SYNAPSE 11:lO-19 (1992)

Regional Distribution and Kinetics of Haloperidol Binding in _ _ Human Brain: A PET

Study With [18F]Haloperidol DAVID J. SCHLYER, NORA D. VOLKOW, JOANNA S. FOWLER, ALFRED P. WOLF,

ROBERT RAULLI, ROBERT HITZEMANN, JONATHAN BRODIE, ABASS A. ALAVI, AND ROBERT R. MAcGREGOR

Departments of Chemistry and Medicine, Brookhaven National Laboratory, Upton. New York 11973 (D.J.S., N.D.V., J.S.F., A.P.W., C.-Y.S., S.L.D., B.B., J.L., R.R., R.R.M.); State University of New York at Stony Brook, Stony Brook, New York 11 794, and Psychiatry Service, Veterans Administration Medical Center,

Northport, New York 11768 (R.H.); New York University Medical Center, New York, New York 10016 (J.B.); University of Pennsylvania, Philadelphia, Pennsylvania 19104 (A.A.A.)

CHYNG-YANN SHIUE, STEPHEN L. DEWEY, BERNARD BENDRIEM, JEAN LOGAN,

KEY WORDS Dopamine receptors, Neuroleptics, Sigma receptors, Schizophrenia

ABSTRACT The regional distribution and the kinetics of haloperidol uptake in hu- man brain were examined using [18Flhaloperidol and PET in 9 controls and 5 schizo- phrenics while on haloperidol medication and after haloperidol washout. The regional distribution of [lsFIN-methylspiroperidol, a tracer for D, receptors, was measured in 1 normal subject for comparison. The uptake of [lsFlhaloperidol in the whole brain in normals was high (6.6% of the injected dose at 2 hr), and regional distribution was much more extensive than could be accounted for by the distribution of dopamine D, receptors. In normals, the cerebellum, basal ganglia, and thalamus showed a greater concentration than the cortex, and there was minimal clearance of "F from the brain during the 10-hr period of the study. Medicated schizophrenics showed a total brain uptake of 4.0% and had a significant clearance of [lsF]haloperidol from brain and a higher concentration of [18F]haloperidol in plasma. After withdrawal from medication, [18Flhaloperidol clearance from brain became slower than while on medication. These results are discussed in terms of the pharmacokinetics of haloperidol in the human brain and its binding to dopamine D, receptors and to sigma receptors. o 1992 Wiley-Liss, Inc.

INTRODUCTION Haloperidol is one of the most frequently used anti-

psychotic drugs in the United States and its use has been increasing dramatically over the past 10 years (Wysowski, 1989). Although the mechanism of action and antipsychotic efficacy of haloperidol is generally attributed to the blockade of dopamine D, receptors (Carlsson, 1978; Creese, 1976; Seeman, 19871, its high affinity for sigma receptors and the known psychotomi- metic and motor effects of sigma agonists have lead to the suggestion that blockade of the sigma receptor may also account for some of the therapeutic actions and side effects of antipsychotic drugs (Snyder, 1989; Sonders, 1988; Tam, 1984). For example, the adminis- tration of sigma agonists has been shown to produce hallucinations (Keats, 19641, delirium in the chronic, nondependent spinal dog (Martin, 19761, and abnormal motor movements in animals (Goldstein, 1988; Walker, 1988).

Haloperidol's receptor binding profile is complex. For example, it binds to D, receptors (K,, = 2.7 nM, See- PUBLISHED 1992 WILEY-LISS, INC.

man, 19751, to sigma receptors (KD = 0.95 nM, Weiss- man, 19881, to alpha 1 adrenergic receptors (K, = 6.2 nM, Black, 19871, and 5HT2 receptors (Ki = 22 nM, Leysen, 1982) with moderate to high affinity. Although haloperidol lacks specificity for any single receptor sys- tem, it is one of the most potent sigma ligands yet re- ported and has been suggested as a radioligand for mapping sigma receptor populations in vivo (Weiss- man, 1988,1990).

