D-Alanine Added to Antipsychotics for the Treatment of Schizophrenia

D-Alanine Added to Antipsychotics for the Treatment of Schizophrenia Guochuan E. Tsai, Pinchen Yang, Yue-Cune Chang, and Mian-Yoon Chong Background: Hypofunction of the N-methyl-d-aspartate (NMDA) subtype glutamate receptor had been implicated in the pathophysiology of schizophrenia. Treatment with D-serine, glycine, endogenous full agonists of the glycine site of the NMDA receptor (NMDA-glycine site), D-cycloserine, a partial agonist, or sarcosine, a glycine transporter-1 inhibitor, improves the symptoms of schizophrenia. D-alanine is another endogenous agonist of the NMDA-glycine site that might have beneficial effects on schizophrenia. Methods: Thirty-two schizophrenic patients enrolled in a 6-week double-blind, placebo-controlled trial of D-alanine (100 mg/kg/day), which was added to their stable antipsychotic regimens. Measures of clinical efficacy and side effects were determined every other week. Results: Patint who received D-alanine treatment revealed significant reductions in their Clinical Global Impression Scale and Positive and Negative Syndrome Scale (PANSS) total scores. The Scale for the Assessment of Negative Symptoms and PANSS subscores of positive and cognitive symptoms were improved. D-alanine was well tolerated, and no significant side effect was noted. Conclusions: The significant improvement with the D-alanine further supports the hypothesis of hypofunction of NMDA neurotransmission in schizophrenia and strengthens the proof of the principle that NMDA-enhancing treatment is a promising approach for the pharmacotherapy of schizophrenia.

Key Words: N-methyl-D-aspartate, glutamate, schizophrenia, treatment-resistant

S

chizophrenia is a severe chronic brain disease that afflicts approximately 1% of the global population during their lifetimes and causes a high degree of disability. In addition to dopaminergic and serotonergic neurotransmission, glutamatergic neurotransmission has been implicated in the pathophysiology of schizophrenia (Olney and Farber 1995; Tsai et al 1998a; Tsai and Coyle 2001). N-methyl-D-aspartate (NMDA) receptor, a subtype of ionotropic glutamate receptor, plays an important role in neurocognition and neuroplasticity. Glycine serves as a coagonist at the NMDA receptor, with activation of both the glutamate and glycine sites required for channel opening (Thomson et al 1989). Studies of the action of the psychomimetic compounds phencyclidine (PCP) and ketamine offer the most compelling link between the NMDA system and schizophrenia (Coyle 1996; Halberstadt 1995; Javitt and Zukin 1991). Phencyclidine and ketamine bind to a site within the NMDA channel and act as noncompetitive antagonists. Use of ketamine produces a psychotic condition with compelling similarities to schizophrenic psychosis (Krystal et al 1994; Lahti et al 2001; Malhotra et al 1997). The psychosis induced by NMDA antagonists PCP or ketamine not only causes positive symptoms similar to the action of dopaminergic agonists but also negative symptoms and cognitive deficits associated with schizophrenia. Accordingly, potentiation of NMDA receptor–mediated neurotransmission has been proposed as a treatment alternative in schizophrenia (Deutsch et al 1989). Several studies have demonstrated the clinical benefits of treatment for chronic schizophrenia targeting the glycine site of the NMDA receptor (NMDA-

From the Department of Psychiatry (GT), Harbor-UCLA Medical Center, Torrance, California; Department of Psychiatry (PY), Kaoshiung Medical University; Department of Psychiatry (M-YC), Kaohsiung Chang Gung Memorial Hospital, Kaohsiung; and Institute of Life Sciences and Department of Mathematics (Y-CC), Tamkang University, Tamsui, Taiwan. Address reprint requests to Guochuan E. Tsai, M.D., Ph.D., Harbor-UCLA Medical Center, Department of Psychiatry, Box 462, 1000 W. Carson Street, Torrance, CA 90509; E-mail: [email protected]. Received February 8, 2005; revised June 2, 2005; accepted June 27, 2005.

