Antiepileptic Drug Development Program1
Roger J. Porter, M.D. The modern era of antiepileptic drug therapy began with the James J. Cereghino, M.D. use of phénobarbital in 1912. In the years thereafter, many new Gill D. Gladding, R.Ph. drugs were introduced, including other barbiturates, hydantoins, Bettiejean Hessie, B.S. succinimides, and oxazolidinediones. Then, for various reasons, Harvey J. Kupferberg, Ph.D. the marketing of new antiepileptic drugs was dramatically cur- tailed. To help reverse this trend, the Epilepsy Branch of the Barrett Scoville, M.D. National Institute of Neurological and Communicative Disorders Billy G. White, Ph.D. and Stroke sponsored clinical trials of drugs which had already been marketed abroad, resulting in the distribution of clonazepam, carbamazepine, and valproic acid in the U.S. These trials were followed by the establishment of the Antiepileptic Drug Develop- ment Program, which encompasses both the preclinical and clini- cal elements of drug development, including the Anticonvulsant Screening Project, the Toxicology Project, and support for con- trolled clinical trials. Index terms: Anticonvulsants • Epilepsy Cleve Clin Q 51:293-305, Summer 1984
Approximately 2.5 million (1%) of all Americans have epilepsy; 200,000 have seizures more than once a month, making epilepsy second only to stroke as the leading neu- rological disorder. It often begins in childhood, with 75% 1 Epilepsy Branch, National Institute of Neurolog- of patients having their first seizure before the age of 18. ical and Communicative Disorders and Stroke, Be- Most patients are dependent on drugs for seizure control, thesda, MD. Submitted and accepted for publication but therapy is often inadequate. For some patients with Oct 1983. intractable seizures, hope may lie in the development of more effective antiepileptic drugs; for others, a new drug 0009-8787/84/02/0293/13/$4.25/0 may reduce the side effects they must often tolerate to gain Copyright © 1984, The Cleveland seizure control with current treatment. The purpose of Clinic Foundation. this paper is to describe the Antiepileptic Drug Develop- 293
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Table 1. Antiepileptic drugs marketed in the phenobarbital soon became the drug of choice. United States Since the barbituric acid molecule is easily mod- Year in- International nonproprietary U. S. trade ified, many analogues of phenobarbital were syn- troduced name name Company thesized, of which approximately 50 were mar- 1912 Phenobarbital Luminal Winthrop keted in the first 35 years of this century. One of 1935 Mephobarbital Mebaral Winthrop these analogues, mephobarbital, demonstrated 1938 Phenytoin Dilantin Parke-Davis good antiepileptic activity and was marketed in 1946 Trimethadione Tridione Abbott the U.S. in 1935. 1947 Mephenytoin Mesantoin Sandoz 1949 Paramethadione Paradione Abbott In the absence of experimental models of sei- 1950 Phethenylate* Thiantoin Lilly zures which could be used to test anticonvulsant 1951 Phenacemide Phenurone Abbott activity, the discovery of the antiepileptic effect 1952 Metharbital Gemonil Abbott of bromide and phenobarbital was serendipitous. 1952 Benzchlorpropamidef Hibicon Lederle 1953 Phensuximide Milontin Parke-Davis Later, with the development of seizure models, 1954 Primidone Mysoline Ayerst the search for new antiepileptic drugs was based 1957 Methsuximide Celontin Parke-Davis on scientific screening programs. 1957 Ethotoin Peganone Abbott One of the earliest models of epilepsy was 1960 Aminoglutethimidef Elipten Ciba developed in 1882, when seizures were elicited 1960 Ethosuximide Zarontin Parke-Davis 1968 Diazepam§ Valium Roche in dogs by direct faradic stimulation of the motor 1974 Carbamazepine Tegretol Geigy cortex and used to test chemicals for anticonvul- 3 1975 Clonazepam Clonopin Roche sant activity. Later, other seizure models involv- 1978 Valproic acid Depakene Abbott ing convulsant chemicals were developed, includ- 1981 Clorazepate dipotas- Tranxene Abbott ing the naturally occurring picrotoxin, bicucul- sium§ line, and strychnine and the synthetic compound, * Withdrawn in 1952 pentylenetetrazol. The use of systemic chemical t Withdrawn in 1955 convulsants as experimental models has been well $ Withdrawn in 1966 described.4 § Approved by the FDA as an adjunct The year 1937 marked the beginning of the experimental evaluation of promising anticon- ment Program, the federal government's effort vulsant chemicals prior to clinical use. Using a to develop more effective and less toxic drugs for seizure model based on a new electroshock tech- epilepsy, and to review the highly successful era nique for producing convulsions in animals,5 of the 1940s and 1950s when many major anti- Merritt and Putnam^7 screened a group of com- epileptic drugs were marketed, as well as the 12- pounds supplied to them by Parke-Davis and year hiatus which took place between 1961 and discovered the anticonvulsant properties of phe- 1973. nytoin, then called diphenylhydantoin. Because phenytoin was well tolerated by laboratory ani- History of antiepileptic drug development mals, it was subjected to clinical trials in 1938 During the mid-1800s, a number of inorganic and marketed that same year. The absence of a bromide salts were reported to produce good sedative effect and the dramatic control of sei- sedative effects and were accepted into medical zures observed when phenytoin was added to practice. Potassium bromide, used by Locock to barbiturate therapy were the key factors in its treat catamenial seizures,1 largely replaced ear- rapid marketing. In addition, its entry into the lier drugs when it was found to reduce seizure market was not delayed by regulatory require- frequency in many patients after other forms of ments, since at that time the introduction of new therapy had failed. Although it was used regu- drugs was still regulated by the Federal Food and larly during the next 50 years, it was found to Drugs Act of 1906, which required only that cause severe skin eruptions and psychosis, drugs be accurately labeled without requiring 8 prompting a search for less toxic drugs. proof of safety or efficacy. The modern history of antiepileptic drugs mar- The reliability and quantitative capacity of keted in the U.S. (Table 1) begins in 1912 with Merritt's method demonstrated the feasibility of the introduction of phenobarbital, a synthetic testing new chemicals for anticonvulsant activity.6 sedative-hypnotic drug which was shown to re- Administration to humans, a more costly, time- duce seizure frequency.2 As it proved to be more consuming, and risky procedure, could confi- effective and less toxic than potassium bromide, dently be reserved for the most effective experi-
