Evolution of molecular targets for antipsychotic therapy

S. MARENCO, D.R. WEINBERGER

Clinical Brain Disorders Branch, National Institute of Mental Health, IRP, DHHS, Bethesda, MD

Key words: Psychosis � Antipsychotic drugs � Dopamine � Glutamate � Molecular targets

Correspondence: Dr. Stefano Marenco, CBDB/NIMH, 10 Center Drive, Room 4S235, Bethesda, MD 20892- E-mail: marencos@intra.nimh.nih.gov

Treatment of psychosis has been one of the major medical challenges in the last century. Despite 70 years of efforts to treat these disorders, we are still only partially successful in controlling the symptoms of schizophrenia and a cure has eluded us. The pathophysiology of schizophrenia and other “functional” psychoses remains largely a mystery, although promising steps are being made in the identification of genes that may be important in the causation or mediation of the illness and new treatment strategies will certainly emerge from these advances.

We will review here the progression of attempts to treat functional psychoses as they relate to the identification of molecular targets that have pathophysiological significance for the expression of the illness. Figure 1 provides a synthetic overview.

The initial attempts to treat psychosis go back to insulin-induced coma, introduced by Manfred Sakel in 1933. In the late 1940s and early 1950s frontal lobotomies and electroconvulsive treatment (ECT (1)) were also attempted. Although some degree of effectiveness was claimed for all these procedures, they all fell out of favor after the introduction of Chlorpromazine in the early 1950s. Today, insulin coma has been abandoned, while psychosurgery is still used as an experimental procedure with smaller, less destructive and more targeted operations for severe cases of obsessive compulsive disorder and depression. ECT survives in the treatment of major depression and occasionally patients with severe treatment-refractory psychosis and affective symptoms receive a trial of ECT in the US. All of these initial attempts to treat psychosis had no molecular target.

Dopamine as a Molecular Target for the Treatment of Psychosis

The first indication of a drug that had some success in treating schizophrenia symptoms came with the use of reserpine. Its use was first advocated in an Indian medical journal in 1931, but it was not until 1954 that the western medical community recognized its potential effectiveness in treating psychosis. Reserpine was later discovered to have multiple mechanisms of action, but the best candidate to explain its sporadic effectiveness for psychosis is its ability to deplete monoamines in neurons, thereby also depleting dopamine.

With the discovery of chlorpromazine in the early 1950�s, the era of molecular targets for schizophrenia began. Chlorpromazine was initially developed as an anesthetic and the first to use this drug to treat psychosis were Paraire and Sigwald in Paris in 1951-1952. In the following years, Delay and Deniker (2) and later Lehmann and Hanrahan (3) showed its clinical usefulness in treating agitated and psychotic patients. These studies led to the introduction of Chlorpromazine on the US market and throughout the world. Other compounds with neuroleptic properties were discovered shortly and haloperidol was in 1958. The pharmacology of these compounds continued to be studied and understood in more detail in the 1960�s. Particularly important was the contribution by Janssen and his co-workers (e.g. (4) (5)). The classes of drugs with neuroleptic action grew from the phenothiazines, to the thioxanthenes, to the butyrophenones. Introduction of similar drugs to the US market continued until the early 1980s with pimozide. The development of these new agents was driven largely by market forces and by the search for differences in side-effect profiles. Thus, the higher potency agents, such as the thioxanthenes and particularly the butyrophenones, caused less autonomic and cardiovascular side effects, but they carried a greater risk for extrapyramidal motor effects.

However, it was not until the 1970s that the molecular target of these drugs was identified. Work by Carlsson (6), Bunney et al. (7), Creese et al. (8) and especially Seeman et al. (9)-(11) contributed to the discovery that all neuroleptics reduced dopamine transmission. Of note, the work from the labs of Snyder and Seeman identified methods for the measurement of dopamine receptor occupancy in vitro and showed that all neuroleptics available at that time bind to these molecules. Moverover, in a landmark observation by Creese, Burt and Snyder (8), it was shown that, with the exception of clozapine, the affinity of these drugs for dopamine receptors in vitro strongly predicted their clinical potencies. Moreover, the average daily doses of neuroleptics correlated well with their potency in binding to the dopamine D2 receptors.

