An evaluation of chronic 6- and 12-month rat toxicology studies as predictors of 2-year tumor outcome.

Chronic 6- and 12-month rat toxicology studies were evaluated for their ability to predict tumor outcome in 2-year rat carcinogenicity studies for 80 pharmaceuticals from commercial and Merck databases. The data consisted of 62 (6-month) and 54 (12-month) studies, which included 30 rat carcinogens and 50 noncarcinogens in 2-year studies. The histopathology findings considered as evidence of potential preneoplasia in the chronic studies were hyperplasia, cellular hypertrophy, and atypical cellular foci. The authors hypothesized that a whole animal-based approach should be taken, wherein (1) evidence of potential preneoplasia in any tissue may serve as a sensitive predictor of tumor outcome in any tissue in the whole animal and not necessarily the same tissue and (2) the absence of evidence for potential preneoplasia in all tissues may serve as a strong negative predictor of tumor outcome in any tissue. Based on this whole animal approach, 25 of 30 rat carcinogens showed histopathologic signals in chronic toxicology studies, yielding a test sensitivity of 83%. The negative predictivity of the absence of histopathology findings in chronic toxicology studies of 50 nontumorigenic compounds was 88%. The value of the extra 6-month treatment was not apparent. The 5 false negatives (negative chronic studies but positive 2-year studies) are for marketed drugs approved for non-life-threatening conditions and associated with rat-specific mechanisms. The absence of preneoplasia in the whole animal is a reliable predictor of negative tumor outcome in 2-year rat studies, and approximately 50% rat carcinogenicity studies could be eliminated for the 80 pharmaceuticals examined, with no risk to humans and with a substantial reduction in animal usage and drug development time.

Preclinical predictors of clinical safety: opportunities for improvement.

Toxicology studies in animals are required by regulatory authorities worldwide to provide assurances that clinical testing of pharmaceutical candidates can be conducted safely. Safety concerns from animal studies account for over 20% of attritions from drug development. As discordance between humans and animals is expected, two goals of safe and efficient drug development must be (1) to improve the human relevance of animal testing with new models and technologies, and (2) to advance quickly to clinical testing armed with improved safety biomarker tools.

Evaluation of Therapeutics for Advanced-Stage Heart Failure and Other Severely-Debilitating or Life-Threatening Diseases.

Severely-debilitating or life-threatening (SDLT) diseases include conditions in which life expectancy is short or quality of life is greatly diminished despite available therapies. As such, the medical context for SDLT diseases is comparable to advanced cancer and the benefit vs. risk assessment and development of SDLT disease therapeutics should be similar to that of advanced cancer therapeutics. A streamlined development approach would allow patients with SDLT conditions earlier access to therapeutics and increase the speed of progression through development. In addition, this will likely increase the SDLT disease therapeutic pipeline, directly benefiting patients and reducing the economic and societal burden of SDLT conditions. Using advanced-stage heart failure (HF) as an example that illustrates the concepts applicable to other SDLT indications, this article proposes a streamlined development paradigm for SDLT disease therapeutics and recommends development of aligned global regulatory guidance.

Predictive value of preclinical toxicology studies for platinum anticancer drugs.

Rodent and nonrodent toxicology studies are currently expected to support Phase I trials of antineoplastic drugs in the United States. To determine the predictive value of these studies, we initiated a project to compare preclinical and clinical toxicity data within various drug classes. The first class analyzed was the platinum anticancer drugs. Twelve platinum analogues that had both preclinical (mice, rats and/or dogs) and clinical data from matching drug administration schedules were identified. The rodent LD10 (the dose that causes lethality in 10% of treated animals) or dog toxic dose high (a dose that when doubled causes lethality in dogs) correlated well with the human maximally tolerated dose on a mg/m2 basis. For every platinum analogue investigated, one-third the rodent LD10 or one-third the dog toxic dose high in mg/m2 gave a starting dose and a first escalation dose that did not exceed the clinical maximally tolerated dose. The dose-limiting toxicities in patients were previously observed in 7 of 7, 7 of 8, and 9 of 11 mouse, rat, and dog studies, respectively. Our data indicate that mice, rats, and dogs all had value in predicting a safe starting dose and the qualitative toxicities in humans for platinum anticancer compounds. The efficiency of Phase 1 trials could have been improved without sacrificing patient safety by allowing higher starting doses for this drug class than conventionally permitted.

