Applications for patents of 5-alpha reductase inhibitors began in the mid-1970s, after an influential 1974 study by Imperato-McGinley et al., as described in ‘How was finasteride invented?

From 1980 through the mid-1990s, most patents were filed by Merck and SmithKline Beckham, a precursor of GlaxoSmithKline. Merck’s application for Proscar (finasteride 5 mg) was approved by FDA in 1992. Its application for Propecia (finasteride 1 mg) was approved in December 1997. GlaxoSmithKline’s application for Avodart (dutasteride 0.5 mg) was approved in 2001.

Merck’s key patent for Propecia appears to be Patent No. 5,571,817, granted Nov 5, 1996: “Methods of treating androgenic alopecia with finasteride…

This is a selection of patents which appear to be most relevant to drug development. More may be found via USPTO Public Search.

Patent #DateTitleAssignee
3,917,829Nov 4, 1975Novel methods of inhibiting the activity of testosterone 5.alpha.-reductaseResearch Corporation
4,088,760May 9, 1978Testosterone 5.alpha.-reductase inhibitorsRichardson-Merrell
4,191,759Mar 4, 1980N-substituted-17.beta.-carbamoylandrost-4-en-3-one 5.alpha. reductase inhibitorsMerck

Sep 2, 1980Preparation of 4-aza-17-substituted-5.alpha.-androstan-3-ones useful as 5.alpha.-reductase inhibitorsMerck
4,317,817Mar 2, 1982Novel steroid 5.alpha.-reductase inhibitorsRichardson-Merrell
4,361,578Nov 30, 1982D-HomosteroidsAlig, Leo et al. (Switzerland & Germany)
4,377,584Mar 22, 19834-Aza-17.beta.-substituted-5.alpha.-androstan-3-one-reductase inhibitors Merck
4,732,897Mar 22, 1988Steroidic 5.alpha.-reductase inhibitorsFarmitalia; Consiglio Nazonale Delle Ricerche
4,760,071Jul 26, 198817.beta.-N-monosubstituted carbamoyl-4-aza-5.alpha.-androst-1-en-3-ones which are active as testosterone 5.alpha.-reductase inhibitorsMerck
4,845,104Jul 4, 1989Oxidized analogs of 17.beta.-N-monosubstituted-carbamoyl-4-aza-5-.alpha.-androstan-3-onesMerck
4,859,681Aug 22, 198917.alpha.-Acyl-4-aza-5a-androst-1-en-3-ones as 5 alpha-reductase inhibitorsMerck
4,882,319Nov 21, 1989Phosphonic acid substituted aromatic steroids as inhibitors of steroid 5-.alpha.-reductaseSmithKline Beckman
4,888,336Dec 19, 1989Steroid 5-alpha-reductase inhibitorsSmithKline Beckman
4,910,226Mar 20, 1990Steroid 5-alpha-reductase inhibitorsSmithKline Beckman
4,937,237Jun 26, 1990Phosphinic acid substituted aromatic steroids as inhibitors of steroid 5-60 -reductaseSmithKline Beckman
5,017,568May 21, 1991Steriod [sic] 5-alpha-reductase inhibitorsSmithKline Beecham
5,061,802Oct 29, 199117.beta.-aminobenzoyl-4-aza-5.alpha.-androst-1-en-3-ones as benign prostatic hypertrophy agentsGlaxo
5,098,908Mar 24, 199217.beta.-hydroxybenzoyl-4-aza-5.alpha.-androst-1-en-3-ones as testosterone reductase inhibitorsMerck
5,302,589Apr 12, 1994Heterocyclic inhibitors of 5-.alpha.-testosterone reductaseGlaxo
5,571,817Nov 5, 1996Methods of treating androgenic alopecia with finasteride [17.beta.-N-mono-substituted-carbamoyl-4-aza-5-.alpha.-androst-1-en-ones]Merck
5,962,442Oct 5, 1999Irreversible inhibition of human 5ɑ-reductaseMerck