Antiepileptic Drugs

1. Antiepileptic Drugs Gisvold’s book • Barbiturates • Hydantoins • Oxazolidinedienos • Succinimides • Phenacemides • Glutethemides • Miscellaneous • Benzodiazepines

2. Mechanism of Actions • Positive allosteric modulation of action of g-aminobutyric acid (GABA) at GABAA receptor site - benzodiazepines and barbiturates • Phenobarbital, may also block voltage gated Na+-channel • 5,5-dialkylbarbiturates, may also block Ca2+ T-channel • Oxazolidine-2,4-diones and succinimides appear to act via Ca2+ T-channel block • Phenyl-substituted succinimide may also cause some Na+-channel block • The major mode of action for phenytoin, carbamazepine, oxacarbazepine, valproic acid, and felbamate is reported to be voltage-gated Na+-channel blocker

3. Antiepileptic General Structure • Overall, R1 and R2 should be hydrocarbon • Lower alkyls tend to be active against absence seizures and not active against generalized tonic-clonic or partial seizures • If one of the hydrocarbon substituent is an aryl group activity tends to be directed toward generalized tonic-clonic and partial seizures and not absence seizures

4. Barbiturates • First synthesized in 1864 by German researcher Adolf von Baeyer, the founder of Bayer pharmaceuticals company by combining urea with malonic acid • The first pharmacologically active compound discovered (1903) was barbital which was very effective in putting dogs to sleep • Barbital was then marketed by Bayer under the trade name Veronal(after a peaceful Italian City name) and then Phenobarbital, under the trade name Luminal, as a sedative-hypnotic • In the 1950s and 1960s, reports began to be published about side effects and dependence related to barbiturates • In 1970 several barbiturates were designated as controlled substances with the passage of the Controlled Substances Act of 1970 Which are under schedules III and IV?

5. Barbiturate Structures • The main antiepileptic drug is Phenobarbital major metabolite of which is the p-hydroxyl and/or the p-hydroxyglucuronide; about 25% excreted unchanged • Mephobarbital is N dealkylated to phenobarbital which many think is the active drug and thus mephobarbital is a prodrug • N1 and N3 are not distinguishable. • Both drugs being substituted with an aromatic ring at R2 are effective against generalized tonic-clonic and partial seizures.

6. Secobarbital possesses anaesthetic, anticonvulsant, sedative and hypnotic properties and sometimes used in physician assisted suicides. The primary metabolites of secobarbital are (ω-1)-hydroxysecobarbital (36.5%) and dihydroxydihydrosecobarbital (secodiol, 15.7%), both of which are excreted also as glucuronide conjugates (26.2%). Thiopental is an ultra-short-acting barbiturate and has been used commonly in the induction phase of general anesthesia and historically to induce medical comas. It is also used intravenously for the purposes of euthanasia. Along with pancuronium bromide and potassium chloride, thiopental is used in 34 states of the U.S. to execute prisoners by lethal injection. Thiopental is still used in some places as a truth serum during interrogation. As with all lipid soluble anaesthetic drugs, the short duration of its action is almost entirely due to its redistribution away from central circulation towards muscle and fat tissue. Once redistributed the free fraction in the blood is metabolized in the liver. Sodium thiopental is mainly metabolized to pentobarbital.

7. Barbiturate Relatives • Primidone is a pyrimidinedione and not a barbiturate but is related where C2 oxidation leads to conversion into Phenobarbital in vivo which is thought to be the active constituent • Glutethimide is another non barbiturate sedative hypnotic used as safe alternative to barbiturates to treat insomnia. It is however a schedule II drug due to dependency.

