Biochemistry and pharmacology of 7alpha-substituted androstenediones as aromatase inhibitors.

The inhibition of aromatase, the enzyme responsible for converting androgens to estrogens, is therapeutically useful for the endocrine treatment of hormone-dependent breast cancer. Research by our laboratory has focused on developing competitive and irreversible steroidal aromatase inhibitors, with an emphasis on synthesis and biochemistry of 7alpha-substituted androstenediones. Numerous 7alpha-thiosubstituted androst-4-ene-3,17-diones are potent competitive inhibitors, and several 1,4-diene analogs, such as 7alpha-(4'-aminophenylthio)-androsta-1,4-diene-3,17-di one (7alpha-APTADD), have demonstrated effective enzyme-activated irreversible inhibition of aromatase in microsomal enzyme assays. One focus of current research is to examine the effectiveness and biochemical pharmacology of 7alpha-APTADD in vivo. In the hormone-dependent 7,12-dimethylbenz(a)anthracene (DMBA)-induced rat mammary carcinoma model system, 7alpha-APTADD at a 50 mg/kg/day dose caused an initial decrease in mean tumor volume during the first week, and tumor volume remained unchanged throughout the remaining 5-week treatment period. This agent lowers serum estradiol levels and inhibits ovarian aromatase activity. A second research area has focused on the synthesis of more metabolically stable inhibitors by replacing the thioether linkage at the 7alpha position with a carbon-carbon linkage. Several 7alpha-arylaliphatic androst-4-ene-3,17-diones were synthesized by 1,6-conjugate additions of appropriate organocuprates to a protected androst-4,6-diene or by 1,4-conjugate additions to a seco-A-ring steroid intermediate. These compounds were all potent inhibitors of aromatase with apparent Kis ranging between 13 and 19 nM. Extension of the research on these 7alpha-arylaliphatic androgens includes the introduction of a C1-C2 double bond in the A-ring to provide enzyme-activated irreversible inhibitors. The desired 7alpha-arylaliphatic androsta-1,4-diene-3,17-diones were obtained from their corresponding 7alpha-arylaliphatic androst-4-ene-3,17-diones by oxidation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). These inhibitors demonstrated enzyme-mediated inactivation of aromatase with apparent k(inact)s ranging from 4.4 x 10(-4) to 1.90 x 10(-3) s(-1). The best inactivator of the series was 7alpha-phenpropylandrosta-1,4-diene-3,17-dione, which exhibited a T(1/2) of 6.08 min. Aromatase inhibition was also observed in MCF-7 human mammary carcinoma cell cultures and in JAr human choriocarcinoma cell cultures, exhibiting IC50 values of 64-328 nM. The 7alpha-arylaliphatic androgens thus demonstrate potent inhibition of aromatase in both microsomal incubations and in choriocarcinoma cell lines expressing aromatase enzymatic activity. Additionally, the results from these studies provide further evidence for the presence of a hydrophobic binding pocket existing near the 7alpha-position of the steroid in the active site of aromatase. The size of the 7alpha-substituent influences optimal binding of steroidal inhibitors to the active site and affects the extent of enzyme-mediated inactivation observed with androsta-1,4-diene-3,17-dione analogs.

Boronated starburst dendrimer-monoclonal antibody immunoconjugates: evaluation as a potential delivery system for neutron capture therapy.

Boron neutron capture therapy (BNCT) is based on the nuclear capture reaction that occurs when boron-10, a stable isotope, is irradiated with low-energy or thermal neutrons (< or = 0.025 eV) to yield high LET alpha particles and recoiling 7Li nuclei [10B + nth-->[11B]-->4He(alpha) + 7Li + 2.39 MeV]. Approximately 10(9) boron-10 atoms must be delivered to each target cell in order to sustain a lethal 10B(n,alpha)7Li reaction. If MoAbs are to be used for targeting boron-10, then it is essential that they recognize a surface membrane epitope that is highly expressed on tumor cells and that a large number of boron-10 atoms be attached to each antibody molecule. In order to heavily boronate MoAbs, we have utilized starburst dendrimers (SD), which are precise, spherical macromolecules composed of repetitive poly(amidoamino) groups. Second- and fourth-generation dendrimers, having 12 and 48 reactive terminal amino groups and molecular weights of 2414 and 10,632 Da, respectively, were boronated using an isocyanato polyhedral borane, Na(CH3)3NB10H8NCO. The boronated starburst dendrimers (BSD), in turn, were derivatized with m-maleimidobenzoyl N-hydroxysulfosuccinimide ester (sulfo-MBS). The MoAbIB16-6, which is directed against the murine B16 melanoma, was derivatized with N-succinimidyl 3-(2-pyridyldithio)propionate (SPDP). The MBS-derivatized BSD and SPDP-derivatized MoAb were reacted to yield stable immunoconjugates.(ABSTRACT TRUNCATED AT 250 WORDS)

Steroidal inhibitors as chemical probes of the active site of aromatase.

