Steroid 5a-reductase inhibitors

Our second example of drugs that function as mechanism-based inactivators is the steroid 5a-reductase inhibitors finasteride and dutasteride. The mechanism of inactivation by these compounds is an interesting departure from the typical target enzyme covalent modification seen with most mechanism-based inactivators. 

Males with deficiency of the 5a-reductase isoenzyme do not develop acne, male pattern baldness, or enlarged prostates.274 The last fact was some of the impetus for development of the steroid 5a-reductase inhibitor finasteride, which is widely used to treat benign prostate enlargement 274/277/278 It is an enzyme-activated inhibitor in which the NADH reduces the C= C bond in the A ring, which is not in the same position as in the substrate. The resulting anion cannot become protonated but instead adds to the NAD+ as shown in Eq. 22-15. 

We saw in Chapter 3 that bisubstrate reactions can conform to a number of different reaction mechanisms. We saw further that the apparent value of a substrate Km (KT) can vary with the degree of saturation of the other substrate of the reaction, in different ways depending on the mechanistic details. Hence the determination of balanced conditions for screening of an enzyme that catalyzes a bisubstrate reaction will require a prior knowledge of reaction mechanism. This places a necessary, but often overlooked, burden on the scientist to determine the reaction mechanism of the enzyme before finalizing assay conditions for HTS purposes. The importance of this mechanistic information cannot be overstated. We have already seen, in the examples of methotrexate inhibition of dihydrofolate, mycophenolic acid inhibiton of IMP dehydrogenase, and epristeride inhibition of steroid 5a-reductase (Chapter 3), how the [5]/A p ratio can influence one s ability to identify uncompetitive inhibitors of bisubstrate reactions. We have also seen that our ability to discover uncompetitive inhibitors of such reactions must be balanced with our ability to discover competitive inhibitors as well. 

Evidence that BPH could be hormone related came from studies of a population of pseudohermaphrodites in the Dominican Republic. These individuals are genetically male, but do not display normal male genitalia until the onset of puberty. They are therefore raised as females until puberty. Studies revealed that these pseudohermaphrodites are deficient in an isoform of the enzyme steroid 5a-reductase, which is responsible for catalyzing the conversion of testosterone to dihydrotestosterone (DHT). In addition to the overt sexual manifestations of this condition, affected individuals show no incident of male pattern baldness, mild or no acne, and underdevelopment of the prostate. These observations led researchers to postulate that a selective inhibitor of steroid 5a-reductase would be an effective treatment for BPH. 

Finasteride (Figure 8.18A) was designed as a mimic of the substrate testosterone. Preliminary studies suggested that the compound was a slow, tight binding reversible inhibitor of steroid 5a-reductase. A combination of detailed kinetic and... 

After a decrease in the importance of steroids in the field of drug research during the last 20 years, a renewal is now being observed. Recently, some fluorinated and fluoroalkylated steroids have been launched or are in advanced phases of clinical development, such as the dutasteride (5a-reductase inhibitor), CCD-3693 (GABA agonist), fulvestrant and antiprogestine (antihormone), and fluasterone (diabetes) (Figure 4.14). Details on the synthesis on these compounds can be found in Chapter 8. 

Steroid synthesis inhibitors 5a reductase inhibitors Testosterone receptor inhibitors... 

Enzyme kinetic studies of inhibitor are very important for considering as a therapeutic agent. It is interesting to note that isoprenoid-substituted flavonoids having non-steroidal structures are potent un-competitive inhibitors of 5a-reductase. So, it would be expected that the isoprenoid-substituted flavonoid derivative would be an interesting lead compounds for testosterone 5a-reductase inhibitor. 

Avodart1M Dutasteride 0.5 mg SEC Benign prostatic hyperplasia A selective inhibitor of both the type 1 and type 2 isoform of steroid 5a-reductase Mono-di-glycerides of caprylic/ capric acid, butylated hydroxytoluene Glaxosmithkline (GSK)... 

Merck also patented a variety of steroidal SARMs. These are a variation of the 4-azasteroidal template of finasteride, a 5a-reductase inhibitor. Modifications at... 

FIGURE 12.1 Biosynthesis of estrogens. Abbreviations DHEA, dehydroepiandrosterone El, estrone E2, estradiol T, testosterone DHT, dihydrotestosterone. Letrozole is an aroma-tase inhibitor, Finasteride is a 5a-reductase inhibitor. Androstane-3p,17P-diol (SPO) is an estrogenic steroid and its synthesis is blocked by 5a-reductase inhibitor, but not affected by aromatase inhibitor."... 

Ishikawa, T., Glidewell-Kenney, C., and Jamerson, J. L. 2006. Aromatase-independent testosterone conversion into estrogenic steroids is inhibited by a 5a-reductase inhibitor. J. Steroid Biochem. Mol. Biol. 98 133-38. 

Finasteride is a clinically useful agent in the treatment of prostate hyperplasia and male pattern baldness. It is a potent inhibitor of human steroid-5a-reductase, the enzyme responsible for the reduction of testosterone to dihydrotestosterone (Fig. 5.19). (see Chapter 45) The inhibitory action of finasteride has been attributed both to its similarity in structure to testosterone, which allows it to bind to the enzyme and be reduced to dihydrofinasteride in place of testosterone, as well as to its ability to act as a mechanism-based inhibitor, during which it can tie up the cofactor, NADPH, by forming a covalent NADP-dihydrofinasteride adduct (Fig. 5.19) (see Chapter 45). This adduct very slowly releases dihydrofinasteride with a half-life of one month (37). 

Bull HG, Garcia-Calvo M, Andersson S, etal. Mechanism-based inhibition of human steroid 5a-reductase by finasteride enzyme-catalyzed formation of NADP-dihydrofinasteride, a potent bisubstrate analogue inhibitor. J Am Chem Soc 1996 118 2359-2365. 

On the other hand, quinolinyl nucleophiles as the quinolinylzinc derivatives presented in the following example have been prepared by the in situ transmetallation of quinolinyllithium salts realized by direct halogen-metal exchange of 6-bromo quinoline derivative. The zinc salt was allowed to react with several aryl bromides, affording clean Negishi reactions for the formation of 6-substituted derivatives, which are potent inhibitors of steroid 5a reductases of types 1 and 2 ... 

In addition, the influence of oral contraceptives as well as 5a-reductase inhibitors (common active principles in drugs used for the treatment of benign prostate hyperplasia as well as alopecia) was described. The first mentioned class of drugs leads to an increase of the T/EpiT ratio due to the suppression of EpiT elimination however, other parameters such as A/E and Adiol/Bdiol remain stable. The use of 5a-reductase inhibitors strongly interferes with steroid profile interpretations as particularly A and Adiol are significantly reduced and, thus, the formerly mentioned intra-individually stable ratios of A/E and Adiol/Bdiol are considerably modified. ... 

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