The present study was carried out to examine the behavior of this widely used antipsychotic drug in the living human brain with PET, with a specific view to- ward measuring its regional distribution and time course of binding, both at a tracer dose in normal sub- jects and in schizophrenic subjects receiving chronic haloperidol medication. The regional distribution of

Received April 1,1991; accepted in revised form September 10,1991

Presented in part at the 35th Annual Meeting of the Society of Nuclear Medi- cine, San Francisco, California, June 14, 1988.

HALOPERIDOL BINDING IN HUMAN BRAIN 11

TABLE I. Demographics of control and schizophrenic populations

Duration Subject no. Age Diagnosis of illness Medication

Controls 1 2 3 4 5 6 I 8 9

41 Not applicable 23 42 30 24 45 23 22 33

Not applicable , None

Schizophrenics on medication 10 28 Chronic undifferentiated 6 yr Haloperidol (15 mg/day)

11 37 Chronic paranoid 15 yr Haloperidol (15 mg/day)

12 33 Chronic undifferentiated 12 yr Haloperidol (10 mg/day)

13 37 Chronic paranoid 16 yr Haloperidol (10 mg/day)

14 24 Schizophreniform 4 mo Haloperidol (15 mg/day)

schizophrenia Benztropine mesylate (3 mg/day)

schizophrenia

schizophrenia Benztropine mesylate (6 mg/day)

schizophrenia

[18F]N-methylspiroperidol was measured in a normal subject for comparison. These human PET studies of [ "F]haloperidol binding include a measurement of plasma [lsF]haloperidol levels and an examination of the metabolism of [18Flhaloperidol in mouse brain to provide information on the short-term brain metabo- lism of the drug.

SUBJECTS AND METHODS Subject selection

Nine normal male subjects and 5 patients with a DSM-IITR diagnosis of chronic schizophrenia or schizo- phreniform disorder were studied with [lsF]haloperidol (see Table I for demographics). One additional normal subject (male, 24 years) was studied with ["FI-N-meth- ylspiroperidol as described previously (Amett, 1986). Informed consent was obtained from all subjects. Other details are included below. Table I gives the clinical and demographic characteristics of the normal controls and of the patients.

Design of PET studies

Normal controls The regional distribution and kinetics of [laF]halo-

peridol in the normal brain were measured in 9 healthy controls, 6 of which were studied with the PETT VI camera (FWHM 1.2 cm x 1.2 x 1.5 cm) for a total of 3-4 hr (subjects 1-6). The other 3 normal subjects (sub- jects 7-9) were studied with the CTI tomograph (FWHM 0.6 x 0.6 x 0.65 cm). One of the studies using the CTI tomograph was 4 hr (subject 7) , another was 10 hr (subject 8), and one was 15 hr (subject 9) in duration. The [18F1N-methylspiroperidol study was carried out for 4 hr on the CTI tomograph. Table I1 contains infor- mation on the design of the PET studies.

Schizophrenics The regional distribution and kinetics of ["Flhalo-

peridol were measured in 5 male patients who met DSM IIIR criteria for schizophrenia or schizophreni- form disorder (see Table I). All of the patients were on haloperidol medication and had been on the same dose of medication for at least 2 weeks prior to the study. Three of the patients (subjects 10,12, and 14) were also on benztropine mesylate medication. For each patient, the first PET study was carried out 2 hr after the last dose of haloperidol. Information on the instrument, study duration, medication, and washout period are summarized in Table 11.

Normal subjects and schizophrenic patients on halo- peridol medication were studied using the following procedure. An arterial catheter was inserted into the radial artery for blood sampling, and a venous catheter was inserted into the antecubital vein for injection of the radiotracer. Alignment in the tomograph (PETT VI or CTI 931) was effected through the use of a custom- made head holder. ['8Flhaloperidol (4.5-9.0 mCi; spe- cific activity, 1 CYFM at the time of injection; injected dose, 1-2 Fg) was injected, and serial blood samples were taken from the arterial catheter from the time of injection through the end of the study. Using an auto- matic blood sampling instrument (Ole Dich, Denmark), 0.3 mL samples were taken every 2.5 sec for 2 min and then larger samples were taken at less frequent inter- vals for a total of 4 hr. The total amount of blood taken averaged 40 mL. Samples were centrifuged for 45 sec at 10,OOOg to obtain plasma, and the concentration of ra- dioactivity in these plasma samples was measured. Se- lected plasma samples withdrawn at 40 sec and at 4,10, 30, 60, 120, and 180 min after injection were retained for the determination of the amount of unchanged

