# Dexamethasone in cattle: pharmacokinetics and ion on adrenal glande.de

P.L. Toutain, R.A. Brandon, M. Alvinerie, Rafael Garcia Villar, Y.

Ruckebusch

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P.L. Toutain, R.A. Brandon, M. Alvinerie, Rafael Garcia Villar, Y. Ruckebusch. Dexamethasone in cattle: pharmacokinetics and ion on adrenal glande.de. Journal of Veterinary Pharmacology and Therapeutics, 1982, 5, pp.33-43. hal-02717695

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# Dexamethasone in cattle: pharmacokinetics and action on the adrenal gland

P. L. TOUTAIN, R. A. BRANDON, M. ALVINERIE, R. GARCIA-VILLAR & Y. RUCKEBUSCH

Institut National de la Recherche Agronomique, Station de Pharmacologie Toxicologie, 180, chemin de Tournefeuille, Toulouse, France.

Toutain, P.L., Brandon, R.A., Alvinerie, M., Garcia-Villar, R. & Ruckebusch, Y. Dexamethasone in cattle: pharmacokinetics and action on the adrenal gland. *J. vet. Pharmacol. Therap.* 5, 33-43.

The pharmacokinetics of Dexamethasone (DXM) was studied in four cows all of which received DXM alcohol and DXM 21 isonicotinate (as a solution) by the intravenous and intramuscular routes. Concentrations of DXM and cortisol were determined using high performance liquid chromatography. An additional study was made in a second group of four cows which received intramuscular DXM 21 isonicotinate suspension for the assessment of DXM suppression of adrenal gland function. This was determined by measurements of base-line and ACTH-stimulated cortisol concentrations, before and following DXM administration.

Following intravenous administration, the disposition kinetics of both formulations were described by a two-compartment open model. The half-times of elimination were similar; 335 and 291 min, respectively, for DXM alcohol and DXM 21 isonicotinate. All other pharmacokinetic parameters were not statistically different indicating that DXM was almost totally available (from DXM 21 isonicotinate). Following intramuscular administration, no significant difference in parameters was observed between the two formulations. Peak plasma concentrations were reached at 3 to 4 h post injection and bioavailability was approximately 70%. DXM was not detected in the plasma after the intramuscular administration of the suspension.

The mean control plasma cortisol concentration was 8.8 ± 3.03 ng/ml. Following intravenous and intramuscular administrations of DXM alcohol and DXM 21 isonicotinate (solution), cortisol concentrations initially increased. However, at 120 min (intravenous) and 2-4 h (intramuscular), concentrations were negligible; 24-72 h and 48-96 h, respectively elapsed before concentrations returned control values. Following DXM 21 isonicotinate (suspension) there was no initial increase and concentrations had not returned to normal in all four cows until 52 days post administration. Similarly, ACTH-stimulated plasma cortisol concentrations decreased progressively and significantly post administration. At 52 days, response to ACTH was normal in all animals.

Dr P. L. Toutain, Institut National de la Recherche Agronomique, Station de Pharmacologie Toxicologie, 180, chemin de Tournefeuille, 31300 Toulouse, France.
## INTRODUCTION

Dexamethasone (DXM) is a long-acting synthetic analogue of cortisol used in cattle either alone or in combination with antibiotics for the treatment of several diseases. In addition, it is frequently used for the induction of parturition.

The pharmacological properties of this drug are well known. In contrast, the availability of information on the pharmacokinetics and the duration and extent of suppressive effects of DXM on the bovine adrenal gland is scant. This paucity of information can be explained by the analytical difficulties encountered in the evaluation of DXM and endogenous corticosteroids in cattle. Following administration of therapeutic doses of DXM to cattle, concentrations of the drug in the plasma are very low; in addition, concentrations of the endogenous corticosteroids, cortisol and corticosterone are lower in cattle than in any other domestic species (Heitzman, Adams & Hunter, 1970; Macadam & Eberhart, 1972). Amongst the available techniques of evaluation of plasma concentrations of corticosteroids (fluorometry, gas chromatography, radiocompetition, radioimmunoassay, high performance liquid chromatography (HPLC)) it has been found that HPLC is the most accurate (Tsuei, Ashley, Moore & McBride, 1978; Rose & Jusko, 1979).