Approaches to the development and marketing approval of drugs that prevent cancer.

The broad concept of chemoprevention applies to the prevention of clinical cancer by the administration of chemical agents. Current approaches to the development and marketing approval of drugs to prevent cancer are described by a Working Group from the National Cancer Institute and the Food and Drug Administration. A strategy is presented that identifies candidate drugs, with examples that illustrate how drugs are characterized for efficacy through in vitro transformation modulation and mechanistic assays, and in vivo tumor modulation models of carcinogenesis. Requirements and recommendations for safety evaluation in toxicology testing are given, and the evaluation of pharmacokinetic and pharmacodynamic drug effects and potential surrogate end point biomarkers in Phase I trials are discussed. Appropriate subject populations are identified. Phase II trials should emphasize the evaluation of surrogate end point biomarkers that are highly correlated with cancer incidence and may serve as an estimate of cancer incidence reduction. In Phase III trials the interim analysis of a validated surrogate end point of cancer incidence may facilitate timely and cost-effective marketing of efficacious drugs.

Differences in rates of incorporation of intravenously injected radiolabeled fatty acids into phospholipids of intracerebrally implanted tumor and brain in awake rats.

This study investigates the incorporation of three intravenously administered radiolabeled fatty acids, [9,10-3H]palmitate (3H-PAM), [1-14C]arachidonate (14C-ACH) and [1-14C]docosahexaenoate (14C-DHA), into lipids of intracerebrally implanted tumor and contralateral brain cortex in awake rats. A suspension of Walker 256 carcinosarcoma cells (1 x 10(6) cells) was implanted into the right cerebral hemisphere of an 8- to 9-week-old Fischer-344 rat. Seven days later, the awake rat was infused intravenously for 5 min with 3H-PAM (6.4 mCi/kg), 14C-ACH (170 microCi/kg) or 14C-DHA (100 microCi/kg). Twenty min after the start of infusion, the rat was killed and intracranial tumor mass and brain cortex were removed for lipids analysis. Each radiolabel was incorporated more into tumor than into brain cortex. Ratios of net incorporation rate coefficients (k*) into tumor as compared with brain were 4.5, 3.4 and 1.7 for 3H-PAM, 14C-ACH and 14C-DHA, respectively. Lipid radioactivity comprised more than 80% of total tumor or brain radioactivity for each probe. Phospholipids contained 58%, 89% and 68% of tumor lipid radioactivity, and 58%, 82% and 74% of brain lipid radioactivity, for 3H-PAM, 14C-ACH and 14C-DHA, respectively. Incorporation coefficients (k*i) for a phospholipid class (i)--choline phosphoglycerides (PC), inositol monophosphoglycerides (PI), ethanolamine phosphoglycerides (PE), serine phosphoglycerides (PS), and sphingomyelin (SM)--were greater in tumor than in brain for each fatty acid probe, except that values for k*PE and k*PS using 14C-DHA were equivalent. Differences in k*i between tumor and brain were largest for SM and PC and the change in k*PC accounted for 65-90% of the increase in the net phospholipid incorporation rate for each probe. Differences in k*PI, k*PE and k*PS were smaller than those in were smaller than those in k*PC and k*SM, and varied with the probe. Differences in k*i were related to differences in tumor and brain phospholipid composition and metabolism. The results indicate that suitably radiolabeled fatty acids may be used to image and characterize metabolism of lipid compartments of a brain tumor in vivo using positron emission tomography.

Intravenously injected radiolabelled fatty acids image brain tumour phospholipids in vivo: differential uptakes of palmitate, arachidonate and docosahexaenoate.

