Antipsychotic Drugs Which Elicit Little Or No Parkinsonism Bind More Loosely

Home , Antipsychotic, Atypical antipsychotic, Benperidol, Butaclamol, Desmethylclozapine, Droperidol

PERSPECTIVE Antipsychotic drugs which elicit little or no Parkinsonism bind more loosely than dopamine to brain D2 receptors, yet occupy high levels of these receptors P Seeman1,2 and T Tallerico1

Departments of 1Pharmacology and 2Psychiatry, University of Toronto, Toronto, Canada M5S 1A8

This review addresses two questions. First, why does clozapine apparently occupy low levels of dopamine D2 receptors in patients, in contrast to all other antipsychotic drugs which occupy 70–80% of brain dopamine D2 receptors? Second, what is the receptor basis of action of antipsychotic drugs which elicit low levels of Parkinsonism? Antipsychotic doses of clozapine occupy between 0% and 50% of D2 receptors, as measured in patients by a variety of radioligands. It has recently been found, however, that the percent occupancy of a receptor by a drug depends on the radioligand used to measure that receptor. Based on this new finding, this review concludes that clozapine clinically occupies high levels of D2 receptors in the absence of any radioligand. This occupancy is estimated to be of the order of 70–80% in the dopamine-rich region of the human striatum, and even higher in the limbic D2-containing regions which are low in endogenous synaptic dopamine. This conclusion arises from two different approaches. One approach is to relate the reported clozapine occupancies in the human striatum with the dissociation constants of the various radioligands at the D2 receptor. This relation extrapolates to approximately 70–80% occupancy by clozapine when clozapine competes with endogenous dopamine at the D2 receptor. The second approach is to calculate the D2 occupancy of each antipsychotic drug, using the average spinal fluid concentration and the correct dissociation constant of the antipsychotic, thereby revealing that all antipsychotic drugs, including clozapine, occupy approximately 70– 80% of dopamine D2 receptors in the human striatum, and possibly higher in the limbic regions. As determined by the new dissociation constants, antipsychotic drugs which elicit Parkin- sonism (trifluperazine, chlorpromazine, raclopride, haloperidol, fluphenazine, risperidone) bind more tightly than dopamine to D2, while those antipsychotic drugs which elicit little or no Parkinsonism (melperone, seroquel, perlapine, clozapine, remoxipride, molindone, sulpiride, olanzapine, sertindole) bind more loosely than dopamine to D2 receptors. Compared to the tightly bound antipsychotic drugs, the more loosely bound antipsychotics generally require higher clinical doses, require fewer days for clinical adjustment, but may dissociate from the D2 receptor more rapidly and could lead to clinical relapse somewhat earlier than that found with the traditional tightly bound antipsychotic drugs. Keywords: antipsychotics; clozapine; atypical neuroleptics; risperidone; D2 receptor occupancy

1. Apparent low occupancy of dopamine D2 0% and °50% of brain dopamine D2 receptors, as mea- receptors by clozapine sured by a variety of radioligands using either positron tomography1,2,8–13 or single photon tomography.14–19 Antipsychotic drugs, when given at clinically effective Although the apparently low occupancy of D2 by doses, generally occupy between 70% and 80% of clozapine might suggest that D2 may not be the major brain dopamine D2 receptors in patients,1–7 as meas- antipsychotic target for clozapine (because clozapine ured in the human striatum (ie the caudate nucleus binds to many other neurotransmitter receptors)20 it is and/or the putamen). Clozapine, however, has consist- necessary to analyse these low occupancy data in the ently been an apparent exception. For example, in light of recent evidence showing that the occupancy of patients taking therapeutically effective antipsychotic a receptor by a drug depends on the radioligand used doses of clozapine, this drug only occupies between to measure that receptor.21–26 This latter finding shows that the occupancy is higher when radioligands with higher dissociation constants are used. Correspondence: P Seeman, Pharmacology Department, Medical Using this latter finding, therefore, one objective in Science Building, 8 Taddle Creek Road, Toronto, Ontario, Canada M5S 1A8 this mini-review was to determine indirectly the occu- Received 23 April 1997; revised 15 and 21 July 1997; accepted pancy by clozapine of brain dopamine D2 receptors 26 August 1997 that must in fact be taking place in vivo in patients Atypical antipsychotics bind loosely to D2 receptors P Seeman and T Tallerico 124 being treated with clozapine in the absence of any pine, for example, has a K of 35–59 nM for D2 using radioligand. This analysis indicates that clozapine, at [3H]raclopride,22 compared to a value of 44 nM for the therapeutic doses, occupies high levels (of the order radioligand-independent dissociation constant (Table of 70–80%) of dopamine D2 receptors in the striatum, 1). Similarly, haloperidol has a K of 0.5–0.6 nM, using similar to the occupancies found with therapeutic the very large volumes,22 compared to the radioligand- doses of other antipsychotic drugs, and that clozapine independent dissociation constant of 0.4 nM in may possibly occupy even higher levels in the limbic Table 1. Hence, the large-volume method and the radi- regions where the endogenous synaptic dopamine is oligand-independent method provide data which con- lower than that in the striatum. firm one another.

2. Potencies of antipsychotic drugs at dopamine 3. Clinical antipsychotic potencies correlate with D2 receptors true dissociation constants at D2 The occupancy of receptors can be determined directly These antipsychotic drug dissociation constants, by radioligand tomography or indirectly by measuring which are independent of the radioligand used the concentration, C, of the antipsychotic drug in the (Table 1), correlate with the antipsychotic doses used spinal fluid. Using the latter method, the percent of clinically (Figure 1). This correlation agrees with pre- receptors occupied, f, is equal to C/(C + K), where K is vious work.29–32 the dissociation constant (or inhibition constant) of the Of the many antipsychotic drugs shown in Figure 1, antipsychotic drug at the D2 receptor. only chlorpromazine and thioridazine deviate signifi- The concentration method, therefore, provides an cantly from the overall correlation. The deviations for independent estimate of receptor occupancy by anti- these two drugs, however, disappear when the spinal psychotic drugs. The free concentrations of antipsy- fluid concentrations of the various antipsychotic drugs chotics in the cerebrospinal fluid and in the plasma are considered (next section). water (ie, not bound to plasma proteins) have previously been reviewed,27 although an important 4. Therapeutic levels of antipsychotic drugs revision to the data for clozapine is added below. occupy high levels of brain dopamine D2 In order to derive the clinical receptor occupancies receptors from the antipsychotic concentrations, it is necessary to have accurate K values, as indicated in the above The occupancy of brain dopamine D2 receptors, under equation for the occupancy, f. These K values, how- clinically therapeutic conditions, can now be ever, differ considerably between laboratories for a var- indirectly determined from the antipsychotic drug iety of technical reasons.27,28 Hence, it is essential to average concentrations in the spinal fluid and using have K values for all antipsychotic drugs from a single the radioligand-independent dissociation constants in laboratory using standard conditions. Table 1. As noted above, recent evidence shows that the anti- The dissociation constant, K, in Table 1 is defined as psychotic occupancy or the antipsychotic K (which is the antipsychotic concentration required for 50% occu- the concentration for 50% occupancy of a receptor) pation of the receptor in the absence of dopamine or depends on the radioligand used to measure that recep- any other ligand. In reality, however, because the anti- tor.21–26 For example, antipsychotic drugs which bind psychotic drug must compete with dopamine within loosely (ie, with high K) to the receptor are less able the synaptic space, the antipsychotic concentration to to compete against tightly bound radioligands (ie, with block 50% of the receptors in the presence of dopa-