8. SAR of Barbiturates • Both hydrogen atoms at C5 must be substituted • There is a decrease in onset time and a decrease in duration as C5 alkyl chain length increases. • Due to increasing lipid solubility increases rate of CNS penetration for shorter onset and increases susceptibility to microsomal metabolism due to penetration into hepatic cells • Common metabolic pathway is ω and ω-1 oxidation • Except for those with very high lipid solubility (thiobarbiturates), the barbiturates have short duration • Thiobarbiturates undergo slow metabolism; most are in the adipose tissue (depot) and not available to hepatic enzymes which can be converted to corresponding oxybarbiturate by desulfuration • Bulk on C5(i.e., aromatic ring) is a common feature for drugs with activity for generalized seizures and also for partial seizures and status epilepticus, but not good for absence seizures

9. Hydantoins Hydantoin was first isolated in 1881 by Adolf von Baeyer in the course of his study of uric acid. He obtained it by hydrogenation of Allantoin hence the name • Close structural relatives of barbiturates • Only lacking the 6-oxo group and are cyclic monoacylureas rather than diacylureas • As a consequence of losing a carbonyl group weaker organic acids than barbiturates and thus their sodium salt (e.g., phenytoin sodium) generates stronger alkaline solution SAR of Hydantoins Most of the clinically used drugs in this class possess bulky aromatic ring in position C5 that confers usefulness in generalized seizures, partial seizures and status epilepticus but not well for absence seizures

10. Hydantoin Drugs Phenytoin is metabolized by p-hydroxylation followed by conjugation similar to Phenobarbital. Mephenytoin is the hydantoin analogue of mephobarbital which is also a prodrug, converted into the dealkylated derivative. Metabolism is also by p-hydroxylation and then glucuronidation Ethotoin is dealkylated to the active drug. In this case there is free hydrogen at C5, which explains its very low potency. Metabolism is also by p-hydroxylation and then glucuronidation Fosphenytoin is Phosphate ester of phenytoin, rapidly hydrolyzed to phenytoin in vivo. Phenytoin sodium must be buffered to an alkaline pH to maintain solubility, thus is very irritating when injected. Fosphenytoin is neutral (pH~7) so is less irritating

11. Oxazolidinediones Replacement of the N-H group at position 1 of the hydantoin with an oxygen atom yields the oxazolidine-2,4-dione system Trimethadione is useful for absence seizures. Note the absence of bulky substituents at the C5 position which are useful in absence seizures. It is metabolized to 5,5 dimethyl oxazolidine 2,4 dione (dimethadione) which is also active. Both trimethadione and dimethadione are excreted in the urine and are very toxic Paramethadione is also N dealkylated, half life is 12-24 hours. Some excreted by kidney. The metabolite is active and probably accounts for most activity the half life of which is 14 days and is excreted by the kidney. Also it is fairly toxic

12. Succinimides This group of drugs resulted from a search for a less toxic version of the oxazolidinediones by replacing the “O” with CH2 Ethosuximide is lacking bulky groups attached at C3 which corresponds to C5 in the other related structures and thus is good for absence seizures. Major metabolite is from oxidation of the ethyl group, hydroxyethyl and conjugated hydroxyethyl, both are inactive Methsuximide has a bulky group at C3 which is good for absence but also picks up some partial seizures activity. It is N-dealkylated to an active metabolite. Half life of methsuximide is 1.4 h, the N demethyl has a half life of 38 h. So most activity are due to metabolite, followed by p-hydroxylation and conjugation Phensuximide possesses the bulky group at C3 which is good for absence but also picks up some generalized tonic-clonic activity. Because of the free hydrogen at C3, it is much weaker than the disubstituted compounds. N-dealkylated to an active metabolite, but the half life is about the same as the parent (5-12 hr) and the activity is due to both species. Followed by p-hydroxylation and conjugation

13. Miscellaneous Dibenzazepines structurally related to the TCAs. The H2NCO function is referred to as a carbamoyl or carboxamide. If the N in the ring is included we have a urea derivative. So it is also a ureide Oxcarbazepine does not undergo such epoxidation so is expected to be less toxic

