Kinetic analysis of LY320236: competitive inhibitor of type I and non-competitive inhibitor of type II human steroid 5alpha-reductase

Specific pharmacokinetic aspects of the urinary tract

This chapter reviews the evidence for "specific" pharmacokinetics playing a role in currently marketed drugs intended to treat lower urinary tract (LUT) symptoms. Principles of drug targeting include intrinsic properties of drugs or organs as well as drug formulations to modify drug release or to create confinement of drug presence. Prodrugs and specific formulations to deliver high drug concentrations at the site(s) of action as well as other ways to manipulate drug distribution to achieve enrichment in target tissues are considered. In overactive bladder (OAB), specific formulations for oxybutynin have been introduced to reduce the level of side effects of the active drug. Extended release tablet formulations and a topical gel formulation have been introduced, with efficacy similar to immediate release (IR) tablets, but with a reduction in anticholinergic adverse effects. However, these modifications have not led to outstanding performance parameters compared to other anticholinergic drugs marketed as IR formulations. Urinary excretion is discussed as potential mechanism for targeting LUT symptoms, but no strong indications appear to exist that this mechanism would contribute for currently available drugs. Intravesical administration of drugs is not a preferred option and only considered for drugs like botulinum toxin, where the inconvenient application compensates for a reasonable degree of long-term efficacy in severe refractory OAB. Alpha acid glycoprotein binding is discussed as a potential factor to influence drug tissue distribution, and it is concluded that there is reasonable evidence that for tamsulosin this mechanism is responsible for the difference in free fraction of the drug observed in plasma and prostate, which could contribute to its relative absence of blood pressure effects in patients with LUT symptoms related to benign prostate hyperplasia (LUTS-BPH). The principle of irreversible inhibition of type II 5α-reductase as a tool to develop drugs to reduce prostatic levels of dihydrotestosterone is employed by both dutasteride and finasteride for treatment of LUTS-BPH. Of the mechanisms discussed, the principles employed for the 5α-reductase blockers and tamsulosin in this respect can be considered relatively specific for its urological indication.

Pharmacologic Basis for the Enhanced Efficacy of Dutasteride against Prostatic Cancers

PURPOSE

Prostatic dihydrotestosterone (DHT) concentration is regulated by precursors from systemic circulation and prostatic enzymes of androgen metabolism, particularly 5alpha-reductases (i.e., SRD5A1 and SRD5A2). Therefore, the levels of expression SRD5A1 and SRD5A2 and the antiprostatic cancer growth response to finasteride, a selective SRD5A2 inhibitor, versus the dual SRD5A1 and SRD5A2 inhibitor, dutasteride, were compared.

EXPERIMENTAL DESIGN

Real-time PCR and enzymatic assays were used to determine the levels of SRD5A1 and SRD5A2 in normal versus malignant rat and human prostatic tissues. Rats bearing the Dunning R-3327H rat prostate cancer and nude mice bearing LNCaP or PC-3 human prostate cancer xenografts were used as model systems. Tissue levels of testosterone and DHT were determined using liquid chromatography-mass spectrometry.

RESULTS

Prostate cancer cells express undetectable to low levels of SRD5A2 but elevated levels of SRD5A1 activity compared with nonmalignant prostatic tissue. Daily oral treatment of rats with the SRD5A2 selective inhibitor, finasteride, reduces prostate weight and DHT content but did not inhibit R-3327H rat prostate cancer growth or DHT content in intact (i.e., noncastrated) male rats. In contrast, daily oral treatment with even a low 1 mg/kg/d dose of the dual SRD5A1 and SRD5A2 inhibitor, dutasteride, reduces both normal prostate and H tumor DHT content and weight in intact rats while elevating tissue testosterone. Daily oral treatment with finasteride significantly (P < 0.05) inhibits growth of LNCaP human prostate cancer xenografts in intact male nude mice, but this inhibition is not as great as that by equimolar oral dosing with dutasteride. This anticancer efficacy is not equivalent, however, to that produced by castration. Only combination of dutasteride and castration produces a greater tumor inhibition (P < 0.05) than castration monotherapy against androgen-responsive LNCaP cancers. In contrast, no response was induced by dutasteride in nude mice bearing androgen-independent PC-3 human prostatic cancer xenografts.