12 D.J. SCHLYER ET AL.

TABLE II. Experimental parameters of control and schizophrenic populations

Scan Plasma’ % Dose2 Uptake3 Norm.4 Subject Tomo. length int w. brain global uptake

Controls 1 PETT V1 2 PETT VI 3 PETT V1 4 PETT VI 5 PETT V1 6 PETT VI 7 CTI 931 8 CTI 931 9 CTI 931

Average S.D.

10 PE?T VI 11 PETT VI 12 PETT VI 13 CTI 931 14 CTI 931 Average S.D.

Schizophrenics on medication

Washout studies (washout period) 10 (1 day) PETT VI 11 (2 days) PETT VI 12 (2 days) PETT VI 13 (2 days) CTI 931 135 (9 weeks) CTI 931 Average6 Iins

4 hr 4 hr 4 hr 4 hr 3 hr 4 hr 4 hr

10 hr 15 hr

4 hr 4 hr 4 hr

10 hr 10 hr

4 hr 4hr 4 hr

10 hr 10 hr

158.9 193.0 190.4 237.4 125.7 250.7 211.7 209.5 159.8 193.0 39.9

321.5 311.4 265.2 543.7 281.7 344.7 113.5

259.1 306.7 218.0 478.4 280.8

7.3 7.5 7.8 6.0 4.7 6.1 5.5 6.7 7.8 6.6 1.1

3.8 3.9 3.4 5.1 3.9 4.0 0.6

4.2 3.9 3.8 5.4 5.0

48.6 50.1 52.3 39.9 31.6 40.7 45.8 54.8 52.2 46.2 7.5

25.7 26.1 22.6 54.1 31.2 31.9 12.8

28.3 26.1 25.5 63.7 46.7

0.306 0.260 0.275 0.168 0.251 0.162 0.216 0.262 0.327 0.247 0.056

0.080 0.084 0.085 0.099 0.111 0.092 0.013

0.109 0.085 0.117 0.133 0.166

315.6 4.3 35.9 0.111 -.-. 114.5 0.7 18.6 0.020

‘Plasma integral a t 120min corrected for the percentage unchanged tracer in the plasma and normalized to a unit dose. The units are nCi-min/mCi-cc. ‘Average computed as described in text a t 120 min after injection. “Activity in region a t 120 min postinjection normalized to a unit dose. Units of nCi/cc-mCi. 4Global uptake normalized by the plasma integral. “Subject on haloperidol decanoate. “These averages do not include the 9 week washout subject.

tracer using a modification of the (2-18 sep pak extrac- tion method reported previously (Amett, 1985a, 1985b). PET measurements of regional radioactivity concentration were made at 4,12, 18, 59, 70, 119,130, 160, 181, and 220 min postinjection. In addition, stud- ies at times greater than 10 hr were performed on 2 normals and 2 schizophrenics in order to evaluate the washout of the haloperidol in this time period (see Table I1 for details of the scan duration). In another normal subject, [18F]N-methylspiroperidol (5.6 mCi; 1.6 CIIkM at the time of injection; injected dose, 1.4 kg) was injected, and PET measurements of regional radioac- tivity concentration were made periodically from the time of injection until 160 min postinjection.

