Finasteride: a slow-binding 5 alpha-reductase inhibitor

The 5α-reductase inhibitor finasteride reduces opioid self-administration in animal models of opioid use disorder

Opioid use disorder (OUD) has become a leading cause of death in the United States, yet current therapeutic strategies remain highly inadequate. To identify potential treatments for OUD, we screened a targeted selection of over 100 drugs using a recently developed opioid self-administration assay in zebrafish. This paradigm showed that finasteride, a steroidogenesis inhibitor approved for the treatment of benign prostatic hyperplasia and androgenetic alopecia, reduced self-administration of multiple opioids without affecting locomotion or feeding behavior. These findings were confirmed in rats; furthermore, finasteride reduced the physical signs associated with opioid withdrawal. In rat models of neuropathic pain, finasteride did not alter the antinociceptive effect of opioids and reduced withdrawal-induced hyperalgesia. Steroidomic analyses of the brains of fish treated with finasteride revealed a significant increase in dehydroepiandrosterone sulfate (DHEAS). Treatment with precursors of DHEAS reduced opioid self-administration in zebrafish in a fashion akin to the effects of finasteride. These results highlight the importance of steroidogenic pathways as a rich source of therapeutic targets for OUD and point to the potential of finasteride as a new treatment option for this disorder.

The role of 5-reduction in physiology and metabolic disease: evidence from cellular, pre-clinical and human studies

The 5-reductases (5α-reductase types 1, 2 and 3 [5αR1-3], 5β-reductase [5βR]) are steroid hormone metabolising enzymes that hold fundamental roles in human physiology and pathology. They possess broad substrate specificity converting many steroid hormones to their 5α- and 5β-reduced metabolites, as well as catalysing crucial steps in bile acid synthesis. 5αRs are fundamentally important in urogenital development by converting testosterone to the more potent androgen 5α-dihydrotestosterone (5αDHT); inactivating mutations in 5αR2 lead to disorders of sexual development. Due to the ability of the 5αRs to generate 5αDHT, they are an established drug target, and 5αR inhibitors are widely used for the treatment of androgen-dependent benign or malignant prostatic diseases. There is an emerging body of evidence to suggest that the 5-reductases can impact upon aspects of health and disease (other than urogenital development); alterations in their expression and activity have been associated with metabolic disease, polycystic ovarian syndrome, inflammation and bone metabolism. This review will outline the evidence base for the extra-urogenital role of 5-reductases from in vitro cell systems, pre-clinical models and human studies, and highlight the potential adverse effects of 5αR inhibition in human health and disease.

In silico and functional studies reveal novel loss-of-function variants of SRD5A2, but no variants explaining excess 5α-reductase activity

Androgens are steroid hormones essential for human male and female development. Steroid reductases 5α (SRD5As) are key enzymes in androgen biosynthesis. Mutations in the human SRD5A2 are known to cause loss-of-function and severe 46,XY undervirilization. Gain-of-function variants have been suggested in androgen excess syndromes, but have not been found so far. Therefore we searched for gain-of-function mutations in the human SRD5A2 gene which might explain hyperandrogenic disorders such as the polycystic ovary syndrome, premature adrenarche and prostate cancer. We screened databases for candidate variants and characterised them in silico with the help of a novel SRD5A2 model. We selected 9 coding SNPs (A49T, R50A, P106L, P106A, N122A, L167S, R168C, P173S, R227Q) that have not been described in manifesting individuals, and assessed their enzyme kinetic properties in HEK293 cells. SRD5A2 activity was assessed by conversion of testosterone (T), progesterone (Prog) and androstenedione (Δ4A) to their 5α-reduced metabolites. Variants R50A and P173S showed partial activity with substrates T (34% and 28%) and Δ4A (37% and 22%). With substrate Prog variants P106L, P106A, L167S and R168C in addition showed partial activity (15% to 64%). Functional testing of all other variants showed loss-of-function. As predicted in our in silico analysis, all coding SNPs affected enzyme activity, however none of them showed gain-of-function. Thus excess 5α-reductase activity might be rather regulated at the (post)-transcriptional and/or post-translational level. However through this work seven new coding SNPs were characterised which might be of clinical relevance. It is possible that individuals carrying these SNPs show a minor phenotype that is not yet identified.

Discovery of Natural Steroid 5 Alpha-Reductase Inhibitors

Human steroid 5 alpha-reductases (S5αRs) and NADPH irreversibly reduce testosterone to the more potent dihydrotestosterone (DHT). S5αR inhibitors are useful treatments for DHT-dependent diseases, including benign prostatic hyperplasia, androgenic alopecia and hair growth, and acne. There are three S5αR isozymes, and there is a need for safer and more isozyme selective inhibitors than finasteride and dutasteride currently licensed. In this study, we review the methods used to screen for S5αR inhibitory activity and describe studies that characterize the ability of herbal preparations and their constituents to inhibit S5αRs. We identified enormous variations between studies in IC₅₀s for finasteride and dutasteride used as standards. Accordingly, we make several recommendations: Stable isozyme specific transfection systems need creating a standardized enzyme/microsome preparation and all three isozymes, as well as androgen receptor binding, should be tested; agreed reaction conditions, especially the substrate concentrations, and separation/quantitation method optimized for high throughput screening; systematic screening of herbal compounds and most extensive use of leads to develop more potent and isozyme specific inhibitors.

Development of optimized self-nanoemulsifying lyophilized tablets (SNELTs) to improve finasteride clinical pharmacokinetic behavior

OBJECTIVE

Preparation of an optimized finasteride (FSD) lyophilized tablets loaded with self-nanoemulsifying drug delivery system (SNEDDS).

SIGNIFICANCE

Enhance FSD bioavailability in male pattern baldness and benign prostatic hyperplasia.

METHODS

Two-step optimization was implemented to achieve the study goals. First; the mixture design was used to develop an optimized SNEDDS through which the effect of cosurfactant number of carbon atoms on SNEDDS particle size and thermodynamic stability has been tested. Second; the different tablet excipients have been used to develop an optimized self-nanoemulsifying lyophilized tablets (SNELTs). The prepared tablets have been fully characterized. Interaction among tablet components has been studied. Finally, FSD clinical pharmacokinetic has been investigated on human volunteers.