low K). mine (C50%) will be higher than that in the absence of Hence, it has been necessary to obtain true dis- dopamine, in accordance with the equation = × + sociation constants which are independent of the radi- C50% K [1 D/D2High], where D is the effective dopa- oligand used. This has now been done for many anti- mine concentration in the synapse and where D2High is psychotic drugs at human cloned dopamine D2, D3, D4 the dissociation constant of dopamine at the high- and serotonin 2A receptors, and a list of these values affinity state of the dopamine D2 receptor. is given in Table 1 (summarized from Refs 21–26). Although the effective concentration of dopamine in Although the technical explanation for the apparent the synapse is not known, it is considered to be any- dependence of the antipsychotic dissociation constant where between 1 nM and 45 nM. The basal level of on the radioligand is not known, detailed experiments synaptic dopamine in the rat nucleus accumbens has have shown that it is not a result of radioligand been estimated to be 4 nM.35 At a firing frequency of depletion.21 Although most laboratories use final incu- five impulses per second, the synaptic dopamine in rat bation volumes of between 1 and 2 ml, larger volumes striatum has been estimated to be about 200 nM in the reduce this apparent dependence of the antipsychotic first few millisec and then to rapidly fall to 1–2 nM dissociation constant on the radioligand.21 In fact, within 200 millisec.36 The resting synaptic dopamine using very large volumes of 10 ml leads to experi- concentration in the human striatum has been mental dissociation constants for the antipsychotic indirectly estimated to be about 45 nM.37 drugs22 which are almost identical to the radioligand- The dopamine D2 receptor can exist in either a high- independent dissociation constants in Table 1. Cloza- or a low-affinity state for dopamine, wherein the high- Atypical antipsychotics bind loosely to D2 receptors P Seeman and T Tallerico 125 Table 1 Dissociation constants for antipsychotic drugs at the human cloned dopamine D2, D3, D4 and serotonin-2A receptors. The binding of three different radioligands was inhibited by each antipsychotic drug. These three inhibition constants were then related to the membrane/buffer partition coefficients of the radioligands. By extrapolating to ‘zero’ partition coefficient, one obtained the true dissociation constant which was independent of the radioligands.21–26 This dissociation constant agreed with that determined directly using the radioactive form of the same antipsychotic drug. Abbreviations: Ȱ YM-09151-2; ՚ SM 9018; * rat cortex tissue

Human clone: D2 D3 D4 K values (radioligand-independent) D2 D3 D4 5- nM nM nM nM nM nM HT2A 5-HT2A Human clone: nM nM

1. Amoxapine 16 2 0.5 23. Moperone 1.8 2 2. Benperidol 0.027 0.066 24. [3H]Nemonapride Ȱ 0.068 0.097 0.165 3. Bromocriptine 2 25. Norclozapine 100 20 4. Butaclamol−(+) 0.05 4.5 26. Olanzapine 3 7.8 1.6 3 5. Chlorpromazine 0.66 0.84 1.2 1.8 [3H]Olanzapine 2.7 1.6 1.6 ([3H]Chlorpromazine 0.72 1.15 27. Perlapine 60 100 30 13 6. Chlorprothixene 3.3 0.64 28. Perospirone ՚ 0.6 0.09 °1.3* 7. Clozapine 44 150 1.6 3.5 29. Perphenazine 0.16 0.13 17 [3H]Clozapine 1.6 3.5 30. Pimozide 0.6 8. Droperidol 0.25 0.84 31. Prochlorperazine 1.2 5.4 9. Epidepride °0.01 32. Raclopride 0.64 2 620 4000 10. Flupentixol-cis 0.14 0.3 1.6 [3H]Raclopride 1.6 1.6 11. Fluperlapine 21 33. Remoxipride 30 640 2800 5200 12. Fluphenazine 0.32 0.11 50 3.2 34. Risperidone 0.3 2.5 0.25 0.21 13. Glaxo 1192U90 4 0.8 35. Ritanserin 10 30 0.54 14. Haloperidol 0.35 6.2 0.84 46 36. Seroquel 78 160 3000 110 [3H]Haloperidol 0.4 0.84 37. Sertindole 2.6 2.5 °1 0.3 15. Iloperidone (HP873) 3.5 14 2.5 0.12 [3H]Sertindole 1.9 0.84 0.28 16. Isoclozapine 6 11.5 5.8 1.8 38. SM 13496 1.2 0.3 8.2 17. Isoloxapine 6 4.5 3.7 39. [3H]Spiperone 0.065 0.32 0.089 0.57 18. Loxapine 5.2 18 7.8 2 40. Sulpiride-S 5 6.4 200 19. MDL 100, 907 9000 0.14 41. Thioridazine 0.4 1.5 1.5 1.1 20. Melperone ෂ50 160 410 150 42. Thiothixene-cis 0.12 6.4 21. [3H]Methylspiperone 0.094 43. Triﬂuperazine 0.96 0.45 44 7.4 22. Molindone 6 20 2400 5800 44. Triﬂuperidol 0.35 45. Ziprasidone 1.2 1.1 0.8 3.3

= × + affinity state, D2High, is the physiologically functional above equation of C50% K [1 D/D2High] reduces to 38 ° × state. The dissociation constant of dopamine at D2High C50% 2 K. is between 2 and 10 nM38–45 with an average of 7– In addition, the fraction, f, of D2 receptors occupied 7.5 nM, when using [3H]spiperone.38–45 by an antipsychotic at a concentration C is C/(C + K). However, in the same way as the dissociation con- Using this formula, it may be shown that the concen- stant of an antipsychotic drug depends on the radioli- tration of antipsychotic drug needed to occupy 75% of 21–26 gand used (see above), the dissociation constant of the D2 receptors (ie C75%) is about three times higher dopamine at the D2 receptor also depends on the radio- than that required to occupy 50% of the receptors, = × = × ligand used. For example, the dissociation constants of C50%. In other words, C75% 3 C50%,orC75% 6 K. antipsychotic drugs, as well as dopamine agonists, are Hence, using the K values in Table 1, the antipsy- 3 consistently lower by a factor of 3 when using [ H]ra- chotic C75% concentrations were calculated according clopride, compared to the values obtained when using to the latter equation and graphed in Figure 2. These [3H]spiperone.21–26 Thus, the radioligand-independent values were graphed vs the therapeutic concentrations values are even lower, by a factor of 5, than the dis- of the antipsychotic drugs in the cerebrospinal fluid or sociation constants obtained using [3H]spiperone.21–26 in the plasma water (ie, corrected for drug binding to In fact, the radioligand-independent dissociation con- the plasma proteins). stant of dopamine at the high-affinity state of the D2 The therapeutic concentrations used in Figure 2 have receptor is indeed one-fifth that of 7.5 nM or 1.5 nM previously been summarized,27 with the exception of (data not shown). clozapine for which new data have appeared.46 The Although the synaptic average concentration of average concentration of clozapine in plasma found in dopamine, D, is not known, it appears that D is of the patients taking clinically effective doses of clozapine −1 same order of magnitude as the dopamine K for D2High. is 292 ng ml or 894 nM. The concentration of clozap- ° 46–48 Hence, with this single assumption that D D2High, the ine in the spinal fluid is 20% of that in the plasma Atypical antipsychotics bind loosely to D2 receptors P Seeman and T Tallerico 126 dopamine D4 receptors did not reveal any constant percent occupancy for all the antipsychotic drugs (Figure 3).

