Neuroleptic drugs represent a wide range of psychotropic agents, sometimes also referred to as the major tranquilizers. The two most common subclasses of neuroleptic drugs are the phenothiazines and butyrophenones. These drugs are prescribed extensively throughout the world for the treatment of schizophrenia, the relief of certain forms of depression, and as antiemetics. Dosage requirements can vary widely among different patients. In part the variable dose requirement is related to marked differences in uptake of the drug among various individuals.
It is generally felt that a simple and sensitive technique to measure neuroleptics in blood and other body tissues would greatly facilitate selection of optimal doses or a particular drug for individual patients. This is important in ensuring that patients have adequate blood and brain levels to attain therapeutic effects and also to make sure that a patient does not receive a higher dose of the drug than is necessary for treating his or her symptoms.
Long term treatment with high doses of these drugs is sometimes associated with abnormal motor movements referred to as tardive dyskinesia, a disfiguring and frequently permanent side effect.
Detecting neuroleptic levels in body fluids ideally should employ a technique which can be used with all neuroleptics. Moreover, because frequently metabolites of the parent drug have therapeutic activity, an ideal method should be able to measure pharmacologically active but not inactive metabolites in addition to the parent drug.
Presently available techniques, such as gas-liquid chromatography, cannot be applied routinely in clinical laboratories because they are quite costly and very time consuming. Moreover these techniques are applicable only for individual drugs rather than for the whole class of neuroleptics. Such prior art methods also do not specifically detect active metabolites.
A variety of indirect evidence accumulated since about 1962 had suggested that the therapeutic actions of neuroleptic drugs in schizophrenic patients involved a blockade of receptor sites for dopamine in the brain. In none of these investigations had researchers been able to measure the dopamine receptor directly. Using .sup.3 H-haloperidol or .sup.3 H-dopamine a group of researchers succeeded in labeling the dopamine receptor so that it could be measured directly. See the publications Creese et al., LIFE SCIENCES, Vol. 17, pp 993-1002 (1975), Snyder et al., Psychopharmacology Communications, 1 (6), 663-673 (1975), Enna et al., NATURE Vol. 263, Sept. 23, 1976, pp 338-344, and Seeman et al., NATURE, Vol. 261, June 24, 1976 pp 717-719.
In these same investigations (see the above noted publications) it was shown that neuroleptic drugs do indeed compete for the binding of .sup.3 H-dopamine and .sup.3 H-haloperidol to the receptor sites in proportion to their clinical efficacy thus showing that the therapeutic actions of the drugs are associated with a blockade of dopamine receptors.
None of these publications of the dopamine receptor itself disclosed anything beyond the fact that the dopamine receptor can be measured with .sup.3 H-haloperidol and .sup.3 H-dopamine and that neuroleptic drugs inhibit the binding of the .sup.3 H-agents. Moreover the information contained in these above mentioned publications does not provide a tool for measuring amounts of neuroleptic drugs in body fluids of human patients, because a number of needed elements, all of which were yet to be discovered, had to be discovered to exist for a successful assay for levels of neuroleptic drugs. For a successful assay for neuroleptic drug levels it was necessary to discover the non-specific effects of body fluids on the binding properties of the dopamine receptor and discover means of reducing or abolishing them. It was also necessary to discover that neuroleptic drugs added to body fluids could be recovered in a form that would still interact with the dopamine receptor. It was also necessary to show that in the presence of body fluids increasing levels of neuroleptic would in a predictable fashion produce progressively greater blockade of dopamine receptors. Only after making a series of discoveries as disclosed herein which reduced nonspecific effects of body fluids on the dopamine receptor, permitted recovery of added neuroleptics and resulted in reproducible augmentations in receptor blockade with increasing amounts of neuroleptics in body fluids was it possible to measure neuroleptic drugs in body fluids with this invention.
One study (Enna and Snyder J. Neurochem., 1976 26: 221-224) coauthored by the present inventor would on its face appear to be relevant to the present invention. This publication discloses the use of a GABA receptor as a means to measure GABA itself in brain tissue. However, this method can only be used for GABA, because neuroleptic drugs do not bind with appreciable potency to the GABA receptor. In competing for binding to the GABA receptor, major neuroleptic drugs are less than 1/1000th as potent as GABA so that circulating levels of neuroleptics could not be detected at all. Because of the limited sensitivity of the GABA radioreceptor assay, one would not expect an assay using another receptor to detect the low levels of neuroleptics which occur in the body fluids of patients. Moreover, in actual clinical practice the most important body fluid for measuring drugs is blood. Plasma proteins in the blood prevent binding of GABA to its receptor so that on the basis of the GABA assay one would not expect to be able to measure GABA or drugs in blood using the GABA receptor or any other receptor.
The present invention departs from the above in that it has been discovered that it now is possible to detect neuroleptic drugs with sensitivities thousands of times greater than achieved with the GABA assay in blood plasma and other body fluids. The GABA receptor does not have sufficient affinity for any known drug used in patients to permit their assay. Thus the GABA assay is restricted to the measurement of GABA itself, a neurotransmitter contained in the brain, but cannot be used to measure any clinically employed drug.
It is also believed that drugs which are not neuroleptics do not bind to the dopamine receptor sites.