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U.S. Department of Health and Human Services

Animal & Veterinary

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NADA 141-099 CYDECTIN® (moxidectin) 0.5% Pour-On for Cattle - original approval (continued)

V. TARGET ANIMAL SAFETY

 

Specific nonclinical laboratory experiments were carried out in accordance with the applicable Good Laboratory Practices (GLP) regulations (i.e., 21 CFR Part 58) in the target species to address the following potential safety-in-use considerations: toxic syndrome as defined by the results of a drug tolerance test, margin of safety as delineated by a toxicity study with consecutive-day treatment at up to five times the recommended level, ocular exposure and three separate trials designed to evaluate reproductive safety in breeding bulls and estral and pregnant cows. All studies were performed with the final moxidectin pour-on formulation which was evenly applied in accordance with label directions.

A. Drug Tolerance Test - Study Number B-93-7

1. Type of Study: The drug tolerance test is designed to determine the dose level of an animal drug product which will elicit a toxic syndrome and provide the basis for recognition of a drug reaction attributable to product use in the target species.

2. Investigator: W. B. Epperson, D.V.M.
Cyanamid Agricultural Research Center
Princeton, NJ

3. General Design:

a. Purpose: This study was designed to evaluate the clinical and pathological effects of application of moxidectin 0.5% pour-on at dose rates of 5X the recommended dose (2.5 mg moxidectin/kg body weight) for five consecutive days, 10X the recommended dose (5.0 mg moxidectin/kg body weight) for two consecutive days, or 25X the recommended dose (12.5 mg moxidectin/kg body weight) in a single application.

b. Animals: Eight Angus crossbred steers (4) and heifers (4), approximately 12 months of age, and weighing 257 to 284 kg were allotted to four treatment groups (one male, one female per group).

c. Control: Vehicle

d. Dosage Form: Moxidectin 0.5% pour-on, 5 mg/mL

e. Route of Administration: Topically along the back from the withers to the tailhead.

f. Doses: Pour-on vehicle (no moxidectin) - 5 mL/10 kg body weight (providing 0 mg moxidectin/kg body weight) applied daily for five consecutive days.

5X dose of moxidectin 0.5% pour-on - 5 mL/10 kg body weight (providing 2.5 mg moxidectin/kg body weight) applied daily for five consecutive days.

10X dose of moxidectin 0.5% pour-on - 10 mL/10 kg body weight (providing 5 mg moxidectin/kg body weight) applied daily on two consecutive days.

A single application of a 25X dose of moxidectin 0.5% pour-on - 25 mL/10 kg body weight (providing 12.5 mg moxidectin/kg body weight).

g. Test Duration: 19 days

h. Pertinent Measurements/Observations: Physical examinations were conducted on Day -1 (day before treatment) and on the day of sacrifice. Body weight was measured on the day of allotment to treatment groups, on Experimental Day 1 which was the first day treatment was administered, and on the day of sacrifice. Venous blood samples, urine and feces were collected on Day -1 pre-treatment and Days 1, 2, 3, 4, 7, and 14 after treatment for hematology, clinical chemistry, urinalysis and fecal analysis. Health observations were made once hourly in the four hours after treatment on each treatment day. Thereafter, health observations were recorded twice daily and unforeseen circumstances were recorded once daily. Amounts of feed offered and consumed were recorded throughout the experiment. Necropsies were performed on Day 12, (7 days posttreatment) or Day 19 (14 days posttreatment) and all animals were evaluated for macroscopic pathology. All tissues from all treatment groups were examined microscopically.

4. Results:

a. Clinical Observations: Run-off (loss of product to the ground) was observed in the immediate post-application period in the test animals which received the 10X and 25X doses. Transient, mild salivation was observed in the two animals receiving the 5X dose level following the first treatment. The hypersalivation dissipated within one hour of treatment. A slight increase in salivation was again seen in one of the 5X-treatment animals following the Day 3 application.

The excessive salivation once more subsided to normal levels within one hour posttreatment. No test animals showed any other clinical signs attributable to pour-on treatment in the 7- to 14-day posttreatment observation period. No signs of irritation at the treatment site were noted at any observation throughout the study.

b. Feed and Water Consumption: Feed intakes were considered to be within normal variation for all animals in the study.

c. Body Weight: All calves in all groups gained weight over the time from treatment to necropsy.

d. Hematology/Serum Chemistry: Blood samples obtained from test animals at critical points throughout the study revealed no significant serum chemistry or hematology abnormalities.

e. Urinalysis: No test article-related effects were observed.

f. Fecal Examination: No test article-related effects were observed.

g. Gross and Histopathologic Observations: Euthanasia and postmortem examinations were performed on one animal per group on Day 7 following the last treatment. The remaining test animals were necropsied on Day 14 following the last treatment. No gross lesions suggestive of treatment-related toxicity were observed at necropsy. Similarly, microscopic evaluation of 41 different tissues (representative of all major organ systems) obtained at necropsy revealed no histopathologic changes indicative of a toxic drug effect.

5. Statistical Analysis: None

6. Conclusion: Application of moxidectin 0.5% pour-on solution at 5X the recommended dose for 5 consecutive days, 10X for 2 consecutive days and 25X for one day did not produce any significant adverse clinical or pathological effects.

B. Toxicity Test (1X, 3X, 5X Study) - Study Number B-93-6

1. Type of Study: The objective of the target animal toxicity study is to determine the safety of the drug product to the target animal and the signs and predictable effects (if any) of product-related toxicity.

2. Investigator: W. B. Epperson, D.V.M.
Cyanamid Agricultural Research Center
Princeton, NJ

3. General Design:

a. Purpose: The purpose of this study was to evaluate the clinical and pathological effects of application of moxidectin 0.5% pour-on at 0.5, 1.0, and 2.5 mg moxidectin/kg body weight (1X, 3X, and 5X, respectively) along with a control (0 mg/kg body weight) for three consecutive days in cattle.

b. Animals: Sixteen Angus crossbred steers (8) and heifers (8) weighing between 266 and 296 kg at treatment and 10-12 months of age were randomly assigned to four treatment groups (2 males and 2 females).

c. Control: Vehicle

d. Dosage Form: Moxidectin 0.5% pour-on, 5 mg/mL

e. Route of Administration: Topically along the back from the withers to the tailhead.

f. Doses: Pour-on vehicle (no moxidectin) - 5 mL/10 kg body weight (providing 0 mg moxidectin/kg body weight) applied daily for three consecutive days.

1X dose of moxidectin 0.5% pour-on - 1 mL/10 kg body weight (providing 0.5 mg moxidectin/kg body weight) applied daily for three consecutive days.

3X dose of moxidectin 0.5% pour-on - 3 mL/10 kg body weight (providing 1.5 mg moxidectin/kg body weight) applied daily for three consecutive days.

5X dose of moxidectin 0.5% pour-on - 5 mL/10 kg body weight (providing 2.5 mg moxidectin/kg body weight) applied daily for three consecutive days.

g. Test Duration: 25 days

h. Pertinent Measurements/Observations: Physical examinations were conducted on Day -1 (day before treatment) and on the day of sacrifice. Body weight was measured on the day of allotment to treatment groups, on Experimental Day 1 which was the first day treatment was administered, and on Day 22. Venous blood samples, urine (free catch) and feces were collected on Day -1 pretreatment and Day 22 after treatment for hematology, clinical chemistry, urinalysis and fecal analysis.

Health observations were made once hourly for four hours after treatment on each treatment day. Thereafter, health observations were recorded twice daily and unforeseen circumstances were recorded once daily. Amounts of feed offered and consumed were recorded throughout the experiment. Necropsies were performed on Day 23 to Day 25 and all animals were evaluated for macroscopic pathology. All tissues from all treatment groups were examined microscopically.

