Structure-activity Comparisons Research Articles

Review of the Zebrafish as a Model to Investigate per- and Polyfluoroalkyl Substance Toxicity.

The existence of thousands of per- and polyfluoroalkyl substances (PFAS) and evidence that some cause adverse health effects has created immense need to better understand PFAS toxicity and to move beyond one-chemical-at-a-time approaches to hazard assessment for this chemical class. The zebrafish model enables rapid assessment of large libraries of PFAS, powerful comparison of compounds in a single in vivo system, and evaluation across life stages and generations, and has led to significant advances in PFAS research in recent years. The focus of this review is to assess contemporary findings regarding PFAS toxicokinetics, toxicity and apical adverse health outcomes, and potential modes of action using the zebrafish model. Much of the peer-reviewed literature has focused on a small subset of PFAS structural subclasses, such as the perfluoroalkyl sulfonic acids and perfluoroalkyl carboxylic acids. However, recent data on more diverse PFAS structures are enabling prioritization of compounds of concern. Structure-activity comparisons and the utilization of modeling and 'omics technologies in zebrafish have greatly contributed to our understanding of the hazard potential for a growing number of PFAS and will surely inform our understanding and predictive capabilities for many more PFAS in the future.

Abstract 4021: The evaluation of carcinogenicity evidence of dibenzanthracenes

Abstract Dibenzanthracenes (DBAs) are identified as carcinogens and are on the California Proposition 65 list. DBAs are five-ring polycyclic aromatic hydrocarbons (PAHs). This chemical group consists of three isomers: DB[a,c]A, DB[a,h]A, and DB[a,j]A. DBAs are products of the incomplete combustion or pyrolysis of organic matter and can be formed during high temperature cooking. They are air pollutants present in engine exhaust, tobacco and marijuana smoke, and may also contaminate soil and water. DB[a,h]A is a well-studied International Agency for Research on Cancer (IARC) Group 2A carcinogen, whereas fewer studies relevant to carcinogenicity have been conducted with DB[a,c]A and DB[a,j]A. Carcinogenicity evidence from multiple data streams was integrated in the hazard identification of this chemical group. These data streams include animal cancer bioassays, mechanistic studies and other relevant data, including high-throughput studies (e.g., ToxCast), toxicogenomic studies, and information from quantitative structure-activity relationship (QSAR) models. Structure activity comparisons across the three DBAs and related carcinogenic PAHs served to bridge the data gaps for the two less studied isomers. The results of our analyses showed that each of the DBAs share similar biological activities in animals: all induce tumors in mice, are mouse skin tumor initiators, form DNA adducts, are genotoxic, and form metabolites that are also genotoxic and initiators of mouse skin tumors. All three DBAs activate the aryl hydrocarbon receptor (AhR) in reporter gene assays, and DB[a,h]A and DB[a,c]A have been shown to bind to the AhR. In addition to strong evidence of genotoxicity, other carcinogenic mechanisms are likely also operative, including receptor activation (e.g., AhR), immune suppression, altered cell proliferation, apoptosis, and cell cycle regulation. There are strong structure-activity similarities among the DBAs and several carcinogenic 4-, 5- and 6-ring PAHs, including benzo[a]pyrene and dibenzo[a,h]pyrene. In addition, DB[a,c]A and DB[a,j]A were predicted to be mutagenic and carcinogenic in several QSAR models, including VEGA, Lazar and OECD QSAR Toolbox. Integration of the evidence from these multiple data streams resulted in the identification of the DBAs as a chemical group as carcinogens. Citation Format: Feng Tsai, Jennifer C. Hsieh, Gwendolyn Osborne, Martha Sandy. The evaluation of carcinogenicity evidence of dibenzanthracenes [abstract]. In: Proceedings of the American Association for Cancer Research Annual Meeting 2018; 2018 Apr 14-18; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2018;78(13 Suppl):Abstract nr 4021.

Dandamycin and chandrananimycin E, benzoxazines from Streptomyces griseus.

Two new benzoxazines were isolated from Streptomyces griseus (HKI 0545) and assigned as chandrananimycin E (1) and dandamycin (2). Although a number of phenoxazinone-type compounds have been reported from nature, phenoxazines are rarer, and carbon substitution at N-10 such as in 1 is unprecedented. The cyclopentene-containing ring structure of dandamycin (2) is also unique. Chandrananimycin E (1) was found to possess moderate antiproliferative activity against HUVEC cells (GI50 35.3 μM) and weak cytotoxic activity towards HeLa cells (CC50 56.9 μM). Dandamycin showed neither antiproliferative activity nor cytotoxicity towards these cell lines. Structure activity comparisons with phenoxazinones isolated from S. griseus HKI 0545 suggested that the alteration of the core ring systems in 1 and 2 diminishes their activity. Natural products 1 and 2 are interesting additions to the rich secondary metabolome of S. griseus and constitute an important addition to the body of knowledge on phenoxazinone-derived metabolites.

