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PH1.1-13 | General Pharmacology Foundations — PBL Case

CLINICAL SETTING

Mrs. Padmavathi, a 55-year-old homemaker from a semi-urban district of Tamil Nadu, is brought to the district general hospital emergency department by her daughter. She was apparently well until 3 days ago, when she was started on carbamazepine 200 mg twice daily by a private practitioner for trigeminal neuralgia. Today she has developed widespread painful blistering of the skin and erosions of the oral and genital mucosa, with high fever (39.8°C). Her past medical history includes epilepsy controlled for the past 6 years on phenobarbitone 60 mg twice daily. The attending physician immediately suspects Stevens-Johnson Syndrome (SJS) and contacts the pharmacovigilance unit. Relevant information: - Ethnicity: Tamil (South Indian population) - Weight: 58 kg; otherwise healthy kidneys and liver - Drug list: Phenobarbitone (chronic, 6 years); Carbamazepine (3 days, new) - Laboratory: Serum creatinine 0.8 mg/dL; liver enzymes normal; phenobarbitone level 18 μg/mL (therapeutic range 15–40 μg/mL)

Trigger 1: Trigger 1: The Emergency Presentation

Mrs. Padmavathi is brought to the ED with high fever, diffuse skin blistering, and mucositis 3 days after starting carbamazepine. The dermatologist confirms Stevens-Johnson Syndrome. Carbamazepine is immediately stopped. The attending physician notes her Tamil ethnicity and her 6-year phenobarbitone history.

DISCUSSION POINTS

  • What type of adverse drug reaction (ADR) is SJS, using the A–E classification? What distinguishes this type from Type A reactions, and why is simple dose reduction NOT the appropriate management?
  • What is the pharmacogenetic basis for increased SJS risk with carbamazepine in South and Southeast Asian populations? Which HLA allele is involved, and what does this tell us about inter-individual variation in drug response?
  • The prescriber had carbamazepine on the district formulary under its generic (INN) name. Why is the INN system important for patient safety, and could brand-name confusion have played a role in any element of this case?
Click to reveal Trigger 2: Trigger 2: Drug Interaction Concern (discuss previous trigger first!)

Trigger 2: Trigger 2: Drug Interaction Concern

While reviewing the patient's medications, the clinical pharmacologist notes that phenobarbitone is a powerful CYP enzyme inducer. The team wonders whether phenobarbitone altered the carbamazepine's pharmacokinetics in a way that contributed to the clinical picture. Carbamazepine is also noted to be an auto-inducer — it induces its own metabolism. The patient's carbamazepine level on admission is 4.2 μg/mL (reference range 4–12 μg/mL).

DISCUSSION POINTS

  • Phenobarbitone is a broad CYP inducer (CYP2B6, CYP3A4, CYP2C9). Carbamazepine is metabolised primarily by CYP3A4. What effect would phenobarbitone have on carbamazepine plasma levels, and is this the direction of interaction that led to toxicity here?
  • Carbamazepine is an auto-inducer — it induces CYP3A4 over 2–4 weeks of use. How does auto-induction affect carbamazepine's own plasma levels over time? What clinical implication does this have for dose titration at initiation?
  • Conversely, carbamazepine could affect phenobarbitone levels. Explain the direction and mechanism of this interaction. Using the patient's phenobarbitone level (18 μg/mL), does the current level suggest significant impact?
Click to reveal Trigger 3: Trigger 3: Pharmacovigilance and the Route of Management (discuss previous trigger first!)

Trigger 3: Trigger 3: Pharmacovigilance and the Route of Management

Mrs. Padmavathi cannot swallow due to oral mucositis. The team needs to continue her anti-seizure medication. Phenobarbitone is available as IV injection (60 mg/mL ampoule). The pharmacovigilance officer arrives to assist with the ADR report. She explains that SJS due to carbamazepine must be reported to the national ADR monitoring centre under India's PvPI programme.

DISCUSSION POINTS

  • Mrs. Padmavathi cannot take oral phenobarbitone. Compare the oral and IV routes for phenobarbitone: discuss differences in bioavailability, first-pass metabolism, onset, and whether IV dosing requires dose adjustment relative to oral dosing.
  • The pharmacovigilance officer uses the WHO-UMC causality framework to assess the carbamazepine–SJS reaction. Apply this framework to this case: what evidence supports the temporal relationship, drug plausibility, dechallenge outcome, and absence of alternative explanations? Which causality category would you assign?
  • India's PvPI (Pharmacovigilance Programme of India) collects ADR reports. Who is mandated to report in India? Through which system are reports submitted, and what information is required on the form? Why does individual case reporting matter for rare reactions like SJS?