Data analysis For [18Flhaloperidol studies, global values and re-

gions of interest were selected from the image plane in which the thalamus and the basal ganglia can be visu- alized most prominently (corresponding to 4.0-5.4 cm above the cantho-meatal (CM) plane). The image plane located 1.2 cm above the CM plane was used for the cerebellar areas. The percentage of haloperidol in the brain was calculated in the PETT VI studies by multi- plying the global percentage injected dose per cubic centimeter (cc) in the plane containing the basal gan-

glia by 1500 cc for the volume of the average brain. In the CTI studies, the percentage in the brain was calcu- lated by summing the global value in each plane of the tomographic image over all planes. Regions of interest were outlined directly onto the PET images using a neuroanatomical atlas (DeArmond, 1976). Regions were obtained €or the frontal, temporal, and occipital cortices, for the left and right basal ganglia, for the thalamus, and for the cerebellum. “Relative” values for [lsF]haloperidol uptake for the different brain regions were obtained by dividing the average concentration of 18F in a given region by the average concentration of l8F in the 4.0 and 5.4 CM slices. Differences in the relative values between normal subjects and schizophrenics were tested using Students t-test.

To control for differences in [lsFlhaloperidol plasma concentration among different subjects, the arterial plasma input function was measured from the total radioactivity in the plasma corrected for unchanged tracer using a 2-component exponential to fit the mea- surements of the percent of unchanged tracer. The inte- gral of the plasma input function was obtained by using a trapezoidal approximation of the area under the me- tabolite corrected plasma curve between each set of time points. Tissue uptake was normalized for the amount of tracer in the plasma by dividing the regional

HALOPERIDOL BINDING IN HUMAN BRAIN 13

uptake at a given time by the integral of plasma activity from the time of injection to that point in time (Patlak, 1981).

The half-time of brain clearance of radioactivity after [‘sFlhaloperidol in medicated schizophrenics was de- termined by fitting the clearance curve at times beyond 40 min to a single-component exponential with the lin- ear regression technique.

Radiotracer synthesis

[lsFlhaloperidol [ lsF]haloperidol was prepared following the proce-

dure for [lsF]N-methylspiroperidol (Shiue, 1986) through the hydrolysis step to obtain a pentane solu- tion of gamma-chloro-p-~18F]fluorobutyrophenone. The pentane solution was added to a mixture of 4-(p-chlo- rophenyl)-4-hydroxypiperidine (3.5 mg), potassium io- dide (7-9 mg), and DMF:THF (1:3, 0.2 ml), and the pentane was carefully removed by heating in a 140°C oil bath. Following the removal of the pentane, heating was continued for 5 min at 140°C followed by extraction (C-18 sep-pak, waste washes with water and pentane, and product elution with CH,Cl,) to produce a crude solution of [18F]haloperidol. The solvent was removed and the residue was dissolved in 1 ml of methanol, followed by 1 ml of water, and purified by HPLC (silica gel, 10 x 250 mm, CH,CN: 0.004 M (NH,),HPO, (70:30), 5 mumin, retention time: 10 min). The HPLC solvent containing the product was evaporated to dry- ness, and the residue was dissolved in sterile isotonic saline. The specific activity as determined by HPLC using a calibration curve was 2 Ci/pM at the end of cytoplasm bombardment and 1 Ci/pM at the time of injection, and the yield averaged lo%, corrected to the end of bombardment. The radiochemical purity was greater than 95%, as determined by thin-layer chroma- tography (acetonitrile:methanol, 2:l; R, = 0.21). The total synthesis time averaged 100 min.

[‘sFIN-methylspiroperidol

previously (Shiue, 1986). [18F]N-methylspiroperidol was prepared as described

Analysis of [“F]haloperidol in mouse brain In order to assess the fraction of the brain activity

that was associated with unmetabolized tracer, [18F]haloperidol (60-200 pCi, 0.01-0.04 pg) was in- jected through a tail vein into 6 mice. Two of the mice were killed by cervical dislocation at 5 min, 2 were killed at 30 min, and 2 were killed at 60 min. Brains were removed immediately and homogenized in 1 ml MeOH. The homogenate was centrifuged, separated, and counted. Unlabeled haloperidol was added to the supernatant, which was analyzed by HPLC [C-18 col- umn, flow of 0.7 ml/min, and MeOH:0.2 M NH,HCO2 (8515) solvent]. The [18F]haloperidol was taken as the

ratio of the radioactivity in the peak coeluting with the haloperi’dol standard and the total radioactivity in- jected.