0006-3223/06/$32.00 doi:10.1016/j.biopsych.2005.06.032

glycine site). These agents include D-serine (Tsai et al 1998a), glycine (Heresco-Levy et al 1996, 1999, 2004), and D-cycloserine (Goff et al 1999; Heresco-Levy et al 2002). D-cycloserine can reduce negative symptoms (Goff 1999; Heresco-Levy 2002). D-serine and glycine improve both negative and cognitive symptoms (Tsai et al 1998b; Heresco-Levy et al 1996, 1999, 2004). Another approach to enhance NMDA neurotransmission is through increasing the availability of synaptic glycine by the attenuation of the glycine reuptake through the glycine transporter-1 (GlyT-1) (Tsai et al 2004b). N-methylglycine (sarcosine) is a potent endogenous inhibitor of the GlyT-1 (Herden et al 2001). Results from our recent study suggest that add-on treatment with sarcosine to stable antipsychotic regimens improves all the critical symptom clusters of chronically stable schizophrenia (Tsai et al 2004a). Furthermore, both D-serine and sarcosine can improve positive symptoms in patients with chronic schizophrenia taking stable doses of antipsychotics (Tsai et al 1998b, 2004a). Not all the control studies of the NMDA-enhancing agents revealed positive therapeutic results, however. Several glycine and D-cycloserine trials failed to show clinical efficacy (Goff et al 2004; Potkin et al 1992; van Berckel et al 1999). One recent large series multicenter trial did not show efficacy when comparing glycine or D-cycloserine with placebo treatment (Carpenter et al 2004). Because the proof of principle is not well established, it is important to investigate other NMDA-enhancing compounds to further test the NMDA hypofunction hypothesis of schizophrenia. D-alanine, like D-serine, is a D-amino acid present in the human central nervous system at a concentration of approximately 10 ␮m (Fisher et al 1991). D-alanine is also a selective and potent agonist at the NMDA-glycine site (Mcbain et al 1989). No other neurotransmitter system is known to be affected by Dalanine. The amino acid is metabolized by D-amino acid oxidase and shares the same amino acid transporter, alanine-serinecysteine transporter 1, as D-serine (Chairoungdua et al 2001). Consistent with the hypo-NMDA hypothesis of schizophrenia, D-alanine inhibits PCP-induced locomotor activity (Tanii et al 1994), PCP-facilitated cortical dopamine metabolism (Umino et al 1998), and methamphetamine-induced hyperactivity in animals (Hashimoto et al 1991). To further test the NMDA hypofunction hypothesis of schizophrenia, in the present study we examined BIOL PSYCHIATRY 2006;59:230 –234 © 2005 Society of Biological Psychiatry

BIOL PSYCHIATRY 2006;59:230 –234 231

G. Tsai et al the supposition that, similar to D-serine and glycine, D-alanine is a therapeutic agent for schizophrenia owing to its activity as a full agonist on the NMDA-glycine site. If so, D-alanine would be predicted to improve the symptom clusters of schizophrenia as do the other NMDA-glycine site agonists.

Methods and Materials Subjects Patients were recruited from the affiliated day program and inpatient unit of Kaoshiung Medical University, which is one of the major medical centers in Taiwan. D-alanine is considered a natural product and regulated as a food supplement, and the Investigational New Drug application is not required in Taiwan. Written informed consent was obtained from all participants after a detailed description of the study, which was approved by the institutional review board of Kaohsing Medical University. Thirtytwo schizophrenic patients were enrolled in this study. Of these, 31 completed the double-blind, placebo-controlled study. One patient in the placebo group was not compliant with the study and dropped out after week 2. Patients’ demographics are shown in Table 1. Patients were evaluated by a research psychiatrist after a thorough medical and neurological workup. The Structured Clinical Interview for DSM-IV (American Psychiatric Association 1994a) was conducted for the diagnosis. Patients with Axis I diagnoses other than schizophrenia, significant depressive symptoms (Hamilton Depression Scale score ⬎20), or serious medical or neurological illness were not included. All the enrolled patients had normal results on physical examination, neurological examination, and laboratory screening tests. All enrolled patients fulfilled the DSM-IV diagnosis of schizophrenia (American Psychiatric Association 1994b). Except for two patients in the placebo group, all also fulfilled the criteria of primary deficit syndrome (Kirkpatrick et al 1989). All the study participants had poor responses to treatment with antipsychotics, as defined by at least two previous adequate antipsychotic treatments (4 – 6 week trials of ⬎400 – 600 mg of chlorpromazine equivalent dosage from at least two different classes) without satisfactory response (Clinical Global Impression Scale [CGI] score ⱖ4) (Conley and Buchanan 1997; Guy 1997). Their antipsychotic doses remained stable for at least 3 months before the enrollment of the study, and the subjects remained taking the same antipsychotic regimen for the period of the D-alanine trial. The antipsychotics received Table 1. Characteristics of Schizophrenic Patients Assigned to Placebo and D-alanine Treatment