ChemInform Abstract: Additional Nucleotide Derivatives of Mitosenes. Synthesis and Activity Against Parental and Multidrug Resistant L1210 Leukemia.

Cytidine 5'-monophosphate and 5'-ara-CMP conjugates of 2,7-diaminomitosene, with the phosphate groups linked to C-1, were prepared by treating mitomycin C with the appropriate nucleotides. 5'-UMP conjugates were prepared from mitomycin A, 7 (M-83), and 8 (BMY-25282) by similar procedures. A conjugate could not be prepared from mitomycin C and 6-MPRP, but a sulfur-linked derivative was made with 6-MP ribonucleoside. The corresponding 1-hydroxy-2-aminomitosenes were prepared from the parent mitomycin analogues for structure-activity comparisons. All compounds were tested against L1210 murine leukemia in the MTT tetrazolium dye assay. In general, the conjugates were less potent than the parent mitomycins; however 5'-ara-CMP conjugate 14 derived from mitomycin C was more potent than the parent compound or any mitomycin tested except mitomycin A. It also was more potent than ara-C. This result establishes the value of this approach to prodrugs, at least in cell culture. Against a multi-drug-resistant L1210 cell line, all of the conjugates derived from mitomycin C were more potent than the parent compound. 6-Mercaptopurine ribonucleoside conjugate 15 was more active against the resistant cells than it was against the parental cell line.

Examination of Two Independent Kinetic Assays for Determining the Inhibition of Carbonic Anhydrases I and II: Structure–Activity Comparison of Sulfamates and Sulfamides

Enzyme inhibition assays often require deviations from physiological conditions. For carbonic anhydrases, procedures involving native CO(2) and non-native substrates have been used. We compared a native and a non-native substrate in the context of inhibition of human carbonic anhydrases I and II by examining various sulfamate and sulfamide compounds in two kinetic assays: hydration of CO(2) and hydrolysis of 4-nitrophenylacetate. For carbonic anhydrase II, the two assays consistently generated similar K(i) values, with the relative difference between the assays never exceeding 2.5-fold. However, for carbonic anhydrase I there was more variability between the two assays, with K(i) values for three compounds differing by more than 2.5-fold, up to eightfold. In the CO(2) hydration assay, some sulfamates and sulfamides exhibited mixed kinetics or partial inhibition. Our results indicate that K(i) or K(d) values from carbonic anhydrase assays involving non-native substrates should be confirmed by assays that use CO(2) (or HCO), to establish pharmacological relevance. From structure-activity comparisons, the sulfamate is more effective than the sulfamide in inhibiting carbonic anhydrase I and II, but the sulfamate does not confer selectivity. In contrast, the sulfonamide confers selectivity for carbonic anhydrase I (10- to 30-fold). Selectivity for carbonic anhydrase II occurred with the substituted fructose moiety, especially the d-enantiomer (>100-fold).

Silanediol Protease Inhibitors: From Conception to Validation

AbstractSilanediols are isosteric with the unstable hydrated carbonyl group, but are most commonly associated with polymerization to give silicone polymers. Placement of a silanediol in a dipeptide analogue yields a new kind of nonhydrolyzable transition‐state‐analogue protease inhibitor. Both metallo and aspartic protease inhibitors have been prepared using silanediols, with enzyme inhibition in the low nanomolar range. Structure–activity comparisons with known inhibitors, efficacy in whole cell assays, and a crystal structure of a silanediol inhibitor bound to the thermolysin active site establish these silanediol inhibitors as effective and predictable new protease inhibitor tools. Recent chemistry developments have led to efficient and streamlined preparation of these inhibitors. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)

Characterizing the “Shell Phase” Formed from Amphiphilic Picolinates

The shell phase forms when certain picolinates are subjected to energy input (via sonication or vortexing) while exposed to a water/toluene mixture. A shell, about 600 A thick and containing the picolinate and (very likely) toluene, surround the water droplets that are always produced during the mixing process. Solubility in either phase appears to be deleterious to shell formation. The shells, stable for months, are not easily distorted but can be punctured, even skewered, with a syringe needle without destroying the sphere, yet there is enough mobility among the molecules to repair the physical damage after the needle is removed. This, plus the absence of evidence for crystallinity, suggests a solid or semisolid film forms when picolinates, with the aid of an aromatic solvent, are provided the energy to rearrange themselves on water droplet surfaces. Structure-activity comparisons among the 10 compounds studied indicate that chain-chain association and intermolecular hydrogen bonding are dominant forces in a side-by-side self-assembly of the molecules within the shells.

