Maintenance N-acetyl cysteine treatment for bipolar disorder: A double-blind randomized placebo controlled trial

Bipolar disorder is a recurrent illness with the vast majority of individuals experiencing relapses throughout their lives. The prevention of further episodes is of critical importance to individuals with the disorder, as recurrent episodes can result in hospitalization, suicide and loss of functionality. There appears to be an active process of neuroprogression associated with acute episodes of illness [1]. Maintenance of well-being is, therefore, of paramount importance [2]. Existing agents are imperfect, as many have limitations in terms of either efficacy or tolerability for long-term treatment. Lithium is the mainstay of prophylaxis in bipolar disorder although it is more effective in preventing manic relapses than depression [3] though recent data suggest that it is more effective in relapse prevention than valproate [4]. Interestingly, lithium is more effective in preventing manic relapses as opposed to depression whereas lamotrigine is more effective in the prevention of depressive episodes [3]. Atypical antipsychotics also appear to have maintenance properties, although all except quetiapine are less effective in depression than mania. Given the limitations of these agents, polypharmacy is the routine rather than the exception [5] and most have significant tolerability issues that require routine safety monitoring [6]. In this context there are preliminary data from preclinical studies that N-acetyl cysteine (NAC) might prevent lithium-induced renal dysfunction in animal models. This makes it an attractive adjunct therapy both because of its potential clinical benefits and the reduction of iatrogenic adverse effects [7].

This is particularly interesting because it is consistent with evidence of dysregulated redox biology in bipolar disorder. Data supporting this comes from five main areas; i) evidence of dysregulated oxidative defenses, ii) effects of oxidative stress on cellular constituents (particularly lipids, proteins and nuclear and mitochondrial DNA), iii) concordant structural evidence of neuroprogressive processes, iv) studies showing that established bipolar disorder treatments have significant influences on oxidative processes, and v) association studies of polymorphisms of key genes in the glutathione pathway [8]. In particular, glutathione, which is the principal endogenous antioxidant in the brain, is vulnerable to depletion, and is substantially reduced in bipolar disorder [9]. NAC provides L-cysteine, the rate limiting factor in glutathione synthesis, and thereby increases central and peripheral glutathione [10]. Additionally, NAC modulates glutamate, has anti-inflammatory properties and enhances neurogenesis and mitochondrial function [11].

Given this context, the aim of this study was to investigate the efficacy of adjunctive NAC, in addition to treatment as usual, in the maintenance treatment of bipolar depression in a double-blind randomized multi-center placebo-controlled trial. Time to intervention for a mood episode was the primary outcome measure. Secondary outcome measures included changes in mood symptoms, functioning and quality of life (QoL). It was hypothesized that NAC would reduce the recurrence of episodes in the maintenance phase of the disorder.

This maintenance study included participants screened for the presence of depression at trial entry (beginning of the open-label phase) [12]. All participants received 2 grams of NAC (1 gram twice daily) for eight weeks and were subsequently randomized to continued NAC treatment or placebo in a double blind design for a further 24 weeks. The assessment schedule is shown in Additional File 1. In addition to the clinical interviews, some participants provided blood samples for peripheral analysis of oxidative stress markers and a sub-group was involved in a magnetic resonance spectroscopy study (data presented elsewhere). In order to reduce enrichment bias, response to NAC in the open label phase was not an inclusion criterion, and all participants proceeded to the randomized phase (week 8 to week 32). Methodological details and data from the open-label phase of the study are presented elsewhere [13]. All participants remained on treatment as usual for the duration of the trial. This included any pharmacological or psychological intervention (stable for at least one month as per the inclusion criteria). In order to capture the diversity of treatment settings and enhance generalizability, potential participants were recruited through a variety of avenues, including the participants' case clinicians, newspaper advertisement, flyers in public areas (including flyers placed at shopping centers and pathology collection centers) and web-based advertisements on bipolar disorder-relevant websites, as well as referral from private clinicians including family physicians and specialists. The trial was approved by relevant research and ethics committees (Barwon Health, Bendigo Health and the University of Melbourne, in Victoria, Australia; Royal North Shore Hospital, Sydney, Australia, and Porto Allegre, Brazil) and was conducted in accordance with the Helsinki Declaration as revised in 1989. A preliminary interview was conducted with potential participants to obtain written informed consent and assess inclusion and exclusion criteria, following which the trial proper commenced.