RESULTS Regional distribution and total uptake

The upper images in Figure 1 are for [‘sF]haloperidol in a normal subject studied with CTI at the planes corresponding to the basal ganglia and cerebellum. The lower images in Figure 1 show the corresponding planes for the images obtained with [18F]N-methyl- spiroperidol (Arnett, 198513; [18FlNMS), a ligand with high affinity and specificity for the dopamine D, recep- tors, as a comparison. Haloperidol showed a wide- spread distribution throughout the brain, very differ- ent from the selective accumulation in basal ganglia that is observed with [‘8F]N-methylspiroperidol. In particular, notice the high uptake of [lsF]haloperidol in the cerebellum, an area devoid of dopamine receptors. Since the data obtained with the PETT VI gave the same results as those obtained with the CTI tomo- graph, we report the average values for the studies carried out on both tomographs.

The total uptake of [l’F]haloperidol in the normal brain was high and averaged 6-7% at 120 min. The uptake of haloperidol in the whole brain of schizophren- ics was lower. It averaged 4.0% during medication and is statistically different from the control group using Student’s t-test (P < .001). The average uptake in whole brain increased slightly to 4.3% at 24-48 hr after discontinuation of haloperidol therapy and to 5.0% at 9 weeks after the last haloperidol decanoate administra- tion. The average uptake in the brains of the schizo- phrenics after washout is not statistically different from the average of the same subjects on medication. These data are shown in Table 11.

Figure 2 shows the averaged data for the “relative” regional concentration of [18F]haloperidol for the nor- mal subjects and the schizophrenic patients (“relative” values are the “F concentration in a particular region normalized by the average concentration of “F in the 4.0 and 5.4 CM slices). The areas with the highest con- centration of [‘sFlhaloperidol were found in the cerebel- lum, basal ganglia, and thalamus. The regional distri- bution of [l’F]haloperidol in the brain of the medicated schizophrenics was more homogeneous than that of normal controls, with significantly lower relative val- ues in the cerebellum.

After haloperidol washout there was a variable in- crease in the “relative” regional concentrations of [18F]haloperidol in the 4 subjects who had been on halo- peridol therapy. The basal ganglia increased the most (range 15-23%), followed by thalamus (range 8-23%), followed by cerebellum (range 3 to 11%) and cortex (range 3 to 9%) (data not shown for individual regions). A paired t-test of the “relative” regional concentration data did not demonstrate that these changes were sig-

14 D.J. SCHLYER ET AL

Fig. 1. A comparison of the PET images in 2 normal subjects 160 min after the injection of ['8Flhalo- peridol (subject 7, top row) and ['8FJN-methylspiroperidol (bottom row). The images show 3 levels of the brain, including the basal ganglia, thalamus, and cerebellum. Images were taken on the CTI 931 positron tomograph with a resolution of 6.5 cm.

nificant, whereas a paired t-test of the uptake data normalized by the plasma integral did show a signifi- cant increase in the cerebellum and striatum (P < .Ol), but not in the other regions.

Kinetics The time course of binding of haloperidol in the brain

differed between normals and schizophrenics receiving haloperidol medication. In normal controls, the maxi- mal uptake of haloperidol occurred at 120 min, and this value changed little over a 10-hr period. In the case of the schizophrenics, maximal uptake occurred at about 40 min after injection, with clearance beginning 60 min after injection. Figure 3 provides an example of the time course of ['8Flhaloperidol in the brain of a normal subject and of a schizophrenic patient studied while on medication. The inset in this curve is the plasma inte- gral for the 2 subjects, showing a significantly greater amount of tracer in the plasma of the medicated schizo- phrenic patient when compared with the control over the time course of the study. The values for global brain uptake of [18F]haloperidol normalized for tracer in

plasma (Table 11) are significantly lower for schizo- phrenics than for normal controls.

Patients studied while on medication had a faster clearance of ['8Flhaloperidol from their brain than when they were studied after medication washout. The average half-life of washout from peak uptake is 295 & 81 min (range 184-410 min) for the medicated schizophrenics who were scanned 2 hr after the last haloperidol administration (data not shown). After medication washout, clearance of [18Flhaloperidol de- creased, as can be seen in Figure 4, which shows the time activity curves for ['8Flhaloperidol in a schizo- phrenic patient (subject 13) studied at 3 different times: while on medication, 48 hr after the discontinuation of haloperidol, and 9 weeks after the last injection of halo- peridol decanoate.