Several forms of DXM can be administered, for example, DXM alcohol, salt or ester (phenylpropionate, acetate, 21 isonicotinate). For each form of DXM, various pharmaceutical preparations are available, both long and short acting, e.g. DXM as a solution or a suspension. It is presumed that each pharmaceutical preparation has its own particular pharmacokinetic disposition because of the clinically observed differences in pharmacological action.

The aim of this experimental work was to compare the pharmacokinetic dispositions of intravenously (i.v.) and intramuscularly (i.m.) administered DXM, as both DXM alcohol and DXM 21 isonicotinate (solution) and intramuscularly administered DXM 21 isonicotinate (suspension). The second objective was to determine the duration and magnitude of suppressive effects of these preparations on the bovine adrenal gland.

## MATERIALS AND METHODS

### *Animals*

Eight healthy, non-lactating, dairy cows of mixed breeding, weighing between 310 and 540 kg were used in this study. Each cow was stalled and allowed hay and water *ad libitum*. Four cows were used for intravenous and intramuscular studies of DXM alcohol and DXM 21 isonicotinate (as both a solution and a suspension) and the remaining four, for the assessment of adrenal gland suppression using ACTH response tests following the intramuscular injection of a suspension of DXM 21 isonicotinate.

### *Drug administration and blood sampling*

Each experiment commenced between 7.00 and 8.00 a.m. A control blood sample from the first group of four cows was taken from the tail vein with as little disturbance to the animals as possible. Each cow was then placed in a stanchion for 24 h; catheters were inserted (right jugular vein) and a second control blood sample was taken immediately before drug administration.

DXM alcohol in propylene glycol (Cortamethasone, Vetoquinol, Lure, France) or DXM 21 isonicotinate as a solution in propylene glycol (Voren solution, Boehringer, Ingelheim, Reims, France) at a dose of 0.1 mg/kg (expressed as DXM alcohol) was then injected intravenously via the left jugular vein or intramuscularly at two different locations in the gluteus muscle. The order of administrations was randomly assigned and there was a delay of at least 10 days between each administration. Samples were collected via the intravenous catheter at 1, 2, 4, 8, 16, 30 and 60 min, 2, 4, 6, 8, 10, 12 and 24 h after intravenous or intramuscular administrations. At 24 h, cows were returned to their stalls and further blood samples (tail vein) were collected each 24 h for 3 days. After completion of these four experiments DXM 21 isonicotinate as a suspension (Voren Depot, Boehringer, Ingelheim, Reims, France) was administered intramuscularly (0.1 mg/kg) and blood samples were collected as previously described.

The duration and magnitude of adrenal
glandular suppression after intramuscular administration of DXM 21 isonicotinate as a suspension was determined in the remaining four cows. Two control blood samples were collected as described above before the commencement of the following procedure. At two days before the intramuscular administration of DXM, a control ACTH response test was performed by injecting 50 i.u. of ACTH (ACTH Choay, Choay, Paris) via the left jugular vein. Blood samples from the right jugular vein were taken at 0.5, 1, 2 and 3 h post injection. Following intramuscular injection of DXM 21 isonicotinate as a micro-particulate suspension (15 to 30 µm diameter) similar ACTH response tests were performed at 24 h, 7, 14, 21, 30, 45 and 52 days after administration. In addition following DXM injection, blood samples were collected at 0.5, 1, 2, 6, 10, 12, 24, 48, 72, 96, 120 and 144 h for the assessment of plasma cortisol concentrations.

Blood for analysis was collected into heparinized tubes and centrifuged within 4 h after sampling. Plasma was stored at -20°C until chromatographic analysis.

### Analytical technique

The plasma concentrations of DXM and cortisol were simultaneously measured by the HPLC method described elsewhere by Alvinerie & Toutain, 1981. The threshold sensitivity was 2 ng/ml for DXM and cortisol; corticosterone control values approximated the threshold too closely and thus were not measured.

### Mathematical analysis

The mathematical analysis of the results was performed using a programmable calculator (Hewlett Packard 41C). Individual and pooled plasma concentrations were used for pharmacokinetic analyses according to the generally accepted methods (Wagner, 1975). Values in the text are reported as mean ± standard deviation.

