Vol. 294, Issue 3, 1088-1098, September 2000

Uptake of Imipramine in Rat Liver Lysosomes In Vitro and Its Inhibition by Basic Drugs¹

Junko Ishizaki, Koichi Yokogawa, Fujio Ichimura and Shoji Ohkuma

Hospital Pharmacy (J.I., F.I.), Department of Pharmacology and Pharmaceutics, Graduate School of Natural Science and Technology (K.Y.), and Department of Molecular and Cellular Biology, Faculty of Pharmaceutical Sciences (S.O.), Kanazawa University, Kanazawa, Japan

We investigated the uptake of imipramine (IMP) in highly purified lysosomes from rat liver and its inhibition by a variety of basic drugs in vitro. The uptake of [³H]IMP into lysosomes peaked in less than 20 s, showing little temperature dependency or countertransport phenomena. It was accelerated by increase of extralysosomal pH, stimulated by Mg²⁺-ATP in KCl buffer, and suppressed by acidic ionophores. However, the uptake of [³H]IMP in lysosomes was approximately 140-fold higher than the value expected from the pH-partition theory. IMP and other weak lipophilic bases like chlorpromazine and propranolol raised the intralysosomal pH, and their potency was stronger than that of NH₄Cl, a typical pH-perturbing weak base. A variety of basic drugs inhibited the uptakes of [³H]IMP and [¹⁴C]methylamine into lysosomes, their 50% inhibitory concentrations (IC₅₀) being almost the same for [³H]IMP and [¹⁴C]methylamine uptake (r = 0.842). A high correlation (r = 0.946) was observed between the IC₅₀ values (for the inhibition of [³H]IMP uptake) and the lipophilicity (P_oct values). These results suggest that the accumulation of lipophilic basic drugs is driven primarily by the transmembrane pH difference (pH-partition theory) but with the involvement of some additional mechanism(s) related to drug lipophilicity, possibly binding (partition or adsorption) to lipophilic substance(s) and/or aggregation within lysosomes. Based on this idea, we have established a model that described and successfully simulated the weak base-induced pH increase, the accumulation of a lipophilic weak base (IMP), and the inhibition of accumulation of IMP by lipophilic basic drugs.