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Urinary Antiseptics - Complete MBBS Pharmacology

What Are Urinary Antiseptics?

Sources: Katzung's Basic and Clinical Pharmacology, 16th ed. p. 1395 | Goodman & Gilman's Pharmacological Basis of Therapeutics, p. 1192

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Drugs Classified as Urinary Antiseptics

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1. NITROFURANTOIN (Most Important for MBBS)

Mechanism of Action

1. Nitrofurantoin enters the bacterial cell
2. Bacterial reductases rapidly convert it to highly reactive intermediates (nitroso and hydroxylamine derivatives)
3. These intermediates react nonspecifically with ribosomal proteins and disrupt multiple processes: protein synthesis, RNA synthesis, DNA synthesis, and cell metabolism
4. Bacteria reduce nitrofurantoin far more rapidly than mammalian cells - this selectivity accounts for its antibacterial (not host-toxic) action at therapeutic urinary concentrations
5. The antibacterial activity is higher in acidic urine (pH < 5.5)

Spectrum of Activity

• Bacteriostatic at ≤32 µg/mL; bactericidal at ≥100 µg/mL
• No cross-resistance with other antimicrobials - resistance emerges very slowly

Pharmacokinetics

Formulations

• Standard: 50-100 mg, four times daily (with meals)
• Macrocrystalline (Macrobid): 100 mg, twice daily - contains 25% macrocrystalline nitrofurantoin (slower absorption, better tolerated) + 75% monohydrate in a gel matrix

Clinical Uses

• First-line for uncomplicated cystitis (especially E. coli)
• UTI prophylaxis: single 50-100 mg dose at bedtime
• Not for: pyelonephritis, prostatitis, or upper UTIs (drug levels are inadequate in renal parenchyma)
• Pediatric dose: 5-7 mg/kg/day; prophylaxis: 1 mg/kg/day

Contraindications & Precautions

• Renal insufficiency (creatinine clearance <30-60 mL/min): drug fails to reach adequate urine concentrations AND toxic blood levels accumulate
• Pregnancy at term / neonates <1 month: risk of hemolytic anemia (immature erythrocyte enzyme systems)
• G6PD deficiency: hemolytic anemia

Adverse Effects (High-Yield for Exams)

Important Drug Interaction

• Nitrofurantoin antagonizes nalidixic acid and some fluoroquinolones (norfloxacin, ciprofloxacin) - do not combine

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2. METHENAMINE (Mandelate & Hippurate)

Chemistry & Mechanism of Action

N₄(CH₂)₆ + 6H₂O + 4H⁺  →  6HCHO + 4NH₄⁺
         (Methenamine)         (Formaldehyde)

• At pH 7.4: virtually no decomposition; no formaldehyde generated
• At pH 6.0: ~6% formaldehyde yield
• At pH 5.0: ~20% formaldehyde yield
• Requires ~3 hours to reach 90% completion of the reaction
• Formaldehyde is a non-specific denaturant of proteins and nucleic acids - bactericidal to virtually all bacteria at 20 µg/mL
• No resistance develops to formaldehyde - organisms cannot become resistant to protein denaturation

Two Formulations

Pharmacokinetics

• Well absorbed orally (but 10-30% decomposes in gastric acid, so enteric-coated tablets preferred)
• Urine excretion is nearly quantitative
• Daily dose of 2 g with urine pH ≤6 → urine formaldehyde concentration of 18-60 µg/mL (above MIC for most uropathogens)

Clinical Use

• Only for suppression/prophylaxis of chronic or recurrent UTI - NOT for acute treatment
• Dose: Methenamine mandelate 1 g QID; Methenamine hippurate 1 g BID
• Children (6-12 years): 500 mg QID or BID respectively
• Best for E. coli; also useful for common gram-negative organisms, S. aureus, S. epidermidis
• Proteus and Enterobacter aerogenes are often resistant (Proteus is urease-producing → raises urine pH → prevents formaldehyde release)

To Achieve Adequate Urinary Acidification

• Ascorbic acid 4-12 g/day
• Ammonium chloride
• Methionine
• Cranberry juice (mild effect)

Contraindications

• Hepatic insufficiency - produces ammonia (NH₄⁺) as a byproduct
• Urinary tract obstruction - drug must contact bacteria in urine for sufficient time
• Renal failure (no urine concentration)

Drug Interactions

• Do NOT combine with sulfonamides - formaldehyde + sulfonamides form an insoluble precipitate in urine → crystalluria
• Falsely elevates catecholamine metabolite tests (urine 17-hydroxycorticosteroids, VMA)

