| dc.description.abstract | The tetracyclines have, next to the penicillins"been
the most widely used antibiotics. There are still, however,
problems and less well investigated aspects associated'
with their use.
The present investigation compares the pharmacokinetics
and some postulated advantages of more recently introduced
oxytetracycline (OTC) preparations with a conventional
OTC product. The suitability of deep dewlap injection
of OTC as an alternative route to intramuscular (IM)
injection of the drug was also examined. A further aim
has been to examine some potential adverse effects,
especially a recent claim that tetracycline (TC) deposits
in the eye may cause corneal discolouration and lens
opacities in new-born animals if administered during
pregnancy.
The pharmacokinetic studies required adequate methods
for the determir.ation of OTC levels in biological
specimens. A microbiological agar diffusion method,
with Staphylococcus aureus NCTC 6571 as test organism,
proved to give reproducible and satisfactory results
in preliminary experiments with rats and rabbits. The
lowest detectable concentrations for OTC were 0.1 pg/ml
serum and 0.2 pg/g tissue. While the addition of
various cations reduced the OTC activity in the test
system, the sensitivity of the system was increased when
EDTA was added in a concentration between 0.12 and 1 mmol.
Comparison of the microbiological assay with high pressure
liquid chromatography (HPLC) gave similar results and
sensitivities with respect to OTC in plasma and
tissue samples, while the determination of OTC in
ruminal fluid was only possible with the microbiological
assay, since the fluid contained substances which interferred
with the HPLC determination. Autoradiography with
H-labelled TC was also introduced to trace the distribution
and accumulation of the drug in various tissues.
A cross-over trial on 8 calves was conducted to
compare two OTC preparations, a "long-acting", OTC-LA
(TerramycinR/LA long acting injectable solution, "Pfizer")
and a "conventional" formulation, OTC-C (TerramycinR-IOO
injectable solution, "Pfizer"), with regard t.o the
serum levels after 1M injection and after injection in
the dewlap, as well as the extent and nature of local
tissue damage and the drug residue concentrations at
the sites of injection. After 1M injection of the same
dose of OTC-C as recommended for OTC-LA (20 mg OTC/kg
bwt) , the resulting serum concentrations were not
significantly different. Maximum values were reached
after 4 hours and averaged 6.7 +2.2 ug OTC/ml after
OTC-C, compared with 7.5 + 2.5 ug OTC/ml after OTC-LA
administration. After injection in the dewlap, the
serum concentrations were also similar for the two
preparations, but reaching peak coricentrations of only
1.8 ~ 0.2 and 1.7 + 0.3 ug OTC/ml 12 hours after injectionsof OTC-C
and OTC-LA respectively. The therapeutic minimum
concentration of OTC was assumed to be 0.5 ug OTC/ml.
Following IM and dewlap injection of OTC-C, mean levels
above this concentration were maintained for 45 and
57 hours respectively, while the corresponding values
after OTC-LA were 52 and 62 hours. Only a very limited
retard effect of OTC-LA could be demonstrated, and the
study does not support the claim of the manufacturers
that therapeutic serum concentrations are maintained
for 3 to 5 days after a single injection of OTC-LA.
At post mortem examinations performed 44 and 63 days
after the 1M administration, no macroscopic changes were
detected after any of the two preparations, and neither
UV-illumination, the microbiological assay, or the
HPLC analyses revealed any residue of OTC at the 1M
injection sites. In contrast, post mortem examination
30 and 49 days after injection of OTC-LA in the dewlap,
revealed pronounced tissue damage. When illuminated
by UV-light, intense fluorescence indicated the presence
of OTC. This was confirmed by both HPLC analysis and
the microbiological assay, which revealed that 2.1 and
1.6 mg OTC remained at the site of injection 30 and
49 days following administration of OTC-LA. After
injection of OTC-C in the dewlap, there was far less
tissue damage, and the OTC residues after 30 and 49 days
were only 0.3 and 0.1 mg respectively.
According to these results, OTC-LA does not seem
to offer significant advantages over OTC-C. The dewlap
has been suggested as an alternative injection site,
which compared to the 1M route offers advantages from a
residue point of view. It proved, however that after
dewlap injection the area under OTC serum concentrationtime
curve was only about 56% of that after 1M injection.
