Walter E. Tucker

Preclinical toxicology profile of acyclovir: An overview

Abstract Numerous and extensive toxicologic studies of acyclovir using animal and in vitro assays indicated a low order of toxicity for this antiviral nucleoside analog. Primary cytotoxic effects occurred only at extremely high drug concentrations in plasma or tissue cultures. This correlates well with the high selectivity of the drug for herpes viruses and its low toxicity for uninfected cells in tissue culture. Secondary toxicity in the form of precipitation of acyclovir crystals in the lower nephron of the kidney occurred after rapid intravenous bolus injection causing transient impairment of renal function in the dog, and obstructive nephropathy in the rat. These secondary effects were related to the relatively low solubility of acyclovir in the urine. Dermal and ophthalmic formulations were well-tolerated. No significant primary irritation nor systemic toxic manifestations were associated with the application of these materials. A test in guinea pigs gave no indication that acyclovir has sensitization potential. Acyclovir had no effect on reproductive processes or prenatal, perinatal, or postnatal development of offspring in studies with rats, rabbits, and mice. Positive results occurred in two of 11 in vitro mutagenicity assays. These involved chromosomal damage at extremely high drug concentrations. In contrast, no evidence of chromosomal damage occurred in three in vivo assays at nontoxic dosage levels and under conditions more relevant to clinical use of the drug. While one of two cell transformation assays gave positive results, no oncogenic potential for acyclovir was indicated by lifetime carcinogenicity bioassays in rats and mice. Numerous in vitro and in vivo assays gave no indication that acyclovir might significantly alter immunologic responses at clinically relevant doses.

Preclinical toxicology studies with acyclovir: teratologic, reproductive and neonatal tests.

Five studies were done to define the potential of Acyclovir (ACV), a new nucleoside analog for antiviral chemotherapy, to produce adverse effects on reproduction and development in laboratory animals. ACV produced no adverse effects when given by gavage to F0 generation mice at 50, 150 and 450 mg/kg/day in a two generation reproduction/fertility study. Some mice were evaluated for teratologic effects and others for postnatal development, including behavior, with negative results. ACV was not embryotoxic and did not increase the incidence of fetal malformations when given by subcutaneous injection to pregnant rats and rabbits at dose levels of 12, 25 and 50 mg/kg/day during the periods of major organogenesis. A comparative LD50 study revealed that 3-day-old rats were not more sensitive to acute toxic effects of ACV than more mature rats. Finally, in a comprehensive multidose toxicity study ACV was given subcutaneously to neonatal rats at 5, 20 and 80 mg/kg/day for 19 consecutive days. There was minimal effect on body weight gain in neonates treated at 20 mg/kg/day and a significant decrease in body weight gain at 80 mg/kg/day. Minimal renal lesions occurred at 80 mg/kg/day but no other signs of adverse effects on developing organ systems were observed. Except for decreased body weight gain in neonatal rats treated at 80 mg/kg/day, ACV did not produce adverse effects on mammalian development when tested in a variety of preclinical toxicology studies.

Preclinical toxicology studies with acyclovir: Acute and subchronic tests

Acyclovir (ACV), a new antiherpes drug, was evaluated for toxicity in a series of acute and subchronic toxicity tests. Oral LD50 values were greater than 10 000 mg/kg in male ICR mice and greater than 20 000 mg/kg in male Long Evans rats. When ACV was given iv, the LD50 was 405 mg/kg for male mice and greater than 600 mg/kg for male rats. Additionally, LD50 values for male rats treated sc were 1070, 790, 678, and 650 mg/kg in rats that were respectively, 3, 10, 28 and 71 days old indicating that very young rats were not more sensitive to acute toxic effects of ACV. There were no signs of toxicosis in CD-1 mice given ACV by gavage at dose levels of 50, 150 and 450 mg/kg/day for 1 month. Obstructive nephropathy occurred in rats given 20, 40 and 80 mg/kg/day once each day by rapid iv injection for 3 weeks. Both 5 and 10 mg/kg/day were no effect dose levels. Renal damage caused by precipitation of drug crystals in renal tubules and collecting ducts in rats given ACV by rapid iv injection was readily reversible within 2 weeks. Beagle dogs were given doses of 10, 20, 25, 50 and 100 mg/kg b.i.d. by rapid iv injection for 1 month. All 8 dogs given 100 mg/kg b.i.d. died by the 8th day of treatment; 5 of 8 dogs given 50 mg/kg b.i.d. died after 21 to 31 days of treatment.(ABSTRACT TRUNCATED AT 250 WORDS)

Preclinical toxicology studies with acyclovir: Carcinogenicity bioassays and chronic toxicity tests

