Neurotoxic Effects of the Anti-Varicella Zoster Virus Agent 2'-Valeryl-6-methoxypurine Arabinoside in Monkeys and Rats Dosed Orally for 90 Days

The 2'-valerate ester of 6-methoxypurine arabinoside (170U88),a nucleoside analog with anti-varicella zoster virus (VZV) activity,was given to monkeys and rats. In subchronic preclinical toxicitystudies, dosing was by gavage to monkeys (distilled water vehicle)and rats (0.5% methylcellulose vehicle) for 90 days. Groupsof 5 male and 5 female monkeys (Macaca fascicularis) were given170U88 at 0, 25, 50, or 100 mg/kg/day. The daily dose was givenin two equal portions with 6 hr between doses. Monkeys in thehigh-dose group lost weight. Food consumption was decreasedfor mid- and high-dose monkeys and for low-dose female monkeys.Slightly decreased values for erythrocyte and leukocyte countsat the mid- and high dose were fully reversed during an 8-weekrecovery period. Two high-dose male monkeys and a middose femalemonkey developed signs of central nervous system toxicity andwere necropsied before dosing was complete. These signs werefirst observed in the fifth week of dosing and included bodytremors, incoordination, reduced activity, sleepiness, stupor,and lack of eye tracking. Axonal lesions were observed in histologicsections of sciatic nerve in monkeys at all dose levels. Neitherthe signs of central nervous system toxicity nor the axonallesions reversed during the 8- week recovery period. Groupsof 14 male and 14 female CD rats (Sprague-Dawley derived) weregiven single daily doses of 170U88 at 0, 150, 300, or 600 mg/kg.Body weights were decreased at all dose levels and food consumptionwas decreased for mid- and high-dose rats. Small increases invalues for erythrocyte count, hemoglobin, and packed cell volumeand small decreases in values for glucose, serum protein, andserum albumin were limited to high-dose rats and reversed duringa 4- week recovery period. High-dose rats also had reversibleliver lesions consisting of necrosis of individual hepatocytes,megalocytosis, occasional mitotic figures, and biliary stasis.Clinical and morphologic indications of central nervous systemtoxicity in the rats consisted of altered exploratory behaviorat the high dose and groups of small vacuoles in cerebellarwhite matter at all dose levels. Cerebellar vacuolation wasobserved in rats examined at the end of the exposure periodand also in rats examined after the 4-week postdose recoveryperiod. Signs interpreted as peripheral nervous system toxicitywere limited to rats in the high-dose group and consisted ofhindquarter weakness, slow righting and placing reflexes, andataxia. Again, these findings did not reverse during the recoveryperiod. Thus, signs of both central and peripheral nervous systemtoxicity were observed in both monkeys and rats. These neurotoxiceffects resulted in the decision to stop further developmentof 170U88.     

Preclinical Toxicology Studies with Acyclovir: Carcinogenicity Bioassays and Chronic Toxicity Tests

Preclinical Toxicology Studies with Acyclovir: CarcinogenicityBioassays and Chronic Toxicity Tests. Tucker, W.E., Jr., Krasny,H.C., de Miranda, P., Goldenthal, E.I., Elion, G.B., Hajian,G. and Szczech, G.M. (1983). Fundam. Appl. Toxicol. 3:579–586.Acyclovir (ACV), a nucleoside analog that is a new herpes-specificantiviral drug, was given by gavage at 50, 150 and 450 mg/kg/dayto Sprague Dawley rats and Swiss mice for most of their lifetimeto assess chronic toxicity and carcinogenicity. Treatment withACV did not shorten the lifespan of either rats or mice. Infact, female mice given 150 and 450 mg/kg/day had significantlylonger mean durations of survival than control female mice whenanalyzed by the life table technique. There were no signs oftoxicosis produced by chronic exposure to ACV in either therats or mice, and there was no drug-related increase in neoplasmsin either species. Four groups of Beagle dogs were initiallygiven daily oral doses of 15, 45 or 150 mg/kg ACV in a 1 yearchronic toxicity study. Dogs treated at 150 mg/ kg/day vomited,had diarrhea, consumed less feed and lost weight within 2 weeks.Dogs treated at 45 mg/kg/day also had minimal signs of gastrointestinaltoxicosis. These dose levels were then decreased to 60 and 30mg/kg/day for the rest of the one year test period. With theexception of occasional and inconsistent emesis and diarrhea,the 60 mg/kg/day dose level was well tolerated. Some mid andhigh dose dogs had sore paws due to erosion of footpads andcracking, splitting and loosening of the nails first becomingevident during the 13th week of the study. Several of thesedogs subsequently lost the keratin from some claws. There wasnail regeneration and healing of footpads as the study progressed,with all claws appearing essentially normal at the end of 1year. Nails and footpads of dogs given 15 mg/kg/day were normalthroughout the study.     

