2013 Volume 61 Issue 10 Pages 997-1001
Human microsomal cytochrome P450 (CYP) 2A6 contributes extensively to nicotine detoxication but also activates tobacco-specific procarcinogens to mutagenic products. We prepared a series of benzofuran and coumarin derivatives that have inhibitory effects on the activity of human CYP2A6. The reported compounds methoxalen and menthofuran had potent inhibitory effects on the activity of CYP2A6 with IC50 values of 0.47 µM and 1.27 µM, respectively. Synthetic benzofuran (4-methoxybenzofuran: IC50=2.20 µM) and coumarin (5-methoxycoumarin: IC50=0.13 µM and 6-methoxycoumarin: IC50=0.64 µM) derivatives, which have more selective effects than those of methoxalen and menthofuran, exhibited comparable activities against CYP2A6. These compounds can be used as a lead compounds in the design of CYP2A6 inhibitor drugs to reduce smoking and tobacco-related cancers.
Cytochrome P450 (CYP) 2A6, the major coumarin 7-hydroxylase present in the human liver, is known to metabolize a variety of compounds, including nicotine, cotinine, quionoline, and aflatoxin B1.1,2) CYP2A6 does not seem to have an extensive role in human drug metabolism, but it has been shown to be involved in the mutagenic activation of promutagens such as the tobacco-specific nitrosoamines, 4-(methylnitrosoamino)-1-(3-pyridyl)-1-butanone (NNK) and N′-nitrosonornicotine (NNN).3–5) It has also been shown that methoxalen, a potent human CYP2A6 inhibitor, is a strong chemopreventive agent against NNK induction of lung tumorigenesis.3–5) Pharmacogenomic studies have suggested that male smokers completely lacking CYP2A6 were more resistant to lung cancer. In addition, inhibition of CYP-mediated metabolism would lead to slower elimination of nicotine from the bodies of smokers and, consequently, to a decrease in the number of cigarettes needed to maintain a constant level of nicotine in the body. This decrease in smoking would decrease the exposure to carcinogenic nitrosoamine. For this reason, CYP2A6 inhibitors have been proposed as a novel targets for reducing tobacco-related cancer risk. An ideal drug candidate for use as a CYP2A6 inhibitor and a smoking cessation agent must be highly potent and selective to avoid undesirable effects.6)
Although a number of compounds, including methoxalen, have been used as inhibitors of CYP2A6, these compounds generally lack selectivity for targeting CYP2A6 and many of these compounds also inhibit activities of other drug-metabolizing enzymes (such as CYP3A4 and CYP2D6) and therefore can result in untoward drug-drug interactions.7) Herein we report the synthesis of benzofuran and coumarin derivatives and results of assays of their inhibitory effects on CYP2A6 activity with a view to defining the relationship between structures and inhibitory effects on CYP2A6 activity.
Compounds 1, 2, and 12 were commercially available.