Downloaded from www.ccjm.org on September 29, 2021. For personal use only. All other uses require permission. Summer 1984 Antiepileptic Drug Development Program 295 mental compounds that emerged from such test- ing programs. In addition, the process through which phenytoin came onto the market demon- strated that academic investigators could work successfully with the pharmaceutical industry, en- couraging a relationship that flourished for the next 20 years. C=0 Several pharmaceutical firms began molecular modification projects dealing with phenytoin and its analogues, and numerous hydantoins were synthesized and tested during this period. In addition, investigators from the pharmaceutical 0=C n-r3 industry as well as academic researchers began to Fig. 1. R,, R-i, and R% indicate different side-chain members. explore new and improved methods of provoking X refers to components of different drug groupings: hydantoinates (-NH-), barbiturates (-CO-NH-), oxazolidinediones (-0-), succinim- seizures. ides {-CH. ), and acetylureas (-NH?-). In 1944, Richards and Everett9 reported that r trimethadione, a potent analgesic compound that was to become the first anti-absence drug, pre- phethenylate and benzchlorpropamide were vented pentylenetetrazol-induced threshold sei- withdrawn by 1960, as the former was associated zures in rodents. They also showed that these with a high incidence of hepatic necrosis and the seizures were prevented by phénobarbital, but latter demonstrated toxic effects with long-term not by phenytoin. Goodman et al10 confirmed use in experimental animals. Aminoglutethimide these results and showed that phenytoin and phé- was withdrawn in 1966 after it was linked to a nobarbital modified the pattern of maximal elec- high incidence Of goiter. troshock seizures while trimethadione did not. Interestingly, all antiepileptic drugs developed These findings demonstrated the varying anti- from 1912 to 1960 were based on a sirtiple het- convulsant actions of these drugs and the quali- erocyclic ring Structure (Fig. 1). During this pe- tative difference between threshold and maximal riod, genuinely novel structures were ignored in seizures. the development of antiepileptic drugs; instead, Between 1945 and 1950, several investigators attention centered on the hydantoins, barbitu- conducted tests with a variety of seizure models, rates, oxazolidinediones, succinimides, and ace- but failed to find one in which all drugs were tylureas. active. However, these tests uncovered profiles After 1938, marketing of all drugs in the of anticonvulsant activity which, with few excep- United States was regulated by the Federal Food, tions, correlated well with clinical efficacy and Drug, and Cosmetic Act,13 which required proof specificity.11 In 1951, Chen et al12 investigated of safety in addition to the 1906 labeling provi- the anticonvulsant activity of approximately 65 sions. Definitive questions about efficacy usually phenylsuccinimides and found that among the were not resolved until after a new drug was most potent antipentylerietetrazol compounds marketed. were phensuximide and methsuximide, both of which were later approved for treatment of ab- Decline in antiepileptic drug development sence seizures (1953 and 1957, respectively). A The highly productive era of antiepileptic drug third succinimide, ethosuximide, was introduced development in the 1940s and 1950s was fol- for the same purpose in 1960. lowed by a dormant period lasting for 12 years, During the same period (1938-1960), two an- from 1961 to 1973, during which the only new alogues of phenytoin (mephenytoin and etho- drug of interest was diazepam, an adjunctive toin), two of phénobarbital (metharbital and drug used mostly in status epilepticus (Table 1). primidone), and one of trimethadione (parame- There were many reasons for this. For one thing, thadione) were marketed in the U.S. Each had a some clinicians believed that improvements in spectrum of activity comparable to that of its therapy depended mainly on better use of exist- parent drug and was marketed for similar use. ing drugs, and this belief helped strengthen the Three other drugs, phethenylâte, benzchlor- impression that there was not a substantial need propamide, and aminoglutethimide, were also for new drugs, even though many patients with introduced during this period; however, both common types of seizures and most of those with
