Metrifonate for Alzheimer's disease

BACKGROUND

Metrifonate is a long-acting irreversible cholinesterase inhibitor, originally used to treat schistosomiasis. Its potential to enhance central nervous system cholinergic neurotransmission led to clinical trials for the treatment of people with Alzheimer's disease (AD). Although low incidence of serious side effects occurred during short-term use as an antihelmintic, in studies of the treatment of AD extending over 6 months, 20 patients experienced respiratory paralysis and problems with neuromuscular transmission. These findings have led to a halt to trials of metrifonate for AD and Bayer, the pharmaceutical company, has withdrawn its FDA application.

OBJECTIVES

1) To establish the efficacy of metrifonate for patients with Alzheimer's disease, in terms of cognition, global impression, functional activity, non cognitive symptoms, rate of institutionalization and mortality.2) Assess the safety and tolerability of metrifonate.

SEARCH STRATEGY

The Cochrane Dementia and Cognitive Improvement Group's Specialized Register was searched on 5 December 2005 using the term metrifonat*. This Register is regularly updated with records from all major health care databases (MEDLINE, EMBASE, CINAHL, PsycINFO) and many trials databases. One of the authors (LS), as member of the Metrifonate Study Group has had the opportunity to contact other metrifonate trialists to obtain data from potentially non published data of metrifonate clinical trials.

SELECTION CRITERIA

All unconfounded, randomized double-blind clinical controlled trials comparing metrifonate to placebo in people with AD.

DATA COLLECTION AND ANALYSIS

Data were extracted by the two reviewers, cross-checked, and pooled when appropriate and possible.

MAIN RESULTS

Most studies assessed changes in cognitive function, global function, activities of daily living, behavioural problems, severity of disease and adverse events. Occasionally the results were not reported in sufficient detail to allow extraction of data for the meta-analyses. The treatment regimens were varied: loading doses were used in some trials. The range of maintenance doses and studies were not pooled unless the treatment regimens were considered comparable. The lengths of treatment varied from 6 to 26 weeks and studies were not pooled unless the treatment duration was similar. The results are derived from the ITT populations. Metrifonate at various doses, fixed and loading doses, was associated with significant cognitive improvement compared to placebo, except for weekly doses where there was no difference from placebo: MMSE (metrifonate 60-80 mg/day with initial loading at 26 weeks; metrifonate 50 mg/day fixed dose with no initial loading at 26 weeks MD 1.85, 95% CI 1.06 to 2.64, p<0.00001); ADAS-Cog (metrifonate 60-80 mg/day with initial loading at 26 weeks MD -3.24, 95% CI -4.40 to -2.08, p<0.00001)In most trials, there was improvement in clinical global impression: CIBIC-Plus (metrifonate 30-55 mg/day, approximately 0.65 mg/kg body weight, with initial loading at 26 weeks MD -0.25, 95% CI -0.41 to -0.09 p=0.002; metrifonate 50 mg/day fixed dose with no initial loading at 26 weeks MD -0.20, 95% CI -0.39 to -0.01, p=0.04). There were generally-significant drug-placebo differences in activities of daily living but this often depended on sample size and the characteristics of the instrument used: DAD (metrifonate 30-55 mg/day, 0.65 mg/kg body weight, with initial loading at 26 weeks MD 2.72, 95% CI 0.66 to 4.77, p=0.01; metrifonate 50 mg/day fixed dose with no initial loading at 26 weeks MD 4.07, 95% CI 0.29 to 7.85, p=0.03)Also there were differences associated with metrifonate compared with placebo for different doses of metrifonate in scores on a behavioural symptom scale, caregiver burden scale, and severity of disease scale. Adverse events occurring more often with metrifonate included abdominal pain, bloating, bradycardia, diarrhoea, leg cramps, nausea and rhinitis and were described as mostly mild and transient, but occasionally moderately severe, and infrequently severe and serious. Analysis of the number of patients suffering at least one mild, moderate, severe or serious adverse event before the end of treatment showed that there was usually no difference between placebo and metrifonate.

