The hydrolysis of estramustine phosphate; in vitro studies

Identification and validation of novel human pregnane X receptor activators among prescribed drugs via ligand-based virtual screening

Human pregnane X receptor (hPXR) plays a key role in regulating metabolism and clearance of endogenous and exogenous substances. Identification of novel hPXR activators among commercial drugs may aid in avoiding drug-drug interactions during coadministration. We applied ligand-based computational approaches for virtual screening of a commonly prescribed drug database (SCUT). Bayesian classification models were generated with a training set comprising 177 compounds using Fingerprints and 117 structural descriptors. A cell-based luciferase reporter assay was used for evaluation of chemical-mediated hPXR activation in HepG2 cells. All compounds were tested at 10 μM concentration with rifampicin and dimethyl sulfoxide as positive and negative controls, respectively. The Bayesian models showed specificity and overall prediction accuracy up to 0.92 and 0.69 for test set compounds. Screening the SCUT database with this model retrieved 105 hits and 17 compounds from the top 25 hits were chosen for in vitro testing. The reporter assay confirmed that nine drugs, i.e., fluticasone, nimodipine, nisoldipine, beclomethasone, finasteride, flunisolide, megestrol, secobarbital, and aminoglutethimide, were previously unidentified hPXR activators. Thus, the present study demonstrates that novel hPXR activators can be efficiently identified among U.S. Food and Drug Administration-approved and commonly prescribed drugs, which should lead to detection and prevention of potential drug-drug interactions.

Phase I clinical and pharmacologic trial of intravenous estramustine phosphate

PURPOSE

To determine the dose-limiting toxicities, maximum-tolerated dose, and pharmacokinetics of intravenous estramustine phosphate (IV EMP).

PATIENTS AND METHODS

A total of 31 patients with hormone-refractory prostate cancer received IV EMP as a 30- to 90-minute infusion weekly (n = 28) or for 3 consecutive days followed by a single weekly dose (n = 3). IV EMP dose was escalated from 500 to 3,000 mg/m(2). Pharmacokinetics of EMP and the metabolites estramustine (EaM), estromustine (EoM), estradiol, and estrone were assessed after weeks 1 and 4 of treatment.

RESULTS

The initial IV EMP infusion caused perineal discomfort that was ameliorated by lengthening the infusion time. Other common toxicities were grade 1 to 2 hepatotoxicity, nausea or vomiting, and fatigue or malaise. Lower-extremity thrombosis occurred in one patient, and two others developed upper-extremity thrombosis associated with venous infusion catheters. Dose-limiting fatigue and hypotension occurred at 3,000 mg/m(2), and cumulative fatigue developed after multiple cycles at 2,500 mg/m(2). Mean EMP clearance, estimated steady-state volume of distribution, and elimination half-life were 3.7 L/h, 10.6 L, and 3.7 hours, respectively. Variability of EMP clearance was 21%, and variation in area under the curve per dose for the metabolites was 28% to 36%. Elimination half-lives of EoM and EaM were 110 hours and 64 hours, and peak plasma concentrations of these active metabolites exceeded 10 micromol/L after IV EMP doses greater-than-or-equal 2,000 mg/m(2).

CONCLUSION

High-dose IV EMP can be administered safely as a weekly short infusion to patients with HRPC. High peak concentrations of active metabolites after IV EMP may provide an advantage over oral EMP in antimicrotubule drug combinations.

Androgen antagonistic effect of estramustine phosphate (EMP) metabolites on wild-type and mutated androgen receptor

Estramustine phosphate is used frequently, alone or in combination with other antitumor agents, for the treatment of hormone-refractory prostate cancer. Estramustine phosphate is metabolically activated in vivo, and its metabolites, estramustine, estromustine, estrone, and beta-estradiol inhibit the assembly of microtubules [for review see: Kreis W, In: Concepts, Mechanisms, and New Targets for Chemotherapy (Ed. Muggia FM), pp. 163-184. Kluwer Academic Publishers, Boston, 1995]. We investigated, by displacement of [3H]methyltrienolone in the presence of 2.5 mM of triamcinolone acetonide, the binding of estramustine phosphate and its metabolites, estramustine, estromustine, estrone, and beta-estradiol, as well as other antiandrogen agents including alpha-estradiol, bicalutamide, and hydroxyflutamide, to the mutated androgen receptor (m-AR) in LNCaP cells and to the wild-type androgen receptor in wild-type AR cDNA expression plasmid (w-pAR0) cDNA-transfected HeLa cells. Analogous to the antiandrogens, bicalutamide and hydroxyflutamide, binding of estramustine phosphate metabolites to the androgen receptor was observed. The EC50 values (in microM) were: estramustine phosphate, > 10; estramustine, 3.129 +/- 0.312; estromustine; 2.612 +/- 0.584; estrone, 0.800 +/- 0.090; alpha-estradiol, 1.051 +/- 0.096; beta-estradiol, 0.523 +/- 0.028; bicalutamide, 4.920 +/- 0.361; and hydroxyflutamide, 0.254 +/- 0.012. The transactivation assay demonstrated that, analogous to bicalutamide, estramustine could not induce luciferase activity in either w-pAR0 or m-pARL transfected HeLa cells. In contrast, a strong induction of the reporter activity by dihydrotestosterone was observed. Down-regulation of prostate-specific antigen (PSA) expression, an AR-target gene, by estramustine and bicalutamide was accompanied by the blockade of the mutated androgen receptor. Exposure of LNCaP cells to estramustine for 24 hr caused transcriptional inhibition of PSA in a concentration-dependent manner. The levels of PSA mRNA decreased 56 and 90% when LNCaP cells were treated with 5 and 10 microM of estramustine, respectively (IC50 = 10.97 +/- 1.68 microM). Binding of hydroxyflutamide to m-AR in LNCaP cells resulted in a concentration-dependent stimulation of PSA expression, suggesting that hydroxyflutamide acted as an agonist of the m-AR. Our data indicate that estramustine phosphate metabolites perform as androgen antagonists of AR, an additional mechanism involved in the therapeutic effect of estramustine phosphate in patients with prostate cancer.