5. What is the receptor basis of antipsychotic drugs which elicit little or no Parkinsonism? Does blockade of serotonin receptors alleviate Parkinsonism arising from D2 block? The D2-blocking action of antipsychotic drugs, depending on the type and dose of drug used, com- monly elicits Parkinsonism and other extrapyramidal clinical signs in patients. It has often been suggested that the blockade of serotonin-2A receptors may alleviate the Parkinsonism caused by D2 blockade. It has been difficult, however, to obtain clear behavioural data supporting this important principle. For example, neuroleptics which block both D2 and serotonin-2A receptors also elicit Parkinsonism or catalepsy.56 Second, a high degree of serotonin-2A receptor occupancy (95%) by risperidone (6 mg day−1) did not prevent extrapyramidal signs in 6 out of 7 patients.57 Third, there is no significant correlation between the cataleptic doses of neuroleptics and the ratio of the antipsychotic dissociation constants at D2 and at serotonin-2A receptors, as shown in Figure 4, using the Figure 1 The clinical antipsychotic doses27,33,34 correlate values in Table 1.24,25 with the antipsychotic dissociation constants in Table 1. The Fourth, ritanserin (2 mg kg−1 s.c.) had no effect on deviations for chlorpromazine and thioridazine disappear raclopride-induced catalepsy, using either maximal when the spinal fluid concentrations of the antipsychotic − − (4 mg kg 1 s.c.) or sub-maximal (0.2 mg kg 1 s.c.) doses drugs are considered (Figure 2). The clinical dose for cis-flu- 58 pentixol was taken as half the standard dose used clinically of raclopride. However, in apparent conflict with this 59 (which is the racemate). latter negative result, Lucas et al found that 1.25 mg kg−1 ritanserin i.p., given 15 min prior to haloperidol (1 mg kg−1 s.c.), reduced haloperidol-induced or 179 nM. In addition, the average ratio between catalepsy. norclozapine and clozapine is 0.59,49–55 resulting in a If anything, selective serotonin-2A receptor blockade norclozapine concentration of 106 nM in the spinal markedly enhances the catalepsy of submaximal doses fluid. However, because norclozapine has a dis- of raclopride, as shown in Figure 5 (Wadenburg M-LG, sociation constant of 100 nM at D2, compared to a Hicks PB, Young KA, 1997, personal communication, value of 44 nM for clozapine (Table 1), the spinal fluid with permission). concentration of 106 nM norclozapine is equivalent to It is also uncertain whether serotonin-2A receptors a ‘clozapine-like D2-blocking concentration’ of have any role in the antipsychotic process, because the 106 nM × 44 nM/100 nM or 47 nM. Hence, the average blockade of serotonin-2A receptors ‘is not a prerequi- D2-blocking concentration of clozapine (including nor- site for the antipsychotic effect’.60,61 clozapine in clozapine equivalents) in the spinal fluid Although MDL 100 907 selectively blocks serotonin- is 179 nM + 47 nM or 226 nM. 2A receptors,62 ritanserin blocks both serotonin-2A The line in Figure 2 is the line for identical values and -2C receptors.63,64 Although clozapine blocks sero-

between the C75% and the therapeutic free concen- tonin-2C receptors at concentrations of the order of tration of antipsychotic drug in the spinal ﬂuid or 100 nM,64,65 no clear relation was found between the

plasma water. It may be seen that the C75% values for Parkinson-avoiding potencies of antipsychotics and all the antipsychotic drugs (for which data are their potencies at the serotonin-2C receptor64 (note, available) fall on this line of identity. In other words, although Refs 64 and 65 refer to serotonin-1C, this based on the single assumption that the effective syn- receptor was renamed serotonin-2C in keeping with its aptic dopamine is similar to the K value of dopamine serotonin-2-like amino acid sequence).66 for the high-afﬁnity state of D2, one may conclude that It should also be mentioned here that clozapine is approximately 70–80% of dopamine D2 receptors are of the order of 20- to 50-fold more potent in blocking occupied in the human striatum by antipsychotic drugs muscarinic acetylcholine receptors than blocking under therapeutic conditions. dopamine D2 receptors, making clozapine an The results in Figure 2 are not a coincidence, extremely potent anticholinergic drug. Clozapine because similar calculations for the dopamine D3 and blocks muscarinic receptors between 1.5 nM and Atypical antipsychotics bind loosely to D2 receptors P Seeman and T Tallerico 127

Figure 2 The concentrations of various neuroleptics needed to occupy 75% of the D2 receptors, C75%, are essentially identical to the therapeutic concentrations of neuroleptics found in the spinal ﬂuid or in the plasma water in patients. Because endogenous synaptic dopamine competes with the neuroleptic, the C75% values were six times higher than the neuroleptic dissociation constants at the D2 receptor (Table 1).

36 nM (see Refs in Ref 24), possibly accounting for the to be somewhat higher than that in the human stria- atypical action of clozapine. Isoclozapine, however, is tum. Such data are not yet available. equally anticholinergic (see Refs in Ref 24), yet elicits The case of raclopride is interesting because many catalepsy in low doses, in contrast to clozapine. studies67–69 have shown that radioraclopride can be displaced by 10–30% by endogenous dopamine.67–70 The data in Table 1 indicate that the dissociation con- 6. Antipsychotic drugs which bind more loosely stant for raclopride is approximately 1 nM (between than dopamine to dopamine D2 receptors elicit 0.64 and 1.6 nM), very close to the value of 1.5 nM for little or no Parkinsonism dopamine at the high-afﬁnity state of the D2 receptor A dominant factor in determining whether a particular (Figure 6). Thus, while some raclopride is actually dis- antipsychotic drug elicits Parkinsonism is whether it placed by endogenous dopamine, raclopride is known binds more tightly or more loosely than dopamine at to cause extrapyramidal side-effects. It is possible, the dopamine D2 receptor. This is illustrated in therefore, that the magnitude of displacement by dopa- Figure 6. mine is important clinically in order to avoid Parkin- Thus, for those antipsychotic drugs which elicit little sonism. Previous work68 has shown that dopamine less or no Parkinsonism, it appears that the high endogen- readily displaces the more hydrophobic radioligands of ous dopamine in the human striatum must out- the haloperidol-type. Thus, there appears to be a crude compete the more loosely bound antipsychotic at the correlate between the magnitude of displaceability D2 receptor. The endogenous dopamine in the limbic and Parkinsonism. brain regions (eg, frontal cortex, cingulate gyrus, etc) is This separation of antipsychotic drugs into ‘loose’ of the order of one-tenth that in the striatum, and so and ‘tight’ binding to D2, relative to that for dopamine, the lower synaptic endogenous dopamine in the limbic is consistent with the ﬁndings by Kalkman et al.71 regions would not be as effective in out-competing the These authors were able to reverse catalepsy induced administered antipsychotic drug. Thus, the D2 occu- by olanzapine and loxapine (both more loosely bound pancy in the human limbic regions would be expected than dopamine), but were not able to reverse that by Atypical antipsychotics bind loosely to D2 receptors P Seeman and T Tallerico 128

Figure 3 The therapeutic concentrations of various antipsychotic drugs consistently occupy 75–76% of dopamine D2 receptors (data from Figure 2), but similar calculations for the dopamine D3 and dopamine D4 receptors did not reveal any constant percent occupancy for all the antipsychotic drugs. CPZ, chlorpromazine; Cloz., clozapine; Halo., haloperidol; Molin., molindone; Olan., olanzapine; Raclo., raclopride; Remox., remoxipride; Sulp., S-Sulpiride; Thior., thioridazine; Flupen., cis-ﬂupen- tixol; per., perphenazine.