The local tissue reaction, especially after dewlap
injection of OTC-LA, also indicates that this route
is not recommendable for the administration of tetracyclines.
The dewlap might, however, be a suitable site for testing •
tissue reactions caused by various drug formulations.
AquacyclineR "Rosco" (OTC-A), a recently introduced
aqueous OTC-formulation, has been claimed to render
several advantages including low tissue irritation at
the site of injection. In an experiment with 12 calves,
AquacyclineR in a 5% (OTC-AS) and a 10% (OTC-AIO)
solution,was compared with a "conventional" 10%
OTC-preparation, OTC-C (TerramycinR~100 injectable
solution, "Pfizer", by injecting 20 mg OTC/kg bwt of
these preparations in the dewlap and thereafter monitoring
serum concentrations as well as tissue reactions and
residues at the site of injection. OTC serum levels
above O.S pg/ml were maintained for about 60 ho~rs after
all three preprations. During this period, however,
OTC-AS and OTC-AIO resulted in higher initial blood
levels, reflected in 39 and 20% larger areas under the
serum concentration-time curves as compared to OTC-C.
The OTC-A preparations caused less swelling at the site
of injection. A tendency towards less pronounced tissue
reaction after OTC-A, was also observed at post mortem
examinations 28 and 42 days after injectiori. The OTC
residues at the injection site were smaller, after
OTC-AS, but none of the preparations resulted in OTCresidues
exceeding 0.3 mg. Accordingly the present
investigation gives support to the claims that OTC-A
offers advantages with regard to absorption characteristics
and tissue tolerance.
Gastrointestinal disturbances may accompany
TC therapy, but only few studies have been performed
concerning its effect on ruminal fermentative activity.
A study of this, which also involved the examination
of transfer of OTC from blood to the rumen and vice
versa after intravenous (IV) (10 and 20 mg OTC/k'g'''bwt)
and intra rurainal. (IR)(3mg OTC/kg bwt) administration,
was carried out in sheep. Following IV injection, OTC
could not be detected in the ruminal fluid, and neither
was it possible to detect OTC in plasma after IR
administration. The production in the rumen of acetic,
propionic and butyric acids was chosen as an indicator
for the effect of OTC on the fermentative activity of
the ruminal microflora. Volatile fatty acid determinations
were carried out using gas-liquid chromatography.
Although the level of OTC remained above 0.5 pg OTC/ml
for about 40 hours after IR administration, there was no
significant inhibition of the total volatile acid
production.
Development of corneal discolouration "and lens
opacities in new-born rats due to systemic use of TC
in pregnancy was recently reported, Krejci et aZ.,
Ophthalmic Res. 12: 73 - 77, 1980. It was decided to
undertake follow-up studies on this alarming and so
far unsubstantiated adverse drug effect. In the present
studies the potential of TC to be deposited and induce
changes in the foetal eye was investigated by the
administration of isotope-labelled and unlabelled TC
to pregnant rats and rabbits. Administration of
20 mg TCjkg bwt/day from day 13 or 12 of gestation till
term, had no effect on the length of pregnancy, litter
size or gross morphology of the offspring. Examination
by fluorescence m.i.c ros copy of cryostat and paraffin-embedded
sections of eyes from the offspring, did not reveal TC
deposits in the cornea or lenses of either rats or rabbits,
and no morphological changes were seen in the stained
paraffin sections. The possibility of TC deposition
in the foetal eye was also examined with whole-body
autoradiography after IV injection of 3H-labelled TC
in rats on day 9 and 20 and in rabbits on day 12 and 28
of gestation. The animals were killed at various
intervals after administration and the pregnant uteri
and the maternal kidneys and livers were examined by
autoradiographic procedures. Twenty minutes after
administration, 3H-activity had accumulated in the
placenta, uterine wall, maternal kidney and liver. A
low level of activity was present in the foetal skeleton,
while no radioactivity could be detected in the foetal
lenses or the brain. When a longer time had elapsed,
the radioactivity in the foetal/maternal tissues had
decreased, and still no radioactivity was detectable in
the foetal eye. Accordingly, the results of the present
investigations do not support the recent report
that the administration of TC to pregnant animals
results in the deposition of TC in the foetal eye causing
corneal discolouration and lens opacities. | en |
