Silodosin

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# Overview
Silodosin is an alpha-1 adrenergic antagonist that is FDA approved for the treatment of the signs and symptoms of benign prostatic hyperplasia (BPH).   Common adverse reactions include retrograde ejaculation, dizziness, diarrhea, orthostatic hypotension, headache, nasopharyngitis, and nasal congestion.

# Adult Indications and Dosage

## FDA-Labeled Indications and Dosage (Adult)
Silodosin is indicated for the treatment of the signs and symptoms of benign prostatic hyperplasia (BPH)
- Dosage: 8 mg orally once daily with a meal.

## Off-Label Use and Dosage (Adult)

### Guideline-Supported Use
There is limited information regarding Off-Label Guideline-Supported Use of Silodosin in adult patients.

### NonGuideline-Supported Use
There is limited information regarding Off-Label NonGuideline-Supported Use of Silodosin in adult patients.

# Pediatric Indications and Dosage

## FDA-Labeled Indications and Dosage (Pediatric)
Safety and efficacy not established in pediatric patients

## Off-Label Use and Dosage (Pediatric)

### Guideline-Supported Use
There is limited information regarding Off-Label Guideline-Supported Use of Silodosin in pediatric patients.

### NonGuideline-Supported Use
There is limited information regarding Off-Label NonGuideline-Supported Use of Silodosin in pediatric patients.

# Contraindications
- Severe renal impairment (CCr < 30 mL/min).
- Severe hepatic impairment (Child-Pugh score > 10).
- Concomitant administration with strong Cytochrome P450 3A4 (CYP3A4) inhibitors (e.g., ketoconazole, clarithromycin, itraconazole, ritonavir).
- Patients with a history of hypersensitivity to silodosin or any of the ingredients of silodosin.

# Warnings
Postural hypotension, with or without symptoms (e.g., dizziness) may develop when beginning silodosin treatment. As with other alpha-blockers, there is potential for syncope. Patients should be cautioned about driving, operating machinery, or performing hazardous tasks when initiating therapy.
Silodosin has not been tested in patients with severe hepatic impairment, and therefore, should not be prescribed to such patients.
In a drug interaction study, co-administration of a single 8 mg dose of silodosin with 400 mg ketoconazole, a strong CYP3A4 inhibitor, caused a 3.8-fold increase in maximum plasma silodosin concentrations and 3.2-fold increase in silodosin exposure (i.e., AUC). Concomitant use of ketoconazole or other strong CYP3A4 inhibitors (e.g., itraconazole, clarithromycin, ritonavir) is therefore contraindicated.
The pharmacodynamic interactions between silodosin and other alpha-blockers have not been determined. However, interactions may be expected, and silodosin should not be used in combination with other alpha-blockers.
A specific pharmacodynamic interaction study between silodosin and antihypertensive agents has not been performed. However, patients in the Phase 3 clinical studies taking concomitant antihypertensive medications with silodosin did not experience a significant increase in the incidence of syncope, dizziness, or orthostasis. Nevertheless, exercise caution during concomitant use with antihypertensives and monitor patients for possible adverse events.
Caution is also advised when alpha-adrenergic blocking agents including silodosin are co-administered with PDE5 inhibitors. Alpha-adrenergic blockers and PDE5 inhibitors are both vasodilators that can lower blood pressure. Concomitant use of these two drug classes can potentially cause symptomatic hypotension.
Carcinoma of the prostate and BPH cause many of the same symptoms. These two diseases frequently co-exist. Therefore, patients thought to have BPH should be examined prior to starting therapy with silodosin to rule out the presence of carcinoma of the prostate.
Intraoperative Floppy Iris Syndrome has been observed during cataract surgery in some patients on alpha-1 blockers or previously treated with alpha-1 blockers. This variant of small pupil syndrome is characterized by the combination of a flaccid iris that billows in response to intraoperative irrigation currents; progressive intraoperative miosis despite preoperative dilation with standard mydriatic drugs; and potential prolapse of the iris toward the phacoemulsification incisions. Patients planning cataract surgery should be told to inform their ophthalmologist that they are taking silodosin.