RESULTS

Anise oil and tween 80 were selected as oily phase and surfactant, respectively while different aliphatic alcohols were studied as cosurfactants. Percentages of oil, surfactant, and cosurfactants were significantly affecting SNEDDS particle size. Increasing cosurfactant number of carbon atoms achieved smaller particle size and higher stability. The optimized SNEDDS was found to contain 10.3455, 45.8972, and 43.7573% of anise oil, tween 80, and butanol, respectively. Variations in FSD cumulative release and disintegration time, from the prepared tablets, were attributed to change in the percent of plasdone XL, Avicel and silica. No interaction among components was noticed. Clinical pharmacokinetics illustrated significant enhancement in the studied parameters from the optimized lyophilized tablets loaded with drug SNEDDS when compared to marketed FSD product.

CONCLUSION

Lyophilized tablets could be considered as a good alternative for conventional solid dosage forms especially when loaded with drug nanosystems.

Recent Advances in Drug Design and Drug Discovery for Androgen- Dependent Diseases

This article summarizes the importance of different targets such as 5α-reductase, 17β-HSD, CYP17A, androgen receptor and protein kinase A for the treatment of prostate cancer and benign prostatic hyperplasia. It is a well known fact that dihydrotestosterone (DHT) is associated with the development of androgen-dependent afflictions. At the present time, several research groups are attempting to develop new steroidal and non-steroidal molecules with the purpose of inhibiting the synthesis and biological response of DHT. This review also discusses the most recent studies reported in the literature that describe the therapeutic potential of novel compounds, as well as the new drugs, principally inhibitors of 5α-reductase.

Preparation of finasteride capsules-loaded drug nanoparticles: formulation, optimization, in vitro, and pharmacokinetic evaluation

In this study, optimized freeze-dried finasteride nanoparticles (NPs) were prepared from drug nanosuspension formulation that was developed using the bottom–up technique. The effects of four formulation and processing variables that affect the particle size and solubility enhancement of the NPs were explored using the response surface optimization design. The optimized formulation was morphologically characterized using transmission electron microscopy (TEM). Physicochemical interaction among the studied components was investigated. Crystalline change was investigated using X-ray powder diffraction (XRPD). Crystal growth of the freeze-dried NPs was compared to the corresponding aqueous drug nanosuspension. Freeze-dried NPs formulation was subsequently loaded into hard gelatin capsules that were examined for in vitro dissolution and pharmacokinetic behavior. Results revealed that in most of the studied variables, some of the quadratic and interaction effects had a significant effect on the studied responses. TEM image illustrated homogeneity and shape of the prepared NPs. No interaction among components was noticed. XRPD confirmed crystalline state change in the optimized NPs. An enhancement in the dissolution rate of more than 2.5 times from capsules filled with optimum drug NPs, when compared to capsules filled with pure drug, was obtained. Crystal growth, due to Ostwald ripening phenomenon and positive Gibbs free energy, was reduced following lyophilization of the nanosuspension formulation. Pharmacokinetic parameters from drug NPs were superior to that of pure drug and drug microparticles. In conclusion, freeze-dried NPs based on drug nanosuspension formulation is a successful technique in enhancing stability, solubility, and in vitro dissolution of poorly water-soluble drugs with possible impact on the drug bioavailability.

In vitro analysis of finasteride activity against Candida albicans urinary biofilm formation and filamentation

Candida albicans is the 3rd most common cause of catheter-associated urinary tract infections, with a strong propensity to form drug-resistant catheter-related biofilms. Due to the limited efficacy of available antifungals against biofilms, drug repurposing has been investigated in order to identify novel agents with activities against fungal biofilms. Finasteride is a 5-α-reductase inhibitor commonly used for the treatment of benign prostatic hyperplasia, with activity against human type II and III isoenzymes. We analyzed the Candida Genome Database and identified a C. albicans homolog of type III 5-α-reductase, Dfg10p, which shares 27% sequence identity and 41% similarity to the human type III 5-α-reductase. Thus, we investigated finasteride for activity against C. albicans urinary biofilms, alone and in combination with amphotericin B or fluconazole. Finasteride alone was highly effective in the prevention of C. albicans biofilm formation at doses of ≥16 mg/liter and the treatment of preformed biofilms at doses of ≥128 mg/liter. In biofilm checkerboard analyses, finasteride exhibited synergistic activity in the prevention of biofilm formation in a combination of 4 mg/liter finasteride with 2 mg/liter fluconazole. Finasteride inhibited filamentation, thus suggesting a potential mechanism of action. These results indicate that finasteride alone is highly active in the prevention of C. albicans urinary biofilms in vitro and has synergistic activity in combination with fluconazole. Further investigation of the clinical utility of finasteride in the prevention of urinary candidiasis is warranted.

Prostate cancer cells differ in testosterone accumulation, dihydrotestosterone conversion, and androgen receptor signaling response to steroid 5α-reductase inhibitors

BACKGROUND

Blocking 5α-reductase-mediated testosterone conversion to dihydrotestosterone (DHT) with finasteride or dutasteride is the driving hypothesis behind two prostate cancer prevention trials. Factors affecting intracellular androgen levels and the androgen receptor (AR) signaling axis need to be examined systematically in order to fully understand the outcome of interventions using these drugs.

METHODS

The expression of three 5α-reductase isozymes, as determined by immunohistochemistry and qRT-PCR, was studied in five human prostate cancer cell lines. Intracellular testosterone and DHT were analyzed using mass spectrometry. A luciferase reporter assay and AR-regulated genes were used to evaluate the modulation of AR activity.

RESULTS

Prostate cancer cells were capable of accumulating testosterone to a level 15-50 times higher than that in the medium. The profile and expression of 5α-reductase isozymes did not predict the capacity to convert testosterone to DHT. Finasteride and dutasteride were able to depress testosterone uptake in addition to lowering intracellular DHT. The inhibition of AR activity following drug treatment often exceeded the expected response due to reduced availability of DHT. The ability to maintain high intracellular testosterone might compensate for the shortage of DHT.

CONCLUSIONS

The biological effect of finasteride or dutasteride appears to be complex and may depend on the interplay of several factors, which include testosterone turnover, enzymology of DHT production, ability to use testosterone and DHT interchangeably, and propensity of cells for off-target AR inhibitory effect.

