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WorksheetsIsoniazid: Chemistry and Mechanism
Total questions: 83
Worksheet time: 42mins
Which description best identifies isoniazid (INH) chemically?
Ester of pyridine carboxylate
Amide of nicotinic aldehyde
Hydrazide of isonicotinic acid
Ketone of isonicotinamide
What is the functional role of KatG in Mycobacterium tuberculosis with respect to INH?
Blocks INH binding to ribosomes
Activates INH by catalase–peroxidase
Exports INH via efflux transporters
Degrades INH through amidase
INH primarily exhibits which type of activity against Mycobacterium tuberculosis?
Fungicidal in mixed cultures
Bacteriostatic in dormant cells
Bactericidal against active bacilli
Virucidal against co-infections
Which enzyme target is directly inhibited by activated INH to disrupt mycolic acid biosynthesis?
DNA gyrase topoisomerase
Dihydropteroate synthase
RNA polymerase beta subunit
InhA enoyl-ACP reductase
Mycolic acids are important in the mycobacterial cell wall primarily because they
Provide a permeability barrier to hydrophilic solutes
Function as cytosolic signaling hormones
Serve as ribosomal assembly scaffolds
Act as primary energy storage lipids
INH is best described as a prodrug because it
Acts only as a competitive antagonist
Is directly active without modification
Requires enzymatic activation to a reactive species
Needs acid-catalyzed hydrolysis in stomach
The NADH-dependent process affected by INH involves
Methylation of rRNA nucleotides
Phosphorylation of cell wall arabinans
Reduction during fatty acid elongation
Oxidation of peptidoglycan chains
Activated INH interferes with NADH by
Cleaving NADH into AMP and ribose
Forming an acylated NADH adduct
Blocking NADH transport across membrane
Promoting NADH overproduction
Which statement about INH’s spectrum within the host is accurate?
Inactive in intracellular environments
Targets only intracellular latent bacilli
Limited to extracellular bacilli exclusively
Effective against extracellular and intracellular bacilli
What combined regimen is commonly preferred to enhance TB treatment when using INH?
INH with amphotericin
INH with acyclovir
INH with rifampin
INH with vancomycin
What catalytic activities are attributed to KatG that facilitate INH activation?
Protease–peptidase activities
Catalase–peroxidase activities
Lyase–decarboxylase activities
Kinase–phosphatase activities
Blocking InhA by INH ultimately prevents
Conversion of pyruvate to lactate
Replication of circular DNA plasmids
Assembly of ribosomal 50S subunits
Synthesis of long-chain mycolic acids
Which structural modification of isoniazid’s hydrazide most consistently retains antitubercular activity?
N2 substitution with alkyl groups
N1 hydrogen retained without alkyl group
Replacement of pyridine ring with benzene
Conversion to stable hydrazone derivatives
Why were isoniazide hydrazones found unsuitable as therapeutic agents?
They lacked GI absorption entirely
They were unstable in the GI tract
They inhibited NAT2 excessively
They formed insoluble glycine conjugates
Which statement best describes the role of NAT2 in isoniazid metabolism?
Microsomal enzyme hydrolyzing hydrazides
Peroxisomal enzyme producing radicals
Mitochondrial enzyme controlling oxidation
Cytosolic enzyme catalyzing acetylation
Individuals with high NAT2 activity are referred to as:
Slow acetylators prone to toxicity
Rapid acetylators requiring higher doses
Rapid acetylators needing dose adjustment
Intermediate acetylators without risk
Which metabolite formation is most directly linked to hepatotoxicity during isoniazid therapy?
Acetylhydrazine generation
Isonicotinic acid conjugation
N-acetylisoniazid production
Diacetylhydrazide formation
In rapid acetylators, acetylhydrazine tends to be:
Deacetylated back to isoniazid
Oxidized to isonicotinic acid faster
Further acetylated to diacetylhydrazide
Hydrolyzed to hydrazine quickly
Which pathway explains liver protein acylation leading to necrosis?
Acetylhydrazine as CYP450 substrate
Isonicotinic acid glycine conjugation
Hydrazone stabilization in plasma
Direct NAT2 oxidation to radicals
What is the proposed reactive intermediate causing hepatotoxicity from acetylhydrazine?
N1-alkylated isoniazid derivative
Glycine-bound isonicotinic acid
Acetyl cation or radical species
Stable diacetylhydrazide conjugate
Which combination of N-substitutions typically destroys isoniazid activity?
R1 and R2 = H, R3 = alkyl
R1 and R2 = alkyl, R3 = H
Any substitution on N1 hydrogen
No substitution on N2 nitrogen
Which metabolite is commonly detected in urine as a glycine conjugate during isoniazid metabolism?
