First draft prepared by
Dr M.A. Miller
Center for Veterinary Medicine
Food and Drug Administration
Rockville, Maryland, USA
Four analogues of natural bovine somatotropin (bST) that are
produced by recombinant DNA techniques (rbSTs) were reviewed by the
Committee. These were somagrebove, sometribove, somavubove, and
somidobove. Injection of bST or rbSt into lactating dairy cows
stimulates milk production. In the past, commercial uses of
endogenous bST to increase milk production in dairy cows was limited
because there was no practical method for producing large quantities
of this product. Recent advances in biotechnology have allowed the
economical production of commercial quantities of the hormone. These
products had not been previously evaluated by the Joint FAO/WHO
Expert Committee on Food Additives.
2. BIOLOGICAL DATA
2.1 Biochemical Aspects
Bovine pituitary somatotropin exists as four variants comprised
of 190 or 191 amino acids. The products differ in the amino acids
which are substituted for the terminal alanine residue (Table 1)
(Hammond et al., 1990).
Table 1. Products marketed as recombinant bovine somatotropins
Products Amino Acid Substituted to
Like other protein hormones, bST and rbSTs elicit a biological
response by binding with high affinity to specific receptors on the
cell membrane of target cells (OTA, 1991). Although the chemistry of
the recombinant products varies slightly from pituitary bST, it is
appropriate to consider them together in this human food safety
evaluation because they all act by binding with high affinity to the
bovine somatotropin receptor. The administered compounds are
designated as "rbSTs" in this monograph, even though bST elicits the
same responses in most cases.
2.1.1 Absorption, distribution and excretion
Like most dietary proteins, rbSTs are degraded by digestive
enzymes in the gastrointestinal tract and are not absorbed intact.
In vitro studies on the metabolism of rbSTs demonstrate that they
are readily cleaved by digestive enzymes (Heiman & Harris, 1989a).
The progressive cleavage of peptide bonds results in the loss of
biological activity because both the C- and N-terminal and the
appropriate tertiary structure are required for receptor binding.
In vivo studies in laboratory animals confirm that proteolytic
fragments of rbSTs have no biological activity (Hammond et al.,
As discussed above, rbSTs are not absorbed intact following
oral administration and therefore they display no biological
activity by this route. When commercial doses of rbSTs are injected
into dairy cows, blood levels of somatotropin increase, which does
not translate into a biologically significant increase of
somatotropin in muscle or liver because somatotropin is not stored
in tissues but is rapidly degraded by cytosolic proteases and
lysosomal enzymes (Hammond et al., 1990; OTA, 1991).
2.1.3 Effects on enzymes and other biochemical processes
rbSTs increase milk production by altering the metabolism of
several body tissues so that more nutrients are provided to the
mammary gland for milk production, i.e., repartitioning of nutrients
(OTA, 1991). Since rbSTs alter the metabolism of several tissues,
e.g., adipose tissue, intestine, liver and muscle, the circulating
levels of several nutrients and hormones are changed. Generally, the
increase in milk production is sufficient to utilize all available
nutrients. Milk composition data indicate that bST treatment has no
biologically-significant effect on the major milk components (OTA,
1991). This eliminates concern for changes in milk nutritional value
or quality and processing characteristics.
The morphology of the lactating mammary gland, particularly the
tight junction between the alveolar cells, provides an effective
barrier against blood constituents entering the milk. However, small
amounts of plasma proteins and circulating protein and peptide
hormones do enter the milk (Malven, 1977). For example, trace
amounts of insulin-like growth factor, IGF-I (0.7 to 8.1 ng/ml) are
normally present in bovine milk (Collier et al., 1991).
IGF-I is a 70 amino acid polypeptide hormone of the somatomedin
family. Human and bovine IGF-I are identical. The IGFs
(or somatomedins) have acute metabolic effects similar (but
considerably less potent) to insulin and long-term growth-promoting
effects in that IGF-I mediates many of the growth effects of
somatotropin. Plasma concentrations of IGF-I are regulated by
somatotropin and it is the circulating hormone which changes the
most dramatically following rbST treatment (Juskevich & Guyer,
Although treatment of dairy cattle with rbSTs was not expected
to cause a biologically significant increase in milk IGF-I,
toxicology studies were carried out to determine the oral activity
of IGF-I and residue studies were conducted to quantitate the change
in milk IGF-I levels following treatment with rbSTs (Juskevich &
Guyer, 1990). The results of several studies on milk IGF-I values in
rbST-treated and control cows demonstrate that, following treatment
with rbSTs, milk IGF-I levels were not outside the range of values
found naturally in milk (Miller et al., 1991).
