Transcript بسم الله الرحمن الرحيم

Antimycobacterial Agents

Mycobacteria are a genus of acid-fast bacilli belonging to
the
Mycobacteriaceae
which
include
the
organisms
responsible for tuberculosis and leprosy, as well as a
number of other less common diseases.

The
cell
complexity
envelope
and
is
is
unique
in
responsible
both
for
structure
and
mycobacterium
pathogenicity or virulence, multiple drug resistance, cell
permeability, immunonoreactivity and inhibition of antigen
responsiveness,
recrudescence.
and
disease
persistence
and
Antimycobacterial Agents
Tuberculosis

Tuberculosis is a disease characterized as a chronic
bacterial infection caused by Mycobacterium tuberculosis.

The cell wall has a high lipid content resulting in a high
degree of hydrophobicity and resistance to alcohol, acids,
alkali, and some disinfectants.

It is transmitted via the respiratory route.

The organism appears in water droplets expelled during
coughing, sneezing or talking.

TB is a disease which mainly affects the lungs but the
organism can spread through the bloodstream and the
lymphatic system to the brain, bones, eyes and skin
(extrapulmonary tuberculosis; which is much more common
in HIV-infected patients).
Tuberculosis

In pulmonary tuberculosis, the bacilli reach the alveoli and
are ingested by pulmonary macrophages which secrete
substances stimulate surrounding fibroblasts to enclose the
infection
tubercles.
site
leading
to
formation
of
granulomas
or
leprosy (Hansen's disease)

Leprosy (Hansen's disease) is recognized as a chronic
granulomatous infection caused by Mycobacterium leprae.
The
disease
may
consist
of
lepromatous
leprosy,
tuberculoid leprosy, or a condition with characteristics
between these two poles referred to as borderline leprosy.

Person-to-person contact appears to be the means by which
the disease is spread with entrance into the body occurring
through the skin or the mucosa of the upper respiratory
tract.

Skin and peripheral nerves are the regions most susceptible
to attack where the first signs of the disease consist of
hypopigmented or hyperpigmented macules.
leprosy (Hansen's disease)

Neuronal involvement in the extremities will lead to muscle
atrophy,
resorption
of
small
bones,
and
spontaneous
amputation.

When facial nerves are involved corneal ulceration and
blindness may occur.

The identification of the organism in skin or blood samples
is not always possible, but the detection of the antibody to
the organism is an effective diagnostic test especially for
the lepromatous form of the disease.
Mycobacterium Avium-intracellular Complex

Disseminated Mycobacterium avium and Mycobacterium
intracellular complex (MAC or MAI) is the most common
bacterial opportunistic infection seen in AIDS patients.

The lungs are the organs most commonly involved in nonAIDS patients but the infection may involve bone marrow,
lymph nodes, liver and blood in AIDS patients.

MAC grow within macrophages and therefore the drug must
be capable of penetration of the macrophage.

Treatment of MAC, both prophylactically and for diagnosed
infections requires the use of multiple drug therapy and for
disseminated MAC this treatment is for the life of the
patient.
General approaches to drug therapy

The pathogenic mycobacterial organism can be divided into
organisms that are:
 Actively metabolizing and rapidly growing,
 Semi dormant in acidic intracellular environment
 Semi dormant in a nonacidic intracellular environment,
 Dormant.

The dormant being the most problematic and responsible
for treatment failures. Thus, successful chemotherapy calls
for drugs with bactericidal action against rapidly growing
organisms and the ability to destroy semi dormant and
dormant populations.

The use of combination therapy over an extended period of
time is one answer to successful treatment.
Classification of Anti- TB
Drugs:
1.

First–Line Agents:
They are also known as basic or primary agents which are
used in the initial treatment.

They are the strongest and most effective agents (lowest
toxicity) by which most of TB bacilli are killed, e.g., Isoniazid
(INH), Rifampin, Pyrazinamide, Ethambutol and Streptomycin.
2.

