Many physiological and biochemical measurements can be performed noninvasiv
ely in humans with modern imaging techniques like magnetic resonance imagin
g (MRI), positron emission tomography (PET) or single-photon emission compu
ted tomography (SPECT). This review focuses on the monitoring of drug-recep
tor interactions in patients and healthy volunteers with PET. Such studies
depend on the availability of a suitable radioligand; they are already poss
ible for classical and atypical neuroleptics, anxiolytics, antidepressants,
anticholinergics, antihistamines, antiepileptics, beta-blockers and hypnot
ic drugs. In Phase I-II human studies, measurements of plasma pharmacokinet
ics can be combined with images of receptor occupancy and be quantitatively
related to pharmacologic effects which are induced in the same subjects. O
ptimal dosing schedules can be defined and valuable information for the des
ign of Phase III studies can be acquired. Moreover, the effect of intervent
ions (e.g. change of dose, additional medication) can be predicted. Medical
imaging techniques will play an increasing role in clinical pharmacology a
nd allow well-informed go/no-go decisions in future drug development.