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rare types failed to respond to available drugs nicative Disorders and Stroke, in collaboration and many suffered side effects. Furthermore, the with other investigators, attempted to reverse the attention of the pharmaceutical industry shifted decline in antiepileptic drug development by con- to other areas of central nervous system (CNS) ducting controlled clinical trials of seven drugs therapy following the remarkable financial suc- (albutoin, carbamazepine, clonazepam, cloraze- cess of tranquilizers and sedative-hypnotic drugs. pate dipotassium, mexiletine, sulthiame, and val- In addition, because of the relatively large num- proic acid) that needed proof of efficacy before ber of effective drugs already available, many they could be marketed in this country. Support representatives of the pharmaceutical industry of these trials not only decreased the cost of questioned whether a new drug could capture a development for the pharmaceutical industry, large enough market to justify the cost of devel- but also provided an opportunity for the Epilepsy opment. This cost, already on the increase, rose Branch to develop a methodology and standards still more steeply with the addition of the Drug for the conduct of such trials. The resulting data Amendments Act of 196214 to the 1938 regula- eventually supported new drug applications tion. Known as the Kefauver-Harris amend- (NDA) for carbamazepine, clonazepam, and val- ment, this legislation required that efficacy be proic acid, which were marketed in this country established as a prerequisite for marketing ap- as primary antiepileptic drugs in 1974, 1975, and proval in the U.S. and restricted the conditions 1978, respectively, and for clorazepate dipotas- under which drugs could be tested. It had a sium, which was approved in 1981 as an adjunc- serious impact on drug development, not only tive drug for treatment of epilepsy. Trials of for epilepsy but also for any other disease affect- albutoin, mexiletine, and sulthiame failed to sup- ing a relatively limited population, wherein the port their efficacy in the populations studied, and market was correspondingly small and the return these drugs have not been marketed in the U.S. on corporate investment doubtful. In addition to clinical trials of available drugs, Problems with clinical testing also contributed the need for involvement in the development to the reluctance of pharmaceutical firms to de- process at the preclinical stage soon became ap- velop new drugs. For example, proof of the effi- parent, and a federally sponsored antiepileptic cacy of new antiepileptic drugs was seriously ham- drug development program was formally estab- pered by the almost total lack of patient popula- lished with the introduction of the Anticonvul- tions whose seizure types and frequencies were sant Screening Project in 1975. well defined. Many patients withdrew from con- trolled clinical studies following either dramatic Antiepileptic Drug Development Program improvement or increased seizures. In addition, Several other government-sponsored drug de- the common use of multiple drugs complicated velopment programs set the precedent for fed- the design of controlled clinical trials that could eral assistance in antiepileptic drug development. establish the efficacy of a new drug used alorte. While these programs vary, they are concerned Moreover, many clinicians did not believe in the primarily with drugs that, for various reasons, need for controlled trials, and the resulting lack would not be developed independently by drug of scientific data was a major impediment to the companies. The two fundamental elements of development of new antiepileptic drugs accord- such programs are (a) preclinical screening, often ing to the newer, more rigorous standards of the seeking potential drugs in an effort to entice drug 1960s. A 1967 survey of pharmaceutical firms15 companies to become interested in them, and (b) revealed that most had no new antiepileptic drugs controlled clinical trials of new drugs. under development due to prohibitive cost, while The Antiepileptic Drug Development (ADD) several drugs had not gained the approval of the Program, sponsored by the Epilepsy Branch, en- U.S. Food and Drug Administration (FDA) be- compasses both the preclinical and clinical ele- cause of inadequate proof of efficacy. Further- ments of drug development (Fig. 2). The preclin- more, an informal survey of academic medicinal ical segment comprises the Anticonvulsant chemists revealed that those synthesizing poten- Screening Project (ASP) and the Toxicology tial anticonvulsant agents had no access to appro- Project, and the clinical element is represented priate pharmacologic testing. by sponsorship of controlled clinical trials. Dur- ing the preclinical process, the ADD program Renewed interest in antiepileptic drugs receives compounds from both academic medic- Beginning in 1968, the Epilepsy Branch of the inal chemists and the pharmaceutical industry for National Institute of Neurological and Commu- screening, which is divided into seven phases (see
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Fig. 2. Antiepileptic drug development process above). When a compound shows exceptional the ASP for evaluation of anticonvulsant activity, anticonvulsant activity in the later phases, toxi- description of neurotoxic effects, and delineation cology studies are recommended to the sponsor. of possible mechanisms of action (Fig. 3). The In general, no drug can be administered to hu- testing is done at no cost to them, and in addition, mans until one study has been conducted in ro- they retain the patent rights. As of October 1983, dents and another in other mammals (usually more than 8,100 compounds have been tested in dogs), with the ADD program supporting one a standardized, consistent manner, resulting in a study and the sponsor supporting the other. The data base by which the structure-activity rela- clinical segment of the program begins after the tionships of other anticonvulsants can be pre- sponsor is granted an investigational exemption dicted. The seven phases of the ASP are de- for a new drug (IND). Studies in healthy volun- scribed in Table 2. The most commonly used teers (phase I) are supported by the sponsor, or antiepileptic drugs marketed in the United States infrequently by the ADD program. Initial con- were tested in phases 2-6, and the results are trolled clinical trials in epileptic patients (phase shown in Table 3. II) are supported by the ADD program. Finally, Methodology: The clinical usefulness of the broader clinical trials (phase III) are conducted