Alkoxypsoralens, novel nonpeptide blockers of Shaker-type K+ channels: synthesis and photoreactivity

Identification of lead chemotherapeutic agents from medicinal plants against blood flukes and whipworms

Schistosomiasis and trichuriasis are two of the most common neglected tropical diseases (NTD) that affect almost a billion people worldwide. There is only a limited number of effective drugs to combat these NTD. Medicinal plants are a viable source of parasiticides. In this study, we have investigated six of the 19 phytochemicals isolated from two Bhutanese medicinal plants, Corydalis crispa and Pleurospermum amabile, for their anthelmintic properties. We used the xWORM technique and Scanning Electron Microscope-based imaging to determine the activity of the compounds. Of the six compounds tested, isomyristicin and bergapten showed significant anthelmintic activity against Schistosoma mansoni and Trichuris muris with bergapten being the most efficacious compound one against both parasites (S. mansoni IC₅₀ = 8.6 μg/mL and T. muris IC₅₀ = 10.6 μg/mL) and also against the schistosomulum stage of S. mansoni. These two compounds induced tegumental damage to S. mansoni and affected the cuticle, bacillary bands and bacillary glands of T. muris. The efficacy against multiple phylogenetically distinct parasites and different life stages, especially the schistosomulum where praziquantel is ineffective, makes isomyristicin and bergapten novel scaffolds for broad-spectrum anthelmintic drug development that could be used for the control of helminths infecting humans and animals.

Identification of QTLs affecting scopolin and scopoletin biosynthesis in Arabidopsis thaliana

BACKGROUND

Scopoletin and its glucoside scopolin are important secondary metabolites synthesized in plants as a defense mechanism against various environmental stresses. They belong to coumarins, a class of phytochemicals with significant biological activities that is widely used in medical application and cosmetics industry. Although numerous studies showed that a variety of coumarins occurs naturally in several plant species, the details of coumarins biosynthesis and its regulation is not well understood. It was shown previously that coumarins (predominantly scopolin and scopoletin) occur in Arabidopsis thaliana (Arabidopsis) roots, but until now nothing is known about natural variation of their accumulation in this model plant. Therefore, the genetic architecture of coumarins biosynthesis in Arabidopsis has not been studied before.

RESULTS

Here, the variation in scopolin and scopoletin content was assessed by comparing seven Arabidopsis accessions. Subsequently, a quantitative trait locus (QTL) mapping was performed with an Advanced Intercross Recombinant Inbred Lines (AI-RILs) mapping population EstC (Est-1 × Col). In order to reveal the genetic basis of both scopolin and scopoletin biosynthesis, two sets of methanol extracts were made from Arabidopsis roots and one set was additionally subjected to enzymatic hydrolysis prior to quantification done by high-performance liquid chromatography (HPLC). We identified one QTL for scopolin and five QTLs for scopoletin accumulation. The identified QTLs explained 13.86% and 37.60% of the observed phenotypic variation in scopolin and scopoletin content, respectively. In silico analysis of genes located in the associated QTL intervals identified a number of possible candidate genes involved in coumarins biosynthesis.

CONCLUSIONS

Together, our results demonstrate for the first time that Arabidopsis is an excellent model for studying the genetic and molecular basis of natural variation in coumarins biosynthesis in plants. It additionally provides a basis for fine mapping and cloning of the genes involved in scopolin and scopoletin biosynthesis. Importantly, we have identified new loci for this biosynthetic process.

Low-energy collision-induced dissociation tandem mass spectrometry of 7-acetonyloxycoumarins

RATIONALE

Coumarins are naturally occurring, oxygen-containing heterocycles with considerable pharmaceutical potential. For structural elucidation of natural or synthetic coumarins, tandem mass spectrometry (MS(n)) represents an essential tool. In this study, fragmentation characteristics of twenty-two 7-acetonyloxycoumarins, having promising anti-inflammatory properties, were investigated with low-energy collision-induced dissociation (CID).

METHODS

Accurate mass measurements were performed on a 12-T Fourier transform ion cyclotron resonance (FT-ICR) instrument. Most CID-MS(n) measurements were performed on a quadrupole ion trap (QIT) instrument, except some additional CID-MS(2) measurements performed on the FT-ICR instrument for further confirmation of some fragment ions. Positive-ion electrospray ionization (ESI) was employed throughout. Density functional theory (DFT) calculations (B3LYP) were carried out to analyze putative ion structures/fragmentation channels.