4. Results:

a. Clinical Observations: Four animals (one in the 5X, one in the 3X and two in the vehicle group) showed very mild signs of increased salivation after the first treatment. Salivation started after treatment administered and continued through one hour posttreatment. On the second and third days of treatment only one animal in the vehicle group exhibited excess salivation. Twice daily observations indicated no noticeable adverse effects in the posttreatment period.

b. Feed and Water Consumption: There was no difference in feed intake between treated groups over the treatment and posttreatment period.

c. Body Weight: Average weight gains were similar for all groups. Statistical analysis of body weight gain revealed no significant differences between treatment groups.

d. Hematology/Serum Chemistry: A comparison of pre- and posttreatment hematology, and clinical chemistry values indicated no biologically significant changes.

e. Urinalysis: No test article-related effects were observed.

f. Fecal Examination: No test article-related effects were observed.

g. Gross and Histopathologic Observations: Euthanasia and necropsy were performed on all animals between 20 and 22 days following the last treatment. No gross lesions suggestive of treatment-related toxicity were observed at necropsy. Similarly, microscopic evaluation of 41 different tissues (representative of all major organ systems) obtained from the high-dose and placebo treatment groups at necropsy revealed no histopathologic changes indicative of a toxic drug effect.

5. Statistical Analysis: Results of hematology, clinical chemistry variables, feed intake and body weight gain of cattle were examined by analysis of variance (ANOVA) and analysis of covariance (ANCOVA).

6. Conclusion: Application of moxidectin 0.5% pour-on solution at 1X, 3X and 5X the recommended dose for three consecutive days did not produce any significant adverse clinical or pathological effects.

C. Ocular Safety - Study Number B-93-8

1. Type of Study: The study was performed to evaluate the irritant effects of direct exposure of the bovine eye to moxidectin pour-on.

2. Investigator: W. B. Epperson, D.V.M.
Cyanamid Agricultural Research Center
Princeton, NJ

3. General Design:

a. Purpose: The purpose of this study was to evaluate the effect of accidental application of moxidectin 0.5% pour-on into the eyes of cattle.

b. Animals: Four Angus crossbred steers weighing approximately 280 kg and between 10 and 12 months of age were used.

c. Control: Sterile saline

d. Dosage Form: Moxidectin 0.5% pour-on, 5 mg/mL

e. Route of Administration: Intraocular instillation directly into the lower fornix of the eye.

f. Doses: Three test animals were exposed to direct intraocular instillation of different volumes (0.25, 0.5 and 1 mL) of the moxidectin pour-on solution. The fourth animal served as a sham-treated control and was administered a similar 1 mL sterile saline treatment.

g. Test Duration: 7 days

h. Pertinent Measurements/Observations: Ocular exams were performed on Day -1 (day before treatment) and Day 0 (day of treatment) prior to treatment and one hour posttreatment. Ocular exams were conducted daily until Day 6. Fluoroscein staining was performed as part of all ocular exams with the exception of the two exams performed on Day 0. A physical exam was conducted Day -1, and on Day 7, the final day of the study. The animals were observed twice daily, starting on Day -7 and ending on Day 7.

4. Results: Results of ocular and physical exams before treatment indicated that all animals were normal. In all cases the moxidectin pour-on was quickly removed from the eye by tearing and it resided in the periocular region below the eye by one hour posttreatment. No ocular abnormalities, aside from discharge, were noted at one hour posttreatment. There was no evidence of ocular inflammation caused by any of the treatments at any of the posttreatment examinations.

5. Statistical Analysis: None

6. Conclusion: A single application of up to a 5X dose of moxidectin 0.5% pour-on placed in the bovine eye did not create an adverse ocular reaction.

D. Breeding Bull Safety - Study Number B-92-24

1. Type of Study: Reproductive safety study in bulls.

2. Investigator: Robert G. Mortimer, M.S., D.V.M. Colorado State University Agricultural Campus Fort Collins, CO

3. General Design:

a. Purpose: To assess the effects of moxidectin at 3X the recommended dose on seminal quality and breeding performance of bulls.

b. Animals: Twelve sexually mature, virgin Hereford bulls, 20 to 25 months of age, were randomly divided into two treatment groups, with ten bulls in the moxidectin treatment group and two bulls in the control group. A total of 120 heifers, approximately 1 to 2 years old and weighing 302 to 497 kg were used for the breeding phase.

c. Control: Vehicle

d. Dosage Form: Moxidectin 0.5% pour-on, 5 mg/mL

e. Route of Administration: Topically along the back from the withers to the tailhead.

f. Doses: Ten bulls were treated with a 3X dose of moxidectin pour-on [3 mL/10 kg body weight (1.5 mg moxidectin/kg body weight)] which was applied in accordance with label directions. The two remaining bulls received a similar placebo treatment (vehicle) containing no moxidectin.

g. Pertinent Measurements/Observations: Breeding soundness exams (BSE) were conducted in accordance with the 1992 Society of Theriogenologists guideline. Two consecutive ejaculates of semen per day were collected and evaluated on Days -5, -3, and -1 pretreatment and Days 7, 14, 21, 28, 35, 42, 49, 56, 63, 70, 77, 84, 91, 98, 105, 112 and 119 posttreatment. Semen volume and concentration were also determined for each BSE day. Each bull was pastured with ten prostaglandin synchronized (dinoprost tromethamine administered twice 13 days apart) heifers for a 60-day breeding period. Libido assessments were made throughout the initial week of the breeding period. Pregnancy status of each heifer was determined 45 days following the end of the breeding period.

4. Results: Results of scrotal circumference, percent progressive mortality, sperm count per ejaculate, percent normal morphology, and sperm abnormalities are summarized in the table below. There were no significant differences in these parameters between the control and moxidectin-treated groups.

Response VariableGroup LS Mean/Mean for Control BullsGroup LS Mean/Mean for Moxidectin-Treated Bulls
Scrotal Circumference (cm)37.32a37.08a
*Progressive Motile (%)70.88a69.08a
Sperm per Ejaculate (106)4499.873860.11
Sperm per Ejaculate (log transformed count)3.6532a3.5866a
Morphology, Normal (%)88.22a88.17a
SPERM ABNORMALITY:  
Head (%)3.31a5.21a
*Midpiece (%)0.11a0.18a
Distal Midpiece Reflex (%)5.41a3.88a
*Tail (%)0.02a0.18a
Proximal Droplet (%)0.95a1.16a
*Distal Droplet (%)1.00a0.50a
*Other (%)0.86a0.10a
*Other Cells0.00a0.00a
*Covariate was not significant alpha=0.05

aLS Mean/Mean within a row with the same superscript did not differ at alpha=0.10.

There was no adverse effect on libido and ability to inseminate and impregnate females. The overall conception rates for the moxidectin group was 97% and 100% for the control group. There was no significant difference in the pregnancy rate between the treated and control animals. The proportion of bulls with 100% conception rate for the controls was not significantly different from that for the moxidectin-treated group, as analyzed by a Chi-square test at alpha=0.1.

5. Statistical Analysis: Weighted averages of two ejaculates per day were calculated from percent motility, sperm per ejaculate, percent normal sperm and percent individual sperm cell abnormalities, and a single measure of scrotal circumference was used for statistical analyses. Statistical analysis of covariance was applied to determine the treatment effect, with the pretreatment value as a covariate. A two-sided Student's t-test was used to compare the mean pretreatment values for different response variables in the control and the treated groups at alpha=0.10. The proportion of cows confirmed pregnant and the proportion of bulls making 100% of the assigned cows pregnant were compared between the control and the treated groups by Chi-square test at alpha=0.10.

6. Conclusion: A single topical application of moxidectin 0.5% pour-on at 1.5 mg/kg body weight (3X the recommended dose) on bulls did not have an adverse effect on scrotal circumference, serum quality, libido, and breeding performance.

