United States procedures for screening drugs


I. Introduction
II. Submission of compounds for:
III. Procedures Employed in Dependence Studies


Pages: 11 to 17
Creation Date: 1970/01/01

United States procedures for screening drugs

Testing for dependence liability in animals and man *

I. Introduction

It is not the policy generally of the National Academy of Sciences - National Research Council to become involved in screening programmes, but a unique situation has developed in the field of drug dependence and drug abuse. The first National Research Council Committee on Drug Addiction, operative from 1929 to 1939, conducted its own research programme, chemical, pharmacological and clinical, and was closely concerned with the development of research at the USPHS Hospital for the treatment of addicts at Lexington, Kentucky which led to the establishment of the National Institute of Mental Health Addiction Research Centre. In the course of this development there were established the techniques for the evaluation of morphine-like effects and physical dependence properties in man. When the National Research Council Committee was reorganized in 1947 under the name of the Committee on Drug Addiction and Narcotics, it was given authority to accept funds and to support research in various aspects of drug addiction. One of the first projects supported was that at the University of Michigan to study physical dependence in the monkey and this in turn developed the basic screening programme for physical dependence capacity of potential analgesic agents. It was recognized very early in connexion with the projects in man and the monkey that there was needed an impartial body to accept compounds to be tested and to referee the results, relieving the investigators of any hint of bias or pressure from interested parties. With the concurrence, indeed at the request of the investigators at the Addiction Research Centre and at the University of Michigan, the National Research Council Committee, now known as the Committee on Problems of Drug Dependence, accepted this role. The following pages describe how the Committee interprets and carries out this role. There is also described the specific techniques employed by the co-operating laboratories.

This paper is reprinted from Minutes of Committee on Problems of Drug Dependence, Division of Medical Sciences, National Academy of Sciences - National Research Council, Washington D.C., USA. Twenty-eighth Meeting, New York, N.Y. 9, 10 and 11 February, 1966.

The Committee thus accepts as one of its functions assistance in the determination of drug dependence properties of whatever type and the abuse liability of new substances which act on the central nervous system and are potentially medicinal agents. It supports screening programmes in animals, reviews the data on new compounds with a view to recommendation of tests in man and advises on initiation of narcotics control procedures when, in its opinion, these are required. The Committee welcomes submission of compounds for screening tests and gladly advises the producer on the results and subsequent developments. There are no fees involved. The Committee receives financial assistance from pharmaceutical industry both in the United States and abroad, which supports the above programme and other research projects in the Committee's field of interest. Contributions from industry, new and continued, are welcomed and are essential to the carryingon of the present scope of the Committee's work. All communications with respect to the testing programme should be directed to the Committee's Executive Secretary.1

II. Submission of compounds for:


  1. Time. A compound may be submitted at any time in its development, but for the result to be most helpful this should be at an early stage.

  2. Information required. The name of the compound and its structural formula. A summary of physiochemical data, including base or salt form, solubility, stability and any limiting properties. A summary of pharmacological data and an indication of the producer's specific interest and purpose with respect to the compound.

  3. Quantity. Generally not less than one gramme for an initial sample, but potency obviously will be a major factor in the amount required. If the substance is reputed to have analgesic activity a portion will be used to test for this property by the mouse hot-plate method in the Laboratory of Medicinal Chemistry at the National Institutes of Health, that we may have data on this parameter by a uniform method for all compounds tested. A good yardstick for the quantity needed is not less than 2 grammes for a compound with an analgesic potency approximately equal to that of morphine, with adjustment upwards or downwards according to the ratio of activity to that of morphine. If comparison only with some other standard is available, an initial sample of not less than 5 grammes should be submitted if possible.

  1. Coding and identification. Upon receipt the compound is given an NIH code number and under that number only is sent to the testing laboratory, Department of Pharmacology, The University of Michigan Medical Centre. It is referred to by code number only until all work with the compound is completed. Correspondence with the producer is by means of the producer's code number.