Gender (female/male) Age (y) Education (y) Age of onset (y) Duration of illness (y) Subtype Paranoid Disorganized Undifferentiated Residual Chlorpromazine equivalence (mg)

Placebo (n ⫽ 18)

D-alanine (n ⫽ 14)

Differencea

7/11 31.8 (7.4) 12.0 (2.3) 22.7 (5.7) 8.8 (6.0)

10/4 30.9 (6.5) 11.9 (1.6) 21.4 (4.3) 9.5 (6.1)

ns ns ns ns ns

6 3 2 7 364 (220)

2 2 2 8 468 (478)

by the patients included sulpiride (n ⫽ 6), haloperidol (n ⫽ 4), risperidone (n ⫽ 10), pipotiazine (n ⫽ 7), pipotiazine and chlorpromazine (n ⫽ 1), pipotiazine and sulpiride (n ⫽ 1), thioridazine (n ⫽ 1), trifluoperazine and chlorpromazine (n ⫽ 1), and fluphenazine decanoate and chlorpromazine (n ⫽ 1). All patients were randomly assigned under double-blind conditions to receive a 6-week trial of placebo or D-alanine (100 mg/kg daily). The D-alanine was first dissolved in water as a stock solution. The desired amount of D-alanine was then established in orange juice before administration. Patients taking placebo received orange juice diluted with the same amount of water. The D-alanine-containing and D-alanine-free orange juice presented the same color and taste. Assessments A variety of baseline scale determinations was performed. For the overall symptom assessment, the Positive and Negative Syndrome Scale (PANSS; Kay et al 1987) and CGI were administered. The PANSS-positive, -general, or -cognitive subscale (Lindenmayer et al 1994), Scales for the Assessment of Negative symptoms (SANS; Andreasen 1983), and the Hamilton Depression Rating Scale (Hamilton 1960) were administered for the assessment of various symptom domains. Clinical ratings were performed by a research psychiatrist who was trained and experienced in the use of the various rating scales. All assessments were blind to treatment assignment and completed at baseline and at the end of every 2-week period. Biweekly side effect assessments included the Simpson-Angus Rating Scale for extrapyramidal side effects (Simpson and Angus 1970), the Abnormal Involuntary Movement Scale (AIMS) for dyskinesia (Guy 1997), and the Barnes Akathisia Scale (Barnes 1989). Systemic side effects of D-alanine treatments were reviewed by administering the Udvalg for Kliniske Undersogelser Side Effect Rating Scale (Lingjaerde et al 1987). Data Analysis The demographic and clinical characteristics of the patients and side effects among groups were compared by Kruskal-Wallis tests for continuous variables and by ␹2 tests for categorical variables. To assess the efficacy in various clinical domains and to take into account the subjects’ effects, mixed-effects models were used (with intercept as the random effects) for all normally distributed outcomes, with main effects for treatment (D-alanine or placebo), time (0, 2, 4, and 6 weeks), and the treatment ⫻ time interaction. Significance of the percentage reductions of the measurements was obtained from the t values of the solution for fixed effects. The reduction values could be evaluated from the mean values in Table 2. The symptom domain of Table 2 focused on the interaction terms of treatment ⫻ time; the corresponding p and F values from the tests of fixed effects were shown. For the outcome variables with non-normal distributions, Mann-Whitney tests were used. Significance was assessed by comparing endpoint data while controlling for baseline data. All hypothesis tests were two sided and conducted at the .05 ␣ level.

Results

ns

Data are n, with SD in parentheses. a As assessed by two-sample t test, and/or ␹2 test where appropriate, all df ⫽ 1, 29.

As shown in Table 1, the characteristics of the schizophrenic illness were similar in the patients who received D-alanine and the patients who received placebo. Concomitant risperidone versus other antipsychotics was similar between the two groups (for risperidone, placebo group n ⫽ 6, D-alanine group n ⫽ 4). The clinical changes in outcomes are presented in Table 2. www.sobp.org/journal

232 BIOL PSYCHIATRY 2006;59:230 –234

G. Tsai et al

Table 2. Clinical Measures and Side Effects for the Six-week Placebo-controlled D-alanine Trial Treatment ⫻ Time Scale Overall Outcome CGI PANSS-Total Symptom Domains SANS PANSS-positive PANSS-cognitive PANSS-general Hamilton Side Effects SAS Barnes AIMS

Treatment

Baseline

Week 2

Week 4

Week 6

F

P

D-alanine Placebo D-alanine Placebo

4.1 (0.3) 4.2 (0.4) 83.2 (10.2) 82.4 (9.8)

