Teratogenic drugs inhibit tumour cell attachment to lectin-coated surfaces

Alternative Tests for Teratogenicity

Summary

The major features of the tests surveyed are shown in Table I. In a tier system of tests for teratogenicity, the Chernoff test is at a different level than the other assays described here. It is not appropriate for screening large numbers of chemicals, but may be useful for studies of smaller groups of agents, for example to confirm data from a prescreen. Although the test is certainly easier, cheaper and uses less than half the animals of a Segment II test, it is still much more expensive and time-consuming than most alternative tests.

Of the remaining alternatives, whole embryos or organs in culture encompass the widest range of mammalian developmental events and are invaluable in the study of teratogenic mechanisms. They are, however, also inappropriate for screening large numbers of chemicals. The methods are technically demanding, relatively expensive and use reasonably large numbers of pregnant mammals. To screen a group of, say, 20 chemicals involves a considerable investment of time and, in fact, no study of this size has been reported. In certain specific circumstances, they may be a useful adjunt to testing; for example, if treated human serum samples are freely available, if a drug has a unique action on rodent dams which confounds evaluation of the standard in vivo tests, or if human metabolism is important and can be mimicked in vitro.

Sub-mammalian and sub-vertebrate species offer considerable advantages; reduced cost, relative rapidity and no requirement for laboratory animals. FETAX provides some indication of teratogenicity in relation to embryotoxicity, while CHEST and the planarian and Drosophila assays measure only teratogenic potential, or more strictly speaking, embryotoxic potential, although it should be possible to derive some assessment of hazard with each of the latter systems. The Hydra system is cheap, quick and easy and is commercially available. It is the only assay specifically designed to estimate teratogenic hazard and may offer considerable advantages as an alternative screen.

The metabolic cooperation assay has not generated sufficient data to enable evaluation. The neural crest cell assay is not well developed as a routine screen, and objective endpoints which are not measures of general cytotoxicity must be devised. The viral morphogenesis and Drosophila embryo cell assays have both produced encouraging validation data. With further assessment, the viral system may be shown to be useful, but it is a relatively complex assay and its relevance to teratogenesis is obscure. The Drosophila system is easier, has been used with more chemicals and is developmentally relevant. However, it has not produced dose-response data to evaluate potency or hazard, and must be improved so that it can more clearly distinguish cytotoxicity. The measurement of endpoints in the neuroblastoma cell line assay requires further refinement, and contributions of growth inhibition or stimulation to effects on differentiation must be examined. In combination, tumour cell attachment and HEPM may prove valuable. Alone, HEPM appears to be an assay for cellular toxicity, not teratogenicity, and the attachment assay suffers from a high rate of false negatives because it measures only one cell phenomenon. Although micromass cultures use mammalian tissue, are not the cheapest assays and require some skill for full evaluation of the results obtained, they show considerable promise. Validation data are encouraging, the assay includes several developmental processes and the use of multiple endpoints permits specific developmental toxicities to be evaluated.

Implications for the Predictivity of Cell-Based Developmental Toxicity Assays Developed Two Decades Apart

Many in vitro developmental toxicity assays have been proposed over several decades. Since the late 1980s, we have made intermittent attempts to introduce in vitro assays as screening tests for developmental toxicity of in-house candidate products. Two cell-based assays which were developed two decades apart were intensively studied. One was an assay of inhibitory effects on mouse ascites tumor cell attachment to a concanavalin A-coated plastic sheet surface (MOT assay), which we studied in the early days of assay development. The other was an assay of inhibitory effects on the differentiation of mouse embryonic stem cell to beating heart cells (EST assay), which we assessed more recently. We evaluated the suitability of the assays for screening in-house candidates. The concordance rates with in vivo developmental toxicity were at the 60% level. The EST assay classified chemicals that inhibited cell proliferation as embryo-toxic. Both assays had a significant false positive rate. The assays were generally considered unsuitable for screening the developmental toxicity of our candidate compounds. Recent test systems adopt advanced technologies. Despite such evolution of materials and methods, the concordance rates of the EST and MOT systems were similar. This may suggest that the fundamental predictivity of in vitro developmental toxicity assays has remained basically unchanged for decades. To improve their predictivity, in vitro developmental toxicity assays should be strictly based on elucidated pathogenetic mechanisms of developmental toxicity.

