A pilot study of oral physostigmine plus yohimbine in patients with Alzheimer disease

Yohimbine: a clinical review

Although yohimbine (YOH) has been available for the treatment of male erectile dysfunction (ED) for longer than Viagra, there is a perception that little is known about the clinical performance of the drug. This review attempts, by comprehensive analysis of the literature, to cover the clinical, pharmacological, and therapeutic profiles of YOH, relevant to its potential utility in the management of patients with ED. Relatively few well-designed studies have been completed. From these, however, it can be concluded that YOH as monotherapy possesses only modest efficacy in ED patients. In acute and chronic (long-term) studies, YOH has been found to be relatively free of side effects over the dose range predicted to be effective in ED. At much higher doses, the most frequently observed effects, consistent with the primary pharmacological action of the drug, are elevation of blood pressure, a slight anxiogenic action, and increased frequency of urination. These side effects are all easily reversible on termination of YOH therapy. There is increasing evidence that the erectogenic action of YOH can be augmented by concomitant administration of agents that augment the release and/or action of nitric oxide in the corpus cavernosum. YOH has yet to be studied in female sexual dysfunction. Overall, the benefit risk profile of YOH would indicate that it has potential, more probably as part of a combination strategy, e.g., with a drug that enhances the nitric oxide pathway, in the treatment of ED.

Physostigmine for Alzheimer's disease

BACKGROUND

The main pharmacological approach for the treatment of Alzheimer's disease (AD) has been based on the use of agents potentiating cholinergic transmission, particularly by inhibiting acetylcholinesterase (AChE), the enzyme that destroys acetylcholine after it has been secreted into the synaptic clefts. Physostigmine is an AChE inhibitor originally extracted from calabar beans. It is licensed in many countries as an agent for reversing the effect of drugs and poisons causing the anticholinergic syndrome. Studies conducted more than 20 years ago suggested that physostigmine could improve memory in people with or without dementia. Investigation of this property has been limited by the very short half-life of physostigmine. Various forms of administering the drug have been tried to overcome this problem, most recently a controlled-release (CR) oral formulation, and a skin patch. It has been proposed as a potential drug for the symptomatic treatment of AD.

OBJECTIVES

To determine whether there is evidence of beneficial effects for the use of physostigmine in Alzheimer's disease. To assess the incidence and severity of adverse effects.

SEARCH STRATEGY

The Cochrane Controlled Trials Register was searched using the following terms: 'physostigmine', 'physostigmine salicylate', 'Synapton' and 'Antilirium' in accordance with the Cochrane Dementia and Cognitive Improvement Group's search strategy. The pharmaceutical company was contacted.

SELECTION CRITERIA

All relevant unconfounded, double-blind, randomized, placebo-controlled trials in which physostigmine was administered for more than one day to patients with dementia of Alzheimer type. Trials in which the allocation to the treatment was not randomized, or in which the allocation to the treatment was not concealed were excluded.

DATA COLLECTION AND ANALYSIS

Data were extracted independently by two reviewers (JMC & JB), pooled where appropriate and possible, and the weighted or standardized mean differences or Peto odds ratios (95% CI) were estimated. Where possible, intention-to-treat analysis was used.