RESULTS

The most favourable dissociation channel for [M + H](+) ions of 7-acetonyloxycoumarins was the elimination of a C3H5O(●) radical (57 Da) from the 7-acetonyloxy group via homolytic bond cleavage. The resulting phenolic radical ion was the primary fragment ion for the most compounds studied. Losses of even-electron neutrals, C3H4O and C3H6O (56 and 58 Da), were also observed. These primary eliminations were accompanied with other characteristic neutral losses from the coumarin skeleton, including H2O, CO, CO2, and C2H2O (ketene). In addition, propene (C3H6) loss was also observed for 4-propyl or 3-ethyl-4-methyl-substituted compounds.

CONCLUSIONS

The studied coumarins showed interesting characteristics in low-energy CID due to the presence of a 7-acetonyloxy group, leading to both even- and odd-electron product ions. The main dissociation channels observed for each compound were highly dependent on the substituents in the benzopyranone ring. The present results will advance our knowledge on the dissociation characteristics of both synthetic and natural coumarins.

Structure-activity relationships for naturally occurring coumarins as β-secretase inhibitor

The present study was demonstrated to evaluate the effects of naturally occurring coumarins (NOCs) including simple coumarins, furanocoumarins, and pyranocoumarins on the inhibition of β-secretase (BACE1) activity. Of 41 NOCs examined, some furanocoumarins inhibited BACE1 activity, but simple coumarins and pyranocoumarins did not affect. The most potent inhibitor was 5-geranyloxy-8-methoxypsoralen (31), which has an IC(50) value of 9.9 μM. Other furanocoumarin derivatives, for example, 8-geranyloxy-5-methoxypsoralen (35), 8-geranyloxypsoralen (24), and bergamottin (18) inhibited BACE1 activity, with the IC(50) values <25.0 μM. Analyses of the inhibition mechanism by Dixon plots and Cornish-Bowden plots showed that compounds 18, 31 and 35 were mixed-type inhibitor. The kinetics of inhibition of BACE1 by coumarins 24 was non-competitive inhibitors.

Synthesis and biological evaluation of fused oxepinocoumarins as free radicals scavengers

Some fused dihydrooxepino[f]-, [g]-, and [h]coumarins were obtained from the ring-closing metathesis of the corresponding o-allyl-allyloxycoumarins under the treatment with the first generation Grubbs' catalyst. These compounds were tested in vitro for their antioxidant activity, and they present significant scavenging activity. They were also showed to inhibit in vitro soybean lipoxygenase.

Differentiation and apoptosis in UVA-irradiated cells treated with furocoumarin derivatives

In this review we summarize the structure and biological effects of linear and angular psoralens. These compounds exhibit interesting biological effects on the cell cycle, apoptosis and differentiation. These molecules should be considered promising drugs in the therapy of several diseases, including psoriasis, mycosis fungoides and cancer. Also, preclinical data demonstrate a possible use of these molecules for the treatment of beta-thalassemia and other hematological disorders.

Determination of oxygen heterocyclic components in citrus products by HPLC with UV detection

The study of oxygen heterocyclic compounds (coumarins, psoralens, polymethoxylated flavones) in natural matrices such as citrus oils is not easy due to the difficulty of obtaining standards at the required level of purity and the diversity of structures present in each kind of sample. In this work, standards were either isolated by preparative LC methods from citrus oils or synthesized, then characterized by their physicochemical parameters and spectroscopic techniques, and further used for qualitative and quantitative calculations in citrus essential oils and products made with them (Earl Grey tea, liquors, juices). An HPLC method using an innovative partially porous particle HPLC column enabled baseline separation of all analytes. The method developed was validated in terms of detection limit, quantitation limit, linearity, and precision as repeatability and intralaboratory reproducibility.