Adverse Effects

• GI irritation
• Dysuria (irritation from formaldehyde at high doses)
• Bladder irritation

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3. FOSFOMYCIN

Mechanism of Action

• This mechanism is unique among antibacterials, so no cross-resistance with other drug classes
• Spectrum: E. coli, Proteus, Enterococcus, S. saprophyticus; variable against Klebsiella, Enterobacter, Serratia; Pseudomonas and Acinetobacter are resistant

Pharmacokinetics

• Available in USA as oral powder (fosfomycin tromethamine)
• Oral bioavailability ~40%, t½ = 5-8 hours
• Single oral dose of 3 g → urine concentrations of 1000-4000 µg/mL (very high!)
• Systemic concentrations are low at oral doses

Clinical Use

• Single-dose (3 g) therapy for uncomplicated cystitis - excellent compliance
• Some clinicians use 3 g every other day × 3 doses for complicated UTI
• 3 g every 10 days for prophylaxis

Adverse Effects

• Generally well tolerated
• GI distress, headache, dizziness, vaginitis

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4. NALIDIXIC ACID (First-Generation Quinolone)

Mechanism

• Inhibits DNA gyrase (topoisomerase II), preventing DNA replication in bacteria
• Bactericidal; active mainly against gram-negative organisms
• Only active at urinary concentrations (too low systemic levels for systemic infections)

Important Limitation

• Resistance emerges rapidly (single-step mutations in DNA gyrase gene) - a major clinical limitation
• Now largely replaced by fluoroquinolones in clinical practice

Clinical Use

• Uncomplicated lower UTIs caused by gram-negative organisms
• Not for gram-positive infections (Pseudomonas is intrinsically resistant)

Adverse Effects

• GI symptoms
• CNS effects: headache, dizziness, visual disturbances, rarely convulsions (especially in children with febrile seizure history)
• Photosensitivity
• Not recommended in children, pregnancy, and patients with seizure disorders

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Quick Comparison Table (High-Yield)

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Key Exam Points to Remember

1. Definition: Urinary antiseptics work in urine, NOT in blood/tissues
2. Nitrofurantoin = first-line for uncomplicated cystitis; avoid in renal failure (CrCl <30-60 mL/min)
3. Methenamine = prophylaxis ONLY, not acute treatment; needs acidic urine (pH <5.5)
4. Fosfomycin = single-dose convenience; unique MurA mechanism; no cross-resistance
5. Nalidixic acid = historical significance; resistance emerges rapidly; largely replaced
6. Never combine methenamine + sulfonamides (precipitate forms)
7. Nitrofurantoin + nalidixic acid = antagonism
8. Acidic urine (pH <5.5) enhances activity of both nitrofurantoin AND methenamine
9. Urea-splitting organisms like Proteus alkalinize urine → resist methenamine
10. Pulmonary fibrosis is a chronic ADR specific to nitrofurantoin (not other UTI drugs)

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Antiretroviral Drugs - Complete MBBS Pharmacology

HIV Life Cycle - The Foundation of ART

1. Attachment - HIV gp120 binds to CD4 receptor on T-helper cells
2. Co-receptor binding - gp120 binds to chemokine co-receptors (CCR5 or CXCR4)
3. Fusion - gp41 mediates viral membrane fusion; viral RNA enters the cell
4. Reverse transcription - Viral RNA → DNA (by reverse transcriptase, RT)
5. Integration - Viral DNA integrates into host chromosome (by integrase)
6. Transcription & translation - Host machinery produces viral proteins as long polyproteins
7. Maturation/Cleavage - HIV protease cleaves polyproteins into functional proteins
8. Budding - New virions released

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Classification of Antiretroviral Drugs

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I. NUCLEOSIDE/NUCLEOTIDE REVERSE TRANSCRIPTASE INHIBITORS (NRTIs)

Mechanism of Action

1. Enter the host cell passively
2. Phosphorylated by cellular kinases to the active triphosphate form
3. Incorporated into the growing viral DNA chain by reverse transcriptase (RT)
4. Because the 3'-OH group is absent, a 3',5'-phosphodiester bond cannot form → chain termination
5. They also competitively inhibit RT (compete with natural nucleotide substrates)

Key concept: NRTIs are prodrugs - they require intracellular phosphorylation to become active. They are competitive substrate analogs + chain terminators.

