Intraspecies individuality for the metabolism of steroids

Pharmacokinetics of phenylbutazone and its metabolite oxyphenbutazone in miniature donkeys

OBJECTIVE

To describe the pharmacokinetics of phenylbutazone and oxyphenbutazone after IV administration in miniature donkeys.

ANIMALS

6 clinically normal miniature donkeys.

PROCEDURE

Blood samples were collected before and 5, 10, 20, 30, 45, 60, 90, 120, 180, 240, 300, 360, and 480 minutes after IV administration of phenylbutazone (4.4 mg/kg of body weight). Serum was analyzed in triplicate by use of high-performance liquid chromatography for determination of phenylbutazone and oxyphenbutazone concentrations. The serum concentration-time curve for each donkey was analyzed separately to estimate model-independent pharmacokinetic variables.

RESULTS

Serum concentrations decreased rapidly after IV administration of phenylbutazone, and they reached undetectable concentrations within 4 hours. Values for mean residence time ranged from 0.5 to 3.0 hours (median, 1.1 hour), whereas total body clearance ranged from 4.2 to 7.5 ml/kg/min (mean, 5.8 ml/kg/min). Oxyphenbutazone appeared rapidly in the serum; time to peak concentration ranged from 13 to 41 minutes (mean, 26.4 minutes), and peak concentration in serum ranged from 2.8 to 4.0 mg/ml (mean, 3.5 microg/ml).

CONCLUSION AND CLINICAL RELEVANCE

Clearance of phenylbutazone in miniature donkeys after injection of a single dose (4.4 mg/kg, IV) is rapid. Compared with horses, miniature donkeys may require more frequent administration of phenylbutazone to achieve therapeutic efficacy.

Participation of a cytochrome P450 enzyme from the 2C subfamily in progesterone 21-hydroxylation in sheep liver

Progesterone 21-hydroxylation in hepatic microsomes from adult male sheep is a quantitatively important metabolic pathway (0.27 +/- 0.08 nmol deoxycorticosterone formed/min/mg protein; representing 13-25% of total progesterone conversion). This study was undertaken to determine whether the ovine hepatic progesterone 21-hydroxylase may be another member of the P450 2C subfamily, normally associated with progesterone 21-hydroxylation in rodent liver. An IgG preparation raised in rabbits against purified rat liver microsomal cytochrome P450 2C6 was found to recognize a single antigen (MW 52 kDa) in sheep liver microsomes. This protein was present in sheep liver (apparent concentration 16 +/- 4 ng/micrograms microsomal protein) representing approx. 28% of the corresponding content of P450 2C6 in untreated rat liver. Preincubation of the anti-P450 2C6 IgG with hepatic microsomes was found to decrease the rate of progesterone 21-hydroxylation to 50-80% of uninhibited control. Taken together, from these findings it is apparent that a P450 enzyme, most likely from the 2C subfamily, catalyses deoxycorticosterone formation from progesterone in sheep liver and that this is a quantitatively important pathway of progesterone hydroxylation in these fractions.

Human embryonic cytochrome P450S: phenoxazone ethers as probes for expression of functional isoforms during organogenesis

Human embryonic tissues were investigated during the period of organogenesis with a combination of substrate probes, selective inhibitors and immunoprobes in terms of their capacity to express functional P450 isoforms. A series of phenoxazone ethers utilized as substrate probes revealed that human embryonic hepatic, pulmonary, renal, adrenal and cardiac tissues each contained a complement of functional P450 isoforms when analyzed between days 50 and 60 of gestation. Preparations of each of these tissues contained isoforms capable of catalyzing O-demethylation, O-deethylation, O-depentylation and O-debenzylation of the respective phenoxazone ethers. Investigations with chemical inhibitors and inhibitory antibodies as well as comparisons with vector-expressed, human P450 isoforms suggested that isoforms of P450 subfamilies 1A, 2B, 2C or 3A were not major contributors to any of the observed reactions. The P450-dependent reactions studied exhibited several unexpected and unusual characteristics including a preference for NADH over NADPH as the initial electron donor. Results were consistent with the concept that conceptal-specific P450 isoforms participate in the human embryonic O-dealkylation/debenzylation probe reactions investigated.

Metabolic fate of [3H]deoxycorticosterone and [14C]progesterone--I. Early tissue metabolites and analysis of 21-hydroxysteroids by high pressure liquid chromatography

Rabbits injected with mixtures of [3H]deoxycorticosterone and [14C]progesterone had significant levels of both 3H and 14C in several tissues and fluids extracted 10-45 min later. The distribution of radioactivity between 21-deoxysteroid, 21-hydroxysteroid, steroid acid and steroid glucuronide fractions was determined by alumina adsorption chromatography. Steroid acids derived from both steroids accumulated in the liver, kidney and urine, but were quantitatively less significant in the bile, duodenum, uterus, spleen and lung and were detected in the blood for the first time. Different 21-hydroxysteroid profiles were detected in the tissue and fluid extracts by reverse and straight phase high pressure liquid chromatography. [3H]Deoxycorticosterone accumulated in the kidney, lung, spleen and uterus, whereas tetra and hexahydro reduced metabolites predominated in the liver, bile and duodenum. By contrast, [14C]progesterone was metabolised to more polar 21-hydroxylated metabolites which were detected in the liver, kidney and urine. These results show the influence of a steroid 21-hydroxyl function, when administered, as opposed to being formed in in vivo, on the metabolic fate and excretory pathways of 21-hydroxysteroids by the rabbit.

Selective inactivation by 21-chlorinated steroids of rabbit liver and adrenal microsomal cytochromes P-450 involved in progesterone hydroxylation

The inactivation by 21-chlorinated steroids of rabbit liver cytochromes P-450 involved in the hydroxylation of progesterone has been investigated in intact microsomes encompassing two phenotypes of 21-hydroxylase activity, two phenotypes of 16 alpha-hydroxylase activity, and three phenotypes of 6 beta-hydroxylase activity. In liver microsomes from outbred New Zealand White male rabbits exhibiting a high content of cytochrome P-450 1, 21,21-dichloropregnenolone caused a time- and NADPH-dependent loss of 21-hydroxylase activity. This loss of activity exhibited a number of characteristics of mechanism-based inactivation, including irreversibility, saturation with increasing inhibitor concentrations, and protection by substrate, and was also documented with purified P-450 1 in a reconstituted system. 21,21-Dichloropregnenolone caused no time-dependent loss of 6 beta-hydroxylase activity in microsomes from the New Zealand White rabbits or from control or rifampicin-treated rabbits of the inbred B/J strain. In contrast, in the microsomes from the B/J rabbits, some inactivation of the 16 alpha-hydroxylase was observed (k = 0.04 min-1), regardless of the rifampicin treatment. The other two compounds tested, 21-chloropregnenolone and 21,21-dichloroprogesterone, were less effective than the dichloropregnenolone as inactivators of cytochrome P-450 1. On the other hand, 21,21-dichloroprogesterone, but not 21,21-dichloropregneolone, caused a rapid time-dependent loss of 21-hydroxylase activity in rabbit adrenal microsomes. The results indicate that the introduction of a dichloromethyl group into a substrate bearing a methyl group normally hydroxylated by only one or a few forms of cytochrome P-450 may be a rational means of designing selective inhibitors of the enzyme.