Development of an in vitro embryotoxicity test using murine embryonic stem cell cultures

Mouse embryonic stem (ES) cell line D3 was used to establish conditions for a reproducible differentiation of ES cells in culture. ES cells can be maintained in an undifferentiated state by cultivation on a feeder layer of embryonic fibroblasts. ES cells form aggregates in suspension and can spontaneously differentiate into complex organized embryoid bodies (EBs), which in many respects resemble early postimplantation mouse embryos. Under appropriate culture conditions various cell and tissue types will develop in EBs: these include myocardial and skeletal muscle, nerve cells, chondrocytes and blood cells. Retinoic acid (RA) was used as an embryotoxic substance to test the application of ES cell cultures in in vitro embryotoxicity testing. RA (1 x 10(-8)m) induced an increase in skeletal muscle cell differentiation, which followed a characteristic pattern: day 10 is characterized by the first appearance of mononucleated myoblasts; day 12 shows the fusion of myoblasts; on day 13, multinucleated myotubes can be detected, and on day 25 contractile myofibres are present in ES cell cultures. The development of blood islands with red cells enhanced by erythropoietin in EBs has encouraged the hope that, subsequently, more mature stages of erythroid, myeloid and lymphoid cell development could occur in vitro. These data provide further support for the use of ES cells in an in vitro assay for embryotoxicity testing.

Comparison of rat and chick limb bud micromass cultures for developmental toxicity screening

This study compares the responses of rat and chick limb bud micromass cultures to chemical treatment. Eight chemicals, of diverse structure, potency and mechanism, were tested, using two endpoints: extractable alcian blue stain as a measure of differentiation to chondrocytes, and extractable neutral red stain as an index of proliferation. Each chemical reduced differentiation and proliferation in a concentration-related manner. IC(50)s, concentrations that reduced staining by 50%, ranged from 10 nm (colchicine) to 4 mm (acetazolamide). Rat and chick responses to acetazolamide, colchicine and diazepam were indistinguishable. For diphenhydramine and sulphisoxazole, concentration-response curves were very similar, but rat IC(50)s were half that of chick. For two chemicals, concentration-response slopes were markedly steeper for chick; in the case of beta-aminopropionitrile, IC(50)s were similar, but rat cultures were three-fold more sensitive than chick to cytosine arabinoside. 6-Aminonicotinamide gave a U-shaped response curve, for both endpoints and both species, so IC(50)s may be misleading, but the IC(50) for proliferation was lower for chick (0.6 mum) than rat (4 mum). In vivo and in vitro parameters for validation of developmental toxicity screens are contentious. Diphenhydramine apart, these chemicals can be teratogenic in vivo, although their 'hazard' can be debated. An IC(50)-proliferation/IC(50)-differentiation ratio > 2 has been suggested to predict specific developmental toxicity. Only sulphisoxazole and 6-aminonicotinamide had significantly different IC(50)s for proliferation and differentiation, with ratios of 4.4 (both species), and 10.4 for rat and 1.9 for chick, respectively. All other ratios were close to 1. The general consistency of this ratio, and the concentration-responses, in the two species suggests that the chick is a viable alternative to laboratory mammals, but the predictive ability of micromass remains to be determined.

Alternative methods for developmental toxicity testing

Alternatives to animal testing in developmental toxicology have been the subject of three decades of research. The aims of these investigations have been to reduce animal experimentation, to refine effect assessment and mechanistic studies, and to accelerate and simplify safety testing in an area of toxicology that uses relatively many animals. Many alternatives have been developed over the years with different compexities, using biologic material ranging from continuous cell lines to complete embryos. The validation of alternatives and their application in testing strategies is still in its infancy, although significant steps towards these aims are currently being made. The introduction of the genomics technology is a promising emerging area in developmental toxicity testing in vitro. Future application of alternatives in testing strategies for developmental toxicity may significantly gain from the inclusion of gene expression studies, given the unique programme of gene expression changes in embryonic and foetal development.

Discriminative power of an assay for automated in vitro screening of teratogens

Screening for potential teratogenicity of 20 test compounds was performed using a computerised microscope workstation for determination of cytotoxicity, proliferation and morphology of fibroblastoid murine L929-cells. The test compounds, which were divided into four classes according to teratogenicity were: 5-bromo-2(')-deoxyuridine, 6-aminonicotinamide, acrylamide, boric acid, D-(+)-camphor, dimethadione, dimethyl phthalate, diphenhydramine, hydroxyurea, isobutyl-ethyl-valproic acid, lithium chloride, methyl mercury chloride, methotrexate, methoxyacetic acid, penicillin G, all-trans-retinoic acid, pentyl-4-yn-valproic acid, saccharin, salicylic acid and valproic acid. All compounds, with the exception of dimethadione inhibited proliferation in a linear dose-dependent manner, and there were statistically significant compound class-dependent differences between the IC(50)-values for the compounds (p<0.0374), the strongest teratogens being the most potent. Furthermore, the average efficacies (maximum relative change) for 10 parameters describing cell morphology exhibited statistically significant compound class-dependent differences (p<0.0001), the class I and II compounds exhibiting significantly lower efficacies than the class III and IV compounds (p<0.01). Thus, test compounds affected both the proliferation and morphology of L-cells in manner demonstrating a general relationship with the teratogenic potency of the compounds. However, the moderate teratogens dimethadione and lithium chloride only had minor effects on the morphology and proliferation of the cells whereas the non-teratogen diphenhydramine had effects on both proliferation and morphology comparable to the strong teratogens.

