FDA points-to-consider documents: the need for dietary control for the reduction of experimental variability within animal assays and the use of dietary restriction to achieve dietary control

Standard protocols for conducting chronic toxicity and carcinogenicity studies have been refined over the years to carefully control for many variables. Nevertheless, over the last 2 decades, there has been a steady increase in variability, a decrease in survival, an increase in tumor incidence rates, and an increase in the average body weight of control animals among the various rodent species and strains used for toxicity testing. These observations have prompted an evaluation of chronic study designs to determine what factor(s) may be responsible for such confounding changes. Ad libitum feeding and the selection of successful breeders with rapid offspring growth is believed to be at least partially responsible for the heavier, obese rodents with which many laboratories are coping today. As a result of these changes, some studies used for the evaluation of safety have been deemed inconclusive or inadequate for regulatory purposes and either additional supportive studies have been requested and/or studies per se have been repeated. Research on the molecular mechanisms of caloric restriction and agent-induced toxicity at the Food and Drug Administration (FDA) National Center for Toxicological Research stimulated the first international conference on the biological effects of dietary restriction in 1989; this was followed in 1993 by an FDA workshop exploring the utility of dietary restriction in controlling reduced survival in chronic tests and an international conference in 1994 exploring the implications for the regulatory community of using dietary restriction in toxicity and carcinogenicity studies used in support of a sponsor's submission or in risk assessments. The outcome of that conference was the FDA's commitment to develop Points-to-Consider documents that address the issue of dietary control in chronic toxicity and carcinogenicity studies.

P48: a novel nuclear protein possibly associated with aging and mortality

The synthesis ([35S]-incorporation) of stress proteins (sps, i.e., 24, 25, 70, 90 Mr) and of nuclear protein 48 (p48) was investigated in the heart and bone marrow cells of three groups of male Fischer 344 rats following administration of isoproterenol (IPR). Two groups of rats, young ad libitum (Y/AL-3 1/2 months) and old/AL (O/AL-28 months), had full access to rat chow; a third group of old diet restricted (O/DR-28 months) rats was maintained on a diet restricted intake of 40% of the Y/AL animals. Sp synthesis in the bone marrow (25, 70, 90 Mr) and heart (24, 70, 90 Mr) nuclei of O/AL was significantly reduced, as compared with Y/AL and O/DR rats, following their induction with IPR. A unique sp24 was expressed in heart following IPR dosing. A 1 mg/kg dose of IPR was lethal for O/AL, but not for Y/AL or O/DR animals. This lethal dose induced synthesis of p48 in heart and bone marrow nuclei of O/AL rats only. P48 existed in isoform states in bone marrow, and when a lethal dose of IPR was administered in this tissue, it was expressed in O/AL rats in a cell-cycle regulated pattern. Stress proteins and other non-sps were seen as cell cycle regulated following IPR administration. P48 in bone marrow and heart nuclei from O/AL rats showed an antigenic response identical to that of p48 in HL60 nuclei. The presence of p48 is correlated with mortality and with an ad libitum diet in old rats, since it is absent in old diet restricted animals; therefore, DR may impede the expression of p48 through a mechanism(s) that is undisclosed at this time.

Influences of age and dietary restriction on gastrocnemius electron transport system activities in mice

Alterations in the mitochondrial electron transport system (ETS) may contribute to aging. Dietary restriction (DR) provides a model to investigate retarded aging. ETS activities were measured in gastrocnemius from 10- and 20-month-old B6C3F1 female mice fed either ad libitum (AL) or DR diets (40% < AL). Older (26 month old) AL mice were studied for complex IV. Activities of complexes I, III, and IV decreased 54-74% from 10 to 20 months of age in AL mice. At 10 months, activities of complexes I, III, and IV were 33-64% lower in DR compared to AL mice. The Km for ubiquinol-2 of complex III increased 29% by 20 months of age in AL mice while no change occurred in DR mice. The Vmax of complex IV declined by 90% from 10 to 26 months of age in AL mice and this change was opposed by DR. Complex IV contains high- and low-affinity binding sites. The Km for high-affinity sites was not influenced by age or diet through 20 months; however, the Km was approximately twofold higher at 26 months in AL mice. The percentage of total binding sites which were of high affinity fell from 68% at 10 months in AL mice to 46% at 20 months and was even lower (33%) at 26 months. This value was 80% for DR mice at 10 and 20 months. These alterations with aging in mitochondrial ETS capacities may contribute to decreases in skeletal muscle function.