Lysophospholipase inhibition by organophosphorus toxicants

Lysophospholipases (LysoPLAs) are a large family of enzymes for removing lysophospholipids from cell membranes. Potent inhibitors are needed to define the importance of LysoPLAs as targets for toxicants and potential therapeutics. This study considers organophosphorus (OP) inhibitors with emphasis on mouse brain total LysoPLA activity relative to the mipafox-sensitive neuropathy target esterase (NTE)-LysoPLA recently established as 17% of the total activity and important in the action of OP delayed toxicants. The most potent inhibitors of total LysoPLA in mouse brain are isopropyl dodecylphosphonofluoridate (also for LysoPLA of Vibrio bacteria), ethyl octylphosphonofluoridate (EOPF), and two alkyl-benzodioxaphosphorin 2-oxides (BDPOs)[( S)-octyl and dodecyl] (IC50 2–8 nM). OP inhibitors acting in vitro and in vivo differentiate a more sensitive portion but not a distinct NTE-LysoPLA compared with total LysoPLA activity. For 10 active inhibitors, NTE-LysoPLA is 17-fold more sensitive than total LysoPLA, but structure–activity comparisons give a good correlation ( r 2 = 0.94) of IC50 values, suggesting active site structural similarity or identity. In mice 4 h after intraperitoneal treatment with discriminating doses, EOPF, tribufos (a plant defoliant), and dodecanesulfonyl fluoride inhibit 41–57% of the total brain LysoPLA and 85–99% of the NTE-LysoPLA activity. Total LysoPLA as well as NTE-LysoPLA is decreased in activity in Nte +/− -haploinsufficient mice compared to their Nte +/+ littermates. The lysolecithin level of spinal cord but not brain is elevated significantly following EOPF treatment (3 mg/kg), thereby focusing attention on localized rather than general alterations in lysophospholipid metabolism in OP-induced hyperactivity and toxicity.

7α-methyl-19-nortestosterone, a synthetic androgen with high potency: structure-activity comparisons with other androgens

Studies of androgen receptor (AR)-mediated events in vivo are often complicated by problems related to hormone metabolism and pharmacokinetics. Compounds can be metabolically transformed to agents with altered potency. We have investigated some aspects of the structure-activity relationships of testosterone (T) and its analogs using in vivo and in vitro assays. The dose response of ventral prostate (VP) and levator ani (LA) to T, dihydrotestosterone (DHT), 19-nortestosterone (19-NT), 7α-methyl-19-NT (MENT), 7α-cyano-19-NT (CNNT) and 7α-acetylthio-19-NT (ATNT), was investigated in castrated rats. The most potent androgenic steroid (VP response) was MENT followed by T, DHT, 19-NT, ATNT, and CNNT. On the other hand, the order of anabolic potency (LA response) was MENT>19-NT>T>DHT>ATNT>CNNT. There was a good correlation between bioactivity and binding affinity to AR for the 7α-substituted androgens compared to T. In contrast, relative to their binding affinity to AR, the androgenic potency of DHT and 19-NT was lower compared to T. The reason for the lower in vivo androgenic activity of 19-NT is attributable to its enzymatic conversion to 5α-reduced-19-NT in the prostate. In the case of DHT, the lower bioactivity could be attributed to its faster metabolic clearance rate relative to T. The correlation was further investigated in vitro by co-transfection of rat ARcDNA expression plasmid and a reporter plasmid encoding the chloramphenicol acetyl transferase (CAT) gene driven by an androgen inducible promoter into CV-1 cells. All the androgens led to a dose-dependent increase in the CAT activity. MENT was found to be the most potent followed by DHT, 19-NT, T, and CNNT. The specificity of the androgenic response was confirmed by its inhibition with hydroxyflutamide, an antiandrogen. Thus, there was a good correlation between binding affinity and in vitro bioactivity in the transient transfection assay for the androgens. This suggests that the in vivo bioactivity of androgens could be influenced not only by binding affinity to receptors but also by factors such as absorption, binding to serum proteins and metabolism. However, the high potency of MENT is primarily related to its higher affinity to AR.