Compound 3: 3-Methyl-4,5,6,7-tetrahydrobenzofuranPyrrolidine enamine of cyclohexanone was alkylated by ethyl 2-bromopropionate to give 3-methyl-5,6,7,7a-tetrahydrobenzofuran-2(4H)-one. This butenolide was reduced by DIBAL-H, affording compound 3. Colorless oil, 1H-NMR (500 MHz, CDCl3) δ: 1.65–1.78 (4H, m), 1.85 (3H, d, J=1.2 Hz), 2.26–2.29 (2H, m), 2.46 (2H, t, J=6.3 Hz), 7.10 (1H, d, J=1.2 Hz). 13C-NMR (125 MHz, CDCl3) δ: 7.2, 20.1, 22.8, 22.9, 23.0, 117.5, 119.4, 136.7, 150.3. Its spectral data are in accordance with previously reported data.8,9)
Compound 4: 7-Methoxy-4,5,6,7-tetrahydrobenzofuranCyclohexa-1,2-dione was treated with chloroacetaldehyde to give 5,6-dihydrobenzofuran-7(4H)-none. This ketone was reduced with LiAlH4 to give an appropriate alcohol (racemic mixture). The obtained alcohol was alkylated by methyl iodide in the presence of NaH, affording compound 4. Colorless oil, 1H-NMR (500 MHz, CDCl3) δ: 1.70–1.95 (3H, m), 2.05 (1H, m), 2.36 (1H, m), 2.56 (1H, m), 3.47 (3H, s), 4.29 (1H, d, J=3.5 Hz), 6.21 (1H, d, J=1.7 Hz), 7.31 (1H, d, J=1.7 Hz). 13C-NMR (125 MHz, CDCl3) δ: 19.2, 22.3, 29.2, 56.9, 70.8, 110.3, 120.8, 142.1, 150.2. Its spectral data are in accordance with previously reported data.10)
Compound 5: 3-Methylbenzofuran3-Methylbenzofuran-2-carboxylic acid (commercially available) was heated with copper metal in quinoline to give compound 5. Colorless oil, 1H-NMR (500 MHz, CDCl3) δ: 2.22 (3H, d, J=1.2 Hz), 7.18–7.26 (2H,m), 7.38 (1H, dd, J=1.2, 8.0 Hz), 7.48 (1H, m), 7.51 (1H, dd, J=1.2, 8.0 Hz). 13C-NMR (125 MHz, CDCl3) δ: 6.4, 110.6, 115.2, 119.0, 121.9, 123.8, 128.4, 141.4, 155.5. Its spectral data are in accordance with previously reported data.11)
Compound 6: 3,6-Dimethylbenzofuran12)2-Hydroxy-4-methylacetophenone was alkylated with ethyl bromoacetate to give ethyl 2-acetyl-5-methylphenoxyacetate. This ester was heated with KOH in dioxane, affording compound 6. Colorless oil, 1H-NMR (500 MHz, CDCl3) δ: 2.20 (3H, d, J=1.2 Hz), 2.42 (3H, s), 6.58 (1H, d, J=1.2 Hz), 6.77 (1H, dd, J=2.3, 8.6 Hz), 6.94 (1H, d, J=2.3 Hz), 7.34 (1H, d, J=8.6 Hz), 7.43 (1H, d, J=1.7 Hz). 13C-NMR (125 MHz, CDCl3) δ: 6.5, 10.3, 111.2, 118.2, 120.0, 121.9, 123.8, 128.7, 141.4, 153.5.
Compound 7: 3-Methoxybenzofuran3-Methoxybenzofuran-2-carboxylic acid (commercially available) was heated with copper metal in quinoline to give compound 7. Colorless oil, 1H-NMR (500 MHz, CDCl3) δ: 3.88 (3H, s), 7.21 (1H, m), 7.30 (1H, m), 7.40 (1H, d, J=8.6 Hz), 7.48 (1H, m), 7.53–7.55 (1H, m) 13C-NMR (125 MHz, CDCl3) δ: 58.2, 111.7, 118.8, 121.8, 122.2, 124.4, 125.1, 145.5, 153.8. Its spectral data are in accordance with previously reported data.13,14)
Compound 8: 4-MethoxybenzofuranCyclohexa-1,3-dione was treated with 3-bromopyruvic acid in the presence of NaOH. The obtained tetrahydrobenzofuran derivative was refluxed with 10% palladium carbon in decalin to give 4-hydroxybenzofuran-2-carboxylic acid. This carboxylic acid was heated with copper metal in quinoline, affording 4-hydroxybenzofuran. Compound 815) was obtained by methylation of alcohol described as compound 4. Colorless oil, 1H-NMR (500 MHz, CDCl3) δ: 3.94 (3H, s), 6.66 (1H, d, J=2.3 Hz), 6.86 (1H, d, J=2.3 Hz), 7.14 (1H, d, J=8.6 Hz), 7.22 (1H, t, J=8.6 Hz), 7.53 (1H, d, J=2.3 Hz) 13C-NMR (125 MHz, CDCl3) δ: 55.6, 103.6, 104.1, 104.8, 117.8, 125.0, 143.6, 153.7, 156.4.