4-Phenoxybutoxy-substituted heterocycles--a structure-activity relationship study of blockers of the lymphocyte potassium channel Kv1.3

The voltage-gated potassium channel Kv1.3 constitutes an attractive pharmacological target for the treatment of effector memory T cell-mediated autoimmune diseases such as multiple sclerosis and psoriasis. Using 5-methoxypsoralen (5-MOP, 1), a compound isolated from Ruta graveolens, as a template we previously synthesized 5-(4-phenoxybutoxy)psoralen (PAP-1, 2) which inhibits Kv1.3 with an IC(50) of 2nM. Since PAP-1 is more than 1000-fold more potent than 5-MOP, we here investigated whether attaching a 4-phenoxybutoxy side chain to other heterocyclic systems would also produce potent Kv1.3 blockers. While 4-phenoxybutoxy-substituted quinolines, quinazolines and phenanthrenes were inactive, 4-phenoxybutoxy-substituted quinolinones, furoquinolines, coumarins or furochromones inhibited Kv1.3 with IC(50)s of 150 nM to 10 microM in whole-cell patch-clamp experiments. Our most potent new compound is 4-(4-phenoxybutoxy)-7H-furo[3,2-g]chromene-7-thione (73, IC(50) 17 nM), in which the carbonyl oxygen of PAP-1 is replaced by sulfur. Taken together, our results demonstrate that the psoralen system is a crucial part of the pharmacophore of phenoxyalkoxypsoralen-type Kv1.3 blockers.

Molecular cloning and functional characterization of psoralen synthase, the first committed monooxygenase of furanocoumarin biosynthesis

Ammi majus L. accumulates linear furanocoumarins by cytochrome P450 (CYP)-dependent conversion of 6-prenylumbelliferone via (+)-marmesin to psoralen. Relevant activities, i.e. psoralen synthase, are induced rapidly from negligible background levels upon elicitation of A. majus cultures with transient maxima at 9-10 h and were recovered in labile microsomes. Expressed sequence tags were cloned from elicited Ammi cells by a nested DD-RT-PCR strategy with CYP-specific primers, and full-size cDNAs were generated from those fragments correlated in abundance with the induction profile of furanocoumarin-specific activities. One of these cDNAs representing a transcript of maximal abundance at 4 h of elicitation was assigned CYP71AJ1. Functional expression in Escherichia coli or yeast cells initially failed but was accomplished eventually in yeast cells after swapping the N-terminal membrane anchor domain with that of CYP73A1. The recombinant enzyme was identified as psoralen synthase with narrow substrate specificity for (+)-marmesin. Psoralen synthase catalyzes a unique carbon-chain cleavage reaction concomitantly releasing acetone by syn-elimination. Related plants, i.e. Heracleum mantegazzianum, are known to produce both linear and angular furanocoumarins by analogous conversion of 8-prenylumbelliferone via (+)-columbianetin to angelicin, and it was suggested that angelicin synthase has evolved from psoralen synthase. However, (+)-columbianetin failed as substrate but competitively inhibited psoralen synthase activity. Analogy modeling and docked solutions defined the conditions for high affinity substrate binding and predicted the minimal requirements to accommodate (+)-columbianetin in the active site cavity. The studies suggested that several point mutations are necessary to pave the road toward angelicin synthase evolution.

Design of PAP-1, a selective small molecule Kv1.3 blocker, for the suppression of effector memory T cells in autoimmune diseases

The lymphocyte K+ channel Kv1.3 constitutes an attractive pharmacological target for the selective suppression of terminally differentiated effector memory T (TEM) cells in T cell-mediated autoimmune diseases, such as multiple sclerosis and type 1 diabetes. Unfortunately, none of the existing small-molecule Kv1.3 blockers is selective, and many of them, such as correolide, 4-phenyl-4-[3-(methoxyphenyl)-3-oxo-2-azapropyl]cyclohexanone, and our own compound Psora-4 inhibit the cardiac K+ channel Kv1.5. By further exploring the structure-activity relationship around Psora-4 through a combination of traditional medicinal chemistry and whole-cell patch-clamp, we identified a series of new phenoxyalkoxypsoralens that exhibit 2- to 50-fold selectivity for Kv1.3 over Kv1.5, depending on their exact substitution pattern. The most potent and "drug-like" compound of this series, 5-(4-phenoxybutoxy)psoralen (PAP-1), blocks Kv1.3 in a use-dependent manner, with a Hill coefficient of 2 and an EC50 of 2 nM, by preferentially binding to the C-type inactivated state of the channel. PAP-1 is 23-fold selective over Kv1.5, 33- to 125-fold selective over other Kv1-family channels, and 500- to 7500-fold selective over Kv2.1, Kv3.1, Kv3.2, Kv4.2, HERG, calcium-activated K+ channels, Na+,Ca2+, and Cl- channels. PAP-1 does not exhibit cytotoxic or phototoxic effects, is negative in the Ames test, and affects cytochrome P450-dependent enzymes only at micromolar concentrations. PAP-1 potently inhibits the proliferation of human TEM cells and suppresses delayed type hypersensitivity, a TEM cell-mediated reaction, in rats. PAP-1 and several of its derivatives therefore constitute excellent new tools to further explore Kv1.3 as a target for immunosuppression and could potentially be developed into orally available immunomodulators.

