Ketoconazole-associated preferential increase in dopamine D2 receptor occupancy in striatum compared to pituitary in vivo: role for drug transporters?

Membrane transporters such as P-glycoprotein (P-gp) and breast cancer resistance protein (BCRP) are efflux pumps that remove drugs from the brain back to the peripheral blood compartment, serving as a functional component of the blood-brain barrier (BBB). We report here that coadministration of the P-gp and BCRP inhibitor ketoconazole with risperidone may preferentially increase D2 receptor occupancy in the striatum compared to pituitary. Four male patients with schizophrenia or schizoaffective disorder who had received at least 4 prior injections of the long-acting risperidone at a stable dose of 25 to 50 mg participated in this positron emission tomography study. Multiple-dose ketoconazole coadministration reduced the P-gp activity as shown by fexofenadine oral challenge. Importantly, we found a strong statistical trend in this sample of 4 subjects who consistently showed a decrease in striatal fluorine 18 (F)-fallypride binding (an indication of increased D2 receptor occupancy) after ketoconazole coadministration (P = 0.057), whereas the pituitary (a region that lies outside the BBB) F-fallypride binding did not change (P = 0.99). These observations warrant further research with selective drug transporter inhibitors. We suggest that in neuroimaging studies, the pituitary drug occupancy can serve as a useful new "positive control" to evaluate whether drug occupancy is preferentially increased in brain regions that fall inside the BBB after cotreatment with P-gp and BCRP inhibitors. This is a noteworthy study design consideration regarding the future clinical testing of novel adjunct interventions aimed at modulating membrane transporter function at the BBB, with the goal of augmenting drug access into the brain compartment, particularly in treatment-resistant psychiatric illness.

Asymmetry in scientific method and limits to cross-disciplinary dialogue: toward a shared language and science policy in pharmacogenomics and human disease genetics.

Pharmacogenomics is a hybrid field of experimental science at the intersection of human disease genetics and clinical pharmacology sharing applications of the new genomic technologies. But this hybrid field is not yet stable or fully integrated, nor is science policy in pharmacogenomics fully equipped to resolve the challenges of this emerging hybrid field. The disciplines of human disease genetics and clinical pharmacology contain significant differences in their scientific practices. Whereas clinical pharmacology originates as an experimental science, human disease genetics is primarily observational in nature. The result is a significant asymmetry in scientific method that can differentially impact the degree to which gene-environment interactions are discerned and, by extension, the study sample size required in each discipline. Because the number of subjects enrolled in observational genetic studies of diseases is characteristically viewed as an important criterion of scientific validity and reliability, failure to recognize discipline-specific requirements for sample size may lead to inappropriate dismissal or silencing of meritorious, although smaller-scale, craft-based pharmacogenomic investigations using an experimental study design. Importantly, the recognition that pharmacogenomics is an experimental science creates an avenue for systematic policy response to the ethical imperative to prospectively pursue genetically customized therapies before regulatory approval of pharmaceuticals. To this end, we discuss the critical role of interdisciplinary engagement between medical sciences, policy, and social science. We emphasize the need for development of shared standards across scientific, methodologic, and socioethical epistemologic divides in the hybrid field of pharmacogenomics to best serve the interests of public health.