Indoles, Benzimidazoles and Benzothiophens

By library screening and similarity searches at Lilly Research Laboratories the trisubstituted indole 7a (Fig. 8) was discovered as NPY Y₁ antagonistic lead with low affinity at human Y₁ receptors expressed in AV-12 cells (Ki 2.1 µM for displacement of [¹²⁵I]PYY)^[69]. This structure was optimized in different positions, leading to some of the most potent Y₁ antagonists known so far. First attempts maintained the 1-methyl-2-(4-chlorophenoxy)methylindole scaffold. Variation of the 3-substituent resulted in markedly improved activity with a 1,4’-bipiperidine group linked by two C-atoms to C-3 (7b: Ki 93 nM; 7c: Ki 26 nM). Based on the C-terminus of NPY, the introduction of an additional basic moiety at N-1 was suggested. Alkylpiperidine side chains with a free NH were optimal in this position. Ki values in the low nanomolar and subnanomolar range were obtained in binding studies with the compounds 7d (Ki

1.9 nM), (R )-7e (K_i 1.4 nM), and (S )-7e (LY 357897, K_i 0.75 nM). The activity of (S )-7e in different functional assays was in a similar range: Ki 1.8 nM for reversal of induced inhibition of forskolin-stimulated cAMP, 3.2 nM for inhibition of NPY-induced Ca²⁺ mobilization in SK-N-MC cells.

The compounds proved to be highly selective for the Y₁ receptor (Y₂, Y₄, Y₅: Ki

values > 10 µM). (S )-7e blocked the food consumption in mice, elicited by a submaximal (230 pmol) icv. administered dose of NPY, with an ED₅₀ of 17 nmol.

7a

Fig. 8: Optimization of 1,3-substituted 2-[(4-chlorophenoxy)methyl]indoles.

Attempts to replace the indole moiety by other nuclei resulted in a series of 2-[(4-chlorophenoxy)methyl]benzimidazoles with an optimal 1-[3-(piperidin-3-ylpropyl)]

substituent^{[70, 71]}. Some of the analogs and their NPY Y₁ antagonistic potencies are summarized in Table 3. The parent compound 8a was weakly active. Comparison with the most potent indoles suggested that their structure may be best matched with appropriate 4-substituents at the benzimidazole moiety. Introduction of a methyl group in 8b produced a seven-fold increase in receptor affinity. 3-piperidinylpropoxy and 2-piperidinylethoxy substituents are more favorable. The structure-activity relationships of the piperidine isomers are not uniform and point to an optimal position of the basic nitrogen relative to the benzimidazole nucleus:

(piperidin-1-yl)- and (piperidin-2-yl)propoxy derivatives are more active, but 3-(piperidin-3-yl)- and 3-(piperidin-4-yl)propoxy substituted compounds are less potent than their ethoxy analogs. Among the diastereomers of 8h, highest affinity was found with the (S,S )-configuration. The most potent compound of Table 3, 8d, approached the nanomolar range in NPY Y1 receptor binding as well as in the functional data for Y₁ antagonism in SK-N-MC cells. Optimization of substituents at the piperidine nitrogen in 2-[(4-chlorophenoxy)methyl]-4-methyl-1-[3-(piperidin-4-yl)propyl]benzimidazoles 9 (Fig. 9) also resulted in very active Y₁ antagonists^[70].

Table 3: In vitro Y1 receptor binding (human Y1 in AV-12 cells) and Y1 antagonistic activity (cAMP assay in SK-N-MC cells) of selected 2-[(4-chlorophenoxy)methyl]benzimidazoles 8^[70].

8 N N

O HN

R

Cl

No. R

Y1 binding data Ki (nM)

Y1 antagonism Ki (nM)

4a^a 4.6 11

8a H 700 -

8b CH3 97 -

8c [2-(piperidin-1-yl)ethyl]oxy 43 240 8d [3-(piperidin-1-yl)propyl]oxy 1.7 2.7 8e [2-(piperidin-2-yl)ethyl]oxy 29 - 8f [3-(piperidin-2-yl)propyl]oxy 16 53 8g [2-(piperidin-3-yl)ethyl]oxy 18 91 8h [3-(piperidin-3-yl)propyl]oxy 30 77

