D2 and 5-HT2 Receptor Effects of Antipsychotics D2 and 5-HT2 Receptor Effects of Antipsychotics: Bridging Basic and Clinical Findings Using PET Gary Remington, M.D., Ph.D., F.R.C.P.C., and Shitij Kapur, M.D., Ph.D., F.R.C.P.C. © Copyright 2000 Physicians Postgraduate Press, Inc. The advent of a number of new antipsychotics has been paralleled by efforts to better delineate their mechanisms of action and, in doing so, further our understanding of schizophrenia and its patho- physiology. Technological advances, such as positron emission tomography (PET), have proven to be powerful tools in this process, allowing us to evaluate in vivo models based primarily on in vitro evi- dence. Combined serotonin-2/dopamine-2 (5-HT2/D2) antagonism represents one such model, and we now have PET evidence available that can be extrapolated to our understanding and clinical use of both conventional and novel antipsychotics. (J Clin Psychiatry 1999;60[suppl 10]:15–19) he pharmacologic feature of combined serotonin- clinical setting,3 although it was unclear as to what ac- 2/dopamine-2 (5-HT /D ) antagonismOne personal has played copy a maycounted be printed for these properties. However, in vitro work by T 2 2 critical role in efforts to explain the clinical properties of Meltzer and colleagues4,5 provided evidence to indicate the second generation antipsychotics. While other bio- that atypicality may be defined by the characteristic of chemical factors have also been implicated in the patho- greater 5-HT2 versus D2 antagonism. Indeed, work with a physiology of schizophrenia and its treatment,1 it is no- number of compounds suggested a specific ratio of ≥ table that greater 5-HT2 versus D2 activity represents the 5-HT2/D2 pKi values in this respect ( 1.12), which could one feature shared in common by all novel, “atypical,” or be used to predict an agent’s potential for atypicality in the second generation antipsychotics currently approved for clinical setting.3,4 It is noteworthy that loxapine, an anti- clinical use in North America (i.e., clozapine, olanzapine, psychotic historically categorized within the conventional quetiapine, and risperidone) as well as others presently un- group, approximated this ratio as well, suggesting that it der investigation or approved elsewhere (e.g., sertindole, too may manifest at least some of the clinical benefits at- ziprasidone, zotepine).2 tributable to this feature. Also of note was the finding that This article offers a brief review of earlier in vitro evi- amoxapine, a tricyclic dibenzoxazepine used as an antide- dence supporting this particular model, followed by an pressant, appeared to fit this model of atypicality,4,5 giving overview of more recent in vivo evidence employing rise to the notion that it might have the potential for atypi- positron emission tomography (PET). cal antipsychotic features.6 COMBINED 5-HT2/D2 ANTAGONISM AND CONVENTIONAL AND ATYPICAL ANTIPSYCHOTICS: “ATYPICALITY”: IN VITRO FINDINGS IN VIVO EVIDENCE USING PET Clozapine, the prototype of atypical antipsychotics, It is now possible, employing PET, to corroborate and quickly became identified as having unique features in the perhaps extend earlier in vitro findings. In doing so, there is an opportunity to better understand the clinical profiles of both conventional and novel antipsychotics. From the Schizophrenia Program and PET Centre, Centre for Addiction and Mental Health, Clarke Division, and the Department of Psychiatry, University of Toronto, Toronto, Haloperidol Ontario, Canada. The synthesis of haloperidol heralded the high-potency Supported by an award through the Medical Research class of conventional antipsychotics. Indeed, the search Council of Canada (Dr. Kapur). Presented at the closed symposium “What Makes an for highly selective D2 antagonists was fueled by the hy- Atipsychotic Atypical?” which was held in Dallas, Tex., May pothesis that schizophrenia reflected a disorder of hyper- 6–7, 1998, and was supported by an unrestricted educational grant from Watson Laboratories, Inc. dopaminergic activity, with the D2 receptor most closely Reprint requests to: Gary Remington, M.D., Ph.D., associated with antipsychotic response.7 Schizophrenia and Continuing Care Program, Centre for Addiction and Mental Health, Clarke Division, Toronto, More recent evidence from our group, as well as the Ontario M5T 1R8, Canada (e-mail: [email protected]). work of others, indicates that antipsychotic response is as- J Clin Psychiatry 1999;60 (suppl 10) 15 Remington and Kapur 8,9 sociated with D2 antagonism exceeding 60% ; conversely, creased, allowing concomitant D2 occupancy to continue exceeding a threshold of 80% leads to a marked increase increasing and “override” the potential benefits of the as- 10 22,23,27,28 in risk of extrapyramidal symptoms (EPS). Moreover, it sociated 5-HT2 activity. Risperidone’s risk of EPS has been established that nonresponders do not demon- demonstrates this phenomenon. At lower doses, the risk is 11,12 strate lower levels of D2 occupancy, arguing against the no greater than that with placebo; however, as the dose is notion