2.2 Toxicological studies with rbSTs
2.2.1 Acute toxicity studies
The results of acute toxicity studies with rbSTs are summarized
in Table 2.
2.2.2 Short-term toxicity studies
Somidobove was administered daily to Fischer 344 rats by oral
gavage at doses of 0.05, 0.5 or 5.0 mg/kg bw/day for two weeks.
There were 15 rats/sex/dose including a control group.
There were no physical or behavioural changes associated with
treatment, and there were no toxicologically-significant changes in
body weight, food consumption, efficiency of food utilization,
haematology, clinical chemistry, or organ weights. There were no
gross or microscopic lesions that were compound-related. Bovine
somatotropin was not detected in the plasma of any of the rats
following two weeks of oral administration. No adverse biological
effects were observed (Fisher, 1985).
Charles River CD (Sprague-Dawley derived) rats received
somagrebove by gavage at concentrations of 0.1, 1 and 10 mg/kg
bw/day for 15 consecutive days. There were 20 rats/sex/group
including a control group. The rats were observed twice daily for
signs of toxicity, morbidity, and mortality. Detailed observations,
individual body weights and individual food consumption were
recorded at 3-day intervals during the study. Haematological and
biochemical tests were performed on 10 rats from each dose at the
end of the study. Organ weights and microscopic examinations were
performed at necropsy.
There were no overt signs of toxicity observed at any treatment
level during the course of the study. Three male rats, one each in
the control group and at 0.1 and 1 mg/kg bw/day died during the
first 3 days of the test. All three deaths were attributed to
animals being misdosed in the lung. Food intake and body-weight gain
of the orally treated animals were generally comparable to those of
the untreated controls at all treatment levels.
Table 2. Acute toxicity of rbSTs
Species Route Sex Maximum Effect References
Rat oral M&F 5000 mg/kg bw none Marcek et al., 1988
Rabbit dermal M 500 mg/site slight Sabaitis et al., 1987
Rabbit ocular M 100 mg/eye slight Leong et al., 1987a
Rat inhalation M&F 1.08 mg/l none Leong et al., 1987b
Guinea-pig dermal M&F 20 mg/day slight Leong et al., 1988
Rat oral M&F 5000 mg/kg bw none Auletta &
Rat oral M&F 90 mg/kg bw none Fisher et al., 1987
Rabbit dermal M&F 36 mg/kg bw slight Fisher et al., 1987
Rabbit ocular M&F 1.8 mg none Fisher et al., 1987
Rat oral M&F 5000 mg/kg bw none Fisher & Garces, 1986
Rabbit dermal M&F 2000 mg/kg bw none Fisher & Garces, 1986
Rabbit dermal M 0.5 g none Fisher & Garces, 1986
Rabbit ocular M 100 mg none Fisher & Garces, 1986
Guinea-pig dermal M 0.4 g/site none Costello, 1986
Haematology parameters were unaffected by the administration of
bST. A statistically-significant decrease in alkaline phosphatase
and a significant increase in cholesterol were observed in female
rats which received 1 mg/kg bw/day. Although statistically
significant, these changes were within normal limits and were not
considered to be directly related to treatment.
Absolute liver weights were significantly increased in female
rats at 0.1 and 1 mg/kg bw/day, but not at 10 mg/kg bw/day. Absolute
liver weights for treated male rats were similar to those of the
untreated controls. Absolute spleen weights were significantly
increased in females only at 0.1 mg/kg bw/day. Subsequent
microscopic examinations of the liver and spleen did not show any
pathological effects. The increase in absolute liver and spleen
weights appeared to be related to increased body weights observed at
these levels. The increase in relative liver weight in females at 10
mg/kg bw/day was considered to be biologically insignificant because
there was no corresponding increase in male rats and no untoward
effects on liver function or histology.