Androstenedione analogs containing 7 alpha-substituents have proven to be potent inhibitors of aromatase in human placental microsomes, in MCF-7 mammary cell cultures, and in JAr choriocarcinoma cells. Recent investigations have focused on the use of mechanism-based inhibitors, such as 7 alpha-substituted 1,4-androstadienediones, to biochemically probe the active site of aromatase. Inhibition kinetics were determined under initial velocity conditions using purified human placental cytochrome P450arom protein in a reconstituted system. Derivatives of 1,4-androstadiene-3,17-dione and 1,4,6-androstatriene-3,17-dione exhibited high affinity in the purified enzyme system. 7 alpha-(4'-Amino)phenylthio-1,4-androstadiene-3,17-dione, abbreviated 7 alpha-APTADD, demonstrated rapid time-dependent, first-order inactivation of reconstituted aromatase activity only in the presence of NADPH. The apparent Kinact for 7 alpha-APTADD is 11.8 nM, the first-order rate of inactivation is 2.72 x 10(-3) sec-1, and the half-time of inactivation at infinite inhibitor concentration is 4.25 min. The values for the rate constant and half-time of inactivation are similar to those observed in the placental microsomal assay system. Further studies were performed with radioiodinated 7 alpha-(4'-iodo)phenylthio-1,4-androstadienedione, 7 alpha-IPTADD, and the reconstituted aromatase system. Incubations with [125I] 7 alpha-IPTADD were followed by protein precipitation, solvent extraction, and column chromatography. Analysis of the isolated cytochrome P450arom by gel electrophoresis and autoradiography demonstrated the presence of only one radioactive band, which corresponded to the protein staining band for cytochrome P450arom. HPLC radiochromatographic analysis of the isolated cytochrome P450aroM confirmed the presence of only one radioactive peak coeluting with the u.v. peak for cytochrome P450arom. Peptide mapping analysis by reverse-phase HPLC of digested inhibitor-cytochrome P450arom complex demonstrates that the radioactive inhibitor is covalently bound to a lipophilic fragment. In summary, these inhibitors produced enzyme-catalyzed inactivation of reconstituted aromatase activity, and radioiodinated 7 alpha-IP-TADD binds covalently to the cytochrome P450arom.

Characteristics of covalent gossypol binding to microsomal proteins.

Gossypol is a potent antifertility agent contained in seeds and other parts of cotton plants. The limit set in 1974 by the FDA for this C30H30O8 compound in consumer products is 450 ppm. The binding characteristics and the nature of the microsomal protein adducts of radiolabeled gossypol were studied using centrifugation, extraction, reverse phase HPLC and filter assay approaches. Results showed a significant amount of radiolabeled gossypol to be associated with the precipitated proteins after aqueous, ethanol, acetone and ether extractions. The nature of binding of these protein adducts involved covalent, covalent but reversible (e.g., Schiff bases), and tightly-bound and trapped noncovalent residues. Non-acid labile binding adducts constituted 40% of the precipitated microsomal proteins. Eight percent of the adducts were covalent, reversible and reducible by NaBH4. A gradient HPLC separation of the acetone extracts resulted in non-gossypolone hydrophilic protein adducts with a mean retention time of 2.3 minutes. Gossypol can bind tightly to hepatic microsomal proteins with a ratio of 80 nmoles/mg protein under physiological conditions. Significant portions of these bindings are not due to simple acid labile Schiff base formation. Purer membrane preparation provided results showing predominant binding of gossypol to endoplasmic reticulum (ER) and mitochondria, followed to a lesser extent by peroxisomes and plasma membranes. Difference spectra of the gossypol-bound rat hepatic microsomal preparations and controls demonstrated a 3 nm shift from 413 to 410 nm caused by gossypol covalent-binding. Results of this study indicate that gossypol binds covalently to microsomal proteins. Its binding to membrane proteins may affect metabolism of sterols, steroids, or fatty acids.