5α-reductases in human physiology: an unfolding story

OBJECTIVE

5α-reductases are a family of isozymes expressed in a wide host of tissues including the central nervous system (CNS) and play a pivotal role in male sexual differentiation, development and physiology.

METHODS

A comprehensive literature search from 1970 to 2011 was made through PubMed and the relevant information was summarized.

RESULTS

5α reductases convert testosterone, progesterone, deoxycorticosterone, aldosterone and corticosterone into their respective 5α-dihydro-derivatives, which serve as substrates for 3α-hydroxysteroid dehydrogenase enzymes. The latter transforms these 5α-reduced metabolites into a subclass of neuroactive steroid hormones with distinct physiological functions. The neuroactive steroid hormones modulate a multitude of functions in human physiology encompassing regulation of sexual differentiation, neuroprotection, memory enhancement, anxiety, sleep and stress, among others. In addition, 5α -reductase type 3 is also implicated in the N-glycosylation of proteins via formation of dolichol phosphate. The family of 5α-reductases was targeted for drug development to treat pathophysiological conditions, such as benign prostatic hyperplasia and androgenetic alopecia. While the clinical use of 5α-reductase inhibitors was well established, the scope and the magnitude of the adverse side effects of such drugs, especially on the CNS, is still unrecognized due to lack of knowledge of the various physiological functions of this family of enzymes, especially in the CNS.

CONCLUSION

There is an urgent need to better understand the function of 5α-reductases and the role of neuroactive steroids in human physiology in order to minimize the potential adverse side effects of inhibitors targeting 5α-reductases to treat benign prostatic hyperplasia and androgenic alopecia.

Advances in the design and discovery of drugs for the treatment of prostatic hyperplasia

INTRODUCTION

Benign prostatic hyperplasia (BPH) is a common medical problem in nearly 80% of geriatric male population severely affecting the quality of life. Several strategies has been suggested in the past for the management of BPH, but only α-blockers and 5α-reductase inhibitors are in clinical use. This review aims to give deep insight into advances in the design and discovery of newer chemical entities as 'druggable' molecule for the management of BPH.

AREAS COVERED

In this review, the authors cover various classes of drugs that have shown their potential for management of BPH. These drugs include α-adrenergic antagonists, 5α-reductase inhibitors, phytochemical agents, phosphodiesterase inhibitor, luteinizing hormone releasing hormone antagonists and muscarinic receptor antagonists. Literature searches were carried out using Google Scholar, SciFinder and PubMed.

EXPERT OPINION

The exact etiology of BPH is unknown; however, several mechanisms may be involved in the progression of the disease. Beside surgery and watchful waiting, medical therapies to treat BPH include α-adrenergic antagonist and 5α-reductase inhibitors. Phytotherapeutic agents are also used in some countries. Various other chemical classes of drugs are proposed for the treatment of the disease, but none of them have reached the clinic. Many classes of drugs are currently undergoing clinical trials such as phosphodiesterase inhibitors, luteinizing hormone releasing hormone antagonists and muscarinic receptor antagonists. The current need is to develop a potent, efficacious and highly selective drug for the treatment of BPH.

Targeting 5α-reductase for prostate cancer prevention and treatment

Testosterone is the most abundant circulating androgen, and can be converted to dihydrotestosterone (DHT), a more potent androgen, by the 5α-reductase enzymes in target tissues. Current treatments for prostate cancer consist of reducing androgen levels by chemical or surgical castration or pure antiandrogen therapy that directly targets the androgen receptor (AR). Although these therapies reduce tumor burden and AR activity, the cancer inevitably recurs within 18-30 months. An approach targeting the androgen-AR axis at different levels could, therefore, improve the efficacy of prostate cancer therapy. Inhibition of 5α-reductase is one such approach; however, the two largest trials to investigate the use of the 5α-reductase inhibitors (5ARIs) finasteride and dutasteride in patients with prostate cancer have shown that, although the incidence of cancer was reduced by 5ARI treatment, those cancers that were detected were more aggressive than in patients treated with placebo. Thus, the best practice for using these drugs to prevent and treat prostate cancer remains unclear.

Finasteride metabolism and pharmacogenetics: new approaches to personalized prevention of prostate cancer

Incidences of prostate cancer in most countries are increasing owing to better detection methods; however, prevention with the use of finasteride, a very effective steroid 5α-reductase type II inhibitor, has been met with mixed success. A wide interindividual variation in response exists and is thought to be due to heritable factors. This article summarizes the literature that attempts to elucidate the molecular mechanisms of finasteride in terms of its metabolism, excretion and interaction with endogenous steroid molecules. We describe previously reported genetic variations of steroid-metabolizing genes and their potential association with finasteride efficacy. Based on the literature, we outline directions of research that may contribute to understanding the interindividual variation in finasteride prevention and to the future development of personalized medicine.

Kinetic and inhibition studies of dihydroxybenzoate-AMP ligase from Escherichia coli

Inhibition of siderophore biosynthetic pathways in pathogenic bacteria represents a promising strategy for antibacterial drug development. Escherichia coli synthesize and secrete the small molecule iron chelator siderophore, enterobactin, in response to intracellular iron depletion. Here we describe a detailed kinetic analysis of EntE, one of six enzymes in the enterobactin synthetase gene cluster. EntE catalyzes the ATP-dependent condensation of 2,3-dihydroxybenzoic acid (DHB) and phosphopantetheinylated EntB (holo-EntB) to form covalently arylated EntB, a product that is vital for the final assembly of enterobactin. Initial velocity studies show that EntE proceeds via a bi-uni-uni-bi ping-pong kinetic mechanism with a k(cat) equal to 2.8 s(-1) and K(m) values of 2.5, 430, and 2.9 microM for DHB, ATP, and holo-EntB-ArCP, respectively. Inhibition and direct binding experiments suggest that, during the first half-reaction (adenylation), DHB binds first to the free enzyme, followed by ATP and the release of pyrophosphate to form the adenylate intermediate. During the second half-reaction (ligation), phosphopantetheinylated EntB binds to the enzyme followed by the release of products, AMP and arylated EntB. Two hydrolytically stable adenylate analogues, 5'-O-[N-(salicyl)sulfamoyl]adenosine (Sal-AMS) and 5'-O-[N-(2,3-dihydroxybenzoyl)sulfamoyl]adenosine (DHB-AMS), are shown to act as slow-onset tight-binding inhibitors of the enzyme with (app)K(i) values of 0.9 and 3.8 nM, respectively. Direct binding experiments, via isothermal titration calorimetry, reveal low picomolar dissociation constants for both analogues with respect to EntE. The tight binding of Sal-AMS and DHB-AMS to EntE suggests that these compounds may be developed further as effective antibiotics targeted to this enzyme.