In addition to a decrease in the clearance rate from brain with increasing time after the last dose of halo- peridol, the amount of ['8Flhaloperidol in plasma de- creased after washout (Figure 4 inset) for the patient on haloperidol decanoate. Even after a 9-week washout period, one can see that the kinetics of [18F]haloperidol

1.50

0.001

0.000

0.50

o Control Schizophrenic

l ' l ' l ' i ' l ' l ' l ' l ' l ' l

HALOPERIDOL BINDING IN HUMAN BRAIN 15

T 1

*

-r T

I normal schizophrenic

Pre occ t h bg cbl

Fig. 2. Relative values for the regional distribution of ['8F]haloperidol in brain, comparing both the normal controls (n = 9) and schizophrenics (n = 5) receiving haloperidol medication. Values represent the average regional concentration of ["Flhaloperidol normalized to the averages in the whole slices. The cerebellum showed significantly lower uptake in schizophrenics than in normals (*P < 0.05). pre = prefrontal cortex; occ = occipital cortex; th = thalamus; bg = basal ganglia; cbl = cerebellum.

0.008

0.007

0.006

Time after Injection (min.)

Fig. 3. Comparison of the percentage injected dosdcc (global) vs. time for a normal control (subject 9) and for a schizophrenic patient (subject 14) after the injection of [18Flhaloperidol. The plasma integrals for [18F]haloperidol over the time course of the studies for the same 2 subjects are given as an inset.

in brain are still different from normals and that the total amount of tracer in the plasma is less after wash- out.

The clearance of total radioactivity from plasma was rapid and showed considerable intersubject variability.

Interestingly, the rate of metabolism as measured by the amount of unchanged tracer in the plasma did not differ significantly between the normal and the medi- cated schizophrenic populations. However, the medi- cated schizophrenics generally had a higher integrated

16

0.002-

D.J. SCHLYER ET AL

I1 o Medicated 2 Day Washout

0.010

0.008

0 0

2 0.006 vl 0 0 .- U

0.004

R

Fig. 4. Comparison of the percentage injected dose/cc (basal gan- glia) for ['*FF]haloperidol for a period of 10 hr after injection of the radiotracer. This study followed 1 schizophrenic patient (subject 13) over a 3-month time course. The inset in the curve is the integral of I I'FlhaIoperidol concentration in the plasma over the time course of

amount of unchanged [lsFlhaloperidol in plasma over the time course of the experiment relative to nor- mal subjects at time periods greater than 60 min (Table 11). This effect was due to a slower clearance of total radioactivity from the plasma in the medicated schizophrenic patients rather than a slower metab- olism.

Analysis of I'sFlhaloperidol in mouse brain HPLC analysis of mouse brain homogenates at 30

and 60 min after injection showed that 95% of the radio- activity was intact tracer, in agreement with other studies measuring haloperidol metabolism in the brain (Miyazaki, 1986).

COMMENT This PET study of haloperidol binding represents the

first direct measurement of the total uptake, regional distribution, and kinetics of this widely used antipsy- chotic drug in normal human brain and in schizophren- ics medicated with haloperidol. Previous studies of [ 18F]haloperidol in animals examined its behavior from the perspective of its potential use as a radioligand for probing D2 receptors (Amett, 1985a; Tewson, 1980; Welch, 1983).

