View source for Biology:Discovery and development of 5α-reductase inhibitors

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This article is about the discovery and development of 5α-reductase inhibitors (5-ARIs), also known as [[Chemistry:Dihydrotestosterone|dihydrotestosterone]] (DHT) blockers.

==Development of 5α-reductase inhibitors==
These are two types of 5-ARIs, categorized as [[Biology:Steroid|steroid]]al and [[Medicine:Nonsteroidal|nonsteroidal]] 5-ARIs.<ref name=":0">{{Cite journal|last1=Aggarwal|first1=Saurabh|last2=Thareja|first2=Suresh|last3=Verma|first3=Abhilasha|last4=Bhardwaj|first4=Tilak Raj|last5=Kumar|first5=Manoj|title=An overview on 5α-reductase inhibitors|journal=Steroids|volume=75|issue=2|pages=109–153|doi=10.1016/j.steroids.2009.10.005|pmid=19879888|year=2010|s2cid=44363501}}</ref>

===Steroidal 5α-reductase inhibitors===
Steroid 5α-reductase is a membrane-associated enzyme in an [[Biology:Oxidoreductase|oxidoreductase]] family and has an important role in biological actions towards steroid metabolism. If the steroid 5α-reductase is overexpressed it causes overproduction of DHT that can lead to androgenic disorders in humans.<ref name=":2">{{Cite journal|last1=Karnsomwan|first1=Wiranpat|last2=Rungrotmongkol|first2=Thanyada|last3=De-Eknamkul|first3=Wanchai|last4=Chamni|first4=Supakarn|date=2016-06-01|title=In silico structural prediction of human steroid 5α-reductase type II|journal=Medicinal Chemistry Research|language=en|volume=25|issue=6|pages=1049–1056|doi=10.1007/s00044-016-1541-y|s2cid=12096090|issn=1054-2523}}</ref>

The 5α-reductase isozymes possess a similar steroidal catalytic site.<ref name=":2" /> The only available information about the 5α-reductase isozymes is their primary sequence estimated from c-DNAs and that affects the design of the novel inhibitors. The crystal structure of the 5α-reductase isozymes is not known because the nature of the 5α-reductase enzyme is so unstable during purification. The first 5-ARIs were designed by modifying the structure of natural substrates, including the substitution of one carbon atom of the rings of the steroids by a [[Chemistry:Heteroatom|heteroatom]] such as nitrogen thereby forming [[Chemistry:Azasteroid|azasteroid]]s.<ref name=":0" /> The receptor is known to consist of two hydrogen bond donors, where the C3 and 17β-side chain of the ligands connect, as well as three hydrophobic groups distributed over the steroidal structure. The best receptor inhibitors comply with these factors.<ref>{{Cite journal|last1=Chen|first1=Grace Shiahuy|last2=Chang|first2=Chih-Shiang|last3=Kan|first3=Wai Ming|last4=Chang|first4=Chih-Long|last5=Wang|first5=K. C.|last6=Chern|first6=Ji-Wang|date=2001-11-01|title=Novel Lead Generation through Hypothetical Pharmacophore Three-Dimensional Database Searching: Discovery of Isoflavonoids as Nonsteroidal Inhibitors of Rat 5α-Reductase|journal=Journal of Medicinal Chemistry|volume=44|issue=23|pages=3759–3763|doi=10.1021/jm010433s|pmid=11689062|issn=0022-2623}}</ref> Azasteroids are a type of steroid derivatives which have nitrogen atoms replaced at various positions for one of the carbon atoms in the steroid ring system. Two 4-azasteroids, finasteride and dutasteride are marketed as 5-ARIs.<ref name=":0" /> Finasteride (Proscar or Propecia) was the first steroidal 5α-reductase inhibitor approved by the U.S. [[Organization:Food and Drug Administration|Food and Drug Administration]] (USFDA). It inhibits the function of two of the isoenzymes (type II and III).<ref name=":3">{{Cite journal|last1=Yamana|first1=Kazutoshi|last2=Labrie|first2=Fernand|last3=Luu-The|first3=Van|date=2010-08-01|title=Human type 3 5α-reductase is expressed in peripheral tissues at higher levels than types 1 and 2 and its activity is potently inhibited by finasteride and dutasteride|journal=Hormone Molecular Biology and Clinical Investigation|language=en|volume=2|issue=3|pages=293–9|doi=10.1515/hmbci.2010.035|pmid=25961201|s2cid=28841145|issn=1868-1891}}</ref> In human it decreases the prostatic DHT level by 70–90% and reduces the prostatic size.<ref name=":0" />

Dutasteride (Avodart) was the second steroidal 5α-reductase approved after finasteride. It is a competitive inhibitor of all three 5α-reductase isoenzymes<ref name=":3" /> and it inhibits types 1 and 2 better than finasteride, leading to it causing further reduction in DHT, with  >90% recuded DHT levels following 1 year of oral administration.<ref name=":0" />