Compound 9: 5-Methoxybenzofuran4-Methoxyphenol was alkylated with 2-bromoacetaldehyde diethyl acetal in the presence of KOH. The obtained ether was subsequently cyclized with PPA, affording 5-hydroxybenzofuran. Compound 9 was obtained by methylation of alcohol described as compound 4. Compound 9 was obtained by methylation of alcohol described as compound 4. Colorless oil, 1H-NMR (500 MHz, CDCl3) δ: 3.85 (3H, s), 6.71 (1H, d, J=2.3 Hz), 6.92 (1H, dd, J=2.3, 9.2 Hz), 7.06 (1H, d, J=2.3 Hz), 7.41 (1H, d, J=9.2 Hz), 7.60 (1H, d, J=2.3 Hz). 13C-NMR (125 MHz, CDCl3) δ: 56.0, 103.6, 106.8, 111.9, 113.2, 128.1, 145.8, 150.0, 156.0. Its spectral data are in accordance with previously reported data.16)
Compound 10: 6-MethoxybenzofuranBenzen-1,3-diol was treated with chloroacetonitrile in the presence of HCl to give 6-hydroxy-coumaran-3-one. This compound was reduced with NaBH4 and dehydrated by acid to give 6-hydroxy-benzofuran. Compound 1017) was obtained by methylation of alcohol described as compound 4. Compound 10 was obtained by methylation of alcohol described as compound 4. Colorless oil, 1H-NMR (500 MHz, CDCl3) δ: 3.72 (3H, s), 6.58 (1H, d, J=1.7 Hz), 6.77 (1H, dd, J=2.3, 8.6 Hz), 6.94 (1H, d, J=2.3 Hz), 7.34 (1H, d, J=8.6 Hz), 7.43 (1H, d, J=1.7 Hz). 13C-NMR (125 MHz, CDCl3) δ: 55.7, 96.0, 106.5, 112.1, 120.8, 121.3, 144.2, 156.1, 158.2. Its spectral data are in accordance with previously reported data.18)
Compound 11: 7-Methoxybenzofuran19)2-Hydroxy-3-methoxybenzaldehyde was treated with ethyl chloroacetate in the presence of K2CO3 to give 6-methoxybenzofuran-2-carboxylic acid. This carboxylic acid was heated with copper metal in quinoline, affording compound 11. Colorless oil, 1H-NMR (500 MHz, CDCl3) δ: 4.00 (3H, s), 6.76 (1H, d, J=2.3 Hz), 6.80 (1H, d, J=8.0 Hz), 7.16 (1H, t, J=8.0 Hz), 7.20 (1H, d, J=8.0 Hz), 7.62 (1H, d, J=2.3 Hz). 13C-NMR (125 MHz, CDCl3) δ: 56.1, 106.4, 107.0, 113.6, 123.6, 129.2, 144.4, 145.0, 145.7. Electrospray ionization-low resolution-mass spectrometry (ESI-LR-MS) m/z: 148 (M+). ESI-high resolution (HR)-MS m/z: 148.0520 (Calcd for C9H8O2: 148.0524). Anal. Calcd for C9H8O2: C, 72.96; H, 5.44. Found: C, 72.92; H, 5.34.