3.5 (0.5) 4.2 (0.9) 80.8 (7.8) 82.4 (11.5)

3.1 (0.6) 4.0 (0.9) 77.0 (8.9) 83.3 (13.9)

2.7 (0.8) 4.0 (0.9) 73.9 (11.1) 82.2 (12.9)

⫺3.38

.001

7.69

⬍.0001

D-alanine Placebo D-alanine Placebo D-alanine Placebo D-alanine Placebo D-alanine Placebo

51.4 (6.3) 53.5 (11.0) 15.8 (3.5) 13.8 (5.2) 12.9 (4.4) 12.4 (3.2) 41.1 (5.8) 43.2 (5.5) 6.8 (4.7) 7.0 (5.4)

48.7 (6.5) 54.0 (12.1) 14.8 (4.0) 13.8 (5.5) 12.4 (4.3) 12.4 (3.1) 40.6 (4.7) 42.8 (6.7) 6.4 (4.5) 7.3 (5.8)

45.6 (7.4) 53.2 (11.1) 14.3 (3.2) 14.3 (5.9) 12.2 (3.9) 12.7 (3.2) 39.0 (4.5) 43.6 (8.7) 5.6 (4.3) 7.4 (6.6)

42.9 (9.3) 53.2 (11.2) 13.7 (3.2) 14.3 (5.9) 11.4 (3.2) 12.4 (2.9) 38.1 (5.5) 42.4 (8.3) 7.3 (6.5) 6.5 (6.2)

8.41

⬍.0001

9.1

⬍.0001

5.75

.001

1.85

.14

.82

.41

D-alanine Placebo D-alanine Placebo D-alanine Placebo

1.1 (1.6) 2.1 (2.2) .2 (0.6) .4 (1.1) .1 (0.5) .3 (0.8)

1.1 (1.5) 1.8 (2.0) .2 (0.6) .7 (1.1) .1 (0.5) .7 (1.6)

1.2 (1.7) 1.9 (2.2) .1 (0.4) .5 (0.8) .1 (0.3) .7 (1.4)

1.1 (1.5) 2.0 (2.0) .3 (0.7) .4 (0.9) .1 (0.3) .8 (1.2)

NS NS NS

Data are presented as mean (SD). Mixed-effects models were used for all normally distributed outcomes. Significance of treatment effects over time was assessed by the significance of the treatment ⫻ time interaction while controlling for the main effects (df ⫽ 90). For the outcome variables with non-normal distributions, Mann-Whitney tests between pairs of treatments were used. CGI, Clinical Global Impression; PANSS, positive symptom subscale of the Positive and Negative Syndrome Scale; Hamilton, Hamilton Depression Rating Scale; Barnes, Barnes Akathisia Scale; AIMS, Abnormal Involuntary Movement Scale. SANS, Scales for the Assessment of Negative symptoms; SAS, SimpsonAngus Rating Scale.

For the overall outcome measures, the D-alanine group had better CGI scores (z ⫽ ⫺3.38, p ⫽ .001). In terms of PANSS performance, the D-alanine group showed better response than the placebo group, and the D-alanine group showed 11% reductions in PANSS-total (t ⫽ ⫺4.25, p ⱕ .0001) (Table 2). For the specific symptom clusters, D-alanine adjunctive treatment was superior to placebo, showing a 17% reduction of total scores in SANS (t ⫽ ⫺4.83, p ⱕ .0001), a 13% reduction of the positive symptoms at the end of the 6-week trial, as measured by the PANSS-positive (t ⫽ ⫺4.80, p ⱕ .0001), and a 12% reduction in the scores of PANSS-cognitive (t ⫽ ⫺3.89, p ⫽ .0002). To compare the primary treatment effects among D-alanine and placebo treatment groups on days 14, 28, and 42, the mixed-effects method was also used (with intercept as the random effect term to adjust the subjects’ effects) to examine the significance of the treatment ⫻ time interaction while controlling for the effects of treatment and time. Compared with the placebo group, the D-alanine group showed significantly more reductions in PANSStotal on days 28 and 42. For the symptoms domains, compared with the D-alanine group, the placebo group showed significantly fewer reductions in SANS on days 28 and 42, PANSSpositive on days 28 and 42, PANSS-cognitive on days 28 and 42 (all p ⬍ .05), but not on PANSS-general or Hamilton Depression Scale psychopathology. Excluding the two patients without primary deficit syndrome who received placebo treatment did not change the overall statistical results. The baseline scores of the Simpson-Angus, AIMS, and Barnes Akathisia Scale were small and similar in the two groups (all p ⫽ ns) (Table 2). Both groups revealed only minimal extrapyramidal www.sobp.org/journal