Developing gossypol derivatives with enhanced antitumor activity.

Preclinical and clinical studies have pointed to the antitumor potential of the naturally occurring polyphenolic binaphthyl dialdehyde, gossypol, as well as its purified (-,+) enantiomers. To explore further the antitumor properties of this multifunctional agent, we synthesized several reactive derivatives including the (-,+) enantiomers of gossypolone and four different gossypol Schiff's bases (AR1, AR2, AR3, AR4). The biological activities of these new agents were screened by measuring their in vitro antiproliferative activity against malignant (MCF-7, MCF-7/adr) or immortalized (HBL-100) human breast epithelial cell lines. Racemic gossypolone showed relatively uniform antiproliferative activity against all of the breast epithelial cell lines with 3- to 5-fold less activity than (--)-gossypol against MCF-7 and MCF-7/adr cells. Of interest, the relative antitumor potency of purified gossypolone enantiomers was reverse that of gossypol enantiomers, since (+)-gossypolone showed up to 3-fold greater inhibition of MCF-7 culture growth than (--)-gossypolone. Of the Schiff's base derivatives only AR3 with its isopropyl amine substituent demonstrated cytotoxic activity comparable to that of (--)-gossypol; derivatives with ethyl, propyl, or butyl amine substituents (AR1, AR2, AR4) had little growth inhibitory activity at culture concentrations up to 25 microM. AR3 activity was greatest against HBL-100 and MCF-7 cells [MCF-7 IC50 values: AR3 = 0.9 microM, (--)-gossypol = 2.3 microM]; unlike (--)-gossypol, however, AR3 showed substantially reduced activity against the multidrug-resistant subline, MCF-7/adr. These structure-activity comparisons suggest that isolation of (-,+)-enantiomers of AR3 and additional chemical modifications including the synthesis of an isopropyl amine Schiff's base of gossypolone will likely yield a newer generation of gossypol analogues with enhanced anticancer potential.

Neurotoxicity of artemisinin analogs in vitro.

The sesquiterpene endoperoxide antimalarial agents arteether and artemether have been reported to cause neurotoxicity with a discrete distribution in the brain stems of rats and dogs after multiple doses. The nature and distribution of the brain lesions suggest a specific neuronal target, the identity of which is unknown. In order to further investigate artemisinin analog-induced neurotoxicity, we evaluated several in vitro models: fetal rat primary neuronal cultures, fetal rat secondary astrocyte cultures, and transformed neuronal cultures (rat-derived neuroblastoma NG108-15 and mouse-derived neuroblastoma Neuro-2a). Results indicate that toxicity was specific for neuronal cell types but not glial cells. Neurotoxicity, as indexed by liberation of lactate dehydrogenase and/or inhibition of radiolabelled-leucine uptake, was seen in all three neuronal culture types, implicating a common target. In vitro neurotoxicity was dose and time dependent. Acute exposure to drug results in delayed, but not immediate, manifestations of cell toxicity. Structure-activity comparisons indicate that substitutions at positions 9 and 10 and stereoisomerism at position 10 of the artemisinin backbone influence the degree of toxicity. The endoperoxide is necessary but not sufficient for toxicity. Sodium artesunate and dihydroartemisinin, a metabolite common to all artemisinin analogs currently being developed for clinical use, are the most potent of all analogs tested. These results are consistent with a specific neuronal target, but the identity of the target(s) remains unknown.

Impairment of UDP-glucose dehydrogenase and glucuronidation activities in liver and small intestine of rat and guinea pig in vitro by piperine

