Membrane arrays were fabricated by capturing biMEM at concentrations in the range of 0.02–0.2 mg/mL on SpotReady™ chips. The arrays were then exposed to six drugs (Figure 2). The drugs were chosen because they are commercially available; they are highly soluble in aqueous solvent; their absorption to liposomes had previously been evaluated by SPR; and they represent different categories of drugs with respect to charge and lipophilicity.
The chip was assembled into the SPRimager II and an image of the array was obtained—both to evaluate probe density on each spot, and to serve as a reference image for calculating reflectivity changes as binding occurs.
The array was then exposed sequentially to the six different drug solutions (1 mM in HBS). Drug absorption was considered complete when a plateau in SPR signal was reached. The drug was then washed from the membrane with HBS, and the next drug was introduced. As drug absorption to the membranes was reversible, several drugs could be tested in series on the same array.
A typical drug absorption profile plotted as absolute change in reflectivity (D%R) with time is presented in Figure 2. For five of the six drugs, the binding plateaus were rapidly achieved in the presence of drug followed by rapid release when the array was washed with buffer. The sixth drug, verapamil, did not reach equilibrium during the five minute exposure used.Verapamil also did not completely dissociate after buffer wash, a behavior consistent with previous reports.
As expected, the amount of drug absorbed was higher where the amount of immobilized biMEM was greater. The absorption profiles (Figure 2) were used to calculate D%RDRUG, the normalized signal due to drug binding on the biMEM.
D%RDRUG =D%RbiMEM - D%RSA
D%RSA is the reflectivity change for the streptavidin control (i.e., no membrane) spots.