With the advent of tomographic nuclear medicine techniques in the late 1980�s it became possible to study receptor occupancy by neuroleptics in vivo and their relationship to side effects such as extrapyramidal symptoms. Wolkin et al. (12) showed a sigmoidal relationship between haloperidol plasma levels and receptor occupancy. Their study demonstrated that with haloperidol, occupancy reached a ceiling at about 15 ng/ml plasma level, and then treatment response did not improve with higher blood levels. Studies by Farde et al. (13) and Nordstrom et al. (14) indicated that motor side effects such as parkinsonism emerged when neuroleptics occupied more than approximately 80-90% of D2 receptors, and that therapeutic efficacy was not present below occupancies of approximately 50%. These findings established a therapeutic-toxic window of D2 receptor occupancy for the classical antipsychotic agents, such that within this window response was likely without prominent extrapyramidal effects. Other side effects such as increased prolactin and secondary negative symptoms were also tied to dopamine receptor blockade. Also, tardive dyskinesia was tied to protracted disruption of D2 mediated transmission.

Though it is not widely appreciated, these early PET D2 receptor occupancy studies established many of the basic principles of the clinical pharmacology of these drugs that apply to the later generation of antipsychotic agents as well. First, they established a tight relationship between blood level and brain occupancy until apparent saturation around 90% occupancy, at which point higher doses do not achieve greater D2 binding. Second, they showed that while clinical response was not linearly related to blood level or occupancy at dopamine receptors, there was an apparent threshold relationship, such that a minimal level of occupancy was necessary but not sufficient. Third, they demonstrated that dopamine related side effects, e.g. elevated prolactin and EPS, were also phenomena lawfully predicted by high levels of occupancy.

The next decisive step in understanding more about the molecular targets for schizophrenia treatment was the introduction of clozapine in the early 1970�s. A courageous series of studies by Kane (15)-(17) and others demonstrated the superior effectiveness of clozapine, while putting the serious adverse events associated with its use in a more realistic light. Still today, clozapine is considered the most effective treatment for psychosis although the exact mechanism of its apparently unique benefits remains unknown.

Consisent with the early in vitro studies of Creese and Snyder and Seeman, in the late 80�s-early 90�s it was rediscovered that this drug had very weak binding to the D2 receptor (18), while it had much higher antagonist activity at the D4 and 5HT2a receptor. Meltzer (19) developed a theory that this balance between 5HT2a and D2 receptors was the critical component of clozapine�s success, while the weak blocking of D2 receptors seemed responsible for the lack of extrapyramidal side effects, including tardive dyskinesia.

The reality of clozapine led to the search for novel molecules that would retain its effectiveness while avoiding its side effects, specifically neutropenia. Strong HT2a receptor binding became almost a requirement for drugs developed based on the clozapine experience. Risperidone, olanzapine, quetiapine, sertindole and ziprasidone, all developed in 1990�s, have higher affinity for the HT2a receptor than for D2, as can be seen in Figure 2. Olanzapine was developed by slightly altering the clozapine molecule, and it too showed weaker D2 receptor binding than the “typical” neuroleptics. Risperidone remained the atypical neuroleptic with the strongest D2 binding affinity, while quietiapine became the atypical with the weakest binding to D2 receptors. The rapid development and marketing of these compounds was truly remarkable. After the introduction on the market of the typical neuroleptics 30-40 years earlier there had been no new development in the pharmacology of schizophrenia. In the last 10 years, an explosion of new treatments has taken place that offer the advantage of reduced extrapyramidal side effects and tardive dyskinesia.

Atypicals have been claimed to have another advantage over typicals: they appeared to improve cognition and negative symptoms in patients with schizophrenia (20)-(24). These two symptom domains have been shown to impact on social and vocational function in a much more dramatic way than positive symptoms, therefore constituting a key target for treatment. The reason why atypicals may improve cognition is still unclear, but one possibility is that they increase dopamine release in the prefrontal cortex, at least when administered acutely in animals (25). There is evidence that prefrontal cortical dopamine transmission is reduced in schizophrenia (26)-(29), therefore this explanation has some support in the literature. The mechanism that would mediate increased prefrontal dopamine release remains unclear, though.

