Novel oxotremorine-related heterocyclic derivatives: Synthesis and in vitro pharmacology at the muscarinic receptor subtypes.

A set of novel heterocyclic ligands (6-27) structurally related to Oxotremorine 2 was designed, synthesized and tested at muscarinic receptor subtypes (mAChRs). In the binding experiments at cloned human receptors (hm1-5), compounds 7 and 15 evidenced a remarkable affinity and selectivity for the hm2 subtype. The in vitro functional assays, performed on a selected group of derivatives at M(1), M(2), and M(3) tissue preparations, singled out the 3-butynyloxy-5-methylisoxazole trimethylammonium salt 7 as a potent unselective muscarinic agonist [pEC(50): 7.40 (M(1)), 8.18 (M(2)), and 8.14 (M(3))], whereas its 5-phenyl analogue 12 behaved as a muscarinic antagonist, slightly selective for the M(1) subtype [pK(B): 6.88 (M(1)), 5.95 (M(2)), 5.53 (M(3))]. Moreover, the functional data put in evidence that the presence of the piperidine ring may generate a functional selectivity, e.g., an M(1) antagonist/M(2) partial agonist/M(3) full agonist profile (compound 21), at variance with the corresponding quaternary ammonium salt (compound 22) which behaved as a muscarinic agonist at all M(1-3) receptors, with an appreciable selectivity for the cardiac M(2) receptors.

Design, synthesis, and action of oxotremorine-related hybrid-type allosteric modulators of muscarinic acetylcholine receptors.

A novel series of muscarinic receptor ligands of the hexamethonio-type was prepared which contained, on one side, the phthalimidopropane or 1,8-naphthalimido-2,2-dimethylpropane moiety typical for subtype selective allosteric antagonists and, on the other, the acetylenic fragment typical for the nonselective orthosteric muscarinic agonists oxotremorine, oxotremorine-M, and related muscarinic agonists. Binding experiments in M(2) receptors using [(3)H]N-methylscopolamine as an orthosteric probe proved an allosteric action of both groups of hybrids, 7a-10a and 8b-10b. The difference in activity between a-group and b-group hybrids corresponded with the activity difference between the allosteric parent compounds. In M(1)-M(3) muscarinic isolated organ preparations, most of the hybrids behaved as subtype selective antagonists. [(35)S]GTPgammaS binding assays using human M(2) receptors overexpressed in CHO cells revealed that a weak intrinsic efficacy was preserved in 8b-10b. Thus, attaching muscarinic allosteric antagonist moieties to orthosteric muscarinic agonists may lead to hybrid compounds in which functions of both components are mixed.

Synthesis and in vitro pharmacology of novel heterocyclic muscarinic ligands.

A set of novel heterocyclic ligands (7a-9a, 7b-9b, and 9c) structurally related to oxotremorine 2 was designed, synthesized, and tested at muscarinic receptor subtypes. In the binding experiments at cloned hm1-5, the presence of the 2-methylimidazole/2-methyl-3-alkylimidazolium moiety in place of the pyrrolidine ring revealed, in derivatives 8a, 8b, and 9c, a moderate selectivity for some receptor subtypes. The functional in vitro assays yielded results that correlated closely to binding data. In general, on passing from agonists bearing the pyrrolidine moiety to their analogues carrying the 2-methylimidazole function, the overall pharmacological efficacy profile is shifted from agonism toward partial agonism. The insertion of the 2-methyl-3-alkylimidazolium moiety advances the effect such that the compounds are pure antagonists. Quite similarly, chiral 3-oxo-Delta(2)-isoxazoline (+/-)-10 behaved as a weak antagonist unable to discriminate the different muscarinic receptor subtypes.

Synthesis and functional characterization of novel derivatives related to oxotremorine and oxotremorine-M.

Two subseries of nonquaternized (5a-10a) and quaternized derivatives (5b-10b) related to oxotremorine and oxotremorine-M were synthesized and tested. The agonist potency at the muscarinic receptor subtypes of the new compounds was estimated in three classical in vitro functional assays: M1 rabbit vas deferens, M2 guinea pig left atrium and M3 guinea pig ileum. In addition, the occurrence of central muscarinic effects was evaluated as tremorigenic activity after intraperitoneal administration in mice. In in vitro tests a nonselective muscarinic activity was exhibited by all the derivatives with potencies values that, in some instances, surpassed those of the reference compounds (i.e. 8b). Functional selectivity was evidenced only for the oxotremorine-like derivative 9a, which behaved as a mixed M3-agonist/M1-antagonist (pD2 = 5.85; pA2 = 4.76, respectively). In in vivo tests non-quaternary compounds were able to evoke central muscarinic effects, with a potency order parallel to that observed in vitro.

Synthesis and in vitro characterization of novel amino terminally modified oxotremorine derivatives for brain muscarinic receptors.

A series of novel 2-substituted acetylenic pyrrolidines and piperidines related to oxotremorine (1) were prepared and evaluated in vitro as muscarinic cholinergic agents at brain M1 and M2 receptors. One analogue, 3-(2-oxo-1-pyrrolidinyl)-1-[2(R)-pyrrolidinyl]-1-propyne hydrogen oxalate (6a), was found to be a partial agonist producing a PI hydrolysis response at cortical M1 receptors approximately 3-fold larger than that produced by 1. The intrinsic activity profile of 6a at brain muscarinic receptors is similar to those of azetidine oxo analogue 2 and dimethylamino oxo analogue. All three compounds are partial M1 agonists and full M2 agonists; however, the profile of 6a in binding studies is significantly different. While 2 and 3 exhibit large M2 selectivities ranging between 8-fold to several hundred-fold, the binding profile of 6a shows almost no subtype selectivity.