This paper investigates the incorporation of intravenously (i.v.) administered radiolabelled fatty acids--[9,10(3)-H]palmitate (3H-PA), [1-14C]arachidonate (14C-AA) and [1-14C]docosahexaenoate (14C-DHA)--into intracerebrally implanted tumours in awake Fischer-344 rats. A suspension of Walker 256 carcinosarcoma tumour cells (1 x 10(6) cells) was implanted into the right cerebral hemisphere of 8- to 9-week-old rats. Seven days after implantation, the awake rat was infused i.v. for 5 min with 3H-PA (6.4 mCi/kg), 14C-AA (170 microCi/kg) or 14C-DHA (100 microCi/kg). Twenty minutes after the start of infusion, the rat was killed and coronal brain sections were obtained for quantitative autoradiography and histology. Each fatty acid showed well-demarcated incorporation into tumour tissue. Areas of necrosis or haemorrhage showed no or small levels of incorporation. The ratios of incorporation into the tumour to incorporation into contralateral brain regions were 2.8-5.5 for 3H-PA, 2.1-3.3 for 14C-AA and 1.5-2.2 for 14C-DHA. The mean ratios differed significantly between the fatty acids (P < 0.01). 3H-PA was not incorporated into necrotic tumours despite the presence of an open blood-tumour barrier, indicated by extravasated horseradish peroxidase. The incorporation rate constant of 3H-PA was similar for small intracerebral and large extracerebral tumours. The results show that 3H-PA, 14C-AA and 14C-DHA are incorporated more readily into tumour tissue than into brain, and that the increase is primarily due to increased utilization of fatty acids by tumour cells and not due to a high blood-tumour permeability. The relative increases in rates of incorporation for the different fatty acids may be related to lipid composition of the tumour and to the requirement of and specific role of these fatty acids in tumour cell growth and division.

In vivo transgenic bioassays and assessment of the carcinogenic potential of pharmaceuticals.

There is general agreement in the scientific community on the need to improve carcinogenicity testing and the assessment of human carcinogenic risk and to incorporate more information on mechanisms and modes of action into the risk assessment process. Advances in molecular biology have identified a growing number of genes such as protooncogenes and tumor-suppressor genes that are highly conserved across species and are associated with a wide variety of human and animal cancers. In vivo transgenic rodent models incorporating such mechanisms are used to identify mechanisms involved in tumor formation and as selective tests for carcinogens. Transgenic methods can be considered an extension of genetic manipulation by selective breeding, which long has been employed in science and agriculture. The use of two rodent species in carcinogenicity testing is especially important for identifying transspecies carcinogens. The capacity of a substance to induce neoplasia across species suggests that the mechanism(s) involved in the induction of the neoplasia are conserved and therefore may have significance for humans. Based on available information there is sufficient experience with some in vivo transgenic rodent carcinogenicity models to support their application as complementary second species studies in conjunction with a single 2-year rodent carcinogenicity study. The optional substitution of a second 2-year rodent carcinogenicity study with an alternative study such as an in vivo transgenic carcinogenicity study is part of the International Conference on Harmonization guidance S1B: Testing for Carcinogenicity of Pharmaceuticals. This guidance is intended to be flexible enough to accommodate a wide range of possible carcinogenicity assessment models currently under consideration or models that may be developed in the future. The use of an in vivo transgenic mouse model in place of a second 2-year mouse study will improve the assessment of carcinogenic risk by contributing insights into the mechanisms of tumorigenesis and potential human relevance not available from a standard 2-year bioassay. It is envisioned that this will stimulate the further development of more efficient and relevant methods for identifying and assessing potential human carcinogenic risk, which will benefit public health.

The duration of non-rodent toxicity studies for pharmaceuticals. International Conference on Harmonication (ICH).

At the present time, there are no uniform standards for the duration of non-rodent chronic toxicity studies. The European Union (EU) requires a 6-month non-rodent study. In Japan, a 6-month study is sufficient for most, but not all, compounds. The U.S. Food and Drug Administration (FDA) maintains its standard duration of 12 months for non-rodents, with 6-month studies accepted for some clinical indications on a case-by-case basis. To achieve harmonization on the duration of non-rodent toxicity studies, each member regulatory region (EU, U.S., and Japan) of the International Conference on Harmonization (ICH) collected non-rodent studies with significant new toxicological findings that had occurred after 6 months. An ICH expert working group was organized that included representatives from the regulatory authorities of each ICH region, to jointly review all available case studies for the purpose of arriving at a consensus on the best duration time for non-rodent toxicity studies. Eighteen case studies were identified and evaluated (16 original cases plus 2 additional FDA cases); most of the toxicities identified fell into the following categories: (1) toxicities identified at 6 months; (2) toxicities observed at 12 months, which were absent or considered isolated and not noteworthy findings at 6 months; (3) drug-related deaths or morbidity that occurred between 6 and 12 months, with a pattern of toxicity that permitted the interpolation of findings to an intermediate interval between 6 and 12 months; and (4) a shift in the dose response for toxicity with increasing duration of drug exposure. Of the 18 cases evaluated, 11 supported a study-duration of 9-12 months, 4 supported a duration of 12 months, and the 3 remaining cases indicated that a 6-month study would be adequate. The working group concluded that there was sufficient evidence to support a harmonized 9-month duration for non-rodent toxicity studies, which would be applicable for most categories of pharmaceuticals.

