Comparison of intracellular methotrexate kinetics in red blood cells with the kinetics in other cell types

Abstract

Aim: To assess the similarities in intracellular pharmacokinetics (PK) of methotrexate (MTX) in red blood cells (RBCs) and other cell lines.

Methods: Three previously published PK models for intracellular MTX and MTX polyglutamate (MTXGlu2-5 ) concentrations were used: (i) a model for the kinetics in RBCs, (ii) a model for the kinetics in human breast cancer cells (HBCCs) and (iii) a model for the kinetics in various white blood cell (WBC) lines. All three models were used to simulate the response in a typical individual receiving 10 mg oral MTX once weekly and the predicted steady-state concentrations (Css ) and time to Css (tss ) were compared.

Results: The HBCC model showed a lower Css for MTXGlu2 and 3 and higher Css for MTXGlu4 and 5 compared with the RBC PK model, while tss and overall intracellular MTX exposure appeared similar. The WBC PK model showed much lower Css for the parent MTXGlu1 and of tss for all MTXGlun , as well as a much lower cumulative Css for MTXGlu2-7 for the majority of the WBC cell lines.

Conclusion: RBC kinetics of MTX differ from the kinetics in other cell types such as WBCs and HBCCs to a variable degree. It is possible that similarly diverse profiles may exist across other cell lines, including those on the causal path in rheumatoid arthritis. Hence, there may not necessarily be a clear link between RBC MTX concentrations and disease control in rheumatoid arthritis.

Keywords: cell lines; intracellular kinetics; methotrexate; polyglutamates; rheumatoid arthritis; simulations.

Figure 1

Structure ofthe parent-metabolite pharmacokinetic model for MTXGlu_n in RBCs (adapted from [6]). k_a,oral and k_a,s.c. = absorption rate constants after oral and subcutaneous administration, respectively; F_oral and F_s.c. = bioavailability after oral and subcutaneous administration, respectively; V₁ = apparent volume of distribution of MTXGlu₁ in the plasma compartment, CL₁ = clearance of MTXGlu₁ from the plasma compartment; k₁₂ and k₂₁ = intercompartmental transfer rate constants; k_in = rate constant of MTXGlu₁ uptake into RBCs; V_RBCs = apparent volume of distribution of MTXGlu_n in RBCs; rate constants; rate constants; CL_RBCs = clearance of MTXGlu_n from RBCs

Figure 2

Structure of the WBC PK model published by Panetta et al. , adapted to include the dosing seen in the RBC MTX PK study . Parameters in the plasma PK model are defined as in Figure 1. V_max = maximum velocity and K_m = Michaelis constant of the active uptake transport of MTXGlu₁, k_p = rate constant of the passive uptake component (fixed by Panetta et al. to 0.4 h⁻¹), k_eff = rate constant of MTXGlu₁ efflux, = maximum velocity and = of the enzymatic polyglutamation, k_γGH = deglutamation rate constant, V_intracell = apparent intracellular volume of distribution of MTXGlu_n