Pharmacokinetics and metabolism of a vitamin D analogue (Seocalcitol) in rat and minipig

Prediction of in vitro metabolic stability of calcitriol analogs by QSAR

The metabolic stability of a drug is an important property for potential drug candidates. Measuring this property, however, can be costly and time-consuming. The use of quantitative structure-activity relationships (QSAR) to estimate the in vitro stability is an attractive alternative to experimental measurements. A data set of 130 calcitriol analogs with known values of in vitro metabolic stability was used to develop QSAR models. The analogs were encoded with molecular structure descriptors computed mainly with the commercial software QikProp and DiverseSolutions. Variable selection was carried out by five different variable selection techniques and Partial Least Squares Regression (PLS) models were generated from the 130 analogs. The models were used for prediction of the metabolic stability of 244 virtual calcitriol analogs. Twenty of the 244 analogs were selected and the in vitro metabolic stability was determined experimentally. The PLS models were able to predict the correct metabolic stability for 17 of the 20 selected analogs, corresponding to a prediction performance of 85%. The results clearly demonstrate the utility of QSAR models in predicting the in vitro metabolic stability of calcitriol analogs.

The tissue-specific distribution of 3H-seocalcitol (EB 1089) and 3H-calcitriol in rats

Seocalcitol (EB 1089) is under development for the treatment of hepato-cellular carcinoma (HCC). The tissue distribution of 3H-seocalcitol was investigated in comparison to 3H-calcitriol in rats. Quantitative whole-body autoradiography was used to quantify the tissue distribution. The greatest difference in distribution between the two compounds was observed in the bloodstream. For most tissues the ratio seocalcitol/calcitriol varied between 0.2 and 3.1. The concentration of radioactivity in the liver was almost the same for the two compounds. For seocalcitol the concentration in the liver was 10 times higher than in serum. Assuming that the liver/serum concentration ratio is the same in rats and humans, the concentration of seocalcitol in the human liver is expected to be higher than the concentration resulting in more than 50% inhibition of cancer cell proliferation, and thus pharmacologically effective in HCC. It is questionable whether calcitriol would be present in the human liver in sufficient concentrations to be effective for the treatment of HCC, as the antiproliferative activity of calcitriol is generally more than 10-fold lower compared to that of seocalcitol and as calcitriol can only be administered at a dose that is ca. three-fold lower than the dose of seocalcitol.

Sensitive analysis of 1alpha,25-dihydroxyvitamin D3 in biological fluids by liquid chromatography-tandem mass spectrometry

A liquid chromatographic-tandem mass spectrometric assay using 5% bovine serum albumin as the calibration matrix has been developed for the quantitative analysis of 1alpha,25-dihydroxyvitamin D3 [1alpha,25(OH)2D3] in biological fluids. The analyte was extracted from the matrix after protein precipitation using an automated solid-phase extraction procedure involving both a reversed-phase and normal-phase procedure on a single C18 cartridge. The analytical chromatography was performed using a Symmetry C8 50 x 2.1 mm, 3.5 microm column. The mobile phase was a linear gradient from 75 to 99% methanol with a constant concentration of 2 mM ammonium acetate. 1alpha,25(OH)2D3 and the internal standard [2H6]1alpha,25(OH)2D3 were detected by using MS-MS. The ion source was operated in the positive electrospray ionisation mode. The assay is specific, sensitive, and has a capacity of more than 100 samples per day, with a limit of quantitation of 20 pg ml(-1) for a 1.0-ml sample aliquot. The assay has been used for the analysis of 1alpha,25(OH)2D3 in serum from rats and pigs simultaneously with the analysis of the vitamin D analog seocalcitol.