A diffusion cell adapted to nuclear imaging instruments for the measurement of molecular release and pharmacokinetics across membranes

Abstract

Diffusion cells are routinely used in pharmacology to measure the permeation of pharmaceutical compounds and contaminants across membranes (biological or synthetic). They can also be used to study drug release from excipients. The device is made of a donor (DC) and an acceptor (AC) compartment, separated by a membrane. Usually, permeation of molecules across membranes is measured by sampling from the AC at different time points. However, this process disturbs the equilibrium of the cell. Furthermore, analytical techniques used in association with diffusion cells sometimes lack either accuracy, sensitivity, or both. This work reports on the development of nuclear imaging - compatible diffusion cells. The cell is made of a polymer transparent to high-energy photons typically detected in positron emission tomography (PET). It was tested in a finite-dose set-up experiment with a pre-clinical PET system. Porous cellulose membranes (3.5, 25 and 300 kDa), a common excipient in pharmacology, as well as for dialysis membranes, were used as test membranes. The radioisotope ⁸⁹Zr chelated with deferoxamine B (DFO; 0.65 kDa), was used as an imaging probe (7-10 MBq; 0.2-0.3 nMol ⁸⁹Zr-DFO). In medicine, DFO is also commonly used for iron removal treatments and pharmacological formulations often require the association of this molecule with cellulose. Permeation profiles were obtained by measuring the radioactivity in the DC and AC for up to 2 weeks. The kinetic profiles were used to extract lag time, influx, and diffusion coefficients of DFO across porous cellulose membranes. A sensitivity threshold of 0.005 MBq, or 3.4 fmol of ⁸⁹Zr-DFO, was revealed. The lag time to permeation (τ) measured in the AC compartment, was found to be 1.33, 0.5, and 0.19 h with 3.5, 25, and 300 kDa membranes, respectively. Diffusion coefficients of 3.65 × 10^-6, 8.33 × 10^-6, and 4.74 × 10^-5 cm² h^-1 where revealed, with maximal pseudo steady-state influx values (J_pss) of 6.55 × 10^-6, 1.76 × 10^-5, and 1.29 × 10^-5 nmol cm^-2 h^-1. This study confirms the potential of the technology for monitoring molecular diffusion and release processes at low concentrations, high sensitivities, in real time and in a visual manner.

Keywords: Diffusion cell; Diffusion coefficient; Diffusion measurements; Nuclear imaging; Permeation measurements; Polymer membranes; Positron emission tomography; Radioisotopes.