• Affinities for human DNA polymerases are lower than for HIV RT (explains selectivity)
• Exception: Mitochondrial DNA polymerase γ is also inhibited → cause of mitochondrial toxicities

Individual NRTIs

1. Zidovudine (AZT, ZDV)

• Thymidine analogue
• First FDA-approved antiretroviral drug
• Active against HIV-1, HIV-2, HTLV-I and II
• Phosphorylated to ZDV-5'-triphosphate; terminates viral DNA elongation
• Also inhibits mitochondrial DNA polymerase γ (→ toxicity)
• Resistance mutations: at RT codons 41, 44, 67, 70, 210, 215, 219 (Thymidine Analogue Mutations = TAMs)
• M184V mutation (lamivudine resistance) restores sensitivity to ZDV
• Oral bioavailability: 64%; crosses blood-brain barrier; excreted in breast milk and semen
• Adverse effects:
  • Bone marrow suppression - anemia, granulocytopenia (most important)
  • Fatigue, malaise, nausea, headache, insomnia (early, usually transient)
  • Myopathy with long-term use (mitochondrial damage)
  • Lipoatrophy, lactic acidosis, hepatic steatosis (class effect)
• Drug interaction: Stavudine competes for intracellular phosphorylation - never combine ZDV + stavudine
• Special use: Prevention of mother-to-child transmission (PMTCT); IV formulation available

2. Lamivudine (3TC)

• Cytidine analogue; active against HIV-1, HIV-2, and Hepatitis B virus (HBV)
• Enters cells by passive diffusion → phosphorylated to 3TC-5'-triphosphate
• Low affinity for human DNA polymerases → low host toxicity
• Resistance: M184V or M184I mutation in RT → high-level resistance, but restores ZDV sensitivity
• Oral bioavailability: 86-87%; minimal toxicity
• Adverse effects: Generally very well tolerated; rare peripheral neuropathy
• Very commonly used as backbone of regimens

3. Emtricitabine (FTC)

• Cytidine analogue; structurally similar to lamivudine
• Active against HIV-1 and HBV
• Oral bioavailability: 93% (highest of all NRTIs)
• Resistance: same M184V/I mutation as lamivudine; cross-resistance with 3TC
• Well tolerated; may cause skin hyperpigmentation
• Available as fixed-dose combination with tenofovir (Truvada, Descovy)

4. Tenofovir (TDF & TAF)

• Nucleotide analogue (acyclic phosphonate) - unique among NRTIs because it already has one phosphate group → requires only two phosphorylation steps instead of three
• Available in two prodrug forms:
  • TDF (Tenofovir Disoproxil Fumarate): Oral bioavailability ~25% (with high-fat meal); prodrug converted in gut/plasma
  • TAF (Tenofovir Alafenamide): Improved intracellular delivery to lymphoid cells - 5-7x higher intracellular diphosphate levels; lower plasma tenofovir → fewer systemic adverse effects
• Active against HIV-1, HIV-2, HBV
• Adverse effects of TDF: Nephrotoxicity (proximal tubular dysfunction - Fanconi syndrome), decrease in bone mineral density
• TAF has significantly reduced renal and bone toxicity compared to TDF
• TDF + emtricitabine (Truvada) = approved for PrEP (pre-exposure prophylaxis)

5. Abacavir (ABC)

• Guanosine analogue
• High oral bioavailability (83%); metabolized by alcohol dehydrogenase and glucuronyl transferase
• Does NOT require dose adjustment in renal failure (only NRTI that is primarily hepatically metabolized)
• Resistance: K65R mutation
• Critical adverse effect: Hypersensitivity Reaction (HSR)
  • Occurs in ~5% of patients
  • Multi-system: fever, rash, GI symptoms (nausea, vomiting, diarrhea), malaise, respiratory symptoms
  • Linked to HLA-B*5701 allele - genotyping is mandatory before prescribing abacavir
  • NEVER rechallenge a sensitized patient - can be rapidly fatal
• Used in pediatric HIV treatment

6. Didanosine (ddI)

• Adenosine analogue; must be taken on empty stomach (reduced absorption 55% with food/acidity)
• Metabolism impaired by acid → enteric-coated formulation used
• Adverse effects: Peripheral neuropathy, pancreatitis, lipoatrophy (all due to mitochondrial toxicity)
• Rarely used today due to mitochondrial toxicity profile

Class Adverse Effects of NRTIs (All NRTIs)

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II. NON-NUCLEOSIDE REVERSE TRANSCRIPTASE INHIBITORS (NNRTIs)

Mechanism of Action

• Bind to an allosteric hydrophobic pocket adjacent to (but NOT at) the active site of RT
• Cause a conformational change in the enzyme → reduce RT activity
• Do NOT require intracellular phosphorylation (not prodrugs)
• Only active against HIV-1 (not HIV-2 - HIV-2 RT lacks the allosteric binding pocket)
• Cross-resistance is common among first-generation NNRTIs (they share the same binding pocket)