Group Task Assignments

Group Group 1: ADR Classification and Pharmacogenetics

  • Classify carbamazepine-induced SJS using the A–E ADR framework and justify the classification
  • Research and present the pharmacogenetic basis: HLA-B*1502 allele, its prevalence in South/Southeast Asian populations, and the FDA black-box warning for carbamazepine
  • Explain why pharmacogenetic testing before carbamazepine initiation is a rational, evidence-based approach in this population

Competencies: PH1.11, PH1.12

Group Group 2: Drug Interactions: CYP Induction and Auto-induction

  • Map the CYP enzyme interactions between phenobarbitone and carbamazepine, including direction of induction and net expected plasma level changes for each drug
  • Explain carbamazepine auto-induction: timeline (2–4 weeks), mechanism (CYP3A4 upregulation), and clinical consequence for dose titration
  • Discuss whether the phenobarbitone–carbamazepine interaction contributed to the SJS in this case, supported by the carbamazepine level

Competencies: PH1.6, PH1.13

Group Group 3: Routes of Administration and Pharmacokinetics

  • Compare oral vs IV phenobarbitone: bioavailability, first-pass metabolism, Vd, protein binding, onset, and whether dose conversion is 1:1
  • Explain the pharmacokinetic basis of phenobarbitone's long half-life (~80–100 hours) and why this has implications for managing seizure prophylaxis during the acute admission
  • Discuss what happens to the plasma level of a drug at steady state when an inducer (phenobarbitone) is added — draw a conceptual plasma level vs time curve

Competencies: PH1.5, PH1.6

Group Group 4: Pharmacovigilance and Rational Prescribing

  • Apply the WHO-UMC causality framework to the carbamazepine–SJS case and assign a causality category with full justification for each criterion
  • Describe India's PvPI programme: reporting obligations, submission pathway, and the role of individual case reports in signal generation for rare reactions like SJS
  • Propose a pharmacovigilance-informed prescribing protocol for carbamazepine initiation in Tamil Nadu hospitals — incorporating HLA testing, monitoring schedule, and ADR reporting plan

Competencies: PH1.2, PH1.12, PH1.3

Group Group 5: Mechanisms of Action and Drug Selection

  • Describe the mechanism of action of carbamazepine (sodium channel blockade) and phenobarbitone (GABA-A receptor potentiation) — why are these rational for trigeminal neuralgia and epilepsy respectively?
  • If the treating neurologist had prescribed the carbamazepine more carefully, which alternative (drug or formulation route) might have been safer in a Tamil patient with trigeminal neuralgia? Justify with pharmacological and pharmacogenetic reasoning.
  • Explain the concept of rational drug use applied to this case: what went wrong in the prescribing chain, and what principles should guide future prescribing for this patient?

Competencies: PH1.1, PH1.7, PH1.9

Learning Issues

Research these questions and bring your findings to the discussion.

  1. [PH1.11] What are the A–E types of ADRs, and what distinguishes a Type B (bizarre) reaction — such as carbamazepine SJS — from a Type A (augmented) reaction in terms of mechanism, dose-dependency, and management?
  2. [PH1.12] What is pharmacovigilance, and what is the role of the Pharmacovigilance Programme of India (PvPI)? Who is required to report ADRs in India, and why is individual case safety reporting important for detecting rare reactions like SJS?
  3. [PH1.13] How does CYP enzyme induction (by phenobarbitone) alter carbamazepine's pharmacokinetics? What is CYP auto-induction, and how does carbamazepine's self-induction affect its own plasma levels over the first weeks of treatment?
  4. [PH1.6] What determines a drug's plasma half-life and how is steady state reached? Why does phenobarbitone's long t½ (~80–100 h) create a specific clinical management challenge during this acute admission?
  5. [PH1.5] How does switching from oral to IV phenobarbitone affect its pharmacokinetics (bioavailability, first-pass metabolism, dose equivalence, onset)? When is IV administration clinically indicated, and what monitoring is required?
  6. [PH1.3] What is the clinical and safety rationale for using the INN (generic name) in prescribing? How does brand-name variation create risk for patients managed across different healthcare facilities?