Dehydroepiandrosterone (DHEA) treatment in vitro inhibits adipogenesis in human omental but not subcutaneous adipose tissue

Dehydroepiandrosterone (DHEA), a precursor sex steroid, circulates in sulphated form (DHEAS). Serum DHEAS concentrations are inversely correlated with metabolic syndrome components and in vivo/in vitro studies suggest a role in modulating adipose mass. To investigate further, we assessed the in vitro biological effect of DHEA in white (3T3-L1) and brown (PAZ6) preadipocyte cell lines and human primary preadipocytes. DHEA (from 10(-8)M) caused concentration-dependent proliferation inhibition of 3T3-L1 and PAZ6 preadipocytes. Cell cycle analysis demonstrated unaltered apoptosis but indicated blockade at G1/S or G2/M in 3T3-L1 and PAZ6, respectively. Preadipocyte cell-line adipogenesis was not affected. In human primary subcutaneous and omental preadipocytes, DHEA significantly inhibited proliferation from 10(-8)M. DHEA 10(-7)M had opposing effects on adipogenesis in the two fat depots. Subcutaneous preadipocyte differentiation was unaffected or increased whereas omental preadipocytes showed significantly reduced adipogenesis. We conclude that DHEA exerts fat depot-specific differences which modulate body composition by limiting omental fat production.

Deoxycorticosterone's anticonvulsant effects in infant rats are blocked by finasteride, but not by indomethacin

Deoxycorticosterone (DOC) is a steroid hormone that suppresses seizures in both humans and animals. At higher doses, DOC's anticonvulsant actions are accompanied by sedation and ataxia. The mechanism of DOC's anticonvulsant actions is not known, although it has been suggested that they may relate to DOC's secondary metabolite 3-alpha-5-alpha-tetrahydrodeoxycorticosterone (THDOC). The present study was designed to study the relation of DOC's anticonvulsant actions to its primary and secondary metabolites in 15-day-old rats. It was found that DOC's anticonvulsant and ataxic effects were suppressed by finasteride, which blocks the formation of DOC's primary metabolite, 3-alpha-5-alpha-dehydrodeoxycorticosterone (DHDOC). They were not suppressed by indomethacin (INDO), which blocks the conversion of DHDOC into THDOC. The direct anticonvulsant effects of DHDOC and THDOC were also tested. DHDOC and THDOC were both potent anticonvulsants in 15-day old rats. Both also caused ataxia at high doses. DHDOC had a therapeutic index (TI) of 3.2, however, which was better than either DOC (TI = 1.2) or THDOC (TI = 1.5). It appears that DOC itself is not anticonvulsant, but that its anticonvulsant effects may relate to both its primary and secondary metabolites. DOC's primary metabolite, DHDOC--with its good TI--deserves a test in the treatment of childhood seizures.

Benzo[c]quinolizin-3-ones Theoretical Investigation: SAR Analysis and Application to Nontested Compounds

We investigate with the use of theoretical methodologies the activity of a set of 41 benzo[c]quinolizin-3-ones (BC3), some of them explored as selective inhibitors of the human 5alpha-reductase steroid. For the structure-activity study we have considered dividing the molecules into groups of tested and nontested compounds. Semiempirical calculations and pattern recognition methods such as Electronic Indices Methodology (EIM), Principal Components Analysis (PCA), Hierarchical Cluster Analysis (HCA), and K-Nearest Neighbors (KNN) have been applied to search for a correlation between experimental activity and theoretical descriptors. Our results show that it is possible to directly correlate some molecular quantum descriptors with BC3 biological activity. This information can be used in principle to identify active/inactive untested compounds and/or to design new active compounds.

Androgens and male physiology the syndrome of 5alpha-reductase-2 deficiency

Dihydrotestosterone (DHT), a potent androgen, is converted from testosterone by 5alpha-reductase isozymes. There are two 5alpha-reductase isozymes, type 1 and type 2 in humans and animals. These two isozymes have differential biochemical and molecular features. Mutations in type 2 isozyme cause male pseudohermaphroditism, and many mutations have been reported from various ethnic groups. The affected 46XY individuals have high normal to elevated plasma testosterone levels with decreased DHT levels and elevated testosterone/DHT ratios. They have ambiguous external genitalia at birth so that they are believed to be girls and are often raised as such. However, Wolffian differentiation occurs normally and they have epididymides, vas deferens and seminal vesicles. Virilization occurs at puberty frequently with a gender role change. The prostate in adulthood is small and rudimentary, and facial and body hair is absent or decreased. Balding has not been reported. Spermatogenesis is normal if the testes are descended. The clinical, biochemical and molecular genetic analyses of 5alpha-reductase-2 deficiency highlight the significance of DHT in male sexual differentiation and male pathophysiology.

Pharmacokinetic parameters and mechanisms of inhibition of rat type 1 and 2 steroid 5alpha-reductases: determinants for different in vivo activities of GI198745 and finasteride in the rat