Second–Line Agents:
These drugs are utilized in case of resistance, retreatment
or intolerance to the first-line agents, e.g. Ethionamide, pAmino salicylic acid (PAS), Cycloserine.
1.Isoniazid (Isonicotinic Acid Hydrazide, INH)

INH is a synthetic orally active antibacterial agent with
bactericidal action against M. tuberculosis.
Mechanism of action

INH is a prodrug which is activated through an oxidation
reaction catalyzed by an endogenous enzyme, katG, which
exhibits catalase-peroxidase activity converting INH to a
reactive species capable of acylation of an enzyme system
found exclusively in the M. tuberculosis.
Isoniazid (Isonicotinic Acid Hydrazide, INH)
Isoniazid (Isonicotinic Acid Hydrazide, INH)

The reaction of catalase-peroxidase activated INH with a
portion of the enzyme inhA (involved in the biosynthesis of
the mycolic acids which are important constituents of the
mycobacterial cell wall in that they provide a permeability
barrier to hydrophilic solutes).

The enzyme inhA is a NADH-dependent enoyl reductase
protein involved in double bond reduction during fatty acid
elongation.
Isoniazid (Isonicotinic Acid Hydrazide, INH)

INH specifically inhibits long chain fatty acid synthesis,
How?

INH is activated to an electrophilic species which acylates
the 4- position of the NADH so it is no longer capable of
catalyzing
the
reduction
of
unsaturated
essential for the synthesis of the mycolic acids.
fatty
acids
Isoniazide (INH):
O
C - NH - NH2
N
Isonicotinyl Hydrazide
Synthesis
Isoniazid (Isonicotinic Acid Hydrazide, INH)
Structure-activity Relationships:

Isoniazid hydrazones possess activity but were unstable in
the G.I. tract releasing the active INH i.e. their activity
resulted from the INH and not the derivatives.

Substitution of the hydrazine portion of INH with alkyl and
aralkyl substituents resulted in a series of active and
inactive derivatives.
Isoniazid (Isonicotinic Acid Hydrazide, INH)
Structure-activity Relationships:

Substitution
on
the
N2
position
resulted
in
active
compounds (R1 = R2 = alkyl; R3 = H), whereas any
substitution of the N1 hydrogen with alkyl groups destroyed
the activity (R, and R, = H; R3 = alkyl).

None of these changes produced compounds with superior
activity over INH.
Isoniazid (Isonicotinic Acid Hydrazide, INH)
Metabolism:

INH is extensively metabolized to inactive metabolites.

The major metabolite is N-acetylisoniazid.

The
enzyme
responsible
acetyltransferase.
for
acetylation,
Individuals
cytosolic
possessing
N-
high
concentrations of the enzyme are referred to as rapid
acetylators while those with low concentrations are slow
acetylators. This may result in a need to adjust the dosage
for fast acetylators.

Acetylhydrazine serve as a substrate for CYP450 resulting
in the formation of a reactive intermediate capable of
acetylating liver protein resulting in the liver necrosis.
Isoniazid (Isonicotinic Acid Hydrazide, INH)
Isoniazid (Isonicotinic Acid Hydrazide, INH)
Metabolism

Acetylhydrazine serve as a substrate for CYP450 resulting
in the formation of a reactive intermediate capable of
acetylating liver protein resulting in the liver necrosis.
2.Rifamycin Antibiotics

The
rifamycins
are
natural
products
produced
by
Streptomyces mediterranei.

This chemical class is an aliphatic chain forming a bridge
between two nonadjacent positions of an aromatic moiety.

Semisynthetic derivatives are prepared via conversion of
the
natural
rifamycins
to
3-formylrifamycin
which
is
derivatized with various hydrazines to give products such
as rifampin and rifapentine.

Rifampin and rifapentine have significant benefit over
previously investigated rifamycins in that they are orally
active, highly effective against a variety of gram-positive
and
gram-negative
organisms,
and
have
efficacy in the oral treatment of tuberculosis.
high
clinical
Rifamycin Antibiotics
Rifamycin Antibiotics
Mechanism of Action:

The
rifamycins
inhibit
bacterial
DNA-dependent
RNA
polymerase (DDRP) by binding to the β-subunit of the
enzyme leads to a blocking of the initiation of chain
formation in RNA synthesis.

Rifamycins
are
highly
active
against
rapidly
dividing
intracellular and extracellular bacilli.