currently available antiepileptic drugs is indi- by the sponsor. Successful completion of clinical cated by their ability experimentally to prevent evaluation is generally followed by marketing of the spread of seizures and/or increase the mini- the drug. mal seizure threshold. Those applicable to gen- To avoid confusion, the seven phases of the eralized tonic-clonic seizures and partial seizures, ASP are designated by Arabic numerals (phases such as phenytoin, prevent the spread of seizures 1-7), and the three phases of clinical evaluation and may or may not increase the minimal seizure are designated by Roman numerals (phases I- threshold, while those used against absence sei- III). zures, such as ethosuximide, elevate the thresh- old and have little or no ability to prevent spread. Anticonvulsant Screening Project Thus initial screening involves (a) the maximal Academic chemists and representatives of the electroshock seizure test to detect agents that pharmaceutical industry submit compounds to prevent spread of seizures, and (b) the seizure
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90° angle to the plane of the body) indicates that the compound can prevent MES-induced seizure spread. In the subcutaneous pentylenetetrazol seizure threshold test (scMet), the convulsant dose (CD97) of pentylenetetrazol (85 mg/kg in mice, 70 mg/ kg in rats) is injected at the time of peak effect of the test compound. The animals are isolated and observed for 30 minutes to see whether seizures occur. Absence of clonic spasms persist- ing for at least five seconds indicates that the compound can elevate the pentylenetetrazol-in- duced seizure threshold. In the subcutaneous bicuculline seizure threshold test (scBic), the CD97 of bicuculline (2.70 mg/kg) is injected into mice at the time of peak effect of the test compound. They are then isolated and checked for seizures for 30 minutes. Absence of seizures indicates that the compound can elevate the bicuculline-induced seizure threshold. In the subcutaneous picrotoxin seizure threshold test (scPic), the CD97 of picrotoxin (3.15 mg/kg) is injected into mice at the time of peak effect of threshold test with subcutaneous pentylenetetra- the test compound, after which the mice are zol (Metrazol) to detect agents that elevate the isolated and any seizures noted for 45 minutes. minimal seizure threshold. If a compound shows Absence of seizures indicates that the compound significant anticonvulsant activity and demon- can elevate the picrotoxin-induced seizure strates minimal neurotoxicity on the rotorod threshold. ataxia test, further screening is performed. All In the subcutaneous strychnine seizure pattern test compounds tested are either dissolved in 0.9% (scStr), the CDgy of strychnine (1.20 mg/kg) is sodium chloride or suspended in a mixture of injected into mice at the time of peak effect of 30% polyethylene glycol 400 and 70% water. the test compound. The mice are placed in iso- Except for a specific interaction between certain lation cages and observed for 30 minutes for the drugs and polyethylene glycol in the Metrazol hind-leg tonic-extensor component of a seizure; test, which results in increased activity of the test abolition of this component indicates that the compound, the solvents introduce no significant compound can prevent strychnine-induced bias. The compounds are administered intraper- spread of seizures. itoneal^ (i.p.) or orally (p.o.) to Carworth Farms Acute anticonvulsant drug-induced toxicity in # 1 mice (in a volume of 0.01 ml/g of body weight) laboratory animals is usually characterized by or Sprague-Dawley rats (in a volume of 0.004 some type of neurologic abnormality. In mice, ml/g of body weight). Times of peak effect and these abnormalities are easily detected by the peak neurologic deficit are determined before rotorod ataxia test, which is somewhat less useful the anticonvulsant tests are administered. in rats. When a normal mouse is placed on a In the maximal electroshock seizure test (MES), knurled rod rotating at 6 rpm, it can maintain its corneal electrodes primed with a drop of electro- equilibrium for a long time. The neurologic def- lyte solution (0.9% sodium chloride) are applied icit is indicated by inability to maintain equilib- to the eyes and an electrical stimulus (50 mA in rium for one minute in each of three trials. Rats mice, 150 mA in rats; 60 Hz) is delivered for 0.2 are examined by the positional sense test and gait second at the time of peak effect of the test and stance test. In the positional sense test, one compound. The animals are restrained by hand hind leg is gently lowered over the edge of a and released at the moment of stimulation in table, whereupon the animal will quickly lift it order to permit observation of the entire seizure. back to a normal position. Inability to do so Abolition of the hind-leg tonic-extensor compo- rapidly indicates a neurologic deficit. In the gait nent (hind-leg tonic extension does not exceed a and stance test, a neurologic deficit is indicated
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Table 2. Test phases in the Anticonvulsant Screening Project Phase 1: Anticonvulsant identification to determine the level of activity [active (<100 mg/kg) to inactive (>300 mg/kg)] (mice, i.p.) 1. Maximal electroshock (MES) test—seizure spread 2. Subcutaneous pentylenetetrazol (scMet) test—seizure threshold 3. Rotorod ataxia test—neurotoxicity