8i [2-(piperidin-4-yl)ethyl]oxy 7 15 8j [3-(piperidin-4-yl)propyl]oxy 152 119 (S,S)-8h [3-(piperidin-3-yl)propyl]oxy 6 87 (R,R)-8h [3-(piperidin-3-yl)propyl]oxy 41 153 (R,S)-8h [3-(piperidin-3-yl)propyl]oxy 27 137 (S,R)-8h [3-(piperidin-3-yl)propyl]oxy 17 65

a BIBP 3226

The highest NPY Y₁ receptor affinity was obtained by introduction of an additional basic nitrogen separated by 3 – 4 C atoms from the piperidine-N (cf. Fig. 9; 9a: Ki 5 nM; 9b: K_i 6 nM). It was a reasonable extension of this work to combine such 1-substituents with the 4-[3-(piperidin-1-yl)propoxy] group present in 8d^[70]. Indeed, a

number of compounds of the common structure 10 displayed Y₁ receptor affinities in the subnanomolar range^[72]. K_i values lower than 0.3 nM were found for derivatives with higher alkyl groups (cf. Fig. 9; 10a,b). Phenylalkyl or phenylalkenyl substitution (10c,d) or the attachment of a moiety with a polar group as in 10e,f led to even higher affinities (Ki 0.1 – 0.2 nM). The derivative with a p-iodophenylethyl substituent R (10g) is among the most potent non-peptide NPY Y₁ receptor ligands known so far (Ki 0.05 nM).

Fig. 9: General structures of highly active NPY Y1 antagonistic benzimidazole (9, 10) and benzo-thiophene (11) derivatives. Example structures: 9a: R = 3-(piperidin-1-yl)propyl; 9b: R = 4-(piperi-din-1-yl)butyl; 10a: R = isobutyl; 10b: R = cyclohexylmethyl; 10c: R = 2-phenylethyl; 10d: R = 3-phenylprop-2-en-1-yl; 10e: R = 3-(piperidin-1-yl)propyl; 10f: R = 4-oxo-4-phenylbutyl; 10g: R = 2-(4-iodophenyl)ethyl; 11a: R² = CH2OH, R⁴ = Br; 11b: R² = CH2OCH3, R⁴ = Br; 11c: R² = CN, R⁴ = Br.

With benzothiophene derivatives a third nucleus was used at Lilly as scaffold for the design of NPY Y₁ antagonists^[73]. Optimization of both side chains in 2- and 3-position resulted in the common structure 11 (Fig. 9). A 4-chlorophenoxymethyl group as present in the indole and benzimidazole series does only lead to moderate affinity (Ki 310 nM). However, the potency may be significantly increased by appropriate multiple substitution at the phenyl ring. The most active Y₁ antagonists in this series (Ki values: 11 – 15 nM) are those with polar groups in ortho position (R²

= CH₂OH, CH₂OMe, CN) and an additional para-Br substituent (11a-c).

3.4. Y1 Antagonists Based on other Common Structures 3.4.1. 6-ARYLSULFONYL-5-NITROQUINOLINES

Arylsulfonyl compounds from Parke-Davis/Warner-Lambert with a nitroquinoline nucleus are only weakly basic and do not obviously overlap with NPY^[74]. The 8-amino-5-nitroquinoline and the phenylsulfonyl moieties in the general structure 12 (Fig. 10) are essential for Y₁ antagonistic activity. Whereas the parent compound PD 9262 (12a) is not very potent (Ki 282 nM for displacement of [¹²⁵I]PYY from SK-N-MC membranes), ortho-alkyl or halogen substituents at the phenyl ring enhance affinity up to a Ki value of 48 nM for the ortho-isopropyl derivative PD 160170 (12b).

3.4.2. PHENYLPIPERAZINES

Neurogen and Pfizer have patented 1-(1-phenylcyclohexyl)-4-phenyl-piperazines with amide, amine and ether substituents R (13, Fig. 10) as novel class of NPY Y₁ specific ligands^{[75, 76]}. No biological data were given for the new amides (e.g., 13a).

An ether derivative (13b) displaced [¹²⁵I]PYY with an IC50 value of 30 nM.

CH₃

12b: R = 2-isopropyl (PD 160170) ¹⁴

R X

Fig. 10: 6-Arylsulfonyl-5-nitroquinolines 12, phenylpiperazines 13 and bis[diamino(phenyl)triazines]

14.

3.4.3. BIS[DIAMINO(PHENYL)TRIAZINES]

At Alanex different structures were identified as non-peptide NPY Y₁ receptor antagonists by pharmacophore-based approaches and by screening of combinatorial libraries^[77]. Examples of discovered compounds are symmetric bis[diamino(phenyl)-triazines] 14 with a disubstituted central benzene ring as spacer (Fig. 10). The K_i values for Y₁ receptor binding are 117 nM (meta derivative AXC01829) and 150 nM (para analog AXC011018). A certain similarity of the compounds to benextramine (2a) and analogs is obvious.