The interaction of baculovirus expressed rat steroid 5alpha-reductase types 1 and 2 (r5AR1 and r5AR2) with 17beta-N-(2,5-bis(trifluoromethyl)phenyl)carbamoyl-4-aza-5alpha-androst-1-en-3-one (GI198745) was investigated at pH 7 and 37 degrees. This 5alpha-reductase inhibitor was found previously to be a time-dependent inhibitor of the two human 5alpha-reductase isozymes. In contrast, we demonstrate in the present study that although GI198745 is a potent time-dependent inhibitor of r5AR2, it is a classical rapid-equilibrium inhibitor of r5AR1. This type of behavior with human and rat 5alpha-reductases has been shown for the inhibitor 17beta-(N-tert-butylcarbamoyl)-4-aza-5alpha-androst-1-en-3-one (finasteride), a current therapy for benign prostatic hyperplasia. Inhibition of r5AR1 by GI198745 was competitive with testosterone and followed Michaelis-Menten kinetics with a K(i) value of 0.3 +/- 0.02 nM. Data for the inhibition of r5AR2 by GI198745 were consistent with a two-step mechanism, where K(i) is the dissociation constant for an initial enzyme-inhibitor complex and k(3) is the rate constant for the second slow step. The pseudo-bimolecular rate constant (k(3)/K(i)) for the association of GI198745 with r5AR2 was (2.0 +/- 0.4) x 10(7) M(-1) sec(-1). The high affinity of this inhibitor for r5AR2 was further demonstrated by the inability of the enzyme-inhibitor complex to dissociate after approximately 7 days of dialysis at 4 degrees. Both GI198745 and finasteride appear to inactivate r5AR2 by apparent irreversible modification, but are classical, reversible inhibitors of r5AR1. Therefore, we hypothesize that because of its pharmacokinetic parameters and increased potency against r5AR1, GI198745 is more effective than finasteride in preventing the growth of the rat prostate.

Neurosteroids mediate habituation and tonic inhibition in the auditory midbrain

Habituation of the behavioral response to a repetitive stimulus is a well-established observation in perceptual studies and is considered a basic form of nonassociative learning. There is also a long history of physiological studies suggesting that central nervous system habituation is mediated by inhibition. At higher levels of the sensory pathways, such inhibition is mainly contributed by GABAa receptor mechanisms. Concepts of modification of synaptic efficacy that apply to excitatory amino acid synaptic transmission do not have direct parallels with these inhibitory synapses: quantal release of GABA rapidly saturates available receptors at a synapse, placing an upper limit on responsiveness to increased transmitter release. However, pharmacological modulation of GABAa-receptor efficacy with exogenous agents (e.g., benzodiazepines and beta-carbolines) is known to occur through allosteric mechanisms that modulate the effectiveness (positive and negative) of GABA at this receptor. The most potent endogenous modulators are 5alpha-reduced steroids. Production of these steroids was attenuated in adult rats with systemic injection of Finasteride, a competitive substrate for 5alpha-reductase. This treatment was sufficient to block habituation of the evoked midbrain response to repetitive presentation of an acoustic click. This result confirms that simple habituation is due to an increase in active inhibition, the increase being mediated by steroid modulation of the GABAa-receptor. Finasteride treatment also brought about a 23% increase in the evoked response to a click stimulus, suggesting that 5alpha-reduced steroids normally contribute to tonic inhibition in the rat inferior colliculus.

Linear relationships between the ligand binding energy and the activation energy of time-dependent inhibition of steroid 5alpha-reductase by delta 1-4-azasteroids

The inhibition of steroid 5alpha-reductase (5AR) by Delta(1)-4-azasteroids is characterized by a two-step time-dependent kinetic mechanism where inhibitor combines with enzyme in a fast equilibrium, defined by the inhibition constant K(i), to form an initial reversible enzyme-inhibitor complex, which subsequently undergoes a time-dependent chemical rearrangement, defined by the rate constant k(3), leading to the formation of an apparently irreversible, tight-binding enzyme-inhibitor complex (Tian, G., Mook, R. A., Jr., Moss, M. L., and Frye, S. V. (1995) Biochemistry 34, 13453-13459). A detailed kinetic analysis of this process with a series of Delta(1)-4-azasteroids having different C-17 substituents was performed to understand the relationships between the rate of time-dependent inhibition and the affinity of the time-dependent inhibitors for the enzyme. A linear correlation was observed between ln(1/K(i)), which is proportional to the ligand binding energy for the formation of the enzyme-inhibitor complex, and ln(1/(k(3)/K(i))), which is proportional to the activation energy for the inhibition reaction under the second order reaction condition, which leads to the formation of the irreversible, tight-binding enzyme-inhibitor complex. The coefficient of the correlation was -0.88 +/- 0.07 for type 1 5AR and -1.0 +/- 0.2 for type 2 5AR. In comparison, there was no obvious correlation between ln(1/K(i)) and ln(1/k(3)), which is proportional to the activation energy of the second, time-dependent step of the inhibition reaction. These data are consistent with a model where ligand binding energies provided at C-17 of Delta(1)-4-azasteroids is fully expressed to lower the activation energy of k(3)/K(i) with little perturbation of the energy barrier of the second, time-dependent step.

A model for the turnover of dihydrotestosterone in the presence of the irreversible 5 alpha-reductase inhibitors GI198745 and finasteride

OBJECTIVE

To develop a pharmacokinetic-pharmacodynamic model that characterizes the conversion of testosterone to dihydrotestosterone (DHT) by 5 alpha-reductase types 1 and 2 and the irreversible inhibition of 5 alpha-reductase by finasteride, a 5 alpha-reductase type 2 inhibitor and by GI198745 (dutasteride), a potent and specific dual 5 alpha-reductase inhibitor.

METHODS

Healthy men (n = 48) received doses of 0.1 to 40 mg GI198745 (n = 4 subjects per dose), 5 mg finasteride (n = 8), or placebo (n = 8) in a parallel-group study. Plasma concentrations of GI198745, finasteride, and DHT were measured frequently up to 8 weeks after dosing. Models were fitted with mixed-effects modeling with the NONMEM program.

RESULTS

The pharmacodynamics were well described with a model that accounted for the rates of DHT formation and elimination, 5 alpha-reductase turnover, relative capacity of the 2 5 alpha-reductase isozymes, and the rates of irreversible inhibition of one (finasteride) or both (GI198745) types of 5 alpha-reductase. The model indicated that type 2 5 alpha-reductase contributed approximately 80% of plasma DHT. GI198745 was about 3-fold more potent than finasteride on 5 alpha-reductase type 2. Nearly full blockade of both isozymes was achieved at doses of 10 mg or more GI198745, although the potency of this agent on 5 alpha-reductase type 1 was less than on type 2.

CONCLUSIONS

A physiologically based model for the turnover and irreversible inhibition of 5 alpha-reductase and for formation and elimination of DHT described the data well. This model helps explain differences in the rates of onset and offset of effect and offers a way to determine the relative potency of the irreversible 5 alpha-reductase inhibitors.