Drugs

1. Efavirenz (EFV)

• Most widely used NNRTI globally
• Potent, once-daily dosing
• CNS adverse effects (most characteristic): vivid dreams, nightmares, dizziness, impaired concentration, psychiatric symptoms (usually resolve after 2-4 weeks)
• Teratogenic in first trimester - avoid in pregnancy (neural tube defects in monkeys; category D)
• Strong inducer of CYP450 → multiple drug interactions
• Advantage: Compatible with rifampicin (TB treatment) - lower interaction risk than other NNRTIs
• Metabolized by CYP2B6; poor metabolizers have higher plasma levels and more CNS toxicity

2. Nevirapine (NVP)

• Older first-generation NNRTI
• Adverse effects: Severe hepatotoxicity (especially in women with CD4 >250 and men with CD4 >400 at initiation), life-threatening skin rash (Stevens-Johnson syndrome)
• Rarely used in adults today; still used for PMTCT (single-dose NVP at delivery in resource-limited settings)
• Inducer of CYP450

3. Rilpivirine (RPV)

• Second-generation NNRTI
• Smallest tablet size - good for patients with dysphagia
• Must be taken with a meal (≥390 kcal)
• Limitation: Inferior efficacy at baseline HIV RNA >100,000 copies/mL or CD4 <200
• Good CNS tolerability vs. efavirenz
• Available as long-acting injectable form (with cabotegravir) for monthly dosing

4. Etravirine (ETR)

• Second-generation; active against many HIV strains resistant to first-generation NNRTIs
• Reserved for treatment-experienced patients with multidrug-resistant HIV

5. Doravirine (DOR)

• Newest NNRTI; once daily
• CNS tolerability advantage over efavirenz
• Fewer drug interactions than efavirenz or rilpivirine
• Efficacy maintained even with high viral load (advantage over rilpivirine)

Class Adverse Effects of NNRTIs

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III. PROTEASE INHIBITORS (PIs)

Mechanism of Action

1. HIV protease cleaves large viral polyproteins (Gag-Pol polyprotein) into functional structural proteins (p24, p17) and enzymes during viral maturation
2. PIs are peptidomimetics - they mimic the peptide substrate of HIV protease and competitively inhibit the enzyme
3. Without protease activity, virions bud but remain immature and non-infectious
4. PIs have high genetic barrier to resistance - multiple mutations needed → preferred in patients with uncertain adherence

Key concept: PIs act AFTER integration - they block viral maturation, not transcription. New virions form but are non-functional.

Pharmacokinetic Boosting

• Ritonavir at therapeutic doses = PI with severe toxicity (rarely used as antiviral)
• Ritonavir at low dose (100-200 mg) = CYP3A4 inhibitor booster only
• Cobicistat = pharmacokinetic enhancer with no antiviral activity; less drug interactions than RTV

Individual PIs

Class Adverse Effects of PIs

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IV. INTEGRASE STRAND TRANSFER INHIBITORS (INSTIs)

Mechanism of Action

1. After reverse transcription, HIV integrase processes the viral DNA ends (3'-processing)
2. The processed viral DNA enters the nucleus and integrase catalyzes strand transfer - inserting viral DNA into host chromosomal DNA
3. INSTIs chelate the Mg²⁺ ions in the catalytic site of integrase and block the strand transfer step
4. Viral DNA cannot integrate → replication halted

Drugs

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V. ENTRY INHIBITORS

Three Subclasses Based on the Step of Entry Blocked

A. Attachment Inhibitors

• Prodrug → active drug temsavir
• Binds to HIV gp120 envelope glycoprotein → prevents conformational change needed for attachment to host CD4 receptor
• Indicated for heavily treatment-experienced adults with multidrug-resistant HIV
• Oral, twice daily
• CYP450 substrate - contraindicated with strong inducers

B. CCR5 Antagonists (Co-receptor Antagonists)

• Binds to the host CCR5 co-receptor (not the virus) → blocks HIV from using CCR5 as an entry co-receptor
• Only active against CCR5-tropic ("R5") HIV strains
• Before use, a tropism test (genotypic or phenotypic) is MANDATORY - if patient has CXCR4-tropic or dual/mixed-tropic virus, maraviroc will not work
• Adverse effects: Cough, fever, upper respiratory infections, hepatotoxicity
• Drug interactions: CYP3A4 substrate; dose adjustment needed with RTV/cobicistat

