Terazosin
Finasteride (Propecia) Interactions
NOTE: Finasteride is metabolized by hepatic CYP3A4 isoenzymes. The clinical significance of theoretical interactions with CYP3A4 inhibitors is unknown. In general, finasteride has not been associated with clinically important drug interactions during clinical trials, including a 4-year safety trial of 3040 males aged 45 - 78 years.
No drug-drug interactions were demonstrated when finasteride was used with antipyrine, digoxin, propranolol, theophylline, or warfarin. Although no specific drug interaction studies were conducted, finasteride was administered with alpha-blockers, ACE inhibitors, analgesics, anticonvulsants, benzodiazepines, beta-blockers, calcium channel blockers, diuretics, H2-blockers, HMG-CoA-reductase inhibitors, nitrates, NSAIDs, and quinolone antibiotics without significant interactions.
Finasteride is a synthetic analog of testosterone and inhibits an intracellular enzyme responsible for converting testosterone to the potent androgen 5-alpha-dihydrotestosterone (DHT). Since finasteride appears to interfere with the activation of testosterone to a more active form, thereby blocking the clinical effects of testosterone, it would be illogical to administer finasteride concurrently with testosterone or other androgens.
Drug interactions with Saw palmetto, Serenoa repens have not been specifically studied or reported. Saw palmetto may inhibit 5 alpha-reductase to prevent the conversion of testosterone to dihydrotestosterone. This action is similar to the action of 5-alpha reductase inhibitors (i.e., dutasteride, finasteride). Co-use of these agents is likely to be common by patients, but the effects of co-use are not known. In theory, the effects could be additive, but it is not known if the added effects would be beneficial or harmful. Clinicians should be alert for any unusual effects if patients ingest saw palmetto supplements while taking 5-alpha reductase inhibitors.
Terazosin has been reported to increase peak concentrations of finasteride by 16% and AUC by 31% when the two agents are coadministered. The significance of this effect is unknown, but it is probably of minor importance. Evidently, doxazosin does not interact.
In vitro, both genistein and daidzein inhibit 5 alpha-reductase isoenzyme II, resulting in decreased conversion of testosterone to the potent androgen 5-alpha-dihydrotestosterone (DHT) and a subsequent reduction in testosterone-dependent tissue proliferation. The action is similar to that of finasteride, but is thought to be less potent. Theoretically, because the soy isoflavones appear to inhibit type II 5-alpha-reductase, the soy isoflavones may have additive effects with other 5-alpha reductase inhibitors (i.e., dutasteride, finasteride).
[ Last revised: 3/2/2005 7:51:00 PM ]
References
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. Propecia® (finasteride) package insert. Whitehouse Station, NJ: Merck & Co., INC.; 2003 Oct.
. Marks LS, Hess DL, Dorey FJ, et al. Tissue effects of saw palmetto and finasteride: use of biopsy cores for in situ quantification of prostatic androgens. Urology 2001;57:999 - 1005.
. Vashi V, Chung M, Hilbert J, et al. Pharmacokinetic interaction between finasteride and terazosin, but not finasteride and doxazosin. J Clin Pharmacol 1998;38:1072 - 6.
. Samara EE, Hosmane B, Locke C, et al. Assessment of the pharmacokinetic-pharmacodynamic interaction between terazosin and finasteride. J Clin Pharmacol 1996;36:1169 - 78.
. Proscar® (finasteride) package insert. Whitehouse Station, NJ: Merck & Co., Inc.; 2004 Apr.
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