14. Valproic Acids • Discovered accidentally • Valproic acid is a liquid and so is used as a liquid filled capsule • Being an organic solvent not soluble in water for intravenous use. Valproate sodium was developed as a water soluble salt, but too hygroscopic for solid oral dosage forms. Also causes GI irritation and cannot formulate a liquid into sustained release forms • Divalproex sodium is a stable salt for oral tablets and less irritating to the stomach

15. Felbamate is a dicarbamate. Carbamates are salts or esters of the hypothetical carbamic acid (H2NCOOH), similar to meprobamate. But felbamate has a phenyl (fel-) instead of the methyl, propyl groups as in meprobamate Gabapentin is GABA plus 5 carbons. The idea was to make GABA more lipid soluble for better CNS penetration. But works through a non GABA agonist unknown mechanism. Widely used for neuropathic pain where it is thought to involve voltage-gated N-type calcium ion channels. The more potent successor drug that is also used to treat neuropathic pain (fibromyalgia) is pregabalin approved in 2007. Its S-isomer is the active form. Baclofen modulates mammalian (but not fruit fly) GABAB receptor. It is used for the treatment of spastic movement, especially in instances of spinal cord injury, spastic diplegia, multiple sclerosis, amyotrophic lateral sclerosis and trigeminal and glossopharyngeal neuralgias. It appears to have reduced abuse and dependence potential. The drug is rapidly absorbed after oral administration and is widely distributed throughout the body. Biotransformation is low and the drug is predominantly excreted in the unchanged form by the kidneys.

16. New Molecular Entity in 2009: Vigabatrin • (±)-4-amino-5-hexenoic acid • A GABA analog and is dosed as a racemic compound, with the S-enantiomer being the pharmacologically active form. • The alkene group forms an irreversible, covalent bond with the gamma-aminobutyric acid transaminase (GABA-T) and irreversibly inhibits it. The enzyme (GABA-T) is responsible for the metabolism of the inhibitory neurotransmitter GABA; its blockade leads to increased levels of GABA in the central nervous system. • Thus, it is an antiepileptic drug indicated as a monotherapy for pediatric patients 1 month to 2 years of age with infantile spasms (IS) and as an adjunctive therapy for adult patients with refractory complex partial seizures (CPS) who have inadequately responded to several alternative treatments. • It is essentially completely orally absorbed and widely distributed throughout the body. It is not significantly metabolized (80% of a dose is recovered as parent drug), although it does induce CYP2C9, and it is eliminated primarily through renal excretion.

17. The Benzodiazepines (sedative-hypnotics) • 1,4-benzodiazepine-4-oxide • 1,4-benzodiazepine-2-one • Annelated benzodiazepines

18. BDZ Acid-Base Character • All are basic; most are weakly basic • N4 is basic (imine) in all except for chlordiazepoxide (because the n-electrons are utilized in forming the N-oxide). The imine is weak because the carbon atom is attached to two electron withdrawing aromatic rings (ring A and C). e.g., Lorazepam pKa 1.3. • Some are more basic due to substituents at C2 or N1. e.g., chlordizepoxide – the nitrogen substituent at C2 produces an amidine, pKa 4.76; flurazepam – the N1 substituent is a tertiary amine, pKa 8.16. • Some are more basic due to fused ring D. e.g., midazolam – the nitrogens’ positions 1 an 2 are pyridine–like nitrogen, pKa 6.15 • Some are weakly acidic due to the amide (N1–C2): The nitrogen must be unsubstituted to tautomerize. Since amides have a low tendency to tautomerize, they have a low tendency to ionize, thus are weak acids. e.g., lorazepam pKa 11.5 • Only one is strongly acidic: clorazepate is the potassium salt of a carboxylic acid, thus is highly ionized