Khellinone derivatives as blockers of the voltage-gated potassium channel Kv1.3: synthesis and immunosuppressive activity

The voltage-gated potassium channel Kv1.3 constitutes a promising new target for the treatment of T-cell-mediated autoimmune diseases such as multiple sclerosis. In this study, we report the discovery of two new classes of Kv1.3 blockers based on the naturally occurring compound khellinone, 5-acetyl-4,7-dimethoxy-6-hydroxybenzofuran: (1) khellinone dimers linked via the alkylation of the 6-hydroxy groups and (2) chalcone derivatives of khellinone formed by Claisen-Schmidt condensation of the 5-acetyl group with aryl aldehydes. In particular, the chalcone 3-(4,7-dimethoxy-6-hydroxybenzofuran-5-yl)-1-phenyl-3-oxopropene (16) and several of its derivatives inhibited Kv1.3 with K(d) values of 300-800 nM and a Hill coefficient of 2, displayed moderate selectivity over other Kv1-family K(+) channels, suppressed T-lymphocyte proliferation at submicromolar concentrations, and showed no signs of acute toxicity in mice. Because of their relatively low molecular weight and lipophilicity and their high affinity to Kv1.3, aryl-substituted khellinone derivatives represent attractive lead compounds for the development of more potent and selective Kv1.3 blocking immunosuppressants.

Kv1.3-Blocking 5-Phenylalkoxypsoralens: A New Class of Immunomodulators

The lymphocyte potassium channel Kv1.3 is widely regarded as a promising new target for immunosuppression. To identify a potent small-molecule Kv1.3 blocker, we synthesized a series of 5-phenylalkoxypsoralens and tested them by whole-cell patch clamp. The most potent compound of this series, 5-(4-phenylbutoxy)psoralen (Psora-4), blocked Kv1.3 in a use-dependent manner, with a Hill coefficient of 2 and an EC50 value of 3 nM, by preferentially binding to the C-type inactivated state of the channel. Psora-4 is the most potent small-molecule Kv1.3 blocker known. It exhibited 17- to 70-fold selectivity for Kv1.3 over closely related Kv1-family channels (Kv1.1, Kv1.2, Kv1.4, and Kv1.7) with the exception of Kv1.5 (EC50, 7.7 nM) and showed no effect on human ether-a-go-go-related channel, Kv3.1, the calcium-activated K+ channels (IKCa1, SK1-SK3, and BKCa), or the neuronal NaV1.2 channel. In a test of in vivo toxicity in rats, Psora-4 did not display any signs of acute toxicity after five daily subcutaneous injections at 33 mg/kg body weight. Psora-4 selectively suppressed the proliferation of human and rat myelin-specific effector memory T cells with EC50 values of 25 and 60 nM, respectively, without persistently suppressing peripheral blood naive and central memory T cells. Because autoantigen-specific effector memory T cells contribute to the pathogenesis of T cell-mediated autoimmune diseases such as multiple sclerosis, Psora-4 and other Kv1.3 blockers may be useful as immunomodulators for the therapy of autoimmune disorders.

1-Thioangelicin: crystal structure, computer-aided studies and photobiological activity

1-Thioangelicin is a furocoumarin analog synthesized to investigate the role of the substitution of sulfur for oxygen in the parent compound angelicin. The compound was examined by X-ray diffraction, and its interaction with DNA, both in the dark and by UVA irradiation, studied by means of linear flow dichroism, chromatography and (1)H NMR. Further insight into the steric and electronic features of 1-thioangelicin has been reached through theoretical calculations, including molecular mechanics optimization, docking studies and frontier molecular orbital investigations. The experimental data indicate that thioangelicin is able to intercalate in the DNA helix and subsequent irradiation yields a cis-syn adduct, in agreement with theoretical calculations within the lower/higher singly occupied molecular orbital formalism. Antiproliferative activity has been assessed on Balb/c 3T3 cultured cells.