The effect of aging and dietary restriction on the retinoylation of nuclear matrix proteins in rats

The labeling in vivo of young ad libitum (Y/AL) and old diet restricted (O/DR) rats with [3H]retinoic acid (RA) for 6 hours, and the exposure of electrophoretically separated nuclear matrix proteins from bone marrow tissue on film for 48 days revealed the presence of eleven retinoylated proteins. Dosing with RA (100 mg/kg body weight) for 96 hours and exposure to [3H]RA enhanced the levels of radioactive incorporation of several nuclear matrix proteins, including p51, and p55, similarly in Y/AL and O/DR rats. Dosing of old ad libitum (O/AL) rats with [3H]RA for 6 hours showed the incorporation of six proteins following 48 days of exposure on film. Long-term dosing of RA (96 hours) did not increase [3H]RA incorporation in these proteins, including p51 and p55, in O/AL rats. Increasing the level of RA by two-fold (200 mg/kg body weight) in Y/AL and O/DR rats elicited an increase in the incorporation levels of [3H]RA in five proteins. This dose response following increased levels of RA was not seen in the retinoylated proteins of O/AL animals. Analysis by the Western blotting technique showed p51 and p55 from rat bone marrow cells to have the same immunochemical determinates with proteins of identical molecular masses in HL60 cells. The levels of retinoylation of nuclear matrix proteins in O/DR animals, altered by age- and diet-dependent factors, suggests a condition that is more reminiscent of Y/AL than of O/AL animals.

Inhibitory effect of caloric restriction on tumorigenicity induced by 4-aminobiphenyl and 2-amino-1-methyl-6-phenylimidazo-[4,5-b]pyridine (PhIP) in the CD1 newborn mouse bioassay

The tumorigenicity of 4-aminobiphenyl (4-ABP) and 2-amino-1-methyl-6-phenylimidazo[4,5-b]pyridine (PhIP) were studied in combination with caloric restriction in the male neonatal CD1 mouse bioassay. 4-ABP and PhIP exhibited moderate and weak tumorigenicity, respectively, in ad libitum fed mice; however, none of the caloric restricted mice developed tumors. These results indicate that caloric restriction, even when begun 3 months after the conclusion of compound treatment, markedly inhibited 4-ABP- and PhIP-induced tumors in the CD1 mouse.

MPP(+)-induced neurotoxicity in mouse is age-dependent: evidenced by the selective inhibition of complexes of electron transport

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), has been demonstrated to cause selective neurotoxicity by inhibiting complex I in mitochondria, through its toxic metabolite 1-methyl-4-phenylpyridine (MPP+) which is formed during the bioactivation of MPTP by monoamine oxidase B. In this report, we have evaluated the effect of MPP+ on the 4 mitochondrial respiratory chain complexes by incubating brain mitochondria of mice at 3 different age groups with MPP+ (200 microM) and monitoring enzyme activities of complexes I, II, III, and IV at 5, 10, 15, 30, 60, and 120 min. Complexes I, III, and IV showed significant inhibition within 15 min in all the age groups studied, followed by some recovery in enzyme activities upon further incubation for complexes I and IV. However, complex II was not affected by MPP+ at any age. Our data suggest that inhibition of complexes I, III, and IV by MPP+ efficiently restrict the transport of electrons down the respiratory chain which ultimately leads to decreased ATP production. This could further aggravate oxidative stress as ATP is required for the synthesis of glutathione (GSH), one of the important scavengers of free radicals. In this study, inhibition was more severe in mitochondrial preparations from older rather than younger mice. Additionally, young animals showed faster recovery following inhibition than old animals for complex I. Impaired respiratory chain function in older animals compared to younger ones supports the hypothesis of accumulation of age-related mitochondrial DNA mutations which partly encode for subunits of complexes I, III, and IV. From this study, it seems that inhibition of complexes I, III, and IV may be the underlying cause of neurotoxicity due to MPP+ which could be intensified by age-associated dysfunction of electron transport.