Automated in vitro screening of teratogens

We present a new in vitro assay for screening of potential teratogens, based on staining of cultured mouse fibroblastoid L929 cells for the determination of number of live and dead cells and of cell morphology, employing automatic video recording, followed by detection of the stained specimen and calculation of endpoint values by the use of a computerized microscope workstation. Ten different parameters were combined empirically into a single index describing general alterations in cell morphology, and, subsequently, measurements of alterations in morphology and proliferation were combined to produce a single empirical index aimed at predicting teratogenic potency. The assay was employed in two different laboratories on 10 coded compounds; 7 compounds that have demonstrated in vivo teratogenic potentials: valproic acid (VPA), pentyl-4-yn-VPA, retinoic acid (RA), 13-cis-RA, AM580, thalidomide, and alpha-EM12 and 3 compounds for which no teratogenic potential has been demonstrated: isobutyl-4-yn-VPA, phytanic acid, and beta-EM12. Within each of the three groups of compounds the nonteratogens generally caused smaller alterations in cell morphology than the teratogens, although the effects of thalidomide and related compounds generally were minor or insignificant. The data support the hypothesis that cell morphology and proliferation in combination with other endpoints may be employed for in vitro screenings of potential teratogens, although studies of additional compounds are needed in order to establish the general validity of the procedure.

The Galileo Data Bank on Toxicity Testing with In Vitro Alternative Methods. II. Toxicology Profiles of 20 Chemicals

The identification of the hazard of chemicals to man has relied on the use of several animal models. However, the availability of various cell toxicity models as alternatives to the use of animals has stimulated attempts to evaluate in vitro data for use in the prediction of human toxicity.

The cell toxicity models developed previously are capable of indicating a variety of endpoints susceptible to the activity of various chemical substances.

The in vitro data derived so far from testing a variety of types of chemicals, have been used to develop toxicology profiles for twenty chemicals, which are presented in this paper. Data have been selected from among those already entered in the Galileo Data Bank, a computerised data system containing all the available existing data derived using in vitro methods.

In vitro tests for teratogens: desirable endpoints, test batteries and current status of the micromass teratogen test

Information from in vitro tests can be usefully used as a component of the risk/hazard assessment process. In vivo studies will be required to confirm the in vitro data. If the in vitro test system is designed around endpoints that reflect changes following in vivo toxic insult then it may be possible to modify the in vitro system to account for some of the discrepancies observed between in vivo and in vitro outcomes. When the discrepancy can be accounted for by low bioavailability in vivo, pharmacokinetic studies may be required to determine the relevance of the in vitro toxic concentrations. Reproductive hazard, especially teratogenicity, has been the subject of intensive in vitro test development. The observation of teratogenicity may affect the development of new products more significantly than any other type or category of reproductive toxicity. The micromass test, involving culture of differentiating rat embryo limb and midbrain cells exposed to test agents, may be useful as part of a battery of in vitro tests for teratogens. The most recent protocol for the micromass test is described, followed by a summary of validation and mechanistic studies confirming its usefulness. The test is robust in its transfer to new laboratories. Interlaboratory variability is small.

Cytotoxicity test using blastocyst-derived euploid embryonal stem cells: a new approach to in vitro teratogenesis screening

To develop a mammalian in vitro system for teratogenicity testing, cytotoxicity of xenobiotics was evaluated in pluripotent euploid embryonal stem cells (ESC) derived from mouse blastocysts. The dimethyl-thiazol-diphenyl tetrazolium bromide (MTT) assay was the most appropriate test system for cytotoxicity determinations with ESC. Only compounds that do not require metabolic activation were selected for testing from the database for validation of in vitro teratogenesis assays by Smith et al. Results obtained with ESC were compared to corresponding data from fibroblasts from day-14 mouse embryos to detect differences in sensitivity between undifferentiated and differentiated cells. ESC showed a higher sensitivity to known teratogens than fibroblast cultures, which allows calculation of a sensitivity ratio of "adult" cells (differentiated fibroblasts) to embryonal cells (undifferentiated ESC) in a mammalian system similar to the hydra assay. Although some xenobiotics had to be classified as false negatives in our system, the ESC cytotoxicity assay holds promise as a new in vitro screening assay in teratology.