19. Table: Log P Values of Selected BDZs Drug Calc Expt Drug Calc Expt Alprazolam 3.87 2.12 Halazepam 3.73 3.97 Bromazepam 1.93 2.05 Lorazepam 2.41 2.39 Chlordiazepoxide 2.42 2.44 Medazepam 4.43 4.41 Clonazepam 2.53 2.41 Midazolam 4.33 — Clorazepate 2.04 — Nordazepam 2.87 2.93 Demoxepam 1.87 1.49 Oxazepam 3.32 2.24 Diazepam 2.7 2.82 Prazepam 3.99 3.73 Estazolam 3.32 — Quazepam 4.3 4.03 Flumazenil 1.03 1.0 Temazepam 2.15 2.19 Flunitrazepam 1.91 2.06 Triazolam 3.96 2.42 Flurazepam 3.02 — from http://esc.syrres.com

20. BDZ Metabolism ???? • Currently there are only four drugs available in injectable form only one is in pure aqueous solution • The other three utilized organic co-solvents which are irritating to tissue • Two factors are responsible; the low water solubility and the susceptibility of ring B to hydrolysis. Benzodiazepines in solution exist in equilibrium with the inactive benzophenone analog produced by the opening of the N4-C5 double bond (a Shiff base). Low pHs favor the benzophenone structure, which also favor aqueous solubility. In this form the amide is susceptible to hydrolysis. Recall that low pHs catalyze hydrolysis. Thus benzodiazepines are unstable in acid pHs. Fortunately at blood pH of 7.4 benzodiazepines exists in the closed ring B form.

21. BDZ SARs IC50 = 1.8 IC50 = 350 1) Applicable to 4-oxides only a) nitrogen at position 2 is not essential (does increase basic character) b) increasing size of 2 substituent beyond methyl decreases potency. This group can not bind with Site 2 and may introduce steric hindrance. c) the N-oxide is not essential and decreases potency. Since it is a polar group it has a repulsive interaction with site 4. 2) Ring A SARs a) Small e/w substituents at C7 increase potency: NO2 > CF3 > Br > Cl > H > phenyl b) Substituents on other ring A positions decrease potency. e/w groups make ring A slightly electron deficient. This increase the London interaction with Site 1. Groups at other positions have the same electronic effects. c) Large groups or electron donors decrease potency. These introduce steric factors. Clonazepam with a 7-nitro has an IC50 of 1.8.

22. 3) Ring B SARs a) Removal of phenyl at C5 decreases potency b) Replacement of phenyl with isostere at C5 is allowed. Ring C provides a London interaction with Site 5. Loss activity if this interaction decreases binding with the receptor c) Isosteric replacement of O by S at C2 decreases potency. Generally this isosteric replacement results in an increase in potency. However, since the benzodiazepeines are so lipid soluble, the increase in lipid solubility makes the log P too high d) Saturation of atoms at 4 and 5 form sp2 hybridized to sp3. This changes the shape of ring B. Further studies have shown that the shape of ring B is the most important factor determining binding. Dihydrodiazepam has an IC50 greater than 1000 vs 8.1 for diazepam.

23. IC50 of 8.4 (8.1 for diazepam) IC50 of 92 (14.8 for flurazepam) IC50 of 18 (14.8 for flurazepam) e) A methyl is the best substituent at N1. 1) By removing the methyl the interaction with Site 1 is lost. (nordiazepam) 2) Increasing the size of the alkyl introduces steric factor, however linear substituents retain sufficient potency to be useful clinically, but bulky groups like t-butyl produce very weak agents. f) A hydroxyl at C3 decreases potency as polar group interacting with the lipophilic Site 3, decrease half-life, as it is susceptible to rapid glucuronidation. The decrease in toxicity is due to the ease of metabolism (oxazepam). g) Annulation with triazole or imidazole markedly increases potency. The ring increases LWPC and increases receptor affinity. Annelated benzodiazepines are more potent than corresponding 2-one derivatives.