Effect of dietary restriction on benzo[a]pyrene (BaP) metabolic activation and pulmonary BaP-DNA adduct formation in mouse

Hepatic microsomal xenobiotic metabolizing enzyme activities of laboratory animals can be modulated by Dietary restriction (DR). The modulation of xenobiotic metabolizing enzyme activities can affect the metabolic activation of chemical carcinogens. Acute DR (60% of the food consumption of ad libitum (AL)-fed mice for 7 weeks) reduced the body weights of the male B6C3F1 mice, and increased mouse pulmonary cytochrome P4501A1-dependent BaP metabolizing enzyme activity. The effects of DR on the formation of the specific BaP-DNA adduct, 10-(N2-deoxyguanosinyl)-7,8,9-trihydroxy-7,8,9,10-tetrahydro-BaP (BaP-N2-dG) in mouse lung can be detected by using 32P-postlabeling technique. In both AL- and DR-mice total BaP-DNA adduct formation in lung reached a peak at 48 hours after treatment with [3H]BaP and the in vivo formation of BaP-N2-dG was greater in DR mouse lung than in that of AL-animals by 22%. DR increased in vitro BaP-N2-dG formation by 39% when calf-thymus DNA was incubated with BaP using liver microsomes obtained from DR- or AL-mice as the enzyme source. The formation of the specific BaP-N2-dG adducts, measured by 32P-postlabeling, was only 20% of the total [3H]BaP-DNA adducts as determined by liquid scintillation counting. The increase of BaP-DNA adduct formation in mouse lung was correlated to the enhancement of the mouse pulmonary BaP metabolizing enzyme activity. Our results indicated that the effect of DR on the metabolic activation of BaP in mouse lung was dependent upon the mouse lung cytochrome P4501A1-dependent BaP metabolizing enzymes activities which was significantly increased by DR.

Longevity, body weight, and neoplasia in ad libitum-fed and diet-restricted C57BL6 mice fed NIH-31 open formula diet

Groups of C57BL6 mice of each sex were assigned to one of 2 dietary regimens, ad libitum (AL) or dietary restriction (DR), to study effects of food restriction on body weight, survival, and neoplasia. The AL and DR groups were subdivided into a scheduled sacrifice group for examination at 6-mo intervals, and a lifetime group to provide longevity data. Necropsies and microscopic examinations were conducted on 911 animals. In the lifetime group food consumption averaged 33.6 and 34.4 g per week by AL males and AL females, respectively; the DR counterparts were given 40% less. The diet contained 4.35 kcal/g. The average lifetime body weights were 34.8, 26.8, 22.6, and 21.6 g for AL males, AL females, DR males, and DR females, respectively, and their age at 50% survival was 27.5, 26.9, 31.7, and 33.5 mo. Maximal lifespan was increased 18% in DR males and females. Lifetime incidence of tumor-bearing mice was 89% and 86% for AL males and females, versus 64% for each sex of DR mice. Dramatic reduction occurred in female DR mice in lymphoma (9% vs 29%), pituitary neoplasms (1% vs 37%), and thyroid neoplasms (0.4% vs 8%). In males, hepatocellular tumors were reduced to 1% from 10% by DR. In contrast, the incidence of histiocytic sarcoma was increased in DR females and unaffected in DR males. Tumor onset was delayed in DR animals; 87% of all neoplasms in males and 95% in females had occurred in the AL mice by 24 mo, whereas the DR animals had only 52% and 39% of their lifetime incidence, respectively, by that age. This study provided comparative AL and DR data from C57BL6 mice examined randomly at 6-mo intervals (cross-sectional group) in parallel with data from animals in similar cohort that was unsampled and allowed to succumb naturally (longevity group). Dietary restriction reduced the lifetime percentage of tumor-bearing animals and the number of tumors per animal, and delayed the age at onset of most neoplasms.

