Differential pharmacokinetics of acetohexamide in male Wistar-Imamichi and Sprague-Dawley rats: role of microsomal carbonyl reductase

Acetohexamide (AH) is reduced to its alcohol metabolite by carbonyl reductase. We have previously shown that carbonyl reductase present in the liver microsomes of rats is a male-specific and androgen-dependent enzyme. In the present study, the role of microsomal carbonyl reductase in the pharmacokinetics of AH was examined in male Wistar-Imamichi (WI) and Sprague-Dawley (SD) rats after its intravenous administration. AH was eliminated more slowly from plasma in the WI strain, which lacks most of the microsomal carbonyl reductase, than in the SD strain. Furthermore, several pharmacokinetic parameters were derived from the data for the plasma concentrations of AH. The plasma clearance (CL(p)) of AH (72.8+/-11.2 ml/h/kg) in male WI rats was significantly smaller than that (105.5+/-11.1 ml/h/kg) in male SD rats. Testectomy caused a marked decrease, from 105.5+/-11.1 to 44.3+/-11.8 ml/h/kg, in the CL(p) of AH in male SD rats. These results indicate that microsomal carbonyl reductase plays a critical role in the differential pharmacokinetics of AH in male WI and SD rats.

Sex-dependent pharmacokinetics of S(-)-hydroxyhexamide, a pharmacologically active metabolite of acetohexamide, in rats

The pharmacokinetic profile of S(-)-hydroxyhexamide (S-HH), a pharmacologically active metabolite of acetohexamide, was examined in male and female rats. S-HH was eliminated more rapidly from plasma in the males than in the females. A significant sex difference was observed in the pharmacokinetic parameters of S-HH in rats. Testectomy caused significant alteration in these parameters of S-HH in male rats, whereas ovariectomy did not in the females. The co-administration of sulfamethazine significantly decreased the plasma clearance (CL(p)) of S-HH in male rats, but had no effect in the females. The plasma concentrations of acetohexamide generated from S-HH showed no sex-related difference. Furthermore, there was no difference in the accumulation of S-HH by renal cortical slices from male and female rats. We propose the possibility that the sex-dependent pharmacokinetics of S-HH in rats is mediated through the male-specific hydroxylation of the cyclohexyl ring catalyzed by a major cytochrome p450 (CYP) isoform (CYP2C11), although the detailed mechanism remains to be elucidated.

Carbonyl reductase activity for acetohexamide in human erythrocytes

Acetohexamide is an oral antidiabetic agent and is metabolized by the reductive conversion of the acetoxy group to a secondary alcohol metabolite. In vivo, many drugs are metabolized by reductase enzymes; however, the characteristics of the enzymes that reduce carbonyl compounds need to be clarified. We tested whether reductase activity for acetohexamide can be found in human erythrocytes. Enzyme activity was monitored by formation of hydroxyhexamide using HPLC methods. In human erythrocytes, reductase activity (6.10 +/- 1.20 nmol/min/g hemoglobin) (mean +/- SD) was indeed observed, when 0.5 mM acetohexamide was used as a substrate. KM values and Vmax at the physiologically important pH 7.4 were 0.70 +/- 0.13 mM and 9.19 +/- 0.88 nmol/min/g hemoglobin, respectively. Separation of protein by gel filtration gave one major peak fraction with reductase activity whose molecular weight was estimated to be 31,000. Known substrates of carbonyl reductase such as menadione, daunorubicin, and ethacrynic acid inhibited the acetohexamide reduction. The acetohexamide reductase in erythrocyte showed characteristics of carbonyl reductase. Furthermore, acetohexamide reductase activity in erythrocyte was approximately 30% activity of that of human liver (0.17 +/- 0.05 nmol/min/mg cytosolic protein). The pattern of inhibitors in human liver was essentially the same as that in erythrocytes. It is plausible that the activity in erythrocytes may predict the activity in the liver. It was concluded that carbonyl reductase in human erythrocyte plays an important role in acetohexamide metabolism.

A case of acetohexamide-induced hypoglycemia: the influence of hypothyroidism on the metabolism of acetohexamide

Prolonged hypoglycemia induced by acetohexamide (AH) in a patient with noninsulin dependent diabetes mellitus accompanied by primary hypothyroidism was presented. A 74-year-old man who had been treated with AH (500mg, daily) for diabetes mellitus since 1973 was admitted to our hospital in Oct. 1988 because of hypoglycemic coma. On admission, the level of blood glucose was 20mg/dl. Continuous intravenous administration of 10 per cent glucose solution led to improvement in the mental state on the second day. However, the level of blood glucose remained between 30 to 45mg/dl for four days after admission. On the fifth day, a fasting blood glucose level finally reached 75mg/dl. In a thyroid function test, the serum levels of thyroid hormone showed the following decreases: T3 68ng/dl, T4 2.8 micrograms/dl, free T4 0.3ng/dl, while basal TSH levels increased to 50.3 microU/ml. Since anti-thyroid microsomal antibody was positive and thyroid 99mTc-pertechnetate uptake was slightly elevated, the hypothyroidism in this patient was considered to be caused by chronic thyroiditis. Urinalysis was positive for protein. In a renal function test, the blood urea nitrogen was 26.7mg/dl and creatinine 1.7mg/dl, and creatinine clearance decreased to 22ml/min. After thyroid function returned to euthyroid, creatinine clearance improved (41 ml/min). To clarify the relationship between hypothyroidism and the metabolism of AH, the serum levels of AH and its metabolite hydroxyhexamide (HH) following oral administration of AH (500mg) were evaluated before and after thyroxine replacement therapy. The blood glucose level before therapy was lower than that after therapy, and hypoglycemic symptoms were observed early in the second and third morning after AH administration.(ABSTRACT TRUNCATED AT 250 WORDS)