symptoms after treatment and did not have significant differences between the groups (Table 2) (all p ⫽ ns). Treatment-emergent adverse events in the placebo group included mild urinary retention (n ⫽ 1); side effects in the D-alanine group included insomnia and nausea (n ⫽ 1). These systemic side effects were all short-lived and resolved spontaneously within days, not warranting medical treatment. They were likely coincidental observations. The vital signs after 6 weeks were similar to the pretreatment measures. The routine blood cell count and chemistry after 6 weeks of D-alanine or placebo treatment remained unchanged and were all within the normal ranges (data not shown).

Discussion Our findings indicate that D-alanine, acting as a full agonist on the NMDA-glycine site, can improve symptoms of schizophrenia, including the positive, negative, and cognitive symptom domains as measured by PANSS and SANS. The therapeutic effects of D-alanine are specific for the core symptoms of schizophrenia. The PANSS-general subscale and the depressive symptoms assessed by the Hamilton Depression Scale are not changed by D-alanine treatment. Because there is no other signal transduction site at which D-alanine acts except the known NMDAglycine site, the effect of D-alanine is likely due to its specific action on the NMDA receptor. Several studies have demonstrated the clinical benefits of sarcosine (Tsai et al 2004a), D-serine (Tsai et al 1998b), glycine (Heresco-Levy et al 1996, 1999, 2004) or D-cycloserine (Goff et al

G. Tsai et al 1999; Heresco-Levy et al 2002) treatment for improving the positive, negative, and cognitive symptoms of schizophrenia. Sarcosine has the most comprehensive efficacy profile, which extends to PANSS-depression and -general measurements (Lane et al, in press; Tsai et al 2004a). Similar to the other NMDA-glycine site (partial) agonists, D-alanine has a similar magnitude of clinical efficacy in improving the negative symptoms of schizophrenia as D-serine, glycine, and D-cycloserine. In addition, D-alanine, D-serine, and glycine but not D-cycloserine also improve the positive and cognitive symptoms of schizophrenia. Because D-alanine treatment did not increase D-serine levels in the brain (Nagata et al 1994), the therapeutic effect of D-alanine, therefore, cannot be explained by its conversion to D-serine. The D-alanine dosage chosen (100 mg/kg/day) was determined by a pilot dose-finding study. It is presently unclear whether 100 mg/kg/day is sufficient to saturate the NMDA-glycine site. It is theoretically possible that doses higher than 100 mg/kg/day can fully saturate the NMDA-glycine site and provide more therapeutic benefits for schizophrenia; however, the therapeutic effects of sarcosine (Tsai et al 2004a), D-serine (Tsai et al 1998b), glycine (Heresco-Levy et al 1996a, 1999, 2004), and D-cycloserine (Goff et al 1999; Heresco-Levy et al 2002) on negative symptoms are all in the similar range of D-alanine’s effect. Whether this is a ceiling effect remains to be determined. D-alanine, glycine, and D-serine are potent agonists for the NMDA-glycine site. In our study, the effective therapeutic dose of D-alanine (100 mg/kg/day) is higher than that of D-serine (30 mg/kg/day) (Tsai et al 1998b). This is consistent with their potency at the NMDA-glycine site. The median effective concentration of D-alanine is twice that of D-serine (Mcbain et al 1989). Moreover, the central bioavailability of alanine is between that of serine and glycine (Oldendorf 1971). Therefore, less D-alanine is required than glycine (800 mg/kg/day) for the treatment of schizophrenia (Heresco-Levy et al 1996a 1999, 2002). Attenuation of NMDA receptor–mediated neurotransmission can result in loss of neuronal plasticity and cognitive deficits. In addition, hypo-NMDA function induced by a NMDA receptor antagonist is neurotoxic and might be etiologically related to schizophrenia (Olney and Farber 1995). Therefore, therapeutic agents targeting at the NMDA receptor can improve the cognitive deficit of schizophrenia not only because of the general role of the NMDA receptor in cognition but also because of the specific role that the NMDA system plays in schizophrenia. Consistent with this, our patients taking D-alanine improved significantly in their PANSS-cognitive symptoms (Table 2). This cognition-enhancing effect of NMDA-glycinergic agents is also supported by the positive finding of our D-serine and sarcosine studies (Tsai et al 1998b, 2004a) and a glycine trial (Heresco-Levy et al 1996a). The PANSS-cognitive subscale is a symptomatic measure correlated with verbal memory and verbal intelligence and does not reflect the whole spectrum of cognitive function (Ehmann et al 2004). Further neuropsychological studies of NMDA-glycine agents are required to explore their therapeutic potential on neurocognition. The subjects enrolled in this study are similar to those in the D-serine study, which also recruited patients with primary deficit syndrome (Tsai et al 1998b). Most schizophrenic patients have residual symptoms. Regardless of their status of deficit syndrome, the therapeutic approach will likely bring improved outcome in their long-term function if the NMDA-enhancing agents can induce long-lasting improvement. It would have significant implications for the chronic disability of schizophrenic patients.