24. IC50 3.5 18 for oxazepam 4) Ring C SARs a) Halogenation only at 2' enhances potency, other positions decrease potency: Cl > F > Br > NO2 > CF3 > H. Electron withdrawing groups have similar electronic effects as discussed for ring A, thus promote binding with site 5. (Lorazepam) 5) 1,2-Annelated SARs a) Introduction of 1–methyl shortens duration of action. The 1-methyl is very susceptible to oxidation, thus easily hydroxylated. The hydroxy metabolite retains potency but may be quickly conjugated and excreted.

25. BDZ Mechanism of Action As with the barbs, the GABAA receptor complex is implicated as the site of action. The GABAA receptor is a ligand gated ion channel composed of different combinations of a, b, g, r subunits. α1–6, β1–3, γ1–3,, ρ1–3. Different combinations result in affinity for different drugs and producing different activities Major subtype (60% ): α1β2γ2: Sedative, amnestic, anticonvulsant Minor subtype (15% ): α2β3γ2: Anxiolytic Minor subtype (10% ): α3βnγ2 To date three BDZ receptors have been identified. The BDZ receptors are also known as omega–1, omega–2 and omega–3. The BDZ–1 receptors are located in areas of the brain that are involved in sedation, and the BDZ–2 receptors are highly concentrated in areas responsible for cognition, memory, and psychomotor functioning. The BDZ—3 receptors are located in peripheral tissues and not involved in hypnotic efficacy. The BDZs bind to all three receptor subtypes. This lack of selectivity allows the benzodiazepines to be used as anticonvulsants, sedatives, hypnotics, anxiolytics muscle relaxants and general anesthetics, the activity is determined by the dose and the desires of the company.

26. Other Benzodiazepine Receptor Ligands: Type 1 selective agents should have less central adverse effects on cognition, memory, and psychomotor function. Compared to benzodiazepine, these agents have little effect on the normal stages of sleep, and few if any anxiolytic, anticonvulsant, muscle relaxant properties or amnesia. Zolpidem, Zaleplon and Eszopicloneselectively bind to BDZ–1 receptors. Hypnotic efficacy may not differ between benzodiazepine and the newer selective agents. Zaleplon has a half-life of 1 hour. This allows dosing up to four hours before a patient needs to be awake. However, it may lead to awakening during the night. Eszopiclone has the advantage of being the only hypnotic approved by the FDA for continual use. Benzodiazepine Receptor Antagonists Benzodiazepine antagonists are useful in treating overdoses and in terminating benzodiazepine induced anesthesia. Flumazenil binds the receptor with high affinity (IC50 2.5) but the lack of ring C and the modifications of ring B prevent activation of the receptor. Activity is terminated by hydrolysis to the inactive acid.

27. Study Guide • The mode of action of all antiepileptic drugs • Critically analyze the differences in structural changes of all the classes. How the bulkiness of groups R1 and R2 affect the activity of the drugs against generalized seizures, partial seizures or absence seizures? • Structures of most important drugs in these classes in the way that you can recognize them • Metabolic pathways and pharmacokinetics of the drugs indicated • SAR in general and also for the individual drugs • SAR of BDZs. What is truth serum? Which drugs are used to cause physician assisted suicide or execute the criminals? • Know all about vegabatrin • Which antiepileptic class of drugs are most toxic? • Which receptor(s) are modulated by BDZs? • Which ones are Type-1 selective agents? What are their advantages? What are benzodiazepine receptor antagonists?

28. Case Study Case 1. JB, a 34-year-old woman, was the recent victim of car accident in which she sustained a severe head injury. She has come to emergency room after suffering a severe, generalized tonic-clonic convulsive episode. She is hospitalized and, within the next few days, two more generalized seizures are experienced. It is decided to initiate chronic anticonvulsant therapy, and your advice is solicited. • Which of the anticonvulsant structures (1-5) should be administered to JB? • What structural features of your drug of choice are responsible for your answer?