  2. Reporting. On occasion interim reports are submitted to indicate the direction of the results and for consultation on modification or extension of the tests to be performed. When all tests are completed a report is sent to the producer and it is expected that he will at that time, or shortly thereafter, authorize revelation of the name and structure of the compound, hitherto held in confidence by the Executive Secretary. After the producer's authorization the nature of the compound and the results with it will be included in an annual report from the screening laboratory to the Committee and reproduced as an addendum to the Committee's annual report, from which they may be quoted.


  1. and 2. Same as under A.

  2. Quantity. If comparison with a barbiturate has been made by another laboratory technique, this can be used as a basis. For a compound with a sedative potency approximately equal to sodium barbital not less than 5 grammes will be required.


A screening programme for psychic dependence properties is not yet available.


  1. Time. Testing in man can be recommended only after the name and structure of the compound have been revealed, preliminary screening for dependence properties in animals has been completed, and pharmacological data to satisfy Food and Drug Administration Regulations are available.

  2. Information required. The producer must submit multiple copies (15) of a brochure giving the nature and properties of the compound and a description with

results of all pharmacological and clinical tests that have been performed with it. This must include acute and chronic (not less than three months) toxicity data in several species, tests for respiratory, circulatory and behavioural effects, information with respect to any incidental effects which have been observed, evidence that the compound has been administered to man and that it may be a useful therapeutic agent, its relative potency in man and indication of the side effects which may be produced, whether or not the actions of the drug are antagonistic to the opiates, or are antagonized by one of the specific antagonists such as nalorphine, the rate and extent of dosage increase in chronic administration to determine the possibility of tolerance, whether or not tolerance develops and cross tolerance and whether or not acute tolerance and physical dependence have been demonstrated in dogs.

The brochure is distributed to the Director of the National Institute of Mental Health Addiction Research Centre, Lexington, Kentucky, and to the members of the Committee who reach a judgment on the adequacy of information on efficacy and safety and the desirability of tests in man at the Centre. The Addiction Research Centre is not bound, but prefers to be guided by the judgment of the Committee.

  1. Quantity. Generally at least 10 to 25 grammes will be required. The quantity and form in which the material is submitted are determined by direct correspondence with the Director of the Addiction Research Centre. Shipment of material also is directly to him.

  2. Sponsorship. Since these are clinical trials, the Food and Drug Administration Regulations require a sponsor; the producer, or if a foreign producer his American counterpart, who assumes responsibility for application for approval of clinical trial and for reporting to the Food and Drug Administration.

  3. Reporting. The Director of the Addiction Research Centre will report on an interim basis and finally to the Committee and through its Secretary to the producer. Protocols on work done will be made available by the Director, when required for compliance with Regulations of the Food and Drug Administration.


Specific programmes under this heading are not yet available.

III. Procedures Employed in Dependence Studies


Evaluation is based upon the fact, well established in animal and man, that any chemical substance capable of complete supression of all the specific signs of morphine abstinence is equally capable of creating physical dependence during chronic administration. In so far as the monkey and man respond similarly to these agents, the test has predictive value in estimating dependence liability for the human.

The primary and principal objective is to ascertain to what extent a drug is capable of suppression of abstinence signs in the morphine-dependent monkey or the degree to which the total spectrum of signs is affected. This is termed the physical dependence capacity of the test drug. Initially a qualitative test, subsequent quantitative studies may be carried out.

Physical dependence capacity is characterized as:

High. The drug produces complete suppression of all abstinence signs with doses which reveal no other overt pharmacological effect.

Intermediate. Complete suppression of all morphine abstinence signs is obtainable but only with doses which elicit other pharmacological actions manifested by signs such as ataxia, stupor, muscarcinic effects, etc.

Low. Although low grade suppression of all signs may occur, attempts to achieve complete suppression with larger doses result in the intervention of toxic reactions, convulsions, coma, etc, before this can be done.

None. The drug fails to produce specific suppression of morphine abstinence signs. Non-specific depressants may obscure individual signs.

  1. Single dose suppression involves utilization of a colony of 50 to 100 morphine-dependent Rhesus monkeys. The animals are maintained in a state of physical dependence by the hypodermic administration of 3 mg/kg of morphine sulphate every six hours without interruption. After a stabilization period of 60 days they can be used for testing at weekly intervals.