E. Estral Cow Safety - Study Number B-92-22

1. Type of Study: Reproductive safety study in cows before, during, and after estrus and during early gestation.

2. Investigator: D. Owen Rae, D.V.M., M.P.V.M.
University of Florida
Beef Research Unit
Gainesville, FL

3. General Design:

a. Purpose: The objective of this study was to assess the effects of moxidectin at 3X the recommended dose on folliculogenesis, ovulation, postovulation, pregnancy, and calving rate in estrous cycling cows.

b. Animals: One hundred and eighty two pastured Angus and crossbred beef cows, averaging 3 years of age were maintained under similar conditions. Cows were randomly assigned to five groups of 28 cows and three groups of 14 cows.

c. Control: Vehicle or no treatment

d. Dosage Form: Moxidectin 0.5% pour-on, 5 mg/mL

e. Route of Administration: Topically along the back of the withers to the tailhead.

f. Doses: After each cow was synchronized for estrous with a norgestomet implant and a norgestomet/estradial valerate injection, the test animals in the 28-cow groups were treated at least once with 3X dose levels of moxidectin pour-on [3 mL/10 kg body weight (1.5 mg moxidectin/kg body weight)] at one of five timepoints calculated to correspond with critical phases of the estrous cycle [folliculogenesis (Day -26 and Day -2), ovulation (Day 0) and 7, 14, 28 days post ovulation]. Two of the 14-cow groups were administered identical volumes of the formulation without moxidectin at multiple points during their estrous cycle consistent with the timing of the 28-cow group treatments. The last 14-cow group received no treatment.

g. Test Duration: 18 months

h. Pertinent Measurements/Observations: All cows were artificially inseminated on Day 0. Any cows returning to estrus after the AI breeding were bred naturally. At Day 48 or 51 (following the AI breeding), each cow was assessed rectally for pregnancy status, days pregnant, and body condition score. Cows determined to be pregnant to the Day 0 insemination were used in the determination of first service conception rate. Cows that calved within 290 days of the first service were used in the determination of the first service calving rate. All cows that calved were used to determine the total calving rate for all cows in the study. Only the cows calving as the result of the first breeding were used for the generation of the calving data (i.e., calf birth and 60 day weights, gross calf abnormalities, and health of the calf at birth and at the 60 day weighing). The primary parameters were first service pregnancy rate, first service calving rate, and calf normality (health, weight, growth to 60 days, and lack of congenital abnormalities). Adverse reactions to treatment, cow body weight, body condition score, and the total pregnancy and calving rates for all cows in the study were secondary parameters.

4. Results: The average pregnancy rate after the first insemination ranged from 46.4% to 64.3%. The total pregnancy rate based on palpation and calving information ranged from 89.3% to 100%. However, there were no significant differences in these pregnancy rates among the 8 treatment groups. No adverse reactions to moxidectin treatment were observed at the application site or in the general condition of the cows on treatment day and for the duration of the trial. The following table summarizes the number of cows per group, pregnancy and calving rates for the first service conception and total pregnancy and calving rates for all cows in the study.

GroupNo. Cows in GroupTime of Treatment# Cows Pregnant 1st servicePreg Rate 1st ServiceTotal Preg RateNo. Cows Calving 1st ServiceCalving Rate 1st Service
128Moxidectin Day -26, and 21657.1%96.4%1657.1%
228Moxidectin Day 01553.6%89.3%1450.0%
328Moxidectin Day 71657.1%89.3%1553.6%
428Moxidectin Day 141346.4%96.4%1346.4%
528Moxidectin Day 281346.4%92.9%1346.4%
614Vehicle Days -26, -2, 7, 28750.0%92.9%750.0%
714Vehicle Days 0, 7, 14964.3%100%964.3%
814No treatment964.3%100%964.3%

The average calf weights at birth were relatively uniform (27.8 to 31.9 kg) and were not significantly different from each other. The majority of calves were healthy at birth except one stillbirth in each of Groups 2, 3, and 8. Another calf in Group 3 was born alive, but was weak and low in birth weight (12.7 kg). The calf was raised as an orphan due to poor maternal traits of the cow. No significant differences were observed in calf health among the eight treatment groups.

At the 60 day weighing, calves from the first service conceptions had an average weight range from 84.8 kg to 90.9 kg. There were no significant differences among the eight treatment groups. All calves were healthy at the 60 day weighing.

5. Statistical Analysis: Pregnancy rates and calving rates were analyzed by a Chi-square test. Calf weights were analyzed by a one-way analysis of variance with treatment groups compared using the least significant difference procedure. All comparisons were performed at the 0.10 level.

6. Conclusion: Cows treated with moxidectin 0.5 pour-on topically at 3 times the recommended dose (1.5 mg moxidectin/kg body weight) at folliculogenesis, ovulation, or postovulation on Days 7, 14, or 28, were not adversely affected in terms of conception, pregnancy maintenance, and the development, growth, or health of the fetus or calf.

F. Pregnant Cow Safety - Study Number B-93-1

1. Type of Study: Reproductive safety study in cows during early, mid, and late gestation.

2. Investigator: Larry R. Cruthers, Ph.D.
Professional Laboratory and Research Services
Corapeake, NC

3. General Design:

a. Purpose: The purpose of this study was to determine the effects of moxidectin 0.5% pour-on at 3X (1.5 mg moxidectin/kg body weight) the recommended dose (0.5 mg moxidectin/kg body weight) on the general health, gestation characteristics, calf weights, calf deformities, and number of live calf births in three different groups of pregnant cows treated during early, mid or late gestation.

b. Animals: One hundred and eighty mixed-breed beef cows of average age between 5 and 6 years that had produced at least one live calf previously were bred by natural service. Upon confirmation of pregnancy, the cows were assigned to four groups, I, II, III, and IV with 45 cows per group. The control group, I, consisted of three subgroups, Ia, Ib, and Ic, with 15 cows per subgroup and were treated with carrier vehicle during the first (Ia), second (Ib), and third (Ic) trimester of gestation. Each subgroup was paired with their respective treatment groups (II, III, and IV) and treated with carrier vehicle at the same time the moxidectin pour-on was applied to groups II, III, and IV. Most of the cows were nursing calves at the time of breeding.

c. Control: Vehicle

d. Dosage Form: Moxidectin 0.5% pour-on, 5 mg/mL

e. Route of Administration: Topically along the back of the withers to the tailhead.

f. Doses: Cows in the first three groups were treated with 3X dose levels of moxidectin pour-on [3 mL/10 kg body weight (1.5 mg moxidectin/kg body weight)] in accordance with label directions on three separate occasions during either early - (Group I - treated on days 55, 80 and 105 of gestation), mid- (Group II - treated on days 130, 155, and 180 of gestation) or late-gestation (Group III - treated on days 205, 230, and 255 of gestation). The fourth group was divided into three 15-cow control subgroups which were each treated with identical volumes of a placebo formulation concurrently with either Group I, II, or III.

g. Test Duration: 1 year

h. Pertinent Measurements/Observations: Calving rate, calf birth weight, 60-day weight, and average daily gain of calves were the parameters of interest.

4. Results: There was no significant difference in the number of healthy calves born to cows in any of the four treated groups or the health of the calves. The following table shows the number of calves born for each treatment group.

 

Treatment GroupGroup ID# of Cows# of Cows w/o Calves (%)# of Live Calves (%)*# of Dead Calves (%)
Early Gestation     
Vehicle controlIa152 (13.3%)13 (86.7%)0
MoxidectinII453 (6.7%)38 (84.4%)4** (8.9%)
Mid Gestation     
Vehicle Control1b152 (13.3%)13 (86.7%)0
MoxidectinIII452 (4.4%)40 (88.9%)3 (6.7%)
Late Gestation     
Vehicle ControlIc15015 (100%)0
MoxidectinIV44**3 (6.8%)40 (90.9%)1 (2.3%)

*Three additional calves were born and orphaned. They were not included in the analysis due to uncertainty over their dams and treatment groups.

**One calf in this group was born alive and died within 2 hours. For statistical purposes, the calf was handled as a dead calf.

***One cow in this group died before treatment and was not replaced.

There were no statistically significant treatment effects on live births, dead births, or cows not producing a calf. Of the 179 cows participating in the trial, 170 cows produced a calf, of which 8 calves did not survive. Five of these calves were dystocias and assistance was necessary to complete the birth. No gross abnormalities were observed in any of the dead calves, with the exception of one calf in Group III that had an atrial septal defect that was not of enough significance to cause death. There were no abnormalities noted in any of the calves at the observations made at less than 48 hours postpartum or at examination at approximately 60 days. The following table summarizes the calf data. The three orphaned calves are not included in the table or the analysis.