C. Fusion Inhibitors

• Peptide (36 amino acid) that mimics the HR2 region of HIV gp41
• Binds to the HR1 region of gp41 → prevents the conformational change needed for fusion of viral and host cell membranes
• Subcutaneous injection only (cannot be given orally - peptide destroyed by gut)
• Twice-daily injections
• Adverse effects: Injection site reactions (almost universal - nodules, cysts, ecchymosis), increased rate of bacterial pneumonia
• Reserved for treatment-experienced patients with multidrug-resistant HIV (expensive, injection-based)

D. CD4-Directed Post-Attachment Inhibitor

• Monoclonal antibody against domain 2 of the CD4 receptor
• Prevents post-attachment conformational changes required for HIV entry
• Inhibits HIV entry without blocking normal CD4 T-cell function (binds to domain 2, not domain 1 where MHC-II binds)
• Given as IV infusion every 2 weeks
• For multidrug-resistant HIV in treatment-experienced patients

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VI. PHARMACOKINETIC ENHANCERS (Boosters)

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VII. COMBINATION THERAPY - ART (Antiretroviral Therapy)

Why Combination Therapy?

• HIV replicates rapidly (~10⁹ virions/day) with high error rate → mutations arise constantly
• Monotherapy → rapid selection of resistant mutants → treatment failure
• Using 2-3 drugs targeting different steps → resistant mutants to all 3 drugs simultaneously are vanishingly rare

Recommended Initial Regimens (General Principles)

• 2 NRTIs + 1 INSTI (most guidelines, e.g., WHO, DHHS)

• TDF/FTC + DTG (Tenofovir/Emtricitabine + Dolutegravir) - WHO preferred
• TAF/FTC + BIC (Tenofovir alafenamide/Emtricitabine + Bictegravir) - single tablet (Biktarvy)
• TDF/FTC + EFV (Tenofovir/Emtricitabine + Efavirenz) - widely used in resource-limited settings

When to Start ART

• All HIV-infected individuals regardless of CD4 count (current WHO/DHHS guidance)
• ASAP in opportunistic infections EXCEPT TB meningitis and cryptococcal meningitis (defer slightly)
• Pregnant women: immediately regardless of CD4 count

Goals of ART

1. Suppress viral load to undetectable (<50 copies/mL)
2. Allow CD4 cell count recovery
3. Prevent opportunistic infections
4. Prevent HIV transmission (Undetectable = Untransmittable, U=U)
5. Prevent development of resistance

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VIII. POST-EXPOSURE PROPHYLAXIS (PEP) & PRE-EXPOSURE PROPHYLAXIS (PrEP)

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IX. SPECIAL SITUATIONS

HIV in Pregnancy

• ART should be started/continued in ALL pregnant women
• ZDV reduces MTCT (given IV during labor + oral to neonate for 6 weeks)
• Avoid efavirenz in first trimester; dolutegravir requires counseling (neural tube defect risk with periconceptional exposure)
• Preferred: TDF/FTC + RAL or LPV/r during pregnancy (well-studied safety data)

HIV + Tuberculosis Co-infection

• Start TB treatment first, then ART within 2-8 weeks (immediately if CD4 <50)
• Preferred NNRTI: Efavirenz (less interaction with rifampicin than other antiretrovirals)
• Avoid PI-based regimens with rifampicin (rifampicin markedly reduces PI levels)
• Dolutegravir requires dose adjustment (increase to twice daily) with rifampicin

HIV + Hepatitis B Co-infection

• Treat both simultaneously
• Choose NRTI backbone that covers both: TDF or TAF + emtricitabine or lamivudine (active against both HIV and HBV)
• Never stop these NRTIs suddenly in co-infected patients → risk of HBV flare

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X. HIGH-YIELD EXAM SUMMARY TABLE

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Key Mnemonics & Exam Points

1. NRTI mechanism: "No 3'-OH = No chain extension" → chain termination
2. Abacavir: Always check HLA-B*5701 before prescribing; rechallenge = potentially fatal
3. NNRTIs = HIV-1 ONLY (no HIV-2 activity)
4. PIs = Peptidomimetic + boosted with ritonavir/cobicistat
5. INSTIs preferred in current guidelines for treatment-naive patients
6. Enfuvirtide = only SC antiretroviral (peptide, cannot be given orally)
7. Maraviroc = check tropism first (only for CCR5-tropic virus)
8. ZDV contraindication: Avoid combining with stavudine (competition for phosphorylation)
9. Never stop TDF/FTC in HBV co-infected patients (risk of hepatitis flare)
10. Atazanavir = only PI that does NOT cause dyslipidemia; causes benign hyperbilirubinemia
11. PEP must start within 72 hours of exposure; continue for 28 days
12. Dolutegravir = current WHO preferred first-line (most countries); concern for neural tube defects in periconceptional use

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