Pharmacokinetics and Pharmacodynamics Flashcards

A 72-year-old patient undergoes a 4-hour abdominal procedure maintained with a remifentanil infusion at 0.2 mcg/kg/min. At emergence, the anesthesiologist expects rapid offset of opioid effect. Which statement MOST accurately explains remifentanil's unique pharmacokinetic profile underlying this prediction?

Answer: Remifentanil undergoes organ-independent ester hydrolysis by non-specific plasma and tissue esterases, resulting in a context-sensitive half-time of ~3–4 minutes that remains constant regardless of infusion duration

Remifentanil is uniquely metabolized by ubiquitous non-specific esterases in blood and tissues (not plasma cholinesterase), yielding an extraordinarily high clearance (~3 L/min) that is independent of hepatic blood flow, renal function, or enzyme capacity. This is why its context-sensitive half-time is invariably ~3–4 minutes whether the infusion runs for 30 minutes or 8 hours — no peripheral accumulation occurs. Option B is incorrect: remifentanil is not a CYP3A4 substrate. Option C is incorrect: remifentanil has a small Vd (~0.35 L/kg). Option D is incorrect: remifentanil acid is pharmacologically inactive and does not regulate esterase kinetics.

Alfentanil has a lower octanol:water partition coefficient than fentanyl, yet it demonstrates faster clinical onset than morphine and a more predictable time-to-peak effect than fentanyl. The pharmacokinetic parameter that BEST explains alfentanil's faster onset compared to morphine is:

Answer: Alfentanil's lower pKa (6.5 vs 8.0 for morphine) results in ~89% existing in the unionized form at physiological pH compared to ~10% for morphine, dramatically increasing its membrane permeability and ke0

The Henderson-Hasselbalch equation reveals that alfentanil (pKa 6.5) at physiological pH 7.4 has ~89% in the unionized, lipid-soluble form, whereas morphine (pKa 8.0) has only ~10% unionized. The non-ionized fraction crosses lipid membranes passively; thus alfentanil equilibrates with the effect compartment far more rapidly, reflected in a much larger ke0 (0.77 min⁻¹ vs ~0.01–0.02 min⁻¹ for morphine). A is partially true but is a consequence of pKa, not the primary mechanism of faster onset — a small Vd does not by itself accelerate CNS penetration. C is a distractor: high protein binding if anything would slow onset. D is irrelevant since these are IV drugs bypassing first-pass metabolism.

Morphine has a hepatic extraction ratio of approximately 0.7. A patient develops acute liver failure with a 60% reduction in hepatic blood flow and near-complete loss of CYP3A4 and glucuronidation capacity. Which physiological change is the DOMINANT driver of reduced morphine clearance in this scenario?

Answer: The 60% reduction in hepatic blood flow, since high-extraction drugs are primarily flow-limited and clearance approximates Q × ER

For high-extraction drugs (ER ≥ 0.7), hepatic clearance is flow-limited: CLH ≈ Q_H × ER. The liver already removes nearly all drug presented to it per pass, so intrinsic clearance (enzyme activity) has minimal additional leverage — even reducing enzyme activity by 80% barely changes the extraction ratio when it starts near 0.7. In contrast, a 60% reduction in hepatic blood flow directly and proportionately reduces CLH regardless of enzyme capacity. Option A is therefore incorrect. Option C is a distractor: M6G is an active metabolite but its accumulation does not meaningfully inhibit morphine's metabolism at clinical concentrations. Option D: for high-ER drugs, changes in protein binding also have little effect on clearance since essentially all drug (bound and free) is extracted during each hepatic pass.

Two neuromuscular blocking agents share identical ED95 values (equivalent potency). Drug A follows the Hill equation with a Hill coefficient (γ) of 1.5; Drug B has γ = 5. During a difficult intubation sequence, an anesthesiologist needs to reliably achieve >95% twitch suppression. Which statement is TRUE about Drug B's concentration-response behavior?

Answer: Drug B's steeper sigmoid curve means the concentration range between 5% and 95% block is compressed into a narrow window, making the transition to profound block faster and more reliable at doses modestly above ED95

The Hill coefficient (γ) governs the steepness of the sigmoid Emax curve without altering EC50 or Emax. With γ = 1 (Langmuir), the concentration range spanning 20–80% effect covers an 81-fold range. With γ = 5, that same 20–80% transition occurs over only a ~2.3-fold concentration range. Critically, the concentrations for 5% and 95% effect are far closer together, meaning doses modestly above ED95 reliably produce profound (>95%) block. This is NOT the same as easier titration — the steep curve means small overdoses give maximal block and small underdoses give minimal block (more threshold-like). Option A is wrong: Drug B is harder to titrate finely. Option C is wrong: γ does not change Emax or the multiple of ED95 needed. Option D is wrong: γ directly affects the dose-response steepness and the dose multiples required for near-maximal effect.

An isobolographic study examines the combination of propofol and dexmedetomidine for sedation scoring (MOAA/S ≤ 2). The experimental isobole is found to bow CONVEXLY away from the origin (i.e., the measured combination doses are consistently above the line of additivity). Which interpretation is CORRECT?

Answer: The drugs are infra-additive (antagonistic): greater combined doses than predicted by additivity are required, suggesting partial competition or opposing mechanisms at the target effect

In isobolographic analysis, the straight line connecting the individual drug doses that produce a defined effect on each axis represents simple additivity. A concave isobole (bowing toward the origin) indicates synergy — combinations below the additive line achieve the effect with less total drug. Conversely, a convex isobole (bowing away from the origin, into the region of greater drug amounts) indicates infra-additivity or antagonism — the combination requires MORE total drug than additivity predicts, suggesting the drugs partially oppose each other's effect or compete for the same pathway. Option A describes a concave isobole (synergy). Option B describes the straight-line additive case. Option D is incorrect — a ceiling effect would appear as inability to achieve the endpoint with either drug alone, preventing the analysis from being constructed at all.

A patient with end-stage renal disease (eGFR < 5 mL/min) is prescribed a drug that is 90% eliminated unchanged by the kidneys and 10% hepatically metabolized. At therapeutic concentrations, the drug displays saturable (Michaelis-Menten) hepatic metabolism operating near Vmax. Which consequence of combining renal failure with saturable kinetics is MOST clinically significant?

Answer: Accumulation will occur due to lost renal clearance, and because hepatic metabolism is already near Vmax, even a 50% dose reduction may cause a disproportionately large fall in steady-state levels — requiring careful titration with therapeutic drug monitoring

With 90% renal elimination lost, drug accumulates substantially and steady-state concentrations rise. Because hepatic metabolism is operating near Vmax (zero-order territory), the residual elimination pathway removes a nearly constant AMOUNT per unit time rather than a constant fraction — this is the hallmark of zero-order kinetics. In this region, the dose-concentration relationship is extremely non-linear: doubling the dose can more than double steady-state levels, and conversely, dose reductions cause disproportionately large drops. This makes safe dosing unpredictable and necessitates TDM. Option A is incorrect: loss of renal clearance causes accumulation, driving plasma levels HIGHER and pushing hepatic enzymes further into saturation, not below Km. Option C is incorrect: it confuses zero-order (constant amount per time) with first-order (constant fraction per time) kinetics. Option D is incorrect: volume of distribution is not determined by GFR.