Compound 13: 9-Methoxy-2H-furo[3,2-g]chromen-7(3H)-oneMethoxalen was reduced with 10% Pd–C to give compound 13. mp 159–161°C. (lit.20) 163°C). 1H-NMR (CDCl3) δ: 3.27 (2H, t, J=8.6 Hz), 4.04 (3H, s), 4.72 (2H, t, J=8.6 Hz), 6.21 (1H, d, J=9.5 Hz), 6.98 (1H, s), 7.58 (1H, d, J=9.5 Hz). 13C-NMR (CDCl3) δ: 29.2, 61.1, 73.3, 112.5, 113.7, 117.4, 125.8, 131.8, 144.0, 147.6, 154.7, 161.0. Electron ionization (EI)-LR-MS m/z: 218 (M+). EI-HR-MS m/z: 218.0580 (Calcd for C12H10O4: 218.0579).
Compound 14: 4-Methoxycoumarin4-Hydroxycoumarin (commercially available) was refluxed with methyl iodide, K2CO3 in acetone to give compound 14. mp 122–124°C. (lit.21) 123.5–124°C) 1H-NMR (500 MHz, CDCl3) δ: 3.99 (3H, s), 5.69 (1H, s), 7.26 (1H, dd, J=7.5, 8.0 Hz), 7.31 (1H, d, J=8.6 Hz), 7.54 (1H, ddd, J=1.2, 7.5, 8.6 Hz), 7.80 (1H, dd, J=1.2, 8.0 Hz). 13C-NMR (125 MHz, CDCl3) δ: 56.4, 90.2, 115.7, 116.9, 123.1, 124.0, 132.5, 153.4, 163.0, 166.5. Its spectral data are in accordance with previously reported data.21)
Compound 15: 5-Methoxycoumarin2,6-Dimethoxybenzaldehyde was treated with methyl (triphenylphosphoranylidene)acetate in toluene to give (E)-methyl-3-(2,6-dimethoxyphenyl)acrylate.22) This ester was cyclized with BBr3 in dichloromethane to give 5-hydroxycoumarin. 5-Hydroxycoumarin was refluxed with methyl iodide, Cs2CO3 in acetonitrile to give compound 15. mp 83°C. (lit.22) 82°C) 1H-NMR (500 MHz, CDCl3) δ: 3.93 (3H, s), 6.33 (1H, d, J=9.7 Hz), 6.70 (1H, d, J=8.1 Hz), 6.91 (1H, d, J=8.6 Hz), 7.43 (1H, m), 8.08 (1H, d, J=9.7 Hz). 13C-NMR (125 MHz, CDCl3) δ: 56.1, 105.2, 109.3, 109.7, 114.7, 132.4, 138.6, 155.2, 156.2, 161.1. Its spectral data are in accordance with previously reported data.23)
Compound 16: 6-Methoxycoumarin6-Hydroxycoumarin (commercially available) was refluxed with methyl iodide, K2CO3 in acetone to give compound 16. mp 99–101°C. (lit.23) 102–103°C) 1H-NMR (500 MHz, CDCl3) δ: 3.84 (3H, s), 6.42 (1H, d, J=9.2 Hz), 6.91 (1H, d, J=2.9 Hz), 7.10 (1H, dd, J=2.9, 8.6 Hz), 7.26 (1H, d, J=8.6 Hz), 7.65 (1H, d, J=9.2 Hz). 13C-NMR (125 MHz, CDCl3) δ: 55.9, 110.1, 117.2, 118.0, 119.3, 119.5, 143.3, 148.6, 156.2, 161.1. Its spectral data are in accordance with previously reported data.23)
Compound 17: 8-Methoxycoumarin2-Hydroxy-3-methoxybenzaldehyde was heated with methyl (triphenylphosphoranylidene)acetate to give compound 17. mp 81–83°C. (lit.22) 89°C) 1H-NMR (500 MHz, CDCl3) δ: 3.96 (3H, s), 6.43 (1H, d, J=9.8 Hz), 7.05 (1H, d, J=8.0 Hz), 7.07 (1H, d, J=8.0 Hz), 7.20 (1H, t, J=8.0 Hz), 7.68 (1H, d, J=9.8 Hz). 13C-NMR (125 MHz, CDCl3) δ: 56.3, 113.8, 117.1, 119.3, 119.6, 124.4, 143.7, 143.8, 147.4, 160.3. Its spectral data are in accordance with previously reported data.22)