3.4.4. BENZAZEPINES AND BENZODIAZEPINES

Hybrid compounds combining a CCK-B receptor antagonistic benzodiazepine and a histamine H₂ receptor blocking roxatidine-like moiety were synthesized at Shionogi and Co.^[78], e.g. the derivative 15 (Fig. 11), which was about equiactive with BIBP 3226 as NPY Y₁ antagonist (K_i 6.4 nM in radioligand binding studies, IC50 95 and 320 nM in functional Ca²⁺ and cAMP assays in SK-N-MC cells). No binding to Y₂ and Y₅ receptors was observed up to concentrations of 1 µM. The hybrid molecule maintains the CCK-B and histamine H₂ antagonistic potency of the components, which were, however, both inactive at NPY Y₁ receptors. Other series of Y₁ antagonists from Shionogi and Co. are based on a 1,3-disubstituted benzazepine nucleus^{[79, 80]}. The common structure 16 was optimized at both positions. Generally, derivatives with urea moieties (R¹ = NH-alkyl) are about ten times more potent than the corresponding carbamates (R¹= O-alkyl). Maximal Y1 receptor binding affinity (Ki 2.9 nM) was observed for the 3-guanidino derivative (X = NH) with an isopropylamino and a 4-hydroxyphenyl group as R¹ and R², respectively, whereas the 3-ureido analog (X = O) was much less potent (Ki 82 nM). Further optimization of R² in a 3-ureido series (R¹ = NH-isopropyl) also led to compounds with K_i values lower than 10 nM (R² = 6-benzofuryl, 6-benzothienyl, 6-benzothiazolyl, 2-F-phenyl, 2,4-di-F-phenyl)^[79]. The 6-benzothiazolyl derivative (Ki 5.1 nM) was functionally

characterized as an Y₁ antagonist and did not show any effects on Y₂, Y₄ and Y₅

Fig. 11: Benzodiazepine 15 and general structure of benzazepines 16 (X = O; R¹ = NH-alkyl, O-alkyl; R² = aryl, heteroaryl).

3.4.5. MORPHOLINOPYRIDINES J-104870 AND J-115814

Two highly potent NPY Y₁ receptor antagonists, the morpholinopyridines J-104870 (17a) and J-115814 (17b, Fig. 12) were disclosed by Banyu^{[81, 82]}. J-104870 displaced [¹²⁵I]PYY binding to cloned human and rat Y₁ receptors with Ki values of 0.29 and 0.54 nM, respectively, and inhibited the NPY-induced intracellular calcium mobilization (IC50 3.2 nM). Ki values determined for the binding at other NPY receptors were greater than 5 µM. Anorexigenic effects on NPY-mediated feeding of rats were demonstrated by both intracerebroventricular and oral administration of the compound^[82]. J-115814 (Ki 1.4 – 1.8 nM) was nearly as potent as J-104870.

Feeding induced by icv. NPY was unaffected by ip. injected J-115814 in Y₁(-/-) mice, but suppressed in wildtype and Y₅(-/-) mice^[81]. Together these findings suggest the contribution of Y₁ receptors in the regulation of food intake. In vitro mutagenesis studies on the human Y₁ receptor^[62] resulted in reduced affinity of J-104870 at alanine mutants of amino acids Trp¹⁶³, Phe¹⁷³, Asn²⁸³, Asp²⁸⁷ and Leu³⁰³, indicating that the compound recognizes a pocket formed by TMs 4, 5 and 6 which only

partially overlaps with the binding site of other antagonists like BIBP 3226 or the

Fig. 12: Morpholinopyridines 17, dihydropyridines 18 and dihydropyrazines 19

3.4.6. DIHYDROPYRIDINES AND DIHYDROPYRAZINES

Recently, dihydropyridine (18) and dihydropyrazine derivatives (19, Fig. 12) from Bristol-Myers Squibb^[83-85] were described as NPY Y₁ antagonists. Generally the dihydropyridines 18 were up to about 100 times more potent in displacing [¹²⁵I]PYY from human Y₁ receptors than the corresponding dihydropyrazine analogs 19.

Highly active compounds 18 (Ki 5 nM) are urea derivatives (Z = O) with 2-methoxy-, 3-methoxy- or 3-hydroxy-substituted phenyl rings as R. Replacement of the urea with a cyanoguanidine group (Z = NCN) results in a further increase in activity (e.g. with R = tert-butyl: Ki < 1 nM). In comparison to the urea analogs, members of the cyanoguanidine series show improved permeability properties in Caco-2 cells^[86]. For the derivative BMS-193885 (Z = O, R = 2-methoxyphenyl) full

functional Y₁ antagonism (Kb 4.5 nM) was observed in a cAMP assay using human Y₁ receptor expressing CHO cells. The compounds are Y₁ selective and specific in spite of the presence of α1 adrenoceptor and calcium channel blocking pharmacophores.

H 394/84 (18a) antagonized vascular responses to exogenous and endogenous, neuronally released NPY with similar potency already at plasma levels of 29 nM with a long duration of action in vivo^{[60, 87]}.