Preclinical Toxicology Studies with Acyclovir: Acute and Subchronic Tests

Preclinical Toxicology Studies with Acyclovir: Acute and SubchronicTests. Tucker, W.E., Jr., Macklin, A.W., Szot, R.J., Johnston,R.E., Elion, G.B., de Miranda, P. and Szczech, G.M. (1983).Fundam. Appl. Toxicol. 3:573–578. Acyclovir (ACV), a newantiherpes drug, was evaluated for toxicity in a series of acuteand subchronic toxicity tests. Oral LD₅₀ values were greaterthan 10 000 mg/kg in male ICR mice and greater than 20 000 mg/kgin male Long Evans rats. When ACV was given iv, the LD₅₀ was405 mg/kg for male mice and greater than 600 mg/kg for malerats. Additionally, LD₅₀ values for male rats treated sc were1070, 790, 678, and 650 mg/kg in rats that were respectively,3, 10, 28 and 71 days old indicating that very young rats werenot more sensitive to acute toxic effects of ACV. There wereno signs of toxicosis in CD-1 mice given ACV by gavage at doselevels of 50, 150 and 450 mg/kg/day for 1 month. Obstructivenephropathy occurred in rats given 20, 40 and 80 mg/kg/day onceeach day by rapid iv injection for 3 weeks. Both 5 and 10 mg/kg/daywere no effect dose levels. Renal damage caused by precipitationof drug crystals in renal tubules and collecting ducts in ratsgiven ACV by rapid iv injection was readily reversible within2 weeks. Beagle dogs were given doses of 10, 20, 25, 50 and100 mg/kg b.i.d. by rapid iv injection for 1 month. All 8 dogsgiven 100 mg/kg b.i.d. died by the 8th day of treatment; 5 of8 dogs given 50 mg/kg b.i.d. died after 21 to 31 days of treatment.At 50 and 100 mg/kg b.i.d. the clinical signs of toxicosis werenumerous and mainly resulted from the underlying morphologicaland functional changes associated with hypoplasia of the esophagealand gastrointestinal mucosa, lymphoid tissue, and bone marrow.At the 20 and 25 mg/kg b.i.d. dose levels the kidney was thetarget organ; the principal indications of altered renal functionwere increased water intake and hyposthenuria. The dose levelof 10 mg/kg b.i.d. was a no effect level for Beagle dogs treatediv. Thus, in subchronic experiments, the rapid iv injectionof acyclovir caused precipitation of crystals in the renal tubules,resulting in obstructive nephropathy in rats and dogs. Primarytoxicity occurred only in the dog where high doses of acyclovircaused hypoplasia of certain tissues with rapid cell turnover.     

Preclinical Toxicology Studies with Acyclovir: Ophthalmic and Cutaneous Tests

Preclinical Toxicology Studies with Acyclovir: Ophthalmic andCutaneous Tests. Tucker, W.E., Jr., Johnston, R.E., Macklin,A.W., Szot, R.J., Elion, G.B., de Miranda, P. and Szczech, G.M.(1983). Fundam. Appl. Toxicol. 3:569–572. Topical formulationsof acyclovir (ACV) were tested in animals to define potentialfor tissue irritation and systemic toxicity. Acyclovir ointments(5 and 10% concentrations in polyethylene glycol vehicle) producedno sign of dermal irritation or systemic toxicity when appliedto shaved abraded and intact skin of guinea pigs for 24 consecutivedays. Solutions (0.9% normal saline vehicle) of ACV did notsensitize guinea pigs when 10 sensitizing doses and a challengedose were injected intradermally. Petrolatum base ophthalmicointments containing 1 and 3% ACV did not produce significantocular irritation when applied to the corneas of New ZealandWhite rabbits 5 times each day for 21 consecutive days. A 6%petrolatum base ointment produced mild conjunctival irritationbut no sign of corneal or iridic toxicity. Mean concentrationsof 2.53 µM ACV were found in aqueous humor 2 hours aftera 1 cm ribbon (21 mg) of 3% ophthalmic ointment was placed inthe eyes of rabbits. A single treatment with a topical ointmentcontaining 5% ACV in polyethylene glycol vehicle produced minimalirritation when placed in the eyes of New Zealand White rabbits.     

Preclinical Toxicology Studies with Acyclovir: Teratologic, Reproductive and Neonatal Tests

Preclinical Toxicology Studies with Acyclovin Teratologic, Reproductiveand Neonatal Tests. Moore, H.L., Jr., Szczech, G.M., Rodwell,D.E., Kapp, R.W., Jr., de Miranda, P. and Tucker, W.E., Jr.(1983).Fundam. Appl Toxicol 3:560–568. Five studies weredone to define the potential of Acyclovir (ACV), a new nucleosideanalog for antiviral chemotherapy, to produce adverse effectson reproduction and development in laboratory animals. ACV producedno adverse effects when given by gavage to F₀ generation miceat 50, 150 and 450 mg/kg/day in a two generation reproduction/fertilitystudy. Some mice were evaluated for teratologic effects andothers for postnatal development, including behavior, with negativeresults. ACV was not embryotoxic and did not increase the incidenceof fetal malformations when given by subcutaneous injectionto pregnant rats and rabbits at dose levels of 12, 25 and 50mg/kg/day during the periods of major organogenesis. A comparativeLD₅₀ study revealed that 3-day-old rats were not more sensitiveto acute toxic effects of ACV than more mature rats. Finally,in a comprehensive multidose toxicity study ACV was given subcutaneouslyto neonatal rats at 5, 20 and 80 mg/kg/day for 19 consecutivedays. There was minimal effect on body weight gain in neonatestreated at 20 mg/kg/day and a significant decrease in body weightgain at 80 mg/kg/day. Minimal renal lesions occurred at 80 mg/kg/ day but no other signs of adverse effects on developingorgan systems were observed. Except for decreased body weightgain in neonatal rats treated at 80 mg/kg/day, ACV did not produceadverse effects on mammalian development when tested in a varietyof preclinical toxicology studies.