 

Treatment GroupGroup IDWeight 1 (lb)Weight 2 (lb)ADG (lb/day)
Early Gestation    
Vehicle controlIa74.6a180.7a1.77a
MoxidectinII74.3a187.6a1.90a
Mid Gestation    
Vehicle controlIb73.7a188.3a1.87a
MoxidectinIII77.1a194.3a1.94a
Late Gestation    
Vehicle controlIc69.4a192.1a1.85a
MoxidectinIV75.8b192.8a1.93a

a,bMeans of control and treated groups for a common variable and the same gestation phase with different superscripts differ at P<0.10.

5. Statistical Analysis: Each of the moxidectin treated groups of cows were compared to their respective controls. Cow weights and body condition scores were tested for statistical significance using a one-way analysis of variance at the 10% level. The effect of treatment on live births produced from cows in the various treatment groups were analyzed by first coding each birth as 1 for a dead birth; 2 for a live birth; or 0 for no birth. These data were analyzed using Fisher's Exact Test for a 2X3 contingency table (2 treatments and 3 categories) and also a Chi-square test at the 10% level. Calf weights, both at less than or equal to 2 days of age (weight 1) and at approximately 60 days of age (weight 2) , as well as average daily gain over this time period, were analyzed using ANOVA with treatment, calf sex, and treatment X calf sex as sources of variation.

6. Conclusion: This trial demonstrated that moxidectin 0.5% pour-on was safe when administered to pregnant beef cows at 3X the recommended dose on three occasions during early, mid, or late gestation. There were no negative effects of treatment on the number of live calves born, the health of the calves, their weights during the first 48 hours postpartum and at approximately 60 days of age, or their average daily gain over this time period.

G. Safety in Youngest Age Tested - Study Number 0863-B-US-10-96

1. Type of Study: The objective of the study was to evaluate the safety of the product in newborn calves.

2. Investigator: Larry R. Cruthers, Ph.D.
Professional Laboratory and Research Services, Inc.
Corapeake, NC

3. General Design:

a. Purpose: The study was designed to evaluate the clinical effects of moxidectin 0.5% pour-on in newborn calves treated at the recommended dose level (0.5 mg/kg body weight) or 3X the recommended dose level (1.5 mg/kg body weight) and concurrently nursing from dams treated at the recommended dose level.

b. Animals: Thirty-one cross-bred beef calves, weighing between 22.7 and 47.6 kg, and their dams were randomly assigned to one of three treatment groups. The study was designed to have ten animals per treatment group; however, the dam of one calf treated with the recommended dose level did not allow its calf to nurse. This calf was removed from the study on Day 8 and a replacement calf was subsequently added to this treatment group.

c. Controls: vehicle

d. Dosage Form: moxidectin 0.5% pour-on, 5 mg/mL

e. Route of Administration: Topically along the back from the withers to the tailhead

f. Doses: pour-on vehicle (no moxidectin) - applied to calves at 3 mL/10 kg body weight (providing 0 mg moxidectin/kg body weight) and their dams at 1 mL/10 kg body weight (providing 0 mg moxidectin/kg body weight) in a single application within 12 hours of parturition.

1X dose moxidectin 0.5% pour-on - applied to calves at 1 mL/10 kg body weight (providing 0.5 mg moxidectin/kg body weight) and their dams at 1 mL/10 kg body weight (providing 0.5 mg moxidectin/kg body weight) in a single application within 12 hours of parturition.

3X dose moxidectin 0.5% pour-on - applied to calves at 3 mL/10 kg body weight (providing 1.5 mg moxidectin/kg body weight) and their dams at 1 mL/10 kg body weight (providing 0.5 mg moxidectin/kg body weight) in a single application within 12 hours of parturition.

g. Test Duration: 14 days

h. Pertinent Parameters Measured: Health observations of the calves and cows were made twice daily, once in the morning and once in the afternoon, for 14 days posttreatment. Blood was sampled from calves pretreatment and on Days 2 and 7 posttreatment for analysis of serum iron. Ocular examinations were performed on calves on Days 1, 2 and 3 posttreatment.

4. Results:

a. Clinical Observations: Twice daily observations over a two-week posttreatment period indicated no noticeable adverse reactions in any of the calves or cows that were attributed to treatment with moxidectin.

b. Ocular Examinations: No treatment-related abnormalities were observed in calves on Days 1, 2 or 3 posttreatment.

c. Serum Iron Analyses: All pretreatment and Day 2 posttreatment serum iron values were within the normal range for calves. The average Day 7 serum iron values were elevated and outside of the normal range (57 to 162 µg/dL) for calves in the control and the two treated groups. There were no significant differences in serum iron between control and treated calves either pretreatment or on Days 2 and 7 posttreatment.

5. Statistical Analysis: None

6. Conclusions: Moxidectin 0.5% pour-on applied at 1X and 3X the recommended dose level to newborn calves (<12 hours old) who were nursing dams concurrently treated with the recommended dose level, did not produce any adverse clinical effects, ocular signs or serum iron concentrations.

 

VI. HUMAN FOOD SAFETY

A. Toxicology

Genotoxicity Studies

Moxidectin was tested in a battery of four different short-term genetic toxicity experiments. These studies included a bacterial/microsome mutagenicity (Ames) test, a mammalian cell CHO/HGPRT mutagenicity assay, an in vivo chromosomal aberration test and an unscheduled DNA synthesis assay. The moxidectin test article used in all four of these tests had a purity of 88.5%. Specific information pertaining to the conduct and outcome of these four genotoxicity studies is briefly summarized below.

1. Bacterial/Microsome Mutagenicity Assay (Ames Test)

a. Identification: Study performed by K. A. Traul and staff at American Cyanamid's Princeton, New Jersey testing facility. Experimental period: July 5, 1989 to March 22, 1990. Cyanamid Genetic Toxicology Study No.: 89-02-01.

b. Procedure: The microbial/microsome mutagenicity plate incorporation assay was used to determine the ability of the test article (moxidectin) at concentrations ranging from 54 to 2000 µg/plate with and without added metabolic activation in the form of Aroclor-induced rat liver S-9 to cause genetic damage (either base-pair substitution or frame shift mutation) in the following six bacterial tester strains: Salmonella typhimurium TA98, TA100, TA1535, TA1537, and TA1538 and E. coli WP-2uvrA-. Five dose levels were used with three replicates per dose point. The high dose level was limited by the solubility of moxidectin in the test system. The assay was done twice to confirm the results.

c. Findings: Moxidectin was not mutagenic at dose levels up to and including 2000 µg/plate in any of the six bacterial tester strains.

2. Mammalian Cell CHO/HGPRT Mutagenicity Test

a. Identification: Study performed by K. A. Traul and staff at American Cyanamid's Princeton, New Jersey testing facility. Experimental period: July 27, 1989 to April 9, 1990. Cyanamid Genetic Toxicology Study No.: 89-05-002.

b. Procedure: The mammalian cell mutagenicity assay was used to determine the ability of the test article (moxidectin) at concentrations ranging from 5 to 10 µg/ml with and without added metabolic activation in the form of Aroclor-induced rat liver S-9 to cause genetic damage (mutations) at the HGPRT locus of the Chinese hamster ovary (CHO) K1BH4 cells present in the test system. Five dose levels were used with two replicates per dose point. The assay was done twice to confirm the results.

c. Findings: The results of the CHO/HGPRT Mutagenicity Test were inconclusive and unresolved.

3. In Vivo Chromosome Aberration Assay in Rat Bone Marrow Cells

a. Identification: Study performed by R. K. Sharma and staff at American Cyanamid's Princeton, New Jersey testing facility. Experimental period: October 27, 1989 to April 10, 1990. Cyanamid Genetic Toxicology Study No.: 89-14-002.

b. Procedure: Male and female Sprague-Dawley rats were given a single oral administration of either 15, 30 or 60 mg moxidectin/kg body weight by gavage and euthanized either 12, 24 or 48 hours post treatment for the collection of bone marrow. The test animals were treated with intraperitoneal injections of colchicine two-three hours prior to sacrifice to arrest dividing cells at metaphase. The bone marrow cells were analyzed microscopically for evidence of chromosomal aberrations.

c. Findings: The results of the In Vivo Chromosome Aberration Assay in Rat Bone Marrow Cells were inconclusive and unresolved.

