Compartment-specific gene regulation of the CAR inducer efavirenz in vivo

Nuclear receptors such as the constitutive androstane receptor (CAR) are central factors that link drug exposure to the activities of drug metabolism and elimination. In order to determine the in vivo effects of efavirenz, a CAR activator, the expression of target genes was determined in duodenal biopsies obtained from 12 healthy volunteers before treatment and after 10 days of treatment with efavirenz; concomitant administration of the cholesterol inhibitor ezetimibe produced no significant difference. However, in in vitro studies, efavirenz significantly increased CYP2B6 expression in several cell types, suggesting that the drug transactivates CAR. This hypothesis is supported by our findings that there is significant induction of CAR target genes in in vivo peripheral blood mononuclear cells (PBMCs) isolated from healthy volunteers treated with multiple doses of efavirenz. The impact of efavirenz on hepatic metabolism in vivo was confirmed by significant changes in plasma 4β-hydroxycholesterol and bilirubin levels and the area under the curve (AUC) of efavirenz. Induction of CYP2B6 mRNA expression correlated with the decrease in the AUC of efavirenz (r = 0.61; P = 0.036). Taken together, our results provide evidence that efavirenz exerts compartment-specific inductive capacity in vivo.

Replacement therapy with levothyroxine plus triiodothyronine (bioavailable molar ratio 14 : 1) is not superior to thyroxine alone to improve well-being and cognitive performance in hypothyroidism

OBJECTIVES

There is evidence from recent controlled clinical studies that replacement therapy of hypothyroidism with T4 in combination with a small amount of T3 may improve the well-being of the patients. As the issue is still the subject of controversial discussion, our study was assigned to confirm the superiority of a physiological combination of thyroid hormones (absorbed molar ratio 14 : 1) over T4 alone with regard to mood states and cognitive functioning.

DESIGN AND PATIENTS

After a run-in period with the T4 study medication for 4 weeks, a controlled, randomized, double-blind, two-period (each 12 weeks), cross-over study without washout between the treatment periods was performed in 23 hypothyroid patients (three males, 20 females, age 23-69 years, 21 subjects after surgery/radioiodine, two with autoimmune thyroiditis) to compare the effects of the previous individual T4 dose (100-175 micro g) with a treatment in which 5% of the respective T4 dose was substituted by T3.

MEASUREMENTS

Standard hormonal characteristics and standardized psychological tests to quantify mood and cognitive performance were measured after the run-in period and at the end of each treatment period. In 12 subjects, the concentration-time profiles of fT3 and fT4 were compared after the last administration of the respective study medication. TSH, fT3 and fT4 were measured with immunological assays.

CLINICAL RESULTS

Replacement therapy with T4 and T4/T3 was not different in all steady-state hormonal, metabolic and cardiovascular characteristics except for TSH, which was more suppressed after T4/T3. The efficacy of replacement therapy with the T4/T3 combination was not different from the T4 monotherapy with regard to all psychological test scores describing mood and cognitive functioning of the patients. Mood was even significantly impaired by the T4/T3 combination in eight subjects, with TSH < 0.02 mU/l, compared to patients with normal TSH (Beck Depression Inventory: 8.25 +/- 5.01 vs. 4.07 +/- 5.60, P = 0.026). PHARMACOKINETIC RESULTS: The area under the concentration-time curve (AUC(0-8h)) of fT3 was significantly higher after T4/T3 compared to the T4 monotherapy (42.8 +/- 9.03 pmol x h/l vs. 36.3 +/- 8.50 pmol x h/l, P < 0.05) and was significantly correlated to serum TSH (r(s) = -0.609, P < 0.05). After T4/T3, patients with a history of Graves' disease or autoimmune thyroiditis had significantly higher serum trough levels of fT3 whereas the fT4 concentrations were significantly lower in patients with a nonautoimmune background.

CONCLUSION

Replacement therapy of hypothyroidism with T4 plus T3 does not improve mood and cognitive performance compared to the standard T4 monotherapy. There is even a higher risk of signs of subclinical hyperthyroidism associated with impaired well-being of the patients, which is clearly caused by significant fluctuations in the steady-state fT3 serum concentrations.