N-acetylisoniazid after conjugation
Acetylhydrazine after oxidation
Isonicotinic acid after hydrolysis
Hydrazine from hydrazide hydrolysis
Which best describes the core mechanism of rifamycins against tuberculosis bacteria?
Inhibit DNA-dependent RNA polymerase beta-subunit
Disrupt peptidoglycan crosslinking in cell wall
Chelate iron to starve bacteria of nutrients
Block 30S ribosomal decoding during translation
Which statement about the macrocyclic structure of rifamycins is most accurate for activity?
Ring opening generally increases antibacterial potency
Linear analogs penetrate gram-negative cell walls better
Cyclization only improves oral absorption, not activity
Intact macrocycle is required to maintain activity
Which functional groups on rifamycin contribute to binding via hydrogen bonding to DDRP?
Tertiary amines at C12 and C14 sites
Free hydroxyls at C21 and C23 positions
Sulfonamides at C3 and C5 positions
Nitro groups at C7 and C9 positions
Which feature of DDRP supports rifamycin binding through metal coordination?
Possesses calcium-stabilized beta-sheet
Contains two zinc atoms for chelation
Has copper-dependent catalytic residues
Requires magnesium in the active pocket
Which pharmacologic effect is characteristic during rifampin therapy?
Blue urine due to riboflavin excretion
Red-orange discoloration of body fluids
Black stools from iron chelation effects
Green discoloration of sweat and tears
Which statement best distinguishes rifampin’s action on transcription?
Blocks elongation of full-length RNA transcripts
Destroys promoter sequences on bacterial DNA
Inhibits sigma-factor binding to the core enzyme
Prevents ribosome initiation on mRNA templates
A common resistance mechanism to rifamycins in bacteria involves:
Efflux pumps removing drug from cytosol
Altered porin expression in outer membrane
Mutations in DDRP beta-subunit gene
Enzymatic hydrolysis by beta-lactamase
Which SAR statement aligns with rifamycin activity requirements?
Double bond reduction increases intrinsic potency
Free OH groups at C1, C8, C21, C23 are needed
Acylation at C21–C23 enhances binding strength
Substituting C3 and C4 removes antibacterial effects
Which best explains rifampin’s limited utility against some gram-negative infections?
Instability in acidic outer membrane milieu
Rapid metabolism by periplasmic enzymes
Weak binding to gram-negative ribosomes
Poor penetration through gram-negative cell wall
Which metabolic pathway primarily inactivates rifampin?
N-oxidation of hydrazine producing toxic species
Glucuronidation at C3 creating inactive conjugates
Deacetylation at C25 forming active metabolites
O-demethylation at C5 followed by oxidation
Which semisynthetic modification differentiates rifampin from rifapentine?
Variation in R group attached to hydrazone
Presence of extra macrocyclic ether ring
Substitution of beta-lactam side chain
Addition of fluorine at the C7 position
Which statement best describes pyrazinamide’s relationship to nicotinamide?
Prodrug of nicotinamide adenine dinucleotide
Unrelated aliphatic amide compound
Direct inhibitor of nicotinamide transport
Structural bioisostere of nicotinamide
What is the active metabolite generated from pyrazinamide inside mycobacteria?
Isonicotinic acid
Pyrazinoic acid
5‑hydroxypyrazinamide
Nicotinic acid
Which enzyme in mycobacteria hydrolyzes pyrazinamide to its active form?
Xanthine oxidase
Transpeptidase
Arabinosyl transferase
Pyrazinamidase
At what environmental condition does pyrazinoic acid exert notable biological activity around M. tuberculosis?
Alkaline pH above 8.5
Neutral pH around 7.0
Hypoxic pH-independent state
Acidic pH near 5.5
Which mechanism contributes to pyrazinamide’s bactericidal effect against semi‑dormant bacilli?
Direct inhibition of DNA gyrase
Disrupting ribosomal assembly at 50S
Blocking folate synthesis pathway
Intracellular acidification by pyrazinoic acid
A key resistance mechanism to pyrazinamide involves mutation in which gene?
rpoB polymerase gene
pyrazinamidase gene
embCAB operon
katG peroxidase gene
Which pharmacokinetic statement about pyrazinamide is most accurate?
Poor oral absorption, mostly unchanged excreted
Readily absorbed orally, little unchanged excreted
Parenteral only, minimal systemic absorption
Extensive renal secretion of intact drug
Which hepatic process converts pyrazinamide to pyrazinoic acid in humans?
Peroxisomal deamination
Microsomal hydrolysis
Cytosolic methylation
Mitochondrial oxidation
Introducing pyrazinamide into combination TB therapy primarily achieves which clinical outcome?