[[Chemistry:Epristeride|Epristeride]] is the third marketed steroidal 5-ARI. It is a noncompetitive, specific inhibitor. It potency is not as significant as finasteride or dutasteride and thus it is only marketed in China.<ref>{{Cite book|title=Integration of Pharmaceutical Discovery and Development|last=Borchardt|display-authors=et al|first=Ronald T.|publisher=Kluwer Academic Publishers|year=2006|isbn=978-0-306-47384-5|location=Google Books|pages=398}}</ref>

===Nonsteroidal 5α-reductase inhibitors===
Various pharmaceutical and academic groups have conducted the synthesis of nonsteroidal compounds that inhibit human 5α-reductases due to the unwanted hormonal side effects of steroidal compounds. Nonsteroidal inhibitors can be categorized due to their structure. Many have been obtained from azasteroid inhibitors by taking away one or more rings from the steroid structure.

Four main categories of nonsteroidal 5-ARIs have been described:

* Benzo(c)quinolizinones
* Benzo(f)quinolonone
* Piperidones
* Carboxylic acids

Nonsteroidal inhibitors are thought to act as competitive inhibitors on the 5α-reductase isozymes, except for epristeride analogues (carboxylic acids), which are noncompetitive inhibitors.<ref name="Azz">{{cite journal|display-authors=0|year=2012|title=The 5 alpha-reductase isozyme family: a review of basic biology and their role in human diseases|journal=Adv. Urol.|volume=2012|page=530121|doi=10.1155/2012/530121|pmc=3253436|pmid=22235201|vauthors=Azzouni F, Godoy A, Li Y, Mohler J|doi-access=free}}</ref>

[[Chemistry:Bexlosteride|Bexlosteride]] falls into the category of benzo(f)quinolonones, and is probably the derivative that has come closest to being marketed. It functions as a 5-ARI1 inhibitor which inhibits testosterone stimulated [[Biology:LNCaP|LNCaP]] cell growth but without testosterone the compound shows no effect and was therefore never marketed.<ref>{{Cite book|title=Prostate Cancer: Basic Mechanisms and Therapeutic Approaches|url=https://archive.org/details/prostatecancerba00chan|url-access=limited|last=Chang|first=Chawnshang|publisher=World Scientific Publishing|year=2005|isbn=978-981-256-067-4|location=Singapore|pages=[https://archive.org/details/prostatecancerba00chan/page/n270 250]}}</ref>

==Structure–activity relationships==

===4-Azasteroids===
Many steroidal 5-ARIs have been researched but only 3 are marketed. Two of them are 4-azasteroids and will be covered here. As mentioned above, the third one, epristeride is only marketed in China and will not be covered here. The basic SAR of 4-azasteroids is shown below. For competitive inhibiting functions there are two functions considered crucial, 4-en-3-one function and a lipophilic 17β-side chain with one or more oxygen atoms. The main problems for 4-azasteroids is the rapid conversion into inactive 4,5-dihydro form, which is done by the enzyme.<ref name=":0" />

[[File:Basic SAR of 4-azasteroids.png|alt=Basic SAR of 4-azasteroids|center|thumb|350x350px|Basic SAR of 4-azasteroids]]

Finasteride is considered similar to the transition state of reduced testosterone and is thus a slow-offset, irreversible inhibitor. The similarity to the transition state is a formation of an enzyme-NADP-dihydrofinasteride adduct by rearrangement on the A-ring of the compound.<ref name=":4">{{Cite journal|last=Kulig, Katarzyna.|first=Malawska, Barbara|date=2006|title=Trends in the Development of New Drugs for Treatment of Benign Hyperplasia|journal=Current Medicinal Chemistry|volume=13|issue=28|pages=3395–2416|doi=10.2174/092986706779010315|pmid=17168713}}</ref>

[[File:Finasteride.svg|alt=Finasteride|thumb|Structure of [[Chemistry:Finasteride|finasteride]]]]

Finasteride mainly inhibits the 5α-R2 (IC50=69 nM) and 5α-R3 (IC50=17.4 nM) with little inhibition of 5α-R1 (IC50=360 nM). As mentioned above, finasteride reduces prostatic DHT levels on a 70-90% range but the detailed reduction of DHT is 70.8% and 85% of intraprostatic DHT.<ref name="Azz" />