For testing, regular morphine injections are withheld until abstinence signs of intermediate intensity are present (12 to 14 hours). If left untreated, the intensity of abstinence will increase progressively over the next several hours, but the administration of morphine or of any drug with morphine-like physical dependence capacity results in a partial or complete suppression of the signs of abstinence. If the dose is in excess of that required for complete suppression, typical signs of narcotic depression result.

A preselected quantity of the test drug and morphine sulphate (3 mg/kg) are administered subcutaneously by someone other than the observer, each drug to two withdrawn monkeys. The intensity of abstinence signs or of drug depression is graded just prior to and at intervals of 1/2, 1, 2, 3, etc., hours after administration until the monkeys have returned to the preinjection level of excitability. If the dose of the test drug produces more effect than morphine the dose is halved, if less effect the dose is doubled for a subsequent test. The procedure is repeated until a dose which is approximately equivalent to 3 mg/kg of morphine is determined or until side effects appear in such intensity as to preclude further testing. Thus it is determined if a drug has high, intermediate, low or no physical dependence capacity.

A further assay is conducted with all drugs having high or intermediate physical dependence capacity: (1) to extend the series of animals to confirm the original estimate of physical dependence capacity; and (2) to determine more accurately the potency of the test drug relative to morphine. Physical dependence capacity and single dose suppression potency are not synonymous and may not even parallel each other in relation to morphine. Only the former is of major predictive significance in relation to dependence in man. A compound of relatively low potency, 20 mg equivalent to 3 mg/kg of morphine (ratio 0.15) may have a high physical dependence capacity and in this respect be equal to morphine in its potential as a drug of abuse, only requiring large quantities to be effective.

In the assay procedure five single dose suppression treatments are each administered to five withdrawn monkeys at weekly intervals in a Latin square pattern. The treatments include the dose (X) which is approximately equivalent to 3 mg/kg of morphine, 1/2X, 2X, and two controls, 3 mg/kg of morphine sulphate and a placebo. The assay is conducted on a double-blind basis. From the average time-effect curves which are obtained for each treatment a dose-effect curve is established for the test agent based on the maximum decrease in excitability produced by each dose. The intercept of this curve with the line indicating the effect of morphine in the same animals indicates the relative potency to morphine of the test agent.

  1. Primary physical dependence. When it is deemed advisable (most often in cases of questionable significance of physical dependence capacity) the ability of a drug to produce physical dependence is determined directly by chronic administration to monkeys which have not previously received morphine or a related agent. If determinable, the dose equipotent with 3 mg/kg of morphine sulphate in the single dose suppression test, otherwise an arbitrary dose based on the drug's effectiveness for some pertinent action, is administered for 31 days at intervals (usually two to six hours) which correspond to the drug's duration of action. If the condition of the animals warrants, the dose is increased stepwise to two or more times the original level by the last week of administration. A group of three or more monkeys is employed. The development of physical dependence is monitored at the 14th and 28th days of treatment by administering 2 mg/kg of nalorphine HCl subcutaneously. The administration of nalorphine precipitates an abstinence syndrome of one-half to four hours duration, the severity of which corresponds to the degree of physical dependence developed at the time. On the 31st day drug administration is terminated and the monkeys are observed for 7 to 14 days for signs of abstinence. From this experiment physical dependence is graded, low if only mild abstinence signs develop, intermediate if the abstinence signs are of intermediate severity and high if severe or very severe abstinence signs result during withdrawal or after nalorphine administration.

  1. Evaluation for specific antagonistic properties. Some drugs submitted may be reported to have specific opiate antagonistic action or are suspected of such action on the basis of prompt exacerbation of the abstinence signs in the 14-hour withdrawn monkey during routine single dose suppression testing. In either case the drug is used in a definitive test for antagonistic action in non-withdrawn morphine-dependent monkeys. The drug is injected subcutaneously and the animal observed for abstinence signs. If such occur the tests are repeated at successively lower doses until one is reached which is ineffective.

