A new oral antipsychotic drug, paliperidone extended-release (ER), has recently been developed and might represent an innovative approach in the treatment of schizophrenia. Paliperidone is 9-hydroxyrisperidone, the chief active metabolite of risperidone. Although paliperidone possesses a pharmacological profile very similar to that of its parent compound, it has many different pharmacokinetic and pharmacodynamic characteristics compared with risperidone (Pani and Marchese, 2009). First, paliperidone ER utilizes an osmotic controlled-release oral delivery system (OROS), resulting in a more stable serum concentration and reduced likelihood of causing unexpected over- or under-dosages due to CYP2D6 genetic variability. Second, paliperidone does not undergo significant hepatic metabolism, and the drug is predominantly excreted by the kidney as an unchanged drug; whereas risperidone is extensively metabolized by the CYP2D6 hepatic enzyme. Third, the off-rate for dissociation from human cloned D2 receptors in tissue culture cells is faster for paliperidone (60 s) compared with risperidone (27 min) (Seeman, 2005). Due to its looser binding to D2 receptors, paliperidone should be associated with a reduced risk of extrapyramidal side effects compared with its parent drug. Fourth, ex vivo studies have indicated that paliperidone injections induce relatively smaller H1 occupancy levels in the brains of animals when compared with similar dosages of risperidone (Schotte, et al. 1995 and 1996). This may contribute to a reduced sedative effect and less weight gain secondary to paliperidone when compared with risperidone. Finally, paliperidone has no relevant affinity toward muscarinic receptors, resulting in the absence of anticholinergic side effects; this is another important benefit compared with risperidone (Schotte, et al. 1996). Therefore, paliperidone would be the drug of choice in young psychotic patients for whom preservation or improvement of cognitive function is critical. All of these properties might be associated with improved efficacy and better tolerability of paliperidone ER compared with risperidone. In support of this view, recent studies (Canuso, et al. 2008 and 2010) demonstrated that switching from risperidone to paliperidone ER resulted in improvements in medication satisfaction, Positive and Negative Syndrome Scale (PANSS) (Kay, et al. 1987) score, and abbreviated Extrapyramidal Symptoms Rating Scale (Chouinard and Margolese, 2005) score. However, to date, no clinical trials have investigated the relative superiority of paliperidone ER over risperidone in terms of effects on cognitive function. Recently, interest in the negative subjective experiences secondary to antipsychotic medications has been renewed, as these experiences are key factors in adherence and clinical outcomes (Awad, 1993; van Putten and May, 1978). Artaloytia et al. (2006) reported that risperidone induced negative symptoms in healthy volunteers, which might be termed secondary negative symptoms. Therefore, we hypothesized that paliperidone ER has a better safety profile in terms of subjective experiences and cognitive function compared with risperidone. Research on the effects of antipsychotic drugs on subjective experiences and cognitive function in patients with schizophrenia may be hampered by numerous confounding factors, such as pathological features of the illness, patient motivation, or concomitant medications. Investigating the effects of antipsychotic drugs in healthy subjects provides a method for controlling these variables. Therefore, the main objective of this study was to assess subjective experiences related to secondary negative symptoms and cognitive performance in healthy volunteers in response to multiple doses of paliperidone ER and risperidone in a double-blind, placebo-controlled trial. Adverse events caused by these drugs were also evaluated.