MPTP- and MPP(+)-induced effects on body temperature exhibit age- and strain-dependence in mice

1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is toxic toward the dopaminergic nigrostriatal system of a plethora of species including rodents, nonhuman primates and humans. The present study was designed to evaluate if systemic administration of MPTP or its metabolite, 1-methyl-4-phenylpyridinium ion (MPP+), has significant effects on body temperature (BT) and whether such effects might play a role in the neurotoxicity. A single intraperitoneal (i.p.) dose of either MPTP (50 mg/kg) or MPP+ (12.5 mg/kg) leads to a decrease in BT in both C57BL/6N (C57) and CD-1 mice. The hypothermia induced by MPTP can be blocked by pretreatment with deprenyl (30 mg/kg, i.p.), an MAO-B inhibitor. However, the hypothermia elicited by MPP+ is refractive to MAO-B inhibition. These findings suggest that MPP+ is responsible for the BT reduction and that the primary site of action lies outside the blood-brain barrier. An initial hyperthermic phase in the CD-1 mice, which leads to the induction of heat shock protein-72 (HSP-72) throughout the brain, differentiates their response to MPTP from that of C57 mice. This initial hyperthermia appears to be protective since its prevention by dosing at a low ambient temperature enhances striatal dopamine (DA) depletion in CD-1 mice. The temperature effects of both MPTP and MPP+ also display an age-dependence in the C57 strain of mice, with the magnitude of the effects correlating positively with age. However, profound hypothermia could be induced by MPP+ in the absence of striatal DA depletion. The latter finding suggests that while a positive correlation was found between age and the magnitude of the hypothermia, DA depletion and hypothermia are not causally related. The apparent protective effect of the initial hyperthermia in the CD-1 strain of mice, however, suggests that BT is an important parameter in the neurotoxicity of MPTP.

Effect of dietary restriction on glutathione S-transferase activity specific toward aflatoxin B1-8,9-epoxide

Dietary restriction (DR) reduced the metabolic activation of aflatoxin B1 (AFB1) in rats. This reduction may be attributed to the decrease of cytochrome P-450-mediated AFB1 epoxidation and/or increase in the detoxification of AFB1 catalyzed by hepatic glutathione S-transferase (GST) and other phase II detoxification enzymes. In this study the effect of DR on male rat liver cytosolic GST activity toward AFB1-8,9-epoxide was studied. The chemically-synthesized AFB1-8,9-epoxide was used as the substrate in this assay, and the formation of AFB1-GSH conjugate was analyzed by HPLC. Male Fischer 344 rats fed DR diets (60% of the food consumption of ad libitum (AL)-fed rats) showed a 2.4-fold increase in GST activity when AFB1-epoxide was used as the substrate. The results from the enzyme kinetic study showed that DR increased Vmax of the liver cytosolic GST but not the Km. Acute DR has little or no impact on GST activity when 1-chloro-2,4-dinitrobenzene and 2,4-dichloronitrobenzene were used as substrates. The mouse liver GST activity toward AFB1-epoxide was 3-fold greater than that of phenobarbital-induced rats, 4.5-fold greater than DR rats, and 14.7-fold greater than the GST activity of AL rats. This direct assay of liver GST activity using AFB1-epoxide as the substrate is useful for studying AFB1-induced biomarkers, such as AFB1-GSH conjugation and AFB1-DNA adducts.