4. Unscheduled DNA Synthesis in Primary Hepatocytes

a. Identification: Study performed by R. D. Curren and staff at testing facilities operated by Microbiological Associates, Inc. in Rockville, Maryland. Experimental period: December 12, 1989 to October 18, 1990. MBA Study No.: T9090.380025.

b. Procedure: Five doses of moxidectin were tested. Test plates seeded with 5 x 105 rat hepatocytes/plate were treated with 0.1 to 5 µg/ml of moxidectin (doses > 10 mg/ml were not analyzed for UDS due to excessive toxicity). All plates received 3H-thymidine at a concentration of 10 µC/ml. Positive and negative controls were run concurrently. After 18-20 hours the plates were processed and slides prepared for microscopic evaluation. Evaluation for UDS was determined by a standardized procedure involving quantification of increases in mean net nuclear count.

c. Findings: Rat hepatocyte cell cultures exposed to moxidectin concentrations of up to 5 µg/ml for 18-20 hours showed no significant increase in unscheduled DNA synthesis.

Repeated-Dose Toxicity Studies

Two dietary toxicity studies were conducted in which moxidectin was fed continuously to laboratory rats and dogs for three months. In both experiments, moxidectin test article with a purity of 81.3% was incorporated in the test animal diet at constant concentrations measured in parts per million (ppm) of moxidectin. Diets were formulated to correct for purity. The dose levels (expressed in mg/kg body weight/day) were calculated from food consumption and body weight data collected as part of each experiment. Specific information regarding the conduct and outcome of these two feeding studies is briefly summarized below.

1. 13-Week Rat Continuous Feeding Study

a. Identification: Study performed by J. E. Fischer and staff at American Cyanamid's Princeton, New Jersey testing facility. Experimental period: April 29, 1988 to August 5, 1988. Cyanamid Toxicology Report No.: AX89-1.

b. Design: Twenty Sprague-Dawley [Crl:CD(SD)BR] rats/sex/group were fed diets with either 0, 25, 50, 100 or 150 ppm concentrations of moxidectin for 13 weeks.

c. Findings: Slight depression in body weight gain in all 100 ppm test animals, increased kidney and adrenal gland weights in 100 ppm females and slight increase in hypersensitivity to touch during first two weeks of experimental period for all 100 and 150 ppm test animals. No signs of toxicity were noted in the 50 and 25 ppm treatment groups.

d. Conclusion: The NOEL for this study was 50 ppm (~3.9 mg/kg body weight/day).

2. 91-Day Dietary Toxicity Study in Dogs

a. Identification: Study performed by G. E. Schulze and staff at testing facilities operated by Hazelton Laboratories America, Inc. in Vienna, Virginia. Experimental period: January 26, 1989 to April 28, 1989. HLA Study No. 362-198.

b. Design: Four purebred Beagle dogs/sex/group were continuously fed diets with either 0, 10, 30 or 60 ppm concentrations of moxidectin for 91 days.

c. Findings: A dose response increase in incidence of lacrimation was observed in treated dogs. At 60 ppm, lacrimation was associated with salivation, tremors, and ataxia. Tremors were also observed in one low-dose male during weeks 1-3 and 13. Statistically significant reductions in food intake and body weight gains were reported in both male and female dogs in the 30 and 60 ppm treatment groups.

d. Conclusion: Based on the dose-response increase in incidence of lacrimation in treated animals and the occurrence of tremors in low-dose males, a NOEL could not be established for this study.

Reproductive Toxicity Studies

The effects of continuous treatment with various levels of moxidectin on fertility and reproductive performance were evaluated in a three generation (two-litter) study carried out in laboratory rats. The moxidectin test article with a purity of 88.6% was incorporated in the test animal diet at constant concentrations measured in parts per million (ppm) of moxidectin. Diets were formulated to correct for purity. The four treatment levels were selected on the basis of a pilot study. The dose levels (expressed in mg/kg body weight/day) were calculated from food consumption and body weight data collected as part of the experiment. This reproductive toxicity experiment is summarized below.

1. Three Generation (Two-Litter) Rat Reproduction Study

a. Identification: Study performed by R. E. Schroeder and staff at testing facilities operated by Bio/dynamics, Inc. in East Millstone, New Jersey. Treatment period: October 11, 1989 to July 16, 1991. Bio/dynamics Project No.: 89-3496.

b. Design: Twenty-five Sprague Dawley [Crl:CD(SD)COBS] rats/sex/group were continuously fed diets with either 0, 1, 2, 5 or 10 ppm concentrations of moxidectin beginning at 10 weeks prior to mating and extending to terminal sacrifice. As diagrammed below, males and female from each treatment group were mated to produce two successive litters. Offspring from the second litter were randomly selected, given the same level of moxidectin their parents had received and mated to also produce two successive second generation litters.

aTreatment periods: P1 generation males (October 11, 1989 to April 26, 1990) and females (October 11, 1989 to May 11, 1990); F1 generation males (May 16 to November 21, 1990) and females (May 16 to December 12, 1990); F2 generation males (December 19, 1990 to June 26, 1991) and females (December 19, 1990 to July 16, 1991).

bF1 parents selected randomly from second P1 litter (F1b) of corresponding treatment group; F2 parents selected randomly from second F1 litter (F2b) of corresponding treatment group.

c. Findings: Reduced body weights in the 10 ppm treatment group were observed in the F1 (statistically significant) and F2 (not statistically significant) parental males. In the 10 ppm treatment group, decreased pup survival was observed in F1a pups during the entire lactation period, in F2a pups mainly between Days 0-4 of lactation and in F3a pups between Days 4-21 of lactation. The decreased pup survival resulted in increased percentages of litters with pup mortality in the 10 ppm F1a, F2a and F3a litters during the same respective times during the lactation period. The 1, 2 and 5 ppm dose level resulted in no treatment-related effects in the P1, F1 and F2 parents, reproductive performance, litter characteristics or the rat pups.

d. Conclusion: The NOEL for this study was 5 ppm (~0.4 mg/kg body weight/day). Developmental Toxicity Studies The teratogenic potential of moxidectin was evaluated in two experiments involving rodent and non-rodent laboratory animal species (rat and rabbit). In both studies, the moxidectin was administered by gavage as part of a corn oil solution which was formulated to correct for the 88.5% (rat) and 85.3% (rabbit) purity of the test material.

Specific information regarding the conduct and outcome of these two teratology studies is briefly summarized below.

1. Oral Developmental Toxicity Study in Rats

a. Identification: Study performed by A. M. Hoberman and staff at testing facilities operated by Argus Research Laboratories, Inc. in Horsham, Pennsylvania. Treatment period: January 2-17, 1989. Argus Project No.: 101-006.

b. Design: Between 23 and 25 pregnant Sprague Dawley [Crl:CD(SD)BR] rats/group were administered either 0, 2.5, 5, 10 or 12 mg/kg body weight/day dose levels of moxidectin in a corn oil solution by gavage on Days 6 through 15 of their gestation period. Cesarean delivery of each dam's litter was accomplished on Day 20 of gestation.

c. Findings: Statistically significant reductions in food consumption were recorded in both the 10 and 12 mg/kg/day treatment groups with consequent decreased maternal body weights in these test animals. Increases in the incidence of fetal alteration (cleft palate) were noted in the 10 mg/kg/day (not statistically significant) and 12 mg/kg/day (statistically significant) treatment groups. Malformations were only seen at maternally toxic levels. No treatment-related effects were noted in the dams or offspring in the 2.5 and 5 mg/kg body weight/day treatment groups.

d. Conclusions: The maternal and developmental NOEL for this study was 5 mg/kg body weight/day. Moxidectin is neither a selective developmental toxicant nor a teratogen in rats.