# Adverse Reactions

## Clinical Trials Experience
Because clinical trials are conducted under widely varying conditions, adverse reaction rates observed in the clinical trials of a drug cannot be directly compared to rates in the clinical trials of another drug and may not reflect the rates observed in clinical practice.
In U.S. clinical trials, 897 patients with BPH were exposed to 8 mg silodosin daily. This includes 486 patients exposed for 6 months and 168 patients exposed for 1 year. The population was 44 to 87 years of age, and predominantly Caucasian. Of these patients, 42.8% were 65 years of age or older and 10.7% were 75 years of age or older.
In double-blind, placebo controlled, 12-week clinical trials, 466 patients were administered silodosin and 457 patients were administered placebo. At least one treatment-emergent adverse reaction was reported by 55.2% of silodosin treated patients (36.8% for placebo treated). The majority (72.1%) of adverse reactions for the silodosin treated patients (59.8% for placebo treated) were qualified by the investigator as mild. A total of 6.4% of silodosin treated patients (2.2% for placebo treated) discontinued therapy due to an adverse reaction (treatment-emergent), the most common reaction being retrograde ejaculation (2.8%) for silodosin treated patients. Retrograde ejaculation is reversible upon discontinuation of treatment.
The incidence of treatment-emergent adverse reactions listed in the following table were derived from two 12-week, multicenter, double-blind, placebo-controlled clinical studies of silodosin 8 mg daily in BPH patients. Adverse reactions that occurred in at least 2% of patients treated with silodosin and more frequently than with placebo are shown in Table 1.
In the two 12-week, placebo-controlled clinical trials, the following adverse events were reported by between 1% and 2% of patients receiving silodosin and occurred more frequently than with placebo: insomnia, PSA increased, sinusitis, abdominal pain, asthenia, and rhinorrhea. One case of syncope in a patient taking prazosin concomitantly and one case of priapism were reported in the silodosin treatment group.
In a 9-month open-label safety study of silodosin, one case of Intraoperative Floppy Iris Syndrome (IFIS) was reported.

## Postmarketing Experience
The following adverse reactions have been identified during post approval use of silodosin. Because these reactions are reported voluntarily from a population of uncertain size, it is not always possible to reliably estimate their frequency or establish a causal relationship to drug exposure:
- Skin and subcutaneous tissue disorders: toxic skin eruption, purpura, skin rash, pruritus, and urticaria.
- Hepatobiliary disorders: jaundice, impaired hepatic function associated with increased transaminase values.
- Immune system disorders: allergic-type reactions, not limited to skin reactions including swollen tongue and pharyngeal edema resulting in serious outcomes.

