Transcript ANTI-TUBERCULOSIS DRUGS

ANTI-TUBERCULOSIS DRUGS
 Tuberculosis (TB) is a disease that has affected
mankind for centuries and dates as far back as ancient
Egyptian times.
 is caused by a slow growing
bacterium called M. tuberculosis.
 The disease is spread by coughing,
talking, spitting or sneezing which
spreads the mycobacteria through the air in tiny
droplets of water which are inhaled in to the lungs.
ANTI-TUBERCULOSIS DRUGS
 The primary location of the disease is in the lungs,
called pulmonary TB, where bacterial growth destroys
tissue making it very hard for the patient to breathe
resulting in death.
 symptoms can also include meningitis, legions on the
skin and degradation of the heart, bones and
intestines.
 The difficulty in managing tuberculosis is the
prolonged treatment duration, the emergence of drug
resistance and co-infection with HIV/AIDS.
ANTI-TUBERCULOSIS DRUGS
 Tuberculosis control requires new drugs that act at
novel drug targets to help combat resistant forms of
Mycobacterium tuberculosis and reduce treatment
duration.
 Drug resistant strains of the mycobacterium are not
susceptible to the current cocktail of drugs available.
 now infects approximately one third of the world’s
population and causes 8 million new cases of TB each
year resulting in around 2 million deaths worldwide.
The resurgence has been caused due to three main
reasons:
The available chemotherapy is not very efficacious and
has to be given as a precise combination over a period
of months (poor compliance).
2. Drug resistant strains of the mycobacterium are not
susceptible to the available drugs.
3. There is also a strong epidemiological coexistence
between TB with HIV
1.
Mycobacterium tuberculosis
 is the principal causative agent of TB in human.
 is a slow growing Gram-positive rod-shaped bacterium
that has a thick, rigid, and hydrophobic cell wall which
serves to protect the organism from the environment,
making it highly impermeable to conventional
antimicrobial agents.
The current treatment of TB
 TB is treated in two phases. There is an initial phase
and a continuation phase and depending upon the
patients ability to comply with the drug regime:
Phase-I: Here there is the concurrent use of at least
three drugs to reduce the bacterial population as
rapidly as possible in order to prevent resistance.
1.
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as a combination preparation or “triple therapy”.
isoniazid (INH), rifampicin (RIF) and pyrazinamide (PZA).
Streptomycin (SM) may be used in cases where resistance to
INH has been established.
The initial phase drugs are normally used for two months
The current treatment of TB
2. Phase-II: After the initial phase, a further four
months of chemotherapy is carried out using
preferably a combination of RIF and INH.
Potential targets in M.
tuberculosis
 Targeted pathways should be unique to the
Mycobacterium.
 Many agents have been introduced to target
certain metabolic sites:
1. Inhibit cell wall biosynthesis.
2. Affect protein biosynthesis.
3. Affect DNA replication and transcription.
4. Inhibit fatty acid synthesis (FAS).
Mycobacterial cell wall
 a very complicated cell wall which is sometimes
described as “waxy” because of its complex fatty acid
barrier which makes getting a drug molecule into the
cytoplasm extremely difficult.
 Consists mainly from special type of fatty acids called
mycolic acids which make mycobacterium
exceptionally different from other gram +ve bacteria
regarding the cell wall permeability.
Mycobacterial cell wall
Mycolic acids
Mycolic acids
OH
O
OH
 are β-hydroxy C54-63 fatty acids with a long α-alkyl
side chain of C22-24 in length.
 play important roles in the mycobacterium,
including resistance to chemical injury; resistance
to dehydration, low permeability to polar
molecules and allow the bacterium to grow readily
inside macrophages
 α-mycolic acids are the predominant form (70%)
Synthesis of α-mycolic acid
Targeting mycolic acid biosynthesis
Isoniazid (INH) – 1952
 a purely synthetic agent.
 Has bacteriostatic action at lower dose and
bactericidal at higher concentration.
 Inhibits fatty acid biosynthesis by interfering with
InhA enzyme (essential one).
 It is a prodrug and has to be activated before killing
the bacteria.
Activation of INH
isonicotinic acyl-NADH complex will tightly
bound to the active site of InhA thus preventing
access of the natural enoyl-AcpM substrate
INH pharmacokinetic profile
 INH is well absorbed orally or intramuscularly and
distributes well throughout the body (Why?).
