The antipsychotic effect of neuroleptic drugs is well established.1 The most widely accepted hypothesis of antipsychotic drug action states that the effect of neuroleptic drugs is related to their ability to antagonize the action of the neurotransmitter dopamine.2 Dopamine binds to two types of receptors in the central nervous system, the Dl- and the D2dopamine receptors.3 The development of radioligand binding techniques and the availability of 3Hlabeled neuroleptics has enabled a more direct study of neuroleptic binding sites in vitro in brain homogenates and brain slices. A linear correlation has been demonstrated between drug affinity for the D2dopamine receptor subtype in animal brains in vitro and clinical antipsychotic potency of the drugs in man.4 No such correlation has been found for the Dl-dopamine receptor or any other receptor system so far studied.5
Previously it was not possible to test the dopamine hypothesis by a direct examination of dopamine receptor binding in patients treated with neuroleptic drugs. The development of positron emission tomography (PET) has made it feasible to study the binding of a radiolabeled ligand to receptors in the living human brain.6,7 Despite the fact that thousands of receptor binding compounds are synthesized every year, only a very few are suitable as radioligands for PET studies. Nevertheless, several such ligands have been developed for examination of dopamine receptor binding with PET.6,7 In addition, the receptor binding of unlabeled drugs can be studied by examining the way these drugs interfere with the receptor binding of well-characterized radiolabeled compounds. The dopamine receptor blockade,therefore, can be studied in vivo for all neuroleptic drugs.
This article focuses on the following three issues:
* the identification of receptors in the human brain that mediate the antipsychotic effect of neuroleptics,
* the relationship between neuroleptic drug concentration in plasma (or the brain) and the degree of central dopamine receptor binding, and
* the relationship between the degree of dopamine receptor binding and antipsychotic effect.
PET camera systems consist of detectors arranged in one or several rings. After intravenous injection of Iigands labeled with positron-emitting isotopes the system measures the uptake of radioactivity in tissue as a function of time. Reconstructed images show the distribution of radioactivity in horizontal brain sections. In the studies reviewed, systems with a resolution of 7 mm to 12 mm have been used.8 However, technical improvements mean that with recently developed commercially available PET systems, the spatial resolution is about 4 mm.
For the study of receptor binding, the most commonly used isotopes are 1 IC (which has a half-life of 20 minutes), 18F (half-life, 110 minutes) and 76Br (half-life, 16.2 hours). A PET experiment with HC-labeled ligand usually lasts up to 90 minutes; experiments with 18F or 76Br may last for several hours. A head-fixation device is used to keep positioning of the head constant during a PET experiment. To standardize and optimize the positioning of the brain, several PET groups use the head-fixation device in a computed tomography (CT) examination prior to the PET examination.
The first radioligand used to visualize receptor binding in the living human brain was 11C-N-methylspiperone.6 This ligand has a high affinity for D2-dopamine receptors but also a considerable affinity for 5HT2-receptors. Spiperone and several of its derivatives have also been labeled with HC, 18F, or 76Br for PET studies. The regional radioactivity curves obtained from a PET experiment with radioligands must be interpreted in terms of quantitative models. For spiperone and its derivatives, a kinetic analysis with a three-compartment model has provided valuable information.9,10
Considerable advances followed these studies. One of the most important involves raclopride; a selective D2-dopamine receptor antagonist with negligible affinity for other central receptors.11 Raclopride has been labeled with 1 IC and used to develop a procedure for the quantitative determination of D2-dopamine receptor density (Bmax) and affinity (Kd) in the caudate-putamen of the living human brain.12 The saturation procedure is based on the occurrence of equilibrium for HC-raclopride binding to D2-dopamine receptors. For HC-raclopride, equilibrium occurs within the time-span of a PET experiment.
