Isopimpinellin

Several naturally occurring coumarins previously found to be potent inhibitors of mouse hepatic ethoxyresorufin-O-deethylase (EROD) and/or pentoxyresorufin-O-dealkylase (PROD) were examined for their effects on formation of benzo[a]pyrene (B[a]P) and 7,12-dimethylbenz[a]anthracene (DMBA) DNA adducts in mouse epidermis, as well as, their effects on skin tumor initiation by these polycyclic aromatic hydrocarbons (PAH). Bergamottin, a potent inhibitor of hepatic EROD, given topically 5 min prior to an initiating dose of B[a]P, significantly decreased total covalent binding of B[a]P to DNA in a dose-dependent manner 24 hr after treatment. A dose of 400 nmol bergamottin reduced covalent binding of B[a]P by 72%. Coriandrin, at a dose of 400 nmol also significantly reduced total covalent binding of B[a]P by 59%. In addition, formation of the major (+)anti-B[a]P-diol epoxide-N2-dGuo adduct was selectively reduced by both of these coumarins. In contrast, bergamottin and coriandrin did not significantly decrease covalent binding of DMBA to epidermal DNA at doses of either 400 nmol or 800 nmol. Imperatorin and isopimpinellin, which are more potent inhibitors of hepatic PROD activity, significantly reduced overall binding of DMBA to epidermal DNA by 67% and 52%, respectively, when applied at doses of 400 nmol. These two coumarins also inhibited B[a]P-DNA adduct formation at similar doses but to a lesser extent. Imperatorin at a dose of 400 nmol dramatically decreased formation of covalent DNA adducts derived from both the anti and syn diol epoxides of DMBA. Bergamottin was a potent inhibitor of tumor initiation by B[a]P while coriandrin was less effective in this regard. Imperatorin was an effective inhibitor of skin tumor initiation by DMBA and also inhibited complete carcinogenesis by this PAH. At dose levels higher than those effective against DMBA, imperatorin also inhibited tumor initiation by B[a]P. The results demonstrate that several naturally occurring coumarins possess the ability to block DNA adduct formation and tumor initiation by PAHs such as B[a]P and DMBA. The mechanism for reduced DNA adduct formation and tumor initiation appears to involve inhibition of the P450s involved in the metabolic activation of these hydrocarbons. Finally, the differential effects of certain coumarins on B[a]P vs DMBA DNA adduct formation and tumor initiation may be useful for dissecting the role of specific cytochromes P450 in their metabolic activation.

Naturally occurring coumarins (NOCs) inhibit polycyclic aromatic hydrocarbon-induced skin tumor initiation in mice by blocking cytochrome P450 (P450)-mediated bioactivation of benzo[a]pyrene (B[a]P) and 7,12-dimethylbenz[a]anthracene (DMBA). Bergamottin selectively inhibits tumor initiation by B[a]P, whereas imperatorin and isopimpinellin inhibit tumor initiation with both carcinogens. The goals of the current study were to examine the ability of NOCs to inhibit human P450s in vitro and to establish whether NOCs, which are anticarcinogenic in mice, can block carcinogen bioactivation in cultured human cells. For the initial experiments, incubations containing 5 uM P450, P450 substrate, an NADPH generating system, and NOCs were used to determine the concentrations of each inhibitor that blocked 50% of P450 activity (IC(50)). These results confirmed that NOCs are capable of inhibiting multiple human P450s and that they exhibit selectivity for certain isoforms of human P450s. In subsequent experiments, we examined the effects of bergamottin, imperatorin, and isopimpinellin on DMBA and B[a]P DNA adduct formation in the human breast MCF-7 adenocarcinoma cell line. Coincubation of cells with the three different NOCs significantly inhibited DMBA DNA adduct formation by 29-82% at doses ranging from 2 to 10 uM and significantly inhibited B[a]P DNA adduct formation by 37-80% at doses ranging from 20 to 80 uM. HPLC analysis of the DNA hydrolysates demonstrated that inhibition of DNA adducts corresponded to inhibition of the major B[a]P and DMBA diol-epoxide-derived adducts. Although bergamottin was not effective at blocking DMBA bioactivation in the mouse skin model, it was similar in effectiveness to imperatorin and isopimpinellin in MCF-7 cells. These results demonstrate that NOCs, which are present in citrus fruits and other components of the human diet, are capable of inhibiting carcinogen metabolizing enzymes and blocking bioactivation of both B[a]P and DMBA in MCF-7 cells.