Regulatory considerations for preclinical development of anticancer drugs.

The entry of new anticancer treatments into phase I clinical trials is ordinarily based on relatively modest preclinical data. This report defines the battery of preclinical tests important for assessing safety under an Investigational New Drug application (IND) and outlines a basis for extrapolating starting doses of investigational anticancer drugs in phase I clinical trials from animal toxicity studies. Types of preclinical studies for the support of marketing of a new anticancer drug are also discussed. This report addresses differences and similarities in the preclinical development of cytotoxic drugs (including photosensitizers and targeted delivery products), drugs used chronically (chemopreventive drugs, hormonal drugs, immunomodulators), and drugs intended to enhance the efficacy (MDR-reversing agents and radiation/chemotherapy sensitizers) or diminish the toxicity of currently used anticancer therapies. Factors to consider in the design of preclinical studies of combination therapies, alternative therapies, and adjuvant therapies in the treatment of cancer, and to support changes in clinical formulations or route of administration, are also discussed.

Wheat germ agglutinin inhibits nerve fiber growth and concanavalin A stimulates nerve fiber initiation in cultures of dorsal root ganglia neurons.

Treatment of dorsal root ganglion (DRG) neurons with the lectin wheat germ agglutinin (WGA; 25 micrograms/ml), which binds to N-acetylglucosamine containing glycoconjugates, inhibits nerve fiber growth in culture. DRG neurons treated with WGA have significantly reduced total output in nerve fiber length per neuron as well as reductions in the average length of the individual nerve fibers extended. The inhibition of nerve fiber growth by WGA is concentration-dependent, specific, reversible and not mimicked by treatment with several metabolic poisons. In contrast, treatment with the lectin concanavalin A (25 micrograms/ml), which binds to mannose-containing glycoconjugates, increases the number of nerve fibers produced per neuron. These results suggest that lectins which bind to distinct carbohydrate moieties can differentially regulate nerve fiber growth.

In vivo brain incorporation of [1-14C]arachidonate in awake rats, with or without cholinergic stimulation, following unilateral lesioning of nucleus basalis magnocellularis.

Regional brain incorporation of a radiolabeled unsaturated fatty acid, [1-14C]arachidonic acid (14C-AA), was measured in awake rats following unilateral lesioning of the nucleus basalis magnocellularis (NBM). Right-sided lesions were produced in 3-month-old, male rats by stereotaxic injection of 10 micrograms ibotenic acid. Two weeks after lesioning, rats were subjected to one of two protocols: (1) 5 min intravenous infusion of 14C-AA (170 microCi/kg); or (2) i.p. injection of arecoline (5 mg/kg), a cholinergic agonist, followed by 5 min intravenous infusion of 14C-AA. All animals were killed 15 min postinfusion. Brains were frozen and sectioned for quantitative autoradiography or were stained for acetylcholinesterase (AChE). Animals with unilateral NBM lesions displayed reduced AChE staining in prefrontal, frontal and parietal cortices of the lesioned side, but there was no right-left difference in incorporation of 14C-AA without cholinergic stimulation. Arecoline administration increased 14C-AA incorporation into the prefrontal and frontal cortices ipsilateral to the NBM lesion as compared to the contralateral side and the increase was most prominent in deeper cortical layers such as layers IV and V. Right-left differences in incorporation were not apparent in parietal, temporal, or occipital cortices, where reduction of AChE activity was minimal or absent, nor in subcortical structures. The results suggest that the intravenous 14C-AA technique combined with cholinergic stimulation can be used to detect compensatory regulation of phospholipid-coupled signal transduction caused by a deficit in cholinergic input into the cerebral cortex.