Figure 4 Lack of clear correlation between the rat catalepsy doses and the ratio of the dissociation constants at the dopamine D2 and the serotonin-2A receptors for antipsychotic drugs.24,25 Atypical antipsychotics bind loosely to D2 receptors P Seeman and T Tallerico 129 7. Therapeutic levels of clozapine occupy high levels of D2 receptors, using brain imaging

The finding of high occupancy of D2 receptors by maintenance doses of clozapine, shown in Figure 2, can be confirmed independently from published data using PET (positron emission tomography) or SPET (single photon emission tomography). It has been consistently claimed that clozapine only occupies between 0% and 50% of brain dopamine D2 receptors, as measured by a variety of radioligands using either positron tomography1,2,8–13 or single photon tomography.14–19 This is in contrast to the D2 occupancies of 70 –80% for the typical neuroleptics. Figure 5 The selective blockade of serotonin-2A receptors As noted above, it is necessary to analyse these low by MDL 100 907, while enhancing the anti-avoidance potency of D2-blockade by raclopride, also markedly enhances the occupancy data in the light of evidence showing that catalepsy potency of raclopride in rats58 (Wadenburg M-LG, the occupancy of a receptor depends on the radioli- 21–26 Hicks PB, Young KA, 1997, personal communication, with gand used to measure that receptor. This latter permission). finding shows that the occupancy rises when radioligands with higher dissociation constants are used. This important principle also holds true for the occupancy of D2 by clozapine when using various radioligands with PET or SPET, as illustrated in Figure 7. This Figure shows that clozapine occupied low levels of D2 receptors, between 0 and 29%, when radiomethylspiperone congeners were used ([18F]methylspiperone,8 [11C]methylspiperone11,12 or [18F]fluorethylspiperone9,10), all of which have a dissociation constant of 0.092 nM (this lab, unpublished).72 Clozapine occupied higher levels of D2 receptors, between 30% and 47%,14–19 when [123I]iodobenzamide was used with its higher dissociation constant of 0.43–0.49 nM.73,74 Finally, the highest level of D2 receptors, °50%, was occupied by clozapine2 (Kapur S, personal communication, 1996) when [11C]raclopride was used with its dissociation constant of 0.6–1 nM (Table 1). The observation that the D2 occupancy by clozapine depends on the radioligand used (Figure 7) is similar to that found in measuring the occupancy of the dopamine transporter.75 Cocaine, with its dissociation con- ° 76,77 123 stant, Kd,of 60 nM, does not displace [ I]RTI-55 ° 77 3 (Kd of 0.6 nM) but does displace 40% of [ H]cocaine (Refs in Ref 75). Extrapolating the relation shown in Figure 7 to the dissociation constant of 1.5 nM for dopamine (at the high-affinity state of D2) reveals that 75% of D2 receptors must in fact be occupied by clozapine in patients taking maintenance doses of clozapine in the absence Figure 6 Antipsychotic drugs which bind more tightly than of any radioligand. dopamine at the dopamine D2 receptor elicit Parkinsonism, while those which bind more loosely than dopamine elicit little or no Parkinsonism or other extrapyramidal clinical signs in patients. The radioligand-independent value for the 8. Clinical implications and conclusions dissociation constant of dopamine at the high-affinity state of the dopamine D2 receptor is 1.5 nM (see text). Data from Clozapine fits the rule of high levels of blockade of Table 1. dopamine D2 receptors by neuroleptics when patients take effective maintenance doses of antipsychotic medication, a rule worked out by Farde and col- leagues.1–3 There are a number of clinical implications haloperidol (Kalkman HO, Tricklebank MD, 1997, per- with respect to the properties of antipsychotic drugs sonal communication). summarized in this review, as follows: Atypical antipsychotics bind loosely to D2 receptors P Seeman and T Tallerico 130

Figure 7 The occupancy of D2 by clozapine rises when radioligands with higher dissociation constants are used with PET or SPET. The radiomethylspiperone congeners were [18F]methylspiperone (K),8 [11C]methylspiperone (C)11,12 or [18F]ﬂuorethylspi- perone (L),9,10 all of which have a dissociation constant of 0.092 nM (this lab, unpublished).72 Clozapine occupied higher levels of D2 receptors (D19;S17,18;B14;P15;M16) when [123I]iodobenzamide was used (dissociation constant of 0.43–0.49 nM).73,74 The highest level of D2 receptors was occupied by clozapine (N:2; R: Kapur S, personal communication, 1996) when [11C]raclopride was used (dissociation constant of 0.64–1 nM; Table 1). Extrapolating this relation to the dissociation constant of 1.5 nM for dopamine (at the high-afﬁnity state of D2) reveals that 75% of D2 receptors must be occupied by clozapine in patients in the absence of any radioligand.

Is D2 block a necessary minimum for clinical antipsychotic action of clozapine appears to antipsychotic action? reside in its D2-blocking ability with its ‘loose’ blockade of D2 at 44 nM, primarily based on the findings in The consistent finding of high occupancy of D2 for all Figures 2 and 7 that clozapine fits the universal rule of neuroleptics, including clozapine, suggests that the blockade of D2 is an essential minimal requirement for 75% blockade for clinical antipsychotic action. clinical antipsychotic action in those patients who It has often been stated that D2 blockade is inad- respond to neuroleptics. It is, of course, well known equate to explain the clinical antipsychotic action of that many schizophrenia patients may not clinically clozapine because clozapine has a low occupancy of improve despite high occupancy (Ͼ75%) of their D2 D2, and, second, because clozapine is clinically effec- receptors.78–81 This supports the long-standing assump- tive in 30% of patients who are resistant to haloperidol 82 tion that there may be more than one subtype of schizo- and similar traditional D2-blocking drugs. Such phrenia. treatment-resistant patients, however, also respond (sometimes dramatically) to remoxipride,83–85 which is extremely selective for D2 (Table 1). In fact, remoxi- Do therapy-resistant patients respond to clozapine pride clinically improves at least 30% of treatment- or remoxipride via D2-blockade? resistant schizophrenia patients.83 This experience Clozapine is selective for the dopamine D4 receptor with remoxipride indicates that treatment-resistant with a K value of 1.6 nM, compared to 3.5 nM for the patients may still improve via D2-blockade when using serotonin-2A receptor and 44 nM for the D2 receptor a low potency drug such as clozapine (K = 44 nM at D2) (Table 1). Despite the selectivity for D4, however, the or remoxipride (K = 30 nM at D2). Atypical antipsychotics bind loosely to D2 receptors P Seeman and T Tallerico 131 Why do many patients suddenly relapse when clozapine with this radioligand (Figure 7). Also using stopping clozapine? [123I]iodobenzamide, sertindole (dose not specified) occupies at least 55% of D2 receptors. using [11C]raclo- Patients taking clozapine often relapse within days of pride, ziprasidone occupies 45–75% of D2 receptors.92 stopping clozapine (see Refs in References 86 and 87). Hence, once sufficient data become available using Because clozapine is loosely bound to the dopamine other radioligands, it appears that these neuroleptics D2 receptor (K = 44 nM), clozapine is readily displaced (remoxipride, seroquel, sertindole and ziprasidone) by any sudden pulse of endogenous dopamine arising would also extrapolate under therapeutic conditions to from emotional or physical activity. In fact, both R occupy approximately 75% of D2 receptors under Conley86 and Pickar et al88 observed that the D2 occu- therapeutic conditions. pancy by clozapine readily decreased upon clozapine withdrawal, in contrast to the 2 weeks or more of residual occupancy of D2 by traditional neuroleptics.89 In treating L-DOPA-induced psychosis, does Any sudden surge of impulse-triggered release of clozapine or remoxipride also block 75% of D2 endogenous dopamine will quickly displace any receptors? residual clozapine and may lead to a sudden clinical The psychosis caused by L-DOPA or bromocriptine in relapse. Parkinson’s Disease can be readily treated by low doses Table 2 summarizes this point that the relatively of either clozapine49,93–97 or remoxipride.98 The average more rapid offset of drugs such as clozapine could − dose for clozapine is 55 mg day 1 (for 51 patients),93–97 result in a higher risk of earlier relapse when compared − while that for remoxipride is 150 mg day 1,98 much to the more traditional haloperidol and chlorpromaz- lower than that used in schizophrenia for either drug ine drugs which are more tightly bound to the D2 (Figure 1). receptor. These lower doses follow directly from the well- known fact that 90–99% of the brain dopamine has How can neuroleptic-induced Parkinsonism be been depleted in Parkinson’s disease. Hence, there is minimized? virtually no endogenous dopamine to compete against clozapine or remoxipride. The low dose of clozapine Although it may be trite to repeat the adage that low corresponds to a low spinal fluid concentration of cloz- doses of neuroleptics elicit low levels of extrapyrami- apine and norclozapine, estimated to be of the order of dal signs, the present analysis emphasizes this prin- 60 nM (given that the unbound clozapine is 20% of the ciple once again. Hence, although at least 70–75% total plasma clozapine).46 Under these conditions, the occupancy is needed for antipsychotic action, D2 occu- fraction of D2 receptors occupied would be about pancy needs to remain below 80% in order to avoid 60%*. This D2 occupancy by clozapine may be even extrapyramidal signs. Clinical dosing schedules, there- lower, possibly 20%, if the estimated concentration of fore, may be adjusted accordingly by building up the 10 nM (unbound) is used from four Parkinson patients dose to a level where EPS occurs after which the dose who developed psychosis,49 and using 20% as the frac- may be reduced by half. This maintains the 75% occu- tion of unbound clozapine. Because the D2 occupancy pancy of D2 and is a classical clinical strategy which by clozapine in Parkinson patients has not been has been used since the advent of neuroleptics. directly examined, however, the D2 occupancy by clozapine in such patients remains uncertain. Does the present analysis for clozapine extend to Despite this uncertainty about the occupancy of D2 other new atypical neuroleptics? by clozapine in Parkinson patients, the occupancy of the dopamine D4 receptors by clozapine would be at Insufficient D2 occupancy data are available for remox- least 75%†, because the K for clozapine at D4 is 1.6 nM ipride,1 seroquel,90 sertindole91 or ziprasidone.92 (Table 1). At the lowest clozapine concentration, the Remoxipride occupies about 70% of D2, using [11C]ra- D4 occupancy would still be at least 86%‡. However, clopride.1 Using [123I]iodobenzamide, seroquel occu- the possible antipsychotic effect of D4 receptor block- pies 23% of D2 receptors,90 similar to that occupied by ade is still not known. The treatment of L-DOPA psychosis by 150 mg day−1 of remoxipride, on the other hand, would be expected Table 2 Clinical aspects of antipsychotics which have tight § or loose binding to D2 to be associated with about 73% occupancy of dopamine D2 receptors, in accord with the general rule of Tight Loose 75% D2 block for antipsychotic action.

