Safety of tacrine: clinical trials, treatment IND, and postmarketing experience.

The safety of tacrine (Cognex), a centrally active, reversible acetylcholinesterase inhibitor approved in 1993 for the treatment of mild to moderate dementia of the Alzheimer type, was evaluated in 2,706 patients with Alzheimer disease (AD) in clinical trials and in 9861 patients with AD in a treatment investigational new drug (TIND) program. More than 190,000 patients in the United States received tacrine during the first 2 years following marketing approval. The most common tacrine-associated adverse events were elevated liver transaminase levels [alanine aminotransferase (ALT) and, to a lesser degree, aspartate aminotransferase] and peripheral cholinergic events involving primarily the digestive system (nausea, vomiting, diarrhea, dyspepsia, anorexia, and weight loss). Based on clinical trial experience, potentially clinically significant (>3 x upper limit of normal) ALT elevations occurred in 25% of patients, requiring routine monitoring early in treatment. The elevations were almost always asymptomatic, rarely accompanied by significant increases in bilirubin, and related to time on drug rather than to dose (90% occurred within the first 12 weeks of treatment). Gastrointestinal events were related to dose and generally of mild to moderate intensity. Tacrine-associated events, including ALT elevations, were reversible. Cholinergic events were manageable with dosage adjustment. Tacrine was not associated with permanent liver injury in clinical trials or a TIND setting.

Structure-activity studies on benzhydrol-containing nipecotic acid and guvacine derivatives as potent, orally-active inhibitors of GABA uptake.

The introduction of lipophilic groups onto the ring nitrogen of nipecotic acid and guvacine, two known GABA uptake inhibitors, afforded potent, orally-active anticonvulsant drugs. A series of compounds is reported which explores the structure-activity relationships (SAR) in this series. Among the areas explored: side-chain SAR (aromatic-, heterocyclic-, and tricyclic-containing side chains) and modifications to the tetrahydropyridine ring. The benzhydrol ether-containing side chains afforded the most potent compounds with several exhibiting in vitro IC50 values for GABA uptake of < 1 microM (including 5, Table I; 37, 43, Table IV; and 44, Table V). Compound 44 was selected for extensive evaluation and subsequently progressed to Phase 1 clinical trials with severe adverse effects seen after single dose administration to humans.

N-phenyl-N'-pyridinylureas as anticonvulsant agents.

A series of N-phenyl-N'-pyridinylureas was examined for anticonvulsant activity. Extensive structure/activity investigations revealed optimal activity in the N-(2,6-disubstituted-phenyl)-N'-(4-pyridinyl)urea series, with 37 exhibiting the best overall anticonvulsant profile. Compound 37 was effective against seizures induced by maximal electroshock but did not protect mice from clonic seizures produced by the convulsant pentylenetetrazol. The overall pharmacological profile suggests that 37 would be of therapeutic use in the treatment of generalized tonic-clonic and partial seizures. Compound 37 was selected for Phase 1 clinical trials.

6-Alkyl-N,N-disubstituted-2-pyridinamines as anticonvulsant agents.

The anticonvulsant effect of a series of 6-alkyl-N,N-disubstituted-2-pyridinamines is described. An investigation was carried out to optimize the anticonvulsant activity and reduce behavioral side effects in this series. Three compounds (7, 8, 10; Table I) were selected from initial screening for a more complete pharmacological evaluation. While each of these compounds was a potent anticonvulsant agent with ED50 values from 5 to 10 mg/kg, the activity was accompanied by significant behavioral side effects including decreased spontaneous locomotion, ataxia, and ptosis.

3-Phenoxypyridine 1-oxides as anticonvulsant agents.

The anticonvulsant activity of a series of 3-phenoxypyridine 1-oxides is described. An investigation carried out to optimize the activity/side effect ratio provided 4-methyl-3-phenoxypyridine 1-oxide, 3, as the derivative of choice. Overall, 3 has a pharmacological profile that is very similar to phenytoin. It exhibited significant anticonvulsant activity at doses that did not produce ataxia or sedation but caused increased spontaneous behavioral activity not seen with most anticonvulsants. The short duration of pharmacological effect of 3 was attributed to metabolic hydroxylation at the C-4 pyridine methyl group; however, structural modifications designed to inhibit this metabolic pathway were unsuccessful.

