Pyrilutamide

Pyrilutamide (developmental code KX-826) is a nonsteroidal antiandrogen of the thiohydantoin structural class under clinical development by Suzhou Kintor Pharmaceuticals for the topical treatment of androgenetic alopecia (AGA) and acne vulgaris. The compound acts as a selective, high-affinity silent antagonist of the androgen receptor (AR), competitively displacing dihydrotestosterone (DHT) and testosterone from the ligand-binding domain without inducing any agonist-mediated transcriptional activity. This mechanism distinguishes pyrilutamide from systemic 5-alpha-reductase inhibitors (finasteride, dutasteride) that reduce circulating DHT concentrations, and from first-generation nonsteroidal antiandrogens (bicalutamide, flutamide) that possess residual partial agonist activity and are administered systemically. In cell-based AR transactivation assays, pyrilutamide exhibits an IC50 of approximately 0.28 nM, a Ki of 24 nM in competitive AR binding assays (compared to 48 nM for enzalutamide), and an IC50 of 264 nM for inhibition of prostate-specific antigen secretion in LNCaP prostate cancer cells, establishing it as one of the most potent topical antiandrogens characterized to date.

The compound was designed for topical delivery to the scalp and skin, with molecular properties selected to favor dermal retention and limit transdermal penetration. Preclinical pharmacokinetic evaluation in rats demonstrated rapid distribution to skin and adipose tissue following topical application, with low transdermal bioavailability and negligible plasma exposure. In human clinical studies, peak plasma concentrations following application of the 0.5% topical formulation remained below 0.5 ng/mL, with an estimated dermal half-life of approximately 2 hours. Toxicological assessments established no-observed-adverse-effect levels (NOAELs) of greater than 5000 mg/kg orally in rats and 90 mg/kg dermally in minipigs, yielding systemic safety margins of 168- to 222-fold over levels associated with therapeutic efficacy.

Clinical development of pyrilutamide has proceeded through multiple trials across China and the United States. A Phase I ascending-dose safety study in 40 men in the United States (0.3% to 9.6% concentrations) demonstrated tolerability with only mild contact dermatitis as the principal adverse event. A Phase II randomized, placebo-controlled trial in 120 Chinese men with androgenetic alopecia (Norwood grades 3V, 4, and 5) reported that 0.5% pyrilutamide applied twice daily for 24 weeks produced a mean increase of 22.73 hairs per square centimeter in target area hair count (TAHC) from baseline, representing a 15.34 hairs per square centimeter advantage over placebo. A Phase II trial in Chinese women with female-pattern hair loss met its primary endpoint. A subsequent pivotal Phase III trial in Chinese men, however, failed to demonstrate statistical significance on the primary TAHC endpoint versus placebo at 24 weeks (announced November 2023). Long-term safety extensions at 52 weeks reported that 46% of patients achieved at least a 10 hairs per square centimeter increase and 20% achieved at least a 20 hairs per square centimeter increase, with no drug-related sexual dysfunction and a favorable overall safety profile. A reformulated 1.0% tincture is now under evaluation in a Phase II/III pivotal trial initiated in 2025, with enrollment exceeding 750 participants at more than 20 hospital sites. A Phase II trial for acne vulgaris has completed enrollment in China. No regulatory approval has been granted in any jurisdiction as of the date of this monograph.

This monograph reviews the chemistry, structural class, and synthesis of pyrilutamide; the androgen receptor antagonist mechanism in molecular detail; the topical pharmacokinetic profile including systemic exposure characterization; the preclinical pharmacology in androgen-driven disease models; the clinical evidence base across androgenetic alopecia in men and women, acne vulgaris, and related dermatological indications; sourcing and quality verification considerations for research-grade material; reconstitution and handling; stack-interaction considerations with other dermatological and hormonal agents; the adverse-event and safety signal; and a comparative assessment of five alternative agents (finasteride, dutasteride, topilutamide, RU-58841, and minoxidil) against pyrilutamide on five competency standards (novelty, effect size, promising potential, side-effect profile, and overall validation).