The effect of 4MA, a potent inhibitor of 5 alpha-reductase, on the growth of androgen-responsive human genitourinary tumors grown in athymic nude mice

Therapeutic targeting of the androgen receptor (AR) and AR variants in prostate cancer

Prostate cancer (PCa) accounted for over 300 000 deaths world-wide in 2018. Most of the PCa deaths occurred due to the aggressive castration-resistant PCa (CRPC). Since the androgen receptor (AR) and its ligands contribute to the continued growth of androgen-dependent PCa (ADPCa) and CRPC, AR has become a well-characterized and pivotal therapeutic-target. Although AR signaling was identified as therapeutic-target in PCa over five-decades ago, there remains several practical issues such as lack of antagonist-bound AR crystal structure, stabilization of the AR in the presence of agonists due to N-terminus and C-terminus interaction, unfavorable large-molecule accommodation of the ligand-binding domain (LBD), and generation of AR splice variants that lack the LBD that impede the discovery of highly potent fail-safe drugs. This review summarizes the AR-signaling pathway targeted therapeutics currently used in PCa and the approaches that could be used in future AR-targeted drug development of potent next-generation molecules. The review also outlines the discovery of molecules that bind to domains other than the LBD and those that inhibit both the full length and splice variant of ARs.

Serially heterotransplanted human prostate tumours as an experimental model

* Introduction * Serially heterotransplanted human tumours in immunosuppressed mice: similarity to the tumour of origin - Cytological and histological analysis - Karyotype - Marker expression - Other PC markers - Tumour cell proliferation and frequency of mitosis - Vasculature - Stromal compartment - Heterotransplant hormone dependency - Androgen dependent - Partially androgen dependent - Androgen independent - Metastases * Conclusions Preclinical research on prostate cancer (PC) therapies uses several models to represent the human disease accurately. A common model uses patient prostate tumour biopsies to develop a cell line by serially passaging and subsequent implantation, in immunodeficient mice. An alternative model is direct implantation of patient prostate tumour biopsies into immunodeficient mice, followed by serial passage in vivo. The purpose of this review is to compile data from the more than 30 years of human PC serial heterotransplantation research. Serially heterotransplanted tumours are characterized by evaluating the histopathology of the resulting heterotransplants, including cellular differentiation, karyotype, marker expression, hormone sensitivity, cellular proliferation, metastatic potential and stromal and vascular components. These data are compared with the initial patient tumour specimen and, depending on available information, the patient's clinical outcome was compared with the heterotransplanted tumour. The heterotansplant model is a more accurate preclinical model than older generation serially passaged or genetic models to investigate current and newly developed androgen-deprivation agents, antitumour compounds, anti-angiogenic drugs and positron emission tomography radiotracers, as well as new therapeutic regimens for the treatment of PC.

Androgen-induced regrowth in the castrated rat ventral prostate: role of 5alpha-reductase

Testosterone (T), the major circulating androgen, must be converted to dihydrotestosterone (DHT) by the enzyme 5alpha-reductase (5alpha-R) to be maximally active in the prostate. The present study was designed to determine the relative potency of T and DHT on regrowth of the involuted prostate and to elucidate the role of 5alpha-R in the growing prostate. To create dose-response curves for intraprostatic T or DHT, rats were castrated for 2 weeks to allow their prostates to fully regress and then given T implants of various sizes in the presence or absence of the 5alpha-R inhibitor, finasteride. Markers for androgen effects on regrowth of the prostate were prostate weight, duct mass (a measure of secretory activity) and DNA content (a measure of cell number). To assess the relative uptake of T and DHT by the prostate, a comparison was made of intraprostatic DHT levels resulting from T and DHT implants. In the prostate, 1.6-1.9 times more T than DHT was required to achieve a half-maximal response for each of the three markers of prostate regrowth. The dose-response curves revealed that thresholds for intraprostatic T and DHT had to be attained before significant growth was observed. The threshold for T was 2- to 3-fold greater than that for DHT. However, at high intraprostatic concentrations, the effects of T mimicked those of DHT. When the relationship between serum T levels and prostate regrowth was considered, 13 times more serum T was required for half-maximal prostate regrowth when its conversion to DHT was blocked by finasteride. This is partly due to decreased androgen accumulation in the prostate when T was the major intraprostatic androgen. Finally, T or DHT implants in the absence of finasteride resulted in similar intraprostatic DHT levels, indicating that uptake of each serum androgen into the prostate was similar. However, to achieve similar levels of DHT or T in serum, much larger DHT pellets were needed, suggesting more rapid metabolism of DHT in tissues other than the prostate. We conclude that the role of 5alpha-R is 2-fold: it converts testosterone into a modestly more potent androgen and enhances prostatic accumulation of androgen. DHT, in principle, could serve equally well as T as the circulating androgen, although the rate of DHT production would have to be considerably higher to counter the apparent rapid clearance from serum. In addition, we hypothesize that T has arisen as the major circulating androgen instead of DHT because it can be aromatized to estradiol, which itself has important roles in male reproductive function and bone physiology.

