. Mycobacterium
leprae causes leprosy. Mycobacterium avium-
intracellulare (M avium complex, or MAC) and other
nontuberculous mycobacteria (NTM) frequently infect
patients with AIDS, are opportunistic pathogens in other
immunocompromised persons, and occasionally cause
disease in patients with normal immune system.

4. Mycobacteria
Mycobacteria cannot be classified as either gram
positive or gram negative. When stained by basic
dyes, they cannot be decolorized by alcohol,
regardless of treatment with iodine.
True tubercle bacilli are characterized by “acid
fastness”—that is, 95% ethyl alcohol containing 3%
hydrochloric acid (acid-alcohol) quickly decolorizes
all bacteria except the mycobacteria. This is due to
the presence of mycolic acid in their cell wall

5. Cell wall

6. Infection
There are marked differences in the ability of
different mycobacteria to cause lesions in various
host species. Humans and guinea pigs are highly
susceptible to M tuberculosis infection, but fowl and
cattle are resistant.
M tuberculosis and Mycobacterium bovis are
equally pathogenic for humans. The route of
infection (respiratory vs intestinal) determines the
pattern of lesions.

7. Infection
Mycobacteria tend to be more resistant to chemical
agents than other bacteria because of the hydrophobic
nature of the cell surface and their clumped growth.
Dyes (eg, malachite green) or antibacterial agents (eg,
penicillin) that are bacteriostatic to other bacteria can be
incorporated into media without inhibiting the growth of
tubercle bacilli. Acids and alkalies permit the survival of
some exposed tubercle bacilli and are used to help
eliminate contaminating organisms and for
“concentration” of clinical specimens. Tubercle bacilli are
resistant to drying and survive for long periods in dried
sputum

8. Infection
If inhaled mycobacteria can infect the alveoli of the
lungs, the immune system reacts by recruiting
monocytes, macrophages, and dendritic cells.
The bacilli are phagocytosed by macrophages and
DCs and can replicate within the phagosome.
The maturation of the phagosome is inhibited as is
the presentation of antigen on MHC molecules

9. Lesions
 Two Principal Lesions
1. Exudative type—This consists of an acute
inflammatory reaction with edema fluid;
polymorphonuclear leukocytes; and, later, monocytes
around the tubercle bacilli. This type is seen particularly
in lung tissue, where it resembles bacterial pneumonia. It
may heal by resolution so that the entire exudate
becomes absorbed; it may lead to massive necrosis of
tissue or may develop into the second (productive) type
of lesion. During the exudative phase, the tuberculin test
result becomes positive.

10. 2. Productive (proliferative) type—When fully
developed, this lesion, a chronic granuloma, consists of
three zones: (1) a central area of large, multinucleated
giant cells containing tubercle bacilli; (2) a mid zone of
pale epithelioid cells, often arranged radially; and (3) a
peripheral zone of fibroblasts, lymphocytes, and
monocytes. Later, peripheral fibrous tissue develops, and
the central area undergoes caseation necrosis. Such a
lesion is called a tubercle. A caseous tubercle may break
into a bronchus, empty its contents there, and form a
cavity. It may subsequently heal by fibrosis or
calcification.

11. Lesions

12. Antimycobacterial agents

13. Antimycobacterial drugs
The major drugs used in tuberculosis are isoniazid
(INH), rifampin, ethambutol, pyrazinamide, and
streptomycin.

14. Isoniazid
Inhibition of the synthesis of mycolic acids, essential
components of mycobacterial cell walls.
Must be activated by a bacterial catalase-peroxidase
enzyme in Mycobacterium tuberculosis called KatG.
KatG couples the isonicotinic acyl with NADH to form
isonicotinic acyl-NADH complex. This complex binds
tightly to the enoyl-acyl carrier protein reductase known
as InhA, thereby blocking the natural enoyl-AcpM
substrate and the action of fatty acid synthase. This
process inhibits the synthesis of mycolic acids, which are
required components of the mycobacterial cell wall.

15. Resistance
Resistance can emerge rapidly if the drug is used
alone. High-level resistance is associated with
deletion in the katG gene that codes for a catalase-
peroxidase involved in the bioactivation of INH.
Low-level resistance occurs via deletions in the inhA
gene that encodes the target enzyme. INH is
bactericidal for actively growing tubercle bacilli, but
is less effective against dormant organisms.

