A histopathology peer review already has become an integral part of industrial toxicologic pathology in the USA and Europe. Nevertheless, the review is unfamiliar to toxicologic pathologists in Japan. This report provides detailed information on convenient, useful procedures for peer review in toxicologic histopathology and describes its necessity and type. Histopathology peer review is either informal or formal. Formal review means that the target organs to be re-estimated and the reviewing pathologist's name are stated somewhere in a protocol or study report. The resolution of all diagnostic discrepancies between studies and reviewing pathologists must be clarified, and the review records need to be preserved. No audit trails need to be kept for informal peer review. Based on the purpose of the review, histopathology peer reviews may be classified into 3 major types:“complete", “problem-solving", and a“quality-monitoring"reviews. For a“Complete"review, all histopathologic findings crucial to the conclusion of the study are re-examined. A“problem-solving"review serves to re-evaluate anomalous findings. A“quality-monitoring"review is a periodical monitor of the quality of histopathologic diagnoses at pathology laboratories, particularly in toxicology contract research organizations. The procedures for histopathology peer review consist of 5 stages. At the first stage, the target organs to be reviewed are selected from the draft summary tables of the original findings diagnosed by the study pathologist, and the reviewing pathologist evaluates all selected organs. At the second stage, the study pathologist re-examines all the findings with diagnoses not accepted by the reviewing pathologist and re-considers the appropriateness of these original diagnoses. At the third stage, the study pathologist and the reviewing pathologists discuss all diagnostic differences until they reach a consensus opinion. At the fourth stage, the whole process (stages 1 to 3) of peer review of all target organs is tabulated, including the study pathologist's and reviewing pathologist's diagnoses, the study pathologist's opinions, and consensus diagnoses. At the fifth stage, Peer Review Certification is issued with the study pathologists' and reviewing pathologists'signatures. The reviewing pathologist must share the responsibility for the final diagnoses with the study pathologist. In order to gain greater credibility form regulatory agencies, a peer review performance is best made by an independent external pathologist.

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To clarify whether levofolinate calcium (1-LV) enhances 5-fluorouracil (5-FU) toxicity, a 4-week toxicity study of 5-FU (10 mg/kg/day) in combination with 1-LV (6, 20 or 60 mg/kg/day) was conducted in rats. In the 5-FU alone group, a decrease in body weight gain, food consumption, RBC parameter and WBC counts were detected. Histopathologically, lymphoid depletion of lymphatic organs, hematopoiesis enhancement of the spleen and myelosuppression were observed. In the group for which 5-FU was combined with 1-LV, the RBC counts decreased, extramedullary hematopoiesis increased and the suppression of lymphatic organs was enhanced. Changes in the lymphatic organs were observed at 20 mg/kg/day of 1-LV and above. In monitoring of blood drug concentrations of 1-LV, 5-methyl tetrahydrofolic acid, a metabolite of 1-LV, and 5-FU after the 1st and 14th dosings, there was no apparent difference between 5-FU alone and 5-FU combined with 1-LV in C_max and AUC_0-∞. The potentiation induced by 1-LV in the toxicity of 5-FU appeared to be mainly immuno-suppression and myelosuppression, which were related to the anti-tumor activity of 5-FU. Plasma concentrations of 5-FU and 1-LV in this study overwhelmed the concentrations that enhancement of thymidylate synthetase (TS) inhibition due to 5-FU was observed by addition of 1-LV in vitro. Therefore toxic potentiation of 5-FU due to simultaneous 1-LV dosing is presumed to be concerned with an increased ternary complex (FdUMP-TS-5, 10-methylenetetrahydrofolate) formation and a greater extent of TS inhibition.

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A single dose toxicity study of magnesium sulfate by intravenous administration was conducted in rats and dogs. The results are summarized in the following. Magnesium sulfate was administered once at dose levels of 90, 130, 200, 300 and 450 mg/kg to Crj:CD(SD) rats at 6 weeks of age. Deaths occurred in the 200 mg/kg and above groups in both sexes. The LD₅₀ values were 206 mg/kg for males and 174 mg/kg for females. In the surviving animals, in the 130 mg/kg and above groups, tonic convulsions, abnormal gait and tachypnea were seen. However, these signs disappeared gradually and all animals returned to a normal state by 15 min after dosing. There were no treatment-related changes in the body weight or gross pathology. Magnesium sulfate was infused for 6 hr at dose levels of 75, 300 and 1200 mg/kg (12.5, 50 and 200 mg/kg/hr) to female beagle dogs at 6 months of age. No deaths were observed in any of the dose groups and it was considered that the lethal dose level would be higher than 1200 mg/kg (200mg/kg/hr). In the 1200 mg/kg group, vomiting, decreased spontaneous movement, staggering gait, prone position and flush of the conjunctiva and ear auricles were seen. However, these signs disappeared gradually and animals returned to a normal state by 1 hr after dosing. There were no treatment-related changes in the body weight, food consumption or gross pathology.