that higher doses will convert nonresponders to re- increased, risk of EPS rises and even begins to approxi- 29,30 sponders. Thus, a threshold between 60% to 80% D2 occu- mate that of haloperidol. pancy appears to represent the optimal range to effect anti- This finding suggests that a threshold exists with com- psychotic response while minimizing risk of EPS. bined 5-HT2/D2 antagonism and that benefits attributable The level of D occupancy associated with hyper- to serotonergic activity may be diminished or even lost at © Copyright2 2000 Physicians Postgraduate Press, Inc. prolactinemia may further narrow this threshold. Occu- the point where 5-HT2 antagonism of the serotonergic sys- pancy greater than 50% has been associated with hyper- tem reaches saturation while D2 blockade continues to in- prolactinemia,13 although preliminary work at our PET crease. center has suggested that risk is associated with levels greater than 72%.14 Olanzapine Evidence with haloperidol indicates that the aforemen- Olanzapine is similar to risperidone in demonstrating tioned thresholds represent a rather narrow dose range. relatively high D2 occupancy at therapeutic doses, i.e., Haloperidol at 1 mg falls below an average occupancy of greater than 70% at doses of 10 mg/day. It too has high 8,15 65%, although by 2.5 mg this level is exceeded. More- 5-HT2 activity at those doses and, like risperidone, it can over, haloperidol at 5 mg exceeds the noted thresholds for approach saturation of the 5-HT2 receptors at higher doses 15,16 31,32 hyperprolactinemia and EPS. These findings dovetail while D2 occupancy continues to rise. The fact that nicely with earlier clinical evidence indicatingOne personal that even copy in mayEPS be seem printed less of an issue with olanzapine versus risperi- acute treatment there is a lack of evidence to indicate that done at comparatively higher therapeutic doses is thought doses in excess of 12 mg/day offer superior benefit,17 as to reflect, at least in part, its added antimuscarinic activ- well as with more recent evidence that even doses lower ity,33 a feature devoid in risperidone.34 than 12 mg/day may be as effective.18–20 Unfortunately, ha- loperidol is largely devoid of 5-HT2 antagonism, thereby Quetiapine failing to gain the clinical benefits associated with such Like clozapine, and unlike risperidone and olanzapine, blockade. quetiapine has relatively low D2 antagonism. Indeed, evi- dence to date suggests that its level of D2 activity is even Clozapine less than that of clozapine at its recommended therapeutic A striking finding with clozapine has been its relatively doses,35,36 and in this sense it appears to parallel clozapine low D2 activity, reported in a number of studies to be in the in terms of a “glass ceiling” with respect to D2 occupancy. range of 20% to 67%.10,21 Indeed, even doses at the high Once more, this attribute may reflect these compounds’ end of the therapeutic range fail to lead to a substantial in- capacity to bind “loosely” to the D2 receptor. In keeping crease in D2 occupancy, suggesting a “glass ceiling” to de- with this argument, both clozapine and quetiapine cause 37,38 scribe the relationship between dose and D2 occupancy only transient increases in prolactin elevation, and the and implying the possible involvement of other pharmaco- ability of quetiapine to be detected at the level of the D2 kinetic factors. receptor following drug administration is considerably 39 In combination with this profile of relatively low D2 ac- shorter than for its 5-HT2 occupancy. tivity is higher concomitant 5-HT2 antagonism, with occu- Unlike clozapine, risperidone, and olanzapine, though, pancy in the range of 85% to 90%, even at doses of quetiapine’s 5-HT2 antagonism, while comparatively 21 125–200 mg daily. To this extent, the in vivo profile of higher than its D2 blockade, is not as great as that seen clozapine closely parallels what was predicted based on with these other compounds, and even at higher therapeu- earlier in vitro evidence. tic doses its occupancy is well below saturation.36 Risperidone Loxapine and Amoxapine Unlike clozapine, risperidone demonstrates a profile of Loxapine, like risperidone and olanzapine, has higher D2 occupancy more in keeping with conventional agents D2 occupancy at therapeutic doses. In addition, though, it 22,23 such as haloperidol. It exemplifies the importance of has high 5-HT2 occupancy at these doses that approxi- corroborating in vitro findings with in vivo evidence, as in mates its degree of D2 antagonism (Figure 1) and vitro work clearly predicted atypicality clinically.24–26 Al- distinguishes it from other conventional agents such as ha- 41 though risperidone does manifest relatively higher 5-HT2 loperidol. Evidence from PET corroborates the recom- activity at therapeutic doses, e.g., 2 to 6 mg daily, the mendation of a relative potency relationship of 15:2 for 42 5-HT2 activity approaches saturation as the dose is in- loxapine:haloperidol, and also indicates that doses in the 16 J Clin Psychiatry 1999;60 (suppl 10) D2 and 5-HT2 Receptor Effects of Antipsychotics Figure 1.