## RESULTS

The plasma concentrations (ng/ml) of DXM

after intravenous administration of DXM alcohol and DXM 21 isonicotinate are shown in a semi-logarithmic plot in Fig. 1. Three phases appeared in the decrease of plasma concentrations; the first and second phases were rapid (0--16 and 16--60 min, respectively) whereas the third and final phase was longer and represented elimination of the drug. In some cattle, we were able to detect concentrations in the plasma at 24 h but these values were not used in the calculation of pharmacokinetic parameters because they were too near the limit of sensitivity of our analytical technique. The plasma concentrations obtained between 1 and 720 min were fitted to both equations 1 and 2:

$$Cp^t = Ae^{-\alpha t} + Be^{-\beta t} \quad (1)$$

$$Cp^t = Ae^{-\alpha t} + Be^{-\beta t} + Ce^{-\gamma t} \quad (2)$$

where $Cp^t$ is the concentration of drug in plasma at time $t$, A, B and C are coefficients and $\alpha$, $\beta$ and $\gamma$ are exponential coefficients. To select the most suitable equation for the description of the experimental data, coefficients of determination were calculated for each. It was found that the improvement in the coefficient of determination using equation 2 was limited (Wagner and coworkers, 1977) and equation 1 was used for the description of the experimental data. Table I presents the values of the pharmacokinetic parameters for the two forms of DXM calculated from averaged plasma concentration. There was no significant difference between the pharmacokinetic parameters of the two formulations (paired t test, $P>0.05$). The half-times of elimination were 335 ± 37.5 and 291 ± 29.6 min (mean ± SD) respectively for DXM alcohol and DXM 21 isonicotinate. Variations between subjects were moderate with extreme values ranging from 202 to 369 min. The volumes of distribution ($V_d$ area) were large and similar for both (1.1--1.2 litres/kg) and the ratio $Fc = k_{12}/k_{21}-\beta$ (the ratio of drug level in the peripheral compartment to drug level in the central compartment after pseudodistribution equilibrium has been attained) indicated that DXM was largely distributed in the tissues with approximately 75%--80% of the drug in the peripheral compartment for both formulations. The area under the curve (AUC) was
<figure><img src="image_2.png" /><figcaption>FIG. 1. The semi-logarithmic plot of mean plasma concentration (ng/ml) versus time (h) for Dexamethasone alcohol (○) and Dexamethasone 21 isonicotinate (●) after intravenous administration at a dose rate of 0.1 mg/kg.</figcaption></figure>

<table><caption>TABLE I. Kinetics of Dexamethasone in cattle; intravenous route (0.1 mg/kg)</caption><thead><tr><th>Pharmacokinetic parameters</th><th>DXM alcohol</th><th>DXM 21-isonicotinate</th></tr></thead><tbody><tr><td>C<sub>p</sub><sup>0</sup> (ng/ml)</td><td>400.5</td><td>352</td></tr><tr><td>A (ng/ml)</td><td>328</td><td>279</td></tr><tr><td>α (min<sup>-1</sup>)</td><td>0.0859</td><td>0.0612</td></tr><tr><td>t<sub>1/2</sub>α (min)</td><td>8.06</td><td>11.3</td></tr><tr><td>B (ng/ml)</td><td>72.5</td><td>72.9</td></tr><tr><td>β (min<sup>-1</sup>)</td><td>0.00206</td><td>0.00238</td></tr><tr><td>t<sub>1/2</sub>β (min)</td><td>335</td><td>291</td></tr><tr><td>V<sub>d</sub> area (litre/kg)</td><td>1.18</td><td>1.105</td></tr><tr><td>ClB (ml/min<sup>-1</sup>/kg<sup>-1</sup>)</td><td>2.43</td><td>2.62</td></tr><tr><td>k<sub>12</sub> (min<sup>-1</sup>)</td><td>0.0605</td><td>0.0390</td></tr><tr><td>k<sub>21</sub> (min<sup>-1</sup>)</td><td>0.0172</td><td>0.0145</td></tr><tr><td>kel (min<sup>-1</sup>)</td><td>0.0102</td><td>0.0100</td></tr><tr><td>Fc</td><td>4.0</td><td>3.2</td></tr><tr><td>AUC (ng/ml/min)</td><td>41 179</td><td>38 772</td></tr></tbody></table>

C<sub>p</sub><sup>0</sup>, extrapolated zero time concentration; A, extrapolated zero time plasma drug concentration of the distribution phase; α, rate constant of the distribution phase; t<sub>1/2</sub>α, half-time of distribution; B, extrapolated zero time plasma drug concentration of the elimination phase; β, overall rate constant of the elimination phase; t<sub>1/2</sub>β, half-time of elimination; V<sub>d</sub> area, apparent specific volume of distribution; ClB, clearance; k<sub>12</sub>, k<sub>21</sub>, transfer rate constants; kel, rate constant of elimination; Fc, ratio of the drug located in the peripheral compartment and the central compartment; AUC, area under the curve.
calculated by the trapezoidal rule using Cp*t*/β as an estimate of the infinite part of the curve. Cp*t* represents the last measured concentration of DXM and β is the overall elimination rate constant. There was no difference found between the two formulations thus indicating that DXM was totally available from DXM 21 isonicotinate.

