Endoxifen, an Estrogen Receptor Targeted Therapy: From Bench to Bedside

Abstract

The selective estrogen receptor (ER) modulator, tamoxifen, is the only endocrine agent with approvals for both the prevention and treatment of premenopausal and postmenopausal estrogen-receptor positive breast cancer as well as for the treatment of male breast cancer. Endoxifen, a secondary metabolite resulting from CYP2D6-dependent biotransformation of the primary tamoxifen metabolite, N-desmethyltamoxifen (NDT), is a more potent antiestrogen than either NDT or the parent drug, tamoxifen. However, endoxifen’s antitumor effects may be related to additional molecular mechanisms of action, apart from its effects on ER. In phase 1/2 clinical studies, the efficacy of Z-endoxifen, the active isomer of endoxifen, was evaluated in patients with endocrine-refractory metastatic breast cancer as well as in patients with gynecologic, desmoid, and hormone-receptor positive solid tumors, and demonstrated substantial oral bioavailability and promising antitumor activity. Apart from its potent anticancer effects, Z-endoxifen appears to result in similar or even greater bone agonistic effects while resulting in little or no endometrial proliferative effects compared with tamoxifen. In this review, we summarize the preclinical and clinical studies evaluating endoxifen in the context of breast and other solid tumors, the potential benefits of endoxifen in bone, as well as its emerging role as an antimanic agent in bipolar disorder. In total, the summarized body of literature provides compelling arguments for the ongoing development of Z-endoxifen as a novel drug for multiple indications.

Breast cancer is the most diagnosed cancer worldwide, with nearly 80% of tumors classified as estrogen receptor positive (ER+), based on the expression of ER alpha (ERα). In the United States alone, nearly 284 200 new cases of breast cancer are estimated with a projection of 44 130 deaths in 2021, making it the second most common cause of cancer-related deaths among women (1). Despite these staggering numbers, ER+ breast cancer mortality rates have steadily declined over the years, in part because of the use of highly effective endocrine treatment regimens such as tamoxifen. Tamoxifen, a selective estrogen receptor modulator (SERM), was originally developed as an antifertility drug but failed in this capacity (2). Its efficacy for the treatment of postmenopausal metastatic breast cancer was first reported by Cole et al in the early 1970s, with promising antitumor activity and manageable side effects (3), resulting in the US Food and Drug Administration approval of tamoxifen for the postmenopausal treatment of metastatic ER+ breast cancer in 1977. Results from ensuing randomized clinical trials over the next three decades firmly established the therapeutic utility of tamoxifen for the treatment of early and advanced breast cancer, including both premenopausal and postmenopausal women (4-13), the adjuvant treatment of both pre- and postmenopausal breast cancer (14-17) and breast cancer in men (18, 19). Tamoxifen is also the only US Food and Drug Administration-approved drug for use in the prevention of breast cancer in high-risk women (20, 21). The results of the Early Breast Cancer Trialists’ Collaborative Group, which summarized multiple clinical trials of women with early breast cancer receiving chemotherapy and hormonal therapy, concluded that 5 years of adjuvant tamoxifen therapy significantly improves disease-free survival by reducing the risk of disease recurrence and death by upwards of 50%, as well as decreasing the risk of contralateral breast cancer (22), making tamoxifen a reliable endocrine agent for reducing breast cancer mortality.