Phase 2: Anticonvulsant quantification to determine the level of activity at the ED50, TD50, and protective index (TD50/ED50) (mice, i.p.) 1. Maximal electroshock (MES) test—seizure spread 2. Subcutaneous pentylenetetrazol (scMet) test—seizure threshold 3. Rotorod ataxia test—neurotoxicity Phase 3: Toxicity profile to assess general behavior and selected pharmacologic response at toxic doses (mice, i.p.) 1. Median lethal dose (LD50) 2. Median hypnotic dose (HD50) Phase 4: Anticonvulsant quantification to measure activity by the usual clinical route of administration and indicate the absorption and metabolic characteristics of the compound (mice, p.o.) 1. Maximal electroshock (MES) test—seizure spread 2. Subcutaneous pentylenetetrazol (scMet) test—seizure threshold 3. Rotorod ataxia test—neurotoxicity Phase 5: Antiepileptic drug differentiation and comparison with known effective drugs to help determine the mechanism of action (mice, i.p.) 1. Pentylenetetrazol seizure threshold test 2. Picrotoxin seizure threshold test 3. Bicuculline seizure threshold test 4. Strychnine seizure threshold test 5. Special in vitro receptor binding studies on selected candidate compounds Phase 6: Anticonvulsant quantification to measure activity in another species at the ED50, TD50, and protective index (TD50/ED50) (rats, p.o.) 1. Maximal electroshock (MES) test—seizure spread 2. Subcutaneous pentylenetetrazol (scMet) test—seizure threshold 3. Positional sense test—neurotoxicity 4. Gait and stance test—neurotoxicity Phase 7: Estimation of minimal lethal dose (LD3) and effect of prolonged administration on anticonvulsant activity (rats, p.o.) 1. Estimated LDS in male and female rats following administration once a day for 5 days 2. Administration for 5 days—tolerance 3. Hexobarbital sleep time test—tolerance 4. Microsomal enzyme studies in vitro—tolerance Abbreviations: i.p. = intraperitoneally, ED = effective dose, TD = toxic dose, p.o. = orally. by a circular or zigzag gait, ataxia, abnormal About 15% of all compounds are advanced to spread of the legs, abnormal body posture, phase 2. tremor, hyperactivity, lack of exploratory behav- Phase 2 (anticonvulsant quantification in mice, ior, somnolence, stupor, or catalepsy. i.p.) measures the anticonvulsant activity and Testing protocol: The potency and protective neurotoxicity estimated in phase 1. The median index of active compounds are estimated, and effective dose (ED50) is determined using the inactive or toxic compounds are eliminated from MES and scMet tests and the median toxic dose further testing in phase 1 (anticonvulsant identi- (TD50) using the rotorod ataxia test. When a fication in mice, i.p.). Testing is carried out in 16 compound appears to exhibit anticonvulsant ac- mice at doses of 30, 100, 300, and 600 mg/kg (4 tivity in one test, it is tested in the other model mice apiece) 30 minutes and four hours after up to doses that produce a neurologic deficit. administering the compound. Based on the re- Whenever possible, compounds failing to pro- sults of this test, compounds are divided into four duce a minimal neurologic deficit are tested to groups: (a) those with no anticonvulsant activity doses ten times their lowest anticonvulsant ED50. at doses up to 300 mg/kg, which are not tested The median effective dose is determined at the further; (b) those showing activity at 100 mg/kg, time of peak effect in the MES test except when which are tested further; (c) those showing activ- preliminary testing indicates that scMet activity ity at 300 mg/kg, which may or may not be tested occurs at a different time; TD50 is determined at further depending on the novelty of the struc- the time of peak neurologic deficit. The most ture; and (d) those demonstrating activity and/ promising compounds emerging from phase 2 or toxicity at 30 mg/kg, which are usually re- become candidates for advanced testing. tested and may or may not be evaluated further. Phase 3 (toxicity profile in mice, i.p.) reveals
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the dose-time relationships with regard to overt dent species. The ED50s in the MES and scMet toxic manifestations and determines the median tests and the TD50 are determined after oral hypnotic dose (HD50) and the 24-hour median administration of the compound, and the posi- lethal dose (LD50). Toxicity is determined by tional sense test and gait and stance test are used administering the TD50) two times the TD50, and to determine neurotoxicity. These studies help four times the TD50. Mice are observed for onset, to determine whether the accumulated experi- intensity, and nature of overt toxicity at 10, 20, mental data are promising enough to warrant and 30 minutes and 1, 2, 4, 6, 8, and 24 hours moving the candidate compound into toxicity after administration of the test compound, which studies. in turn, can help clarify its effects on the central Phase 7 (estimation of minimal lethal dose and and autonomic nervous systems. The aforemen- effect of prolonged administration on anticon- tioned neurotoxicity tests are also performed, vulsant activity in rats, p.o.) provides the dosage with abnormal results on at least two of them information which is essential for subsequent tox- indicating overt neurologic toxicity. icity studies in the Toxicology Project as well as Phase 4 (anticonvulsant quantification in mice, an indication of the development of tolerance, p.o.) provides the same kind of information as and hence is limited to those compounds showing phase 2, except that the test compound is given the greatest antiepileptic potential. Both male orally instead of intraperitoneally to see whether and female albino rats are used in order to cor- this makes any difference in the activity of the relate drug response with sex. First the minimal drug. The time of peak effect indicates how lethal dose (LD3) is estimated following oral ad- rapidly it is absorbed, while the ED50 and TD50 ministration once a day for five days. Any phar- disclose how adequately it is absorbed, which is macologic or toxic manifestations, including important because antiepileptic drugs are usually death, are recorded one and four hours after given by mouth. Consequently, test compounds administration and before the next day's dose. that reach this stage and still exhibit a satisfactory Development of tolerance is first measured in 24 anticonvulsant activity, margin of safety, and ad- rats divided into three groups