EPS High Low Dose Low High (1–5 mg) (10–500 mg) Offset wks, mos Days * Where C/(C+K) is 60 nM/(60 nM + 44 nM) = 60% of D2. Relapse Low risk Higher risk † Where C/(C+K) is 60 nM/(60 nM + 1.6 nM) = 97% of D4. Tardive High risk Lower risk ‡ Where C/(C+K) is 10 nM/(10 nM + 1.6 nM) = 86% of D4. § Where C/(C+K) is 80 nM/(80 nM + 30 nM) = 73% of D2. Atypical antipsychotics bind loosely to D2 receptors P Seeman and T Tallerico 132 Would high doses of clozapine or remoxipride, ship to extrapyramidal symptoms: a PET study. Life Sci 1995; 57: exceeding 80% D2 occupancy, cause PL103–PL107. 5 Kapur S, Remington G, Jones C, Wilson A, DaSilva J, Houle S et Parkinsonism? al. High levels of dopamine D2 receptor occupancy with low-dose haloperidol treatment: a PET study. Am J Psychiatry 1996; 153: Although clozapine occupies 75% of D2 receptors 948–950. −1 19 when used at 300–400 mg day , higher doses may 6 Kapur S, Zipursky RB, Jones C, Remington GJ, Wilson AA, Da- 2 and may not result in higher D2 occupancy, but do Silva J et al. The D2 receptor occupancy proﬁle of loxapine deter- not result in Parkinsonism. The situation for remoxi- mined using PET. Neuropsychopharmacology 1996; 15: 562–566. pride may be identical, although insufﬁcient data 7 Knable MB, Heinz A, Coppola R, Gorey J, Weinberger DR. IBZM exist.1 As discussed above, it appears that clozapine SPECT measurement of D2 receptor occupancy by haloperidol and risperidone. (Abstract) Biol Psychiatry 1996; 39: 515. and remoxipride, weakly bound to D2 (see Table 1), are 8 Karbe H, Wienhard K, Hamacher K, Huber M, Herholz K, Coenen readily displaced by synaptic dopamine, which may HH et al. Positron emission tomography with 18 be transiently very high during brief bursts of nerve ( F)methylspiperone demonstrates D2 dopamine receptor bind- impulses known to occur in the motor pathway,99 thus ing differences of clozapine and haloperidol. J Neural Transm obviating Parkinsonism. 1991; 86: 163–173. 9 Louwerens JW, Buddingh JA, Zijlstra S, Pruim J, Korf J, Paans

AMJ et al. Dopamine (D2)-receptor occupancy in clozapine-treated patients as measured by positron emission tomography using Will the new antipsychotic drugs with low affinity 18FESP. In: Brunello N, Mendlewicz J, Racagni G (eds). New Gen- for D2 and with low risk for Parkinsonism also eration of Antipsychotic Drugs: Novel Mechanisms of Action, vol cause less tardive dyskinesia? 4. Int Acad Biomed Drug Res, Basel, Karger, 1993, pp 130–135. 10 Louwerens JW, Slooff CJ, Korf J, Coppens HJ, Paans AMJ. Dopa- It is likely that those antipsychotic drugs which elicit mine2- and serotonin2-receptor-antagonism by antipsychotics in less Parkinsonism by virtue of their low affinity for D2 man. (Abstract) Schizophrenia Res 1996; 18: 141. 11 (in comparison to dopamine) may result in less tardive 11 Conley R, Medoff D, Wong D, Tamminga C. C NMSP receptor 100 occupancy by clozapine and haloperidol in schizophrenic sub- dyskinesia. This is discussed by Casey. Essentially, jects. (Abstract) Schizophrenia Res 1995; 15: 80. by its high affinity for the D2 receptor, the endogenous 12 Conley R, Zhao M, Wong D, Tamminga C. 11C NMSP receptor dopamine effectively removes the D2-blocking action occupancy by clozapine and haloperidol in schizophrenic sub- in the putamen and the caudate nucleus of more jects. (Abstract) Biol Psychiatry 1996; 39: 513. loosely associated antipsychotic drugs, thus sparing 13 Kapur S. SDA antipsychotics: from theory to clinical practice. Les- sons from PET neuroimaging. (Abstract) 20th Collegium Int the lingering and injurious action which antipsychotic Neuropsychopharmacol 1996; S-17-3. 101 drugs have on cell membranes. 14 Busatto GF, Pilowsky LS, Costa DC, Ell PJ, Verhoeff NPLG, Kerwin