Tamoxifen is extensively metabolized in the liver by the cytochrome P450 enzyme system into the primary metabolites 4-hydroxy-tamoxifen (4HT) and N-desmethyltamoxifen (NDT) (Fig. 1). NDT undergoes secondary catabolism to 4-hydroxy-N-desmethyl-tamoxifen (endoxifen). The cytochrome P450 2D6 (CYP2D6) enzyme mediates the oxidation of tamoxifen to 4HT and NDT to endoxifen (23). Studies have identified 4HT and endoxifen as the most active metabolites of tamoxifen (24-27), with preclinical data demonstrating that both metabolites demonstrate equipotent ER binding affinity as well as suppression of estrogen-dependent growth of ER+ breast cancer cells, with 100-fold greater effects compared with the parent drug, tamoxifen (28, 29). The plasma concentrations of tamoxifen, NDT, 4HT, and endoxifen observed in patients taking the 20 mg/d dose of tamoxifen are 133.8 ± 59.16 ng/mL, 278.1 ± 141.5 ng/mL, 3.0 ± 1.5 ng/mL, and 24.4 ± 16.2 ng/mL (30, 31). Studies directly measuring the plasma concentrations of tamoxifen metabolites in tamoxifen-treated patients have shown that low levels of 4HT or endoxifen associates with poor survival outcomes (32-35). Because the plasma concentrations of 4HT are very low compared with the other metabolites in tamoxifen-treated patients, much of the focus has been on evaluating the association between plasma concentrations of tamoxifen, NDT, and endoxifen levels and clinical outcomes.

Figure 1.

Schematic representation of tamoxifen biotransformation into primary and secondary metabolites by the cytochrome P540 system. The thickness of the arrows represents the relative contribution of each of the pathway to the overall oxidation of tamoxifen. Reprinted from Sideras et al. J Clin Oncol. 2010;28(16):2768-2776, with permission from Wolters Kluwer Health, Inc.

The association between polymorphisms in CYP2D6 genotype and clinical benefit of tamoxifen in patients receiving tamoxifen therapy have been the subject of controversy for years (36, 37). Although many studies have demonstrated an association between decreased CYP2D6 metabolism and poor clinical outcomes including shorter time to recurrence and worse relapse-free survival (38-40), other studies have disputed this notion showing no such association (41-44). Perhaps one of the most important issues confounding the retrospective and even prospective tamoxifen pharmacogenetic studies relates to the fact that differences in the concentrations of the active tamoxifen metabolites are rendered irrelevant when additional anticancer therapies are given either before or after tamoxifen. For example, a secondary analysis of the prospective Austrian Breast and Colorectal Cancer Study Group Trial 8 that randomized postmenopausal women with ER+ breast cancer to tamoxifen for 5 years (arm A) or tamoxifen for 2 years followed by anastrozole for 3 years (arm B), demonstrated that reduced CYP2D6 metabolism was associated with a higher risk of breast cancer recurrence, but only during the period of tamoxifen administration and not after switching to anastrozole (45). In another study, the role of CYP2D6 metabolism in patients treated with either tamoxifen monotherapy or chemotherapy and tamoxifen was evaluated. This study showed that in contrast to patients treated with tamoxifen monotherapy, no significant association between CYP2D6 genotype and recurrence-free survival was observed in patients treated with chemotherapy and tamoxifen (46). A recent prospective study validated this finding, showing that variation in endoxifen levels were not associated with recurrence when women with ER+ breast cancer were treated with adjuvant chemotherapy, a short period of tamoxifen (1-2 years), followed by an aromatase inhibitor (47). Therefore, although CYP2D6 genotype and low concentrations of the active metabolites are a critical determinant of tamoxifen clinical efficacy, they are unlikely to be associated with the risk of recurrence when other treatments are administered, which additionally alter the hazard for recurrence. Other additional differences reported to influence outcomes of the tamoxifen pharmacogenetic studies include differences in cohort size, patient recruitment criteria, use of concomitant CYP2D6 inhibitors, ER status, and CYP2D6 genotyping methods and are often cited as potential confounding factors (48).

Regardless, there is consensus that endoxifen is the most abundant active metabolite of tamoxifen, and the development of endoxifen as a novel drug was pursued to avoid issues related to CYP2D6 polymorphisms. Early phase (1/2) studies have demonstrated substantial antitumor activity of endoxifen in patients with prior progression on tamoxifen, providing definitive evidence for the benefit of endoxifen in tamoxifen-resistant patients (49, 50). In this review, we summarize the published literature surrounding the pharmacology and pharmacodynamics of endoxifen compared with tamoxifen in ER+ breast cancer, its effects on nonbreast cancers and nonbreast organs, as well as its emerging role as an antimanic agent in bipolar disorder.