Reduces hepatotoxicity risk
Eliminates need for ethambutol
Increases monotherapy success
Shortens regimen duration
Why is pyrazinamide included in combination therapy from a resistance standpoint?
It slows resistance development
It reverses existing resistance
It avoids all cross‑resistance
It selects for hyper‑susceptible mutants
Ethambutol’s mechanism primarily targets which biosynthetic process in mycobacteria?
Cell wall synthesis
Mycolic acid beta‑oxidation
Protein translation
DNA replication
Ethambutol inhibits which enzyme family essential for arabinan polymer assembly?
Arabinosyl transferases
Transpeptidases
Beta‑lactamases
Topoisomerases
Which mycobacterial cell wall components are impacted by ethambutol’s inhibition of arabinan synthesis?
Lipopolysaccharide and porins
Peptidoglycan and teichoic acids
Capsular polysaccharides only
Arabinogalactan and lipoarabinomannan
Which structural feature is central to ethambutol’s SAR for activity?
Macrolide lactone ring
Ethylene diamine chain
Aromatic pyridine ring
Sulfonamide linkage
Which ribosomal target is primarily affected by streptomycin in Mycobacterium tuberculosis?
70S initiation complex
50S subunit binding site
30S subunit decoding site
23S rRNA peptidyl center
Streptomycin’s action includes which additional effect beyond inhibiting protein synthesis?
DNA strand breaks induction
mRNA misreading and membrane damage
tRNA charging inhibition
Cell wall arabinan blockade
Adenylyltransferase-mediated modification of streptomycin produces which inactive form?
O-6 adenylate derivative
N-1 adenylate conjugate
O-3 adenylate metabolite
C-2 adenylate product
Phosphotransferase inactivation of streptomycin typically yields which product?
C-5 phosphate addition
O-1 phosphorylate analogue
N-6 phosphate ester
O-3 phosphorylate metabolite
Why do chemically modified streptomycin metabolites fail to bind ribosomes?
Loss of positive charge only
Enhanced efflux pump recognition
Steric hindrance at decoding site
Reduced hydrophobic interactions
Dihydrostreptomycin compared with streptomycin is noted for which clinical concern?
Greater potential for delayed deafness
Higher nephrotoxicity risk
More severe hypersensitivity rates
Lower antibacterial spectrum
Oxidation of the aldehyde of streptomycin to a carboxylic group results in what?
Inactive analogue formation
Enhanced ribosomal binding
Improved tissue penetration
Prolonged serum half-life
Modification of the methyl group in L-streptose to methylene-hydroxyl yields an analogue with what property?
Superior activity against M. leprae
Selectively targets 50S subunit
Inactive due to poor uptake
Active but no advantage over streptomycin
Alkyl changes to the amino-methyl group on the glucosamine moiety of streptomycin typically do what?
Guarantee resistance reversal
Enhance carboxylate formation
Remove activity by guanidine deletion
Increase activity with higher alkyl groups
Which list best represents common second-line agents for TB?
Ethionamide, PAS, cycloserine, capreomycin, kanamycin
Isoniazid, rifampin, pyrazinamide, ethambutol
Streptomycin, amikacin, gentamicin, tobramycin
Levofloxacin, moxifloxacin, linezolid, bedaquiline
Second-line TB agents are usually reserved for which situation?
Latent TB prophylaxis solely
Initial induction therapy always
Exclusive use in pediatric TB
First-line intolerance or resistance cases
Ethionamide is best described as which structural relationship?
Fluoroquinolone mimic
Prodrug of ethambutol
Derivative of streptidine
Analog of isonicotinamide
Replacing ethionamide’s ethyl group with propyl gives what agent?
Cycloserine
Protionamide
Kanamycin
Capreomycin
Ethionamide is bactericidal against which organisms?
S. aureus and P. aeruginosa
C. difficile and E. coli
M. kansasii and M. avium
M. tuberculosis and M. leprae
Which statement best explains ethionamide’s mechanism of action?
Direct inhibition of 30S ribosome
Chelation of Mg2+ to disrupt ATP synthase
Activation to sulfoxide that acylates and inactivates InhA enoyl reductase
Binding arabinosyl transferase to block arabinan
Which enzyme system is involved in converting ethionamide to its active form?
Catalase–peroxidase mediated oxidation
Beta-lactamase cleavage reaction
P450 monooxygenase hydroxylation
Glutathione S-transferase conjugation
Which statement best describes para-aminosalicylic acid (PAS) in tuberculosis therapy?