No test substance-related gross or microscopic lesions were
seen in any tissues from any of the treatment groups. The lesions
described among these groups were considered spontaneous in nature,
because the incidence of occurrence was generally similar among all
groups. The NOEL was 10 mg/kg bw/day (Fisher et al., 1984).
Four groups of male and female Sprague-Dawley rats were given
daily oral doses of somavubove at 0 (vehicle), 0.5, 5 or 50 mg/kg
bw/day for 26 days via gastric intubation. The dose groups consisted
of 30, 25, 25 and 30 rats/sex/dose, respectively. A fifth dose group
served as a positive control. This group consisted of 10 rats/sex
given daily subcutaneous injections of 50 µg/rat somavubove, which
were killed on study days 5 and 22 for evaluation of serum
somatotropin and rbST antibody production. Five rats/sex/dose from
the 0 (vehicle control) and the 50 mg/kg bw/day groups were killed
on study day 5; and five rats/sex/dose from all oral dose groups
were killed on day 22 for evaluation and comparison to positive
controls. Clinical observations were made daily. Body weight was
measured twice weekly and food consumption was measured twice during
for week 1 and weekly thereafter. Clinical chemistry, haematology,
body weights, food consumption and organ weights were measured
terminally and were analyzed for treatment-related effects of orally
administered rbST. All orally-treated rats were necropsied. Tissues
from rats of the 0 (vehicle control) and 50 mg/kg bw/day dose groups
were examined histopathologically for the presence of
No treatment-related clinical observations were noted. There
were no statistically-significant differences in the orally-treated
groups relative to the vehicle control group in terms of
body-weight, food consumption or haematology. There was a
statistically-significant decrease in total protein (females only)
values in the 5 mg/kg bw/day dose group compared to controls.
Adrenal weights were significantly heavier in the 0.5 and 5.0 mg/kg
bw/day dose groups (females only) compared to the control group. No
dose-response relationship was observed for any of these
statistically-significant parameters and no corresponding
pathological effects were found microscopically.
Serum somatotropin and rbST antibody production results
indicated that orally ingested bST was not absorbed intact from the
rat gut at the dose levels tested. Antibody titers slightly higher
than background were present in several of the orally-treated rats,
but rbST was not detectable by RIA. These data suggest that the
immune system has access to an antigenic portion of the rbST (Seaman
& Skinner, 1986). Formation of antibodies to dietary protein is a
normal response (Bahna & Heiner, 1980; Hammond et al., 1991).
Eighty Sprague-Dawley Crl CD(SD)BR rats were divided into 4
groups of 20 rats/sex/group. Animals were gavaged daily for 28 days
with 0 (vehicle), 0.06, 0.6 or 6 mg of sometribove/kg bw/day.
Survival, clinical observations, body weight, food consumption,
clinical pathology, gross pathology, organ weights and
histopathology were assessed.
No animals were found dead or were sacrificed during the study.
No compound-related trends were noted in clinical observations of
either sex. Mean growth rate and mean total food consumption values
were not significantly different between control and treated groups.
Clinical and gross pathology findings were unremarkable. There were
no compound-related trends in the mean terminal body-weight of
treated animals. Mean ovary weight and mean ovary/body-weight ratios
were significantly lower in the 0.06 and 6 mg/kg bw/day group
females when compared to controls. Mean kidney/brain weight ratios
in the 0.06 and 6 mg/kg bw/day groups and the mean adrenal/brain
weight ratios in the 6 mg/kg bw/day group females were significantly
higher than in the control animals. These effects were not
considered related to treatment because there was no dose-dependent
relationship. No treatment-related microscopic changes were
observed. The NOEL was 6 mg/kg bw/day (Serota, 1984).
Groups of 15 male and 15 female young adult Fischer 344 rats
were given daily gavage doses of 0 (vehicle), 10, 30, or 100 mg/kg
bw/day of somidobove for three months. Clinical observations were
made daily. Body weight and food consumption were determined weekly.
Clinical chemistry, haematology, urinalysis, organ weights and
histopathology were performed at the end of the study.