The precipitation of abstinence signs by the specific antagonists is not an all or none phenomenon. Small doses precipitate a syndrome of only mild intensity while larger doses precipitate syndromes of increasing severity. When one of maximum intensity is attained further increase in dose prolongs the effect. Determining the effects of several doses which produce less than maximum responses results in a dose-response curve which permits comparison of the potency of the test drug with nalorphine or levallorphan.

  1. The grading of drug effects. In the single dose suppression technique effective doses of test drugs reduce the intensity of abstinence signs partially or completely, or, having completely suppressed the abstinence signs, may then produce typical signs of narcotic depression. The abstinence signs and the depression are graded (two grades for each classification). The classifications are:

Abstinence signs:

Mild. Apprehension, continual yawning, rhinorrhea, lacrimation, hiccup, shivering, perspiration on face, chattering, quarrelling and fighting. Likely to be overlooked or considered of no significance by the untrained observer.

Intermediate. Intention tremor, anorexia, pilomotor activity, muscle twitching and rigidity, holding the abdomen (cramps).

Severe. Extreme restlessness, assumption of peculiar attitudes, vomiting, severe diarrhea, erection and continued masturbation, inflammation of the eyelids and conjunctivae (insomnia), continual calling and crying, lying on side with eyes closed, marked spasticity.

Very severe. Docility in the normally excitable animal, dyspnoea, pallor, strabismus, dehydration, weight loss, prostration, circulatory collapse and occasionally death.

These signs fall into four categories of hyper-irritability; viz., sympathetic, parasympathetic, psychic (behavioural) and somatic (neuromuscular). Whereas morphine will suppress all of the syndrome evenly to an extent dependent upon the dose, many narcotic analgesics fail to suppress one or another category of signs to the same extent as the remainder of the syndrome. For this reason, grades of withdrawal intensity during the suppression test are assigned according to the psychic signs and not on the basis of the entire morphine abstinence syndrome.

Narcotic depression:

Mild. Inattentive to ordinary movements and actions of other monkeys within the cage and of the observer.

Intermediate. Other monkeys within the cage gain the attention of the monkey only by strong threatening gestures while the monkey becomes attentive to the observer only when the latter makes extraordinarily loud noises or opens the latch on the cage door (a strong conditioned stimulus).

Severe. The monkey responds only to such exteroceptive stimuli as shouting or clapping the hands near his ear or being touched by other monkeys or the observer.

Very severe. The monkey is comatose and cannot be aroused by any stimuli.

Central nervous system depression produced in the monkey by narcotic analgesics is a combination of two factors, referred to collectively as stupor - decreased awareness of and diminished responsiveness to environmental stimuli; and a decrease in the degree of apprehension as indicated by alteration in behaviour once his attention has been gained.

  1. Interpretation. If physical dependence capacity is high, unless there are compelling reasons to the contrary, the specific compound had best be abandoned so far as any intent to go on to trial in man is concerned. If it is intermediate, and especially if it is low other properties and considerations will determine whether it should be studied extensively pharmacologically and this in turn initial clinical trial. Only then is it necessaty to consider trials in man for confirmation or otherwise of the physical dependence property and for determination of the likelihood of drug abuse.


As in the determination of morphine-like physical dependence capacity, it is assumed that any substance which will substitute for sodium barbital in a barbital-dependent dog, suppressing completely a barbiturate withdrawal syndrome, may be expected to produce during prolonged administration physical dependence of barbiturate type.

Dogs are made physically dependent on sodium barbital by the daily oral administration of a single dose of 100 mg/kg. After six weeks of this treatment the drug is withdrawn for a period of five days and the ensuing abstinence signs are observed. The abstinence syndrome is somewhat different in detail in each dog. After this five-day period barbital administration is resumed for at least one month. A test drug is then substituted in appropriate dose and at appropriate intervals for five days. All treatment is then discontinued for five days and the abstinence syndrome observed and compared with the control withdrawal. Barbital treatment may be re-established and the dog used for another substitution in about another month.

The sedative potency of a test drug and its duration of action must be determined in normal dogs before substitution in dependent animals. The dose which is estimated to be equivalent in sedative potency to 30 mg/kg of sodium phenobarbital is the dose used in substitution at sufficiently frequent intervals so that no period intervenes between doses when the dog is free of the depressant effect of the test drug.