Chelates iron to disrupt heme synthesis
Blocks ATP synthase in bacterial membranes
Acts as a folate antimetabolite resembling PABA
Directly inhibits DNA gyrase in mycobacteria
PAS co-administered with isoniazid is valuable in rapid acetylators because it
Enhances rifampicin activation by hepatic enzymes
Prevents renal excretion of pyrazinamide metabolites
Reduces acetylation of isoniazid by competing as substrate
Increases gastric absorption of ethionamide significantly
A likely adverse effect of PAS at therapeutic doses is
Gastrointestinal irritation with hypersensitivity risk
Bronchospasm as a common acute reaction
Ototoxicity with irreversible hearing loss
Severe hemolysis due to G6PD deficiency
Cycloserine’s primary pharmacologic target in mycobacteria involves inhibition of
D-alanine racemase and D-alanine ligase enzymes
Protein synthesis at the 30S ribosomal subunit
Topoisomerase IV and DNA gyrase enzymes
Transcription by blocking RNA polymerase
Why is D-alanine critical for mycobacterial cell wall integrity?
It regulates synthesis of lipoarabinomannan lipids
It forms the D-alanine–D-alanine dipeptide for peptidoglycan
It donates electrons in respiratory chain complexes
It cross-links arabinogalactan to mycolic acids
Cycloserine is considered a rigid analog of D-alanine. This structural relationship allows it to
Competitively block binding of D-alanine to target enzymes
Irreversibly alkylate cysteine residues in enzyme active sites
Stimulate efflux pumps that expel cell wall precursors
Disrupt membrane potential by acting as an ionophore
Which sequence correctly outlines the role of D-alanine in peptidoglycan biosynthesis?
Phosphorylation by kinases, polymerization to polyalanine, membrane insertion
Condensation to dihydrodipicolinate, reduction to lysine, cross-linking step
Decarboxylation of pyruvate, transamination to L-alanine, lipid II transport
Racemization of L-alanine to D-alanine, formation of D-Ala–D-Ala, incorporation into cell wall
A patient on cycloserine develops reduced peptidoglycan synthesis. The most proximal biochemical explanation is
Inhibited conversion of L-alanine to D-alanine
Impaired proton motive force across the membrane
Blocked synthesis of mycolic acids from fatty acids
Reduced activity of RNA polymerase sigma factor
PAS metabolism involves which processes at functional groups?
Acetylation of the amino group with conjugation at carboxyl group
Sulfation of the phenolic ring with oxidative deamination
Phosphorylation of the carboxylate with glycine cleavage
Glucuronidation of the amide nitrogen with methylation
In designing combination therapy for TB, cycloserine’s mechanism complements PAS because
Both inhibit the same enzyme increasing potency synergistically
Each amplifies ethionamide toxicity through shared metabolism
One inhibits cell wall synthesis while the other disrupts folate pathway
They jointly block ATP synthase leading to bactericidal action
What is the primary rationale for using combination therapy in tuberculosis treatment?
To avoid drug interactions between agents
To minimize costs by using fewer agents
To increase patient adherence to single drug
To reduce emergence of drug resistance strains
Which factor should guide the choice of antitubercular agents for a patient?
Hospital bed availability and cost
Patient’s preference for oral therapy
Seasonal prevalence of tuberculosis
Location of disease and patient condition
Which three drugs constitute a commonly recommended initial 8‑week TB regimen?
Isoniazid, rifampin, pyrazinamide
Ethambutol, streptomycin, kanamycin
Isoniazid, kanamycin, capreomycin
Rifampin, capreomycin, pyrazinamide
Adding pyrazinamide to INH and rifampin primarily leads to which outcome?
Shortening total treatment duration
Increasing hepatotoxicity without benefit
Preventing peripheral neuropathy
Eliminating need for susceptibility testing
In areas with high incidence of drug resistance, which modification is appropriate for initial therapy?
Reduce treatment duration to 12 weeks
Avoid pyrazinamide during induction
Use monotherapy with rifampin
Include ethambutol in the regimen
Which statement reflects a cardinal rule for TB regimens?
Delay susceptibility testing for weeks
Start with one highly effective drug
Begin therapy with at least three drugs
Never add drugs to a failing regimen
What is the proven prophylaxis for tuberculosis and typical duration?
INH for 6–12 months
RIF for 2–4 weeks
PZA for 3–5 months
EMB for 1–2 months
Which patients are considered high risk warranting prophylaxis?
Young adults without exposure
Individuals with normal radiographs
HIV infection and close contacts
Patients with healed fractures
Which adverse effect is most concerning during long‑term INH therapy?
Severe liver damage and hepatitis
Irreversible ototoxicity and deafness
Profound hypoglycemia and coma
Acute renal failure and stones
How can INH‑related peripheral neuropathy be prevented?
Avoiding pyrazinamide completely
Adding capreomycin weekly
Dose reduction of rifampin
Co‑administration of pyridoxine