Phenyl-substituted analogues of oxotremorine as muscarinic antagonists.

A series of phenyl-substituted analogues of the muscarinic agent oxotremorine (1) have been prepared. The new compounds (3b-11b and 9c) were assayed for antimuscarinic activity on the isolated guinea pig ileum and in intact mice. They were also evaluated for ability to inhibit the binding of the muscarinic antagonist (-)-[3H]-N-methylscopolamine to homogenates of the rat cerebral cortex. The phenyl-substituted derivatives were devoid of intrinsic muscarinic activity. Instead, they behaved as competitive muscarinic antagonists in these assays with similar or lower affinity for muscarinic receptors than the corresponding methyl-substituted analogues. The succinimide (8b) and the pyrrolidone (3b) derivatives of 1 substituted with a phenyl group at position 1 of the butynyl chain showed the highest antimuscarinic potency with dissociation constants (KD) of 0.10 and 0.20 microM, respectively, in the ileum assay. The phenyl-substituted analogues showed an approximately 10-fold lower in vivo antimuscarinic potency than their corresponding methyl-substituted positional isomers. A correlation was observed between in vitro and in vivo potency within subsets consisting of methyl- and phenyl-substituted derivatives.

5-Methyl-2-pyrrolidone analogues of oxotremorine as selective muscarinic agonists.

A series of N-(4-amino-2-butynyl)-5-methyl-2-pyrrolidones modified only in the amino group was synthesized. The compounds were agonists, partial agonists, and antagonists on the isolated guinea pig ileum. They had greater affinity and lower intrinsic efficacy at ileal muscarinic receptors than the identically modified N-(4-amino-2-butynyl)-2-pyrrolidones and N-(4-amino-2-butynyl)succinimides. Dissociation constants in the three series were correlated, suggesting that the compounds had similar mode of binding to muscarinic receptors. The 5-methyl-2-pyrrolidones were 10- to 20-fold less potent as muscarinic agonists on the guinea pig urinary bladder than on the ileum and also elicited lower relative maximal responses on the bladder. For example, the trimethylammonium (9) and azetidino (10) analogues were equipotent (EC50 = 0.2 microM) with the selective muscarinic stimulant N-(1-methyl-4-pyrrolidino-2-butynyl)-N-methylacetamide, BM 5 (2), as agonists on the ileum, but on the bladder 9 and 10 were relatively weak partial agonists, whereas 2 was an antagonist. Compound 10, like 2 and the dimethylamino analogue 8, also differentiated between centrally mediated muscarinic effects in vivo as it was potent in producing analgesia and hypothermia but did not elicit tremor. Instead, 10 antagonized oxotremorine-induced tremor. Thus, 10 resembled 2 in its actions except that the greater intrinsic efficacy of 10 shifted the balance between agonist and antagonist properties slightly toward agonism. Manipulation of intrinsic efficacy by minor changes in chemical structure is emphasized as a means of attaining selectivity.

The conversion of 2-chloroalkylamine analogues of oxotremorine to aziridinium ions and their interactions with muscarinic receptors in the guinea pig ileum.

Two mustard analogues of oxotremorine, N-[4-(2-chloroethylmethylamino)-2-butynyl]-2-pyrrolidone (BM 123) and N-[4-(2-chloromethylpyrrolidino)-2-butynyl]-2-pyrrolidone (BM 130), were synthesized. BM 123 and BM 130 cyclize in neutral aqueous solution to aziridinium ions. These aziridinium ions are potent stimulants of the guinea pig ileum. This agonist activity is unaffected by hexamethonium but is inhibited by methylatropine and by pretreatment of solutions of BM 123 and BM 130 with thiosulfate. The parent 2-chloroalkylamines and the alcohols formed by hydrolysis of the aziridinium ions have very weak pharmacological effects. Incubation of the isolated guinea pig ileum for 30 min with BM 123 at 20 microM and 2 microM caused 94% and 48% receptor alkylation, respectively, as calculated from the shift in the agonist dose-response curve. Similar incubations with BM 130 at 20 microM and 5 microM alkylated 85% and 63% of the receptors. No recovery from this apparent blockade was observed over a 4-hr time period. These calculated receptor occupancies by BM 123 and BM 130 agreed with those estimated from reduction of [3H] (-)-3-quinuclidinyl benzilate binding to homogenates of the ileum following exposure to BM 123 and BM 130. Methylatropine (20 nM) protected against the irreversible actions of BM 123 and BM 130. In homogenates of the guinea pig ileum, BM 123 and BM 130 also caused a selective reduction in the binding capacity of [3H]N-methylscopolamine without significantly affecting the apparent affinity. This inhibitory effect persisted after extensive washing. BM 130 was a weak agonist at nicotinic receptors of the frog rectus abdominis muscle, whereas BM 123 was almost inactive. In conclusion, BM 123 and BM 130 are potent and specific muscarinic agonists that bind irreversibly to muscarinic receptors.