of eight. Group 1 equate absorption usually proceed to phase 5, receives the ED50 once a day for five days, with particularly if they also have a novel chemical anticonvulsant activity at the time of peak effect structure. being determined on the fifth day using either Phase 5 (antiepileptic drug differentiation in the MES or scMET test. Group 2 receives saline mice, i.p.) delineates antiepileptic potential in or suspension media for four days, followed by vivo and in vitro. The in vivo portion tests the the ED50 on day five, with anticonvulsant activity compound in seizures induced by pentylenetetra- being determined by MES or scMet at the time zol, bicuculline, picrotoxin, and strychnine. Be- of peak effect. Group 3 is given saline or suspen- cause each of these convulsants acts via a some- sion media for five days and tested on the fifth what different neurotransmitter system, the re- day. A greater number of seizures in group 1 than in group 2 indicates the development of sulting ED50s may reflect the activity profile of the test compound, which can be compared with tolerance. Another method of determining tol- those of clinically effective drugs. In the in vitro erance is the hexobarbital sleep time test, which portion, receptor binding of the compound is measures total duration (in minutes) of loss of the correlated with its anticonvulsant activity. This righting reflex in male rats following intraperi- involves evaluation of its ability to displace radi- toneal administration of a hypnotic dose of hex- olabeled flunitrazepam and gamma-aminobutyric obarbital on day six. Development of tolerance is acid from membranes that have been isolated as indicated by a shorter sleep time in group 1 and a P2 fraction from whole-mouse-brain homoge- may reflect induction of microsomal hepatic en- nates using standardized ultracentrifuge tech- zymes. Following completion of the sleep time niques. The estimated displacing potency of the test, the same rats are each treated with the compound is given as K\ (affinity constant of the original regimens for two more days before being inhibitor) and IC50 (inhibitor concentration that killed. The liver is removed and weighed and the displaces 50% of the radiolabeled ligand from endoplasmic reticulum oxidative enzyme (hepatic the membranes). microsome) isolated. Changes in liver weight, total liver protein, cytochrome P-450, enzyme Phase 6 (anticonvulsant quantification in rats, activity of jb-nitroanisole demethylase, and cyto- p.o.) was added to the protocol to verify anticon- chrome C reductase are measured, and these in vulsant activity and neurotoxicity in another ro-
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vitro findings are used to confirm the results of FDA regulations require that clinical investiga- the in vivo studies. tions be conducted in three phases: determina- tion of safety in humans, usually healthy volun- Toxicology Project teers (phase I), tests to see whether the drug If the results from all phases of the ASP are treats or prevents the disease for which it is favorable, the ADD program (with the assistance intended, as well as estimation of clinical safety of a priority evaluation by the Epilepsy Advisory and efficacy (phase II), and evaluation of long- Committee) schedules compounds for further term efficacy and safety in extensive clinical trials testing in the Toxicology Project, which provides (phase III). Generally speaking, two adequate and the toxicity evaluation required by the FDA be- well-controlled phase II trials by independent fore humans can be exposed to the drug. Selec- investigators (or a multiclinic study in which data tion of compounds for toxicity studies is based from at least three investigators can be evaluated on four criteria: (a) adequate absorption after independently) are considered minimal in estab- oral administration to mice and rats; (b) adequate lishing the efficacy of a new drug. Although not protective indices after oral and intraperitoneal required, it is usually implied that at least one administration to mice and rats; (c) a compound trial must be performed in the U.S. if the drug is with a novel chemical structure; and (d) absence to be approved for marketing in this country. of tolerance to the anticonvulsant effects. The Phase I clinical trials: Phase I studies provide greatest emphasis is given to potent compounds data on the safety, pharmacologic effects, phar- having a novel structure, i.e., compounds whose macokinetics, and side effects of a given drug. activity is equal to or better than that of known These studies are usually performed by the phar- drugs and belonging to a different chemical fam- maceutical company rather than the ADD pro- ily. Toxicity studies consist of a 91-day evaluation gram, since they do not require expertise in of the compound after oral administration to rats clinical epilepsy research. However, performance and beagles. The compound is administered in of these studies is entirely within the scope of the doses exceeding those anticipated for clinical use. ADD program, especially if it would accelerate Urine, blood, and tissue samples from more than clinical testing of a highly promising compound 25 organs are examined for abnormal changes, or if the pharmaceutical company is unable to and any cardiovascular or autonomic abnormali- perform such studies on a timely basis, as for ties are noted. example if it is a foreign company lacking a firm Since one of the goals of the ADD program is understanding of the need for extensive phase I cost sharing between the government and the studies. pharmaceutical industry, the ADD program as- Usually, healthy ("normal") adult volunteers sumes only part of the cost of the toxicology are involved in phase I testing. Since abnormality studies. Before the studies begin, both the ADD need not be considered, subjects are more readily program and the sponsor commit themselves to available and