Some of the views expressed in the following notes on newly approved products should be regarded as preliminary, as there may have been limited published data at the time of publication, and little experience in Australia of their safety or efficacy. However, the Editorial Executive Committee believes that comments made in good faith at an early stage may still be of value. Before new drugs are prescribed, the Committee believes it is important that more detailed information is obtained from the manufacturer's approved product information, a drug information centre or some other appropriate source.
1 mg, 2 mg, 3 mg and 4 mg film-coated tablets
Benzisoxazole derivatives such as risperidone represent a new class of antipsychotic drugs. Risperidone is an antagonist at serotonin (5-HT2) and dopamine (D2)receptors. It also binds to alpha1 adrenoceptors, but has no affinity for cholinergic receptors.
Risperidone is well absorbed from the gut, even in the presence of food. The elimination half-life depends on whether the patient is an extensive (3 hours) or a poor (19 hours) metaboliser (8% of the Causasian population are poor metabolisers).The metabolism is partly carried out by cytochrome P450 and produces an active metabolite. A twice daily dose is recommended. Although clinical data are lacking, the company advises halving the recommended dose in the elderly and in patients with hepatic or renal impairment.
The drug is more effective than placebo in the treatment of schizophrenia and has a similar efficacy to haloperidol.
As risperidone blocks alpha1 adrenoceptors, hypotension can occur and practitioners are advised to titrate the dose slowly over several days.
Blockade of D2 receptors by antipsychotics causes extra pyramidal adverse effects and stimulates prolactin release. Risperidone may cause fewer extra pyramidal symptoms than other drugs, but the elevation of prolactin can cause amenorrhoea.
Other adverse effects include insomnia, lethargy, headache and sexual dysfunction. In clinical trials, approximately 7% of the patients had to cease risperidone therapy because of adverse effects.