The semi-logarithmic plots of the plasma concentrations of DXM (ng/ml) versus time (h) after the intramuscular administration of DXM alcohol and DXM 21 isonicotinate as a solution are shown in Fig. 2. For both formulations plasma concentrations increased up until 3–4 h post injection; at 4–5 h a plateau of plasma concentrations was observed and then subsequently, the concentrations decreased as was observed after intravenous administration. The Bateman equation was used to mathematically describe the data:

$$C_p^t = \frac{ka Bo}{ka - kel} (e^{-kelt} - e^{-kat})$$

where ka is the absorption rate constant, kel is the elimination rate constant and Bo is the theoretical initial concentration. The distribution phase of the drug is not considered in the equation because it is masked by the phase of absorption. Consequently, the central and peripheral compartments are combined and $kel = β$. Table II presents the values of the pharmacokinetic parameters calculated for the two formulations.

No significant difference was noted between the two formulations when the pharmacokinetic parameters were compared (paired t test; $P>0.05$). The half-time of the absorption phase for both was approximately 1.3 h, the maximal concentration (42–44 ng/ml) being obtained between 3.5 and 4.2 h post injection; systemic availability (the percentage of the entire dose that effectively reaches the central compartment after the intramuscular injection) was evaluated using the ratio of the areas under the intravenous and intramuscular curves. No significant difference was noted between the two formulations and approximately 70% of the DXM was available. After the intramuscular administration of DXM 21 isonicotinate as a suspension, concentrations of DXM were not detected in the plasma.

The mean plasma cortisol concentration in samples taken from the tail vein of each of the four cows was 8.8 ± 3.03 ng/ml with a range from 4.8 to 12.0 ng/ml. Cortisol concentrations in plasma samples from the jugular vein of cows in stanchion were significantly higher and more variable; 18 ± 12.2 with a range from 5 to 41 ng/ml.

Figure 3 represents the histogram of plasma cortisol concentrations (ng/ml) versus time (h) after the intravenous administration of DXM 21 isonicotinate; similar results were observed for DXM alcohol. For both formulations two phases were seen. During the first

<figure><img src="image_3.png" /><figcaption>FIG. 2. The semi-logarithmic plot of mean plasma concentration (ng/ml) versus time (h) for Dexamethasone alcohol (○) and Dexamethasone 21 isonicotinate (●) after intramuscular administration at a dose rate of 0.1 mg/kg.</figcaption></figure>
<table><caption>TABLE II. Kinetics of Dexamethasone in cattle; intramuscular route (0.1 mg/kg)</caption><thead><tr><th>Pharmacokinetic parameters</th><th></th><th>DXM alcohol</th><th>DXM 21-isonicotinate</th></tr></thead><tbody><tr><td>ka (min<sup>-1</sup>)</td><td></td><td>0.00786</td><td>0.00933</td></tr><tr><td>t<sub>1/2</sub> ka (min)</td><td></td><td>94</td><td>74</td></tr><tr><td>Cmax (ng/ml)</td><td></td><td>42.8</td><td>44.1</td></tr><tr><td>Tmax (min)</td><td></td><td>256</td><td>214</td></tr><tr><td>β = kel (min<sup>-1</sup>)</td><td></td><td>0.001748</td><td>0.001881</td></tr><tr><td>t<sub>1/2</sub> β (min)</td><td></td><td>396</td><td>368</td></tr><tr><td>AUC (ng/ml/min)</td><td></td><td>27 614</td><td>28 953</td></tr><tr><td>Bioavailability(%)</td><td></td><td>67</td><td>72</td></tr></tbody></table>

ka, absorption rate constant; t₁/₂ ka, half time of absorption; Cmax, maximal concentration; Tmax: peak concentration time.

phase (0–60 min) cortisol concentrations increased, however, important interindividual variations existed. In one animal the administration of both preparations induced a rise in concentrations of cortisol with maximal values of 80 ng/ml at 8 min post injection. The elevation in the concentration of cortisol was the highest in this animal and she became recumbent within a few seconds after the rapid injection of the drug. In all animals we observed tachypnoea and ataxia. The second phase represented the suppressive effect of DXM on the adrenal cortex. In one of the animals, concentration of cortisol was negligible 16 min after the injection of DXM 21 isonicotinate; at 120 min cortisol concentrations in all subjects were negligible. For both

formulations 24–72 h elapsed before the cortisol concentrations returned to normal.