In vitro screening for anticonvulsant-induced teratogenesis: drug alteration of cell adhesivity

Anticonvulsant-induced alteration in C6 glioma cell adhesivity has been evaluated in two independent in vitro assay systems. A centrifugal shear assay was employed to determine drug-induced change in cell-substratum adhesivity. Valproate and clonazepam were found to significantly increase cell-substratum adhesivity when cells were cultured at concentrations which were within twice their therapeutic plasma level. A second assay evaluated change in affinity for concanavalin A lectin coated surfaces to determine change in cell surface glycoconjugate expression. Valproate and clonazepam and, to a lesser extent, diazepam significantly decreased drug-exposed C6 glioma cell affinity for concanavalin A lectin coated surfaces. Valproate and clonazepam had approximate IC50 values of 0.75 mM and 75 microM, respectively. These findings are compared and discussed in relation to those obtained with an anti-proliferative assay which has been suggested to predict teratogen potential.

Limb bud cell culture for in vitro teratogen screening: validation of an improved assessment method using 51 compounds

Rat embryo limb bud cells multiply and undergo chondrogenesis in micromass culture. Teratogenic agents are identified from their inhibition of chondrogenesis, which is quantified by determination of cartilaginous foci number or proteoglycan production. In other in vitro systems, the detection is based on their ability to affect cell proliferation. So far, these methods have failed to distinguish among true inhibition of differentiation, inhibition of cell proliferation, and nonspecific cytotoxicity. The improved technique involves simultaneous measurement of cartilage synthesis and cell multiplication. Differentiation was evaluated by measurement, using an Artek Counter, of nodule areas after Alcian blue staining and proliferation by spectrophotometric quantification of Crystal-Violet bound to micromass cells. Using this method, retinoic acid was shown to inhibit chondrogenesis without affecting cell multiplication, whereas 6-aminonicotinamide preferentially inhibited cell multiplication without affecting nodule size. Doxylamine (succinate), a known nonteratogen, induced inhibition of chondrogenesis, but with a parallel inhibition of cell multiplication, reflecting a nonspecific toxic effect. This improvement increases the specificity of the micromass culture test. Validation was performed using 51 compounds. Compounds were classified according to their inhibitory activity and their active concentration. The sensitivity of the test was 61%; the specificity, 100%; and the final accuracy, 75%. The method is fully miniaturised, automated, and computerised, allowing numerous compounds to be rapidly tested at very low cost.

In vitro toxicology: a commercial proposition?

1. The commercial value of any test system must be equated with its scientific value. In vitro toxicity tests only have scientific value when they are accurate models of toxicity in vivo. 2. Toxicity is a complex event so that the simple tests of cell viability are unlikely to be useful models as these tests often use cell lines which are functionally remote from cells in tissues of the whole animal. 3. Primary cultures retain their differentiated function for hours (hepatocytes) or days (nerve cells) and may acquire differentiated functions in vitro (embryo cells) and are the models of choice for predicting toxic hazard. 4. When the in vitro test is a satisfactory predictive model of toxicity there are sound commercial reasons for its use. It saves time and effort and can be used at an early stage in compound development to sort out the toxic from the non-toxic candidate compounds. Large numbers of structurally similar compounds can be tested (QSAR studies) and a greater understanding of the mechanism of toxic action can often be achieved than in vivo because of the numbers of animals required and the interplay of different organ systems in the toxic events that make interpretation difficult. Thus the in vitro test can not only save animals but can also save compounds.

Human embryonic cell growth assay for teratogens with or without metabolic activation system using microplate