Age-related changes in the intrinsic rate of apoptosis in livers of diet-restricted and ad libitum-fed B6C3F1 mice

Cancer incidence increases progressively with age. This observation suggests that a mechanistic relationship may exist at the cellular level between these two apparently diverse processes. Indirect evidence for this fundamental relationship is derived from the fact that interventions that retard the rate of aging simultaneously retard the incidence of many forms of cancer. Dietary restriction of rodents is a noninvasive manipulation that reproducibly retards most physiological indices of aging as well as the incidence of spontaneous and chemically induced tumors. As such, it provides a powerful model in which to study common mechanistic processes associated with both aging and cancer. In a recent study, we established that chronic dietary restriction induces an increase in the spontaneous rate of apoptotic cell death in hepatocytes of 12-month-old B6C3F1 mice and is associated with a significant reduction in the subsequent development of spontaneous hepatoma in this genetically susceptible strain. The purpose of the present investigation was to extend and confirm these original observations by determining whether the increased rate of spontaneous apoptosis with chronic dietary restriction is maintained throughout the life span in this strain. We quantified the spontaneous apoptotic rate by histological examination of liver sections from diet-restricted and ad libitum-fed B6C3F1 mice at age intervals of 12, 18, 24, and 30 months. The incidence of apoptotic bodies was enumerated in non-tumor-bearing mice by scoring 50,000 hepatocytes per liver by in situ end-labeling immunohistochemistry and was expressed as the mean incidence per 100 cells. The rate of apoptotic cell death was found to be elevated with age in both diet groups; however, the rate of apoptosis was significantly and consistently higher in the diet-restricted mice, relative to the ad libitum-fed mice, regardless of age. It has been proposed that apoptosis, or physiological cell death, provides a protective mechanism whereby DNA-damaged or potentially neoplastic cells are selectively eliminated. Thus, interventions that increase cellular sensitivity to apoptotic cell death would tend to protect genotypic and phenotypic stability with age; on the other hand, the failure to initiate or respond to appropriate signals for apoptosis would tend to accelerate the accumulation of age-associated genetic lesions and age-related neoplasia. An increase in the intrinsic rate of apoptotic cell death may contribute, in part, to decreased tumor incidence and increased life span potential with dietary restriction.

Age-related neoplasia in a lifetime study of ad libitum-fed and food-restricted B6C3F1 mice

Longevity, body weight, and age-specific neoplasia were determined in 1,064 B6C3F1 mice as part of a coordinated study of food restriction (FR). Restricted animals were offered 60% of the diet consumed by the ad libitum (AL) group. Longevity data were derived from a set of 56 animals of each sex from each diet group, which were examined whenever dead or moribund. For cross-sectional data, a parallel set of 210 animals were sacrificed in groups of 12-15 at 6-mo intervals. Lifetime body weight was reduced in the FR mice approximately proportional to restriction (i.e., 40%). Food restriction increased the age at 50% survival (median) by 36% in both sexes and increased the maximal lifespan (mean age of oldest 10%) by 21.5% in males and by 32.5% in females. In 56 males of the longevity groups, there were 89 neoplasms in the AL subgroup versus 53 in FR; 56 AL females had 100, versus 58 in 55 FR females. Increase in lifespan of the restricted animals was achieved primarily by decrease in incidence and delay of onset of fatal tumors, of which lymphoma was the most prominent. This report catalogs all of the neoplasms (1,103) observed in longevity and cross-sectional groups, by diet, sex, and age. These data add to the existing knowledge base needed for future studies of dietary restriction and aging as well to evaluate nutrition of animals used in bioassays.

Effect of dietary restriction on partial hepatectomy-induced liver regeneration of aged F344 rats