CYP2D6 Metabolism of Tamoxifen and Endoxifen Plasma Concentrations

Studies have shown that in patients receiving tamoxifen therapy, CYP2D6 genetic variation is significantly associated with endoxifen levels, with endoxifen plasma concentrations ranging from 1.5 to 3.1 ng/mL for poor metabolizers, to above 22.0 ng/mL in ultrarapid metabolizers (51, 52). Furthermore, endoxifen concentrations are lower in tamoxifen-treated patients who are coprescribed CYP2D6-inhibiting drugs (30, 31, 53). In addition to numerous studies evaluating CYP2D6 genotype and tamoxifen clinical outcomes, several studies have reported an association between endoxifen concentrations and disease-free survival in tamoxifen treated early breast cancer patients (38, 39, 54). Taken together, these findings have led to the implementation of clinical guidelines based on CYP2D6 genotype status in deciding tamoxifen therapy recommendations (48). Notably, in patients with predicted decreased CYP2D6 enzyme activity, recommendations include either increasing the tamoxifen dose from 20 mg/d to 40 mg/d, which has been reported to normalize endoxifen concentration (55) or switching from tamoxifen to an aromatase inhibitor (AI) (48). For all patients, recommendation was provided to avoid concomitant usage of moderate and strong CYP2D6-inhibiting drugs (48).

Based on data that lower concentrations of the active metabolites of tamoxifen are associated with inferior breast cancer outcomes, as well as data from murine studies demonstrating that Z-endoxifen administration demonstrated high oral bioavailability (56, 57), the development of endoxifen as a novel therapeutic for the treatment ER+ breast cancer ensued.

Endoxifen Compared With Tamoxifen as a Potent Antiestrogen

Evidence supporting endoxifen’s superior antiestrogenic effects came from studies demonstrating that endoxifen, at concentrations typically observed in CYP2D6 extensive metabolizers, was not only more potent in terms of inhibition of estrogen-dependent tumor growth, but unlike tamoxifen or 4HT, additionally appeared to target ERα for degradation in breast cancer cells (58). This observation suggested that the mechanism of action of endoxifen may be distinct from that of tamoxifen or 4HT. Supporting this notion, global gene expression profiling of MCF7 cells treated with the tamoxifen metabolite 4HT, endoxifen, or the selective estrogen receptor degrader ICI 182 780 (fulvestrant) in the absence or presence of estrogen revealed substantial differences in the transcriptomes of endoxifen-treated cells compared with those of the other treatment groups in both settings (59). Notably, pathway analysis of differentially regulated genes in endoxifen-treated MCF7 cells indicated that cell-cycle arrest and apoptosis pathways were significantly induced at endoxifen concentrations that mirror those observed in CYP2D6 extensive metabolizers but not poor metabolizers. Other independent groups demonstrated that although clinical concentrations of tamoxifen and its metabolites used in the postmenopausal setting partially blocked estrogen-stimulated genes, the addition of endoxifen was necessary to achieve complete blockade of the expression of these estrogen-stimulated genes (60). In another study, clinical levels of tamoxifen and its metabolites in the premenopausal setting only partially inhibited estrogen-dependent growth and gene modulation. However, supplementation of endoxifen at concentrations corresponding to that of extensive metabolizers additionally augmented the inhibitory effects of tamoxifen and its metabolites on estrogen-stimulated actions (61). These data suggest that the endoxifen concentrations, particularly in the premenopausal setting, might be critical owing to its stronger antiestrogenic actions. Collectively, these preclinical studies established that endoxifen is an important component of tamoxifen’s antiestrogenic effects, with its increasing concentrations associating with improved anticancer effects.