In vivo and in vitro binding of (-)-hydroxyhexamide, a major metabolite of acetohexamide, to rabbit serum

The in vivo and in vitro bindings of (-)-hydroxyhexamide, a major metabolite of acetohexamide, to rabbit serum were examined by using an ultrafiltration method. The in vivo serum protein binding of (-)-hydroxyhexamide was much lower than the in vitro serum protein binding. The in vitro serum protein binding of (-)-hydroxyhexamide was strongly displaced by the addition of acetohexamide. Furthermore, the in vitro serum protein binding of (-)-hydroxyhexamide in the presence of acetohexamide and (-)-hydroxyhexamide at the same concentrations as those found 1.0 h after acetohexamide administration was approximately similar to the in vivo serum protein binding of (-)-hydroxyhexamide. These results lead us to conclude that acetohexamide, the parent drug of (-)-hydroxyhexamide, plays an important role in the in vivo serum protein binding of (-)-hydroxyhexamide.

GLC determination of acetohexamide and hydroxyhexamide in biological fluids

A sensitive and specific GLC assay was developed for acetohexamide and hydroxyhexamide, its major metabolite, in plasma and urine. The assay uses tolbutamide as a mass internal standard. Compounds are extracted from acidified plasma or urine with toluene, converted to methylated derivatives with dimethyl sulfate, and measured by GLC using a flame-ionization detector. With GLC-mass spectrometry, the compounds measured are the N-methylsulfonamides resulting from GLC pyrolysis. Plasma and urine data are presented from a bioavailability study to demonstrate the utility of this method.

Neonatal hypoglycaemia in infants of diabetic mothers given sulphonylurea drugs in pregnancy

Three infants whose diabetic mothers were given chlorpropamide and one infant whose diabetic mother was given acetohexamide up to the time of delivery were studied in the neonatal period because each became severely hypoglycaemic. The sulphonylurea drugs given to the mother crossed the placenta, and fetal plasma concentrations were in the therapeutic range for adults with diabetes mellitus. Each baby had severe hyperinsulinaemia resulting in profound hypoglycaemia. These acutely ill infants needed vigorous and prolonged treatment to correct the hypoglycaemia. In two infants exchange transfusion was performed to remove the drug. These sulphonylurea drugs should not be used to control diabetes mellitus in pregnancy.

Metabolism, blood levels and rate of excretion of acetohexamide in human subjects

The metabolism of acetohexamide was studied in normal and diabetic subjects,utilizing acetohexamide-C-14 and hydroxyhexamide-C-14 in two types of experiments. In one, blood levels were determined by an isotopic dilution analysis (IDA). In the other, following the oral and intravenous administration of acetohexamide-C-14, radiocarbon levels in the blood and urine and the nature of the urinary metabolites were elucidated. IDA showed an average half-life for acetohexamide of 1.6 hrs. (range 0.8 to 2.4) and an average half-time for its principal metabolic product, hydroxyhexamide, of 5.3 hrs. (range 3.7 to 6.4). Half-time for total radiocarbon was 5.3 hrs. (range 3.5 to 11). Urinary recovery ofradioactivity after oral doses averaged 71.6 per cent in twenty-four hours.

Of the urinary metabolites of acetohexamide (AH) recovered, about 65 per cent was L-hydroxyhexamide (HH) and the remainder consisted chiefly of 4-trans-hydroxyacetohexamide (HAH), 4-trans-hydroxyhydroxyhexamide (HHH), and unchanged acetohexamide with small quantities of other hydroxylated isomers. Fecal excretion was measured in one patient followingthe oral administration and found to be 15 per cent. The observation that, even followingintravenous injection, urinary recovery was only 85 per cent suggested that biliary excretion represents the secondary route of elimination of acetohexamide and/or its metabolites. The radioactivity which was not recovered in the urine at the end of 48 hrs., e.g., on the average 25 per cent, was probably excreted in the stools, resulting from the failure of a small quantity of the tablet to be absorbed and from biliary excretion. The blood and urine data agree with those reported by Sheldon, Anderson and Stoner and confirm the report by Smith, Vecchio and Forist that the half-life of acetohexamide (AH) plus hydroxyhexamide (HH) is comparable to that of tolbutamide. In spite of this relatively short half-life, limited blood sugar data indicate definite hypoglycemic effects up to 24 hrs. after a single 1.0-gm. dose. These findings indicate that the duration of action of AH and its metabolites are not necessarily related to drug levels.