Figure 4 represents the histogram of plasma cortisol concentration (ng/ml) versus time (h) after the intramuscular administration of DXM 21 isonicotinate as a solution. After the intramuscular injections there was a large variation in the concentrations of cortisol between individuals until 2–4 h post injection; at this time all cortisol concentrations were decreased. Forty-eight to ninety-six hours elapsed before cortisol concentrations returned to normal which was slightly longer than that observed after intravenous administration. No significant difference was observed between the two formulations.

Because of our inability to detect concen-

<figure><img src="image_4.png" /><figcaption>FIG. 3. Histogram of mean plasma cortisol concentration (ng/ml) versus time (h) after the intravenous administration of Dexamethasone 21 isonicotinate at a dose rate of 0.1 mg/kg; mean and standard deviation.</figcaption></figure>
<figure><img src="image_5.png" /><figcaption>FIG. 4. Histogram of mean plasma cortisol concentration (ng/ml) versus time (h) after the intramuscular administration of Dexamethasone 21 isonicotinate as a solution at a dose rate of 0.1 mg/kg; mean and standard deviation.</figcaption></figure>

trations of DXM in the plasma after the intramuscular administration of the suspension of DXM 21 isonicotinate, a more precise study was performed with this formulation using the second group of four cows. Figure 5 represents the histograms of mean plasma cortisol concentrations over the first 24 h and from 2 to 52 days, respectively, following the administration of DXM 21 isonicotinate. During the first 24 hours, plasma cortisol concentrations decreased from 11.4 ± 1.05 ng/ml (control via the tail vein) to 1.95 ±

0.968 ng/ml, that is, the level of sensitivity of our analysis. From 2 to 7 days, concentrations were very low or not detectable. At 7, 14 and 21 days, the concentrations were 2.8 ± 2.12, 4.2 ± 3.26 and 3.4 ± 4.44 ng/ml, respectively. The plasma cortisol concentration of one of the four cows had returned to normal at 30 days and in the remaining three, concentrations were not detectable or less than 5.5 ng/ml. At 45 days cortisol concentrations were normal in three of the four cows for which the mean was 10.5 ± 5.04 ng/ml.

<figure><img src="image_6.png" /><figcaption>FIG. 5. Histogram of mean plasma cortisol concentration (ng/ml) versus time (h, days) after the intramuscular administration of Dexamethasone 21 isonicotinate as a suspension at a dose rate of 0.1 mg/kg; mean and standard deviation.</figcaption></figure>
Plasma cortisol concentrations in all cows had returned to normal at 52 days.

Figure 6 represents the histogram of base-line plasma cortisol concentrations (tail and jugular) and ACTH-stimulated plasma cortisol concentrations 2 days before and 1, 7, 14, 21, 30, 45 and 52 days after the administration of DXM 21 isonicotinate as a suspension. At days 45 and 52, as was seen 2 days before injection, base-line cortisol concentrations in samples taken with contention (jugular) were significantly greater (0.1>P>0.05; paired t test) than those obtained when there was little disturbance to the animals (tail). In contrast, at days 1, 7, 14, 21 and 30 there was no significant difference between base-line cortisol concentrations before and after contention (P>0.05; paired t test).

Plasma cortisol concentrations increased markedly to 61.7 ± 14.05 ng/ml (mean value at 2 and 3 hours post injection) after ACTH injection 2 days before drug administration (control ACTH test). Following DXM 21 isonicotinate administration, ACTH-stimulated plasma cortisol concentrations decreased progressively and significantly from day 1

(P<0.01; paired t test) to day 21 (P<0.05; paired t test) at which time the value was 5.8 ± 5.02 ng/ml. Thereafter concentrations increased and had returned to normal (56.9 ± 12.78 ng/ml) at 52 days for all cows (P>0.05; paired t test). At 30 and 45 days, one and three of the total four cows, respectively, had returned to normal.