Acyclovir (ACV), a nucleoside analog that is a new herpes-specific antiviral drug, was given by gavage at 50, 150 and 450 mg/kg/day to Sprague Dawley rats and Swiss mice for most of their lifetime to assess chronic toxicity and carcinogenicity. Treatment with ACV did not shorten the lifespan of either rats or mice. In fact, female mice given 150 and 450 mg/kg/day had significantly longer mean durations of survival than control female mice when analyzed by the life table technique. There were no signs of toxicosis produced by chronic exposure to ACV in either the rats or mice, and there was no drug-related increase in neoplasms in either species. Four groups of Beagle dogs were initially given daily oral doses of 15, 45 or 150 mg/kg ACV in a 1 year chronic 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 gastrointestinal toxicosis. These dose levels were then decreased to 60 and 30 mg/kg/day for the rest of the one year test period. With the exception of occasional and inconsistent emesis and diarrhea, the 60 mg/kg/day dose level was well tolerated. Some mid and high dose dogs had sore paws due to erosion of footpads and cracking, splitting and loosening of the nails first becoming evident during the 13th week of the study.(ABSTRACT TRUNCATED AT 250 WORDS)

Postnatal survival in wistar rats following oral dosage with zidovudine on gestation day 10

Groups of 20 female Wistar rats from Charles River Breeding Laboratories (Kingston, NY) were given three oral doses of 100 mg zidovudine/kg at 5-hr intervals on Gestation Day 10 (total dose = 300 mg/kg). Control rats received three oral doses of the vehicle, distilled water. This design approximated that of an earlier study that reported 38% postnatal mortality among the offspring of Wistar rats given zidovudine. In the study reported here, no adverse effects were noted on maternal body weight, food consumption, reproductive capacity, or hematology. Similarly, no effects on growth or survival of the offspring were noted. Hematology and clinical chemistry values were comparable between offspring of treated and control dams, and no treatment-related gross or histopathologic lesions were noted in the weanling rats. The mean concentration of zidovudine in embryonal homogenates, collected 30 min after administration of the third dose to the dam on Gestation Day 10, was 21.1 micrograms/g tissue. This value is approximately one-third of the mean drug plasma concentration (62.6 micrograms/ml) measured in the dams at the same time point. The dramatic difference in results in the two studies may be related to differences in Wistar rats from two different sources or to other unknown factors associated with the design and conduct of the studies. The results of the current study were consistent with other preclinical studies on the reproductive toxicity of zidovudine in rats and rabbits.

Preclinical Toxicology Studies with Acyclovir: Ophthalmic and Cutaneous Tests

Topical formulations of acyclovir (ACV) were tested in animals to define potential for tissue irritation and systemic toxicity. Acyclovir ointments (5 and 10% concentrations in polyethylene glycol vehicle) produced no sign of dermal irritation or systemic toxicity when applied to shaved abraded and intact skin of guinea pigs for 24 consecutive days. Solutions (0.9% normal saline vehicle) of ACV did not sensitize guinea pigs when 10 sensitizing doses and a challenge dose were injected intradermally. Petrolatum base ophthalmic ointments containing 1 and 3% ACV did not produce significant ocular irritation when applied to the corneas of New Zealand White rabbits 5 times each day for 21 consecutive days. A 6% petrolatum base ointment produced mild conjunctival irritation but no sign of corneal or iridic toxicity. Mean concentrations of 2.53 microM ACV were found in aqueous humor 2 hours after a 1 cm ribbon (21 mg) of 3% ophthalmic ointment was placed in the eyes of rabbits. A single treatment with a topical ointment containing 5% ACV in polyethylene glycol vehicle produced minimal irritation when placed in the eyes of New Zealand White rabbits.

Nail loss and Footpad Erosions in Beagle Dogs Given BW 134U, a Nucleoside Analog*

The oral administration of BW 134U to Beagle dogs in a 1-month study was associated with lameness, footpad erosions, and nail loss. Groups of dogs received 60, 120, and 240 mg/kg/day. Compound-related effects were observed in the high dose group and only during the postdose period. Light microscopy revealed prominent cell maturation or radiomimetic defects in the stratum germinativum of the distal phalanges and footpads. The mechanism by which BW 134U produced these cell maturation defects is unknown. There were no signs of adverse effects in other keratin-producing or keratin-containing tissues.

Dose Selection as it Pertains to Testing a Prodrug in Carcinogenicity Bioassays

Toxicologists need more information than is usually available in the early stages of development of a drug in order to choose proper dose levels for testing in the bioassays. The approach most likely to result in successful bioassays involves an early multidisciplinary effort in which there is pharmacokinetic characterization of the test material in both rats and mice. Preliminary 3 month studies are desirable. Periodic sampling of plasma is essential to detect possible non-linear kinetics (as in the example we report herein) reflected as accumulation of the test material or metabolites. This is true regardless of the test substance. However, if one tests prodrugs it may be particularly helpful to know if chemical or enzymatic conversion of the prodrug is linear and if there is reversion to prodrug or other abberant metabolism. Failure to rule out these possibilities could result in subsequent clinically irrelevant organ damage or could compromise longevity or the interpretation of results in lifetime studies. Pharmacokinetic considerations are as valid as the more traditional biologic or morphologic end points used to estimate maximum tolerated or no-effect dose levels.