The History of Estetrol (E4)
The Early Rise and Fall of Estetrol
Estetrol or E4 (because this estrogenic steroid molecule has 4 OH groups) was first identified by the group of Egon Diczfalusy at the Karolinska Institute in Stockholm, Sweden in 1965 (Hagen et al,) in a 24-hour urine specimen of 6 to 13 weeks old infants born with multiple malformations. They reported that estrone (E1) and estradiol (E2) accounted for about 4% and estriol (E3 ) for approximately 12% of the total urinary radioactive material. The surprising finding was the presence of a novel compound. In 1966, Gurpide and colleagues (Gurpide et al., 1966) at the Rockefeller Institute in New York, USA, examined the fetal and maternal metabolism of E2 during pregnancy by isotope dilution methods and found a urinary metabolite more polar than E3. By comparing infrared spectra, they concluded that their compound was identical to that detected by Diczfalusy and coworkers and they named it Estetrol. In 1967 Diczfalusy and colleagues (Zucconi et al.) succeeded in isolating and identifying the novel estrogen E4 by extracting 200 liters of late pregnancy urine. In 1979, Chris Holinka and Elio Gurpide at Rockefeller investigated the in vivo effect of E4 on the immature rat uterus (Biol Reprod 20, 242-6). Interest in E4 completely disappeared halfway the eighties, since it was considered to be a very weak estrogen, also shown to be unsuitable as marker of fetal wellbeing and the estrogen was forgotten.
The Revival of Estetrol as Useful Natural Estrogen
In 2001, Pantarhei Bioscience (PRB) proposed E4 as a potential estrogen for human use and performed the preclinical development of E4, summarized in a special issue of the journal Climacteric in 2008, and in the paper “Estetrol: A unique steroid in human pregnancy (Christian F. Holinka, Egon Diczfalusy, Herjan J.T. Coelingh Bennink; Journal of Steroid Biochemistry & Molecular Biology 110 (2008) 138–143)”. Also the phase I/IIA clinical development of E4 for the Women’s Health (WH) applications Combined Oral Contraception (COC) and Menopausal Hormone Therapy (MHT) was executed by Pantarhei. From 2009-2013 the phase IIB development of E4 for the COC application was performed in collaboration with Mithra Pharmaceuticals in Liège, Belgium in the joint venture Estetra. In 2015 Pantarhei sold the rights for E4 to Mithra, but kept the exclusive world-wide, rights on E4 for oncological and veterinary applications. From 2015-2021 Mithra performed the further phase IIB and III development of E4 for COC and MHT and in 2021 a new E4 containing COC became available for patients in Europe, the USA, Canada and Australia, whereas E4 for MHT is expected to enter the market in 2023.
Celebration 5 years Pantarhei Bioscience in 2006, the inventors of E4 Egon Diczfalusy and Herjan Coelingh Bennink
The Dual Efficacy of High Dose Estetrol in Reproductive Endocrine Cancer
In 2014, Pantarhei Bioscience founded a subsidiary Pantarhei Oncology (PRO) for the development of E4 for the treatment of advanced breast cancer (BC) and advanced prostate cancer (PC).
Based on dose finding studies with E4 in COC and in MHT, a dose of 20 mg E4 was selected for the first BC study in Vienna, Austria, demonstrating that 20 mg E4 induced apoptosis in BC tumor tissue (Singer et al Carcinogenesis, 2014:35(11):2447-51). High doses of E4 (HDE4) of 20 mg, 40 mg and 60 mg E4 were investigated in a dose escalating 3+3 design study in end stage patients in Mainz, Germany. In this study, anti-tumor effects were observed in 5 of 9 patients (stabilization of tumor growth or remission) according to the RECIST criteria. HDE4 was well tolerated and effectively improved wellbeing (Schmidt et al J Cancer Res Clin Onc, 2021:147:1833-42). At present after almost 4 years, one of these 9 end stage patients is still alive and well on her continued E4 treatment. Further studies with HDE4 in BC patients after failure of anti-estrogen treatment and chemotherapy are in preparation.
A daily dose of 40 mg E4 was selected for further development for advanced PC in a E4 dose-finding study by Pantarhei Oncology in healthy older males (Coelingh Bennink et al JCEM, 2018;103:3239-49). Thereafter a phase II study (PCombi) was performed in patients with advanced PC, starting androgen deprivation treatment (ADT) and co-treated with HDE4 or placebo. Additional anti-tumor effects of HDE4 to ADT were demonstrated after 3 and 6 months treatment by further suppression of the biochemical tumor markers total and free T, PSA, FSH and IGF-1. (Coelingh Bennink et al EUROS, 2021;28:52-61). Strong HDE4 estrogen replacement/treatment effects were found on hot flushes (HFs) and on Quality-of Life (QoL) (Zimmerman et al EUROS, 2022;45:59-67) and on bone biochemistry (Coelingh Bennink et al Endocrine Connections, 2022; accepted for publication). The further phase III development has been agreed with the FDA in November 2021. At present the next phase III study is in preparation with a unique design. After a first year of randomized, double-blind, placebo-controlled, 6 month cross-over co-treatment of standard of PC care with 40 mg E4 (A) or placebo (B), the patient himself will choose his treatment for the second study year, either treatment A or B and again blinded.
The Natural Estrogen Estetrol for Human Use
The fetal estrogen E4 is a potential new endocrine treatment for oral contraception in COCs, for postmenopausal estrogen replacement as MHT and for the treatment of advanced prostate cancer and advanced breast cancer. Estetrol has a high oral bioavailability (80% for E4 vs 5% for E2) and is therefore very suitable as an oral drug. Unlike previously used estrogens, it has no (toxic) metabolites and a low impact on coagulation and haemostatic liver factors. In view of these pharmaceutical properties and its favourable safety profile, Pantarhei expects that E4 will be safer than other natural or synthetic estrogens used in the past and at present, also allowing the use of higher dosages for the treatment of cancer compared to the estrogens used for cancer treatment in the past. So far, E4-related cardiovascular events (CVEs) were very rare in clinical studies with E4 for COCs and MHT and in the Pantarhei studies for oncological indications. High Dose Estetrol (HDE4) for advanced prostate and breast cancer treatment has successfully achieved proof-of-concept in humans demonstrating dual efficacy by a combination of anti-tumor activity and major improvements of wellbeing by strong estrogen treatment and substitution.