Rifampin is active against DDRP from both gram-positive
and gram-negative bacteria but due to poor penetration of
the cell wall of gram-negative organisms by rifampin, the
drug has less value in infections caused by such organisms.
Rifamycin Antibiotics
Structure-activity Relationship:
1. Free
OH groups are required at C-l,8,21 and 23 as they are
important binding groups for attachment to DDRP.
2. Acetylation of C-21 and C-23 produces inactive compounds.
3. Reduction of the double bonds in the macro ring results in a
progressive decrease in activity.
4. Opening of the macro ring gives inactive compounds. These
latter
two
changes
greatly
affect
the
conformational
structure of the rifamycins which in turn decreases binding
to DDRP.
5. Substitution at C-3 or C-4 results in compounds with varying
degrees of antibacterial activity.
Rifamycin Antibiotics
Metabolism:

Rifampin and rifapentine are readily absorbed from the
intestine although food in the tract may affect absorption.

The major metabolism of rifampin and rifapentine is
deacetylation which occurs at the C-25 acetate to give
desacetylrifampin and desacetylrifapentine, which are still
active antibacterial agents.
Rifamycin Antibiotics
Rifamycin Antibiotics
Therapeutic Application:

Rifampin
(Rifadin,
Rimactane)
is
always
used
in
combination with one or more other antitubercular agents.
The drug is potentially hepatotoxic and may produce
gastrointestinal disturbances, rash and thrombocytopenic
purpura (low levels of platelets that prevents bleeding).

Rifampin is known to induce CYP3A4 and CYP2C isoforms
and may decrease the effectiveness of oral contraceptives,
corticosteroids, Warfarin, quinidine, methadone, zidovudine,
clarithromycin, and the azole antifungal agents.
Rifamycin Antibiotics
Therapeutic Application:

Rifapentine is introduced for the treatment of pulmonary
tuberculosis and has major advantage over rifampin is the
fact that when used in combination therapy rifapentine can
be orally administered twice weekly during the "intense"
phase of therapy followed by once a week during the
"continuous" phase of therapy.

In contrast, rifampin is normally administered daily during
the "intense" phase of therapy followed by twice a week
dosing during the "continuous" phase of therapy.
3.Pyrazinamide

Pyrazinamide (PZA, pyrazinecarboxamide) is a bioisostere
of nicotinamide and possess bactericidal action against M.
tuberculosis .

It is a heterocyclic amide derivative.
Pyrazinamide
Mechanism of Action

Pyrazinamide may be active totally or in part as a prodrug.

Pyrazinoic acid may lower the pH in the immediate
surroundings of the M. tuberculosis to an extent that the
organism is unable to grow.

The obvious structural similarity between pyrazinoic acid
and nicotinamide would suggest that the pyrazinoic acid
might function as an antimetabolite of nicotinamide and
interfere with the synthesis of NAD.
MOA
 PZA diffuses into the mycobacterium by positive diffusion, then it is converted
into POA by the pyrazineamidase enzyme  accumulation of POA inside the
cell   pH of the surroundings of the M. tuberculosis that the organism is
unable to grow.
 Resistant strains of TB do not produce pyrazinamidase enzyme.
Synthesis
N
COOH
Esterification
COOCH3
N
N
N
POA
NH3
PZA
Pyrazinamide
Structure-activity Relationship

Substitution on the pyrazine ring or the use of alternate
heterocyclic aromatic rings have given compounds with
reduced activity.

Using QSAR a series of analogs have been prepared with
improved biologic activity. The requirements for successful
analogs include:
 Provision
for hydrophilicity to allow sufficient plasma
concentrations such that the drug can be delivered to
the site of infection.
 Lipophilicity
to allow penetration into the mycobacterial
cell.
 Susceptibility
to hydrolysis such that the prodrug is
unaffected by the "extracellular" enzymes but is readily
hydrolyzed at the site of action.
Pyrazinamide
Structure-activity Relationship

Examples; 5-chloropyrazinamide and 2-(2'-methyldecyl)-5chloropyrazinamide
Pyrazinamide
Metabolism
Pyrazinamide
Therapeutic Application

Pyrazinamide is an essential component of combination
therapy for the oral treatment of tuberculosis (component
of Rifater with INH and Rifampin) as it is especially
beneficial
in
that
it
is
active
against
semi-dormant
intracellular tubercle bacilli that are not affected by other
drugs and reduced treatment regimens to 6 months from
the previous 9 month therapy.