2. Oral Developmental Toxicity Study in Rabbits

a. Identification: Study performed by E. A. Lochry and staff at testing facilities operated by Argus Research Laboratories, Inc. in Perkasie, Pennsylvania. Treatment period: July 31, 1989 to August 12, 1989 Argus Project No.: 101-007.

b. Design: Between 14 and 18 pregnant New Zealand White [Hra:(NZW)SPF] rabbits/group were administered either 0, 1, 5, or 10 mg/kg body weight/day dose levels of moxidectin in a corn oil solution by gavage on Days 7 through 19 of their gestation period. Cesarean delivery of each dam's litter was accomplished on Day 29 of gestation.

c. Findings: Statistically significant increases in the incidence of abnormal feces (dried, soft or liquid) and decreases in maternal food consumption and weight gain were reported in both the 5 and 10 mg/kg/day treatment groups with consequent decreased maternal body weights in these test animals. The 10 mg/kg/day group had smaller litter size and higher percentage of dead or resorbed conceptuses per litter than the other groups. No gross lesions, soft tissue, or skeletal abnormalities that could be related to treatment were found in the fetuses.

d. Conclusions: The maternal and developmental NOEL for this study were 1 mg/kg body weight/day and 5 mg/kg body weight/day, respectively. Moxidectin is neither a selective developmental toxicant nor a teratogen in rabbits.

Chronic Toxicity and Carcinogenicity Studies

Chronic dietary toxicity studies were conducted in laboratory mice, rats and dogs. In each experiment the test article was incorporated in the test animal diet at constant concentrations measured in parts per million (ppm) of moxidectin and fed continuously for one year to dogs and for two years to the mice and rats. Diets were formulated to correct for the 88.5% (dog) and 81.5% (mouse and rat) purity of the test article. The dose levels (expressed in mg/kg body weight/day) were calculated from food consumption and body weight data collected as part of each experiment. Study descriptions and conclusions determined from the data collected in these three experiments are summarized below.

1. One-Year Dietary Toxicity Study in Dogs

a. Identification: Study performed by G. E. Schulze and staff at testing facilities operated by Hazelton Washington, Inc. in Vienna, Virginia. Experimental period: July 17, 1989 to July 19, 1990. HLA Study No. 362-200.

b. Design: Six purebred Beagle dogs/sex/group were continuously fed diets with either 0, 10, 20, or 45 ppm concentrations of moxidectin for one year.

c. Findings: Although not statistically significant, the average body weights for both males and females in the 45 ppm treatment group were reduced. Decreases in mean ovary, heart, liver and kidney weights for the 45 ppm females were also noted. No treatment-related effects were seen in the 10 and 20 ppm test animals.

d. Conclusion: The NOEL for this study was 20 ppm (~0.5 mg/kg body weight/day).

2. Two-Year Dietary Chronic Toxicity and Oncogenicity Study in Mice

a. Identification: Study performed by E. I. Goldenthal and staff at testing facilities operated by the International Research and Development Corporation in Mattawan, Michigan. Experimental period: December 15, 1989 to December 17, 1991. IRDC Project No. 141-031.

b. Design: Sixty-five Charles River CD-1 mice/sex/group were continuously fed diets with either 0, 15, 30, or 60/50 ppm concentrations of moxidectin for two years. The high-dose dietary level was reduced from 60 ppm to 50 ppm after the first eight weeks of the study due to a significant number of deaths in the high-dose male and female test animals.

c. Findings: A significant treatment-related increased mortality in female 50 ppm test animals was observed toward the end of the experiment. No treatment-related effects were observed in the 15 and 30 ppm test animals. No evidence of carcinogenicity was noted in this study.

d. Conclusion: The NOEL for this study was 30 ppm (~5.0 mg/kg body weight/day).

3. Two-Year Dietary Chronic Toxicity and Oncogenicity Study in Rats

a. Identification: Study performed by T. Zoetis and staff at testing facilities operated by Hazelton Washington, Inc. located in Vienna, Virginia. Experimental period: January 11, 1990 to January 17, 1992. HWA Study No.: 362-202.

b. Design: Sixty-five Sprague-Dawley [Crl:CD(SD)BR]rats/sex/group were continuously fed diets with either 0, 15, 60, or 120/100 ppm concentrations of moxidectin for two years. The high-dose dietary level was reduced from 120 ppm to 100 ppm due to increased mortality, abnormal clinical signs, and depressed body weight gains in the high-dose female test animals in the first eight weeks of the study.

c. Findings: There were no treatment-related effects at any dose level during the study, except for those associated with the deaths which occurred in the high-dose (120 ppm) female treatment group during the first eight weeks of the experiment. No evidence of carcinogenicity was noted in this study.

d. Conclusion: The NOEL for this study was 100 ppm (~6.0 mg/kg body weight/day).

B. Safe Concentration of Residues

The lowest NOEL in the most sensitive species obtained in the battery of toxicity studies previously described in Section VI A is 0.4 mg/kg bw/day. This NOEL was determined in the three generation (two litter) rat reproduction study. In the absence of any structural relationship to any known carcinogen and the experimentally demonstrated lack of reproductive or oncogenic effects, the appropriate safety factor to be applied to this long-term study NOEL is 100. Applying this 100-fold safety factor to the 0.4 mg/kg bw/day NOEL results in a calculated acceptable daily intake (ADI) of up to 0.004 mg moxidectin/kg body weight/day. The calculation to achieve this value is presented below:

ADI = 0.4 mg moxidectin/kg/day (NOEL) ¸ 100 (safety factor) = 0.004 mg moxidectin/kg/day

The safe concentration of moxidectin residues in each of the four major edible tissues of cattle (muscle, liver, kidney and fat) is determined by using the ADI (in micrograms/kg body weight/day), the weight in kg of an average adult (60 kg) and the estimated amount of each edible tissue consumed per day in grams using the following relationship:

Safe Concentration (ppm) = ADI (µg/kg/day) x 60 kg ¸ edible tissue consumption value (g/day)

The consumption values employed in these calculations are taken from Section IV (B) of the Center for Veterinary Medicine's "General Principles for Evaluating the Safety of Compounds Used in Food-Producing Animals" (revised July 1994). The safe concentration of moxidectin residue for each edible cattle tissue and the associated consumption value used in its calculation are listed below.

Edible Beef TissueDaily Consumption
(grams)
Safe Concentration
(ppm)
Muscle3000.8
Liver1002.4
Kidney504.8
Fat504.8

C. Total Residue and Metabolism 

The levels of total drug-related residues of moxidectin in the edible tissues of cattle were determined in a series of three residue studies conducted using [14C]-labeled moxidectin administered either by subcutaneous injection or topical application. For comparison, similar experimentation was also carried out in laboratory rats. These studies are summarized below.

1. Total Residue and Metabolism in Cattle

1.1 Absorption, Distribution, Excretion and Biotransformation of [14C]-Moxidectin Administered by Injection to Steers

a. Identification: Study conducted by J. A. Zulalian, Ph.D., and staff at American Cyanamid's Princeton, New Jersey testing facility. Experimental period: May 23, 1990 to July 31, 1990. Report No.: PD-M 28-34.

b. Design: Three Hereford steers with an average weight of 224 kg were given a single subcutaneous administration of an injectable formulation providing 0.2 mg [14C]-labeled moxidectin/kg body weight. Total urine and feces were collected daily from these treated steers. A fourth Hereford steer weighing 225 kg which served as a control received a single subcutaneous injection of a comparable volume of the same formulation containing no [14C]-labeled moxidectin. The control steer was sacrificed six days following treatment and the treated steers were sequentially sacrificed on 7, 14 and 28 days posttreatment. Samples of 25 different tissues and body fluids were harvested for analysis. Total [14C] residues were determined in all tissue, feces and urine samples to evaluate the absorption, distribution and excretion of the radioactivity. Selected samples were also used to assess the biotransformation of moxidectin. Additional in vitro metabolism studies using steer live microsomes and Aroclor-1254 S9 rat liver homogenates were performed to confirm the identification of the metabolites derived in the in vivo steer study.

c. Findings: Total radioactivity recovered in all collected samples accounted for 73%, 71% and 77% of the administered dose at 7, 14 and 28 days posttreament, respectively. The primary route of excretion was through the feces accounting for 32%, 41% and 58% of the administered dose at each respective sequential sacrifice time point. A maximum of 3% of the radioactivity was recovered in the urine. Total residue levels in all tissues declined steadily throughout the 28-day time period demonstrating the absence of bioaccumulation and were 10 to 40 times higher in fat than in the other major edible tissues (muscle, kidney and liver). The extractability of the total [14C] was greater than 90% in all edible tissues and feces indicating the absence of significant bound residues. Qualitatively similar profiles of moxidectin and seven metabolites were detected in all tissues at the three sacrifice times. Moxidectin was the major component accounting for between 75% and 90% of the radioactivity in fat. Only two metabolites (C-29/C-30 hydroxymethyl and C-14 hydroxymethyl) contributed more than 5% of the total residual radioactivity in any tissue at any sampling point. The remaining minor metabolites were all mono- and di-hydroxylation products of moxidectin.

d. Conclusions: The primary route of excretion is via feces. Because residue levels in fat are 10-40 times higher than other edible tissues, fat is the appropriate target tissue for residue monitoring purposes. The steady decline of total residue levels over time shows the absence of bioaccumulation and the greater than 90% extractability of total [14C] confirms the absence of significant bound residues. The parent molecule, which generally accounts for 80% of the total residue in fat, is the major residue, establishing unaltered moxidectin as the marker residue. Hydroxylation is the principal route of biotransformation in cattle.