CONCLUSIONS

These results document that testosterone is not as potent as DHT but does stimulate prostate cancer growth, thus combining castration with dutasteride enhances therapeutic efficacy.

Androgen deprivation therapy for prostate cancer: current status and future prospects

Androgens play a major role in promoting the development and progression of prostate cancer. As a result, androgen ablation or blockade of androgen action through the androgen receptor (AR) has been the cornerstone of treatment of advanced prostate cancer. Different strategies involving this hormonal therapy produce a significant clinical response in most of the patients, but most responders eventually lose dependency, resulting in mortality. Thus, whether hormonal therapy contributes to the improvement of overall survival rates, especially in patients with advanced prostate cancer, remains controversial. However, patients with advanced disease clearly have a benefit from androgen deprivation-based treatment for palliating their symptoms and for improving the quality of their lives. In order to improve overall survival, novel treatment strategies that prolong the androgen-dependent state and that are useful for androgen-independent disease based on specific molecular mechanisms need to be identified.

Cutaneous androgen metabolism: basic research and clinical perspectives

The skin, especially the pilosebaceous unit composed of sebaceous glands and hair follicles, can synthesize androgens de novo from cholesterol or by locally converting circulating weaker androgens to more potent ones. As in other classical steroidogenic organs, the same six major enzyme systems are involved in cutaneous androgen metabolism, namely steroid sulfatase, 3beta-hydroxy-steroid dehydrogenase, 17beta-hydroxysteroid dehydrogenase, steroid 5alpha-reductase, 3alpha-hydroxysteroid dehydrogenase, and aromatase. Steroid sulfatase, together with P450 side chain cleavage enzyme and P450 17-hydroxylase, was found to reside in the cytoplasm of sebocytes and keratinocytes. Strong steroid sulfatase immunoreactivity was observed in the lesional skin but not in unaffected skin of acne patients. 3beta-hydroxysteroid dehydrogenase has been mainly immunolocalized to sebaceous glands, with the type 1 being the key cutaneous isoenzyme. The type 2 17beta-hydroxysteroid dehydrogenase isoenzyme predominates in sebaceous glands and exhibits greater reductive activity in glands from facial areas compared with acne nonprone areas. In hair follicles, 17beta-hydroxysteroid dehydrogenase was identified mainly in outer root sheath cells. The type 1 5alpha-reductase mainly occurs in the sebaceous glands, whereby the type II isoenzyme seems to be localized in the hair follicles. 3alpha-hydroxysteroid dehydrogenase converts dihydrotestosterone to 3alpha-androstanediol, and the use of 3alpha-androstanediol glucuronide serum level to reflect the hyperandrogenic state in hirsute women may be a reliable parameter, especially for idiopathic hirsutism. In acne patients it is still controversial if 3alpha-androstanediol glucuronide or androsterone glucuronide could serve as suitable serum markers for measuring androgenicity. Aromatase, localized to sebaceous glands and to both outer as well as inner root sheath cells of anagen terminal hair follicles, may play a "detoxifying" role by removing excess androgens. Pharmacologic development of more potent specific isoenzyme antagonists may lead to better clinical treatment or even prevention of androgen-dependent dermatoses.

Comparative study of human steroid 5alpha-reductase isoforms in prostate and female breast skin tissues: sensitivity to inhibition by finasteride and epristeride

Steroid 5alpha-reductase (5-AR) catalyses the reduction of testosterone (T) to dihydrotestosterone (DHT). The 5alpha-reductase found in human benign prostatic hyperplasia (BPH) has been compared with that found in human breast skin tissue in respect of sensitivity to inhibition by Finasteride and Epristeride. Kinetic studies showed the presence of two isoforms of 5alpha-reductase in benign prostatic hyperplasia indicated by low and high Km isoforms for testosterone, while female breast skin tissue contained only one isoform. The isoforms differ in their affinity for the inhibitors Finasteride and Epristeride, both compounds being more effective for the low Km 5alpha-reductase isoform than the high Km 5alpha-reductase of prostatic tissue, with Finasteride displaying competitive inhibition and Epristeride uncompetitive. Finasteride and Epristeride are also inhibitors of skin 5alpha-reductase, which possesses a comparable Ki for Finasteride to that of the low Km prostatic enzyme, but Epristeride was a less potent inhibitor of the skin enzyme relative to the prostate isoform. These results suggest that the inhibitors have therapeutic potential, other than for treatment of benign prostatic hyperplasia, for treating skin disorders influenced by the action of dihydrotestosterone and warrant further investigation.