AUTHORS' CONCLUSIONS

Metrifonate given once per day appears to be related to clinical response in cognition, global improvement, and activities of daily living in patients with mild to moderate Alzheimer's disease. Tolerability is good with adverse events as expected from a cholinesterase inhibitor, but with a low incidence of neuromuscular dysfunction and respiratory failure, too low to be detected in this review. It has been withdrawn from further development.

Clinically significant drug interactions with cholinesterase inhibitors: a guide for neurologists

Cholinesterase inhibitors are the only pharmacological class indicated for the treatment of mild to moderate Alzheimer's disease. These drugs are also being used off label to treat severe cases of Alzheimer's disease or vascular dementia and other disorders. The widespread use of cholinesterase inhibitors raises the possibility of their use in combination regimens, with the subsequent risk of deleterious drug-drug interactions in high-risk populations. The purpose of this review is to present the possible sources of pharmacokinetic or pharmacodynamic drug-drug interactions involving cholinesterase inhibitors. The four cholinesterase inhibitors (tacrine, donepezil, rivastigmine and galantamine) that are currently available have different pharmacological properties that expose patients to the risk of several types of drug interactions of nonequivalent clinical relevance. The principal proven clinically relevant drug interactions involve tacrine and drugs metabolised by the cytochrome P450 (CYP) 1A2 enzyme, as well as tacrine or donepezil and antipsychotics (which results in the appearance of parkinsonian symptoms). The bioavailability of galantamine is increased by coadministration with paroxetine, ketoconazole and erythromycin. It is of interest to note that because rivastigmine is metabolised by esterases rather than CYP enzymes, unlike the other cholinesterase inhibitors, it is unlikely to be involved in pharmacokinetic drug-drug interactions. Care must be taken to reduce the risk of inducing central (excitation, agitation) or peripheral (e.g. bradycardia, loss of consciousness, digestive disorders) hypercholinergic effects via drug interactions with cholinesterase inhibitors. A review of the literature does not reveal any alarming data but does highlight the need for prudent prescription, particularly when cholinesterase inhibitors are given in combination with psychotropics or antiarrhythmics. Possible interactions involving other often coprescribed antidementia agents (e.g. memantine, antioxidants, cognitive enhancers) remain an open area requiring particularly prudent use.

Development, validation and application of assays to quantify metrifonate and 2,2-dichlorovinyl dimethylphosphate in human body fluids

Gas chromatographic procedures [GC with electron-capture detection (ECD) and GC-MS] for the quantitative analysis of metrifonate and its active metabolite 2,2-dichlorovinyl dimethylphosphate (DDVP) in human blood and urine were developed, validated, and applied to the analysis of clinical study samples. Analysis of metrifonate involved extraction of acidified blood with ethyl acetate followed by solid-phase clean-up of the organic extract. Acidified urine was extracted with dichloromethane and the residue of evaporated organic phase was reconstituted in toluene. ECD and diethyl analogue of metrifonate internal standard (I.S.) were used for quantitation of metrifonate. The metrifonate lower limit of quantitation (LOQ) was 10.0 microg/l. The DDVP metabolite was chromatographed separately after cyclohexane extraction of acidified blood and urine using d6-DDVP I.S. and MS detection. The LOQ of DDVP was 1 microg/l. Stability studies have confirmed that the matrix should be acidified prior to storage at -20 degrees C or -80 degrees C to inhibit chemical and enzymatic degradation of the analytes and to avoid overestimation of DDVP concentrations. Metrifonate was found to be stable in acidified human blood after 20 months of storage at -20 degrees C and after 23 months of storage at -80 degrees C. Under these conditions DDVP was found to be stable after 12 months of storage. Both assay procedures were cross-validated by different world-wide laboratories and found to be accurate and robust during analyses of clinical study samples.