Dogs which are physically dependent on sodium barbital (100 mg/kg/day, orally) begin to show mild abstinence signs about 20 hours after the last dose. The signs become progressively more intense, reaching a peak during the 72 to 96 hour period of withdrawal and then diminish gradually in severity to complete disappearance in about 10 days. The signs may be classified as:

Mild. Tachycardia (10-20 per cent), hyperpnea, weight loss (10-15 per cent), tremors.

Intermediate. Anorexia, nervousness, restlessness, insomnia, fighting.

Severe. Hyperthermia (1.5-2.0 °C.), fasiculations, convulsions, delirium.

Convulsions and delirium are the most dramatic and specific characteristics of the barbiturate abstinence syndrome. Although the other signs may be said to be less specific since they can result from a variety of causes, when the lesser signs are associated with the withdrawal of a drug and are accompanied by convulsions and delirium, they may be considered to be specific and sensitive signs. The lesser signs are also much more uniform in their occurence. Convulsions and delirium are more variable.

Some dogs show convulsions as early as 12 hours after the last dose but they do not usually appear until after 48 hours. The convulsions may be localized or they may spread to involve the entire body. Delirium appears in a variety of forms but is constant in appearance for a particular dog. Some dogs show catatonic behaviour, remaining in one position for many hours. Others snap and snarl at or retreat from non-existent objects.

Sedative action as determined in normal dogs and substitution potency in the dependent dog may not be related dosewise or otherwise. Since the former is used as the starting point for substitution, begun 24 hours after the last dose of barbital, if abstinence signs appear the dose is increased until these are completely suppressed or toxic signs appear.

If complete substitution, complete suppression of the barbiturate abstinence syndrome, is attained at or near the comparable sedative dosage, it is probable that physical dependence will develop in man with the new agent under the same conditions as with a barbiturate. If a much higher dose is required for substitution, or if no suppression of abstinence occurs presumptively the agent is one which produces less or no dependence of barbiturate type. Suppression of convulsions alone (of the abstinence syndrome) cannot be taken as evidence of adequate substitution. Sodium barbital at a dose of 50 mg/kg orally will prevent the convulsions without affecting other elements of the abstinence syndrome.


The subjects in the studies at the Addiction Research Centre are former opiate dependent persons (addicts), male, coloured or white, in good physical condition, who are serving a sentence for violation of the narcotic laws and who have volunteered for the experiment. The objectives are to determine the drug effects and subject's reaction to them in comparison with morphine or another drug in his experience, the ability of the drug to substitute for morphine in a morphine-dependent person, suppressing the morphine abstinence syndrome, and the ability of the drug to produce primarily tolerance and dependence on prolonged administration.

  1. Single dose administration. Starting with small doses, based on previous experience, a new drug is administered by various routes depending upon its solubility and other properties. The dose is increased, two or three men being given the drug at each dose level, until a definite effect is obtained or signs of impending toxicity are seen. To this point administration is single blind, the nature of the drug unknown to the subject, who is, however, allowed to express his feelings about it in comparison with his previous drug experience.

An idea of relative potency having been obtained, a double blind comparison is undertaken. Two dose levels of the new agent, two comparable doses of an appropriate standard and a placebo are given in random order to 8 to 12 individuals, continuing until each subject has received all medications. Pupil size, respiratory rate and minute volume, blood pressure and body temperature are measured before and at intervals after each dose and both the patient and the observer are required to fill out a single dose attitude questionnaire. Both check whether the agent seems to be an opiate or like some other drug, the degree of the patient's liking for the drug and the frequency of opiate-like symptoms.

