THE PHARMACOKINETICS OF DIMINAZENE IN DOMESTIC RUMINANTS AND THE PHENOTYPIC BASIS OF RESISTANCE TO DIMINAZENE IN TRYPANOSOMA CONGOLENSE INFECTIONS IN GOATS
THE PHARMACOKINETICS OF DIMINAZENE IN DOMESTIC RUMINANTS AND THE PHENOTYPIC BASIS OF RESISTANCE TO DIMINAZENE IN TRYPANOSOMA CONGOLENSE INFECTIONS IN GOATS
No Thumbnail Available
Date
1995-01
Authors
MOHAMMED, MAMMAN,
Journal Title
Journal ISSN
Volume Title
Publisher
Abstract
While an effective antitrypanosomal vaccine awaits development, tsetse
control and efficient use of the rather limited repertoire of antitrypanosomal drugs
have remained the only practical methods available for limiting the severe losses due
to animal trypanosomiasis in sub-Saharan Africa. In order to maximise the
therapeutic efficacy of the trypanocidal drug diminazene aceturate (Berenil®) against
the disease, a series of experiments were carried out to properly characterise the
pharmacokinetics of the drug in noninfected and trypanosome-infected cattle, goats
and mice. Studies were also carried out to investigate the mechanisms of resistance of
Trypanosoma congolense to diminazene in vivo.
In noninfected Boran (Bos indicus) cattle administered diminazene aceturate
as a single intramuscular dose, 3.5 mg/kg body weight, the pharmacokinetics of the
drug were characterised by rapid absorption (Ka = 2.56-11.60 h-1), triphasic
disposition, extensive systemic distribution (Vd(ss) - 1.20-2.02 L/kg), long elimination
half-life (t1/2Xz = 86.50-145.48 h) and slow total body clearance (CI - 1.24-1.70
ml/min/kg). Approximately 8.3% of the total administered dose was eliminated via
the urine over the first 24 h of the treatment.
Absorption and disposition of diminazene in noninfected cattle were similar to
those determined in cattle that were treated during the chronic stage of infection with
a diminazene-sensitive T. congolense population. However, when the drug was
administered to the cattle during the acute phase of an infection with a diminazenesensitive
T. congolense population, significant differences in the disposition were
observed. In particular, absorption of the drug was more rapid, and a higher peak
plasma concentration was attained but with lower systemic distribution. At 48 h of
treatment, the mean concentrations of diminazene maintained in the plasma of the
cattle were essentially the same being 0.43 ± 0.07 μg/ml in the noninfected state, 0.43
±0.11 μg/ml during the acute phase of infection, and 0.44 ± 0.09 μg/ml in chronic
infection. Thus, the plasma concentration-versus-time profiles of diminazene in
noninfected cattle were not significantly influenced by concurrent acute or chronic
trypanosome infections. The data would appear to suggest that adjusting the dose of
diminazene aceturate when treating acute or chronic trypanosome infections with
diminazene-sensitive populations may not be necessary. However, a modified dosage
regimen is required in the treatment of infections with trypanosome populations that
are resistant to the recommended single doses of diminazene aceturate.
Based on the pharmacokinetic data obtained from the above studies, therefore,
a two-dose regimen of diminazene aceturate with repeat treatments at 8 h or 24 h was
designed and its efficacy tested against experimental diminazene-resistant
trypanosome infections in goats. With the double treatment, the drug showed a
higher plasma concentration-time profile and prolonged elimination half-life. The
two-dose regimen also proved more effective than the single-dose treatment.
However, while the response of drug-resistant infections to diminazene in vivo is
dependent on the concentrations of diminazene, it is also dependent on a number of
other factors. It was found that the time, after infection, at which the animals were
administered diminazene aceturate significantly influenced the apparent respsonse of
the parasites to the drug. The treatment was curative in all goats treated on day 1 of
challenge but failed to eliminate infection in any goat when instituted after
microscopic demonstration of parasites in peripheral blood on day 19 of infection.
Further experiments indicated that the ability of the parasites to reappear after
treatment was not due to invasion of the central nervous system (CNS). In addition,
compared to plasma, concentrations of diminazene in the cerebrospinal fluid were
significantly lower and would appear unlikely to eliminate the parasites even if they
were to invade the CNS.
Comparative studies on the pharmacokinetics of diminazene in the lymph and
plasma were also undertaken in goats. The data suggested that trypanosomes at the
level of the skin would be exposed to a greater concentration of the drug than
parasites located in the blood. However, whether or not this would explain the
diminazene-sensitivity to the parasites present on day 1 of infection was not
established. It was also not clear whether the efficacy of the treatment on day 1 of
challenge was due to the relatively smaller number of parasites present at the level of
the skin compared to that in the blood on day 19 of infection. Thus, additional studies
were carried out in mice to determine the response of different parasite numbers to
diminazene aceturate as well as the frequency of diminazene-resistant trypanosomes
occurring in vivo. The results indicated that as the parasite number increased, the
diminazene-sensitivity decreased. Diminazene-resistant trypanosomes occurred at an
apparently low frequency in trypanosome populations which reappeared following
treatment with the drug. Quite surprisingly, however, the frequency varied inversely
with the parasite numbers.
In summary, the pharmacokinetic data described in the dissertation have
enhanced our understanding of the disposition of diminazene in noninfected and
trypanosome-infected animals. It also became apparent that, with the aid of an
appropriate multiple dosage regimen, infections with diminazene-resistant
trypanosome populations may be cured. The time at which treatment is commenced
after initiation of a trypanosome infection as well as the number of parasites present at
the time of the treatment significantly influence the apparent response of the parasites
to the drug. The frequency of drug-resistant trypanosomes occurring in vivo is low.
Description
A DISSERTATION
SUBMITTED TO
THE POSTGRADUATE SCHOOL,
AHMADU BELLO UNIVERSITY
IN PARTIAL FULFILMENT OF THE REQUIREMENT FOR THE DEGREE
OF DOCTOR OF PHILOSOPHY
(VETERINARY CLINICAL PHARMACOLOGY)
DEPARTMENT OF PHYSIOLOGY & PHARMACOLOGY,
FACULTY OF VETERINARY MEDICINE,
AHMADU BELLO UNIVERSITY, ZARIA, NIGERIA.
JANUARY, 1995
Keywords
PHARMACOKINETICS,, DIMINAZENE,, DOMESTIC,, RUMINANTS,, PHENOTYPIC,, RESISTANCE,, DIMINAZENE,, TRYPANOSOMA,, CONGOLENSE, INFECTIONS IN GOATS