The theory that the mechanism of the atypical antipsychotic profile of effects is related to the relatively greater affinity at 5HT-2 receptors has remained somewhat unsatisfactory, though. Generally, most double blind studies have not shown superior efficacy of the atypicals (except for clozapine) as compared to traditional neuroleptics at least over the 6-8 weeks traditionally employed for clinical trials, and, as for typical neuroleptics, many patients remained unresponsive. Moreover, 5-HT-2 antagonists are not antipsychotic and atypicals such as risperidone do not seem to benefit from high 5-HT-2 affinity in terms of lower EPS once they are given at doses that exceed the D2 occupancy threshold associated with EPS (30). This observation confirmed clinical experience that as doses were increased to achieve greater clinical response an increase in extrapyramidal symptoms appeared. Moreover, another disappointment that has emerged with many of the atypicals concerns metabolic side effects such as increased incidence of diabetes and glucose intolerance (the mechanisms of which are still unclear), and weight gain with some drugs (especially olanzapine and clozapine).

Towards the end of the 1990�s, PET studies contributed again in a decisive way to the understanding of the psychopharmacology of antipsychotic drugs in the treatment of schizophrenia. Studies by Kapur et al. (31) indicated that 80% 5HT2a receptor occupancy occurred at non-therapeutic levels of clozapine, making it implausible that this receptor would be involved in the antipsychotic efficacy of all the most recent neuroleptics. Knable et al. (30) also showed that 5HT2a receptor occupancy was unrelated to motor side effect profile of neuroleptics.

Seeman (32) reported evidence that D4 receptors were increased in the brains of patients with schizophrenia and, based on the fact that Clozapine has higher binding affinity for D4 than D2, he proposed that D4 antagonism was the key element of atypicality. However, other groups could not confirm this postmortem observation. Also this proposal had to be set aside in light of the fact that pure D4 antagonists proved to be ineffective antipsychotics.

Kapur (33) advocated that the dissociation constant of the neuroleptic from the D2 receptor was the key therapeutic factor in the atypical profile. This proposal followed efforts to explain earlier PET studies suggesting that drugs like clozapine and quetiapine appeared to be associated with clinical response even in some patients who had D2 occupancy levels in the 20-40% range. These low levels of occupancy violated the principle that a certain minimal level of D2 occupancy was necessary for a therapeutic effect, and that this level was in the 40-50% range. What Kapur observed, however, was that the earlier PET data was based on measurements made over twelve hours after the last dose of medication. Because clozapine and quetiapine dissociated from dopamine receptors relatively quickly (i.e. have fast off rates), there was little residual occupancy by this time. However, when the measurements were made only a few hours after drug administration, relatively high level, i.e. greater than 60% were found. Thus, Kapur and Seeman opined that all antipsychotic drugs achieve D2 occupancy levels above the minimal threshold associated with a clinical response, but that drugs like quetiapine and clozapine do so for relatively brief periods. This interpretation was supported by the fact that even a drug like quietiapine with a very short permanence on the receptor was deemed to be an effective antipsychotic. However, as we will see later, this is not the only possible explanation of differences between typical and atypical neuroleptics.

Studies by Marc Laruelle (34)-(37) and Alan Breier (38) demonstrating that patients with schizophrenia had increased dopamine release in the striatum after administration of amphetamine further swung the pendulum of scientific enquiry back to a focus on dopamine physiology. Studies with fluoro-DOPA (39)-(41) demonstrating increased uptake of this precursor of dopamine further supported the dopamine hypothesis of schizophrenia of excessive presynaptic dopamine activity. Thus, drugs that modulate dopaminergic signaling at least in the striatum are rational antipsychotic agents.

One key concept proposed by Kapur and Seeman (33) that merits emphasis is that one of the keys to the difference between atypical and typical neuroleptics is the ability of the former to permit more physiologic dopamine signaling. Whether because of relatively lower D2 occupancy levels or of brief occupancy, excessive dopamine blockade does not appear to have any advantage and causes increased toxicity, hence some degree of physiological activity at the D2 receptor would seem to be a plausible goal for obtaining optimal therapeutic effects without untoward side effects.