Assay. Assay of InhibitionAssays of inhibition of CYP2A6 activity (GENTEST Co., Human CYP2A6 + cytochrome b5+P450 reductase (Baculovirus)) by benzofuran and coumarin derivatives were based on microsomal coumarin 7-hydroxylation. The 7-hydroxylation of coumarin was used as the index of CYP2A6 activity. The rate of coumarin 7-hydroxylation was determined by the method of Yamazaki et al.24) Expressed human CYP2A6 (0.5 pmol) was incubated with coumarin (final concentration of 1 µM) and an inhibitor (0.01–10 µM) at 37°C in the presence of phosphate buffer (100 mM, pH 7.4), nicotinamide adenine dinucleotide phosphate (NADP+) (0.8 mM), glucose-6-phosphate (7.9 mM), and glucose-6-phosphate dehydrogenase (1 unit/mL) in a total volume of 400 µL. The reaction mixture was preincubated for 2 min at 37°C, and the reaction was started by the addition of a reduced nicotinamide adenine dinucleotide phosphate (NADPH)-generating system of the microsomal solution. The reaction was stopped after 6-min incubation with 50 µL of 60% HClO4. After further mixing, the reaction mixture was centrifuged at 10000 rpm for 5 min at 4°C. The supernatant was analyzed by the HPLC-fluorescence method (Ex 338 nm, Em 458 nm). The HPLC system consisted of a Hitachi model L7100 pump, Hitachi D-7500 detector, and Hitachi F-1050 fluorescence spectrophotometer.
Assays of inhibition of CYP3A4 activity (GENTEST Co., Human CYP3A4 + cytochrome b5+P450 reductase (Baculovirus)) by benzofuran and coumarin derivatives were based on microsomal testosterone 6β-hydroxylation. The rate of testosterone 6β-hydroxylation was determined by the method of Guo et al.7) Expressed human CYP3A4 (2.5 pmol) was incubated with testosterone (final concentration of 0.2 mM) and an inhibitor (0.01–10 µM) at 37°C in the presence of phosphate buffer (100 mM, pH 7.4), NADP+ (0.8 mM), glucose-6-phosphate (7.9 mM), and glucose-6-phosphate dehydrogenase (1 unit/mL) in a total volume of 500 µL. The reaction mixture was preincubated for 2 min at 37°C, and the reaction was started by the addition of an NADPH-generating system of the microsomal solution. The reaction was stopped after 15-min incubation with 3 mL of AcOEt. After 11α-hydroxy progesterone (internal standard) had been added, the reaction mixture was centrifuged for 5 min at 2800 rpm. The supernatant was analyzed by the HPLC-UV method (240 nm). The HPLC system consisted of a Shimadzu model LC-10ATvp pump, Shimadzu C-R8A detector, and Shimadzu SPD-10Avp UV spectrophotometer.
Assays of inhibition of CYP2D6 activity (GENTEST Co., Human CYP3A4 + cytochrome b5+P450 reductase (Baculovirus)) by benzofuran and coumarin derivatives were based on microsomal dextromethorphan 3-hydroxylation. The rate of dextromethorphan 3-hydroxylation was determined by the method of Madeira et al.25) Expressed human CYP2D6 (2 pmol) was incubated with dextromethorphan (final concentration of 4 µM) and an inhibitor (50 µM) at 37°C in the presence of phosphate buffer (500 mM, pH 7.4), NADP+ (0.8 mM), glucose-6-phosphate (7.9 mM), and glucose-6-phosphate dehydrogenase (1 unit/mL) in a total volume of 500 µL. The reaction was started by addition of an NADPH-generating system and was terminated after incubation for 10 min by addition of 50 µL of 60% HClO4. After further mixing, the reaction mixture was centrifuged at 10000 rpm for 10 min at 4°C. The supernatant was analyzed by the HPLC-fluorescence method (Ex 280 nm, Em 310 nm). The HPLC system consisted of a Hitachi model L7100 pump, Hitachi D-7500 detector, and Hitachi F-1050 fluorescence spectrophotometer.