The major serious side effect of pyrazinamide is the
potential for hepatotoxicity.
4.Ethambutol (Myambutol)

Ethambutol,
an
ethylenediiminobutanol,
(EMB)
is
administered as its (+)-enantiomer which is 200-500 times
more
active
enantiomer.
as
a
bacteriostatic
agent
than
its
(-)
Ethambutol (Myambutol)
Mechanism of Action

EMB inhibits the synthesis of the AG portion of the cell wall
(inhibits the transfer of mycolic acid into bacterial cell
wall).

EMB inhibits the enzymes arabinosyl transferases which
catalyze the polymerization of D-arabinofuranose leading to
the
unique
outer
envelope
[which
consists
of
arabinofuranose and galactose] AG. Thus EMB blocks both
the synthesis of AG and LAM of the cell wall.

This mechanism of action accounts for the synergism seen
between EMB and intracellular drugs such as rifampin;
Damage to the cell wall created by EMB improves the cell
penetration of the intracellular drugs resulting in increased
biological activity.
Ethambutol (Myambutol)
Structure-activity Relationship
 Extension of the ethylene diamine chain.
 Replacement of either nitrogen.
 Increasing the size of the nitrogen substituents.
 Moving the location of the alcohol groups.

All are changes that drastically reduce or destroy biologic
activity.
Ethambutol (Myambutol)
Metabolism

The majority of the orally administered ethambutol is
excreted
unchanged
(73%),
with
no
more
than
15%
appearing in the urine as either Metabolite A, or Metabolite
B (inactive)
5.Streptomycin

Streptomycin (STM) was isolated from a manure-containing
soil sample and was ultimately shown to be produced by
Streptomyces grisueus.

The hydrophilic nature of STM results in its very poor
absorption from the gastrointestinal tract and therefore
STM is commonly administered IM.
Streptomycin
Mechanism of Action

STM inhibits protein synthesis, but additional effects on
misreading of a m-RNA template and membrane damage
may contribute to the bactericidal action of STM.
Streptomycin
Structure-activity Relationship

Reduction of the aldehyde to the alcohol results in a
compound, dihydrostreptomycin, which has activity similar
to STM but with a greater potential for producing delayed
severe deafness (major side effect).

Oxidation of the aldehyde to a carboxyl group or conversion
to Schiff’s base derivatives (oxime, semicarbazone, or
phenylhydrazone) results in inactive analogs.

Oxidation of the methyl group in α-streptose to a methylene
hydroxy gives an active analog but with no advantage over
STM.
Streptomycin
Structure-activity Relationship

Modification of the aminomethyl group in the glucosamine
portion of the molecule by demethylation or by replacement
with larger alkyl groups reduces activity.

Removal
or
modification
of
either
guanidine
streptidine nucleus results in decreased activity.
in
the
Streptomycin

Metabolism
II- Second-Line Agents

While these agents are active antibacterial agents, they
usually are less well tolerated or have a higher incidence of
adverse effects.

These
agents
are
utilized
in
cases
of
retreatment or intolerance to the first-line drugs.
resistance,
Ethionamide (Trecator-SC)
Mechanism of Action

Similar to INH, ethionamide is considered to be a prodrug
which is converted via oxidation by catalase-peroxidase to
an active acylating agent, ethionamide sulfoxide, which in
turn inactivates the inhA enoyl reductase enzyme.
Ethionamide
S
C
NH2
N
C2 H5
2-Ethyl-thioisonicotinamide
 It is more active than INH as it is more lipid soluble.
 Ethyl group enhances its activity.
Synthesis:
C
S
N
C
NH2
H2S
N
C2H5
2-ethyl-4-cyanopyridine
N
C2H5
Ethionamide (Trecator-SC)
Metabolism
P-Aminosalicylic Acid (PAS)

PAS (bacteriostatic agent) is utilized as a second-line agent
today.

