Lasofoxifene: selective estrogen receptor modulator for the prevention and treatment of postmenopausal osteoporosis

From Pure Antagonists to Pure Degraders of the Estrogen Receptor: Evolving Strategies for the Same Target

Pure antiestrogens, or selective estrogen receptor degraders (SERDs), have proven to be effective in treating breast cancer that has progressed on tamoxifen and/or aromatase inhibitors. However, the only FDA-approved pure antiestrogen, fulvestrant, is limited in efficacy by its low bioavailability. The search for orally bioavailable SERDs has continued for nearly as long as the clinical history of the injection-only fulvestrant. Oral SERDs that have been developed and tested in patients ranged from nonsteroidal ER binders containing an acrylic acid or amino side chain to bifunctional proteolysis-targeting chimera (PROTAC) pure ER degraders. Structural evolution in the development of oral SERD molecules has been closely associated with quantifiable ER-degrading potency, as seen in the structural comparison analysis of acrylic acid and basic amino side-chain-bearing SERDs. Failure to improve on fulvestrant in the clinical trials by numerous acidic SERDs and early basic SERDs is blamed on tolerability and/or insufficient efficacy, which will likely be overcome by the new-generation basic SERD molecules and PROTAC ER degraders with improved oral bioavailability, low toxicity, and superior efficacy of receptor degradation.

Development of concise two-step catalytic approach towards lasofoxifene precursor nafoxidine

We have elaborated a two-step catalytic approach to nafoxidine, a key precursor to lasofoxifene. Firstly, an efficient α-arylation of 6-methoxy-3,4-dihydronaphthalen-1(2H)-one with chlorobenzene was developed, which operates at low 0.1 mol% Pd-132 catalyst loading in the presence of 1.9 equivalents of sodium tert-butoxide at 60 °C in 1,4-dioxane and provides 6-methoxy-2-phenyl-3,4-dihydronaphthalen-1(2H)-one in 90% yield. Secondly, we have demonstrated that 6-methoxy-2-phenyl-3,4-dihydronaphthalen-1(2H)-one can be converted to nafoxidine in 61% yield via CeCl₃ promoted reaction with (4-(2-(pyrrolidin-1-yl)ethoxy)phenyl)lithium, which is formed in-situ from the corresponding arylbromide precursor and n-butyllithium. Altogether, the shortest two-step approach to nafoxidine from simple tetralone commodity starting material has been developed with overall 55% yield. The developed synthetic approach to nafoxidine has several beneficial aspects over the one used in the synthetic route primarily developed for the preparation of lasofoxifene.

Selective estrogen receptor modulators: tissue specificity and clinical utility

Selective estrogen receptor modulators (SERMs) are a diverse group of nonsteroidal compounds that function as agonists or antagonists for estrogen receptors (ERs) in a target gene-specific and tissue-specific fashion. SERM specificity involves tissue-specific expression of ER subtypes, differential expression of co-regulatory proteins in various tissues, and varying ER conformational changes induced by ligand binding. To date, the major clinical applications of SERMs are their use in the prevention and treatment of breast cancer, the prevention of osteoporosis, and the maintenance of beneficial serum lipid profiles in postmenopausal women. However, SERMs have also been found to promote adverse effects, including thromboembolic events and, in some cases, carcinogenesis, that have proven to be obstacles in their clinical utility. In this review, we discuss the mechanisms of SERM tissue specificity and highlight the therapeutic application of well-known and emergent SERMs.

Bone health in persons with haemophilia: a review

Bone density is a measure of equilibrium between bone formation and bone resorption; any imbalance in these two processes might lead to osteopenia and osteoporosis. Osteoporosis in general has been considered to be an important cause of morbidity in both men and women and more so in haemophilia and other bleeding disorders. Specific triggering factors in persons with haemophilia (PWH) are abnormal liver function because of viral infection, bleeding and inflammation, lack of physical and athletic activities, low body weight and others. Although the pathogenesis of osteoporosis is not clear, it has been considered as a severe comorbidity in PWH in both developing and developed countries. This is more evident in developing countries where there is no free access to factor concentrates, and primary prophylaxis is beyond the reach of vast majority of the patients. Only few references are available in the literature on the prevalence and management of osteoporosis in haemophilia population. This review summarises the prevalence, plausible mechanisms and management options of this important morbidity in PWH.

Selective estrogen receptor modulator (SERM) lasofoxifene forms reactive quinones similar to estradiol

The bioactivation of both endogenous and equine estrogens to electrophilic quinoid metabolites has been postulated as a contributing factor in carcinogenic initiation and/or promotion in hormone sensitive tissues. Bearing structural resemblance to estrogens, extensive studies have shown that many selective estrogen receptor modulators (SERMs) are subject to similar bioactivation pathways. Lasofoxifene (LAS), a third generation SERM which has completed phase III clinical trials for the prevention and treatment of osteoporosis, is currently approved in the European Union for this indication. Previously, Prakash et al. (Drug Metab. Dispos. (2008) 36, 1218-1226) reported that similar to estradiol, two catechol regioisomers of LAS are formed as primary oxidative metabolites, accounting for roughly half of the total LAS metabolism. However, the potential for further oxidation of these catechols to electrophilic o-quinones has not been reported. In the present study, LAS was synthesized and its oxidative metabolism investigated in vitro under various conditions. Incubation of LAS with tyrosinase, human liver microsomes, or rat liver microsomes in the presence of GSH as a trapping reagent resulted in the formation of two mono-GSH and two di-GSH catechol conjugates which were characterized by liquid chromatography-tandem mass spectrometry (LC-MS/MS). Similar conjugates were also detected in incubations with P450 3A4, P450 2D6, and P450 1B1 supersomes. Interestingly, these conjugates were also detected as major metabolites when compared to competing detoxification pathways such as glucuronidation and methylation. The 7-hydroxylasofoxifene (7-OHLAS) catechol regioisomer was also synthesized and oxidized either chemically or enzymatically to an o-quinone that was shown to form depurinating adducts with DNA. Collectively, these data show that analogous to estrogens, LAS is oxidized to catechols and o-quinones which could potentially contribute to in vivo toxicity for this SERM.

Progress in endocrine approaches to the treatment and prevention of breast cancer

Tamoxifen had been the only available hormonal option for the systemic treatment for breast cancer from 1973 to 2000. Enormous efforts have led to the development of potent and selective third generation aromatase inhibitors including anastrozole, letrozole and exemestane. Due to their superior efficacy to tamoxifen, aromatase inhibitors are presently approved as first line agents for the treatment of advanced estrogen receptor (ER) positive breast cancer and adjuvant therapy in early ER positive early breast cancer in postmenopausal women. Selective ER Modulators (SERMS), tamoxifen and raloxifene are the only agents presently used in breast cancer prevention in high risk women and their use has increased substantially over the last decade. Third generations SERMS, lasofoxifene and bazedoxifene have shown significant reduction in bone loss compared to placebo in postmenopausal women and are currently approved in the European Union for the treatment of postmenopausal osteoporosis. This review outlines the current strategies employed in the use of endocrine therapy in the management and prevention of breast cancer.