In vitro microassay for the screening of teratogens was investigated on cancer chemotherapeutic agents sterigmatocystins and benzimidazoles using human embryonic palatal mesenchymal (HEPM) cells. Five thousand cells were inoculated into each well of 96-well microtiter plates, and cultivated for 24 hr, after which the media were changed with new ones that contained various amounts of chemicals; after cultivation for an additional 72 hr, the media were discarded, and cells attached to the tissue plate were fixed and stained with Giemsa's solution; the cell number then was counted by colony counter with three readings for each well. For the metabolic activation, the liver S9 obtained from rats pretreated with phenobarbital and 5,6-benzoflavone and cofactors (S9 mix) were added directly to the HEPM cell cultures along with chemicals. After 6 hr, the cultures were exchanged with a fresh medium and incubated for a further 72 hr. The final IC50 (the concentration that inhibits growth 50%) concentration-finding run had 7 to 11 concentration points (mean, three to four wells). Concentrations of the cancer chemotherapeutic agents that inhibited growth by 50% ranged from 0.001 to 10 micrograms/ml. Sterigmatocystins indicated strong inhibition; among three derivatives, O-acetyl sterigmatocystin was the most potent inhibitor. Benzimidazoles also exhibited an inhibitory action on HEPM cell growth; nitro and chloro groups at the 5 position in 2-(2-pyridyl)benzimidazole were found to be potent substituents. As for the activation of cyclophosphamide in the HEPM cell culture, IC50 was decreased to 1.0 ug/ml by the incubation with S9 mix for 6 hr under our experimental conditions, and sterigmatocystin was found to be activated by S9 mix.(ABSTRACT TRUNCATED AT 250 WORDS)

Chemical and toxicological characterization of residential oil burner emissions: II. Mutagenic, tumorigenic, and potential teratogenic activity

Extracts of effluents from a modern residential oil burner have been evaluated in several toxicological assay systems. Bacterial mutagens were detected in extracts from both the particulate and vapor phase emissions. Effluents from continuous operation were an order of magnitude less mutagenic than those from cyclic (5 min on, 10 min off) operations. No difference in the yield of bacterial mutagens per gram of fuel burned was found between cyclic operation under low and moderate sooting conditions. On the basis of elution behavior from alumina it appeared that the bacterial mutagens collected from high sooting effluents were more polar than those from low sooting effluent. An extract that was mutagenic in bacteria did not induce a significant increase in mutation frequency to human lymphoblasts. No evidence of tumorigenicity was observed in a limited number of newborn mice after IP injection of effluent extract when compared to historical control data. Putative nonmutagenic teratogens were detected in effluent using an attachment inhibition assay. The level of these agents was reduced in effluents from continuous oil burner operation.

Toxicological review of busulfan (Myleran)

Busulfan is a bifunctional alkylating agent that appears to be cytotoxic to slowly proliferating or non-proliferating stem cell compartments, although its specific molecular and cellular mechanisms are unknown. It is the drug of preference in treatment of chronic myelogenous or granulocytic leukemia because its cytotoxic activity results in primary damage or destruction of hematopoietic cells. Additional effects resulting from the cytotoxicity of busulfan in hematological and other tissues, as documented by both human and animal model studies, include lethality, sterility, teratogenicity, and alteration of immune function. Busulfan has been shown to be mutagenic to microorganisms, mammalian cells in culture, Drosophila, and rodents. This agent is also considered potentially carcinogenic to humans. Various tissue hyperplasia and preneoplastic cells have been observed in animal model studies with busulfan, and case reports on human patients implicate busulfan as the causative agent in induction of secondary malignancies. Reports from human and animal studies of busulfan's cytotoxicity, teratogenicity, carcinogenicity, and mutagenicity have been reviewed. This information may be useful in a quantitative assessment of the effects of this agent and the identification of significant deficiencies in the data base. Demonstration that busulfan induces mutations in both somatic and germ cells suggests the need to assess its risk to humans.

An in vitro test for teratogens: its practical application

An in vitro test for teratogens has been used successfully for more than 3 years. The method involves exposing undifferentiated rat embryo midbrain and limb cells to test compounds and observing the effect on subsequent cell differentiation. Experience of using the test has confirmed the accuracy of prediction (greater than 90%) suggested by a blind trial. The test has been used at the early stages of pharmacological evaluation in the selection of non-teratogenic pharmaceuticals and 250 compounds are tested on average each year. Maternal metabolism is modelled by the inclusion of Aroclor 1254-induced rat liver homogenate plus cofactors (S-9 mix). The concentrations of S-9 mix (50-100 microliters/ml culture medium) conventionally used in the Ames bacterial mutagenicity test are toxic to rat embryo cells, but greatly reduced concentrations (3-5 microliters/ml) are not toxic but are still able to activate pro-teratogens such as cyclophosphamide. However, most potentially teratogenic compounds tested are toxic in the absence of active preparations of drug-metabolizing enzymes. The conclusion that most teratogens are direct acting may be premature, since evidence has been found for drug-metabolizing activity in the embryo cells themselves.