A combination of bacterial resistance and severe side
effects has greatly reduced its value.
P-Aminosalicylic Acid (PAS)
Mechanism of Action

PAS
act
as
an
antimetabolite
interfering
with
the
incorporation of paraaminobenzoic acid (PABA) into folic
acid synthesis (competitative inhibitor of PABA).

PAS when coadministered with INH is found to reduce the
acetylation of INH, How?

PAS being the substrate for acetylation, thus increasing
the plasma levels of INH. This action may be especially
valuable in rapid acetylators.
Metabolism

PAS is extensively metabolized by acetylation of the amino
group, and conjugation with glucuronic acid and glycine at
the carboxyl group.
Cycloserine (Seromycin)
is a natural product
orchidaceus as the D (+) enantiomer.
from
Streptomyces

Cycloserine

Cycloserine is readily absorbed after oral administration
and widely distributed, including the CNS.

Unfortunately, DCS binds to neuronal N-methylasparate
receptors and affects synthesis and metabolism of 7aminobutyric acid leading to a complex series of CNS
effects, so it should only be used when retreatment is
necessary or when the organism is resistant to other drugs.

Cycloserine should not be used as a single drug, but must
be used in combination with other antituberular drugs.
Cycloserine (Seromycin)
Mechanism of Action

D-cycloserine (DCS) is the active form of the drug with its
action associated with the ability to inhibit two key
enzymes, L-alanine racemase and D-alanine ligase.

DCS is a rigid analog of D-alanine (important component of
the peptidoglycan portion of the mycobacterial cell wall)
and therefore it competitively inhibits the binding of Dalanine to both of these enzymes and its incorporation into
the peptidoglycan.
Cycloserine (Seromycin)
Capreomycin (Capastat)

Capreomycin is a mixture of four cyclic polypeptides of
which Capreomycin Ia (R = OH) and Ib (R=H) make up 90%
of the mixture.

Capreomycin is produced by Streptomyces capreolus and is
quite similar to the antibiotic viomycin.
Capreomycin (Capastat)

It is a potent inhibitor of protein synthesis, particularly that
which depends upon m-RNA at the 70S ribosome. It blocks
chain elongation by binding to either or both the 50S or 30S
ribosomal subunits.

As a polypeptide,
parenterally (IM).
the
drug
must
be
administered
Kanamycin (Kantrex )

kanamycin is a second-line agent with very limited use in
the treatment of M. tuberculosis.

The drug is only utilized to treat resistant organisms and
then should be used in combination with other effective
agents.
Kanamycin (Kantrex )

The parenteral form of the drug is used, since as an
aminoglycoside the drug is poorly absorbed via the oral
route.

The narrow range of effectiveness and the severe toxicity,
especially if the drug is administered over a long period of
time, have limited is usefulness.
Fluoroquinolones

The
fluoroquinolones
are
a
broad-spectrum
class
of
antibacterials have activity against a wide range of gramnegative as well as gram-positive pathogens.
Mechanism of Action

They bind to DNA gyrase-DNA complex inhibiting bacterial
DNA replication and transcription so these drugs exhibit
bactericidal activity.
Fluoroquinolones
Fluoroquinolones

Two biophobes were also reported are shown in the figure.
Fluoroquinolones

Recently, several C8 methoxy substituted fluoroquinolones
have been reported with superior activity over earlier
quinolones, e.g. Moxifloxacin is reported to be active
against M.tuberculosis when combined with INH.

Fluoroquinolone therapy for tuberculosis is predominantly
used
in
organisms.
patients
infected
with
multidrug-resistant
Drug therapy for MAC

For treatment of MAC it is recommended that a combination
of drugs be used include at least two drugs (clarithromycin
or azithromycin plus etharmbutol for life).

Other drugs which can be added to the combination consist
of rifabutin, fluoroquinolones, and amikacin.

It
should
be
noted
that
INH
and
ineffective in treating disseminated MAC.
pyrazinamide
are
Macrolides
Macrolides

Both clarithromycin or azithromycin are considered first-
line agents for the oral prevention and treatment of MAC
and have replaced rifabutin.