All rats survived the three-month study period. One mid-dose
female had chromodacryorrhea of the right eye for one week; another
female in this group had a small growth at the right corner of the
mouth two days prior to necropsy. Because of the low incidence and
the lack of a dose-response relationship, these observations were
not considered to be related to treatment. There were no
compound-related effects on body weight, weight gain, food
consumption or efficiency of food utilization. Sporadic but
statistically-significant changes in haematological parameters,
including decreased nucleated erythrocytes in the high-dose females
and increased monocytes in the high-dose males, were small and not
considered treatment-related. Also, small but
statistically-significant increases in alanine transaminase,
aspartate transaminase and cholesterol occurred in the low and
high-dose males. These changes were not dose-related and therefore
were probably not related to somidobove treatment. There were no
compound-related changes in urinalysis parameters or absolute and
relative organ weights. There were no gross or microscopic lesions
associated with somidobove treatment. In conclusion, no adverse
effects due to the administration of somidobove were observed in
this study. Therefore, the NOEL was 100 mg/kg bw/day (Fisher, 1987).
Sometribove was administered daily by gavage at doses of 0.1,
0.5, 5 or 50 mg/kg bw/day or subcutaneously at 1 mg/kg bw/day
(positive control) to Charles River CD VAF rats (30 rats/sex/group)
for 13 consecutive weeks. The negative control animals were gavaged
with vehicle only. After 13 weeks, 5 animals/sex/group were
maintained without dosing for a 14-week recovery phase.
No mortality was observed in the groups treated orally except
for one male in the mid-dose group which died due to a dosing
accident. After the eighth week of treatment, three deaths occurred
in the positive control group; one animal of each sex was found dead
and one female was killed in extremis because the animal displayed
thin appearance, reduced motor activity and blood discharge around
No treatment-related ocular changes were observed. Oral
treatment did not increase body weight or feed consumption.
Concomitant marked increases in body-weight gain and feed
consumption were observed from week 2 and throughout the treatment
phase in the positive control group animals. During the recovery
phase, body-weights of the male rats in the positive control group
remained higher than in those of the negative control animals, but
feed consumption rapidly returned to normal.
No toxicologically-meaningful changes were observed in clinical
chemistry or haematology parameters in the orally-treated animals.
In animals treated subcutaneously, cholesterol and calcium values
were increased in male rats. In female rats, albumin was slightly
decreased which caused a decrease in total protein and
albumin/globulin ratio. Phosphorus and alkaline phosphatase were
elevated in these females, suggesting an alteration in bone
metabolism. Moderate increases in the mean corpuscular volume and
mean corpuscular haemoglobin were observed in both sexes. In males,
this was accompanied by decreases in red blood cell count,
haematocrit, haemoglobin concentration and mean corpuscular
haemoglobin concentration. In females, white blood cell counts were
increased which correlated with an increased number of lymphocytes.
No changes of toxicological significance were noted in the
No drug-related changes in organ weights were observed in
animals treated orally. In animals treated subcutaneously, almost
all organs had increased absolute weights or organ/brain weight
ratios. In addition, spleen and liver (both sexes) and adrenals
(males) presented increased organ/body weight ratios.
No gross findings were observed in animals treated orally.
Enlarged submaxillary lymph nodes were noted in a few animals
treated subcutaneously. Renal cortical tubular basophilia was
observed microscopically in some males of the high-dose group and
positive control groups, but only in one male of the highest-dose
group. Fibrosis of the pancreatic islets was observed in some males
of the positive control group, and in a few animals treated orally.
Since these latter two findings were not dose-related, and since
these lesions are often found in this rat strain, a causal
relationship with product administration was doubtful.
As a general conclusion, there were no systemic effects following
oral administration of rbST at doses up to 50 mg/kg bw/day (Richard
et al., 1989).
Groups of four male and four female young adult beagle dogs
were given daily oral doses of 0 (vehicle), 1, 3, or 10 mg/kg bw/day
of somidobove by capsule for three months. Groups of two male and
two female beagle dogs were given a single subcutaneous dose of 0 or
1 mg/kg bw/day of somidobove to serve as a positive control group.
All dogs survived the three-month study period. The only
treatment-related clinical sign was soft or mucoid stools. Abnormal
stools were observed in the control dogs and the frequency of
occurrence increased in a dose-dependent manner. The effect was
probably related to a combination of phosphoric acid diluent and
somidobove. The stool changes were considered to be of minor
toxicological significance. There were no adverse ophthalmic effects
attributable to somidobove administration. There were no
compound-related effects upon body weight, food consumption,
haematology, clinical chemistry and urinalysis parameters or
absolute and relative organ weights. There were no gross or
microscopic lesions associated with somidobove treatment.
