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The application of raster image correlation spectroscopy (RICS) as a tool for the characterisation of protein diffusion was assessed using a model protein, bovine serum albumin (BSA), as a function of formulation and denaturing conditions. RICS results were also validated against dynamic light scattering and fluorescence correlation spectroscopy. Results from this study demonstrate correlation between outputs obtained from the three experimental techniques. Ionic strength independency was observed at pH 7, and a reduction in the corresponding diffusion coefficients was noted at pH 4.5 for 1 µM BSA-Alexa Fluor 488. Conversely, at pH 5.2, higher-concentration samples exhibited ionic strength dependency. Buffer composition, sample pretreatment, thermal denaturation and freeze-thaw cycling were also found to influence RICS output, with a reduction in the diffusion coefficient and the number of particles observed for both pH values. In conclusion, RICS analysis of images acquired using a commercial confocal microscope offers a potential scope for application to both quantitative and qualitative characterisation of macromolecular behaviour in solution.

P-glycoprotein is implicated in clinical drug resistance; thus, rapid quantitative analysis of its expression and activity is of paramout importance to the design and success of novel therapeutics. The scope for the application of quantitative imaging and image analysis tools in this field is reported here at "proof of concept" level. P-glycoprotein expression was utilized as a model for quantitative immunofluorescence and subsequent spatial intensity distribution analysis (SpIDA). Following expression studies, p-glycoprotein inhibition as a function of verapamil concentration was assessed in two cell lines using live cell imaging of intracellular Calcein retention and a routine monolayer fluorescence assay. Intercellular and sub-cellular distributions in the expression of the p-glycoprotein transporter between parent and MDR1-transfected Madin-Derby Canine Kidney cell lines were examined. We have demonstrated that quantitative imaging can provide dose-response parameters while permitting direct microscopic analysis of intracellular fluorophore distributions in live and fixed samples. Analysis with SpIDA offers the ability to detect heterogeniety in the distribution of labeled species, and in conjunction with live cell imaging and immunofluorescence staining may be applied to the determination of pharmacological parameters or analysis of biopsies providing a rapid prognostic tool.