DISCUSSION

In man, pharmacokinetic studies of DXM have been performed using many and varied techniques for the determination of DXM concentrations in the plasma. The results of these studies demonstrate important inter-study variations depending upon the analytical technique used. It is now considered that HPLC is the superior method for the quantification of corticosteroid concentrations in the plasma. Using an HPLC technique developed in our laboratory we were able to detect simultaneously plasma DXM and cortisol concentrations in the cow allowing the determination of the relevant pharmaco-

<figure><img src="image_7.png" /></figure>

FIG. 6. Histogram of the mean base-line plasma cortisol concentrations (tail and jugular) and ACTH-stimulated plasma cortisol concentrations 2 days before and 1, 7, 14, 21, 30, 45 and 52 days after the intramuscular administration of Dexamethasone 21 isonicotinate as a suspension at a dose rate of 0.1 mg/kg; mean and standard deviation. ACTH-stimulated plasma cortisol concentration represented here is the mean of the concentrations at 1 and 2 h post ACTH injection. (■) Caudal venous samples; (□) jugular venous samples; (■) ACTH response.
kinetic parameters and the assessment of the effects of this drug on the function of the adrenal gland.

After intravenous administration of DXM alcohol or DXM 21 isonicotinate the half-times of distribution (8–11 min) and elimination (291–335 min) were very similar for the two formulations. DXM 21 isonicotinate is a prodrug which releases DXM as the active moiety. In the present experiment, hydrolysis was very rapid and complete since the plasma DXM concentration was the highest at the first sampling (1 min) and the areas under the curve were not statistically different between the two formulations. Consequently we conclude that DXM alcohol and DXM 21 isonicotinate as a solution can be considered as being equivalent. Similar results were obtained using the same drugs in dogs (unpublished data). In man the conversion of DXM phosphate to DXM was rapid but during the first 10 min after intravenous administration the concentration of DXM phosphate exceeded the concentration of DXM alcohol and a 90% conversion of DXM phosphate to DXM alcohol was suggested (Hare, Yeh, Ditzler, MacMahon & Duggan, 1975). In contrast, Tsuei, Moore, Ashley & MacBride (1979) estimated that conversion of DXM phosphate was so rapid and complete that the process did not influence the disposition kinetics of DXM and it was suggested that chemical dissociation rather than enzymatic hydrolysis was the major decomposition pathway.

Pharmacokinetic data on DXM are scant in domestic animals and consequently the possibilities of interspecies comparisons were limited. In the dog it was found (unpublished data) that the half-time of elimination for both DXM alcohol and DXM 21 isonicotinate was approximately 110–130 min. In sheep after several days of DXM infusion, Leish & Panaretto (1978) demonstrated that there was a 30 h delay before plasma concentrations reached a plateau which suggested a plasma half-time in this species of approximately 5 h. In man, more pharmacokinetic data has been documented but many interstudy differences exist depending upon the analytical technique used. With a similar assay technique and a dose rate equal to that used in this experiment, the half-time of elimination of DXM was 142 min in women and 201 min in men (Tsuei

et al., 1979) both of which are significantly less than values we have obtained in the cow. This could be related to a more rapid catalysis in man as suggested by the differences observed in plasma clearance values between the two species (3–4 ml/min/kg in man versus 2.5 ml/min/kg in cattle). In the rat the value of the half-time has been reported as ranging between 2.6 and 5.4 h (Varma & Mulay, 1980). In the horse, plasma and urine concentrations have been reported but a pharmacokinetic analysis was not made (Seawright, Ng, Steele, Mason & Loth, 1979). Preliminary data obtained in our laboratory in horses indicated a half-time in this species of approximately 2–3 h. Consequently we can conclude that the bovine is a species in which the value of the half-time of elimination is the largest and thus it can be presumed that a longer duration of pharmacological effect and a greater suppressive action on the anterior pituitary occurs.

The volume of distribution was relatively high (1.1–1.2 litres/kg) and similar values have been found in man. After calculating $F_c = k_{12}/k_{21}-\beta$, we concluded that approximately 80% of DXM was present in the peripheral compartment after pseudodistribution equilibrium had been attained. This appeared to be relevant to the mechanism of action of synthetic corticosteroids which must penetrate the cells to exert their effect (Melby, 1977). Furthermore, it explains why after a delay of only 3–4 h, the plasma concentration of DXM was low.

