Hematologic values and plasma and tissue folate concentrations in dogs given phenytoin on a long-term basis

Susan E. Bunch From the Departments of Companion Animal and Special Species Medicine (Bunch), and Microbiology, Parasitology, and Pathology (Easley, Cullen), College of Veterinary Medicine, North Carolina State University, Raleigh, NC 27606.

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J. Roger Easley From the Departments of Companion Animal and Special Species Medicine (Bunch), and Microbiology, Parasitology, and Pathology (Easley, Cullen), College of Veterinary Medicine, North Carolina State University, Raleigh, NC 27606.

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John M. Cullen From the Departments of Companion Animal and Special Species Medicine (Bunch), and Microbiology, Parasitology, and Pathology (Easley, Cullen), College of Veterinary Medicine, North Carolina State University, Raleigh, NC 27606.

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SUMMARY

During earlier investigations of the hepatic effects in dogs of long-term administration of phenytoin alone or in combination with primidone, erythrocytic macrocytosis, neutropenia, neutrophilic hypersegmentation, and thrombocytopenia were observed. Such abnormalities were observed most often in dogs given phenytoin and resembled those known to be attributable to folate deficiency in human beings with epilepsy treated with phenytoin. To pursue the theory that these hematologic aberrations were caused by drug-induced folate deficiency, 12 dogs were given a diet specifically formulated to contain a minimally adequate concentration of folate. After 2 weeks, phenytoin was administered daily (400 mg, po, q 8 h) to 8 of the 12 dogs for 54 weeks. A cbc, bone marrow aspiration biopsy, and measurement of plasma and rbc folate concentrations were done every 3 weeks. Bone marrow aspirates were examined by transmission electron microscopy after 24 and 36 weeks, and at the end of the treatment period. Hepatic folate concentration was also determined in all dogs before and after treatment. Excretion of formiminoglutamic acid, as a marker of folate deficiency, was measured in all dogs at the end of the study.

All dogs remained healthy throughout the treatment phase. Consistent abnormalities were not observed in the blood or bone marrow of treated dogs. Plasma and RBC folate concentrations decreased in control and treated dogs as a result of dietary restriction (P < 0.02), and remained stable until the end of the study. The rbc folate content decreased further in treated dogs (P < 0.02), although the hepatic folate content was similar in control and treated dogs. Treated dogs did not excrete formiminoglutamic acid more rapidly than did control dogs. Gross necropsy or histologic abnormalities were not identified in control or treated dogs. We concluded that long-term administration of phenytoin was not associated with clinical, hematologic, or biochemical evidence of folate deficiency in dogs.

SUMMARY

During earlier investigations of the hepatic effects in dogs of long-term administration of phenytoin alone or in combination with primidone, erythrocytic macrocytosis, neutropenia, neutrophilic hypersegmentation, and thrombocytopenia were observed. Such abnormalities were observed most often in dogs given phenytoin and resembled those known to be attributable to folate deficiency in human beings with epilepsy treated with phenytoin. To pursue the theory that these hematologic aberrations were caused by drug-induced folate deficiency, 12 dogs were given a diet specifically formulated to contain a minimally adequate concentration of folate. After 2 weeks, phenytoin was administered daily (400 mg, po, q 8 h) to 8 of the 12 dogs for 54 weeks. A cbc, bone marrow aspiration biopsy, and measurement of plasma and rbc folate concentrations were done every 3 weeks. Bone marrow aspirates were examined by transmission electron microscopy after 24 and 36 weeks, and at the end of the treatment period. Hepatic folate concentration was also determined in all dogs before and after treatment. Excretion of formiminoglutamic acid, as a marker of folate deficiency, was measured in all dogs at the end of the study.

All dogs remained healthy throughout the treatment phase. Consistent abnormalities were not observed in the blood or bone marrow of treated dogs. Plasma and RBC folate concentrations decreased in control and treated dogs as a result of dietary restriction (P < 0.02), and remained stable until the end of the study. The rbc folate content decreased further in treated dogs (P < 0.02), although the hepatic folate content was similar in control and treated dogs. Treated dogs did not excrete formiminoglutamic acid more rapidly than did control dogs. Gross necropsy or histologic abnormalities were not identified in control or treated dogs. We concluded that long-term administration of phenytoin was not associated with clinical, hematologic, or biochemical evidence of folate deficiency in dogs.

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