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A 2-week toxicity study of magnesium sulfate administered by a 24-hr intravenous infusion at the dosage levels of 0, 12.5, 50, 100 and 200 mg/kg/hr in female beagle dogs was conducted, with 2-week follow-up observation after drug withdrawal. One of 2 animals in the 200 mg/kg/hr group died approx. 32 hr after the start of infusion. At the same time, the remaining 1 animal of the same group was sacrificed in a moribund state. Changes attributable to the treatment of magnesium sulfate were decreased food consumption and body weight gain, anemia, mild prolongation of conduction time in electrocardiogram and tubular basophilia in the kidneys in the animals treated with 100 mg/kg/hr. Furthermore, decreased calcium level was recorded in the animals treated with 50 mg/kg/hr or more. However, these changes disappeared after drug withdrawal, and reversibility was suggested. Judging from the mode of occurrence, since the change in calcium level observed in the group treated with 50 mg/kg/hr was slight, it was considered to be toxicologically insignificant. In conclusion, the nontoxic dosage level of magnesium sulfate was judged to be 50 mg/kg/hr under the condition of the present study.

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A 4-week toxicity study of magnesium sulfate administered by 24-hr intravenous infusion at the dosage levels of 0, 12.5, 50 and 100 mg/kg/hr in female beagle dogs was conducted. No death occurred in any group. Changes attributable to the treatment with magnesium sulfate were decreased food consumption and body weight gain, anemic change, increased urine volume, decreased serum calcium level, increased inorganic phosphorus level, slight prolongation of conduction time in electrocardiogram and tubular basophilia in the kidneys in the group treated with 100 mg/kg/hr. In addition, essentially similar changes were also observed at the same dosage level in the 2-week study of this drug, in which recoverability was recognized with 2-week follow-up observation after drug withdrawal. In conclusion, the nontoxic dosage level was judged to be 50 mg/kg/hr under the condition of the present study.

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Magnesium sulfate, at dose levels of 250, 500 and 1000 mg/kg, was administered subcutaneously three times daily to Crj:CD(SD) female rats from day 15 through day 20 of gestation. The effects of the compound on dams and F₁ animals were examined. In the dams, decreased food consumption was observed in the 500 and 1000 mg/kg groups. Hypolocomotion, pronation, bradypnea and decreased body weight gain were observed in the 1000 mg/kg group. But there were no effects on the delivery or lactation conditions and necropsy from administration of the test article. In the F₁ animals, low body weight, delays in differentiation (eruption of lower incisor, opening of eyelid) and reversible change in ribs (wavy rib) were observed in the 1000 mg/kg group. But there were no effects from administration of the test article in viability, functional examinations, behavior tests or reproductive ability. Based on the above results, under the conditions of this study, it was concluded that the non-toxic dose levels for general toxicological effects on dams was 3×250 mg/kg/day, for reproductive ability of dams was 3×1000 mg/kg/day, and for development of F₁ animals was 3×500 mg/kg/day.

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The mutagenicity potential of magnesium sulfate was re-assessed using the current procedure of the reverse mutation test with bacteria and chromosomal aberration test with mammalian cells (a Chines hamster lung fibroblast cell line;CHL/IU) in culture. In the reverse mutation test with bacteria, Salmonella typhimurium TA100, TA98, TA1535 and TA1537 and Escherichia coli WP2 uvrA were use and the maximum dose level was set at 5000μg/plate irrespective of the absence or presence of metabolic activation. Five dose levels (313-5000μg/plate) were selected for all strains except for TA98 without metabolic activation and for TA1537 with metabolic activation, for which 6 dose levels (156-5000μg/plate) were selected. Magnesium sulfate induced no increase in the number of colonies with reverse mutation in any of the strains irrespective of the absence or presence of metabolic activation in the dose-range-finding study or in the main study. In the chromosomal aberration test with mammalian cells, a Chinese hamster lung fibroblast cell line (CHL/IU) in culture was used and the maximum dose level was set at 5.0 mg/mL both in the direct and metabolic activation methods. Three dose levels (1.25-5.0 mg/mL) were selected. Magnesium sulfate induced no increase in the incidence of cells with chromosomal aberration or those with genome mutation (polyploidy) in any of the strains irrespective of the absence of presence of metabolic activation. Thus, it is concluded that magnesium sulfate does not have mutagenic potential under the presence experimental conditions.

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