Two Compartment Models | IV Bolus Kinetics Of Multiple Dosing | Unit 4 Biopharmaceutics 6th Semester

The video is a lecture by Harshit Pandey covering Unit 4 of Biopharmaceutics and Pharmacokinetics, specifically the Two Compartment Open Model for IV Bolus administration. The instructor emphasizes that students must complete Unit 3 before attempting Unit 4, as foundational concepts are essential for understanding the material.

The Two Compartment Open Model is explained as consisting of a Central Compartment (blood and highly perfused organs like heart, liver, and kidneys) and a Peripheral Compartment (less perfused tissues like muscle and fat). When a drug is administered via IV bolus, it enters the central compartment first, then distributes reversibly to the peripheral compartment. Elimination occurs only from the central compartment, and drug movement follows first-order kinetics. The rate constants K12 (central to peripheral), K21 (peripheral to central), and K10 (elimination from central) are defined.

The instructor walks through the graphical representation of drug concentration over time in both compartments. At time zero, central compartment concentration is at maximum while peripheral is zero. As time progresses, rapid decline in central compartment occurs due to distribution to peripheral, followed by slower decline due to elimination. The peripheral compartment concentration first rises then falls as drug returns to central and is ultimately eliminated.

Mathematical equations are derived for the rate of change of drug concentration in both compartments (dCc/dt and dCp/dt), incorporating K12, K21, K10 terms. After substituting concentration with amount/volume of distribution, integrated equations are presented involving hybrid first-order constants alpha (α) and beta (β) representing distribution and elimination phases respectively. The relationships α + β = K12 + K21 + K10 and α·β = K21·K10 are also noted.

The dosage regimen section covers steady-state drug levels, loading dose, and maintenance dose. Steady-state is explained using a half-life example where repeated fixed dosing eventually results in the rate of administration equaling the rate of elimination, typically achieved after 4-5 half-lives. Loading dose is the initial higher dose given to rapidly achieve therapeutic plasma concentration, calculated as Css × Vd / F. Maintenance dose is the dose required to sustain steady-state concentration, dependent on drug clearance, dosing interval, and bioavailability. Clinical examples include emergency situations (seizures, arrhythmias) for loading doses and long-term conditions (hypertension, epilepsy) for maintenance doses.