We selected menthofuran26) (1) and methoxalen (2) as lead compounds and prepared various benzofuran and coumarin derivatives (Chart 1). A series of benzofuran derivatives (3,8,9) 4,10,11) 5,12) 6,13) 7,14,15) 8,14,15) 9,16) 10,17,18) and 1119)) and coumarin derivatives (13,20) 14,21) 15,22) 16,23) and 1722)) were prepared by the reported procedure. Assays of inhibition of CYP2A6 activity by benzofuran and coumarin derivatives were based on microsomal coumarin 7-hydroxylation.24) The results are shown in Table 1.
| Compound | IC50 values (µM) | Compound | IC50 values (µM) |
|---|---|---|---|
| CYP2A6 | CYP2A6 | ||
| Menthofuran 1 | 1.27 | 10 | 9.62 |
| Methoxalen 2 | 0.47 | 11 | >10 |
| 3 | 4.56 | 12 | >10 |
| 4 | >10 | 13 | >10 |
| 5 | 7.35 | 14 | >10 |
| 6 | 5.90 | 15 | 0.13 |
| 7 | 4.53 | 16 | 0.64 |
| 8 | 2.20 | 17 | 4.50 |
| 9 | 4.12 |
At first, the inhibitory activities of menthofuran analogues (3–6) were compared. None of them exhibited activity against CYP2A6 comparable to the activity of menthofuran. Modification of the cyclohexane ring of menthofuran 1 to afford aromatic ring analogues (5: 3-methylbenzofuran and 6: 3,6-dimethylbenzofuran) decreased the potency of CYP2A6 inhibition. It was found that aromatization (5, 6) of the cyclohexane ring of menthofuran 1 influenced the inhibitory effects on CYP2A6. The inhibitory potency of compound 3, without a methyl group at the 6-position on the ring of menthofuran, was decreased compared with that of menthofuran, indicating that the existence of a methyl group at the 6-position of menthofuran is also important for potency of CYP2A6 inhibition. 7-Methoxy-4,5,6,7-tetrahydrobenzofuran (4) showed poor inhibitory activity against CYP2A6. Methoxalen 2 has a methoxy group at the 9-position on the furanocoumarin ring. Thus, the influence of the location of the methoxy group on the benzofuran ring (7–11) was studied. 4-Methoxybenzofuran 8 had the strongest inhibitory effect on CYP2A6 among the benzofuran derivatives. Interestingly, bergapten 12 (methoxy group at the 4-position on the furanocoumarin ring) showed a much weaker inhibitory effect on CYP2A6 than that of methoxalen 2 (methoxy group at the 9-position on the furanocoumarin ring). A reductive furanocoumarin derivative (13) showed an IC50 value for CYP2A6 of >10 µM. Apparently, a double bond of the furan ring was essential for a potent inhibitory effect on CYP2A6. Finally, the influence of the location of the methoxy group on the coumarin ring (14–17) was studied. It was found that the potency of inhibition was also very sensitive to location of the methoxy group on the coumarin ring. Introduction of a methoxy group at the 5 or 6 position on the coumarin ring resulted in significantly increased inhibitory potency, whereas the same substituent in the 4 or 8 position gave rise to low potency of CYP2A6 inhibition.