The extensive binding of [ 18F]haloperidol in human brain over different brain regions is consistent with early observations in rats with [3H]haloperidol (Hel- meste, 1983; Laduron, 1978) and more recent observa-

the experiment for each of the PET studies. Note that after 48 hr of washout, the uptake is similar to the uptake while on haloperidol but that the plasma integral has decreased significantly. This results in an increased normalized uptake of the labeled haloperidol.

tions in postmortem human brain and in mouse and rat brain (Contreras, 1987; Weissman, 1988). [''FlHalo- peridol's high uptake and retention in the cerebellum, cortices, and thalamus, in addition to the basal ganglia (Figure I), contrasts with that seen with specific D, ligands, such as [ lsF]N-methylspiroperidol, a structur- ally similar butyrophenone that shows uptake almost exclusively in the D,-receptor-rich basal ganglia and rapid clearance from receptor-poor regions, such as the cerebellum (Arnett, 1986). Indirect measurements of haloperidol binding to D, receptors in human basal ganglia in vivo with PET using D,-receptor-specific ligands leave little doubt that haloperidol does bind to D2 receptors in the human striatum in vivo (Cambon, 1987; Farde, 1988; Smith, 1988; Wolkin, 19891, an ob- servation that is thoroughly supported by in vitro bind- ing and whole animal studies (Seeman, 1980). Halo- peridol's extensive distribution to other brain regions must therefore be considered in light of its known bind- ing to sigma sites and to other receptors, as well as to nonreceptor sites. The striking contrast between the regional distribution exhibited by [ "Flhaloperidol and [ 18F]N-methylspiroperidol points out that small struc- tural changes within the same class of drugs can have a profound effect on the regional distribution and the ki- netics of a drug that, in turn, may influence the thera- peutic properties of the drug. These parameters (kinet- ics and distribution) can now be investigated with PET in vivo in the human brain.

HALOPERIDOL BINDING IN HUMAN BRAIN 17

The residence time of [18Flhaloperidol (at tracer doses) in normal brain is long, with minimal clearance of radioactivity from the whole brain (and from differ- ent regions) over periods as long as 15 hr. This observa- tion is consistent with animal studies that have shown that the half-life of clearance of a single dose of halo- peridol from brain is very slow (16.7 days; Cohen, 1988). The long retention of haloperidol in the brains of nonmedicated control subjects is in contrast to a signif- icant clearance of [ 18F]haloperidol from all brain re- gions of medicated schizophrenics. The observed clear- ance of [ "F]haloperidol in medicated schizophrenics is even more pronounced when one considers that they have significantly higher plasma levels (and therefore higher potential influx) of [lsF]haloperidol over the time course of the study. This can be seen in the nor- malized tissue uptake values for the two populations shown in Table I1 and in the integrated plasma activity curve shown in Figure 3.

The more rapid clearance of [18F]haloperidol in medi- cated individuals can be rationalized in a number of ways. One possibility is that high-affinity sites are sat- urated in the presence of circulating unlabeled halo- peridol and the tracer binds to nonreceptor sites, which are of lower affinity, resulting in a measurable clear- ance. Another possible explanation is that chronic halo- peridol treatment may decrease the number of sigma receptors (Itzhak, 1989). This possibility must be con- sidered in light of animal studies that have shown that chronic treatment with haloperidol decreases the num- ber of sigma receptors. Furthermore, postmortem stud- ies of brains of schizophrenic patients who had received neuroleptic medication at some point during their ill- ness have also shown significant reduction in sigma receptors as measured with [3H]haloperidol, irrespec- tive of whether they were or were not receiving neuro- leptic medication at the time of death (Weissman, 1988). However, from the present study it cannot be determined whether there is an inherent difference be- tween normals and schizophrenics or whether the dif- ferences in uptake and clearance of [18F]haloperidol are only due to haloperidol medication.

It should also be noted that 3 of the 5 schizophrenic subjects on haloperidol medication were also receiving benztropine mesylate. This is a possible confounding effect because of recent evidence that the administra- tion of benztropine mesylate reduces the uptake of the D,-selective tracer [18F]N-methylspiroperidol in the striatum, an effect that is presumably brought about by an increase in striatal dopamine levels and one that could presumably also interfere with [18Flhaloperidol binding (Dewey, 1990). However, in this small number of PET studies of medicated schizophrenic patients, there was no significant difference in the regional up- take and clearance pattern between the patients who were receiving haloperidol alone and the patients who were receiving haloperidol and benztropine mesylate.

