Double blind trial of L-dopa in chronic schizophrenia

The past and future of novel, non-dopamine-2 receptor therapeutics for schizophrenia: A critical and comprehensive review

Since the discovery of chlorpromazine in the 1950's, antipsychotic drugs have been the cornerstone of treatment of schizophrenia, and all attenuate dopamine transmission at the dopamine-2 receptor. Drug development for schizophrenia since that time has led to improvements in side effects and tolerability, and limited improvements in efficacy, with the exception of clozapine. However, the reasons for clozapine's greater efficacy remain unclear, despite the great efforts and resources invested therewith. We performed a comprehensive review of the literature to determine the fate of previously tested, non-dopamine-2 receptor experimental treatments. Overall we included 250 studies in the review from the period 1970 to 2017 including treatments with glutamatergic, serotonergic, cholinergic, neuropeptidergic, hormone-based, dopaminergic, metabolic, vitamin/naturopathic, histaminergic, infection/inflammation-based, and miscellaneous mechanisms. Despite there being several promising targets, such as allosteric modulation of the NMDA and α7 nicotinic receptors, we cannot confidently state that any of the mechanistically novel experimental treatments covered in this review are definitely effective for the treatment of schizophrenia and ready for clinical use. We discuss potential reasons for the relative lack of progress in developing non-dopamine-2 receptor treatments for schizophrenia and provide recommendations for future efforts pursuing novel drug development for schizophrenia.

Enhanced cortical dopamine output and antipsychotic-like effect of raclopride with adjunctive low-dose L-dopa

BACKGROUND

Clozapine shows superior efficacy in schizophrenia and enhances prefrontal dopamine (DA) output like other atypical, but not typical, antipsychotic drugs (APDs). Clinical data also suggest an improved effect of typical APDs in schizophrenia by adjunctive treatment with low doses of 3,4-dihydroxyphenylalanine (L-dopa), but experimental support is scarce, and the underlying mechanisms are poorly understood.

METHODS

Antipsychotic efficacy of the D2 antagonist raclopride with or without adjunctive treatment with a low dose of L-dopa was assessed with the conditioned avoidance response paradigm. Extrapyramidal side effects were scored by the catalepsy test. Finally, in vivo microdialysis was used to measure DA efflux in the prefrontal cortex and the nucleus accumbens.

RESULTS

A low dose of L-dopa (3 mg/kg) combined with benserazide, an inhibitor of peripheral DOPA decarboxylase, significantly enhanced the antipsychotic-like effect of raclopride without any associated catalepsy. L-dopa/benserazide alone had no effect. In contrast to raclopride alone, combined L-dopa/raclopride also produced a much larger increase in DA output in the prefrontal cortex than in the nucleus accumbens.

CONCLUSIONS

These data support the clinical observation that low-dose L-dopa might improve the effect of typical APDs in schizophrenia and indicate that increased prefrontal DA output per se enhances the antipsychotic effect of typical APDs.

A meta-analysis of the response to chronic L-dopa in patients with schizophrenia: therapeutic and heuristic implications

RATIONALE

While it is generally believed that administration of the dopamine precursor levodopa ( L-dopa) exacerbates symptoms of schizophrenia, numerous reports suggest that adjunctive L-dopa may be beneficial. This body of literature has not been critically reviewed.

OBJECTIVES

On the basis of published studies, to determine whether L-dopa administered concomitantly with antipsychotic drugs provides a beneficial response in patients with schizophrenia.

METHODS

This review examined 30 studies involving 716 patients. Due to wide methodological variability and limited statistical information, only five studies encompassing 160 patients could be included in a meta-analysis. The others were evaluated qualitatively.

RESULTS

When L-dopa was added to antipsychotic drugs, the overall improvement was moderate ( d=0.71) and highly significant ( P<0.0001). There were 16 other studies in which L-dopa was added to antipsychotic drugs, but which did not meet criteria for inclusion in the meta-analysis. In these, worsening occurred in less than 20% of patients; the percentage of improved patients varied widely but had a central tendency around 50%.

CONCLUSIONS

. In patients already on antipsychotic drugs, the addition of L-dopa can be beneficial. Dopamine agonists merit further consideration as adjuncts to antipsychotic drugs in the treatment of schizophrenia.

Pharmacotherapy for refractory schizophrenia patients

Most schizophrenic patients experience morbidity over the course of their illness, as the illness runs a chronic course and full remissions are infrequent. Therefore, defining treatment resistance among schizophrenia is problematic. Not all patients respond to antipsychotic medication treatment and an estimated 30-50% are considered resistant to treatment. Treatment resistance normally occurs along a continuum and most patients manifest varying degrees of resistance to antipsychotic medications. Essock and colleagues discovered that more than 60% of the patients in state hospitals met the criteria for clozapine therapy and, therefore, they may qualify for treatment resistance.

Free radical mechanisms in schizophrenia and tardive dyskinesia

The present article discusses the distribution of free radical processes in the central nervous system (CNS). Specifically, we discuss the involvement of oxyradicals in the normal metabolism of catecholamine. We also review some proposals related to the possible importance of these compounds in the development of neuropsychiatric and movement disorders such as schizophrenia and neuroleptic-induced tardive dyskinesia (TD), respectively. Clinical studies have shown that antioxidant treatment can attenuate the movement abnormalities observed in TD. Further studies are necessary to evaluate the status of specific scavenging systems in these two disorders. The prophylactic use of antioxidants in patients who are treated with neuroleptics needs also to be considered.