Cloning, expression and characterization of rhesus macaque types 1 and 2 5alpha-reductase: evidence for mechanism-based inhibition by finasteride

The rhesus macaque types 1 and 2 5alpha-reductase (5aR1 and 5aR2) were cloned and expressed in COS cells to facilitate comparison of rhesus and human 5aRs. The deduced protein sequences of the rhesus SaRs shared 94% and 96% identity with the human type 1 and 2 isozymes, respectively. Despite a four amino acid insertion at the N-terminal region of rhesus 5aR1, the biochemical properties of rhesus and human homologs are very similar with respect to pH optimum, Km values for testosterone and progesterone, and inhibition by a variety of inhibitors. As expected, the biochemical properties of the human and rhesus 5aR2 are also very similar. The mechanism of inhibition of the rhesus 5aR1 and 5aR2 by finasteride was investigated in more detail. Finasteride displays time dependent inhibition of the rhesus 5aR1 and 5aR2 with second order rate constants of 4 x 10(3) M(-1) s(-1) and 5.2 x 10(5) M(-1)s(-1). Inhibition of rhesus 5aR2 with 3H-finasteride resulted in 3H bound to the enzyme which is not released by dialysis. Heat denaturation of the [rhesus SaR2:inhibitor] complex releases dihydrofinasteride, a breakdown product presumably related to the NADP+-adduct previously identified with the human SaRs (Bull et al., Mechanism-based inhibition of human steroid 5alpha-reductase by finasteride: Enzyme catalyzed formation of NADP-dihydrofinasteride, a potent bisubstrate analog inhibitor. J. Amer. Chem. Soc., 1996, 118, 2359-2365). Taken together, these results provide good evidence that the rhesus macaque is a suitable model to evaluate the pharmacological properties of finasteride and other 5aR inhibitors.

Inhibitory effects of Serenoa repens on the kinetic of pig prostatic microsomal 5alpha-reductase activity

The pathogenesis of benign prostatic hyperplasia is linked to the accumulation of dihydrotestosterone (DHT), the active form of testosterone (T), in prostatic tissue. We have defined characteristics of 5alpha-reductase enzyme which catalyzes the conversion of T into DHT in prostatic microsomes of growing pigs. Peaks for the 5alpha-reductase activity were found at pH 5.5 and 8.0, which indicates the presence of both type 1 and type 2 isozymes. Kinetic parameters of porcine 5alpha-reductase in the presence of Serenoa repens extracts revealed uncompetitive, noncompetitive, and mixed types of inhibitions. Our results show the inhibitory action of S. repens on prostate porcine microsomal 5alpha-reductase activity.

Natural potent androgens: lessons from human genetic models

Male pseudohermaphroditism due to 17 beta-hydroxysteroid dehydrogenase-3 (17 beta-HSD-3) deficiency and 5 alpha-reductase-2 (5 alpha-RD-2) deficiency provides natural human genetic models to elucidate androgen actions. To date, five 17 beta-HSD isozymes have been cloned that catalyse the oxidoreduction of androstenedione and testosterone and dihydrotestosterone (DHT), oestrone and oestradiol. Mutations in the isozyme 17 beta-HSD-3 gene are responsible for male pseudohermaphroditism due to 17 beta-HSD deficiency. The type 3 isozyme preferentially catalyses the reduction of androstenedione to testosterone and is primarily expressed in the testes. Fourteen mutations in the 17 beta-HSD-3 gene have been identified from different ethnic groups. Affected males with the 17 beta-HSD-3 gene defect have normal wolffian structures but ambiguous external genitalia at birth. Many are raised as girls but virilize at the time of puberty and adopt a male gender role. Some develop gynaecomastia at puberty, which appears to be related to the testosterone/oestradiol ratio. Two 5 alpha-reductase (5 alpha-RD) isozymes, types 1 and 2, have been identified, which convert testosterone to the more potent androgen DHT. Mutations in the 5 alpha-RD-2 gene cause male pseudohermaphroditism, and 31 mutations in the 5 alpha-RD-2 gene have been reported from various ethnic groups. Such individuals also have normal wolffian structure but ambiguous external genitalia at birth and are raised as girls. Virilization occurs at puberty, often with a gender role change. The prostate remains infantile and facial hair is decreased. Balding has not been reported.

Pharmacological and molecular evidence for the expression of the two steroid 5 alpha-reductase isozymes in normal and hyperplastic human prostatic cells in culture

BACKGROUND

Whereas the embryological development of the human prostate is clearly dependent on steroid 5 alpha-reductase (5 alpha-R) type 2 expression, the respective expression of the two known isoforms (types 1 and 2) of 5 alpha-R in the adult human prostate remains unclear.

METHODS

5 alpha-R isoform mRNA expression (Northern blots and reverse transcriptase-polymerase chain reaction [RT-PCR]) and enzyme activity were studied in immortalized epithelial cells (NE) and in fibroblasts from normal (NF) or hyperplastic (BPHF) human prostates.

RESULTS

5 alpha-R activity (fmol/microgram DNA/hr) was 1.43 +/- 0.5 in NE, 10.7 +/- 4.7 in NF, and 79 +/- 37 in BPHF. mRNAs for both 5 alpha-R isoforms were expressed in the three cell types, as shown by Northern blot and RT-PCR analysis. LY306089, a selective 5 alpha-R type 1 inhibitor, strongly inhibited 5 alpha-R activity in all cell types (IC50: 10 nM), confirming the predominant expression of 5 alpha-R type 1 in these cells. Finasteride, a 5 alpha-R type 2 inhibitor, was less efficient (IC50: 45, 35, and 65 nM in NE, NF, and BPHF, respectively). In addition, the inhibition by finasteride decreased with serial subculture in NF only, suggesting an effect of age in culture on the expression of 5 alpha-R type 2 in these cells. SKF105657, also a 5 alpha-R type 2 inhibitor, was a poor inhibitor in this system.

CONCLUSIONS

These studies demonstrate that human prostate cells in culture express both isoforms of 5 alpha-R and suggest a balance in the expression of the two isoforms as a function of various regulatory factors.