To better understand substrate (coumarin) and inhibitor (methoxalen) binding to CYP2A6, Johnson and colleagues determined the structure of the enzyme by X-ray crystallography.27) The CYP2A6 structure shows a compact, hydrophobic active site with one hydrogen bond donor, Asn297. In the crystal structure, Asn297 acts as a hydrogen donor to the carbonyl oxygen of coumarin, and the 7-position of coumarin is closest to the heme iron in active site of CYP2A6 for reigioselective oxidation.28) These results and our findings provide a clue for elucidating the relationship between structures and inhibitory effects on CYP2A6 activity. Compound 6, aromatized analogue of menthofuran 1, showed a reduced inhibitory effect on CYP2A6. Although the structure of CYP2A6-menthofuran has not yet been determined by X-ray crystallography, formation of furan epoxide was detected in menthofuran.29) This means that the furan oxygen is positioned closer than other parts of menthofuran 1 to the heme iron in the active site of CYP2A6. Changes in molecular size and shape caused by aromatization probably prevent access of the furan part of 6 to the heme iron. Substitution of a methoxy substituent for a methyl substituent on the benzofuran ring resulted in increased inhibitory potency toward CYP2A6. It was also found that the potency of inhibition was very sensitive to location of the methoxy group on the benzofuran ring, which is likely to reflect hydrogen-bonding interaction with Asn297 in the active site of CYP2A6.
Next, we examined coumarin derivatives. Furanocoumarin derivative having a dihydrofuran ring 13 showed poor inhibitory activity. This result was reasonable because it is believed that interaction of CYPs with furanocoumarin occurs at the part of the unsaturated furan ring.30) In this experiment, 15 candidate molecules were selected and tested for inhibition potency. 5-Methoxycoumarin 15 was most potent of all tested compounds as a CYP2A6 inhibitor with an IC50 value of 0.13. Rahnasto et al. reported that structural characteristics such as located hydrophobic and hydrogen donor features are important for inhibition potency.31) We speculated that the presence of a hydrogen bond between a 5-methoxy group of coumarin and Asn297 in the active site of CYP2A6 has an effect on inhibition potency. As previously described, coumarin is known as a substrate of CYP2A6.32) We found that coumarin derivatives work not only as substrates of CYP2A6 but also as inhibitors.
In order to further explore the selectivity of CYP2A6 inhibition, compounds 8, 15, and 16—a selection of the most potent inhibitors of CYP2A6 among compounds examined in this study—were tested for their inhibitory potency for the most prominent drug-metabolizing enzymes CYP3A4 and CYP2D6.7) The results are shown in Table 2.
| Compound | IC50 values (µM) | ||
|---|---|---|---|
| CYP2A6 | CYP3A4 | CYP2D6 | |
| Menthofuran 1 | 1.27 | >50 | >50 |
| Methoxalen 2 | 0.47 | 5.46 | >50 |
 | 2.20 | >50 | >50 |
 | 0.13 | >50 | >50 |
 | 0.64 | >50 | >50 |
As stated above, a number of compounds, including methoxalen, have been reported to be inhibitors of CYP2A6. However, these compounds generally lack selectivity for targeting CYP2A6, and many also inhibit other drug-metabolizing enzymes such as CYP3A4A and 2D6.6) Menthofuran, an excellent selective inhibitor of CYP2A6, also has disadvantages in toxicity and stability.33) Fortunately, of the compounds examined, compounds 8, 15, and 16 were observed to have highly selective inhibitory potency for CYP2A6. These compounds may serve as lead structures to develop even more potent and selective inhibitors of CYP2A6 that are more accessible than menthofuran and methoxalen and have comparable activity against CYP2A6.
The dose–response relationship between the number of cigarettes smoked and development of related cancer was established many years ago by epidemiological studies.34) Reducing the number of cigarettes smoked and decreasing the toxicity and carcinogenicity of tobacco will result in less tobacco-related cancer risk. One approach to decrease smoking is to develop an agent that replaces nicotine and that inhibits the formation of nicotine-related carcinogens. Preparation of potent and selective inhibitors of human CYP2A6 may provide an insight into small molecules that can be used as smoking cessation agents.
This study was partially supported by the Hikari Foundation for Medicinal Research (to KO).