In vitro screening assay for teratogens using growth inhibition of human embryonic cells

We have tested 35 teratogenic and 20 nonteratogenic chemicals or drugs in a short-term, in vitro assay that identifies teratogens by their ability to inhibit growth of an established line of human embryonic palatal mesenchymal cells. Only those chemicals that exhibited a dose-dependent inhibition of growth at concentrations less than 1 mM were classified as inhibitory. An Aroclor-induced rat liver S-9 system was effective in metabolizing cyclophosphamide to its teratogenic form in culture. We suggest that this assay, along with the complementary tumor cell-attachment assay of Braun et al. [Braun, A. G., Emerson, D. J. & Nichinson, B. B. (1979) Nature (London) 282, 507-509] may be useful as a short-term in vitro battery for assessment of the teratogenic potential in environmental agents and to prioritize those chemicals which merit further testing in vivo.

An in vitro assay for teratogens with cultures of rat embryo midbrain and limb bud cells

A short-term in vitro assay for teratogens has been evaluated and shown to have a high predictability (greater than 90%). Cultured cells derived from midbrain (CNS) and limb buds (LB) of 34 to 36 somite rat embryos were exposed to 46 compounds (27 teratogens, 19 nonteratogens) in a blind trial. Rat liver post mitochondrial supernatant fraction plus cofactors were included in the cultures to provide metabolizing enzymes. Differentiation of neurons from CNS cells and chondrocytes from LB cells was measured after 5 days of culture. Inhibition of differentiation (assessed by reduction of number of foci) was the indicator of potential teratogenicity. Variation between experiments was limited. In repeat experiments with two direct-acting teratogens, aldrin and diphenylhyantoin, interexperiment variability was low (coefficient of variation; range 7 to 24%). Of 27 teratogens 25 (93%) and only 2 of 19 nonteratogens (11%) inhibited differentiation (CNS or LB). Inhibition of differentiation in one cell type alone was less predictive (CNS: 85%, LB: 82%).

Lectin-coated agarose beads in the investigation of sperm capacitation in the hamster

Sperm surface changes during in vitro capacitation were examined with the help of an assay system using lectin-coated agarose beads. The nature and intensity of binding of epididymal spermatozoa to beads depended entirely on the particular stage of capacitation and the type of lectin attached to the bead surface. Fresh epididymal spermatozoa bound readily to beads coated with Con A, LCA, WGA, and PNA, but not with seven other lectins. During capacitation there was a constant decline in sperm binding to beads, and spermatozoa cultured for 4-5 hr bound only to those coated with Con A. A dramatic increase in sperm binding to Con A-coated agarose beads occurred between 4.5 and 5 hr, when large numbers of hyperactivated spermatozoa adhered, predominantly through their flagellae, to form large clumps on the beads. The clumping of spermatozoa on Con A-coated beads was enhanced in the presence of stimulators of capacitation (i.e., taurine, hypotaurine, and phosphodiesterase inhibitors) and was suppressed in the presence of various metabolic inhibitors (i.e., sodium azide and local anesthetics). The implications of these results are that the carbohydrate components of the entire surface of spermatozoa undergo striking changes during capacitation, and a close relationship may exist between the sperm surface and the metabolic changes occurring within capacitating spermatozoa. Sperm-bead binding assays are clearly able to recognize surface changes in asynchronous populations of motile spermatozoa and, due to their simplicity and speed, should prove to be valuable in gaining a greater understanding of the biochemistry of sperm capacitation.

Teratogen metabolism: activation of thalidomide and thalidomide analogues to products that inhibit the attachment of cells to concanavalin A coated plastic surfaces

Thalidomide metabolites inhibited the attachment of tumor cells to concanavalin A coated polyethylene surfaces. Thalidomide, itself, was non-inhibitory. Thalidomide activation to inhibitory products required hepatic microsomes, an NADPH-generating system, and molecular oxygen. Production of inhibitory metabolites was unaffected by either epoxide hydrolase or 1,2-epoxy-3,3,3-trichloropropane (TCPO), an inhibitor of epoxide hydrolase endogenous to hepatic S9 fraction. Therefore, the attachment inhibitor was probably not an arene oxide. Inhibition was not accompanied by cytotoxicity, as judged by trypan blue exclusion. Although uninduced hepatic microsomes from mice, rats and dogs had similar abilities to activate thalidomide, microsomes from Aroclor 1254 induced rats were relatively inactive in the system. Inhibitory metabolites were generated from the thalidomide analogues EM8 , EM12 , EM16 , EM87 , EM136 , EM255 , E350 , phthalimide, phthalimido-phthalimide, indan, 1- indanone and 1,3- indandione . Glutarimide , glutamic acid and phthalic acid did not activate to inhibitory products.