# Drug Interactions
In a clinical metabolic inhibition study, a 3.8-fold increase in silodosin maximum plasma concentrations and 3.2-fold increase in silodosin exposure were observed with concurrent administration of a strong CYP3A4 inhibitor, 400 mg ketoconazole. Use of strong CYP3A4 inhibitors such as itraconazole or ritonavir may cause plasma concentrations of silodosin to increase. Concomitant administration of strong CYP3A4 inhibitors and silodosin is contraindicated.
The effect of moderate CYP3A4 inhibitors on the pharmacokinetics of silodosin has not been evaluated. Concomitant administration with moderate CYP3A4 inhibitors (e.g., diltiazem, erythromycin, verapamil) may increase concentration of silodosin. Exercise caution and monitor patients for adverse events when co-administering silodosin with moderate CYP3A4 inhibitors.
In vitro studies indicated that silodosin is a P-gp substrate. Ketoconazole, a CYP3A4 inhibitor that also inhibits P-gp, caused significant increase in exposure to silodosin. Inhibition of P-gp may lead to increased silodosin concentration. Silodosin is therefore not recommended in patients taking strong P-gp inhibitors such as cyclosporine.
The pharmacodynamic interactions between silodosin and other alpha-blockers have not been determined. However, interactions may be expected, and silodosin should not be used in combination with other alpha-blockers.
The effect of co-administration of silodosin and digoxin 0.25 mg/day for 7 days was evaluated in a clinical trial in 16 healthy males, aged 18 to 45 years. Concomitant administration of silodosin and digoxin did not significantly alter the steady state pharmacokinetics of digoxin. No dose adjustment is required.
Co-administration of silodosin with a single dose of 100 mg sildenafil or 20 mg tadalafil was evaluated in a placebo-controlled clinical study that included 24 healthy male subjects, 45 to 78 years of age. Orthostatic vital signs were monitored in the 12-hour period following concomitant dosing. During this period, the total number of positive orthostatic test results was greater in the group receiving silodosin plus a PDE5 inhibitor compared with silodosin alone. No events of symptomatic orthostasis or dizziness were reported in subjects receiving silodosin with a PDE5 inhibitor.
The pharmacodynamic interactions between silodosin and antihypertensives have not been rigorously investigated in a clinical study. However, approximately one-third of the patients in clinical studies used concomitant antihypertensive medications with silodosin. The incidence of dizziness and orthostatic hypotension in these patients was higher than in the general silodosin population (4.6% versus 3.8% and 3.4% versus 3.2%, respectively). Exercise caution during concomitant use with antihypertensives and monitor patients for possible adverse events.
In vitro data indicate that silodosin does not have the potential to inhibit or induce cytochrome P450 enzyme systems.
The effect of a moderate fat, moderate calorie meal on silodosin pharmacokinetics was variable and decreased silodosin maximum plasma concentration (Cmax) by approximately 18 to 43% and exposure (AUC) by 4 to 49% across three different studies. Safety and efficacy clinical trials for silodosin were always conducted in the presence of food intake. Patients should be instructed to take silodosin with a meal to reduce risk of adverse events.

# Use in Specific Populations

### Pregnancy
Pregnancy Category (FDA): B
An embryo/fetal study in rabbits showed decreased maternal body weight at 200 mg/kg/day (approximately 13 to 25 times the maximum recommended human exposure or MRHE of silodosin via AUC). No statistically significant teratogenicity was observed at this dose.
Silodosin was not teratogenic when administered to pregnant rats during organogenesis at 1000 mg/kg/day (estimated to be approximately 20 times the MRHE). No maternal or fetal effects were observed at this dose. Rats and rabbits do not produce glucuronidated silodosin, which is present in human serum at approximately 4 times the level of circulating silodosin and which has similar pharmacological activity to silodosin.
No effects on physical or behavioral development of offspring were observed when rats were treated during pregnancy and lactation at up to 300 mg/kg/day.
Pregnancy Category (AUS): 
There is no Australian Drug Evaluation Committee (ADEC) guidance on usage of Silodosin in women who are pregnant.

### Labor and Delivery
There is no FDA guidance on use of Silodosin during labor and delivery.

### Nursing Mothers
There is no FDA guidance on the use of Silodosin in women who are nursing.

### Pediatric Use
Silodosin is not indicated for use in pediatric patients. Safety and effectiveness in pediatric patients have not been established.

### Geriatic Use
In double-blind, placebo-controlled, 12-week clinical studies of silodosin, 259 (55.6%) were under 65 years of age, 207 (44.4%) patients were 65 years of age and over, while 60 (12.9%) patients were 75 years of age and over. Orthostatic hypotension was reported in 2.3% of silodosin patients  65 years of age (1.9% for placebo), and 5.0% of patients > 75 years of age (0% for placebo). There were otherwise no significant differences in safety or effectiveness between older and younger patients.