interpretation of the findings is the time, money, and resources needed for this easier; however, the results may have limited stage of drug development. The sponsor must application to those patients for whom the drug synthesize kilogram quantities of the candidate is being developed. Healthy volunteers may dif- compound and assure its purity and stability be- fer from patients in their ability to tolerate side fore toxicity testing as well as perform some effects from large doses of antiepileptic drugs; pharmacologic studies on the renal, gastrointes- informed consent must be obtained, and close tinal, and cardiovascular systems. If a compound observation and expert supervision are manda- from an academic supplier is selected for toxicol- tory. The investigators must be experienced in ogy studies, that supplier usually seeks a sponsor clinical pharmacology and medicine and be will- from the pharmaceutical industry. ing to perform the necessary tedious, frequent, and thorough examinations. Phase I studies usu- Controlled clinical trials ally consist of both single- and multiple-dose tests Once a compound has progressed through all following a random-assignment, single-blind, or of the aforementioned tests, it is potentially ready double-blind design, since antiepileptic drugs to be used in humans. Nevertheless, because of may produce CNS effects that are difficult to important species-related differences in drug me- evaluate objectively. tabolism and action, the ultimate value of any Phase II clinical trials: Phase II studies are new drug obviously must be proved clinically. used to evaluate the safety and efficacy of a drug
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Table 4. International classification of epileptic seizures16 * I. Partial seizures (beginning locally) A. Simple partial seizures (consciousness not impaired) 1. With motor symptoms 2. With somatosensory or special sensory symptoms 3. With autonomic symptoms 4. With psychic symptoms B. Complex partial seizures (with impairment of consciousness) 1. Beginning as simple partial seizures and progressing to impairment of consciousness a. With no other features b. With features as in simple partial seizures c. With automatisms 2. With impairment of consciousness at onset a. With no other features b. With features as in simple partial seizures c. With automatisms C. Partial seizures secondarily generalized II. Generalized Seizures (bilaterally symmetrical, without local onset) A. 1. Absence seizures 2. Atypical absence seizures B. Myoclonic seizures C. Clonic seizures D. Tonic seizures E. Tonic-clonic seizures F. Atonic seizures III. Unclassified epileptic seizures (data inadequate or incomplete)
* Approved by the International League Against Epilepsy in September 1981.
in epileptic patients and to determine the thera- description given by the patient or an observer, peutic dose range and its variability in individual but this is often inadequate. For drug trials, the patients. The ADD program has concentrated on type of seizure should be diagnosed by video support of phase II trials, at first sponsoring the monitoring whenever possible and the seizures trials which led to the marketing of four drugs categorized according to the International Clas- from 1974 to 1981 and, more recently, evaluat- sification of Epileptic Seizures16 (Table 4). In ad- ing promising compounds emerging from the dition, because some patients experience more screening and toxicology projects. seizures than others, antiepileptic drug testing Design and execution of controlled clinical requires maximal statistical efficiency with mini- trials of antiepileptic drugs are not trivial tasks. mal danger to the patient. The classic two-period The subtleties of seizures and clinical observa- crossover design (Fig. 4) has become a standard tions require close interaction beween clinical means of alleviating this problem. neurologists experienced in controlled clinical Clinical trials are further complicated by the testing and biostatisticians experienced in the fact that many potential participants have several design of clinical trials and with knowledge of types of seizures, and a drug which is effective neurologic disease. The patient population must for one type may have no effect on another. be carefully defined, and any practical limitations Other antiepileptic drugs may be required when imposed on the study design by the problems of multiple types of seizures coexist, but most are patient availability must be considered. A small enzyme-inducers and may affect the metabolism patient sample is typical. Highly efficient statisti- of the test drug. While it would be desirable to cal designs and accompanying analyses are im- market a drug for all types of seizures it is effec- perative in testing new forms of antiepileptic tive against, the cost of mounting controlled stud- therapy if the trials are to yield maximal clinically ies for several types of seizures is considerable. useful information. In practice, an NDA is usually sought for a spe- Accurate classification of the type of seizure is cific type of seizure and additional indications are crucial to success. Because several terms are used requested later. The parameters used to measure to describe seizures, the type of seizure must be drug efficacy must be sensitive and meaningful clearly defined in order to make the trial com- indicators of active treatment effect; sophisti- prehensible to others. Usually this is based on the cated statistical analyses of poorly defined clinical
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Patient Eligible
Randomization (Randomized Permuted Blocks)
Treatment Ni = N/2 N = N/2 Period I receive Ti 2 receive T2 I "Washout" and "Washout " and
crossover to T2 crossover to Tj
Treatment Ni N2 Period il receive T2 receive Tj Fig. 4. Classic two-period crossover design of clinical drug trials. Ti = standard treatment; r2 = new treatment.