Naturally occurring coumarins (NOCs) are anti-carcinogenic in the mouse skin model. To characterize the chemopreventive potential of NOCs against breast cancer, we first examined their effects on 7,12-dimethylbenz[a]anthracene (DMBA)-DNA adduct formation in mouse mammary gland. We hypothesized that those NOCs that both inhibited cytochrome P450 1A1/1B1 and induced hepatic glutathione S-transferases (GSTs) would be the most effective in blocking DMBA-DNA adduct formation in mouse mammary gland. To address this hypothesis, simple coumarins (e.g. coumarin and limettin, which induced mouse hepatic GSTs but had little effect on P4501A1/1B1) and linear furanocoumarins (e.g. imperatorin and isopimpinellin, which induced hepatic GSTs and were potent inhibitors of P4501A1/1B1) were compared. Mice were pretreated with NOCs (150 mg/kg body wt, by gavage) prior to either a single dose of DMBA (50 ug) or multiple doses of DMBA (20 ug daily for 3 and 6 weeks). Mammary DMBA-DNA adduct formation was quantitated by the nuclease P1-enhanced 32P-postlabeling assay. With the single dose of DMBA, coumarin, limettin, imperatorin and isopimpinellin inhibited DMBA-DNA adduct formation by 50, 41, 79 and 88%, respectively. Coumarin, limettin and imperatorin blocked DMBA-DNA adduct formation by 36, 60, and 66% at 3 weeks, and by 0, 49 and 55% at 6 weeks of DMBA dosing, respectively. In a 6 week dose-response study of select NOCs and 7,8-benzoflavone (a potent P4501 inhibitor that had little effect on GSTs), DMBA-DNA adduct formation was inhibited by 0, 43 and 24% in the limettin groups; by 26, 26 and 69% in the isopimpinellin groups; and by 80, 96 and 97% in the 7,8- benzoflavone groups at 35, 70 and 150 mg/kg, respectively. Taken together, these results suggest that linear furanocoumarins had a greater inhibitory effect on DMBA-DNA adduct formation in mouse mammary glands compared with simple coumarins, and that the predominant effect may be P4501 inhibition.

The current study was designed to determine the mechanistic basis for differences in the effects of naturally occurring furanocoumarins on skin tumor initiation by 7,12-dimethylbenz[a]anthracene (DMBA). Female SENCAR mice were pretreated topically with bergamottin, imperatorin, or isopimpinellin (100-3200 nmol), 7,8-benzoflavone (7,8-BF, 5-40 nmol, a known inhibitor of DMBA skin carcinogenesis in mice), or acetone (vehicle control) 5 min prior to topical treatment with DMBA (10 nmol). Imperatorin, isopimpinellin, and 7,8-BF, but not bergamottin, significantly blocked total DMBA-DNA adduct formation. HPLC analysis of DNA adducts revealed that bergamottin preferentially inhibited formation of anti-DMBA diol-epoxide (DMBADE) derived DNA adducts, imperatorin, and isopimpinellin inhibited both anti- and syn- derived adducts, whereas 7,8-BF showed some selectivity for reduction of syn-DMBADE-DNA adducts. Mouse embryo fibroblast C3H/10T1/2 (10T1/2) cells, and mouse hepatoma-derived 1c1c7 (Hepa-1) cells, which preferentially express P450 1b1 and P450 1a1, respectively, were co-incubated with 2 uM bergamottin, imperatorin, isopimpinellin, and 7,8-BF, and with DMBA (2 uM). Hepa-1 cells (P450 1a1) formed mainly anti-DMBADE-DNA adducts. In contrast, 10T1/2 cells (P450 1b1) formed mainly syn-DMBADE-DNA adducts. Bergamottin inhibited DMBA metabolism to DMBA-3,4-diol and blocked DNA adduct formation in Hepa-1 cells, but had little effect in 10T1/2 cells. In contrast, 7,8-BF completely blocked DMBA metabolism and DNA adduct formation in 10T1/2 cells, but had little effect in Hepa-1 cells. Imperatorin and isopimpinellin inhibited DMBA bioactivation in both cell lines. These results indicate that bergamottin is a more selective inhibitor of P450 1a1 and overall a less effective inhibitor of the metabolic activation of DMBA in mouse epidermis. In contrast, imperatorin, isopimpinellin, and especially 7,8-BF, which block metabolic activation of DMBA in mouse epidermis, appear more selective for P450 1b1. On the basis of our studies using 10T1/2 cells and Hepa-1 cells, it appears that P450 1a1 is primarily responsible for converting DMBA-3,4-diol to anti-DMBADE, whereas P450 1b1 is primarily responsible for converting DMBA-3,4-diol to syn-DMBADE. These data demonstrate the role of P450 1a1 and 1b1 in the metabolic activation of DMBA in mouse epidermis and provide a mechanistic explanation for the differential effects of naturally occurring furanocoumarins (and 7,8-BF) on polycyclic aromatic hydrocarbon skin carcinogenesis.