RW. Dopamine D2 receptor blockade in vivo with the novel antipsychotics risperidone and remoxipride—an 123I-IBZM single Acknowledgements photon emission tomography (SPET) study. Psychopharmacology 1995; 117: 55–61. We thank Dr S Kapur for generously providing unpub- 15 Pilowsky LS, Busatto GF, Taylor M, Costa DC, Sharma T, Sig- lished data. Supported by Mr and Mrs Robert Peterson mundsson T et al. Dopamine D2 receptor occupancy in vivo by 123 of The Peterson Foundation (University Park, FL) and the novel atypical antipsychotic olanzapine—a I IBZM single photon emission tomography (SPET) study. Psychopharmacology NARSAD (National Alliance for Research in Schizo- 1996; 124: 148–153. ¨ phrenia and Depression), the Ontario Mental Health 16 Klemm E, Grunwald F, Kasper S, Menzel C, Broich K, Danos P et 123 Foundation, the Medical Research Council of Canada, al.[ I]IBZM SPECT for imaging of striatal D2 dopamine receptors the Medland family (J Aubrey and Helen Medland, in 56 schizophrenic patients taking various neuroleptics. Am J Pamela and Desmond O’Rorke, Janet Marsh and David Psychiatry 1996; 153: 183–190. 17 Su T-P, Breier A, Coppola R, Hadd K, Elman I, Adler C et al.D2 Medland), the National Institute on Drug Abuse, USA, receptor occupancy in risperidone and clozapine-treated schizo- the Stanley Foundation of the NAMI Research Insti- phrenics. Biol Psychiatry 1996; 39: 512–513. tute, and Novartis (Sandoz Pharma Ltd), Basel, Switz- 18 Su T-P, Breier A, Coppola R, Hadd K, Elman I, Alder C et al.D2 erland. receptor occupancy during risperidone and clozapine treatment in chronic schizophrenia: relationship to blood level, efﬁcacy and EPS. (Abstract) Soc Neurosci Abstr 1996; 22: 265. 19 Pickar D, Su T-P, Weinberger DR, Coppola R, Malhotra AK, Knable References MB et al. Individual variation in D2 dopamine receptor occupancy ¨ 1 Farde L, Nordstrom A-L, Wiesel F-A, Pauli S, Halldin C, Sedvall in clozapine-treated patients. Am J Psychiatry 1996; 153: 1571– 1578. G. Positron emission tomographic analysis of central D1 and D2 dopamine receptor occupancy in patients treated with classical 20 Brunello N, Masotto C, Steardo L, Markstein R, Racagni G. New neuroleptics and clozapine. Arch Gen Psychiatry 1992; 49: 538– insights into the biology of schizophrenia through the mechanism 544. of action of clozapine. Neuropsychopharmacology 1995; 13: ¨ 2 Nordstrom A-L, Farde L, Nyberg S, Karlsson P, Halldin C, Sedvall 177–213.

G. D1,D2, and 5-HT2 receptor occupancy in relation to clozapine 21 Seeman P, Van Tol HHM. Deriving the therapeutic concentrations serum concentration: a PET study of schizophrenic patients. Am for clozapine and haloperidol: the apparent dissociation constant J Psychiatry 1995; 152: 1444–1449. of a neuroleptic at the dopamine D2 or D4 receptor varies with ¨ 3 Nyberg S, Nordstrom A-L, Halldin C, Farde L. Positron emission the afﬁnity of the competing radioligand. Eur J Pharmacol – Mol

tomography studies on D2 dopamine receptor occupancy and Pharmacol Section 1995; 291: 59–66. plasma antipsychotic drug levels in man. Int Clin Psychopharma- 22 Malmberg Å, Jackson DM, Eriksson A, Mohell N. Unique binding cology 1995; 10 (Suppl 3): 81–85. characteristics of antipsychotic agents interacting with human