For prevention, the macrolides may be used orally as single
agents although there is a risk of resistant organisms
forming and a cross resistance between clarithromycin and
azithromycin.

The combination of azithromycin and rifabutin proved more
effective than either drug used singley.
Mechanism of Action

The macrolide antibiotics are bacteriostatic agents which
inhibit protein synthesis by binding to the 50 S ribosomal
units.
Macrolides
Metabolism

Clarithromycin is metabolized in the liver to an active
metabolite, 14-hydroxyclarithromycin, which is less active
than the parent molecule.
Rifamycins

Rifabutin (Mycobutin) is the drug of choice for prophylaxis
of MAC patients.

Rifabutin should be used in treatment failures of the
macrolides or can be combined with azithromycin for
prophylaxis
or
treatment
when
clarithromycin
is
unsuccessful.
Sulfones

The diaryl sulfones represent the major class of agents
used to treat leprosy.

The initial discovery of the sulfones came about as a result
of studies directed at exploring the structure-activity
relationship of sulfonamides
Dapsone (DDS)
Mechanism of Action

Dapsone, a bacteriostatic agent, act through competitive
inhibition of p-aminobenzoic acid incorporation into folic
acid (competitive inhibitor of PABA).
Dapsone (DDS)
Structure-activity Relationship

Isosteric replacement of one benzene ring resulted in the
formation of thiazolsulfone. Although still active, it is less
effective than DDS.

Substitution on the aromatic ring, to produce acetosulfone,
reduces activity while increasing water solubility and
decreasing GI irritation.
Dapsone (DDS)
Structure-activity Relationship

Adding methanesulfinate to DDS to give the water soluble
sulfoxone sodium which is hydrolyzed in vivo to produce
DDS.

Sulfoxone sodium is used in individuals who are unable to
tolerate DDS due to GI irritation, but it must be used in a
dose three times that of DDS because of inefficient
metabolism to DDS.
A successful substitution consists of adding
methanesulphinate to DDS  suphoxone sodium.
Dapsone (DDS)
Metabolism

The major metabolic product of DDS results from Nacetylation in the liver by N-acetyltransferase
Clofazimine (Lamprene)

It is commonly used as a component of multiple-drug therapy.
Mechanism of Action

It possesses direct antimycobacterial and immunosuppressive
properties.

Clofazimine increases prostaglandin synthesis and the generation
of antimicrobial reactive oxidants from neutrophils, which in turn
could have a lethal affect on the organism.

It is a tricyclic phenazine dervative.
Clofazimine (Lamprene)
Structure-activity Relationship

The imino group at C-2 appears essential with activity
increased when the imino group is substituted with alkyl
and cycloalkyl groups.

Halogen substitution on the para position of the two
phenyls at C-3 and N-10 enhance activity but are not
essential to activity.

In the analogs studied, the increased activity correlates
well with pro-oxidative activities of the molecule e.g.,
ability to generate superoxide anion, as well as increased
lipophilicity.
Clofazimine (Lamprene)
Metabolism
Rifampin (Rifadin, Rimactane)

Today rifampin is considered an effective antileprosy agent
when used in combination with the sulfones.
Thalidomide (Thalomid)

Racemic thalidomide has been approved by the FDA for treatment
of erythrema nodosum leprosum (ENL) and is considered the drug
of choice.

The mechanism whereby thalidomide produces relief is thought to
be associated with the drug's ability to control inflammatory
cytokines. Specifically, thalidomide inhibits the synthesis and
release of tumor necrosis factor alpha (TNFa).

Thalidomide is a very potent teratogenic agent.
Therapeutic Considerations

Dapsone has proved to be the single most effective agent
and was used as a monotherapeutic agent despite the
recognition
that
resistant
strains
were
beginning
to
emerge.

The combination consists of rifampin (600 mg monthly),
dapsone (100 mg, daily) and clofazimine (300 mg, monthly
and 50 mg, daily) is used for treatment of multibacillary
leprosy.

For
paucibacillary
leprosy,
including
tuberculoid
and
indeterminate cases only rifampin and dapsone are used
with doses as indicated above.

Other combinations which have been reported include
rifampin plus ofloxacin and minocycline or ofloxacin plus
minocycline.