Comparison of testosterone metabolism in benign prostatic hyperplasia and human prostate cancer cell lines in vitro

Pathways of testosterone metabolism in tissue slices and cell suspensions of human benign hyperplastic prostate (BPH) tissue and human prostate cancer cell lines (DU145, HPC-36M, PC-3/MA2 and LNCaP) were investigated. Thin layer chromatography analysis was used to identify the following tritiated metabolites: testosterone, 5 alpha-dihydrostestosterone (DHT), 5 alpha-androstane-3 alpha/3 beta-17 beta-diol (androstanediols), 4-androstene-3,17-dione (androstenedione) and 5 alpha-androstanedione. The predominant pathway for testosterone metabolism in BPH was via 5 alpha-reductase producing 5 alpha-dihydrotestosterone (71% and 75% total metabolites in slices and suspensions incubated for 24 h, respectively). The cancer cell lines DU145 and HPC-36M resembled BPH by metabolizing testosterone predominantly to DHT (68% and 82% total metabolites, respectively), although the rate of metabolism was much lower in the cell lines (0.099 and 0.05 pmol testosterone/mg protein/h in DU145 and HPC-36M) compared to the BPH cell suspensions (6.4 pmol testosterone/mg protein/h). In contrast, PC-3/MA2 contained high 17 beta-HSD activity forming large amounts of 4-androstene-3,17-dione (84% total metabolites), converting testosterone at a rate faster (12.8 pmol testosterone/mg protein/h) than the BPH cell suspensions. LNCaP rapidly converted testosterone exclusively to a glucuronide conjugate (7.4 pmol testosterone/mg protein/h), although after incubation with [3H]-4-androstene-3,17-dione, 5 alpha-reductase activity was demonstrated. LNCaP was the only cell line whose growth and colony-forming ability was stimulated by testosterone and DHT. BPH and all the cell lines tested had 5 alpha-reductase activity, but only the prostate tissue and the cell lines DU145 and HPC-36M converted testosterone predominantly to DHT.

Chemoprevention strategies for prostate cancer: the role of 5 alpha-reductase inhibitors

Prostate cancer is a major health problem for the aging male population. Despite hormonal dependence, the inevitable emergence of androgen insensitive tumors, which have a dismal prognosis, highlights the need to develop prevention strategies such as chemoprevention. An acceptable agent must interfere with either the process of carcinogenesis or tumor growth, and have minimal toxicity. In clinical studies, 5 alpha-reductase inhibitors have been shown to suppress serum and intraprostatic levels of dihydrotestosterone, an important promoter of prostate cancer, leading to reduction in prostate size and suppression of glandular cell activity as measured by prostate specific antigen secretion. In addition, 5 alpha-reductase inhibitors have demonstrated an excellent safety profile and tolerability in 12 month controlled clinical trials. No significant metabolic effects have been observed in gonadotropin secretion, spermatogenesis, serum lipids or glucose tolerance. The efficacy and safety of 5 alpha-reductase inhibitors in studies to date, combined with the androgen dependence of tumor production, strongly supports investigating their use for chemoprevention of prostate cancer.

Consideration of the use of 17 beta-N,N-diethylcarbamoyl-4-methyl-4-aza-5- alpha-androstan-3-one (4MA), a 5 alpha-reductase inhibitor, in prostate cancer therapy

We investigated the effect of 17 beta-N,N-diethylcarbamoyl-4-methyl-4aza- 5 alpha-androstan-3-one (4MA), a 5 alpha-reductase inhibitor, on growth inhibition of androgen-sensitive rat prostatic tumour (R3327-H) and correlated it with changes in weight of normal androgen target tissues and with levels of androgens. Groups of male Copenhagen rats were treated for 28 days with a daily injection of various, increasing doses of 4MA (0.01-4.0 mg/day) and the results were compared with control (vehicle-treated) and with castrated animals. 4MA decreased tumour growth rate in a dose-dependent manner, which was reflected in a decreased incorporation of BrdUrd in DNA of glandular epithelial cells in the tumour. Normal prostate wet weight was also decreased after high-dose 4MA treatment while serum testosterone levels were not affected by 4MA treatment. Contrary to expectations, however, tissue levels of dihydrotestosterone in tumour and ventral prostate were still considerable in 4MA-treated animals. The tumour-inhibiting action of 4MA, therefore, has to be interpreted as not being purely due to 5 alpha-reductase inhibition. On the other hand, it was not possible to demonstrate any direct tumoricidal effect of 4MA in vitro. The relevance of these findings in terms of the endocrine mechanism of action of 4MA on tumour growth is discussed.