4 Kapur S, Remington G, Zipursky RB, Wilson AA, Houle S. The dopamine D2A,D2B, and D3 receptors. Mol Pharmacol 1993; 43:

D2 dopamine receptor occupancy of risperidone and its relation- 749–754. Atypical antipsychotics bind loosely to D2 receptors P Seeman and T Tallerico 133 23 Seeman P. Therapeutic receptor-blocking concentrations of neuro- 47 Szymanski S, Lieberman J, Pollack S, Munne R, Safferman A, leptics. Int Clin Psychopharmacol 1995; 10 (Suppl 3): 5–13. Kane J et al. The dopamine-serotonin relationship in clozapine 24 Seeman P, Corbett R, Van Tol HHM. Atypical neuroleptics have response. Psychopharmacology 1993; 112: S85–S89. low affinity for dopamine D2 receptors or are selective for D4. 48 Lieberman J, Johns C, Pollack S, Masiar S, Bookstein P, Cooper T Neuropsychopharmacology 1996; 16: 91–110. et al. Biochemical effects of clozapine in cerebrospinal fluid of 25 Seeman P, Corbett R, Nam D, Van Tol HHM. Dopamine and sero- patients with schizophrenia. In: Tamminga CA, Schultz SC (eds). tonin receptors: amino acid sequences, and clinical role in neuro- Advances in Neuropsychiatry and Psychopharmacology, Volume leptic Parkinsonism. Jpn J Pharmacol 1996; 71: 187–204. 1: Schizophrenia Research. Raven Press: New York, 1991, 26 Seeman P, Kapur S. Clozapine occupies high levels of dopamine pp 341–349. D2 receptors. Life Sci 1997; 60: 207–216. 49 Meltzer HY, Kennedy J, Dai J, Parsa M, Riley D. Plasma clozapine 27 Seeman P. Dopamine receptor sequences. Therapeutic levels of levels and the treatment of L-DOPA-induced psychosis in Parkin- neuroleptics occupy D2, clozapine occupies D4. Neuropsycho- son’s disease. A high potency effect of clozapine. Neuropsycho- pharmacology 1992; 7: 261–284. pharmacology 1995; 12: 39–45. 28 Seeman P, Ulpian C, Wreggett KA, Wells J. Dopamine receptor 50 Hagesawa M, Gutierrez-Esteinou R, Way L, Meltzer HY. Relation- 3 parameters detected by H-spiperone depend on tissue concen- ship between clinical efficacy and clozapine concentration in tration: analysis and examples. J Neurochem 1984; 43: 221–235. plasma in schizophrenia: effect of smoking. J Clin Psychopharma- 29 Seeman P, Wong M, Lee T. Dopamine receptor-block and nigral col 1993; 13: 383–390. fiber-impulse blockade by major tranquilizers. (Abstract) Fed Proc 51 Olesen OV, Thomsen K, Jensen PN, Wulff CH, Rasmussen N-A, 1974; 33: 246. Refshammer C et al. Clozapine serum levels and side effects dur- 30 Seeman P, Chau-Wong M, Tedesco J, Wong K. Brain receptors for ing steady state treatment of schizophrenic patients: a cross-sec- antipsychotic drugs and dopamine: direct binding assays. Proc tional study. Psychopharmacology 1995; 117: 371–378. Natl Acad Sci USA 1975; 72: 4376–4380. 52 Perry PJ, Miller DD, Arndt SV, Cadoret RJ. Clozapine and norcloz- 31 Seeman P, Lee T, Chau-Wong M, Wong K. Antipsychotic drug apine plasma concentrations and clinical response of treatment- doses and neuroleptic/dopamine receptors. Nature 1976; 261: refractory schizophrenic patients. Am J Psychiatry 1991; 148: 717–719. 231–235. 32 Burt DR, Creese I, Snyder SH. Properties of [3H]haloperidol and 53 Lin S-K, Chang W-H, Chung M-C, Lam F, Jann MW. Disposition [3H]dopamine binding associated with dopamine receptors in calf of clozapine and desmethylclozapine in schizophrenic patients. J brain membranes. Mol Pharmacol 1976; 12: 800–812. Clin Pharmacol 1994; 34: 318–324. 33 Seeman P, Lee T. Antipsychotic drugs: direct correlation between 54 Haring C, Fleischhacker WW, Schett P, Humpel C, Barnas C, Saria clinical potency and presynaptic action on dopamine neurons. A. Influence of patient-related variables on clozapine plasma lev- Science 1975; 188: 1217–1219. els. Am J Psychiat 1990; 147: 1471–1475. 34 Seeman P, Lee T. Neuroleptic drugs: direct correlation between 55 Olesen OV, Poulsen B. On-line fully automated determination of clinical potency and presynaptic action on dopamine neurons. In: clozapine and desmethylclozapine in human serum by solid- Sedvall G (ed). Antipsychotic Drugs, Pharmacodynamics and phase extraction on exchangeable cartridges and liquid chroma- Pharmacokinetics. Karolinska Institute, Pergamon: Oxford, 1976, tography using a methanol buffer mobile phase on unmodified sil- pp 183–191. 35 Parson LH, Justice Jr JB. Extracellular concentration and in vivo ica. J Chromatog 1993; 622: 39–46. recovery of dopamine in the nucleus accumbens using microdi- 56 Casey DE. Serotonergic and dopaminergic aspects of neuroleptic- alysis. J Neurochem 1992; 58: 212–218. induced extrapyramidal syndromes in nonhuman primates. 36 Kawagoe KT, Garris PA, Wiedemann DJ, Wightman RM. Regu- Psychopharmacology 1993; 112: S55–S59. lation of transient dopamine concentration gradients in the 57 Nyberg S, Eriksson B, Oxenstierna G, Halldin C, Farde L. PET microenvironment surrounding nerve terminals in the rat stria- study of D2- and 5-HT2 receptor occupancy induced by risperi- tum. Neuroscience 1992; 51: 55–64. done in schizophrenic patients. (Abstract) Am College Neuropsy- 37 Laruelle M, De Souza C, Baldwin RM, Abi-Dargham A, Bowers M, chopharmacol 1996; 35: 184. Seibyl JP et al. SPECT measurement of human striatal synaptic 58 Wadenberg M-L, Salmi P, Jimenez P, Svensson T, Ahlenius S. dopamine concentration in the resting state. (Abstract) Soc Neuro- Enhancement of antipsychotic-like properties of the dopamine D2 sci Abstr 1996; 22: 266. receptor antagonist, raclopride, by the additional treatment with 38 George SR, Watanabe M, Di Paolo T, Falardeau P, Labrie F, See- the 5-HT2 receptor blocking agent, ritanserin, in the rat. Eur Neuropsychopharmacol 1996; 6: 305–310. man P. The functional state of the dopamine receptor in the ` anterior pituitary is in the high-affinity form. Endocrinology 1985; 59 Lucas G, Bonhomme N, De Deurwaerdere P, Le Moal M, Spampin- 117: 690–697. ato U, 8-OH-DPAT, a 5-HT1A agonist and ritanserin, a 5-HT2A/C 39 Sokoloff P, Giros B, Martres M-P, Bouthenet M-L, Schwartz J-C. antagonist, reverse haloperidol-induced catalepsy in rats indepen- Molecular cloning and characterization of a novel dopamine dently of striatal dopamine release. Psychopharmacology 1997; 131: 57–63. receptor (D3) as a target for neuroleptics. Nature 1990; 347: 146– ¨ 151. 60 Farde L, Nyberg S, Nordstrom A-L, Halldin C, Sedvall G. Dopam- ine and serotonin receptor occupancy examined by PET in anti- 40 Castro SW, Strange PG. Coupling of D2 and D3 dopamine receptors to G-proteins. FEBS Lett 1993; 315: 223–226. psychotic drug treated patients. (Abstract) Am College Neuropsy- chopharmacol 1993; 32: 38. 41 Seeman P, Niznik HB. Dopamine D1 receptor pharmacology. ISI ¨ Atlas of Sci Pharmacol 1988; 2: 161–170. 61 Nyberg S, Nakashima Y, Nordstrom A-L, Halldin C, Farde L. Posi- 42 Chio CL, Lajiness ME, Huff RM. Activation of heterologously tron emission tomography of in vivo binding characteristics of expressed D3 dopamine receptors: comparison with D2 dopamine atypical antipsychotic drugs. Review of D2 and 5-HT2 receptor receptors. Mol Pharmacol 1994; 45: 51–60. occupancy studies and clinical response. Br J Psychiatry 1996; 43 Giros B, Sokoloff P, Martres M-P, Riou J-F, Emorine LJ, Schwartz 168: 40–44.

J-C. Alternative splicing directs the expression of two D2 dopam- 62 Kehne JH, Baron BM, Carr AA, Chaney SF, Elands J, Feldman DJ ine receptor isoforms. Nature 1989; 342: 923–926. et al. Preclinical characterization of the potential of the putative 3 44 Malmberg A, Mohell N. Characterization of [ H]quinpirole bind- atypical antipsychotic MDL 100,907 as a potent 5-HT2A antagonist

ing to human dopamine D2A and D3 receptors: effects of ions and with a favorable CNS safety proﬁle. J Pharmacol Exper Ther 1996; guanine nucleotides. J Pharmacol Exper Ther 1995; 274: 790–797. 277: 968–981. 45 Seeman P, Watanabe M, Grigoriadis D, Tedesco JL, George SR, 63 Labrecque J, Fargin A, Bouvier M, Chidiac P, Dennis M. Sero-

Svensson U et al. Dopamine D2 receptor binding sites for agonists. tonergic antagonists differentially inhibit spontaneous activity Mol Pharmacol 1985; 28: 391–399. and decrease ligand binding capacity of the rat 5-hydroxytryptam- ´ 46 Nordin C, Alme B, Bondesson U. CSF and serum concentrations ine type 2C receptor in Sf9 cells. Mol Pharmacol 1995; 48: 150– of clozapine and its demethyl metabolite: a pilot study. Psycho- 159. pharmacology 1995; 122: 104–107. 64 Roth BL, Ciaranello RD, Meltzer HY. Binding of typical and atypi- Atypical antipsychotics bind loosely to D2 receptors P Seeman and T Tallerico 134 cal antipsychotic agents to transiently expressed 5-HT1C receptors. 81 Wolkin A, Barouche F, Wolf AP, Rotrosen J, Fowler JS, Shiue C- J Pharmacol Exper Ther 1992; 260: 1361–1365. Y et al. Dopamine blockade and clinical response: evidence for ¨ ¨ 65 Kuoppamaki M, Syvalahti E, Hietala J. Clozapine and N-desme- two biological subgroups of schizophrenia. Am J Psychiatry 1989;