The effects of piperine, a major ingredient of black pepper, on UDP-glucose dehydrogenase (UDP-GDH) and glucuronidation potentials of rat and guinea pig liver and intestine were studied. Piperine caused a concentration-related strong inhibition of UDP-GDH (50% at 10 μM) reversibly and equipotently, in both tissues. Partially purified rat liver UDP-GDH was used to obtain the kinetic values at pH optima of 9.4 and 8.6. At pH 9.4: K m UDP-glucose = 15 μM, V max = 5.2 nmol NADH/min/mg protein, K i = 6 μM. With NAD, a K i of 16 μM was obtained. At pH 8.6: K m = 35 μM, V max = 7.5 nmol, ki = 15 μM. In all of these cases, piperine caused non-competitive inhibition. Data from structure-activity comparisons of piperine analogs indicated that the presence of conjugated double bonds in the side chain of the molecule is a factor in piperine inhibition. However, the UDP-glucuronic acid (UDPGA) contents were decreased less effectively by piperine in isolated rat hepatocytes compared with enterocytes of guinea pig small intestine. Piperine at 50 μM caused a marginal decrease of UDPGA in hepatocytes when the rate of glucuronidation of 3-hydroxybenzo[ita]pyrene (3-OH-BP) decreased by about 40%. The decrease obtained at 10 μM piperine in intestinal cells was comparable to that obtained at 50–100 μM in hepatocytes. UDP-glucuronosyltransferase (UGT) activities towards 3-OH-BP (UGT1A1) and 4-OH-biphenyl (UGT2B1) were also determined. Piperine did not affect the rate of glucuronidation of 4-OH-biphenyl in rat liver, whereas that of 3-OH-BP was impaired significantly. In guinea pig small intestine, both these activities were inhibited significantly requiring less than 25 μM piperine to produce a more than 50% inhibition of UGT(s). The results suggested that (i) piperine is a potent inhibitor of UDP-GDH, (ii) inhibition is offered exclusively by the conjugated double bonds of the molecule, and (iii) piperine exerts stronger effects on intestinal glucuronidation than in rat liver.

Additional nucleotide derivatives of mitosenes. Synthesis and activity against parental and multidrug resistant L1210 leukemia

Cytidine 5'-monophosphate and 5'-ara-CMP conjugates of 2,7-diaminomitosene, with the phosphate groups linked to C-1, were prepared by treating mitomycin C with the appropriate nucleotides. 5'-UMP conjugates were prepared from mitomycin A, 7 (M-83), and 8 (BMY-25282) by similar procedures. A conjugate could not be prepared from mitomycin C and 6-MPRP, but a sulfur-linked derivative was made with 6-MP ribonucleoside. The corresponding 1-hydroxy-2-aminomitosenes were prepared from the parent mitomycin analogues for structure-activity comparisons. All compounds were tested against L1210 murine leukemia in the MTT tetrazolium dye assay. In general, the conjugates were less potent than the parent mitomycins; however 5'-ara-CMP conjugate 14 derived from mitomycin C was more potent than the parent compound or any mitomycin tested except mitomycin A. It also was more potent than ara-C. This result establishes the value of this approach to prodrugs, at least in cell culture. Against a multi-drug-resistant L1210 cell line, all of the conjugates derived from mitomycin C were more potent than the parent compound. 6-Mercaptopurine ribonucleoside conjugate 15 was more active against the resistant cells than it was against the parental cell line.

Trail pheromone of the antTetramorium impurum and model compounds: Structure-activity comparisons

The major component of the trail pheromone ofT. impurum is methyl 2-hydroxy-6-methylbenzoate (methyl 6-methyl salicylate). The poison reservoir of each worker contains about 1.0 ng of this substance in the venom. To ascertain the degree of specificity of the substance as pheromone and to determine any correlation between structure and biological activity, 38 synthetic analogs were tested in a bioassay. Sixteen compounds showed activity at 100 ng per artificial trail. The importance of the methyl group and methyl ester were demonstrated, while the hydroxyl group has a relatively small effect.

A novel pathway to the ultimate mutagens of aromatic amino and nitro compounds.

Photolysis of arylazides in aqueous media was recently found to generate presumed nitrenium ions, species which are generally considered as the ultimate mutagens/carcinogens derived from arylamines and nitroarenes. The primary photolysis products of arylazides, the arylnitrenes, can possibly react as electrophiles themselves, or they can be protonated and thus form the electrophilic nitrenium ions. Numerous arylazides and aryldiazides can be photoactivated to short-lived mutagens detectable in Salmonella typhimurium TA98. Structure-activity comparisons between arylazides and the matching arylamines and nitroarenes show correlations; e.g., phenyl azide and methyl-substituted phenyl azides are not mutagenic or only weakly mutagenic like aniline, nitrobenzene, and their methyl homologues, whereas 4-azidodiphenyl, 2-azidofluorene, 1-azidopyrene, azido-IQ, and azido-isoIQ are increasingly mutagenic in that order, like the matching amino and nitro compounds. It is hypothesized on the basis of these data that the nitrene/nitrenium ion is the reactive intermediate common to the three mutagenic pathways and that the reaction of the nitrene/nitrenium ion with DNA is rate limiting for the overall mutagenic process in Salmonella. The photochemical generation from arylazides of the reactive species, the nitrene/nitrenium ions, opens new perspectives for the understanding of the genotoxic activity of arylamines and nitroarenes in general and, specifically, of the food mutagens/carcinogens of the IQ type.