BIOL PSYCHIATRY 2006;59:230 –234 233 As yet, no long-term outcome study has been attempted except a negative study of D-cycloserine (Goff et al 2004). D-alanine does not worsen the side effects of other antipsychotics, which are mediated by D2, 5-HT2, histamine, and muscarinic receptors. The extrapyramidal side effects of akathisia and dyskinetic movement were not affected by D-alanine treatment (Table 2). This favorable side-effect profile of D-alanine is consistent with its selective action on the NMDA-glycine site. Similarly, D-serine, glycine, sarcosine, and D-cycloserine do not induce significant side effects (Goff et al 1999, 2004; HerescoLevy et al 1996a, 1999, 2002, 2004; Tsai et al 1998b, 2004a). High doses of glycine are tolerated by the schizophrenic patients, and no dropout was reported owing to side effects (Heresco-Levy et al 1996a, 1999, 2002). We also did not observe any change in routine blood chemistry and hematology test results during sarcosine or D-serine treatment (Tsai et al 1998b, 2004a). A vigorous review of systemic side effects reveals that D-alanine treatment at a dose of 100 mg/kg/day is well tolerated. The few side effects reported by the patients were minimal, likely coincidental, and resolved spontaneously. D-alanine is a naturally occurring amino acid in humans. Rodents lacking D-amino acid oxidase express much higher peripheral and central concentrations of D-alanine and D-serine but do not show abnormal behavior or decreased life span and body weight, indirectly supporting the safety of using D-alanine as a long-term therapeutic agent of enhancing the NMDA system (Hashimoto et al 1993; Nagata et al 1994). Because the symptom improvement profile of D-alanine is similar to that of D-serine, however, and it requires much higher dose of D-alanine than D-serine to achieve the therapeutic effects, it would be more practical to administer D-serine than D-alanine. Large series comparison studies at their optimal doses are required to draw a conclusion; but before that, this study can be viewed as a pilot investigation. Nevertheless, the positive findings of D-alanine on schizophrenic symptoms further strengthen the principle of augmentation of NMDA neurotransmission through the NMDAglycine site for the pharmacotherapy of schizophrenia. Together with the findings regarding D-serine, sarcosine, glycine, and D-cycloserine treatments, all available evidence indicates that hypo-NMDA neurotransmission in schizophrenia is relevant to the pathophysiology of the disease and should encourage more exploration of the strategies of enhancing NMDA neurotransmission (Tsai and Coyle 2001). D-alanine is protected by U.S. patents 6228875, 6667297, and 6420351, for which GT is an inventor. The authors thank Dr. Cheng-sheng Chen for his participation. American Psychiatric Association (1994a): Structured Clinical Interview for DSM-IV. Washington, DC: American Psychiatric Press. American Psychiatric Association (1994b): Diagnostic and Statistical Manual of Mental Disorders, 4th ed. Washington, DC: American Psychiatric Press. Andreasen NC (1983): Scales for the Assessment of Negative symptoms (SANS). Iowa City, Iowa: University of Iowa. Barnes TRE (1989): A rating scale for drug-induced akathisia. Br J Psychiatry 154:672– 676. Carpenter W, Buchanan RW, Javitt DC, Marder SR, Schooler NR (2004): Is glutamatergic therapy efficacious in schizophrenia? Neuropsychopharmacology 29:S110. Chairoungdua A, Kanai Y, Matsuo H, Inatomi J, Kim DK, Endou H (2001): Identification and characterization of a novel member of the heterodimeric amino acid transporter family presumed to be associated with an unknown heavy chain. J Biol Chem 276:49390 – 49399. Conley RR, Buchanan RW (1997): Evaluation of treatment resistant schizophrenia. Schizophr Bull 23:663– 673.

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