Pharmacokinetics MCQs

Plasma clearance (CL) is the volume of plasma cleared of a substance in a period of time. It is estimated by dividing a substance's rate of elimination by its plasma concentration: CL = (0.02 mg/min)/(0.04 mg/mL) = 0.5 mL/min. In first-order kinetics, the rate of elimination of the substance and the plasma concentration of the substance decrease with time, but the CL remains constant. Alternatively, the CL of a substance with first-order kinetics can be calculated by multiplying the volume of distribution (Vd) by the elimination constant (Ke) of the substance. For a substance that is minimally protein-bound and excreted unchanged only in the urine, as in this case, the CL is essentially the glomerular filtration rate. In humans, inulin is such a substance Reference: Basic & Clinical Pharmacology by Bertram G. Katzung Fourteenth Edition Pg no.52

Explanation: Loading dose (LD) = (Desired steady-state plasma concentration × Vd) ÷ Bioavailability. Half-life determines maintenance dosing frequency and time to steady state, but it is not a direct component of the loading dose equation (Statement 2 ruled out) For oral drugs , bioavailability (F) must be accounted for because oral dosing is reduced by first-pass extraction and incomplete absorption. Formula (practical): LD (oral) = (C_target × Vd) / F Where: C_target = desired plasma concentration (therapeutic target) Vd = volume of distribution F = fraction absorbed (bioavailability)

This drug is almost entirely absorbed into systemic circulation when orally administered, as indicated by the lack of difference in the dose-corrected AUCs for oral and intravenous administration. indicating that this drug has near 100% bioavailability. Isosorbide mononitrate does not undergo first-pass metabolism by the liver and thus has approx. 100% bioavailability. Reference: Basic & Clinical Pharmacology by Bertram G. Katzung Fourteenth Edition Pg no.48

A maintenance dose of 576 mg should be administered at 12 hour intervals to achieve the desired target concentration of 2 mg/L. The corresponding formula needed for calculation requires the following values: Cp = target plasma concentration at steady state (mg/L) = 2 mg/L Cl = clearance (L/h)= 12 L/h τ = dosing interval (hours)= 12 hour F = bioavailability = 50 % Maintenance dose = (Cp * Cl * τ) / F → (2*12*12)/0.50 = 288/0.5 = 576 Reference: Basic & Clinical Pharmacology by Bertram G. Katzung Fourteenth Edition Pg no.52

To calculate the time it takes for a medication to reach 90% of its final steady state, you can use the formula (3.3) * (t1/2). Given that the half-life (t1/2) of fluconazole is 12 hours: Time to Reach 90% of Steady State = (3.3) * (12 hours) = 39.6 hours So, it will take approximately 39.6 hours for the medication to reach 90% of its final steady-state level. Among the given options, the closest one is: C) 40 hours This option is the most accurate representation of the calculated time of approximately 39.6 hours. Reference: Basic & Clinical Pharmacology by Bertram G. Katzung, Fourteenth Edition, Pg no. 51

When a drug is repeated at relatively short intervals, it accumulates in the body until elimination balances input and a steady state plasma concentration ( Cpss ) is attained- Cpss = dose rate/CL From this equation it is implied that doubling the dose rate would double the average Cpss and so on. Further, if the therapeutic plasma Concentration of the drug has been worked out and its CL is known, the dose rate needed to achieve the target Cpss can be determined- Dose rate = target Cpss × CL

Explanation for answer: The LD₅⁰ (Lethal Dose 50%) is a standard measure of acute toxicity. It represents the single dose of a substance required to kill 50% of a population of test animals under controlled conditions. A higher LD₅⁰ indicates a safer drug. Key Takeaways: LD₅⁰ measures acute lethality in animal studies. Therapeutic Index (TI) = LD₅⁰ / ED₅⁰; a higher TI indicates a safer drug.

Certain drugs are degraded in the gastrointestinal tract, e.g. penicillin G by acid, insulin by peptidases, and are ineffective orally. Enteric coated tablets have acid resistant coating, can be Used to overcome acid lability, gastric irritancy and also used to increase absorption of drugs which are absorbed from the intestine(Options A,C,D) Sustained release preparations have drug particles coated with slowly dissolving material that can increases the half-life of the drug.