Metformin's Intrinsic Blood-to-Plasma Partition Ratio (B/P): Reconciling the Perceived High In Vivo B/P > 10 with the In Vitro Equilibrium Value of Unity

Blood cells are considered an important distributional compartment for metformin based on the high blood-to-plasma partition ratio (B/P) in humans (>10 at C_min). However, literature reports of metformin9s intrinsic in vitro B/P values are lacking. At present, the extent and rate of metformin cellular partitioning was determined in incubations of fresh human and rat blood with [¹⁴C]metformin for up to 1 week at concentrations spanning steady-state plasma C_min, C_max, and a concentration associated with lactic acidosis. The results showed that metformin9s intrinsic equilibrium B/P was ∼0.8–1.4 in blood, which is <10% of the reported clinical value. Kinetics of metformin partitioning into human blood cells and repartitioning back into plasma were slow (repartitioning half-life ∼32–39 hours). These data, along with in vivo rapid and efficient renal clearance of plasma metformin (plasma renal extraction ratio ∼90%–100%), explain why the clinical terminal half-life of metformin in plasma (6 hours) is 3- to 4-fold shorter than the half-life in whole blood (18 hours) and erythrocytes (23 hours). The rate constant for metformin repartitioning from blood cells to plasma (∼0.02 h⁻¹) is far slower than the clinical renal elimination rate constant (0.3 h⁻¹). Blood distributional rate constants were incorporated into a metformin physiologically-based pharmacokinetic model, which predicted the differential elimination half-life in plasma and blood. The present study demonstrates that the extent of cellular drug partitioning in blood observed in a dynamic in vivo system may be very different from the static in vitro values when repartitioning from blood cells is far slower than clearance of drug in plasma.