Several selective D2-dopamine receptor antagonists have been available for many years but research on the biochemical characteristics and functional role of the Dl -dopamine receptor had been hampered by the lack of such selective ligands. Recently, the first selective Dl-dopamine receptor antagonist, SCH23390, was discovered.13 Now this has been labeled with HC; 11C-SCH23390 has been used to visualize Dl-dopamine receptors in the human brain with PET.14
DOPAMINE RECEPTOR OCCUPANCY
Drug-induced receptor occupancy is the percentage of a receptor population to which a drug binds at a given point in time. The general strategy for calculating neurolepticinduced dopamine receptor occupancy is to estimate the expected uptake of radioactivity in the caudate-putamen when the patient is not treated with neuroleptics. By comparison with the measured radioligand uptake during treatment, the receptor occupancy is then calculated as the percent reduction in specific radioligand binding.15
* Identification of a receptor population in the human brain by which the antipsychotic effect of neuroleptics is mediated. Several groups have examined the uptake of labeled Iigands in the caudate-putamen during treatment with clinical doses of antipsychotic drugs.12,15-17 Using HC-raclopride, Farde et al15 determined D2-dopamine receptor occupancy in patients with schizophrenia during treatment with conventional doses of 10 chemically distinct classical neuroleptic drugs. The drugs represent all the major chemical classes of antipsychotics used today. In each of the patients there was a marked reduction of radioactivity in the basal ganglia when compared to the values obtained in drug-free patients with schizophrenia (Figure 1). The Table shows the calculated D2-dopamine receptor occupancy for some neuroleptic-treated patients. For all the classical neuroleptics, the occupancy varied between 65% and 89%. No occupancy was found in a patient treated with the antidepressant nortriptyline. The finding that clinical doses of antipsychotic drugs induce a marked occupancy of the central D2-dopamine receptors represents strong support in living patients for the idea that the mechanism of action of antipsychotic drugs is indeed related to a substantial degree of D2-dopamine receptor blockade.
Figure 1: PET images through the caudate/putamen level of a patient with schizophrenia before (left) and during (right) treatment with haloperidol 3 mg twice daily.
D2-Dopamine Receptor Occupancy in Patients Treated with Psychoactive Drugs
Antipsychotic drugs have been subdivided into classical and atypical neuroleptics.18 Atypical refers to drugs that do not produce catalepsia in animals and that have low frequency of extrapyramidal side effects in man. Clozapine is the prototype for a neuroleptic with an atypical profile.18 In a recent clinical study, clozapine was efficacious in 30% of patients who had not responded to treatment with classical neuroleptics.19 It has been suggested in animal experiments that clozapine has a weak activity on the D2-dopamine receptor whereas it has a considerable activity on the Dl-dopamine receptor.20 Interestingly, patients treated with clinical doses of clozapine had lower D2-dopamine receptor occupancy than the patients treated with classical neuroleptics (Table).
Cambon et al16 studied D2-dopamine receptor occupancy in 10 psychiatric patients treated with each of, or two of, five classical neuroleptics. In patients treated with conventional antipsychotic doses there was more than 60% occupancy of the D2dopamine receptors. Smith et al17 used 18F-N-methylspiperone and PET to examine chronic patients with schizophrenia during treatment with various doses of haloperidol and found a high D2-dopamine receptor occupancy during treatment with clinical doses.
11C-SCH23390 has been used in a preliminary study of Dl-dopamine receptor occupancy in the putamen during antipsychotic drug treatment.14 No occupancy was found in a patient treated with sulpiride (400 mg twice daily), a selective D2-dopamine receptor antagonist, whereas treatment with thioridazine (150 mg twice daily) induced a 29% occupancy and cis-flupenthixol (6 mg twice daily) a 35% occupancy. The highest values (40% and 42%) were found in two patients treated with clozapine. Those patients had the lowest D2-occupancy so far determined (Table). These results confirm the findings in vitro that some neuroleptics are selective for the D2-dopamine receptor; the atypical profile of clozapine may be related to a combined blockade of Dl- and D2-dopamine receptors.
* The quantitative relationship between neuroleptic drug concentration in plasma (or the brain) and the degree of central dopamine receptor binding. In a study designed to analyze the relationship between serum drug concentration and central D2dopamine receptor occupancy after treatment withdrawal, two schizophrenic patients were recruited.15 One of them had been treated with sulpiride, 600 mg twice daily for several weeks. PET experiments were made 3, 6, and 27 hours after the last dose was given. The other patient had been treated with haloperidol, 6 mg twice daily for several weeks and PET experiments were made 6, 30, and 53 hours after the last dose. In both patients repeated blood samples were taken to follow serum drug concentrations after drug withdrawal.
In the patient treated with sulpiride, D2-dopamine receptor occupancy remained above 65% for 27 hours in spite of a several-fold reduction in the serum concentration. In the patient withdrawn from haloperidol there was only a few percent reduction in the D2-dopamine receptor occupancy despite a several-fold reduction of the haloperidol serum concentration (Figure 2).
Two hypotheses can be formulated to explain the discrepancy between the time courses of receptor occupancy in brain and drug concentration in plasma:
* the drug bound to the receptor dissociates with a slower rate than the rate by which the concentration of free drug in brain decreases, and
* Receptor binding follows the law of mass action and there is a curvilinear relationship between specific binding and free drug concentration.
A curvilinear relationship is illustrated by the hypothetical hyperbola for drug binding to receptors shown in Figure 3.