The furocoumarin 8-methoxypsoralen is carcinogenic to humans, and possibly 5-methoxypsoralen as well (L135). There is some evidence from mouse studies that other furocoumarins are carcinogenic when combined with exposure to UVA radiation (A15105). The SKLM regards the additional risk of skin cancer arising from the consumption of typical quantities of furocoumarin-containing foods, which remain significantly below the range of phototoxic doses, as insignificant. However, the consumption of phototoxic quantities cannot be ruled out for certain foods, particularly celery and parsnips, that may lead to significant increases in furocoumarin concentrations, depending on the storage, processing and production conditions. (L2157) Furocoumarin photochemotherapy is known to induce a number of side-effects including erythema, edema, hyperpigmentation, and premature aging of skin. All photobiological effects of furocoumarins result from their photochemical reactions. Because many dietary or water soluble furocoumarins are strong inhibitors of cytochrome P450s, they will also cause adverse drug reactions when taken with other drugs.

TERRESTRIAL FATE: Based on a classification scheme(1), an estimated Koc value of 320(SRC), determined from a structure estimation method(2), indicates that isopimpinellin is expected to have moderate mobility in soil(SRC). Volatilization of isopimpinellin from moist soil surfaces is not expected(SRC) given an estimated Henry's Law constant of 2.4X10-9 atm-cu m/mole(SRC), developed using a fragment constant estimation method(2). Isopimpinellin is not expected to volatilize from dry soil surfaces(SRC) based upon an estimated vapor pressure of 6.8X10-7 mm Hg at 25 °C(SRC), determined from a fragment constant method(2). Biodegradation data in soil were not available(SRC, 2019).

AQUATIC FATE: Based on a classification scheme(1), an estimated Koc value of 320(SRC), determined from a structure estimation method(2), indicates that isopimpinellin is expected to adsorb to suspended solids and sediment(SRC). Volatilization from water surfaces is not expected(3) based upon an estimated Henry's Law constant of 2.4X10-9 atm-cu m/mole(SRC), developed using a fragment constant estimation method(2). Isopimpinellin is expected to undergo hydrolysis in the environment due to the presence of functional groups that hydrolyze under environmental conditions(3). According to a classification scheme(4), an estimated BCF of 14(SRC), from an estimated log Kow of 2.22(2) and a regression-derived equation(2), suggests the potential for bioconcentration in aquatic organisms is low. Biodegradation data in water were not available(SRC, 2019).

ATMOSPHERIC FATE: According to a model of gas/particle partitioning of semivolatile organic compounds in the atmosphere(1), isopimpinellin, which has an estimated vapor pressure of 6.8X10-7 mm Hg at 25 °C(SRC), determined from a fragment constant method(2), is expected to exist solely in the particulate phase in the ambient atmosphere. Particulate-phase isopimpinellin may be removed from the air by wet and dry deposition(SRC). Isopimpinellin has a light absorption peak at 313 nm(3) and, therefore, may be susceptible to direct photolysis by sunlight(SRC).

Isopimpinellin was detected in 12 industrial cold-pressed key and Persian lime oils at 350-365 and 169-293 mg/100 g of oil, respectively(1). Lime oil contained 7422 mg/L of isopimpinellin, it was not detected (detection limit 0.026 mg/L) in lemon, bergamot, grapefruit, mandarin or bitter orange oils(2). Isopimpinellin was detected at 1.4 mg/kg fresh weight in clementine peel extract, at a trace in lemon, grapefruit, bergamot and pummelo peel extracts, but not detected (detection limit 0.03 mg/kg fresh weight) in orange peel extract(3).

IDENTIFICATION AND USE: Isopimpinellin is a coumarin derivative isolated from Cnidii Monnieri Fructus extract. It has been tested as an experimental chemopreventive therapy. HUMAN STUDIES: Isopimpinellin inhibited mouse, but not human coumarin 7-hydroxylase (COH) activity. ANIMAL STUDIES: Isopimpinellin was reported to cause persistent photoirritant reactions in mice. However, synthetic isopimpinellin, confirmed to contain as impurities only trace quantities of bergapten (5-methoxypsoralen) and xanthotoxin (8-methoxypsoralen), was not phototoxic when tested in a chick skin bioassay system. These findings are at variance with earlier studies showing isopimpinellin to be phototoxic against chick skin and support the conclusion that isopimpinellin is photobiologically inactive. Several other reports of isopimpinellin photoactivity are most likely attributable to contamination by small amounts of highly active psoralens such as bergapten or xanthotoxin. Isopimpinellin inhibited mouse coumarin 7-hydroxylase (COH) activity both in vitro and in vivo.