Fourteen weeks-old male F344 rats maintained on a reduced caloric diet (60% of ad libitum (AL) food consumption) for 6 weeks or for 14 months did not affect the hepatic cell proliferation in terms of % S phase population, determined by evaluation of DNA synthesis in hepatocytes isolated from either young (5 months) or aged (18 months) rats. However, hepatic basal cellular DNA synthesis estimated by [3H]thymidine incorporation was reduced through acute dietary restriction (DR) in young rats, but increased in aged animals after 14 months restriction. Partial hepatectomy (PH) on aged rats stimulated hepatocyte regeneration and restored some aging-associated biochemical functions, such as drug metabolizing enzyme-dependent xenobiotic metabolic activation which was determined by measuring the formation of carcinogen-DNA adducts. Forty-eight hours after partial hepatectomy, the % of S phase population and the basal nuclear DNA synthesis of hepatocytes isolated from the partial hepatectomized DR-rats were 4- and 2.8-fold, respectively, greater than those of hepatocytes from AL-animals. DR reduced aflatoxin B1 (AFB1) metabolizing enzyme activity and decreased the AFB1-DNA adduct formation in young rats treated with AFB1. In aged AL-rats, the formation of AFB1-DNA adducts diminished to the same level as that of DR-groups and probably was due to the faster decline of drug metabolizing enzymes in aging AL-rats. However, 48 h after PH, the metabolic activation of AFB1 was restored in AL- and DR-groups which resulted in the increase of AFB1-DNA binding by 4.2 and 1.9-fold, respectively. During the liver regeneration of old PH-rats, DR inhibited the AFB1-DNA adduct formation after the PH-rats received a single dose of AFB1. DR increased benzo[a]pyrene (BaP) metabolic activation in both young and aged rats. Aging also decreased BaP-DNA adduct formation in both DR and AL-rats. The increase of BaP-DNA adduct formation in PH-groups was attributed to the restoration of BaP-metabolizing enzyme activity during liver regeneration. The PH-stimulated BaP-DNA adduct formation in AL- and DR-rats was 3.4- and 2.0-fold greater than control aged rats. Our results indicated that the stimulation of PH-induced liver regeneration by DR in aged animals may be attributed to the retardation of aging by DR and the retention of more active biochemical and enzymological functions in old DR-animals.

Caloric restriction and toxicity

The modulatory effects of caloric intake on the rate and extent of both spontaneous and induced disease incidence is well known, but the significance of these effects in the interpretation of testing data has only recently become appreciated. This is especially true relative to the impact of caloric intake on both survival and background incidence for common tumors. In order to enhance the health and survival of animals ongoing chronic toxicity testing it has been suggested that such tests should restrict food consumption. Although this restriction will result in increasing survival of the test animals, it may also effect the expression of toxicity by altering agent metabolism and disease progression. Focus in this symposium is on the necessity to control dietary consumption in toxicity tests (dietary control), and if such a need does exist to what level of consumption should be diet be focused (caloric restriction).

Chronic caloric restriction induces stress proteins in the hypothalamus of rats

The induction of stress proteins (sps) in the hypothalamus of female Fischer 344 rats in response to caloric restriction (CR) and to heat stress was investigated. Caloric restriction was found to elicit sps 27, 34, 70, and 90 in the hypothalamus of both young and old rats while none was found in the hypothalamus of ad libitum (AL) fed controls. Heat stress initiated heat shock proteins (hsps/sps) 27, 70, and 90 in the hypothalamus of the young (AL) fed animals, the same proteins evoked by feeding stress. The same sps were induced in the old (AL) rats although the expression showed substantial decline with age. This reduction was less marked, however, with the old CR rats. Stress protein 34, an infrequently reported protein, was related to feeding and was not induced by heat shock. Recent reports point to the important role sps play in the cellular reaction to stress, as well as their involvement in the higher functions. The findings reported here suggest that sps are involved in the regulatory mechanisms allowing CR animals to tolerate stress related to metabolic substrate deprivation.

Temporal and substrate-dependent patterns of stress protein expression in the hypothalamus of caloric restricted rats

Stress proteins (sps) 27, 34, 70 and 90 (Mr x 10(3)) were induced in the hypothalamus of caloric restricted (CR) rats by feeding stress. A definite time pattern for sps synthesis was observed when their induction was examined at several time points after the rats were fed, and the level of sps expression was found to vary significantly at different times of the day. The same group of proteins was induced in ad libitum fed rats when they were subjected to food deprivation for 48 h. Stress protein 34 expression in the hypothalamus of old caloric restricted rats was found to be dependent on blood glucose levels, and was substantially reduced when insulin was added to the glucose infusion. The expression of sps 27, 70 and 90, however, was little changed with glucose and/or insulin infusion.