### Gender
There is no FDA guidance on the use of Silodosin with respect to specific gender populations.

### Race
No clinical studies specifically investigating the effects of race have been performed.

### Renal Impairment
The effect of renal impairment on silodosin pharmacokinetics was evaluated in a single dose study of six male patients with moderate renal impairment and seven male subjects with normal renal function. Plasma concentrations of silodosin were approximately three times higher in subjects with moderate renal impairment compared with subjects with normal renal function.
Silodosin should be reduced to 4 mg per day in patients with moderate renal impairment. Exercise caution and monitor patients for adverse events.
Silodosin has not been studied in patients with severe renal impairment. silodosin is contraindicated in patients with severe renal impairment.

### Hepatic Impairment
In a study comparing nine male patients with moderate hepatic impairment (Child-Pugh scores 7 to 9), to nine healthy male subjects, the single dose pharmacokinetics of silodosin were not significantly altered in patients with hepatic impairment. No dosing adjustment is required in patients with mild or moderate hepatic impairment.
Silodosin has not been studied in patients with severe hepatic impairment. Silodosin is contraindicated in patients with severe hepatic impairment.

### Females of Reproductive Potential and Males
Treatment of male rats with silodosin for 15 days resulted in decreased fertility at the high dose of 20 mg/kg/day (about twice the MRHE) which was reversible following a two week recovery period. No effect was observed at 6 mg/kg/day. The clinical relevance of this finding is not known.
In a fertility study in female rats, the high dose of 20 mg/kg/day (about 1 to 4 times the MRHE) resulted in estrus cycle changes, but no effect on fertility. No effect on the estrus cycle was observed at 6 mg/kg/day.
In a male rat fertility study, sperm viability and count were significantly lower after administration of 600 mg/kg/day (about 65 times the MRHE) for one month. Histopathological examination of infertile males revealed changes in the testes and epididymides at 200 mg/kg/day (about 30 times the MRHE).

### Immunocompromised Patients
There is no FDA guidance one the use of Silodosin in patients who are immunocompromised.

# Administration and Monitoring

### Administration
Oral

### Monitoring
There is limited information regarding Silodosin Monitoring in the drug label.

# IV Compatibility
There is limited information regarding the compatibility of Silodosin and IV administrations.

# Overdosage
Silodosin was evaluated at doses of up to 48 mg/day in healthy male subjects. The dose-limiting adverse event was postural hypotension.
Should overdose of silodosin lead to hypotension, support of the cardiovascular system is of first importance. Restoration of blood pressure and normalization of heart rate may be accomplished by maintaining the patient in the supine position. If this measure is inadequate, administration of intravenous fluid should be considered. If necessary, vasopressors could be used, and renal function should be monitored and supported as needed. Dialysis is unlikely to be of significant benefit since silodosin is highly (97%) protein bound.

# Pharmacology

## Mechanism of Action
Silodosin is a selective antagonist of post-synaptic alpha-1 adrenoreceptors, which are located in the human prostate, bladder base, bladder neck, prostatic capsule, and prostatic urethra. Blockade of these alpha-1 adrenoreceptors can cause smooth muscle in these tissues to relax, resulting in an improvement in urine flow and a reduction in BPH symptoms.
An in vitro study examining binding affinity of silodosin to the three subtypes of the alpha-1 adrenoreceptors (alpha-1A, alpha-1B, and alpha-1D) was conducted. The results of the study demonstrated that silodosin binds with high affinity to the alpha-1A subtype.

## Structure
The molecular formula is C25H32F3N3O4 with a molecular weight of 495.53. The structural formula of silodosin is:

## Pharmacodynamics
A test for postural hypotension was conducted 2 to 6 hours after the first dose in the two 12-week, double-blind, placebo-controlled clinical studies. After the patient had been at rest in a supine position for 5 minutes, the patient was asked to stand. Blood pressure and heart rate were assessed at 1 minute and 3 minutes after standing. A positive result was defined as a > 30 mmHg decrease in systolic blood pressure, or a > 20 mmHg decrease in diastolic blood pressure, or a > 20 bpm increase in heart rate.
The effect of silodosin on QT interval was evaluated in a double-blind, randomized, active- (moxifloxacin) and placebo-controlled, parallel-group study in 189 healthy male subjects aged 18 to 45 years. Subjects received either silodosin 8 mg, silodosin 24 mg, or placebo once daily for five days, or a single dose of moxifloxacin 400 mg on Day 5 only. The 24 mg dose of silodosin was selected to achieve blood levels of silodosin that may be seen in a worst-case scenario exposure (i.e., in the setting of concomitant renal disease or use of strong CYP3A4 inhibitors). QT interval was measured during a 24-hour period following dosing on Day 5 (at silodosin steady state).
Silodosin was not associated with an increase in individual corrected (QTcI) QT interval at any time during steady state measurement, while moxifloxacin, the active control, was associated with a maximum 9.59 msec increase in QTcI.
There has been no signal of Torsade de Pointes in the post-marketing experience with silodosin outside the United States.

## Pharmacokinetics
The pharmacokinetics of silodosin have been evaluated in adult male subjects with doses ranging from 0.1 mg to 24 mg per day. The pharmacokinetics of silodosin are linear throughout this dosage range.
The pharmacokinetic characteristics of silodosin 8 mg once daily were determined in a multi-dose, open-label, 7-day pharmacokinetic study completed in 19 healthy, target-aged (> 45 years of age) male subjects. Table 3 presents the steady state pharmacokinetics of this study.
The maximum effect of food (i.e., co-administration with a high fat, high calorie meal) on the PK of silodosin was not evaluated. The effect of a moderate fat, moderate calorie meal was variable and decreased silodosin Cmax by approximately 18 to 43% and AUC by 4 to 49% across three different studies.
In a single-center, open-label, single-dose, randomized, two-period crossover study in twenty healthy male subjects age 21 to 43 years under fed conditions, a study was conducted to evaluate the relative bioavailability of the contents of an 8 mg capsule (size #1) of silodosin sprinkled on applesauce compared to the product administered as an intact capsule.  Based on AUC0-24 and Cmax, silodosin administered by sprinkling the contents of a silodosin capsule onto a tablespoonful of applesauce was found to be bioequivalent to administering the capsule whole.
Silodosin has an apparent volume of distribution of 49.5 L and is approximately 97% protein bound.
Silodosin undergoes extensive metabolism through glucuronidation, alcohol and aldehyde dehydrogenase, and cytochrome P450 3A4 (CYP3A4) pathways. The main metabolite of silodosin is a glucuronide conjugate (KMD-3213G) that is formed via direct conjugation of silodosin by UDP-glucuronosyltransferase 2B7 (UGT2B7). Co-administration with inhibitors of UGT2B7 (e.g., probenecid, valproic acid, fluconazole) may potentially increase exposure to silodosin. KMD-3213G, which has been shown in vitro to be active, has an extended half-life (approximately 24 hours) and reaches plasma exposure (AUC) approximately four times greater than that of silodosin. The second major metabolite (KMD-3293) is formed via alcohol and aldehyde dehydrogenases and reaches plasma exposures similar to that of silodosin. KMD-3293 is not expected to contribute significantly to the overall pharmacologic activity of silodosin.
Following oral administration of 14C-labeled silodosin, the recovery of radioactivity after 10 days was approximately 33.5% in urine and 54.9% in feces. After intravenous administration, the plasma clearance of silodosin was approximately 10 L/hour.