MAIN RESULTS

Fifteen studies were included using four different methods of administration of physostigmine. Four studies, involving 29 people in total, used intravenous infusion; seven, involving 131 people, used a conventional oral form; four, involving 1456 participants, used a controlled-release oral form, and one study of 181 people used a verum skin patch. There are no usable results from the intravenous infusion trials, and the few results from the conventional oral form showed no benefit of physostigmine compared with placebo. The results from two of the four studies of the controlled-release physostigmine apply only to a group of patients identified as responders in a pre-randomization titration period. The best dose physostigmine (mean 25mg/day) was associated with a 1.75 point improvement on ADAS-Cog score (mean difference -1.75, 95% confidence interval -2.90, -0.60 on an intention-to-treat basis) and a 0.26 point improvement on the CGIC score (treated as a continuous scale) (mean difference -0.26, 95% confidence interval 0.06, 0.46 on an intention-to-treat basis) compared with placebo at 6 weeks. There were statistically significantly higher numbers of patients from the physostigmine group withdrawing from the trial (22/183 vs 2/183)(OR 5.92, 95% confidence limits 2.59, 13.54) and suffering at least one event of nausea, vomiting, diarhoea, anorexia, dizziness, stomach pain, flatulence or sweating compared with placebo at 6 weeks. The best dose physostigmine (mean 27mg/day) was associated with a 2.0 point improvement on ADAS-Cog score (mean difference -2.02, 95% confidence interval -3.59, -0.45 on an intention to treat basis) compared with placebo at 12 weeks. There were statistically significantly higher numbers of patients from the physostigmine group withdrawing from the trial due to adverse events (13/83 vs 5/93)(OR 3.05, 95% confidence limits 1.15, 8.07) and suffering at least one event of nausea, vomiting, diarhoea, anorexia, dizziness, stomach pain, tremor, asthenia or sweating compared with placebo at 12 weeks. When no attempt was made to identify responders and all relevant patients with Alzheimer's disease were randomized, fixed dose physostigmine (mean 33 mg/day) was associated with a statistically significantly higher number withdrawing (234/358 vs 31/117)(OR 4.82, 95% confidence limits 3.17, 7.33), withdrawing due to adverse events (196/358 vs 10/117) (OR 6.54, 95%confidence limits 4.29, 9.95) and suffering at least one event of nausea, vomiting, diarhoea, anorexia, dizziness, stomach pain, dyspepsia, sweating, asthenia, dyspnoea or abnormal dreaming compared with placebo at 24 weeks. The results from the study of the verum patch physostigmine show that the double dose (delivering mean dose 12mg/day) was associated with statistically significantly higher numbers suffering at least one adverse event of vomiting, nausea or abdominal cramps compared with placebo at 24 weeks, but placebo was associated with statistically significantly greater numbers of gastrointestinal complaints at 24 weeks compared with single-dose physostigmine.

REVIEWERS' CONCLUSIONS

The evidence of effectiveness of physostigmine for the symptomatic treatment of Alzheimer's disease is limited. Even in a controlled release formulation designed to overcome the short half-life, physostigmine showed no convincing benefit and adverse effects remained common leading to a high rate of withdrawal.

Reappraising neurotransmitter-based strategies

A number of observations support the hypothesis that a central deficit in acetylcholine (ACh) may be responsible for the initiation of Alzheimer's disease (AD). For example, cholinergic innervation in AD is reduced in areas of the brain important for processing information. Further, reduced concentrations of choline acetyltransferase (ChAT), the enzyme responsible for the synthesis of ACh, correlate with the number of beta-amyloid senile plaques and cognitive dysfunction in AD patients. Consequently, several strategies to increase cholinergic neurotransmission have been developed, including ACh precursors, ACh release enhancers, cholinesterase (ChE) inhibitors and receptor agonists. Although ChE inhibitors appear to be the most promising, tacrine, the first ChE inhibitor to be registered and approved for the treatment of AD, has significant tolerability problems. Thus, ChE inhibitors with improved side-effect profiles have been developed and subsequently awarded marketing approval. However, in addition to the cholinergic system that is the most severely affected neurotransmitter system in AD, other neurotransmitter systems may be involved (e.g. serotonergic, noradrenergic and glutamatergic). Therefore, bifunctional compounds or combinations of drugs may provide additional therapeutic value.