A short-term screening test for teratogens using differentiating neuroblastoma cells in vitro

Thirty-nine teratogenic and 18 nonteratogenic compounds were tested using an assay based on the ability of agents to interfere with normal growth and differentiation of murine neuroblastoma cells (clone N1E-115) in culture. Induction of differentiation in cells under growth-promoting conditions and inhibition of differentiating cells over a period of 7 days was dose-dependent, with the lowest effective dose not being highly toxic. Eighty-six percent of the compounds were correctly identified by the assay. The proportions of both false negatives (10%) and false positives (22%) were of the same order or better than in current, comparable tests. The possibilities offered by the system in rapid screening for teratogenic potential of environmental agents are discussed.

Differentation of rat embryo cells in culture: response following acute maternal exposure to teratogens and non-teratogens

An in vivo-in vitro test system with high sensitivity to teratogens has been developed and validated. A single acute intra-peritoneal injection of teratogens (18) and non-teratogens (13) was administered to pregnant rats on the 12th day after fertilisation, and uteri were removed after 16 h by laparotomy. 34-36 Embryos somites were selected, and mid-brain (CNS) and fore-limb buds (LB) were dissected free and dispersed as single-cell suspensions in Ham's F12 culture medium. The cells were cultured as micromass cell islands for 5 days, and discrete foci of neuronal cells differentiated in CNS cultures and chondrocytes in LB cultures. After 5 days, differentiation as determined by number of stainable foci of differentiated cells and 3H-GABA incorporation in CNS or 35SO4 incorporation in LB and growth (as determined by total protein) were measured. Both differentiation and growth of CNS and LB cultures were markedly reduced following exposure of the dam to teratogens, whereas no significant effect was observed with non-teratogens. One teratogen (amaranth) and one non-teratogen (nitrilotriacetic acid) were classified as false negative and positive, respectively; the sensitivity of the test (proportion of teratogens correct) was therefore 92% and the specificity (proportion of non-teratogens correct) was 94%. Inhibition of growth and differentiation in the rat embryo cell cultures following maternal exposure forms the basis of a short-term in vitro test for teratogens.

[Progress report. Possibilities and limitations of the in vitro technic for prenatal toxicologic studies]

Considering the variety of problems of the prenatal toxicity test the in vitro methods hitherto existing in this field are discussed according to their degree of complexity (cell culture, mass culture of dispersed cells, organ culture, whole embryo culture) within their limitations and possibilities. In this connection special attention is focussed to the use of human tissue for testing basic mechanisms of the developmental biology and for toxicity tests.

Quantitative correspondence between the in vivo and in vitro activity of teratogenic agents

We have tested 74 teratogenic and 28 nonteratogenic agents in a recently developed in vitro teratogen assay system. The assay identifies teratogens by their ability to inhibit attachment of ascites tumor cells to plastic surfaces coated with concanavalin A. There is a qualitative agreement between in vivo animal data and in vitro activity for 81 of the 102 agents (79%). Quantitative analysis shows a highly significant correlation coefficient of 0.69 between the inhibitory in vitro dose and the lowest reported teratogenic dose for 54 of the 60 inhibitory teratogens. The doses analyzed ranged over 5 orders of magnitude. We interpret these results to mean that attachment inhibition in concert with other, complementary, in vitro assay systems can become a useful method for the assessment of the teratogenic potential of environmental agents.

Inhibition of tumor cell attachment to concanavalin A-coated surfaces as an assay for teratogenic agents: approaches to validation

Four environmental agents have been tested for activity in a recently developed in vitro teratogen assay system. All four agents inhibited attachment. The agents were 40-fold concentrated drinking water (ID50 = 0.45 ml/ml), whole cigarette smoke condensate (ID50 = 85 micrograms/ml), kerosene soot (ID50 = 90 micrograms/ml), and commercial formulations of the pesticide carbaryl (ID50 approximately 150 micrograms/ml). On the basis of these examples appropriate criteria for the validation of in vitro teratogen assay systems are discussed. It is concluded that criteria are critically dependent on the specific applications of the assay system. For example, the false-positive rate must be minimized to make a wide-ranging screen of water samples useful. On the other hand, an investigation of impurities in commercial compounds requires low false-negative rates. In every case a quantitative measure of the potential teratogenic potency, in vivo, is desirable.