Effect of castration, DES, flutamide, and the 5 alpha-reductase inhibitor, MK-906, on the growth of the Dunning rat prostatic carcinoma, R-3327

Male rats bearing implants of the Dunning rat prostatic carcinoma, R-3327, were used in a 42-day study to determine the effect of castration or orally administered flutamide (FL), DES (diethylstilbestrol) or the 5 alpha-reductase inhibitor, MK-906, on the growth of this androgen-responsive cancer. The rate of growth and final weights of the tumor and the ventral prostate (VP) were all reduced (P less than 0.05) by castration. Flutamide (25 mg/kg/day) significantly decreased tumor and VP weights in intact rats and castrates given 100 micrograms/day (SC) of testosterone propionate (TP) or dihydrotestosterone propionate (DHTP). It also significantly retarded tumor growth rate in TP- or DHTP-treated castrates and was marginally effective in intact animals. DES (100 micrograms/kg/day) reduced (P less than 0.05) tumor and VP weights of intact rats but did not significantly affect tumor growth rate or weight in castrates given TP or DHTP. These results indicated that the effect of DES on tumor growth is caused by its inhibition of the secretion or release of the gonadotropins necessary for testicular androgen production. MK-906 (25 mg/kg/day) affected neither the gross nor the histomorphology of the tumor in intact rats or castrates given TP or DHTP. Further, it caused no histological changes in the testes of intact rats. It did, however, significantly reduce VP weight in intact animals and TP-treated castrates but not in those given DHTP. This illustrates that the anti-androgenicity of MK-906 stems from its inhibition of DHT formation. The failure of MK-906 to influence tumor growth in the TP-treated castrates strongly suggests that the R-3327 tumor can respond to testosterone directly. If that is true, then its growth is unlikely to be affected by a pure 5 alpha-reductase inhibitor such as MK-906. In ancillary experiments, tumors from MK-906-treated animals were found to have reduced levels of DHT and, when assayed in vitro, to have a reduced capacity to convert [3H]-T to [3H]-DHT.

Evaluation of Win 49,596, a novel steroidal androgen receptor antagonist, in animal models of prostate cancer

A series of experiments were conducted to evaluate the effects of Win 49,596, a novel steroidal androgen receptor antagonist, in animal models of prostate cancer. In the first experiment, oral administration of Win 49,596 at doses of 30, 100, or 300 mg/kg/day for 28 days inhibited (P less than 0.05) the growth of the androgen-sensitive PAP variant of the Dunning R-3327 prostatic carcinoma in intact male rats relative to intact controls. The degree of inhibition at 100 and 300 mg/kg/day Win 49,596 was similar (P greater than 0.10) to that observed in castrate controls as well as in intact rats administered the nonsteroidal androgen receptor antagonist flutamide orally at 15 mg/kg/day. Castration as well as treatment with either Win 49,596 or flutamide also decreased (P less than 0.05) the weight of the prostate in tumor-bearing animals. Additional studies were conducted to determine the effect of Win 49,596 on the growth of the androgen-dependent PC-82 human prostatic carcinoma xenografted into athymic nude male mice. Oral administration of Win 49,596 at 30, 100, or 300 mg/kg/day for 35 days inhibited (P less than 0.05) tumor growth relative to intact controls. The degree of tumor inhibition was similar to that observed in intact male mice administered the nonsteroidal androgen receptor antagonist flutamide orally at 30 mg/kg/day but was less than that observed following castration. Ventral prostate weights were also reduced (P less than 0.05) in castrate mice as well as in intact mice administered either Win 49,596 or flutamide. In the last experiment, at equivalent total daily dosages of either 150 or 300 mg/kg/day Win 49,596, twice a day (BID) dosing was more effective than once a day (SID) dosing in inhibiting tumor growth. The inhibitory effects of Win 49,596 at 150 mg/kg BID on tumor growth were similar to those observed following castration. Although Win 49,596 treatment reduced (P less than 0.05) ventral prostate weights relative to intact controls, there was no difference (P greater than 0.10) between SID vs. BID dosing. Based on the results of these studies and subject to further testing, Win 49,596 may have utility in the treatment of hormonally dependent metastatic prostate cancer in humans.