3-alkyl- and 3-aryl-(7-oxo-7H-furo[3, 2-g]chromen-5-yl)alkanoic acids as inhibitors of leukotriene B4 biosynthesis

The synthesis and biological properties of 3-alkyl-and 3-aryl-(7-oxo-7H-furo [3, 2-g]chromen-5-yl)alkanoic acids are described. The compounds were evaluated for their ability to inhibit soybean 15-lipoxygenase (15-LO) and the production of leukotriene B(4) (LTB(4)) in bovine polymorphonuclear leukocytes. The furocoumarins were further investigated for their lipophilicity and their capacity to photobleach N, N-dimethyl-p-nitrosoaniline (RNO) after irradiation with UVA light. The synthesised furocoumarins showed only a moderate ability to inhibit the 15-LO and the production of LT B(4). The 3-alkyl substituted furocoumarins had a higher capacity to photobleach RNO than then the 3-aryl substituted furocoumarinso. The inhibitory activity of the newcompounds was quite modest in comparison to other known inhibitors of 15-LO and LTB(4) production. Nevertheless, they may provide the basis for the development of more potent antiinflammatory potential photoreactive furocoumarins.

K(+) channels as therapeutic drug targets

K(+) channels play critical roles in a wide variety of physiological processes, including the regulation of heart rate, muscle contraction, neurotransmitter release, neuronal excitability, insulin secretion, epithelial electrolyte transport, cell volume regulation, and cell proliferation. As such, K(+) channels have been recognized as potential therapeutic drug targets for many years. Unfortunately, progress toward identifying selective K(+) channel modulators has been severely hampered by the need to use native currents and primary cells in the drug-screening process. Today, however, more than 80 K(+) channel and K(+) channel-related genes have been identified, and an understanding of the molecular composition of many important native K(+) currents has begun to emerge. The identification of these molecular K(+) channel drug targets should lead to the discovery of novel drug candidates. A summary of progress is presented.

A novel class of inhibitors for steroid 5alpha-reductase: synthesis and evaluation of umbelliferone derivatives

A series of umbelliferone derivatives was prepared and their 5alpha-reductase type 1 inhibitory activities were evaluated in cell culture systems. Our studies have identified a new series of potent 5alpha-reductase type 1 inhibitors and provided the basis for further development for the treatment of human endocrine disorders associated with overproduction of DHT by 5alpha-reductase type 1. The preliminary structure-activity relationship was described to elucidate the essential structural requirements.

Molecular properties and physiological roles of ion channels in the immune system

The discovery of a diverse and unique set of ion channels in T lymphocytes has led to a rapidly growing body of knowledge about their functional roles in the immune system. Here we review the biophysical and molecular characterization of K+, Ca2+, and Cl- channels in T lymphocytes. Potent and specific blockers, especially of K+ channels, have provided molecular tools to elucidate the involvement of voltage- and calcium-activated potassium channels in T-cell activation and cell-volume regulation. Their unique and differential expression makes lymphocyte K+ channels excellent pharmaceutical targets for modulating immune system function. This review surveys recent progress at the biophysical, molecular, and functional roles of the ion channels found in T lymphocytes.

Angular methoxy-substituted furo- and pyranoquinolinones as blockers of the voltage-gated potassium channel Kv1.3

The voltage-gated potassium channel Kv1.3 constitutes an attractive target for immunosuppression because of its role in T-lymphocyte activation and its functionally restricted expression to lymphocytes. Blockade of Kv1.3 channels by margatoxin has previously been shown to prevent T-cell activation and attenuate immune responses in vivo. In the present study, several furo- and pyranoquinoline derivatives were synthesized and screened for their blocking activities of Kv1.3 channels, stably expressed in mice-fibroblasts L929. In addition the activities of the compounds on Kv currents of the neuroblastoma cell line N1E-115 were determined. The most potent compounds, the angular furoquinolinone 8-methoxy-2-(1'-methylethyl)-5-methyl-4,5-dihydrofuro[3,2-c]quinolin-4-one (8c) and the angular pyranoquinolinone 9-methoxy-2,2,6-trimethyl-2,6-dihydro-5H-pyrano[3,2-c]quinolin-5-one (9a), inhibited Kv1.3 channels with half-blocking concentrations of 5 and 10 microM, respectively, and displayed 8-fold (8c) and 2-fold (9a) selectivity over Kv currents of N1E-115 cells. Thus, compounds 8c and 9a might function as a template for the development of novel immunosuppressants.