Animal virus screens for potential teratogens. I. Poxvirus morphogenesis

The growth of poxvirions in cell culture is considered a teratogen screening test, since this virus has a rapid, simple morphogenetic pathway that is dependent upon cell proliferation. Vaccinia WR-infected BSC 40 monolayers were exposed to 42 known teratogens and 9 nonteratogens at dosages from 1 microM to 100 mM. After 24 h of infection, the number of functional virions was determined by plaque assay. Thirty-three of the 42 teratogens inhibited the virus, 3 teratogens stimulated the virus, and 6 teratogens were false-negatives. Eight of the 9 nonteratogens had no effect on virus proliferation at dosages as high as 600 times the lowest reported teratogenic dosage. The number of new virions could be directly related to the concentration of the teratogen in vitro, thus allowing each compound to be characterized by an RD50. The RD50 dosage in milligrams per liter was 98% correlated with the lowest reported teratogenic dose in vivo in milligrams per kilogram. In sum, vaccinia-infected cells have an easily identifiable endpoint, plaque-forming units, which may be an accurate prognosticator of teratogenesis.

Embryonic limb bud organ culture in assessment of teratogenicity of environmental agents

The limb bud organ culture system offers a variety of endpoints which may be monitored in the screening process. These are: cell proliferation, differential growth, morphogenetic cell death, size and shape of limb parts, chondrogenesis, collagen or proteoglycan biosynthesis, etc. Essentials of the system including various parameters of normal limb bud development in vitro are described. These parameters serve to gauge the effects of test chemicals with unknown hazard potential. Validation has been carried out only to a limited extent. Further, it needs to be combined with an efficient drug metabolizing preparation before it can achieve its full potential as a short-term screening system.

Prescreening for environmental teratogens using cultured mesenchymal cells from the human embryonic palate

Mesenchymal cells from the prefusion human embryonic palate have been established in culture and can be grown in either a serum-free hormone-supplemented medium or a serum-containing medium. The growth of these cells is quite rapid in culture and inhibited in a dose-dependent manner by most teratogens thus far tested, such as dexamethasone. These cells are highly sensitive to a variety of DNA synthetic and mitotic inhibitors. The responses of these cells are complementary to the ovarian tumor cell attachment assay of Braun et al [1, and in this volume]. When used in conjunction with the tumor cells, the overall reliability is greater than 90% with only one false-negative, allopurinol.

Chondrogenesis: a model developmental system for measuring teratogenic potential of compounds

A simple test for determining the teratogenic potential of compounds is described using embryonic limb bud cells in culture. These mesenchyme cells multiply and differentiate into chondrocytes during a 6-day culture period. The extent of chondrogenesis is assessed by staining for the cartilage specific proteoglycan with alcian blue. The amount of stain is then measured spectrophotometrically. Compounds which interfere with growth or differentiation reduce the amount of proteoglycan and as a consequence, reduce alcian blue staining. Compounds can be added directly to the media or be activated using several different metabolizing systems. The dose of a compound needed to reduce alcian blue staining by 50% is designated the teratogenic potential (TP50) of that compound. TP50's of proven teratogens compare favorably with in vivo teratogenic doses of the teratogens.

The possible effect of diazepam on cancer development and growth

Diazepam has the characteristics of a tumour promoter in a number of in vitro systems. The effect is apparent at concentrations of diazepam which are clinically relevant. Diazepam also accelerated tumour growth in two different experimental animal cancers. Tranquillizer use was found to be greater in women with metastatic breast cancer at the time of diagnosis than in those without metastases. Further evaluation of the possible effects of diazepam and related drugs on human and animal cancers is urgently required.

Effect of chlorpromazine on cell proliferation in the developing rat brain. A combined biochemical and morphological study

Chlorpromazine, a widely used drug in current clinical practice, produced a severe reduction of the rate of [3H]thymidine incorporation into brain DNA of 11-day-old rats. The depression of in vivo synthesis rate was detectable by 6 h after chlorpromazine administration (50 mg/kg, s.c.) and the rate was less than 40% and 60% of controls during period 14-30 h in forebrain and 6-30 h in cerebellum respectively. The depression was dose-dependent and half maximal effect was produced with about 10 mg/kg chlorpromazine. The drug caused some retardation in the rate of conversion of [3H]thymidine to [3H]thymine nucleotides in the brain, but the severe depression in DNA labelling was also evident after correcting the values on the basis of [3H]thymine nucleotides concentration. Mitotic activity was significantly reduced in the cerebellar external granular layer. Increased numbers of cell degenerations, shown by Feulgen cytophotometry to be postmitotic, were seen in both layers 12 and 32 h after chlorpromazine. Analysis of cell cycle parameters showed no significant changes. However, the labelling index in subependymal cells was reduced, indicating an increase in turnover time of about 40%. The results are consistent with an action of chlorpromazine on cell proliferation, either by direct effects on the generation and survival of cells, or via its major pharmacological actions on neurotransmitter balance. These effects are potentially of functional and clinical significance.