The homology of a novel polypeptide with stress protein characteristics in embryonic mice brain and in the hypothalamus of caloric restricted rats as determined by ultramicro western blotting

A novel protein (p34) was observed in polyacrylamide gel fluorographs of gestation day 13 embryonic mouse brain following retinoic acid dosing of dams. Another p34 polypeptide with identical gel migratory characteristics was seen in the hypothalamus of old caloric restricted rats after "food deprivation stress". Western blotting, employing an ultramicro trans-blot cell developed in our laboratory, detected identical immunochemical determinants between these proteins, verifying their homology. Peptide mapping and Western blotting further validated the uniqueness of p34 compared with other stress proteins including heme oxygenase.

Caloric restriction profoundly inhibits liver tumor formation after initiation by 6-nitrochrysene in male mice

Caloric restriction (CR) inhibited strongly the incidence of chemically-induced tumors in the neonatal B6C3F1 mouse tumorigenicity bioassay, when begun 3 months after treatment with the potent carcinogen 6-nitrochrysene. These data indicate that CR can have a profound inhibitory effect on tumor development even long after metabolic activation and DNA repair have occurred.

Survival, body weight, and spontaneous neoplasms in ad Libitum-fed and food-restricted Fischer-344 rats

Ad libitum-fed (AL) and food-restricted (FR) Fischer-344 male and female rats were monitored for survival, body weight, and spontaneous neoplasms. Mean and maximal lifespans for each group were inversely related to mean body weights. AL males were the shortest lived (mean lifespan 101 wk) followed by AL females (118 wk), FR males (125 wk), and FR females (132 wk). Gross and microscopic examinations were performed on 851 rats from cross-sectional and longevity components of the study. In FR groups, the incidence of mammary gland fibroadenomas, testicular interstitial cell tumors, and pituitary neoplasms was decreased while the latency of these neoplasms was increased. In longevity components, most FR groups had a higher incidence of leukemia than AL cohorts, but all FR groups had a higher mean age at death for the rats with leukemia. Higher leukemia rate in the FR groups was thought to be a result of their extended mean lifespan.

Acute toxicity of ganciclovir: effect of dietary restriction and chronobiology

The effect of diet, age and time of dose delivery on the mortality of female B6C3F1 mice from ganciclovir sodium (DHPG) was determined for both single (SD; 400 mg DHPG/kg, ip) and multiple doses (MD; same dose ip for 10 additional days) of the drug. Young (7-10 months) and middle-aged (MA; 19-22 months) mice (B6C3F1), both fed ad lib. (AL) and calorie restricted (CR), were dosed at 0, 6, 12 and 18 hr after lights on (HALO; SD study) and at 12.00 hr (MD study). The SD study mortality rate was 38% (AL) and 1.7% (CR) (P < 0.00001). Mortality was 53% (AL, young; P < 0.00001), over 20% (AL, MA), over 1.8% (CR, MA; P = 0.00004) or more than 1.7% (CR, young; P = 0.00002). Effects were independent of lean body mass differences between AL and CR mice. In the SD study, comparing AL mice only, the greatest mortality was seen in young mice at 6 HALO, (73%; P = 0.0034) and lowest mortality in MA mice at 12 HALO (8%; P = 0.026), whereas in the MD study mortality was 63% AL and 33% CR (P = 0.015). By age, MD mortality was 80% (AL, young; P = 0.0035), 50% (CR, MA), 47% (AL, MA), and 15% (CR, young; P = 0.0013). CR protected both young and MA mice in SD and young mice in MD. Lowest mortality for AL was at 12 HALO. It is suggested that dosing at 12 HALO may protect by decreasing DHPG uptake during a period of minimal DNA synthesis in the affected organ(s). CR and timing of DHPG dose may obviate the necessity to discontinue DHPG because of toxicity in humans. The most significant finding of this study is the impact of diet on mortality.