Inhibition of rat alpha-reductases by finasteride: evidence for isozyme differences in the mechanism of inhibition

The mechanism of inhibition of the rat types 1 and 2 5alpha-reductase by finasteride was investigated using recombinantly expressed enzymes. These studies revealed that finasteride is a potent, reversible inhibitor of the rat type 1 5alpha-reductase with Ki=10.2+/-1.3 nM. Finasteride is a potent inhibitor of the rat type 2; however, in this case the compound binds to the type 2 isozyme-NADPH complex to form a ternary complex with Ki=1.19+/-0.10 nM, which then rearranges to a high affinity complex (E:I) with a pseudo first order rate constant of 1.62+/-0.22 x 10(-3)/s. The second order rate constant is k3/Ki=1.37+/-0.31 x 10(6) M/s. Heat denaturation of the (type 2 enzyme:inhibitor) complex releases dihydrofinasteride and presumably the NADP+-adduct previously identified with the human 5alpha-reductases. The effects of finasteride were also studied in intact COS cells transiently expressing the rat types 1 and 2 5alpha-reductase. Results with whole cell assays confirm differences in mechanism of inhibition of rat types 1 and 2 5alpha-reductase by finasteride.

Effects of finasteride, a type 2 5-alpha reductase inhibitor, on fetal development in the rhesus monkey (Macaca mulatta)

In genetic male fetuses, dihydrotestosterone (DHT) plays an important role in normal prostatic and external genital differentiation. The enzyme steroid 5-alpha reductase (5 alpha R) catalyzes the conversion of testosterone (T) to DHT. The importance of 5 alpha R in sexual differentiation is evident from the study of human genetic males who congenitally lack this enzyme and consequently develop ambiguous genitalia. These individuals are specifically deficient in the type 2 isozyme, whereas the normal type 1 isozyme activity has been found. The purpose of this study was to determine 1) the suitability of the rhesus monkey for testing the safety of 5 alpha R inhibitors when administered during pregnancy and 2) the potential risk of administering a known type 2 5 alpha R inhibitor, finasteride, during the critical period of internal and external genital differentiation in rhesus monkeys. In vitro studies were also performed on selected rhesus monkey tissues to determine the distribution of the 5 alpha R isozymes. Gravid monkeys were treated once daily from gestational days (GD) 20 to 100. Sonographic monitoring was performed during the course of gestation to monitor viability, growth, and organ system development. Detailed fetal evaluations for developmental abnormalities were performed at term (GD 152 +/- 2). A group of 13 pregnant monkeys ("positive control") were given a high oral dose (2 mg/kg/day) of finasteride to demonstrate that inhibiting type 2 5 alpha R results in specific external genital abnormalities in male fetuses. Thirty-two pregnant monkeys were administered an intravenous (i.v.) formulation of finasteride at doses of 8, 80, or 800 ng/day. The highest i.v. dose selected was at least 60-750 times the semen levels of finasteride in man given orally 5 or 1 mg/day, respectively. Seventeen vehicle-control pregnant monkeys were also included. Administration of a high oral dose (2 mg/kg/day) of finasteride resulted in external genital abnormalities characterized by hypospadias, preputial adhesions to the glans, a small underdeveloped scrotum, a small penis, and a prominent midline raphe in male fetuses; however, no developmental abnormalities were seen in female fetuses. Similarly, no abnormalities were observed in either male or female fetuses of mothers given iv doses (8, 80, or 800 ng/day) of finasteride during pregnancy. The in utero sonographic findings in fetuses correlated with the gross findings at term. These studies have shown that external genital abnormalities can be produced in male monkey fetuses when exposed to a high oral dose (2 mg/kg/day) of finasteride, whereas no abnormalities were observed in fetuses exposed to the i.v. formulation of finasteride. Detailed in vitro studies demonstrated that the rhesus monkey also has two 5 alpha R isozymes (types 1 and 2) with a tissue distribution similar to that seen in man and, furthermore, that finasteride is a potent, mechanism-based inhibitor with selectivity for both human and rhesus type 2 5 alpha R. These studies have demonstrated that the monkey is a suitable model for assessing the safety of 5 alpha R inhibitors administered during pregnancy.

MK386: a potent, selective inhibitor of the human type 1 5alpha-reductase

Steroid 5alpha-reductase is required for the conversion of testosterone to dihydrotestosterone. Localization of type 1 5alpha-reductase in the sebaceous gland of skin offers the possibility for selective inhibition of this isozyme as a treatment for acne. The goals of these studies are to demonstrate the mechanism of inhibition of MK386 and its selectivity for type 1 5alpha-reductase. The apparent potency of MK386 differed depending on the source of the enzyme (i.e. recombinant vs. native), yet selectivity for type 1 5alpha-reductase was unchanged. Our results indicate that the apparent potency of MK386 is modulated by the membrane concentration of the assay. These results suggest that MK386 has a high affinity for the lipid-rich membrane environment of 5alpha-reductase. MK386 was also found to be a slow binding inhibitor of type 1 5alpha-reductase. However, the cause of this time-dependent inhibition is unrelated to partitioning of the inhibitor into the membrane because similar studies with type 2 5alpha-reductase indicate that MK386 is a reversible, competitive inhibitor. A number of counterscreens were developed to demonstrate that MK386 is a poor inhibitor of other steroid metabolizing enzymes.

In vivo time-dependent inhibition of human steroid 5 alpha-reductase by finasteride

Finasteride (17 beta-(N-t-butylcarbamoyl)-4-aza-5 alpha-androstan-1-en-3- one), a time-dependent, irreversible inhibitor of human steroid 5 alpha-reductase (5AR), may only reduce dihydrotestosterone levels in humans by approximately 60% at the doses used clinically. A theoretical model was developed to aid in understanding the in vivo efficacy data of finasteride. According to the theory, whether an enzyme can be inhibited in vivo by an irreversible inhibitor is dependent on the value of a ratio of the observed rate of enzyme inhibition over the rate constant for inhibitor elimination. As shown, this ratio should be in excess of 3 for > 95% inhibition of the target in vivo. Subsequent application of the theory to evaluate the in vivo efficacy data of finasteride indicates low effective concentration of finasteride at the inhibition sites and suggests complete inhibition of 5AR 2, but insufficient suppression of 5AR 1 at the clinical doses.