In vivo incorporation of [9,10(-3)H]-palmitate into a rat metastatic brain-tumor model.

Lipid metabolism of an intracerebrally implanted brain tumor and normal brain was investigated in awake Fischer 344 rats using intravenously injected [9,10(-3)H]-palmitate as a probe. A suspension of Walker 256 carcinosarcoma cells (250 cells in 5 microliters medium), with or without 1% low-melting-point agar, was implanted into the caudate nucleus of rats 8 to 9 weeks old. Control animals received an intracerebral injection without tumor cells. Seven days after implantation, awake rats were infused intravenously for 5 minutes with [9,10(-3)H]-palmitate (6.4 mCi/kg). The rats were killed 20 minutes after initiation of the infusion and coronal brain slices were obtained for quantitative autoradiography and light histological study. Tumor cell masses were histologically well demarcated from the surrounding brain tissue. Tumor tissue incorporation of [9,10(-3)H]-palmitate was heterogeneous, ranging on average from 3.1- to 6.1-fold greater than in the corresponding contralateral brain. In addition, incorporation corresponded to regional tumor cell density. The incorporation rate constant of [9,10(-3)H]-palmitate in tumor was significantly increased compared to control brain and was independent of tumor size. Necrotic areas within tumors showed no incorporation of radiolabeled palmitate. Brain surrounding the tumors and control injection sites showed reactive gliosis, and possessed 30% greater incorporation of [9,10(-3)H]-palmitate than contralateral normal brain. These results suggest that [9,10(-3)H]-palmitate can be used to image brain tumors in vivo, measuring turnover and/or synthesis of tumor and brain lipid.

Food and Drug Administration viewpoints on toxicokinetics: the view from review.

The importance of drug kinetics for interpretation of toxicity findings and for cross-species toxicity assessment has been long recognized. Recently, an international effort was initiated to standardize guidance on the kinetic data to be collected in conjunction with toxicity studies. The guidance addresses the kinetic data to be included in studies on carcinogenicity, reproduction toxicity, genotoxicity, and single- and repeat-dose toxicity. In various stages of development or implementation, the guidance is intentionally nondetailed regarding the specific kinetic assessments to be performed. This is to allow flexibility in study design and ensures that scientific judgment is used to determine the appropriate kinetic endpoints to achieve study- and drug-specific goals. Some examples of how kinetics have been used at the Food and Drug Administration in review of toxicity studies submitted in drug applications are presented. The examples discussed demonstrate successful and unsuccessful integration of kinetics into study design and interpretation and highlight the impact on the drug development program from a regulatory perspective.

Specificity and valency of concanavalin A in neuron-substratum adhesion and redistribution of cell surface receptors.

Neurons seeded in culture as spherical cells flatten partially to form lamellipodia by which they adhere to the substratum. Lamellipodium formation is stimulated specifically by concanavalin A (Con A) and other mannose-binding lectins in several types of neuronal cells, but not in similarly treated fibroblasts. Conditions that block much of the adsorption of Con A to the substratum have no effect on stimulation of lamellipodium formation by Con A. This suggests that Con A acts in solution on neurons and does not directly bind them to their substrata. Succinylated-Con A (bivalent) binds to the same receptors as native Con A (tetravalent) but does not elicit lamellipodium extension unless crosslinked with anti-Con A IgG. Treatment of neurons with Con A produces local changes in the composition of the cell surface resulting from redistribution of lectin receptor complexes. This redistribution is not as great with SCon A and, like lamellipodium formation, is sensitive to the valency of Con A. A variety of treatments (4 degrees C, trifluoperazine, nordihydroguaiaretic acid, 4-bromphenacyl bromide, and cytochalasin D), inhibit both Con A-receptor redistribution and lamellipodium extension by neurons. Other treatments (colchicine and cycloheximide) prevented neither lamellipodium formation nor redistribution.

Concanavalin A stimulates neuron-substratum adhesion and nerve fiber outgrowth in culture.