7 alpha-substituted androstenediones as effective in vitro and in vivo inhibitors of aromatase.

Research efforts over the past several years have focused on the synthesis of competitive and irreversible aromatase inhibitors and examination of these inhibitors in microsomal preparations, in cell culture, and in vivo. Several 7 alpha-substituted androstenediones have demonstrated high affinity for placental aromatase, with apparent Ki's ranging from 1 to 30 nM. Inactivation of aromatase occurred following incubation with alkylating and enzyme-activated irreversible inhibitors. 7 alpha-(4'-Amino)phenylthio-4-androstene-3,17-dione (7 alpha-APTA) exhibits potent inhibitory activity of aromatase in the MCF-7 human mammary carcinoma cell line with an ED50 of approximately 25 nM. The inhibitor did not bind to the estrogen receptor of the cells in vitro nor induce levels of progesterone receptors in intact cells. In vivo studies of 7 alpha-APTA in the DMBA-induced rat mammary carcinoma model resulted in 80% of the tumors responding completely or partially at doses of 25 and 50 mg/kg body wt/day. Thus, these 7 alpha-substituted steroidal aromatase inhibitors are effective medicinal agents and may be useful for the treatment of estrogen-dependent breast cancer.

7 alpha-substituted derivatives of androstenedione as inhibitors of estrogen biosynthesis.

In an effort to obtain more information on the structure-activity relationship among the 7 alpha-(phenylthio)androstenedione inhibitors of the enzyme aromatase, a series of compounds containing both electron-donating and electron-withdrawing ring substituents was synthesized and tested for aromatase inhibitory activity. No linear correlation between substituent electronic effects and inhibitory activity was observed. The halogen-containing compounds, particularly 8, appeared to be quite potent inhibitors. The 125I analogue of 8 was synthesized in order to evaluate the possibility of side-chain elimination under the assay conditions. Approximately 90% of [125I]-8 remained intact for up to 1 h under assay conditions.

Synthesis and evaluation of 19-aza- and 19-aminoandrostenedione analogues as potential aromatase inhibitors.

Derivatives of 19- azaandrostenedione (10 beta-amino-4- estrene -3,17-dione, 2) and 19-amino-4-androstene-3,17-dione (3) were synthesized as potential inhibitors of aromtase (estrogen synthetase). Compound 2 and its derivatives were synthesized from 3,17-dioxo-4-androsten-19-oic acid (5) via a Curtius rearrangement. Derivatives of 3 were synthesized from the intermediate 3,17-bis(ethylenedioxy)-5-androsten-19-al oxime (14), which was reduced to the corresponding amine (16) with Raney nickel. However, attempts to synthesize the parent compound, 3, from 16 by several different methods were unsuccessful. The compounds obtained were tested for inhibitory activity in the tritiated water assay for aromatase, with human placental microsomes as the enzyme source and [1-3H]-androst-4-ene-3,17-dione (83% 3H 1 beta) as the substrate. All of the compounds caused less than 20% inhibition of enzyme activity when tested at one and five times the substrate concentration (0.25 microM, 1.25 microM) and were poorer inhibitors than two known inhibitors, 7 alpha-[(p-aminophenyl)thio] androstenedione (7- APTA ) and 4-hydroxy-4-androstene-3,17-dione (4-OHA).

7 alpha-Alkyltestosterone derivatives: synthesis and activity as androgens and as aromatase inhibitors.

The 7 alpha-ethyl,propyl,butyl,3'-t-butoxypropyl,allyl,3'-hydroxypropyl 17-acetate, and 3'-chloropropyl 17-acetate derivatives of testosterone and the 7 alpha-3'-t-butoxypropyl, 3'-hydroxypropyl,3'-acetoxypropyl,3'-bromoacetoxypropyl, 3'-chloropropyl, and 2'-oxo-3'-bromopropyl derivatives of 4-androstene-3,17-dione were synthesized. The testosterone derivatives were found to lose androgenic and anabolic activity rapidly as the size of the group at the 7 position increased. Many of the compounds were tested as inhibitors of aromatase. The 17-keto compounds were more active than the corresponding alcohols and the enzyme was found to tolerate at least the bulk of a hydroxypropyl group at the C-7 alpha position.