16. Rifampin
Rifampin, a derivative of rifamycin, is bactericidal
against M tuberculosis. The drug inhibits DNA-
dependent RNA polymerase (encoded by the rpo
gene) in M tuberculosis and many other
microorganisms.
Resistance via changes in drug sensitivity of the
polymerase often emerges rapidly if the drug is used
alone.

17. Ethambutol
Ethambutol (ETB) inhibits arabinosyltransferases
(encoded by the embCAB operon) involved in the
synthesis of arabinogalactan, a component of
mycobacterial cell walls. Resistance occurs rapidly
via mutations in the emb gene if the drug is used
alone.

18. Pyrazinamide
Pyrazinamide diffuses into the granuloma of M.
tuberculosis, where the tuberculosis
enzyme pyrazinamidase converts pyrazinamide to
the active form pyrazinoic acid. Under acidic
conditions of pH 5 to 6, the pyrazinoic acid that
slowly leaks out converts to the protonated conjugate
acid, which is thought to diffuse easily back into the
bacilli and accumulate. The net effect is that more
pyrazinoic acid accumulates inside the bacillus at
acid pH than at neutral pH.

19. Resistance
Resistance occurs via mutations in the gene that
encodes enzymes involved in the bioactivation of
pyrazinamide and by increased expression of drug
efflux systems. This develops rapidly when the drug
is used alone, but there is minimal cross-resistance
with other antimycobacterial drugs.
Mutations in the pncA gene of tuberculosis, which
encodes a pyrazinamidase and converts
pyrazinamide to its active form, is responsible for the
appearance of most pyrazinamide resistant M.
tuberculosis strains.

20. Streptomycin
This aminoglycoside is now used more frequently
than before because of the growing prevalence of
strains of M tuberculosis resistant to other drugs.
Streptomycin is used principally in drug
combinations for the treatment of life-threatening
tuberculous disease, including meningitis, miliary
dissemination, and severe organ tuberculosis

21. Pyrazinamide
A nicotinamide analog, pyrazinamide is an
important bactericidal drug used in the initial phase
of TB treatment.
Pyrazinamide’s antimycobacterial activity is
essentially limited to M. tuberculosis. The drug is
more active against slowly replicating organisms
than against actively replicating organisms.
Pyrazinamide is a prodrug that is converted by the
mycobacterial pyrimidase to the active form,
pyrazinoic acid (POA).

22. Resistance
The basis of pyrazinamide resistance in M.
tuberculosis is a mutation in the pncA gene coding
for pyrazinamidase, the enzyme that converts the
prodrug to active POA. Resistance to pyrazinamide is
associated with loss of pyrazinamidase activity,
which prevents conversion of pyrazinamide to POA.
Of pyrazinamide-resistant M. tuberculosis isolates,
72–98% have mutations in pncA.

23. Fluoroquinolones
Fluoroquinolones inhibit mycobacterial DNA gyrase
and topoisomerase IV, preventing cell replication
and protein synthesis, and are bactericidal. The
later-generation fluoroquinolones levofloxacin and
moxifloxacin are the most active against M.
tuberculosis and are recommended for the treatment
of MDR-TB. They are also being investigated for
their potential to shorten the course of treatment for
TB.

24. Resistance
Mycobacterial resistance can develop rapidly when a
fluoroquinolone is inadvertently administered alone.
Empirical fluoroquinolone therapy for presumed
community-acquired pneumonia is associated with
increased fluoroquinolone resistance in M.
tuberculosis.
Mutations in the genes encoding for DNA gyrase
(gyrA and gyrB) are implicated in the majority of
cases—but not all cases—of clinical resistance to
fluoroquinolones.

25. Oxazolidinones
Linezolid is an oxazolidinone used primarily for the
treatment of drug-resistant gram-positive bacterial
infections. However, this drug is active in vitro
against M. tuberculosis and NTM (Non tuberculosis
mycobacteria).
Linezolid’s mechanism of action is disruption of
protein synthesis by binding to the 50S bacterial
ribosome.

26. Diarylquinolines
Bedaquiline (TMC207 or R207910) is a new
diarylquinoline with a novel mechanism of action:
inhibition of the mycobacterial ATP synthetase
proton pump. TMC207 is bactericidal for drug-
susceptible and MDR strains of M. tuberculosis.
Resistance has been reported and is due to point
mutations in the atpE gene encoding for subunit c of
ATP synthetase

27. Mycobacterium leprae
Never grown in laboratory
Drugs that interfere in the synthesis of folic acid can
be used. An example is Dapsone.
Resistance arises if used in low doses. Usually
prescribed in combination with other drugs such as
rifampin