1.2 Biotransformation of [14C]-Moxidectin in Steers When Applied Topically As A Pour-On

a. Identification: Study conducted by S.-S Wu, Ph.D., and staff at American Cyanamid's Princeton, New Jersey testing facility. Experimental period: April 23, 1992 to August 31, 1992. Report No.: PD-M 29-43.

b. Design: Six Hereford steers with an average weight of 159 kg were given a single application along the backline of a pour-on formulation providing 0.5 mg [14C]-labeled moxidectin/kg body weight. Two additional Hereford steers with an average weight of 156 kg which served as controls and were similarly treated with a comparable volume of the same formulation containing no [14C]-labeled moxidectin. Total urine and feces were collected daily from the treated steers. Groups of one control and three treated steers were sacrificed 2 and 14 days following treatment. Samples of loin muscle, liver, kidney, back fat and omental fat were taken for analysis. Total [14C] residues in these tissues and the excreta were determined by radioassay (limit of detection; 2 ppb). Residues in tissues and feces were extracted and characterized by high performance liquid chromatography.

c. Findings: Total [14C] residues in the tissues from the steers sacrificed two days after treatment were 7-10 ppb in omental fat; <2-7 ppb in back fat; 2-4 ppb in liver and <2 ppb in kidney and muscle Due to these extremely low residue levels, no characterization of these residues was possible. The 14-day posttreatment residues levels were 33-259 ppb in omental fat; 12-129 ppb in back fat; 5-26 ppb in liver, 3-18 ppb in kidney and <2-3 ppb in muscle. Extractability of total [14C] residues was greater than 86% in tissues 90% in feces. The radioactive profiles were qualitatively similar for all tissues. Unaltered moxidectin was the only significant component in both omental and back fat accounting for >75% of the residue. No single metabolite contributed more than 5% of the total residue in fat. A minimum of five metabolites was detected in the other tissues. Of these, the same two monohydroxylated derivatives of the parent compound identified in the previous [14C] injectable study predominated. Feces was the principal route of excretion.

d. Conclusions: The distribution, excretion and biotransformation of moxidectin in steers is similar when administered by injection or applied topically as a pour-on. These data confirm that moxidectin in fat is approximately 80% of the total residue in fat after injectable and pour-on treatment.

1.3 Residue Profile in the Fat of Heifers and Steers Treated By Subcutaneous Injection with [14C]-Moxidectin.

a. Identification: In-life experimentation conducted by Charles Heird, Ph.D., and staff at Southwest Bio-Labs, Inc., Las Cruces, New Mexico. Analytical component performed by S.-S. Wu and staff at American Cyanamid's Princeton, New Jersey testing facility. Experimental period: November 12, 1996 to March 24, 1997. Report No.: MET 97-006.

b. Design: Twelve English crossbred cattle (four steers and eight heifers) with an average weight of 226 kg were given a single subcutaneous administration of an injectable formulation providing 0.2 mg [14C]-labeled moxidectin/kg body weight. An additional English crossbred steer and heifer with an average weight of 225 kg served as untreated controls. The treated cattle were randomly assigned to four groups consisting of one steer and two heifers. These groups were sequentially sacrificed 3, 7, 14 and 28 days after treatment and omental and back fat as well as injection site tissues were collected for analysis. The magnitude of [14C] residues (TRR) was measured by radioassay (limit of quantitation: 5 ppb). The [14C]-moxidectin-derived residue profile and the ratio of parent compound to TRR in fat was determined by high performance liquid chromatography.

c. Findings: Extractability of radiolabeled residues in all omental and back fat samples was 99-100%. Unaltered moxidectin was identified as the major residue accounting for 84% (steers) and 83% (heifers) of the TRR in the omental fat and 80% (steers) and 81% (heifers) of the TRR in the back fat. Overall, moxidectin accounted for 82% of the total residue in fat. The two monohydroxylated moxidectin metabolites (C-29/C-30 hydroxymethyl and C-14 hydroxymethyl) identified in previous studies were observed at levels <10% of the TRR.

d. Conclusions: Unaltered moxidectin represents approximately 82% of the TRR in the omental and back fat of both male and female cattle. The metabolite profiles seen in both steers and heifers are also qualitatively and quantitatively very similar.

2. Comparative Metabolism in Rats 1.1 Absorption, Distribution, Excretion of [14C]-Moxidectin in Rats

a. Identification: Study conducted by S.-S. Wu, Ph.D., and staff at American Cyanamid's Princeton, New Jersey testing facility. Experimental period: May 15, 1990 to July 8, 1991. Report No.: PD-M 28-33.

b. Design: Two groups of five male and five female Sprague-Dawley CD albino rats [Crl:CD(SD)] were given a single low or high oral administration containing 1.5 or 12 mg [14C]-labeled moxidectin/kg body weight, respectively. Urine and feces were collected for seven days following treatment, then the test animals were sacrificed and tissues/organs/body fluids were harvested for analysis along with the urine and feces for determination of mass balance. A third treatment group consisting of 15 male and 15 female CD(SD) albino rats were administered the 1.5 mg [14C]-labeled moxidectin/kg body weight dose level orally for seven consecutive days. Evenly distributed subgroups were sequentially sacrificed a 6, 24, 72, 120 and 168 hours posttreatment and liver, kidney, muscle and fat tissues were collected to determine the nature and depletion of the [14C] residues.

c. Findings: In both the high- and low-dose groups feces was the primary route of excretion accounting for between 59.7-91.3% of the administered radioactivity in all rats over the seven-day test period. Contrastingly, less than 2% of the radioactivity was found in the urine during the seven-day period. Of all the tissues analyzed, the highest residue levels were found in fat. Extractability of residues from tissues and feces was between 85-99% demonstrating the absence of bound residues. Metabolite characterization showed unaltered moxidectin was the only significant residue in both tissues and feces. The minor metabolites consisted of the two monohydroxylated moxidectin derivatives seen in cattle and a third hydroxylated metabolite only identified in the rat. This metabolite profile indicates the principal route of moxidectin metabolism in the rat is hydroxylation.

d. Conclusions: The metabolism of moxidectin by cattle and rats is qualitatively very similar. In both species moxidectin is excreted primarily in feces with only limited amounts voided in the urine. Due to the highly lipophilic nature of the drug, the proclivity for substantially greater moxidectin residues in fat relative to the other major tissue types is also seen in both species. The primary component of these residues in each species is the parent compound accounting for between 75% to 90% of the total residues in the fat of both cattle and rats. The metabolic profiles in both species are also very comparable. In both cattle and rats the primary route of metabolism is hydroxylation. The two monohydroxylated derivatives of the parent compound identified as metabolites in cattle are also found in rats. The only species difference is the presence of a third hydroxylated metabolite which was only identified in rats. These similarities confirm the suitability of utilizing the rat in toxicity studies to evaluate the human food safety of moxidectin.

D. Selection of Marker Residue and Target Tissue

As demonstrated in the previously summarized [14C]-moxidectin metabolism studies in cattle, the predominant residue found in fat of moxidectin-treated cattle is the unaltered parent compound (80% of the total residue) making moxidectin the appropriate marker residue to be used in tissue residue depletion experimentation. Due to the highly lipophilic nature of moxidectin, the highest levels of total drug-related residues and slowest residue depletion are found in fat, establishing fat as the appropriate target tissue.