Substituted benzopyranobenzothiazinones. Synthesis and estrogenic activity on MCF-7 breast carcinoma cells

In the search for new agents with estrogenic activity mediated by estrogen receptors (ER), six 6,12-dihydro-1-benzopyrano[3,4-b][1,4]benzothiazin-6-ones 3a-f were synthesized. These compounds were readily prepared by the addition of 2-aminothiophenol 2 to substituted 4-hydroxycoumarin derivatives 1a-e. The estrogenic effect has been evaluated on the proliferation of MCF-7 breast adenocarcinoma cells and the specificity of described compounds was evaluated by the inhibition of their effect by ICI 182,780, an antiestrogenic compound. Among the compounds tested, 6,12-dihydro-3-methoxy-1-benzopyrano[3,4-b][1,4]benzothiazin-6-one 3e and 6,12-dihydro-3-hydroxy-1-benzopyrano[3,4-b][1,4]benzothiazin-6-one 3f exhibited an ER-dependent proliferation and a high binding affinity to ER, but a moderate capacity to activate the transcription of a reporter gene. Their pharmacological profiles are defined by their binding properties and their mechanism of action by computational modelling studies.

K(+) channel-blocking alkoxypsoralens inhibit the immune response of encephalitogenic T line cells and lymphocytes from Lewis rats challenged for experimental autoimmune encephalomyelitis

Alkoxypsoralens, known as DNA photomodifying agents, have been shown to block voltage-dependent K(+) channels (Kv) as well as to alleviate functional deficits in certain multiple sclerosis (MS) patients in a manner similar to 4-aminopyridine. Since Kv channel blockers are known to inhibit T cell-mediated immune responses both in vitro and in vivo, we investigated the effects of three alkoxypsoralens, 5-methoxypsoralen (5-MOP), 8-methoxypsoralen (8-MOP), and 5,8-diethoxypsoralen (H37), on the following parameters: (1) whole-cell K(+) currents of encephalitogenic, myelin basic protein-specific memory T cell line cells (MBP-TCLC) derived from Lewis rats as measured by patch-clamp technique, (2) proliferation of MBP-TCLC and lymph node cells (LNC) from Lewis rats challenged for experimental autoimmune encephalomyelitis (EAE) by immunisation with spinal cord homogenate as measured by 3H-thymidine incorporation, (3) interferon-gamma (IFN-gamma) secretion of MBP-TCLC as measured by ELISA, and (4) IFN-gamma gene expression of LNC as measured by quantitative reverse transcription polymerase chain reaction (RT-PCR) with ELISA-detection. The examined alkoxypsoralens exhibited suppressive effects on the measured parameters with the same sequence of efficacy: H37>5-MOP>8-MOP. We, therefore, conclude that Kv channel-blocking alkoxypsoralens interfere with voltage-controlled signal transduction in lymphocytes and might thereby suppress immune responses in autoimmune diseases of the central nervous system and most likely also in other autoimmune disorders. Thus, alkoxypsoralens, especially the non-phototoxic substance H37, are new candidates for further studies on K(+) channel blocking immunosuppressive drugs. The agents may exert a dual beneficial effect on demyelinating diseases like MS, because they could attenuate the inflammatory process and improve axonal conductivity.

Photochemical and photobiological studies on methylthioangelicins

4,4',5'-Trimethyl-1'-thioangelicin (1) and 4,6,4',5'-tetramethyl-1'-thioangelicin (2), two newly synthesised isosters of furocoumarins having a sulfur atom in their five-membered ring, were studied in terms of interactions with DNA, both in the ground state and after UVA light absorption. The compounds were able to intercalate the macromolecule and to photobind efficiently, forming C4-cycloadducts with thymine. The antiproliferative effect of this binding was shown in Ehrlich and HeLa cells and by T2 phage inactivation. Tests on Salmonella typhimurium indicated low mutagenic activity. In particular, compound 1 has photobiological activity comparable with that of 4,6,4'-trimethylangelicin, but is less mutagenic.