L-dopa helps positive but not negative features of neuroleptic-insensitive chronic schizophrenia

L-dopa (Sinemet-110 in a final dose equivalent to - 4 g per day) added to maintenance chlorpromazine, produced a small antipsychotic effect in a group of eight severely impaired male chronic schizophrenic in-patients. Negative symptoms were unaffected by L-dopa, although the improvement in psychotic behaviour and positive symptoms was restricted to the four patients with the most severe negative symptoms measured during the control treatment period. These L-dopa responders also tended to improve slightly when the dose of chlorpromazine was halved, an indication of their poor, or even counter-therapeutic response to conventional neuroleptic medication given in relatively high dosage. Signs of increased dopaminergic activity (raised eye blink rate and reduced plasma prolactin) were not observed in subjects showing an antipsychotic response to L-dopa. This raises the possibility that L-dopa may exert an antipsychotic effect in neuroleptic-insensitive subjects by altering noradrenergic activity in the brain.

Effects of mianserin in chronic schizophrenia

1. The efficacy of mianserin as a supplement in treating chronic schizophrenia was tested by monitoring the BPRS and plasma monoamine metabolites. 2. Twenty inpatients with schizophrenia were administered fixed doses of neuroleptics throughout the study. 3. A control BPRS scoring and blood sampling were done before mianserin administration. 4. Fixed doses of 60 mg/day of mianserin for 2 weeks and flexible doses for 4 weeks were given orally in an open study for 6 consecutive weeks, and no treatment followed for 1 additional week. 5. BPRS scoring was carried out once weekly, and blood samples were obtained after mianserin treatment. 6. Both total BPRS scores and scores for negative symptoms were decreased by mianserin treatment as compared with the control values. 7. 5-HIAA concentrations of both responding patients and nonresponding patients to mianserin were increased after medication; however, 5-HIAA values of responding patients were lower than those of nonresponding patients. 8. HVA concentrations of the responding group were slightly increased by mianserin administration. 9. There were no significant changes in MHPG levels between the two groups. 10. These results suggest that the negative symptoms of schizophrenia are partly improved by mianserin treatment.

Bromocriptine for "negative" schizophrenia

The hypothesis that the pathophysiology of negative symptoms in schizophrenia may involve relative hypoactivity of central dopaminergic neurotransmission prompts the exploration of dopamine agonist strategies in the treatment of this condition. Although the use of dopamine agonists in otherwise unmedicated schizophrenic patients often leads to the exacerbation of psychosis, trials of dopamine agonists in combination with neuroleptic agents warrant investigation. We therefore report on open clinical experience involving six patients with chronic negative symptoms of schizophrenia, maintained on neuroleptic medication, who appeared to have favorable responses to the addition of moderate doses of bromocriptine (10 to 20 mg/d orally in divided doses). One particular factor that makes these trials potentially informative is that five of the six patients had failed to respond to standard treatments with anticholinergic antiparkinsonian medication before the bromocriptine trial, making it unlikely that the bromocriptine had its effect purely by counteracting neuroleptic-induced akinesia. A trial of bromocriptine under these circumstances has never been reported. A second unique feature of this report concerns the lengthy period of follow-up. Adjunctive bromocriptine was continued for a total of 27 patient-years in the six individuals, with maintenance of favorable course and minimal incidence of psychotic exacerbation.

Mechanisms of action of atypical antipsychotic drugs. Implications for novel therapeutic strategies for schizophrenia

The mechanisms which contribute to the actions of atypical antipsychotic drugs, such as clozapine and the putative atypical agents remoxipride and raclopride, are reviewed. Examination of available preclinical and clinical data leads to two hypotheses concerning the mode of action of atypical antipsychotic drugs. The first hypothesis is that antagonism of the dopamine D2 receptor is both necessary and sufficient for the atypical profile, but that interaction with subtypes of the D2 receptor differentiates typical from atypical antipsychotic drugs. The second hypothesis has been previously advanced, and suggests that a relatively high ratio of serotonin 5-HT2:dopamine D2 receptor antagonism may subserve the atypical profile. It seems likely that the atypical antipsychotic drug profile may be achieved in more than one way.

L-dopa in the treatment of negative schizophrenic symptoms: a single-subject experimental study

Adjunctive Sinemet (L-DOPA plus carbidopa) therapy was assessed for a neuroleptic nonresponsive schizophrenic with a longstanding negative syndrome. A twenty-seven week double-blind placebo-controlled reversal design found significant improvement specifically for negative symptoms while treated with combined haloperidol-Sinemet as compared against haloperidol-placebo. The eight-week adjunctive treatment reversed a worsening trend in attention, abstract thinking, passive withdrawal, psychomotor retardation, and a cluster of seven negative parameters, while positive symptoms were unaffected. This seemed to represent more than simple reversal of drug-induced akinesia and underscores the importance of distinguishing between positive and negative syndromes in psychopharmacological research.