After intramuscular administration, the rate and extent of absorption was relatively high; half-time of absorption was 1.5–2.0 h and bioavailability was estimated at 65%–75%. Thus this route of administration would be preferred over the intravenous route as it is without side-effects and is equally as efficient.

The control plasma cortisol concentrations were found to be similar to those formerly reported in the literature (Heitzmann *et al.*, 1970; Macadam & Eberhart, 1972) when samples were taken quickly via the tail vein so as to cause as little disturbance to the animal as was possible. In contrast, if samples were taken from the jugular vein with the animal in a stanchion the control values were elevated and more variable. These elevations and variations were related to the classical effects of
stress and a mild disturbance lasting approximately 10 min was sufficient to quadruple the plasma cortisol concentrations in some cattle. Thus it was confirmed that for the quantificative evaluation of endogenous corticosteroids in this species samples must be taken with as little contact to the head and neck as possible (Willet & Erb, 1972).

Immediately after intravenous or intramuscular administration of both formulations, base-line cortisol concentrations were significantly increased. This could be related to side effects observed after drug administration. After intravenous injection ataxia and tachypnoea occurred presumably as a result of a sudden and transient hypotension. Propylene glycol, which was the excipient in both formulations, was probably responsible for these effects. In cattle, propylene glycol administered intravenously induced cardiac arrest due to an increase in vagal tone (Gross, Kitzman & Adams, 1979). A similar effect has been reported in the dog (Bost & Ruckebusch, 1962). After intramuscular injection pain was evident and could explain the immediate elevation of plasma cortisol concentrations following drug administration. These observed increases in plasma cortisol concentrations immediately following DXM administration, which we have attributed to the above, present some difficulties in the determination of the time at which suppressive effects on the adrenal cortex begin to occur. However after a delay of 120 min, plasma cortisol concentrations were lower than control values after both intravenous and intramuscular administrations; similar delays have been reported in the dog and horse (Eiler & Oliver, 1980; Eiler, Oliver & Gobble, 1979). Consequently, as in other species, DXM could be used for the diagnosis of hypercorticism. The duration of adrenal cortical suppression was 24-72 h and 48-96 h after the intravenous and intramuscular administrations, respectively. These values are slightly longer than those reported in the dog and in man and can be related to the greater half-time of DXM in the cow. The duration of anti-inflammatory action of the glucocorticoids has been claimed to approximate the duration of their suppressive effects (Melby, 1977). By accepting this claim we would presume that the duration of anti-inflammatory action of DXM administered

as a solution, would be between 2 to 4 days.

After the intramuscular administration of DXM 21 isonicotinate as a suspension, we were unable to detect concentrations of DXM in the plasma at any time and thus, pharmacokinetic details could not be determined. In order to determine whether or not DXM was sufficiently absorbed from the site of administration and to evaluate functional suppression of the adrenal cortex we measured base-line and post ACTH injection plasma cortisol concentrations. DXM 21 isonicotinate decreased the base-line cortisol concentrations during the interval of from 30 min to 45 days and, in addition, suppressed the response to ACTH of the adrenal cortex from 24 h to 30-45 days post injection. Similarly, the response of the adrenal cortex to stress was depressed from 24 h to 45 days. In contrast to base-line cortisol concentrations following the intramuscular administration of DXM 21 isonicotinate as a solution there was no immediate increase in the cortisol concentrations following the administration of the suspension. This can be explained by the clinical observation that there was no evidence of pain following the injection of the suspension.

Both the static and dynamic functions of the adrenal cortex of the cow were depressed as was determined by the base-line and ACTH-stimulated plasma cortisol concentrations, respectively. Therefore it can be concluded that DXM was absorbed from the site of administration and would have had a therapeutic effect, despite the fact that plasma concentrations were too small to be detected even at the sensitivity of our analysis.

In conclusion, the specificity and sensitivity of the HPLC technique used in this study has allowed the simultaneous detection of endogenous corticosteroids and DXM in the plasma of the cow. As a result, values of the pharmacokinetic parameters have been determined for differing formulations and routes of administration and some insight has been gained into the suppressive effects of this drug on the bovine adrenal gland.

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Eiler, H. & Oliver, J. (1980) Combined dexamethasone suppression and cosyntropin (synthetic ACTH) stimulation test in the dog; new approach to testing of adrenal gland function. *American Journal of Veterinary Research*, **41**, 1243-1246.

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