  1. Substitution for morphine. A group of 5 to 12 individuals is given morphine sulphate subcutaneously regularly, the dose being built up rapidly to 240 mg per day and stabilized at that level for not less than 30 days. Beginning with the evening dose (10 p.m.) and continuing through 24 hours a coded substance (experimental drug, placebo or the stabilization morphine dose given as an unknown) is administered in place of the regular morphine doses. Beginning the next morning, 14 hours after the last regular dose of morphine, observations made at hourly intervals, recorded and evaluated according to the Himmelsbach hourly score for abstinence phenomena. Regular morphine administration is resumed at the end of the 24 hours and the substitution procedure is repeated at weekly intervals until each individual has received in random order at least one dose of the experimental drug, a saline injection as a placebo, the regular stabilization dose of morphine and frequently one or more doses of one or more other agents. The mean hourly Himmelsbach point score for all patients for each coded medication is plotted and comparison is made between the abstinence score for the experimental drug and morphine on the one hand (complete suppression of abstinence) and the placebo on the other hand (no suppression of abstinence) for a determination of the abstinence suppression potency of the new drug relative to morphine. Alternatively, especially if the new drug has not substituted satisfactorily for morphine, its substitution may be continued for 10 days with some dose adjustment to try to attain complete substitution. One gets in this way an impression of the individual's satisfaction with the substitute. At the end of the ten days the new drug is replaced by placebo without the patient's knowledge. Through the ten days and the following week observations are made for morphine withdrawal phenomena with ultimate comparison qualitatively and quantitatively with an abrupt morphine withdrawal in the same individual.

  2. Primary physical dependence. If a compound has been shown to be quantitatively or qualitatively Different from morphine, if it is a new chemical type, and particularly if its introduction into clinical practice is contemplated, chronic administration may be undertaken to determine directly tolerance and dependence development

Individuals previously dependent on morphine or a related drug and free of drugs for some months are given the new agent regularly at an interval and dose and by a route deemed appropriate according to previous experience. The dose is increased as rapidly as possible, avoiding, of course, cumulative effects and toxic reactions. The objective is to increase the dose, if possible, by a schedule paralleling that leading to stabilization on morphine in the substitution procedure. The primary physical dependence attempt may be continued for 30, 60 or 90 days or longer and is followed by abrupt withdrawal; a comparison is then made with similar periods of administration and withdrawal of morphine. A few days before withdrawal patients are challenged by administration of a small dose of nalorphine.

Two other procedures may be employed: A short term cross-over chronic administration, or a patient's choice experiment. In the first a series of medications including morphine and a placebo, are administered to eight drug-free formerly morphine-dependent individuals, double-blind in random order, in ascending dosage each for 18 or 19 days. On the 19th or 20th day unknown to patient or observer a placebo is substituted for the previous medication and continued for 10 days. Then the individual is started on another of the medications and the whole procedure repeated until each person has received each of them. Since no difficulty is experienced in increasing morphine dosage to 240 mg a day in 18 days, such a regimen of administration of morphine is employed and a parallel schedule of increasing dosage for all other drugs, modified as necessary by slower development of tolerance, excessive sedation or signs of toxic effects. The initial dose in each instance (placebo excepted, of course) is that judged to be equivalent in effectiveness to morphine, based on the 24-hour substitution experiment. In the patient's choice experiment, applicable to substances which may be given intravenously, each of six subjects is given at 3-day intervals a sample high dose of each medication to be compared, including morphine, on a single-blind basis. The patient is asked to rate the effects of the drugs in the order of his preference and to elect to take none, one or any in increasing doses for seven days with the further option that he can request discontinuance of a drug and/or further participation in the experiment at any time. At the end the subject again rates the drugs in order of preference. The advantages of the procedure are: it brings out rapidly among drugs characteristics which determine suitability for intravenous administration, it permits the subject to make elections among drugs as to whether or how long he would like to take one before trying another, it brings out differences among subjective effects and the subject's immediate like or dislike of a drug by his usually preferred route of administration and hence whether or not he would be inclined to use it thus if it were available to him.

  1. Interpretation. A review of the data accumulated at the Addiction Research Centre permits a judgment of the likelihood of tolerance and physical dependence development should the compound be used clinically, confirmed heretofore in all instances in which a compound tested has been used clinically, and a judgment on the probable subjective reaction to and abuse liability of an agent should it be available to persons who seek escape in drug use. On the basis of these judgments a recommendation can be made on the need for narcotics control.


Presently all communications in this connexion should be sent to Dr. Everette L. May, National Institutes of Health, Bethesda, Maryland 20014, USA, who acts as Consultant to the Executive Secretary, for the screening programmes.