It is plausible that the latest innovation in the field of dopaminergic treatments of schizophrenia is based on this very concept. Aripiprazole (42) is the latest antipsychotic drug introduced to the US market. Clinical trials show similar efficacy to other neuroleptics without many side effects usually attributed to dopaminergic drugs: for instance, lack of extrapyramidal symptoms and hyperprolactinemia. This agent also appears to not suffer from the metabolic side effects associated with drugs such as clozapine, olanzapine and quetiapine. With this clinical profile, aripiprazole is clearly a very promising drug. But, what makes this drug most unique is its apparent mechanism of action. It is a partial agonist at the D2 receptor. While the drug binds to D2 receptors (and in Table I, one can see that it has one of the highest affinities to D2 receptors compared to most neuroleptics), its molecular behavior at the receptor is different from all other antipsychotic drugs. While preventing binding of endogenous dopamine to the receptor it also maintains a certain degree of biological activity of dopamine by locking the conformation of the post-synaptic machinery linked to the G-protein system into a state of partial activation. This mechanism has led to speculation that a high affinity partial agonist such as aripiprazole modulates or stabilizes dopamine signaling at dopamine D2 synapses, which may be unstable or overactive in patients who are psychotic. This residual continuous signal should not be too high or too low in order to minimize symptoms and side effects at the same time. The information currently available on this drug indicates that aripiprazole achieves this objective.

Retrospectively, one can explain the efficacy of neuroleptics with low affinity for the D2 receptor (such as clozapine and quietiapine) as due to a partial, and therefore incomplete, block of D2 signaling. This may also be the reason why low doses of typical neuroleptics and especially decanoates (which probably expose the receptors to lower antagonist concentrations than oral medication) have been shown to be equally therapeutic to higher doses and to cause less side effects.

In conclusion, high D2 occupancy is not necessary for an antipsychotic response and may cause side effects without benefit. Even continuous D2 blockade may not be indispensable, as noted by Kapur. Finally, drugs that regulate dopamine transmission without blocking it entirely can be effective antipsychotics. Aripiprazole may be the first of a new wave of “dopamine stabilizers” as Carlsson defined these and similar drugs that are currently being developed.

The Search for Nondopamine Based Treatments

Dopamine dysregulation is not likely to be the only therapeutic target in the treatment of psychosis. Schizophrenia is a complex disorder likely to involve multiple pathogenetic steps, each one of them being a potential target for treatment. The importance of dopamine D1 transmission for cognition has been demonstrated in primates (43) (44) and positron emission tomography studies have shown D1 receptor alterations in the prefrontal cortex in patients with schizophrenia (45) (46). Indeed, a recent genetic discovery related to prefrontal dopamine function in schizophrenia (47) has suggested that dopamine D1 signaling in the prefrontal cortex may be an important target for therapeutic intervention.

Other drugs are being investigated that target the glutamate system. The impulse for this development came from the discovery that PCP (a substance that can cause psychosis when used at subanesthetic doses) blocks NMDA receptors. Recently, associations of genes related to the glutamate system with schizophrenia are emerging. Glutamate transmission-related strategies have already been explored as potential adjuvant treatments for schizophrenia, from agonists at the glycine site of the NMDA receptor (glycine, d-cycloserine and d-serine (48)-(52): which seem to be most effective for negative symptoms), to AMPA receptor modulators (53) (54). Other targets are the metabotropic glutamate receptors that regulate glutamate transmission in the prefrontal cortex and are possibly responsible for some of the feedback between NMDA and dopamine-mediated transmission. Finally, drugs that reduce glutamate transmission such as lamotrigine (55)-(57) and riluzole may have some interest, at least as adjuvants.

Other receptor systems where drugs with antipsychotic efficacy may emerge are the cannabinoid and nicotinic systems (58) (59). In both of these fields compounds are being tested that may prove to have some efficacy. It is also possible that muscarinic receptors may play a role in the cognitive aspects of schizophrenia (60).

The field of drug development for the treatment of psychosis is witnessing a new era of hope and opportunity, new molecular targets for drug development are being identified and the entire field of psychiatry may derive a more rational basis for pharmaceutical development in the process.

Fig. 1. Timeline of antipsychotic development.

Fig. 2. Comparative receptor binding profile of the atypical antipsychotics. The total occupancy exerted on all receptors is the total area of each pie chart and each sector represents the relative amount of binding to each receptor subtype.

Tab. I. Kd of several atypical antipsychotics for various subtypes of receptors. Lower numbers represent higher affinity.

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