The cognitive psychopharmacology of Alzheimer's disease: focus on cholinergic systems

The primary pathology in Alzheimer's disease (DAT) occurs in the basal forebrain cholinergic system (BFCS), which provides the major cholinergic innervation to the neocortex, hippocampus and amygdala. Consistent with the 'cholinergic hypothesis' of dementia in DAT, the most effective treatments so far developed for DAT are drugs which act to boost the functions of the BFCS. These include the centrally acting cholinesterase inhibitor tacrine, and the cholinergic agonist nicotine, acute administration of which leads to an improvement in attentional functions, in line with recent animal studies of the role of the BFCS in cognition. We conclude that future research should include the development of more potent, longer-lasting, less toxic cholinergic agents, which appear to be the best candidates for alleviating the cognitive symptomatology of DAT. Such drugs may also be useful in the treatment of a number of other cognitive disorders, including Lewy body dementia, attention deficit/hyperactivity disorder, and schizophrenia.

Cognitive enhancement therapy for Alzheimer's disease. The way forward

Although at present there is no definitive treatment or cure for Alzheimer's disease, different pharmacological strategies are being actively investigated. At present, cholinergic therapy and nootropics and some neuronotrophic agents represent the available approaches to symptomatic treatment of Alzheimer's disease. The use of cholinesterase inhibitors (ChEI) constitutes the best cholinergic approach to increase acetylcholine levels. Available data suggest that about 15 to 40% of Alzheimer's disease patients show a varying degree of cognitive improvement while taking these medications; however, haematological complications (neutropenia or agranulocytosis), together with hepatotoxicity, need to be considered carefully. Recent data suggest that long term administration of nootropics may lead to a significant improvement of cognitive functions in Alzheimer's disease patients compared with untreated individuals, having excellent tolerability. Protocols for the intracerebroventricular administration of neuronotrophic substances are also ongoing. The most promising approaches for the future currently undergoing investigation involve attempts to slow the production of beta-amyloid and/or to inhibit beta-amyloid aggregation. Another rational therapeutic approach would be to inhibit the formation of paired helical filaments (PHF) by increasing and/or modulating the activities of protein phosphatases and kinases. Antioxidant therapy should disrupt or prevent the free radical/beta-amyloid recirculating cascade and the progressive neurodegeneration. Idebenone, a synthetic compound acting as an 'electron trapper' and free radical scavenger, has shown some efficacy in degenerative and vascular dementia; at present, other different molecules having antioxidative properties [lazaroids (21-aminosteroids), pyrrolopyrimidines, nitric oxide blockers, selegiline, some vitamins] are under investigation. Lowering absorption or brain tissue concentrations of aluminium also offers possible therapeutic opportunities for slowing the rate of clinical progression of the disease; in this sense, some evidence exists using the aluminium chelating agent deferoxamine (desferrioxamine). Inflammation also may play a significant pathogenetic role in Alzheimer's disease. As shown by several retrospective analyses, there is an inverse association of anti-inflammatory drug use with the frequency of Alzheimer's disease diagnosis. Consequently, clinical trials using both nonsteroidal and steroidal molecules have been proposed. These lines of pharmacological intervention represent an important premise for future therapeutic strategies capable of counteracting the pathogenesis of Alzheimer's disease.

Treatment of Alzheimer's disease: future directions

Following the introduction of tacrine hydrochloride (Cognex) in the United States and several other countries, researchers are pursuing two broad therapeutic strategies for Alzheimer's disease (AD). The first involves identifying agents or combinations of agents whose actions can compensate for the considerable cerebral damage that has typically occurred by the time the diagnosis of AD is made. Such therapeutic approaches include the development of additional cholinesterase inhibitors, agents that work on the receptors of other systems damaged by the disease process, and anti-inflammatory and immunomodulatory agents. The second and ultimately more promising strategy involves the development of approaches to retard, halt, or even prevent disease progression. Such protective approaches, which depend on the development of more effective methods for predicting and diagnosing AD, include the administration of nerve growth factor and other neurotrophins and the use of pharmacologic or genetic interventions to limit amyloid deposition and the formation of neurofibrillary tangles.