Individuals who had given written consent were assigned, using computer-generated block randomization (in blocks of four) [14], to treatment with NAC or placebo in addition to treatment as usual, in a double-blind fashion. The nature and dose of the primary therapy was monitored. The person generating the randomization schedule was not involved in any aspect of participant interview or data analysis. The investigators, clinicians and statisticians were blind to treatment allocation. The study was registered with the Australian and New Zealand Clinical Trials Registry (registration # 12607000074493). The trial was completed between 2007 and 2010.

NAC was acquired from Zambon Fine Chemicals, Bresso, Italy. Purity was 99.8% as determined by high performance liquid chromatography. Encapsulation of both the NAC and the identical placebo capsules was done by DFC Thompson, Sydney, Australia. Study medication was sealed in identical bottles, labeled as trial medication, and both dispensed and returned by the pharmacy, so that the investigators were not exposed to the contents of the bottles. It is important to note that NAC has a characteristic odor and so to reduce the risk of unblinding on transition to the double blind phase, placebo capsules were dusted with microgram amounts of NAC to capture the distinct odor. Participants additionally were seen separately, minimizing the opportunity to compare experiences.

Although there was a robust decrease in symptoms in the open-label phase of the trial using 2 grams daily of NAC, there were no significant changes in clinical, functioning and QoL measures in the maintenance phase (see Figure 3 for the changes in mean estimates for the BDRS over both the open-label and maintenance phase of the trial). The improvements in depressive symptoms reached a plateau in the open-label phase and symptoms changed little from this very low base in the randomized phase, such that between group differences did not emerge.

These results are not concordant with our previous data examining NAC in bipolar disorder [31]. In a 24-week double blind placebo controlled trial (n = 75) adjunctive NAC was trialled in participants with either bipolar I or II disorder [32]. In this study NAC significantly improved clinical outcomes, particularly depression, quality of life and measures of functioning, with large effect sizes in almost all domains. In that trial, between group differences at endpoint were no longer evident four weeks after treatment discontinuation. In addition, benefits of NAC in other psychiatric disorders, including a large clinical trial of schizophrenia [8], and small scale clinical papers in obsessive-compulsive disorder [33] and compulsive disorders [9, 33, 34], pathological gambling [35] and cocaine dependence [36] have been reported.

NAC reverses models of glutathione depletion, increasing peripheral [37] and brain glutathione [38]. There appears to be reduced neurogenesis in mood disorders, and antidepressants and mood stabilizers enhance neurogenesis [39]. It is noteworthy in this regard that NAC enhances neurogenesis of neuronal stem cells. NAC promotes neuronal survival after injury and has anti-inflammatory effects. In the forced swim test, NAC results in a significant decrease in immobility. A neuroprotective effect of NAC has been suggested by effects in a variety of neurodegenerative disease models and lastly NAC appears to reverse mitochondrial dysfunction, which is concordant with the emerging literature on the role of mitochondria in bipolar disorder.

The characteristics of this study need to be mentioned in order to contextualize these results. The multi-center, randomized, placebo controlled design provides an appropriate forum for evaluation of efficacy in the continuation phase. The sample size, while ample for the secondary outcome measures, was marginal for the ability to evaluate recurrence, which only occurred in a subset of participants. A larger sample size and a longer length of the randomized phase, concordant with similar recent designs, would have increased the signal to noise ratio in this regard. Other studies have used responder analysis to determine benefits during the maintenance phase; these trials only include those who respond in the open-label phase for analysis of the maintenance phase. While not an a priori analysis, this was investigated in the current study with no change in the outcomes reported (data not shown). The absence of significant restrictions on comorbid diagnosis reflects the extent of comorbidity in the disorder; similarly, the absence of restrictions regarding concomitant therapy also enhances the generalizability of the data, as does the inclusion of bipolar I-, II- and NOS - disorder participants.

In conclusion, there was a substantial decrease in symptoms during the eight-week open label phase. During the subsequent double blind phase, there was a very low baseline reflecting improvement across all symptomatic measures, and this did not change significantly in either group. As a consequence, between group differences could not emerge on recurrence, clinical functioning and QoL measures. This lack of a workable symptomatic substrate for the emergence of a signal in the double blind phase suggests that this study could possibly be seen as a failed rather than negative study. As such, the formal maintenance efficacy of NAC in bipolar disorder remains an open question.