(M1130-05-31) A Mathematical Model Describing the Activity of Proton Gradient-Driven MATE Transport

Purpose: The Multidrug and Toxin Extrusion (MATE) transporters, MATE1 and MATE2-K, are located on the apical surface of renal tubule cells and play a crucial role in eliminating organic cations (OCs), including drugs and endogenous compounds. MATE transporters act as electroneutral H⁺/OC antiporters. Their activity is influenced by the concentration gradients of both the transported substance and protons (pH-gradient) across the cell membrane. In multiple in vitro experiments of OC transport in MATE-transfected cells, it has been shown that the alkalinization of the incubation media relative to the intracellular concentration leads to an intracellular OC accumulation and that media acidification facilitates OC excretion. Thus, the concentration gradient allows MATE transporters to function as either uptake or efflux transporters depending on the experimental setup. Given their location in the kidney, these in vitro findings suggest that changes in urine pH could affect how the kidney handles renal OC disposition and how much of MATE substrate drugs and endogenous compounds are present in the body. To better understand this pH-dependent transport, we developed a mathematical model.

Methods: The mathematical model was build using the QSP designer (V2, Certara UK Ltd., Sheffield, UK) based on the general principles of carrier-mediated antiport systems. A key assumption of this model is that at equilibrium, the concentration ratio of OCs and protons is inversely related. We then used this model to describe the transport of several substrates (lamivudine, TEA, memantine, etc.) under different pH conditions in experiments where MATE transporters acted as both uptake and efflux systems. Passive diffusion of these substrates across the cell membrane was first characterized in experiments lacking MATE expression. Subsequently, MATE transporter activity was modelled using a proton gradient-driven (PGD) model. In experiments using cells expressing both MATE and another transporter (OCT2), the OCT2 activity was modelled using an electrochemical gradient-driven (EGD) model.

Results: Our PGD model successfully captured the shift in MATE transporter function from efflux to uptake and the changes in its activity with varying pH, which could not be explained only by traditional Michaelis-Menten kinetics. Experiments with control cells lacking MATE transporters (mock cells) for lamivudine and TEA showed that the observed changes in transport were not simply due to the ionization state of the drugs affecting their passive permeability, highlighting the necessity of considering changes in transporter activity (Figure 1).

Conclusion: While the model is able to describe in vitro MATE activity, the limited in vivo concentration and pH gradient leads to an underestimation of the renal clearance suggesting a further missing mechanism.

Figure 1: Simulated uptake of lamivudine (left) and TEA (right) compared to observed data from MATE-transfected and mock-transfected HEK293 cells. The middle figure shows the simulated lamivudine transcellular transport for the efflux across double transfected MDCK cells.