6-Alkoxy-N,N-disubstituted-2-pyridinamines as anticonvulsant agents.

The anticonvulsant effect of a series of 6-alkoxy-N,N-disubstituted-2-pyridinamines is described. An investigation was carried out to optimize the activity/side-effect ratio in this series of compounds. The most desirable profile was seen with 1-[6-(2-methylpropoxy)-2-pyridinyl]piperazine, 6, and this compound was selected for a more complete pharmacological evaluation. Overall, 6 has a pharmacological profile that is very similar to that of diphenylhydantoin (phenytoin). While nearly equipotent to phenytoin, animal studies suggest a fairly short duration of action. In addition, 6 exhibited some troublesome side effects including central nervous system depression and hypothermia.

Examination of a series of 8-[3-[bis(4-fluorophenyl)amino]propyl]-1-aryl-1,3,8- triazaspiro[4.5]decan-4-ones as potential antipsychotic agents.

8-[3-[Bis(4-fluorophenyl)amino]propyl]-1-phenyl-1,3,8- triazaspiro[4.5]decan-4-one (3) and related compounds have been shown to have antipsychotic profiles in biochemical and behavioral pharmacological test models. The dose of 3 necessary to produce catalepsy in rats is much greater than that required for activity in behavioral tests predictive of antipsychotic efficacy, for example the suppression of high base line medial forebrain bundle self-stimulation in rats. This suggests that 3 would have a reduced propensity for neurological side effects. The effects of substitution on the 1-phenyl moiety and on the N-3 nitrogen atom of the triazaspirodecanone portion of 3 were examined. Results from this study suggest that behavioral activity is sensitive to substituents on the 1-phenyl moiety while substituents on the N-3 nitrogen are more generally tolerated. In both rats and squirrel monkeys compound 3 was found to have a similar separation between doses inhibiting Sidman avoidance activity and those causing catalepsy. However, in an extrapyramidal side effect (EPS) test model using haloperidol-sensitized cebus monkeys, 3 elicited signs of EPS at doses approximating those previously determined to be efficacious.

1-[3-(Diarylamino)propyl]piperidines and related compounds, potential antipsychotic agents with low cataleptogenic profiles.

On the basis of a structural model of the postsynaptic dopaminergic antagonist pharmacophore, a series of 1-[3-(diarylamino)propyl]piperidines and related compounds was synthesized and evaluated for potential antipsychotic activity. For a rapid measure of activity, the target compounds were initially screened in vitro for inhibition of [3H]haloperidol binding and in vivo in a test of locomotor activity. Behavioral efficacy of compounds identified from the initial screens was more accurately measured in rats by using a suppression of high base-line medial forebrain bundle self-stimulation test model. The propensity of these compounds for causing extrapyramidal side effects was evaluated by using a rat catalepsy method. On the basis of these test models, we have shown that the methine carbon of the 1-(4,4-diarylbutyl)piperidines can be advantageously replaced with a nitrogen atom. The 1-[3-(diarylamino)propyl]piperidines were less cataleptic than the corresponding 1-(4,4-diarylbutyl)piperidines. The compounds with the widest separation between efficacious dose and cataleptic dose are 8-[3-[bis(4-fluorophenyl)amino]propyl]-1-phenyl-1,3,8-triazaspiro [4. 5]decan-4-one (6), 1-[1-[3-[bis(4-fluorophenyl)amino]propyl]-4-piperidinyl]-1,3-dihydro- 2H-benzimidazol-2-one (11), 1-[1-[3-[bis(4-fluorophenyl)amino]propyl]-1,2,3,6-tetrahydro-4- pyridinyl]-1,3-dihydro-2H-benzimidazol-2-one (22), and 1-[3-[bis(4-fluorophenyl)amino]propyl]-4-(2-methoxyphenyl)piperazine (26).