The nicotinamide adenine dinucleotide phosphate (NADPH)-dependent membrane protein 5α-reductase irreversibly catalyses the conversion of testosterone to the most potent androgen, 5α-dihydrotestosterone (DHT). In humans, two 5α-reductase isoenyzmes are expressed: type I and type II. Type II is found primarily in prostate tissue. Saw palmetto extract (SPE) has been widely used for the treatment of lower urinary tract symptoms secondary to benign prostatic hyperplasia (BPH). The mechanisms of the pharmacological effects of SPE include the inhibition of 5α-reductase, among other actions. Clinical studies of SPE have been equivocal, with some showing significant results and others not. These inconsistent results may be due, in part, to varying bioactivities of the SPE used in the studies.
The aim of the present study was to determine the in vitro potency of a novel saw palmetto ethanol extract (SPET-085), an inhibitor of the 5α-reductase isoenzyme type II, in a cell-free test system. On the basis of the enzymatic conversion of the substrate androstenedione to the 5α-reduced product 5α-androstanedione, the inhibitory potency was measured and compared to those of finasteride, an approved 5α-reductase inhibitor.
SPET-085 concentration-dependently inhibited 5α-reductase type II in vitro (IC50=2.88±0.45 μg/mL). The approved 5α-reductase inhibitor, finasteride, tested as positive control, led to 61% inhibition of 5α-reductase type II.
SPET-085 effectively inhibits the enzyme that has been linked to BPH, and the amount of extract required for activity is very low compared to data reported for other extracts. It can be concluded from data in the literature that SPET-085 is as effective as a hexane extract of saw palmetto that exhibited the highest levels of bioactivity, and is more effective than other SPEs tested. This study confirmed that SPET-085 has prostate health-promoting bioactivity that also corresponds favorably to that reported for the established prescription drug standard of therapy, finasteride.