parameters could lead to studies which, in them- patients are generally allowed to continue on the selves, are appropriately analyzable but have very new drug for at least a year. Phase III studies are little inferential credence. conducted by the pharmaceutical firm without Finally, moral and ethical considerations limit the ADD program's support. testing of new drugs for some types of seizures Ongoing clinical trials: Currently, the ADD to individuals who are continuing to have them program is supporting multicenter phase II clin- despite optimal therapy with existing drugs, or ical trials of four potential new antiepileptic who are experiencing severe adverse reactions to drugs. Studies of progabide (a GAB A agonist) at these drugs. Comparison of a test drug to a the University of Virginia and University of Min- placebo is usually the best way of revealing drug nesota, begun in 1981, are nearing completion. effects; however, use of a placebo alone is only In 1982, a trial of an imidazole was started at the rarely permissible because of medical and ethical University of Washington and University of Cal- objections to not treating seizures. Thus in clini- ifornia at Los Angeles, and a trial of a carbox- cal trials of antiepileptic drugs, there is an in- amide was started at the University of Michigan creased emphasis on the need for clinically rele- and University of Utah. Following pharmacoki- vant end points and an effective statistical design. netic studies at the University of Washington, a The ADD program has played a major role in dicarbamate will be tried late in 1983 at the advancing clinical trial methodology to help solve University of Virginia and the University of Min- these problems. Controlled trials and double- nesota. Preliminary studies of other promising blind studies are now the general practice,17 and compounds are also underway with the evalua- guidelines for antiepileptic drug testing have tion of a benzisoxazole at the University of Wash- been developed by the Epilepsy Branch and ington and a pyridine derivative at the Clinical adopted by the International League Against Epi- Center of the National Institutes of Health. lepsy as well as the FDA.18 References Phase III clinical trials: Phase III studies eval- 1. Locock C. (discussion of) Sieveking EH. Analysis of 52 cases uate the long-term efficacy of new drugs, usually of epilepsy observed by the author. Lancet 1857; 1:527. in patients treated successfully during the phase 2. Hauptmann A. Luminal bei Epilepsie. Munch Med Woch- II trials. In an open protocol, about 150 of these enschr 1912;59:1907-1909. [Ger]
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3. Albertoni P. Untersuchungen über die Wirkung einiger Arz- trazol) and electroshock in mice, and their modification by neimittel auf die Erregbarkeit des Grosshirns nebst Beiträgen anticonvulsants. J Pharmacol Exp Ther 1953; 108:168-176. zur Therapie der Epilepsie, Arch Exp Pathol Pharmakol 1882; 11. Swinyard EA. Laboratory assay of clinically effective antiepi- 15:248-288. [Ger] leptic drugs. J Am Pharm Assoc 1949; 38:201-204. 4. Stone WE. Systemic chemical convulsants and metabolic de- 12. Chen G, Portman R, Ensor CR, Bratton AC Jr. The anticon- rangements. [In] Purpura DP, PenryJK, Tower DB, Wood- vulsant activity of «-phenyl succinimides. J Pharmacol Exp bury DM, Walter RD, eds. Experimental Models of Epilepsy— Ther 1951; 103:54-61. A Manual for the Laboratory Worker. New York, Raven 13. Federal Food, Drug, and Cosmetic Act of 1938. Public Law Press, 1972, pp 407-432. 717, 75th Congress. 5. Spiegel EA. Quantitative determination of the convulsive re- 14. Drug Amendments Act of 1962. Public Law 87-781,21 USC activity by electric stimulation of the brain with the skull 355. intact. J Lab Clin Med 1937; 22:1274-1276. 15. Public Health Service Advisory Committee on the Epilepsies. 6. Merritt HH, Putnam TJ. A new series of anticonvulsant drugs Minutes of meeting, February 9, 1967. Bethesda, Md.: Na- tested by experiments on animals. Arch Neurol Psychiatry tional Institutes of Health, 1967. 1938;39:1003-1015. 16. Commission on Classification and Terminology of the Inter- 7. Merritt HH, Putnam TJ. Sodium diphenyl hydantoinate in national League Against Epilepsy. Proposal for revised clinical the treatment of convulsive disorders. JAMA 1938; and electroencephalographic classification of epileptic sei- 111:1068-1073. zures. Epilepsia 1981; 22:489-501. 8. Federal Food and Drugs Act of 1906. Public Law 384, 59th 17. Mattson RH. The design of clinical studies to assess the Congress. efficacy and toxicity of antiepileptic drugs. Neurology 1983; 9. Richards RK, Everett GM. Analgesic and anticonvulsive prop- 33 (Suppl l):l-37. erties of 3,5,5-trimethyloxazolidine-2,4-dione (Tridione). Fed 18. U.S. Food and Drug Administration. Guidelines for the clin- Proc 1944; 3:39. ical evaluation of antiepileptic drugs (adults and children). 10. Goodman LS, Grewal MS, Brown WC, Swinyard EA. Com- Rockville, MD, FDA, 1981. parison of maximal seizures evoked by pentylenetetrazol (me-
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