thylclozapine are potent 5-HT1C receptor antagonists. Eur J Pharm- 146: 905–908. acol – Mol Pharmacol Sect 1993; 245: 179–182. 82 Kane J, Honigfeld G, Singer J, Meltzer HY, Clozaril Collaborative 66 Hoyer D, Clarke DE, Fozard JR, Hartig PR, Martin GR, Mylecharane Study Group. Clozapine for the treatment-resistant schizophrenic: EJ. et al. VII. International Union of Pharmacology Classification a double-blind comparison with chlorpromazine. Arch Gen Psy- of receptors for 5-hydroxytryptamine (serotonin). Pharmacol Rev chiatry 1988; 45: 789–796. 1994; 46: 157–203. 83 Scurlock HJ, Robinson PH. Case report 2: remoxipride in resistant 67 Seeman P, Guan H-C, Niznik HB. Endogenous dopamine lowers schizophrenia. J Drug Dev 1993; 6: 71–72. the dopamine D2 receptor density as measured by [3H]raclopride: 84 Scurlock HJ, Singh AN, Catalan J. Atypical antipsychotic drugs in implications for positron emission tomography of the human the treatment of manic syndromes in patients with V-1 infection. brain. Synapse 1989; 3: 96–97. J Psychopharmacol 1995; 9: 151–154. 68 Dewey SL, Smith GS, Logan J, Brodie JD, Yu DW, Ferrieri RA et 85 Vartianen H, Leinonen E, Putkonen A, Lang S, Hagert U, Tolvanen al. GABAergic inhibition of endogenous dopamine release meas- U. A long-term study of remoxipride in chronic schizophrenic ured in vivo with 11C-raclopride and positron emission tomogra- patients. Acta Pschiatr Scand 1993; 87: 1134–117. phy. J Neurosci 1992; 12: 3773–3780. 86 Shore D, Matthews S, Cott J, Lieberman JA. Clinical implications 69 Gifford AN, Gatley SJ, Ashby Jr CH. Endogenously released dopa- of clozapine discontinuation: report of an NIMH workshop. mine inhibits the binding of dopaminergic PET and SPECT Schizophrenia Bull 1995; 21: 333–338. ligands in superfused rat striatal slices. Synapse 1996; 22: 232– 87 Meltzer HY, Lee MA, Ranjan R, Mason EA, Cola PA. Relapse fol- 238. lowing clozapine withdrawal: effect of neuroleptic drugs and cyp- 70 Laruelle M, Abi-Dargham A, Gil R, Van Dyck CH, Erdos J, D’Souza roheptadine. Psychopharmacology 1996; 124: 176–187. D et al. SPECT imaging of synaptic dopamine release in drug free 88 Pickar D, Su T-P, Coppola R, Lee CS, Hsiao J Breier A et al.D2 schizophrenic patients: an experimental validation of the dopam- occupancy and dopamine release determined by 123I-IBZM ine hypothesis of schizophrenia. Schizophrenia Res 1996; 18: SPECT following clozapine dose reduction. (Abstract) Schizo- 198–199. phrenia Res 1995; 15: 96. 71 Kalkman HO, Neumann V, Tricklebank MD. Clozapine inhibits 89 Baron JC, Martinot JL, Cambon H, Boulenger JP, Poirier MF, Cail- catalepsy induced by olanzapine and loxapine, but prolongs cata- lard V et al. Striatal dopamine receptor occupancy during and lepsy induced by SCH 23390 in rats. Naunyn-Schmiedeberg’s following withdrawal from neuroleptic treatment: correlative Arch Pharmacol 1997; 355: 361–364. evaluation by positron emission tomography and plasma prolactin 72 Wong DF, Gjedde A, Wagner Jr HN, Dannals RF, Douglass KH, levels. Psychopharmacology 1989; 99: 463–472. Links JM et al. Quantification of neuroreceptors in the living 90 Tauscher J, Kufferle B, Neumeister A, Asenbaum S, Brucke T, Pod- human brain. II. Inhibition studies of receptor density and affinity. reka I et al. IBZM-SPECT shows low striatal D2 blockade under therapy with seroquel. (Abstract) Biol Psychiatry 1996; 39: 513. J Cereb Blood Flow Metab 1986; 6: 147–153. ¨ 73 Murphy RA, Kung HF, Kung M-P, Billings J. Synthesis and charac- 91 Kasper S, Tauscher J, Kufferle B, Asenbaum S, Barnas C, Hessel- terization of iodobenzamide analogues: potential D-2 dopamine man B et al. Sertindole and dopamine D2 occupancy. (Abstract) receptor imaging agents. J Med Chem 1990; 33: 171–178. Am College Neuropsychpharmacol 1996; 35: 265. 92 Miceli JJ, Gunn KP, Rubin RH, Frackowiak RSJ, Williams SA, 74 Verhoeff NPLG, Bobeldijk M, Feenstra MGP, Boer GJ, Maas MAW, Fischman A. 5HT and D receptor occupancy of ziprasidone in Erdtsieck-Ernste E et al: In vitro and in vivo D2-dopamine receptor 2 2 healthy volunteers. (Abstract) Am College Neuropsychopharmacol binding with [123I]S(−)iodobenzamide [123I]IBZM) in rat and 1996; 35: 269. human brain. Nucl Med Biol 1991; 18: 837–846. 93 Friedman JH, Lannon MC. Clozapine in the treatment of psychosis 75 Gatley SJ, Volkow ND, Gifford AN, Ding Y-S, Logan J, Wang G-J. in Parkinson’s disease. Neurology 1989; 39: 1219–1221. Model for estimating dopamine transporter occupancy and sub- 94 Pfeiffer RF, Kang J, Graver B, Hofman R, Wilson J. Clozapine for sequent increases in synaptic dopamine using positron emission psychosis in Parkinson’s disease. Movement Disorders 1990; 5: tomography and carbon-11-labeled cocaine. Biochem Pharmacol 239–242. 1997; 53: 43–52. 95 Kahn N, Freeman A, Juncos JL, Manning D, Watts RL. Clozapine 76 Madras BK, Fahey MA, Bergman J, Canfield DR, Spealman RD. is beneficial for psychosis in Parkinson’s disease. Neurology 1991; Effects of cocaine and related drugs in nonhuman primates. I. 41: 1699–1700. 3 [ H]Cocaine binding sites in caudate-putamen. J Pharmacol Exper 96 Factor SA, Brown D. Clozapine prevents recurrence of psychosis Ther 1989; 251: 131–141. in Parkinson’s disease. Movement Disorders 1992; 7: 125–131. 77 Boja JW, Mitchell WM, Patel A, Kopajtic TA, Carroll FI, Lewin 97 Rabey JM, Treves TA, Neufeld MY, Orlov E, Korczyn AD. Low- 125 AH et al. High-affinity binding of [ I]RTI-55 to dopamine and dose clozapine in the treatment of levodopa-induced mental dis- serotonin transporters in rat brain. Synapse 1992; 12: 27–36. turbances in Parkinson’s disease. Neurology 1995; 45: 432–434. 78 Coppens HJ, Slooff CJ, Paans AMJ, Wiegman T, Vaalburg W, Korf 98 Sandor P, Lang AE, Singal S, Angus C. Remoxipride in the treat- J. High central D2-dopamine receptor occupancy as assessed with ment of levodopa-induced psychosis. J Clin Psychopharmacol positron emission tomography in medicated but therapy-resistant 1996; 16: 395–399. schizophrenic patients. Biol Psychiatry 1991; 29: 629–634. 99 Grace AA. Phasic versus tonic dopamine release and the modu- 79 Pilowsky LS, Costa DC, Ell PJ, Murray RM, Verhoeff NPLG, Ker- lation of dopamine system responsivity: a hypothesis for the etiol- win RW. Clozapine, single photon emission tomography, and the ogy of schizophrenia. Neuroscience 1991; 41: 1–24. D2 dopamine receptor blockade hypothesis of schizophrenia. Lan- 100 Casey DE. Will the new antipsychotics bring hope of reducing the cet 1992; 340: 199–202; correction, 340: 380. risk of developing extrapyramidal syndromes and tardive dyski- 80 Pilowsky LS, Costa DC, Ell PJ, Murray RM, Verhoeff NPLG, Ker- nesia? Int Clin Psychopharmacol 1997; 12: (suppl 1): S19–S27.

win RW. Antipsychotic medication, D2 dopamine receptor block- 101 Seeman P. Tardive dyskinesia, dopamine receptors, and neurolep- ade and clinical response: a 123I-IBZM SPET (single photon emis- tic damage to cell membranes. J Clin Psychopharmacol 1988; 8: sion tomography) study. Psychol Med 1993; 23: 791–797. 3S–9S.