Identifying treatment effects using trimmed means when data are missing not at random.

Patients often discontinue from a clinical trial because their health condition is not improving or they cannot tolerate the assigned treatment. Consequently, the observed clinical outcomes in the trial are likely better on average than if every patient had completed the trial. If these differences between trial completers and non-completers cannot be explained by the observed data, then the study outcomes are missing not at random (MNAR). One way to overcome this problem-the trimmed means approach for missing data due to study discontinuation-sets missing values as the worst observed outcome and then trims away a fraction of the distribution from each treatment arm before calculating differences in treatment efficacy (Permutt T, Li F. Trimmed means for symptom trials with dropouts. Pharm Stat. 2017;16(1):20-28). In this paper, we derive sufficient and necessary conditions for when this approach can identify the average population treatment effect. Simulation studies show the trimmed means approach's ability to effectively estimate treatment efficacy when data are MNAR and missingness due to study discontinuation is strongly associated with an unfavorable outcome, but trimmed means fail when data are missing at random. If the reasons for study discontinuation in a clinical trial are known, analysts can improve estimates with a combination of multiple imputation and the trimmed means approach when the assumptions of each hold. We compare the methodology to existing approaches using data from a clinical trial for chronic pain. An R package trim implements the method. When the assumptions are justifiable, using trimmed means can help identify treatment effects notwithstanding MNAR data.

A Longitudinal Item Response Theory Model to Characterize Cognition Over Time in Elderly Subjects.

For drug development in neurodegenerative diseases such as Alzheimer's disease, it is important to understand which cognitive domains carry the most information on the earliest signs of cognitive decline, and which subject characteristics are associated with a faster decline. A longitudinal Item Response Theory (IRT) model was developed for the Basel Study on the Elderly, in which the Consortium to Establish a Registry for Alzheimer's Disease - Neuropsychological Assessment Battery (with additions) and the California Verbal Learning Test were measured on 1,750 elderly subjects for up to 13.9 years. The model jointly captured the multifaceted nature of cognition and its longitudinal trajectory. The word list learning and delayed recall tasks carried the most information. Greater age at baseline, fewer years of education, and positive APOEɛ4 carrier status were associated with a faster cognitive decline. Longitudinal IRT modeling is a powerful approach for progressive diseases with multifaceted endpoints.

Efficacy of rivastigmine in Alzheimer's disease patients with rapid disease progression: results of a meta-analysis.

BACKGROUND: Alzheimer's disease (AD) patients experiencing more rapid symptom progression are likely to have a poorer prognosis than those experiencing slow symptom progression. In a recent retrospective analysis, treatment effects of rivastigmine were more pronounced in AD patients with rapid cognitive decline than in those with slow cognitive decline. This warranted further investigation. METHODS: Rapidly and slowly progressing patients were identified by rates of cognitive decline [>/=4 points and <4 points, respectively, on the Alzheimer's Disease Assessment Scale -- cognitive subscale (ADAS-cog)] during 26 weeks of placebo treatment in four randomized controlled trials (weeks 0--26). This meta-analysis evaluated rates of cognitive decline in both subgroups during subsequent open-label rivastigmine 26-week extension studies (weeks 26--52). A longitudinal mixed effects model compared cognitive decline in rapidly and slowly progressing patients, including correction for possible regression to the mean. RESULTS: 180 (75%) rapidly and 337 (78%) slowly progressing patients provided ADAS-cog data after 26 weeks of open-label rivastigmine treatment. Improvements in cognitive symptoms were observed during the first 12 weeks, which were more pronounced in patients with rapid progression than in those with slow progression. Rapidly progressing patients experienced significantly greater cognitive benefits than slowly progressing patients (p=0.029), who experienced a modest decline in cognitive symptoms at the end of the study. COMMENT: Patients experiencing rapid symptom progression may receive greater benefit from rivastigmine than those with slow progression. In this study, cholinesterase inhibition appeared to be of particular utility in the management of AD patients whose symptoms were rapidly worsening.

Rivastigmine for dementia associated with Parkinson's disease.

BACKGROUND: Cholinergic deficits are prominent in patients who have dementia associated with Parkinson's disease. We investigated the effects of the dual cholinesterase inhibitor rivastigmine in such patients. METHODS: Patients in whom mild-to-moderate dementia developed at least 2 years after they received a clinical diagnosis of Parkinson's disease were randomly assigned to receive placebo or 3 to 12 mg of rivastigmine per day for 24 weeks. Primary efficacy variables were the scores for the cognitive subscale of the Alzheimer's Disease Assessment Scale (ADAS-cog) and Alzheimer's Disease Cooperative Study-Clinician's Global Impression of Change (ADCS-CGIC). Secondary clinical outcomes were the scores for the Alzheimer's Disease Cooperative Study-Activities of Daily Living, the 10-item Neuropsychiatric Inventory, the Mini-Mental State Examination, Cognitive Drug Research power of attention tests, the Verbal Fluency test, and the Ten Point Clock-Drawing test. RESULTS: A total of 541 patients were enrolled, and 410 completed the study. The outcomes were better among patients treated with rivastigmine than among those who received placebo; however, the differences between these two groups were moderate and similar to those reported in trials of rivastigmine for Alzheimer's disease. Rivastigmine-treated patients had a mean improvement of 2.1 points in the score for the 70-point ADAS-cog, from a baseline score of 23.8, as compared with a 0.7-point worsening in the placebo group, from a baseline score of 24.3 (P<0.001). Clinically meaningful improvements in the scores for the ADCS-CGIC were observed in 19.8 percent of patients in the rivastigmine group and 14.5 percent of those in the placebo group, and clinically meaningful worsening was observed in 13.0 percent and 23.1 percent, respectively (mean score at 24 weeks, 3.8 and 4.3, respectively; P=0.007). Significantly better outcomes were seen with rivastigmine with respect to all secondary efficacy variables. The most frequent adverse events were nausea (affecting 29.0 percent of patients in the rivastigmine group and 11.2 percent of those in the placebo group, P<0.001), vomiting (16.6 and 1.7 percent, P<0.001), and tremor (10.2 and 3.9 percent, P=0.01). CONCLUSIONS: In this placebo-controlled study, rivastigmine was associated with moderate improvements in dementia associated with Parkinson's disease but also with higher rates of nausea, vomiting, and tremor.

Disease stage in Alzheimer disease and treatment effects of rivastigmine.

The efficacy and tolerability of the cholinesterase inhibitor rivastigmine in the treatment of Alzheimer disease (AD) have been demonstrated in several clinical trials, which included patients with a wide range of dementia severities. To investigate the association between severity of disease and treatment response, the combined data from three large randomized, placebo-controlled trials were analyzed. The pooled patient population was stratified into three cohorts showing moderately severe (Mini-Mental State Examination score [MMSE] < or = 15), moderate (MMSE 16-22), and mild (MMSE > or = 22) dementia. In each cohort, the effects of rivastigmine 6 to 12 mg/day versus placebo were evaluated using the cognitive subscale of the Alzheimer Disease Assessment Scale (ADAS-cog) and the Progressive Deterioration Scale. Rivastigmine 6 to 12 mg/day maintained ADAS-cog scores at or above placebo levels in all cohorts, while cognitive deterioration with placebo was progressive and severity dependent. Activities of daily living showed statistically significant benefits with rivastigmine across all severity cohorts.