Isopimpinelin strongly inhibits insulin-stimulated lipogenesis. It induces hepatic GSTs and is potent inhibitor of cytochrome P450 1A1/1B1. Oral administration of isopimpinellin, blocks DNA adducts formation and skin tumor initiation by 7,12-dimehylbenz[a]anthracene in SENCAR mice and in mouse mammary gland (L579). The mechanism of action many furocoumarins is based on their ability to form photoadducts with DNA and other cellular components such as RNA, proteins, and several proteins found in the membrane such as phospholipases A2 and C, Ca-dependent and cAMPdependent protein-kinase and epidermal growth factor. Furocoumarins intercalate between base pairs of DNA and after ultraviolet-A irradiation, giving cycloadducts. (L579)

Isopimpinellin was detected at 3.17-14.96 ug/g in seven Florida celery cultivar(1). Isopimpinellin was detected in whole and cubed parsnips (Pastinaca sativa L) 53 days after harvest at 14.5 and 53.7 mg/kg(2). Isopimpinellin was detected in the rhizomes and aerial plant parts of Dorstenia vitifolia at 1.3 and 0.6 mg/g dry weight, respectively, it was not detected in D tubicina or D asaroides; plants were harvested from Brazil in 1996(3).

Isopimpinellin concentrations in various plants(1).

Using a structure estimation method based on molecular connectivity indices(1), the Koc of isopimpinellin can be estimated to be 320(SRC). According to a classification scheme(2), this estimated Koc value suggests that isopimpinellin is expected to have moderate mobility in soil(SRC).

/HUMAN EXPOSURE STUDIES/ Isolation of the furocoumarins (psoralens) bergapten, isobergapten, sphondin, isopimpinellin and pimpinellin from the Umbilliferous plant Heracleum laciniatum was carried out by column chromatography, and the structure and absorption spectra for the 5 furocoumarins isolated are described. Photoepicutaneous testing showed the strongest phototoxic effects from bergapten, marked effects from pimpinellin, weak effects from sphondin and none from the others. These in vivo findings were confirmed by the in vitro Candida test. Action spectrum studies demonstrated peak photosensitivity in the range 330-335 nm, bergapten being more than twice as phototoxic as pimpinellin.

/ALTERNATIVE and IN VITRO TESTS/ Coumarin is 7-hydroxylated by the P450 isoform Cyp2a-5 in mice and CYP2A6 in humans. Various drugs, endogenous substances, plant substances and carcinogens, altogether about 90 chemicals, were evaluated as possible inhibitors of coumarin 7-hydroxylase (COH) activity in mouse microsomes. The effects of selected compounds on COH activity in human liver microsomes were also tested. The furanocoumarin derivatives methoxsalen (8-methoxypsoralen) and psoralen proved to be the most potent inhibitors of mouse COH activity (IC50 values 1.0 and 3.1 uM, respectively). The furanocoumarins bergapten (5-methoxypsoralen), isopimpinellin (5,8-dimethoxypsoralen), imperatorin and sphondin also effectively inhibited mouse COH activity (IC50 values 19-40 uM). Methoxsalen, isopimpinellin and metyrapone were also inhibitors in mice in vivo. Methoxsalen was a potent inhibitor of COH activity also in human liver microsomes, (IC50 value 5.4 uM), whereas bergapten, isopimpinellin and imperatorin had no effect. The imidazole antimycotic miconazole was a potent but non-specific inhibitor of COH activity. Several known substrates and inhibitors of members in the CYP1A, CYP2B, CYP2C, CYP2D and CYP3A subfamilies were poor inhibitors of COH activity. These results suggest that (i) the coumarin-type compounds in particular interact with the active sites of Cyp2a-5 and CYP2A6, and (ii) the active sites of Cyp2a-5 and CYP2A6 are structurally different, since a number of compounds inhibited mouse, but not human COH activity.

Not listed by IARC. IARC has assessed other furocoumarins, classifying 8-methoxypsoralen as carcinogenic to humans (Group 1), 5-methoxypsoralen as possibly carcinogenic to humans (Group 2A), and certain other furocoumarins as not being classifiable as to their carcinogenicity to humans (Group 3). (L135)

An estimated BCF of 14 was calculated in fish for isopimpinellin(SRC), using an estimated log Kow of 2.22(1) and a regression-derived equation(1). According to a classification scheme(2), this BCF suggests the potential for bioconcentration in aquatic organisms is low.

The Henry's Law constant for isopimpinellin is estimated as 2.4X10-9 atm-cu m/mole(SRC) using a fragment constant estimation method(1). This Henry's Law constant indicates that isopimpinellin is expected to be essentially nonvolatile from water and moist soil surfaces(2). Isopimpinellin is not expected to volatilize from dry soil surfaces(SRC) based upon an estimated vapor pressure of 6.8X10-7 mm Hg(SRC), determined from a fragment constant method(1).