Assessing neuroreceptor occupancy by continuous infusion of carbon-11 labeled radioligands

In carbon-11 labeled radiotracer studies of neuroreceptors in the brain, a state of practical equilibrium, monitored with a simple dual-probe detector system, was achieved by injection of a bolus of tracer followed by a continuous infusion of the tracer. After investigating several different bolus-to-infusion activity ratios, we observed that a practical equilibrium state could be achieved with a slope of 0.02 ± 0.001 cps x kg/μCi/min for [¹¹C]carfentanil studies in normal human beings. The usefulness of this approach to assess neuroreceptor occupancy was demonstrated in two cases.