Somidobove was not detectable in the serum of dogs treated
orally except for one dog treated with 10 mg/kg bw/day orally, which
had a single positive response 2 h after treatment on day 30. The
serum level returned to below detectable limits by four hours. This
one positive serum sample out of 144 was believed to represent a
false positive. In the serum of dogs treated subcutaneously,
somidobove was detectable at maximum concentrations of 195 to
551 ng/ml. The NOEL was 10 mg/kg bw/day (Fisher & Russell, 1988).
2.2.3 Special studies with rbSTs in hypophysectomized rats
A study was conducted to determine if orally administered rbST
can be absorbed in the hypophysectomized rat in sufficient
quantities to result in weight gain when dosed at greatly
exaggerated doses with respect to amounts in the human diet. rbST
was given orally or subcutaneously to groups of female
hypophysectomized rats daily for 9 days. Ten rats per dose were
given oral dosages of 0 (vehicle), 0.04, 0.4, 2 or 4 mg rbST/kg
bw/day. One replicate each of 10 rats per dose group received
subcutaneous doses of 0 (vehicle), 15, 30 or 60 µg rbST per day.
Rats were weighed daily to observe body-weight gain, a measure of
absorption and activity of somatotropin in the hypophysectomized
Results showed that rbST administered subcutaneously increased
average daily body-weight gain. No daily body-weight gain occurred
in the rats administered rbST orally and little or no gain occurred
in rats receiving subcutaneous or oral vehicle control. This study
showed that rbST displays no growth-promoting activity following
oral administration (Seaman et al., 1986).
Fifty-nine female hypophysectomized Sprague-Dawley rats were
randomly assigned to one of eight treatments. Rats were administered
0 (vehicle), 40, 80 or 120 µg/day rbST daily by gavage or by
subcutaneous injection for seven days. All groups consisted of 7
animals except for the control group, which had 10 animals. The
objective of the study was to utilize a somatotropin-sensitive
model, the hypophysectomized rat, to determine whether ingested rbST
could be absorbed in a biologically-active form. Body weight was
measured on day 0 and on day 7 for determination of body-weight gain
and relative body-weight gain.
Hypophysectomized rats injected with rbST had increased weight
as a log function of the dose of somatotropin administered. Maximal
weight gain, approximately 16 grams above basal level, was achieved
with the 80 and 120 µg/day doses. No growth response was observed
among the hypophysectomized rats administered somatotropin orally.
The results support the hypothesis that ingested somatotropin is
degraded and is not absorbed in a biologically-active form (Lanza &
2.2.4 Special studies on genotoxicity
The results of genotoxicity studies on rbSTs are summarized in
Table 3. Results of genotoxicity assays on rbSTs
Test system Test object Concentration Result Reference
Micronucleus assay1 Mouse bone 250 mg/kg negative Li et al., 1989
marrow cells subcutaneous
Forward mutation Chinese hamster 200-1000 µg/ml negative Vanrell et al., 1989
assay ovary cells
1 Both with and without metabolic activation using rat liver S9 fraction.
2.3 Observations in humans
Humans are naturally exposed to trace levels of bST in beef,
milk and other dairy products. bST, like other dietary proteins, is
degraded and shows no growth-promoting activity. bST also displays
no growth-promoting activity when injected into humans. During the
1950s, clinical trials were conducted injecting large quantities of
farm animal pituitary preparations (10 000 to 160 000 µg/day) into
children and adults for several weeks or months. These pituitary
preparations did not stimulate growth or cause consistent metabolic
effects (Bennett et al., 1950, Froesch et al., 1957). It was
concluded that somatotropins were species-limited with somatotropins
from lower species having no activity in humans. The biological
basis for this species specificity was discovered years later when
it was determined that the binding of bST to the human somatotropin
receptor is several orders of magnitude lower than that of human
somatotropin (Carr & Friesen, 1976, Moore et al., 1985). Since
receptor binding is governed by the laws of mass action, the low
affinity of bST for the human somatotropin receptor renders it
biologically inactive even at grossly pharmacological doses.