 (LogP= -0.64).
 its metabolism occurs initially by liver Nacetyltransferase. This means that patients who are
poor acetylators can experience toxicity problems as
acetylation of INH is first required before the
hydrolysis can occur and the drug cleared by the
kidneys
INH resistance
 Two major resistant mechanism have been reported:
 Resistance to INH generally occurs when the drug is
administered alone for 3 months and is caused mainly
due to the absence of the gene encoding the catalaseperoxidase katG which prevents activation of the drug.
 Mutations in InhA have also been identified and
considered as a reason for resistance to the drug.
Pyrazinamide (PZA) – 1952
 drug is a synthetic analogue of nicotinamide.
 is bactericidal against growing bacteria.
 The exact mechanism of action of this unclear, but
there are two proposed ones:
 Affect memebrane transport.
 Inhibit FAS-I system: This interferes with the
bacterium’s ability to synthesize new fatty acids,
required for growth and replication.
 it is known that PZA is a prodrug which requires
activation to pyrazinoic acid (POA) by the
Pyrazinamidase enzyme:
Pyrazinamide pharmacokinetic
profile
 PZA is well absorbed orally and distributes well
throughout the body (Why?) reaching concentration
levels above that needed to kill the tubercle bacilli.
 LogP = -0.42
 It is metabolized by the liver to give mainly Pyrazinoic
acid which is then excreted by the kidneys.
 It is also known that resistance to PZA rapidly evolves
if the drug is used alone and is associated with
mutation in the pncA gene which encodes the
Pyrazinamidase/ nicotinamidase enzyme.
Pyrazinamide
 More selective on M.tuberculosis than other
mycobacterium species as well as other bacterial
strains such as E. coli (Ying Zhang, 2008).
 This is thought to be due to effective efflux pump in
those bacterial cells compared to M.tuberculosis which
will pump PZA as pyrazinoic acid out of the cell.
D-Cycloserine (CS) – 1952
 possesses a broad range of anti-mycobacterial activity.
 It works by mimicking D-alanine which is the natural
substrate for the enzyme D-alanine racemase (Alr) and
D-alanine: D-alanine ligase (Ddl) thus preventing the
synthesis of the mycolyl peptidoglycan which lead to
cell death.
D-Cycloserine pharmacokinetic and
resistant profile
 CS is readily absorbed orally and widely distributed
amongst tissues before being excreted by the kidneys
with little of the drug being metabolized.
 Alr serves to convert L- aniline (bacteria utilize L-
aniline from the host environment) to D-alanine and
mutation in the active site of this enzyme have been
proposed to be responsible for the resistance of CS.
 Not active against MDR-TB.
Future TB drugs
 Recent research regarding anti-tuberculosis agents is
interested in:
1. shortening treatment time.
2. combating MDR-TB.
3. and finding effective agents against persisting
bacterial infections.
 Most of the work in this field focused in finding
agents selective on inhibiting FAS-II system
(specifically targeting new and essential enzymes),
especially preventing mycolic acid biosynthesis.
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1
3
Possible new targets for
anti-TB agents
4
Case study for the development of
new anti-TB agents.
• TLM; a natural product form Nocardia, inhibits
KasA and KasB (FAS-II).
• Inactive on the whole bacteria (why?).
• difficult to separate and synthesize.
Molecular Modelling and Computer-Aided Drug
Design
 Is the use of the three dimensional structure of
macromolecular targets to design novel inhibitors.
 Then using computational docking programs to dock
hypothetical structures into the binding site.
 the compounds then will be rank-ordered with respect to
their goodness of fit.
 Compounds will be synthesized and tested on both the
isolated enzyme and the whole bacterial cell.
b
OH
c
d
OH
OH
Cl
O
O
O
O
Cl
Cl
NH2
S
O
Cerulenin
Triclosan
Thiolactomycin
N
S
NH2
S
Ethionamide
OH O
B
S
N
O
N
thienodiazoborine
S
O
a
Thiolactomycin
O
O
Cerulenin
Heath et al. 2001
O
NH2
TLM-Enzyme
2-aminobenzimidazole-ecFabB
4-aminoimidazole-ecFabB
Findings form modeling results
 TLM bound in the same pattern as reported in
literature.
 Increasing the hydrophobic character of the
compound improved the binding.
 The carbonyl group is essential for H-bonding.
 The in-vitro results showed promising activity in
submicromolar range against M.tuberculosis.