Bioactivation mechanisms for mitotane and its analogs.
Cai, Wei
1993
Abstract
Mitotane ($o,p\sp\prime$-DDD, 1-(2-chlorophenyl)-1-(4-chlorophenyl)-2,$2$ -dichloroethane) is the only drug currently available that has extended survival in patients with adrenocortical carcinoma. However, its clinical use is limited by toxicity when given in the required large therapeutic doses. The objectives of this investigation are to study the biotransformations and metabolism-dependent macromolecular binding of mitotane and its analogs in an effort to explain their mechanism of action and to provide a better understanding of the structural requirements for maximal adrenolytic action with minimal toxicity. The in vitro biotransformations of $\sp{14}$C-$o,p\sp\prime$-DDD and its $m,p\sp\prime$- and $p,p\sp\prime$-isomers by bovine adrenal cortex homogenate preparations were studied. Aliphatic hydroxylation at the $\alpha$ and $\beta$ carbons are the major metabolic pathways for mitotane and its isomers. Hydroxylation of the $\beta$-carbon is proposed to be the activation pathway leading to acylation of macromolecules and the desired adrenolytic effect. As the chlorine on a phenyl ring is moved from the ortho to the meta and para positions, there is an increase in $\alpha$-hydroxylation with an accompanying decrease in covalent tissue binding. The in vitro biotransformations and protein binding studies of the $\sp{125}$I-analogs of mitotane were conducted in dog adrenal cortex subcellular fractions, human adrenal cortex and tumor homogenates and cultured human adrenocortical carcinoma cell preparations. A single acid metabolite formed through a $\beta$-hydroxylation route is released by exhaustive hydrolysis of metabolite bound proteins, which strongly supports our hypothesis that the covalent tissue binding of mitotane and its analogs is due to the ability of the dichloromethyl group to become metabolically oxidized to an acyl chloride that covalently binds to adrenal proteins. It was shown that the reactive intermediate of the $p\sp\prime$-iodo analog of $o,p\sp\prime$-DDD binds to specific proteins. Among these, a mitochondrial protein of 49.5 KDa is indicated to be cytochrome P-450$\sb{\rm SCC}.$ The active metabolite could function as a cytochrome P-450$\sb{\rm SCC}$ inhibitor to control steroid production by blocking electron transfer from adrenodoxin to cytochrome P-450. Based on literature reports and our previous results on the enhancement of mitotane biotransformations in mitochondria by cytosolic fractions, the $\sp{125}$I-labeled proteins such as those at 32, 30, 26, 13.5, 11.5 and 8.5 KDa might be sterol carrier proteins that help transport mitotane and its analogs into mitochondria or might be insulin-like growth factor-I binding proteins involved in the regulation of cell growth.Other Identifiers
(UMI)AAI9332024
Subjects
Health Sciences, Pharmacology Chemistry, Pharmaceutical Engineering, Biomedical
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