Dorsal root ganglion neurons in culture proceed through a series of shape changes before growing nerve fibers. These shape changes involve: attachment to the substratum, extension of filopodia, and spreading of part of the cell to form broad lamellipodia. With the formation of lamellipodia, neurons adhere firmly to the substratum and retrogradely transport lectins (concanavalin A, wheat germ agglutinin) on their surfaces. In unspread neurons concanavalin A, but not wheat germ agglutinin, rapidly stimulates lamellipodium formation and neuron-substratum adhesion. Neurons treated with concanavalin A also have more, branched nerve fibers than untreated neurons, but otherwise appear similar. These effects of concanavalin A are concentration dependent, blocked by alpha-methyl-D-mannoside (100 mM), and are accompanied by receptor redistribution. Stimulation of lamellipodium extension by concanavalin A is inhibited by low temperature (4 degrees C), 2,4-dinitrophenol (0.2 mM), cytochalasin D (4 microM), or trifluoperazine (10 microM), but not by cycloheximide (360 microM) or colchicine (12.5 microM). Attachment of neurons to the culture substratum was affected little by these treatments. These results indicate differences in the neuron's metabolic requirements for simple attachment to the substratum and the early phases of nerve fiber growth. Moreover, they suggest a convenient system in which to study the cellular and biochemical events of rapid nerve fiber outgrowth in primary neuronal cultures.

Carcinogenicity testing and the evaluation of regulatory requirements for pharmaceuticals.

The results of rat and mouse carcinogenicity studies for 282 human pharmaceuticals in the FDA database were analyzed and compared as part of an International Conference on Harmonization (ICH) evaluation of rodent carcinogenicity studies and their utility for carcinogenicity testing. A majority of the carcinogenicity studies in the FDA database were carried out in Sprague-Dawley-derived rats and Swiss-Webster-derived CD-1 mice in contrast to Fisher 344 rats and B6C3F1 mice employed in National Toxicology Program (NTP) studies. Despite the differences in rodent strains, the relative proportion of compounds with positive findings (44.3%) and the degree of overall concordance between rats and mice (74.1%) in the FDA database were similar to the NTP rodent carcinogenicity database. Carcinogenicity studies in two rodent species are necessary primarily to identify trans-species tumorigens, which are considered to pose a relatively greater potential risk to humans than single species positive compounds. Two-year carcinogenicity studies in both rats and mice may not be the only means of identifying trans-species tumorigens. Sufficient experience is now available for some alternative in vivo carcinogenicity models to support their application as complementary studies in combination with a single 2-year carcinogenicity study to identify trans-species tumorigens. Our analysis of the rodent carcinogenicity studies supports such an approach for assessing carcinogenic potential without compromising the public health.

Nerve growth factor rapidly stimulates arachidonate metabolism in PC12 cells: potential involvement in nerve fiber growth.

Homogenates prepared from pheochromocytoma (PC12) cells that are extending nerve fibers in response to nerve growth factor (NGF) have an increased capacity to metabolize exogenous arachidonate compared with homogenates prepared from cells untreated with NGF. These changes are not a consequence of cell attachment, since they are also seen in NGF-treated PC12 cells grown in suspension and are not found in attached cells grown in the absence of NGF. This NGF-stimulated increase in arachidonate metabolic capacity occurs rapidly and before the extension of nerve fibers. In contrast to NGF, epidermal growth factor does not alter the metabolism of exogenous arachidonate by PC12 cells. Radioimmunoassay of medium from PC12 cultures indicates that intact cells produce and release increased amounts of prostaglandin (PGE) in response to NGF. Drugs that inhibit arachidonate liberation from membrane phospholipids (mepacrine or 4-bromphenacyl bromide) block NGF-stimulated nerve fiber growth by PC12 cells. Selective inhibitors of cyclooxygenase metabolism of arachidonate (indomethacin and aspirin) fail to block growth, but inhibitors of lipoxygenase metabolism (baicalein, BW755, and eicosatetraynoic acid) are potent blockers. In cultures of dorsal root ganglion neurons, inhibitors of arachidonate release (mepacrine, 4-bromphenacyl bromide) or its subsequent metabolism by lipoxygenases (nordihydroquaiaretic acid, eicosatetraynoic acid) also prevent the early morphological events of nerve fiber growth. Our data suggest that NGF rapidly and specifically increases the capacity of PC12 cells to synthesize arachidonate metabolites, and that arachidonate metabolism may be important in nerve fiber growth by both PC12 cells and dorsal root ganglion neurons.