The present studies demonstrate a measurable clear- ance of haloperidol from the brains of medicated pa- tients within a 24- to 48-hr period after the withdrawal of medication. Previous studies have specifically ad- dressed the effects of haloperidol withdrawal on the availability of D, receptors in basal ganglia using D,- specific radioligands and PET (Cambon, 1987; Farde, 1988; Smith, 1988; Wolkin, 1989). The present study differs from these previous studies in that it directly examines the rate of clearance of haloperidol itself un- der tracer conditions and under conditions of haloperi- do1 medication. Since the [18F]haloperidol uptake and clearance that we observe with PET is a superposition of the total binding profile of haloperidol, it is not possi- ble to resolve the component that is due to clearance from the D, receptor and to relate the present findings with the results obtained using D,-specific tracers.

A recent investigation of the regional distribution and pharmacological profile of haloperidol binding in postmortem human brain with [3Hlhaloperidol demon- strated that haloperidol labels a unique population of binding sites (Weissman, 19881, similar to the sigma binding sites identified in rat brain (Contreras, 1987). In addition, in vivo autoradiography in mouse brain with [3Hlhaloperidol showed that 56% of the total brain activity represented binding to sigma receptors (Weiss- man, 1990). The regional distribution that we observe with [18Flhaloperidol in human brain is consistent with postmortem measurements of sigma receptor density, except for the basal ganglia and thalamus (Weissman, 1988). The higher uptake in basal ganglia observed in PET studies and [18Flhaloperidol relative to postmor- tem measurements could be explained by its binding to dopamine D, receptors that were blocked in the autora- diographic studies by using 50 nM spiroperidol (Weiss- man, 1988). The high uptake seen in the thalamus in the PET studies also differs from that of autoradio- graphic studies and could represent binding to other receptor sites.

While the binding to Dz dopamine and to sigma re- ceptors may contribute to the binding profile of halo- peridol, it is also likely that nonspecific binding, as well as the transport properties of [18F]haloperidol in the brain, also contribute to the regional distribution and kinetics as measured by PET. Clearly, the pharmaco- logical profile of haloperidol binding in vivo must be examined using specific blockade with drugs of differ- ent receptor selectivity, along with a critical examina- tion of the effect of pharmacological intervention on the amount of free ['8Flhaloperidol in the plasma. The non- receptor membrane binding of haloperidol and other dopamine antagonists are well known (Oliveira, 1989; Seeman, 1972,1980). Additionally, ifthe rate of seques- tration of haloperidol by nonreceptor membrane inter- actions is rapid relative to the transport of tracer from the plasma to the brain tissue, or if receptor binding is rapid relative to transport of the tracer from the plasma

18 D.J. SCHLYER ET AL.

to the brain, then the PET image will display a blood- flow pattern that possibly masks specific receptor dis- tribution.

CONCLUSIONS This study illustrates an important application of

PET technology, that is, the investigation of the pharma- cokinetics of a drug at its site of action. The major findings of this study are the following: (1) haloperidol has a high uptake and retention in normal brain; (2) the distribution of haloperidol is more extensive than the regional distribution of dopamine D, receptors, with the highest concentration in basal ganglia, cerebellum, and thalamus; and (3) the pattern of haloperidol distri- bution and clearance in brain is changed in patients on haloperidol medication who show a more homogeneous distribution with a rapid washout of haloperidol from brain. However, a detailed examination of the pharma- cological profile of haloperidol binding in the brain in vivo is still required to assess the contribution of haloperidol binding to receptor systems other than D,, as well as to nonspecific sites.

ACKNOWLEDGMENTS This work was carried out at Brookhaven National

Laboratory under contract DE-AC02-76CH00016 with the US. Department of Energy and supported by its Office of Health and Environmental Research, and was also supported by National Institutes of Health grant NS-15638. The authors would like to thank the Babe Barrett, Bob Carciello, and Don Warner for cyclotron operations; Colleen Shea, Karin Karlstrom, and Eliza- beth Jellett for preparing the compounds and doing the metabolite analyses; Payton King for the collection and preparation of the plasma samples; David Christman, Renee Moadel, and Naomi Pappas for PET operations; and Gale Burr, Noelwah Netusil, Katy Pascani, and Theodore Johnson for patient care.

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