Sugar taste reception in mammals

This review evaluates behavioral and electrophysiological investigations in an attempt to show correlation between the taste responses of humans and other mammals to various sugar sweeteners. Methodologies include whole nerve, single neuron and single cell studies. In addition, to enhance understanding of the mechanism of sweet taste reception, attention is focused on inhibition studies, mixture experiments, pH effects, analyses of concentration – response curves and structure–activity comparisons. Results show that sucrose is the most potent naturally occurring disaccharide stimulant, while chloro-sucrose is the most potent synthetic disaccharide. Among the monosaccharides, fructose is the most potent stimulant. Mixture experiments with sucrose and other sugars suggest that these compounds are interacting at a single receptor site whereas mixtures of sucrose and saccharin suggest different receptor sites. Inhibitor experiments suggest that there are at least two major sucrose receptor sites. Single neuron and single cell studies suggest a number of other sugar receptors as well as a saccharin receptor. Concentration–response curve analyses show sigmoidally shaped curves throughout, and pH studies indicate that sucrose and saccharin are interacting with their respective receptors by a different mechanism. From structure–activity comparisons, we conclude that the binding mechanism for sugars in humans and other mammals is very similar.

Chemical structure and mutagenic activity of aminoimidazoquinolines and aminonaphthimidazoles related to 2-amino-3-methylimidazo[4,5-f]quinoline

We have synthesized 11 heterocyclic aromatic amines with chemical structures related to that of 2-amino-3-methylimidazo [4,5-f]quinoline (IQ), a potent mutagen occurring in broiled sardines, fried beef and beef extract. The mutagenic activity of these IQ analogs was studied and compared with that of IQ using the Ames test with strain TA98 of Salmonella typhimurium in presence of a metabolic activation system (S9 mix) derived from rat liver. The mutagenic activities of the IQ analogs vary over a million-fold; structure-activity comparisons indicate major contributions of the methyl substitution in the imidazole ring and of the quinoline- N, and significant contributions of methylation of the exocyclic amino group and of the geometry of the entire ring system.

ChemInform Abstract: ANTIHYPERTENSIVE PYRROLO(1,2‐C)QUINAZOLINES AND PYRROLO(1,2‐C)QUINAZOLINONES

AbstractDie Methylchinazoline (I) kondensieren mit Brombrenztraubensäureethylester (II) zu den Pyrrolochinazolinen (III).

Antihypertensive pyrrolo[1,2-c]quinazolines and pyrrolo[1,2-c]quinazolinones.

The synthesis of a variety of pyrrolo[1,2-c]quinazolines and pyrrolo[1,2-c]quinazolinones is described. Several of these compounds have exhibited antihypertensive properties in the spontaneously hypertensive rat (SHR). Structure-activity comparisons have indicated that optimal activity is obtained in both the 2-carbethoxydihydroquinazoline series (C) and 2-carbethoxyquinazolinone series (D) when there is either a carbethoxy or cyanoethyl group at position 6 and no substitution in the benzene ring, while optimal activity is obtained in the 2-methyl-quinazolinone series (D) when both position 6 and the benzene ring are unsubstituted.

Molecular Structure and Gibberellin Activity. II.* Further Studies on Helminthosporic Acid Analogues

The gibberellin-like activity of twelve synthetic helminthosporins has been examined in the barley endosperm bioassay. (�)-14-Norhelminthospor-7(12)-en-13-oic acid (5) was more potent than GA*9 and elicited similar responses in the assay measuring production of a-amylase, sugar and refractive compounds. Responses to acid (5) and GA*3 were retarded similary by abscisic acid. Log-doseresponse curves after 41 and 65 h incubation were of two types: linear with two C4-cyclohexyl helminthosporins and GA*3; and biphasic with two norhelminthosporins, showing initially a shallow curve then a steep slope at high concentrations. Structure-activity comparisons reveal that an alkyl group at C7 on a helminthosporin bridge contributes significantly to activity, but an olefinic bond has a minor effect. Three different orientations of the bridge carboxyl group did not materially alter activity. Oxygenated cyclohexyl helminthosporins had less biological activity than the deoxygenated analogue, perhaps reflecting a critical orientation of a hydroxyl group in the A ring of the more potent gibberellins.