At a high degree of central D2-dopamine receptor occupancy, as during antipsychotic drug treatment, the horizontal part of the binding hyperbola is approached. At this part of the hyperbola a major change in free drug concentration is only reflected in a minor change in receptor occupancy.
D2-Dopamine Receptor Occupancy in the Putamen and Haloperidol Concentration in Serum of a Schizophrenic Man After Withdrawal of Haloperidol Treatment
To examine the two hypotheses mentioned above, a patient treated with sulpiride was selected15 and the dose was reduced successively in four steps (800 mg, 600 mg, 400 mg, and 0 mg twice daily). A PET experiment was made on each dose level. To ensure steady-state conditions at least one week elapsed between a dose reduction and the day of the next PET experiment, thus providing enough time for the receptor drug complex to dissociate and adjust to the new steady-state condition. The reduction in D2-dopamine receptor occupancy in relation to dose followed a curve with a hyperbolic (curvilinear) shape (Figure 4) while the reduction of the serum concentration of sulpiride had a linear shape. Two weeks after the complete withdrawal of treatment, D2-dopamine receptor occupancy was reduced to zero, indicating the complete dissociation of drug binding D2-dopamine receptors. These findings support the second hypothesis. At a high degree of central D2-dopamine receptor occupancy, as during antipsychotic drug treatment, the horizontal part of the binding hyperbola is approached where a major change in free drug concentration is reflected only in a minor change in receptor occupancy. This explanation, according to the second hypothesis, is sufficient to explain the demonstrated discrepancy between the time courses for receptor occupancy and serum drug concentration (Figure 2).
Cambon et al16 examined 10 patients treated with a wide range of neuroleptic doses. They plotted dose versus D2-dopamine receptor occupancy and again found a curvilinear relationship. Also, Smith et al17 found a curvilinear relationship when plotting D2-dopamine receptor occupancy versus plasma concentration of haloperidol for 26 patients. Thus, theoretically expected curvilinear relationship has been confirmed by several groups.
Curvilinear Relationship Between Free Ligand Concentration in Brain or Serum and Receptor Occupancy*
* The quantitative relationship between degree of dopamine receptor binding and antipsychotic effect. For several drug effects, such as the antipsychotic effect, there are no generally accepted animal models. Thus it is not possible to design animal experiments to examine a quantitative relationship between degree of binding and effect. PET provides the potential for relating the clinical antipsychotic effect to receptor binding in the same patient. The detailed relationship between the degree of D2-dopamine receptor occupancy and antipsychotic effect has not been studied. However, it is oí interest that in all patients treated with clinical doses of classical neuroleptics there was a high D2-dopamine receptor occupancy (Table). By relating D2-dopamine receptor occupancy to antipsychotic effect it may be possible to define a "threshold occupancy" for antipsychotic effect. In this way a new measure could be obtained to guide the selection of optimal doses for antipsychotic drug treatment.
In our studies those patients who developed extrapyramidal side effects all had comparatively high D2-dopamine receptor occupancy. Such data can be regarded as consistent with the idea that there is a higher "threshold occupancy" for extra pyramidal side effects than for the antipsychotic effect.
IMPLICATIONS FOR NEUROLEPTIC DRUG TREATMENT
The finding that clinical doses of classical antipsychotic drugs induce a 65% to 89% occupancy of the central D2-dopamine receptors strongly suggests that the mechanism of action of antipsychotic drugs is related to a substantial degree of D2dopamine receptor blockade (Table). Furthermore, if the D2dopamine receptor is the common target for antipsychotic drug action, it is not likely that concomitant treatment with several neuroleptics would be more efficacious than single therapy.
The high D2-dopamine receptor occupancy that was found in drug treated schizophrenics (Table) indicates that the horizontal part of the hyperbola is approached during antipsychotic drug treatment (Figure 4). At this part of the hyperbola a change in the free drug concentration will be reflected only in a minor change in D2-dopamine receptor occupancy. In several studies, similar antipsychotic effects have been reported on widely different doses and concentration levels of neuroleptic drugs in schizophrenia patients.21"24 This marked variation of doses and concentrations producing a similar effect may be expected if the clinical doses are high enough to give a receptor occupancy that approaches the horizontal part of the hyperbola. An increased or reduced dose at this part of the hyperbola will cause a small alteration of receptor occupancy at the already nearly saturated D2-dopamine receptors. It is questionable if the administration of very high neuroleptic doses, so called megadoses,23 will cause an increase in D2-dopamine receptor occupancy that will be reflected in a more pronounced antipsychotic effect.
D2-Dopamine Receptor Occupancy in the Putamen and Drug Concentration in Serum of a Sulpiride-Treated Patient*
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D2-Dopamine Receptor Occupancy in Patients Treated with Psychoactive Drugs