Dutasteride

Overview

Dutasteride is a synthetic 4-azasteroid compound that functions as a dual inhibitor of both type I and type II isoforms of the enzyme 5-alpha reductase, which converts testosterone to dihydrotestosterone (DHT). Developed by GlaxoSmithKline and marketed under the brand name Avodart, dutasteride was approved by the FDA in 2001 for the treatment of symptomatic benign prostatic hyperplasia (BPH) in men with an enlarged prostate.

The distinction between dutasteride and its predecessor finasteride lies primarily in their selectivity profiles. Finasteride predominantly inhibits the type II isoform of 5-alpha reductase, which is concentrated in the prostate, seminal vesicles, and hair follicles. Dutasteride, by contrast, inhibits both type I (expressed in skin, liver, and sebaceous glands) and type II isoforms, resulting in more comprehensive suppression of DHT production. While finasteride reduces serum DHT levels by approximately 70%, dutasteride achieves suppression of greater than 90%, making it the more potent of the two approved 5-alpha reductase inhibitors.

Dutasteride has an extraordinarily long terminal elimination half-life of approximately 5 weeks at steady state, a consequence of its high lipophilicity and extensive distribution into tissues. This prolonged half-life means that the drug accumulates slowly and that its pharmacological effects persist for months after discontinuation, a factor with important clinical implications for both efficacy and the management of adverse effects.

Mechanism of Action

Dutasteride exerts its therapeutic effects through competitive and specific inhibition of both type I and type II 5-alpha reductase isoenzymes. These enzymes catalyze the NADPH-dependent reduction of the delta-4,5 double bond of testosterone, converting it to the more potent androgen 5-alpha-dihydrotestosterone (DHT).

5-Alpha Reductase Isoforms

Type I 5-alpha reductase (encoded by the SRD5A1 gene) is widely expressed in skin (particularly sebaceous glands), liver, and some brain regions. It operates optimally at a neutral to alkaline pH and is responsible for a significant proportion of circulating DHT. Type II 5-alpha reductase (encoded by SRD5A2) is predominantly expressed in the prostate, seminal vesicles, epididymis, and hair follicles, functioning optimally at acidic pH. By inhibiting both isoforms, dutasteride produces more complete suppression of DHT synthesis than selective type II inhibitors.

DHT Suppression and Prostatic Effects

In the prostate, DHT is the primary androgenic stimulus driving epithelial and stromal growth. DHT binds to the androgen receptor with approximately 2-5-fold greater affinity than testosterone and has a slower dissociation rate, making it a more potent activator of androgen-responsive gene transcription. By reducing intraprostatic DHT concentrations by approximately 94-97%, dutasteride induces prostatic epithelial apoptosis and stromal involution, resulting in measurable prostate volume reduction of 20-25% over 6-12 months of therapy. This reduction in prostate size translates to improved urinary flow rates and reduced lower urinary tract symptoms.

Hormonal Consequences

The near-complete suppression of DHT by dutasteride triggers a compensatory increase in serum testosterone levels of approximately 10-20% through reduced negative feedback on the hypothalamic-pituitary-gonadal axis. Serum estradiol levels may also increase modestly due to the diversion of testosterone toward aromatization rather than 5-alpha reduction. These hormonal shifts are generally clinically insignificant in most patients but represent important physiological consequences of comprehensive 5-alpha reductase inhibition.

Research Summary

Phase III BPH Trials

Clark et al. (2004) published results from the pivotal Phase III trials evaluating dutasteride for BPH in The Journal of Urology. These were three identical two-year, randomized, double-blind, placebo-controlled trials (ARIA studies) enrolling a total of 4,325 men with moderate to severe BPH symptoms. Dutasteride 0.5 mg daily produced a 25.7% reduction in prostate volume at 24 months compared to a 0.5% increase with placebo (p<0.001). Maximum urinary flow rate improved by 2.2 mL/s with dutasteride versus 0.6 mL/s with placebo, and symptom scores improved by 4.5 points versus 2.3 points. The risk of acute urinary retention was reduced by 57% and the risk of BPH-related surgery by 48%.

REDUCE Trial (Prostate Cancer Prevention)

Andriole et al. (2010) reported results from the Reduction by Dutasteride of Prostate Cancer Events (REDUCE) trial in the New England Journal of Medicine. This landmark study randomized 8,231 men with elevated PSA levels (2.5-10 ng/mL) and a prior negative prostate biopsy to dutasteride 0.5 mg daily or placebo for 4 years, with protocol-mandated prostate biopsies at 2 and 4 years. Dutasteride reduced the relative risk of biopsy-detectable prostate cancer by 22.8% (659 cancers in the dutasteride group vs. 858 in the placebo group, p<0.001). However, the trial also noted a numerically higher incidence of Gleason score 8-10 tumors in the dutasteride group (12 vs. 1), generating significant debate about the safety implications that ultimately prevented FDA approval for this chemoprevention indication.

CombAT Study

The Combination of Avodart and Tamsulosin (CombAT) study demonstrated that combination therapy with dutasteride plus the alpha-blocker tamsulosin was superior to either monotherapy for reducing BPH progression, symptom improvement, and the risk of acute urinary retention and BPH-related surgery over 4 years of treatment.

Dosing in Published Research

Dutasteride is an FDA-approved prescription medicine, sold as Avodart. According to the FDA-approved labeling, the dose for symptomatic benign prostatic hyperplasia is one 0.5 mg capsule taken once daily, used alone or in combination with the alpha-blocker tamsulosin. A treatment period of at least 6 months is generally needed to assess response. These figures are drawn from FDA-approved prescribing information.

Safety and Side Effects

Dutasteride is an FDA-approved drug with a characterized safety profile. Its most common adverse effects are sexual: decreased libido, erectile dysfunction, ejaculation disorders, and gynecomastia, meaning breast enlargement or tenderness. Some men report that sexual side effects, and mood symptoms including depressed mood, persist after the drug is stopped, although the frequency and mechanism of persistent effects remain debated. Dutasteride lowers serum prostate-specific antigen (PSA), which must be accounted for in prostate cancer screening, and analyses of 5-alpha-reductase inhibitors have noted an association with a higher incidence of high-grade prostate cancer. Dutasteride is absorbed through the skin and is a known teratogen: it can cause abnormalities of the genitalia in a male fetus, so the capsules should not be handled by women who are or may become pregnant, and treated men should not donate blood until an appropriate interval after stopping.

Current Research Status

Dutasteride is an FDA-approved prescription medication, marketed as Avodart, approved for the treatment of symptomatic benign prostatic hyperplasia, alone or in combination with tamsulosin. Its use for scalp hair loss is off-label; the related drug finasteride is approved for that use at a specific dose, but dutasteride is not. As a prescription drug it should be used under medical supervision.

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