2.4 Toxicology studies with IGF-I
2.4.1 Short-term toxicity studies
This study was designed to evaluate the oral activity of IGF-I
when administered daily by gavage for two weeks at dose levels of
20, 200, or 2000 µg/kg bw/day to three groups of 20 male and 20
female Sprague-Dawley (Crl:CDBR) rats. To examine the efficiency of
uptake and its growth promoting potential, IGF-I was also
administered using an osmotic pump at a release rate of 50 or
200 µg/day to 20 male and 20 female rats. In addition, 20 rats per
sex per group were given vehicle at the same dosing volume by gavage
or at the same release rate via osmotic pumps (negative controls).
As a positive control, 20 rats per sex received somatotropin, a
proven growth promoter, using an osmotic pump at a release rate of
4000 µg/day. Criteria evaluated for compound effect included
survival, clinical pathology, plasma hormone assay, gross pathology,
organ weights, tibia length and epiphyseal width.
All animals survived to termination of the study. There were no
significant treatment-related clinical observations. Mean body
weights were slightly but significantly increased throughout the
study for males receiving 2000 µg IGF-I/kg bw/day by gavage. This
increase in body weight actually occurred in only one of the two
replicates of high-dose males receiving 2000 µg of IGF-I/kg bw/day
and was not considered biologically significant. For animals that
were implanted with an osmotic pump, the 50 µg IGF-I/day males and
both sexes receiving 200 µg IGF-I/day exhibited significant
increases in body weight throughout most of the study. In addition,
both sexes of the positive control animals exhibited a significant
increase in body weight throughout the study. These findings were
Food consumption values were increased for both sexes of rats
receiving 200 µg IGF-I/day by osmotic pump and for both sexes of the
positive control animals at all intervals. A
statistically-significant increase in food consumption was also seen
in the male animals receiving 50 ug IGF-I/day by osmotic pump on
days 1 to 7. These effects were probably related to treatment. Food
consumption was decreased in females in the 20 µg IGF-I/kg/day
gavage group but this result was considered sporadic.
In animals receiving somatotropin by subcutaneous infusion,
both epiphyseal width and tibia length were increased. Epiphyseal
widths were increased in females receiving the high-dose of IGF-I by
osmotic pump. Tibia lengths were increased in the males receiving
IGF-I by osmotic pump. In rats receiving IGF-I by gavage, epiphyseal
widths were decreased in both sexes of the high-dose group and tibia
lengths were increased in the low- and high-dose males. These
contradictory findings in tibia length and epiphyseal width for
gavage-dosed animals suggest this effect is sporadic rather than
The organ weights of animals receiving IGF-I by osmotic pump
were significantly increased compared to control animals in many
instances. For animals receiving IGF-I by osmotic pump, the males
showed a decrease in relative brain weights and in relative
testis/epididymis weights. The females showed decreased relative
brain weights. Significant increases for the males were seen in
final body weights, absolute heart weights, absolute and relative
kidney weights and absolute liver weights. Significant increases in
final body weights, absolute adrenal weights, absolute heart
weights, absolute and relative spleen weights and absolute and
relative kidney weights were observed in females.
No increases in circulating IGF-I levels in the blood of rats
receiving IGF-I by gavage were observed indicating that significant
absorption of IGF-I did not occur. The rats which received IGF-I by
osmotic pump had a significant increase in circulating IGF-I levels.
In conclusion, no biologically-significant effects were
produced in male rats at gavage doses up to 2000 µg/kg bw/day. The
results suggest that no biologically-significant absorption occurs
following gavage administration. Subcutaneous administration of
IGF-I by osmotic pump produced an increase in body weight, food
consumption, blood levels of IGF-I, tibia length, epiphyseal width
and numerous changes in organ weights. The effects seen with IGF-I
administered by osmotic pump were similar to those produced by
somatotropin administered by a similar route, although somatotropin
was more effective (Terrill, 1989).