## Nonclinical Toxicology
In a 2-year oral carcinogenicity study in rats administered doses up to 150 mg/kg/day , an increase in thyroid follicular cell tumor incidence was seen in male rats receiving doses of 150 mg/kg/day. Silodosin induced stimulation of thyroid stimulating hormone (TSH) secretion in the male rat as a result of increased metabolism and decreased circulating levels of thyroxine (T4). These changes are believed to produce specific morphological and functional changes in the rat thyroid including hypertrophy, hyperplasia, and neoplasia. Silodosin did not alter TSH or T4 levels in clinical trials and no effects based on thyroid examinations were noted. The relevance to human risk of these thyroid tumors in rats is not known.
In a 2-year oral carcinogenicity study in mice administered doses up to 100 mg/kg/day in males (about nine times the MRHE based on AUC of silodosin) and 400 mg/kg/day in females (about 72 times the MRHE based on AUC), there were no significant tumor findings in male mice. Female mice treated for 2 years with doses of 150 mg/kg/day (about 29 times the MRHE based on AUC) or greater had statistically significant increases in the incidence of mammary gland adenoacanthomas and adenocarcinomas. The increased incidence of mammary gland neoplasms in female mice was considered secondary to silodosin-induced hyperprolactinemia measured in the treated mice. Elevated prolactin levels were not observed in clinical trials. The relevance to human risk of prolactin-mediated endocrine tumors in mice is not known. Rats and mice do not produce glucuronidated silodosin, which is present in human serum at approximately four times the level of circulating silodosin and which has similar pharmacological activity to silodosin.
Silodosin produced no evidence of mutagenic or genotoxic potential in the in vitro Ames assay, mouse lymphoma assay, unscheduled DNA synthesis assay and the in vivo mouse micronucleus assay. A weakly positive response was obtained in two in vitro Chinese Hamster Lung (CHL) tests for chromosomal aberration assays at high, cytotoxic concentrations.

# Clinical Studies
Two 12-week, randomized, double-blind, placebo-controlled, multicenter studies were conducted with 8 mg daily of silodosin. In these two studies, 923 patients were randomized and 466 patients received silodosin 8 mg daily. The two studies were identical in design except for the inclusion of pharmacokinetic sampling in Study 1. The primary efficacy assessment was the International Prostate Symptom Score (IPSS) which evaluated irritative (frequency, urgency, and nocturia), and obstructive (hesitancy, incomplete emptying, intermittency, and weak stream) symptoms. Maximum urine flow rate (Qmax) was a secondary efficacy measure.
Mean changes from baseline to last assessment (Week 12) in total IPSS score were statistically significantly greater for groups treated with silodosin than those treated with placebo in both studies (Table 4 and Figures 2 and 3).
Mean IPSS total score for silodosin once daily groups showed a decrease starting at the first scheduled observation and remained decreased through the 12 weeks of treatment in both studies.
Silodosin produced statistically significant increases in maximum urinary flow rates from baseline to last assessment (Week 12) versus placebo in both studies (Table 5 and Figures 4 and 5). Mean peak flow rate increased starting at the first scheduled observation at Day 1 and remained greater than the baseline flow rate through the 12 weeks of treatment for both studies.

# How Supplied
Silodosin capsules 8 mg
- Bottle of 30 capsules (NDC 52544-152-30)
- Bottle of 90 capsules (NDC 52544-152-19)
Silodosin capsules 4 mg
- Bottle of 30 capsules (NDC 52544-151-30)
- Bottle of 90 capsules (NDC 52544-151-19)

## Storage
Store at 25C (77F)

# Images

## Drug Images

## Package and Label Display Panel

# Patient Counseling Information
- Patients should be instructed to take silodosin once daily with a meal.
- Patients should be instructed about the possible occurrence of symptoms related to postural hypotension (such as dizziness), and should be cautioned about driving, operating machinery, or performing hazardous tasks until they know how silodosin will affect them. This is especially important for those with low blood pressure or who are taking antihypertensive medications.
- The most common side effect seen with silodosin is an orgasm with reduced or no semen. This side effect does not pose a safety concern and is reversible with discontinuation of the product.
- The patient should be instructed to tell his ophthalmologist about the use of silodosin before cataract surgery or other procedures involving the eyes, even if the patient is no longer taking silodosin.

# Precautions with Alcohol
Alcohol-Silodosin interaction has not been established. Talk to your doctor about the effects of taking alcohol with this medication.

# Brand Names
- Rapaflo 

# Look-Alike Drug Names
There is limited information regarding Silodosin Look-Alike Drug Names in the drug label.

# Drug Shortage Status

# Price