Dutasteride, however, is a so-called dual inhibitor with both 5α-R1 and 5α-R2 inhibition. IC50 for 5α-R1 is 7 nM but 6 nM for 5α-R2. As mentioned above, it reduces DHT > 90% overall, or precisely 94.7% and for intraprostatic DHT the reduction is 97-99%. Dutasteride has also been found to inhibit 5α-R3, in vitro, with IC50=0.33 nM.<ref name="Azz" /> The 2,5-difluorophenyl side chain on the D-ring of the compound shows significant lipophilic features and as increased [[Chemistry:Lipophilicity|lipophilicity]] enhances the potency of the compounds binding at pocket site, its potency is much greater than of finasteride.<ref name=":4" />

[[File:4-MA steroid.svg|thumb|Structure of 4-MA]]

Finasteride is an unsaturated analogue of another 4-azasteroid, or 4-MA. 4-MA is known to have dual inhibiting features with good inhibition on 5α-R1 (IC50=1.7 nM) and 5α-R2 (IC50=1.9 nM).<ref name="Azz" /> However, 4-MA was never marketed as it showed [[Medicine:Hepatotoxicity|hepatotoxicity]]. There is no detailed data about the cause of hepatotoxicity in 4-MA regarding SAR, but a conclusion may be drawn that the R2 group is the cause as there are other 4-azasteroid compounds containing the same R1 group as 4-MA, or CH3, without showing hepatotoxicity.<ref name=":0" />

===Nonsteroidal===
The common factor in nonsteroidal 5-ARI discovery is that the first compounds were all selective inhibitors to 5α-reductase type 1 only, but were then developed in order to get dual inhibition on both type 1 and 2, since inhibition of the type 2 isozyme is a more important factor in treating the disease of BPH.<ref name="Azz" /><ref name=":4" />

Benzo(c)quinolizinones are tricyclic derivatives of 10-azasteroids. The D-ring has been removed and the C-ring substituted for an aromatic one.<ref name="Azz" /> The first compounds developed were selective 5-alpha reductase type 1 inhibitors, but the most potent one inhibits both type 1 and 2. The fluorine atom is an important part of the structure.<ref name=":4" />

Benzo(f)quinolonone are also tricyclic compounds, but derivatives of the 4-azasteroid structure. The compounds that have been designed can be divided into two categories, hexahydro derivatives and octahydro derivatives. The octahydro derivatives have been proven to be more potent.<ref name=":4" /> Compound LY 191704, later named bexlosteride, is the most potent octahydro derivative designed. It is a selective inhibitor to the type 1 isozyme, especially because of the chlorine atom and the amino-methyl group.<ref name="Azz" />
[[File:Bexlosteride.svg|thumb|Structure of [[Chemistry:Bexlosteride|bexlosteride]]|center]]
Piperidones are also 4-azasteroid derivatives but both B- and D-ring have been removed. The original compounds designed were type 1 selective, especially the ones containing a chlorine atom connected to the aromatic ring. By inserting a styryl group to the piperidones type 2 inhibitory activity increased.<ref name=":4" />

Nonsteroidal carboxylic acids are tricyclic compounds designed to resemble steroidal carboxylic acids such as episteride. As with the other nonsteroidal inhibitors, they have been designed by removing steroid ring systems.<ref name=":4" /> As with the piperidones, addition of a styryl group provides good dual inhibition on isozyme 1 and 2, but the nonsteroidal carboxylic acids are mostly type 1 selective.<ref name="Azz" />
[[File:Nonsteroidal derivatives.png|center|thumb|432x432px|Carboxylic acid derivative and piperidone derivative]]

==Natural products==
[[File:Serenoa repens 1zz.jpg|thumb|''Serenoa repens'' (saw palmetto).]]

===Saw palmetto extract===
The [[Organization:European Medicines Agency|European Medicines Agency]] (EMA) has concluded that the extract of the natural product Saw palmetto can be used to treat symptoms of benign prostatic hyperplasia (BPH) as research has shown its 5-ARI effects.<ref>{{Cite web|url=http://www.ema.europa.eu/docs/en_GB/document_library/Herbal_-_Summary_of_assessment_report_for_the_public/2016/04/WC500205465.pdf|title=Herbal medicine: Summary for the Public. Saw Palmetto Fruit.|date=5 April 2016|website=www.ema.europa.eu|access-date=28 September 2017}}</ref> An extract of ''[[Biology:Serenoa repens|Serenoa repens]]'', also known as [[Biology:Saw palmetto extract|saw palmetto extract]], is a 5-ARI that is sold as an over-the-counter dietary supplement. It is also used under the brand name Permixon in Europe as a pharmaceutical drug for the treatment of benign prostatic hyperplasia.

==See also==
* [[Medicine:Discovery and development of antiandrogens|Discovery and development of antiandrogens]]
* [[Biology:List of 5α-reductase inhibitors|List of 5α-reductase inhibitors]]

==References==
{{Reflist}}

{{Androgens and antiandrogens}}

[[Category:5α-Reductase inhibitors]]

{{Sourceattribution|Discovery and development of 5α-reductase inhibitors}}