Kinetic analysis of steroid 5alpha-reductase activity at neutral pH in benign prostatic hyperplastic tissue: evidence for type I isozyme activity in the human prostate

In human benign prostatic hyperplastic (BPH) tissue homogenates 5alpha-reduction of testosterone was examined at neutral pH. As Lineweaver-Burk and Eadie-Scatchard plots of estimated initial velocities against a wide range of substrate concentrations of 2 nM to 3.2 microM were non-linear, the existence of two 5alpha-reductase isozymes in this tissue was surmised. Indeed, enzyme parameters at pH 7.0 suggested the presence of two isozymes with affinity constants of 1995 and 11.8 nM, characteristic of the well established human steroid 5alpha-reductase isozymes type I and II, respectively. The physiological roles of these isozyme activities remain puzzling. The specific activities, Vmax, of these subtypes indicated an approx. 6-fold higher maximum velocity of type I than of type II 5alpha-reductase in the human hyperplastic prostate at this pH. In contrast, the efficiency ratios, Vmax/Km, demonstrated that the type II isozyme had a nearly 27 times higher potential in vivo activity than the type I isozyme, and is therefore most probably quantitatively responsible for dihydrotestosterone formation at physiological testosterone levels in this tissue at neutral pH. This is the first full paper on type I 5alpha-reductase activity in human BPH tissue.

Rat steroid 5 alpha-reductase kinetic characteristics: extreme pH-dependency of the type II isozyme in prostate and epididymis homogenates

Reevaluating the assay for rat steroid 5 alpha-reductase isozymes in prostate and epididymis homogenates we encountered an extreme pH-dependency of the type II isozyme. The time-course of the metabolism of testosterone (T) to 17 beta-hydroxy-5 alpha-androstan-3-one (DHT) at acidic pH shows an initial burst when the homogenate is not brought to pH before the start of the incubation. Therefore, the rat type II 5 alpha-reductase isozyme does not follow Michaelian law under these conditions making a single time point measurement invalid. Assessing the pH-optimum of 5 alpha-reduction in both rat prostate and epididymis homogenates we found a strong substrate dependency: at high substrate concentrations a pH-optimum for the type II isozyme of pH 5.0 was found, whereas at lower concentrations pH 5.5 is optimal. Establishing Vmax (maximum velocities) and Km (affinity constants) for the 5 alpha-reduction of T at pH 4.5-8.0, the efficiency optimum Vmax/Km appeared to be pH 5.5 in both prostate and epididymis homogenates. Specifically at acidic pH these kinetic characteristics of the type II isozyme vary many-fold. Discrepancies in literature concerning 5 alpha-reductase characteristics can, at least in part, be attributed to the choice of optimal pH, or to pH shifts during the assay.

5 alpha-Reductase type 1 is localized to the outer nuclear membrane

The subcellular distribution of the two isozymes of 5 alpha-reductase has been controversial. To resolve this issue which could provide clues about the respective functions of the two isozymes, two antisera were generated, one which was specific for the Type 1 5 alpha-reductase and one which recognized both isozymes. In COS cells transfected separately with the Type 1 or Type 2 cDNA, both isozymes were detected on Western blots at an M(r) of 26,000. Subfractionation of the COS cells resulted in the partitioning of both isozymes between the crude nuclear and cytosolic fractions, while cytoimmunofluorescence localized both reductases to the nuclear periphery. In rat liver homogenate, the 5 alpha-reductase was also detected at M(r) 26,000. The 5 alpha-reductase immunoreactivity was increased after castration of the animals with no further effect when castrated animals were treated with androgens. Although the rat liver expresses only the Type 1 5 alpha-reductase, the 5 alpha-reductase was distributed about equally between crude nuclear and cytosolic subfractions; this distribution could be shifted to the cytosolic fractions with harsher homogenization procedures. Further extensive subfractionation and extraction studies identified the rat liver Type 1 5 alpha-reductase as an integral membrane protein present in the outer nuclear membrane of the nuclear envelope and in rough endoplasmic reticulum. Thus, the subfractionation and cytoimmunofluorescence studies are consistent with the localization of the Type 1 5 alpha-reductase to the outer nuclear membrane of the nuclear envelope which is continuous with and indistinguishable from the endoplasmic reticulum. This study is the first to localize rat liver Type 1 5 alpha-reductase to the nuclear envelope to which the prostatic 5 alpha-reductase activity previously had been localized. We conclude that, contrary to previous tissue distribution studies, but consistent with investigations in transfected cells, both isozymes are similarly localized to the nuclear periphery.

Role of 5 alpha-reductase in health and disease

The mechanism of androgen action varies in different tissues, but in the majority of androgen target tissues either testosterone or 5 alpha-dihydrotestosterone (DHT) binds to a specific androgen receptor to form a complex that can regulate gene expression. Testosterone is metabolized to DHT by the enzyme 5 alpha-reductase. The autosomal recessive genetic disorder of 5 alpha-reductase deficiency has clearly shown that the requirement for DHT formation varies with different tissues. In this syndrome genetic males contain normal male internal structures including testes, but exhibit ambiguous or female external genitalia at birth; at puberty they undergo partial virilization which includes development of a male gender identity even if brought up as females. Their development suggests that testosterone itself is able to stimulate psychosexual behaviour, development of the embryonic wolffian duct, muscle development, voice deepening, spermatogenesis, and axillary and pubic hair growth; DHT seems to be essential for prostate development and growth, the development of the external genitalia and male patterns of facial and body hair growth or male-pattern baldness. How different hormones operate to regulate genes via the same receptor is currently unknown, but appears to involve cell-specific factors. The 5-alpha-reductase enzyme has proved difficult to isolate biochemically, but recently at least two human isoenzymes have been identified using molecular biological methods. All the various 5 alpha-reductase-deficient kindreds have been shown to have mutations in 5 alpha-reductase 2, the predominant form in the prostate. The biological role of 5 alpha-reductase 1 has not yet been ascertained, but at present it cannot be ruled out that some of the actions ascribed to testosterone are indeed in cells producing DHT via this enzyme. The activity of 5 alpha-reductase is also implicated in benign prostatic hypertrophy, hirsutism and possibly male-pattern baldness; recent evidence discounts the role of 5 alpha reductase 2 in sebaceous glands and acne. Specific inhibitors of both enzymes are now available and finasteride, a 5 alpha-reductase 2 inhibitor, has been used successfully in clinical trials of benign prostatic hypertrophy. Knowledge of 5 alpha-reductase is expanding dramatically at the moment with the application of molecular biological methods. The advent of antibodies to the isoenzymes should herald further understanding of their biological and clinical roles.