Inhibitors of steroid 5 alpha-reductase in benign prostatic hyperplasia, male pattern baldness and acne

Benign prostatic hyperplasia is an androgen-dependent disease which afflicts a large percentage of males over the age of fifty, and is usually treated by surgery. Dihydrotestosterone, a 5 alpha-reduced metabolite of testosterone, has been implicated as a causative factor in the progression of the disease, largely through the clinical study of males who are genetically deficient in the dihydrotestosterone-producing enzyme, steroid 5 alpha-reductase. As a result, inhibition of this enzyme has become a pharmacological strategy for the treatment of benign prostatic hyperplasia as well as other dihydrotestosterone-related disorders such as acne and male pattern baldness. In this review, Brian Metcalf and colleagues focus on the chemical and kinetic mechanisms of steroid 5 alpha-reductase, and known inhibitors of this enzyme, and discuss the rationale behind the design of a mechanistically distinct class of steroid 5 alpha-reductase inhibitors.

Effect of a 4-methyl-4-aza steroid on androgen metabolism by rat ventral prostate epithelial and stromal cell cultures: selective inhibition of 5 alpha-reductase activity

The effect of a potent steroid metabolic inhibitor, 17 beta-N,N-diethylcarbamoyl-4-methyl-4-aza-5 alpha-androstan-3-one (DMAA), on androgen metabolism was investigated in primary monolayer cultures of rat ventral prostate epithelial and stromal cells. Using testosterone (T) as substrate, 5 alpha-reductase (5 alpha-R) activity in both cell types was inhibited by greater than 98% at an inhibitor concentration of 1000 nM. The concentrations required to produce a 50% inhibition (IC50) were 7.4 and 9.0 nM for epithelial and stromal cells, respectively. To examine the specificity of this compound, its effect on other steroid-metabolic enzymes was examined. DMAA at a concentration of 1,000 nM had no effect on 3 alpha-hydroxysteroid oxidase (3 alpha-HSORox), 3-ketosteroid reductase (3 alpha-HSORred), and 6/7-hydroxylase (6/7-HSH) activities in both cell types; 17 beta-hydroxysteroid oxidase (17 beta-HSORox) activity, located primarily in epithelial cells, also was not influenced by DMAA. In contrast, epithelial 3 beta-hydroxysteroid oxidase (3 beta-HSORox) and 3-ketosteroid reductase (3 beta-HSORred) activities were inhibited by 65% (P less than .001) and 58% (P greater than .05), respectively, albeit the latter result was not statistically significant. Stromal 3 beta-HSORox and 3 beta-HSORred activities were negligible; hence the effect of the inhibitor of these enzymes could not be assessed. In conclusion, DMAA is a relatively selective and potent inhibitor of 5 alpha-R activity in primary cultures of rat ventral prostate epithelial and stromal cells and should be a useful compound for antagonizing androgen-mediated actions in the prostate and other androgen target tissues.

Transplantable human prostatic carcinoma (PC-82) in athymic nude mice: I. Hormone dependence and the concentration of androgens in plasma and tumor tissue

This paper is the first part of a series of three describing a number of observations made on the PC-82 human prostatic carcinoma, xenografted into nude mice. The previously described androgen-dependence, one of the main properties of this tumor, has been the subject of subsequent studies. The impact of hormonal manipulation on the growth of the tumor and on plasma and tissue concentrations of androgens is discussed in this first part of the series. The great variability of plasma testosterone (T) levels in intact male mice (range: 1-90 nmol/liter) has been levelled out by the use of T-containing Silastic implants, resulting in levels ranging from 18 to 35 nmol/liter. Moreover, this route of administration also facilitated hormonal manipulation of tumor bearing mice. Androgen withdrawal from male mice with growing PC-82 tumors caused 80% tumor regression at ten weeks after androgen deprivation; the decline of the tumor volume followed a biphasic course. Delayed androgen substitution in castrated male mice grafted with PC-82 30 days before resulted in growth of the tumor tissue. This indicates that cells do not die and keep the capability to respond to androgens. It was concluded that the growth of the PC-82 tumor is not compatible with plasma T levels lower than 1 nmol/liter. Variable concentrations of endogenous T and dihydrotestosterone (DHT) were detected in total homogenates of PC-82 tumor tissue. Androgen withdrawal from T-implanted, tumor-bearing female mice caused a rapid reduction (90% within one day) of the tissue-T and a slower decline (up to 90% within seven to ten days) of tissue-DHT concentrations.