5alpha-reductase activity in the prostate

The prostate gland depends on androgen stimulation for its development and growth. However, testosterone is not the major androgen responsible for growth of the prostate. Testosterone is converted to dihydrotestosterone (DHT) by the enzyme Delta(4), 3 ketosteroid, 5alpha-reductase in prostatic stromal and basal cells. DHT is primarily responsible for prostate development and the pathogenesis of benign prostatic hyperplasia (BPH). Inhibitors of 5alpha-reductase reduce prostate size by 20% to 30%. This reduction in glandular tissue is achieved by the induction of apoptosis, which is histologically manifested by ductal atrophy. Inhibition also diminishes the number of blood vessels in the prostate because of a reduction in vascular-derived endothelial growth factor. 5alpha-Reductase occurs as 2 isozymes, type 1 and type 2, with the prostate expressing predominantly the type-2 isozyme, and the liver and skin expressing primarily the type-1 isozyme. Patients have been identified with deficiencies in the type-2 5alpha-reductase, but not type 1. Knockout mice with the type-2 5alpha-reductase demonstrate a phenotype similar to that seen in men with 5alpha-reductase deficiency. Type-1 5alpha-reductase knockout male mice are phenotypically normal. Enzymatic activity for 5alpha-reductase or immunohistochemical detection has been noted in other genitourinary tissues, such as the epididymis, testes, gubernaculum, and corporal cavernosal tissue. Preputial skin predominately expresses the type-1 5alpha-reductase, whereas stromal cells in the seminal vesicle also express type-2 isozyme. However, epithelial cells in the epididymis, but not surrounding stroma, express type-1 5alpha-reductase. In addition to influencing prostatic growth, 5alpha-reductase also influences the expression of neuronal nitric-oxide synthase in the corpus cavernosum. The contribution of DHT in the serum, which is partially derived from type-1 5alpha-reductase in the liver and the small amount of type-1 5alpha-reductase in the prostate, may play a role in maintaining prostatic enlargement. Thus, in an effort to increase efficacy of treatment for BPH, clinical trials are under way using new drugs, such as GI-198745 (Glaxo-Wellcome, Research Triangle Park, NC), PNU 157706 (Pharmacia & Upjohn, Peapack, NJ), FR146687 (Fujisawa, Osaka, Japan), and LY 320236 (Lilly, Indianapolis, IN), which inhibit both the type-1 and type-2 5alpha-reductase.

Androgen antagonists: Potential role in prostate cancer prevention

This article summarizes discussions of the importance of androgens and androgen antagonists in the genesis of prostate cancer. These discussions occurred at a recent symposium on prostate cancer chemoprevention sponsored by the National Cancer Institute. Considerable information exists indicating the importance of androgens in the development of prostate cancer. Trials in breast cancer indicate that estrogen antagonists prevent breast cancer-suggesting, by analogy, that the blockade of androgen action might prevent the emergence of prostate cancer. The 5alpha-reductase inhibitors block the intracellular metabolism of testosterone and inhibit the growth of the prostate. Limited data suggest that 5alpha-reductase inhibitors reduces prostate-specific antigen in men with localized and advanced, primary or recurrent prostate cancer. An ongoing national trial of 18,000 men over 50 years of age has completed accrual and will evaluate whether a standard dose of finasteride will prevent the development of prostate cancer. The toxicity profile of finasteride (Proscar, Merck & Co., West Point, PA), the only approved 5alpha-reductase inhibitor, is favorable leading to its evaluation as a potential chemopreventive agent for prostate cancer. Anti-androgens such as bicalutamide (Casodex, AstraZeneca, Wilmington, DE) are active in the treatment of prostate cancer and comparable, in some trials, to testicular androgen suppression. These data suggest that antiandrogens may be active in the prevention of prostate cancer; however, the toxicity of antiandrogens (gynecomastia, gastrointestinal toxicity) poses concerns for application in prevention studies. Opportunities for study of factors predictive or associated with the development of prostate cancer and new agents that may interrupt this process offer numerous leads that may reduce the incidence of prostate cancer.