2.4.2 Special study with IGF-I on hypophysectomized rats
Hypophysectomized Sprague-Dawley rats (20 sex/group) were given
daily oral doses of 0 (vehicle), 0.01, 0.1 or 1 mg/kg bw/day IGF-I
by gavage for two weeks. An additional group of 20 males and 20
females were given 1 mg/kg bw/day of bovine serum albumin by gavage
as a positive oral protein control and a group of 10 males and 10
females was given 1 mg/kg bw/day IGF-I via subcutaneously implanted
osmotic pumps to serve as a positive parenteral control. Animals
were examined daily for physical condition. Body weight and body
length were measured twice weekly. Haematology, clinical chemistry,
organ weights and pathology were performed at the termination of the
On test day 3 female rats from the negative control group and
one male rat from the 0.01 mg/kg bw/day treatment group died. On
test day 6, one female died and one male from the 0.01 mg/kg bw/day
group was killed in moribund condition. Animals which died appeared
dehydrated. Therefore, deaths were attributed to failure to
acclimate and to the delicate physical condition of
hypophysectomized rats, and not to treatment. Subcutaneous treatment
with IGF-I caused physiologic effects of increased body weight,
increased neutrophil count, decreased BUN, creatinine and albumin
and increased kidney and spleen weights of both males and females.
Oral treatment caused no effects upon body weight, body length,
haematology or clinical chemistry parameters. The only change in
clinical chemistry of the orally treated rats was a 5.7% decrease in
total protein in the males of the 1 mg/kg bw/day group. This finding
was believed to be incidental and not associated with treatment.
There were no statistically-significant changes in organ weights of
the orally treated animals that were compound-related. There were no
treatment-associated gross or microscopic lesions.
In conclusion, no toxicological or physiological effects
occurred in hypophysectomized rats treated orally for two weeks with
IGF-I at doses up to 1 mg/kg bw/day (Fisher & Russell, 1989).
Somatotropins and insulin-like growth factors which mediate
many of the physiological effects of somatotropins are found in all
mammalian species. Pharmacological studies on the four compounds
show that their physiological effects are indistinguishable from
those of naturally occurring bST in dairy cows. The somatotropins
display species specificity and pituitary-derived bST is inactive
even when administered parenterally to humans. However, rats display
a physiological response to parenterally administered bST and rbSTs.
Somatotropins, including rbSTs, are degraded by enzymes of the
gastrointestinal tract and should therefore be inactive when
administered orally. Acute oral toxicity studies in rats with rbST
doses up to 5 g/kg bw failed to show any biological or toxicological
effect. No adverse biological effects were observed in two 2-week
oral feeding studies in rats with doses of rbSTs up to 10 mg/kg
bw/day, and in two 4-week oral feeding studies in rats with doses up
to 50 mg/kg bw/day, rbSTs caused no dose-dependent effects. In two
90-day oral feeding studies in rats with rbSTs at doses up to 100
mg/kg bw/day, no drug-related changes were observed, and in a 90-day
oral feeding study in dogs at doses up to 10 mg/kg bw/day, rbST
treatment caused no adverse toxicological effects. Two 9-day oral
feeding studies in hypophysectomized rats with rbST doses up to 4
mg/kg bw/day demonstrated that ingested rbSTs are degraded and are
not absorbed in a biologically active form. These studies confirm
that rbSTs have no biological activity when administered orally.
The genotoxic potential of the rbSTs was evaluated in two
assays, both of which were negative.
Many of the physiological effects of rbSTs are mediated by
bovine insulin-like growth factor-I (IGF-I), which is structurally
identical to human IGF-I. The liver is the major site of IGF-I
synthesis, but it is also present in human milk, saliva, and
pancreatic secretions. Two feeding studies, one in intact and one in
hypophysectomized adult rats, confirmed that IGF-I at doses up to 2
mg/kg bw/day has no biological activity when administered orally.
Because human and bovine IGF-I are structurally identical, the role
of dietary IGF-I on the gastrointestinal tract was evaluated in
several studies which showed that IGF-I is degraded by digestive
enzymes, is present in human saliva and digestive juices, and is not
active in the upper gastrointestinal tract.
The lack of oral activity of rbSTs and IGF-I and the low levels
and non-toxic nature of the residues of these compounds, even at
exaggerated doses, results in an extremely large margin of safety
for humans consuming dairy products from rbST-treated cows. In view
of the lack of impact on human food safety, the Committee
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