E. Cold Tissue Residue Depletion in Cattle

Depletion of the marker residue in fat and muscle was demonstrated in an experiment specifically designed to evaluate the period of peak residues resulting from treatment with the recommended dose level of the final moxidectin 0.5% pour-on formulation under weather conditions conducive to optimum uptake of the drug, characterize application site residue levels and assess the depletion of tissue residue concentrations over five posttreatment intervals.

a. Identification: Study performed by L. S. de Montigny, DVM, and staff at American Cyanamid's Princeton, New Jersey testing facility. Experimental period: August 1, 1996 to February 13, 1997. Study No.: 0863-B-US-7-96.

b. Design: Thirty-three Angus and Angus crossbred cattle (17 heifers and 16 steers) weighing an average of 335 kg were randomly assigned to five, six-animal treated groups with equal sex distribution and an untreated control group (two heifers and one steer). The treated groups received a single topical application of the final 0.5% pour-on formulation at the recommended dose level of 0.5 mg moxidectin/kg body weight. All cattle comprising a treated group were sacrificed at either 3, 7, 10, 14 or 21 days posttreatment and samples of fat and muscle adjacent to (back fat, loin muscle) or remote from (omental fat and leg muscle) the application site were taken for residue analysis. Two control animals (one heifer and one steer) were sacrificed at the initial time point while the remaining control heifer was sacrifice at the final time point. Identical tissues were harvested for analysis from the control animals. Treatment coincided with the seasonal period when the highest ambient temperatures and humidity leel are typically experienced at the testing facility.

c. Findings: The mean moxidectin (marker residue) concentrations at the five sacrifice times in back and omental fat and loin and leg muscle following treatment with the recommended level of the final moxidectin 0.5% pour-on formulation are summarized below:

Moxidectin Residue Levelsa
Sacrifice Time (days posttreatment)Back Fat (ppb)bOmental Fat (ppb)bLoin Muscle (ppb)bLeg Muscle (ppb)b
356±3290±76c<10<10
763±1871±27<10<10
1063±6965±69<10<10
1428±20c49±26c<10<10
2125±16c31±17<10<10

aValues listed in table are the mean of tissues types taken from six animals (three males and three females) at each sacrifice point plus or minus (±) one standard deviation.

bValidated limit of quantitation (LOQ) of HPLC method of analysis is 10 ppb (mg/kg).

cOne animal had a residue of <10 ppb (LOQ of method); a value of 9 was used to calculate the mean.

d. Conclusions: The data reported in this cold residue depletion study show that concentrations of moxidectin in the target tissue peak at three to seven days posttreatment. Residue levels are not higher in the tissues adjacent to the application site (back fat, loin muscle) when compared to tissues remote from the site of application (omental fat, leg muscle). No significant differences exist in the moxidectin residue depletion profile observed in male and female cattle. Because parent moxidectin is 80% of the total residue, the total residue in the fat samples given in the table above can be calculated by dividing the observed values by 0.8.

F. Assignment of Zero Tissue Withdrawal Period

Because total residue concentrations of moxidectin (marker residue) in both omental and back fat are more than ten-fold below the safe concentration of 4.8 ppm during the period of peak residues, no withdrawal period is required for the edible tissues of cattle treated per label directions with Cydectin moxidectin 0.5% Pour-On for Cattle.

G. Regulatory Methods and Tolerances

Due to the fact that no posttreatment withdrawal time is necessary for the edible tissue of cattle treated with the approved level of 0.5 mg/kg body weight of Cydectin moxidectin 0.5% Pour-On for Cattle, no official regulatory method is required. Sponsor-validated research methods for moxidectin in fat and milk are on file with the Center for Veterinary Medicine.

However, because FSIS/USDA will, in its monitoring program, assay muscle and liver samples of cattle with a multiresidue method capable of detecting and measuring moxidectin, FDA has established tolerances of 50 ppb and 200 ppb for parent moxidectin in muscle and liver, respectively, of cattle. These tolerances were based on data from a study in which FSIS/USDA used its multiresidue method to measure incurred moxidectin residue in samples taken at 3 days after treatment (the time of peak residues). Moxidectin was observed to range from 6.1 to 24.4 ppb in muscle and 33.6 to 101.9 ppb in liver. When these tolerances are used for monitoring, the total residue in edible tissues will be below the calculated safe concentrations, thus ensuring the safe use of moxidectin pour-on.

H. User Safety Statement

Animals used to evaluate the safety to humans involved in handling and administering moxidectin 0.5% Pour-On for Cattle, and the subsequent handling of treated cattle has demonstrated the product is mildly irritating to the eyes and skin and does not cause skin sensitization after repeated exposure. The routine inhalation of vapors associated with normal product use do not pose a significant health risk. This pour-on product is not to be used for medicinal purposes by humans. If accidental ingestion or contact with eyes or skin occurs, users should follow the first aid steps stated on the label. If any symptoms attributable to exposure to the product persist, consultation with a physician is recommended. The product label will instruct users who experience adverse reactions to report these using the 1-800 number provided on the product label.

 

VII. AGENCY CONCLUSIONS 

The data submitted in support of this original NADA satisfy the requirements of section 512 of the Federal Food, Drug and Cosmetic Act and 21 CFR Part 514 of the implementing regulations. The data demonstrate that CYDECTIN® (moxidectin) Pour-On for Cattle is safe and effective for treatment and control of infections and infestations due to internal and external parasites of cattle, when administered topically at a dose of 500 µg/kg bodyweight.

An Acceptable Daily Intake (ADI) of 0.004 mg/kg/day in tissues has been established. Because total residue concentrations of moxidectin (marker residue) in both omental (abdominal) and back fat (target tissue) are more than ten-fold below the safe concentration of 4.8 ppm during the period of peak residues, no withdrawal period is required for the edible tissues of cattle treated according to label directions. Since no withdrawal time is necessary, no official regulatory method is required. Sponsor-validated research methods for moxidectin in fat and milk are on file with the Center for Veterinary Medicine.

However, because FSIS/USDA will, in its monitoring program, assay muscle and liver samples of cattle with a multiresidue method capable of detecting and measuring moxidectin, FDA has established tolerances of 50 ppb and 200 ppb for parent moxidectin in muscle and liver, respectively, of cattle. These tolerances were based on data from a study in which FSIS/USDA used its multiresidue method to measure incurred moxidectin residue in samples taken at 3 days after treatment (the time of peak residues). Moxidectin was observed to range from 6.1 to 24.4 ppb in muscle and 33.6 to 101.9 ppb in liver. When these tolerances are used for monitoring, the total residue in edible tissues will be below the calculated safe concentrations, thus ensuring the safe use of moxidectin pour-on.

The data submitted for CYDECTIN® (moxidectin) Pour-On for Cattle support the marketing of the product as an over-the-counter new animal drug. Adequate directions for use have been written for the layman, and the conditions for use prescribed on the label are likely to be followed in practice. Therefore, the Center for Veterinary Medicine (CVM) has concluded that this product shall have over-the-counter marketing status.

The agency has carefully considered the potential environmental effects of this action and has concluded that the action will not have significant impact on the human environment and that an environmental impact statement is not required. The agency's finding of no significant impact (FONSI) and the evidence supporting that finding contained in an environmental assessment may be seen at the Dockets Management Branch (HFV-305), Park Building (Room 1-23), 12420 Parklawn Dr., Rockville, Maryland 20855.

Under section 512(c)(2)(F)(ii) of the FFDCA, this approval for food producing animals qualifies for THREE years of marketing exclusivity beginning on the date of approval because the supplemental application contains substantial evidence of the effectiveness of the drug involved, any studies of animal safety, or, in the case of food producing animals, human food safety studies (other than bioequivalence or residue studies) required for the approval of the application and conducted or sponsored by the applicant. CYDECTIN® (moxidectin) Pour-On for Cattle is under U.S. patent number 4,916,154, which expires on April 10, 2007.

 

VIII. LABELING (Attached)

Copies of applicable labels may be obtained by writing to the:

Freedom of Information Office
Center for Veterinary Medicine, FDA
7500 Standish Place
Rockville, MD 20855