Endoxifen’s Superior Antitumor Activity Compared With Tamoxifen

In a collaboration between Mayo Clinic and the National Cancer Institute, the first in-human prospective clinical trial of Z-endoxifen, the active isomer of endoxifen, was tested in patients with ER+ breast cancer who had progressed on multiple endocrine therapies (NCT01327781) (49). In this study, administration of oral Z-endoxifen resulted in plasma concentrations of Z-endoxifen reaching as high as 628.9 to 1886.8 ng/mL, greatly exceeding the Z-endoxifen concentrations achieved with tamoxifen (56, 57, 62, 63) and manageable toxicity. Furthermore, Z-endoxifen demonstrated promising antitumor activity, including in patients with ESR1 mutations, which are reported at a higher frequency in ER+ metastatic breast cancers and associates with worse clinical outcomes (64). A maximum tolerated dose was not identified. Importantly, Z-endoxifen clearance was unaffected by CYP2D6 metabolism, implying that in women with low or nonfunctional CYP2D6 enzyme activity, therapeutic concentrations of Z-endoxifen could be achieved thereby eliminating any role for CYP2D6 genotype in determining treatment outcomes in these women. In a second phase 1 clinical trial evaluating Z-endoxifen in patients with hormone receptor-positive breast cancer, Z-endoxifen dose levels as high as 360 mg/d dose were tested (NCT01273168) (50), more than double the highest dose (160 mg/d) studied in the first phase 1 clinical study (49). Although the maximum tolerated dose was not established, the doses used in this study were well tolerated. Regarding the breast cancer patients enrolled onto this trial, 1 of 9 patients (11%), treated with an aromatase inhibitor for a prolonged period before trial enrollment, experienced a partial response, whereas 3 other patients (33%) who were previously on multiple endocrine therapies including tamoxifen and anastrazole, experienced stable disease for ≥ 6 cycles (50).

These encouraging findings led to a randomized phase 2 clinical trial from the Alliance for Clinical Trials in Oncology (A011203) (NCT02311933) directly comparing the efficacy of tamoxifen and Z-endoxifen in women with ER+ and human epidermal growth factor 2 receptor (HER2) metastatic breast cancer that had progressed during endocrine therapy. Preliminary results from this trial were reported at the San Antonio Breast Cancer Symposium 2019 meeting. Although Z-endoxifen was not superior to tamoxifen (median progression-free survival [PFS], 4.3 months vs 1.8 months; hazard ratio [HR], 0.77; 95% CI, 0.49-1.22), marked differences in Z-endoxifen efficacy were observed according to receipt of prior CDK4/6 inhibitor (CDK4/6i) therapy. In patients with progression on CDK4/6i, Z-endoxifen was not superior to tamoxifen (median PFS 2.4 months vs 1.8 months; HR 1.29; 95% CI, 0.66-2.96). In contrast, in CDK4/6i-naïve patients, Z-endoxifen significantly prolonged PFS compared with tamoxifen (median PFS, 7.2 months vs 2.4 months; HR, 0.42; 95% CI, 0.22-0.80; P = 0.002). For patients that progressed on tamoxifen and crossed over to Z-endoxifen, the clinical benefit rate was 28% (14.0%-46.2%). These findings are consistent with emerging data from other prospective trials wherein endocrine monotherapy exhibited minimal antitumor activity in the post-CDK 4/6 inhibitor space. For example, in the recently reported VERONICA study in which patients with endocrine and CDK4/6i refractory metastatic breast cancer were randomized to fulvestrant alone or fulvestrant plus venetoclax, the median PFS was 2.69 months (95% CI, 1.94-3.71) in the combination arm vs 1.94 months (1.84-3.55) in the fulvestrant arm (65). This study also supports the findings from the A011203 study demonstrating lack of endocrine sensitivity in tumors that progressed on CDK4/6i. Although additional studies are needed to pinpoint the cause for the lack of antitumor activity following CD4/6i treatment, based on the preclinical and phase 1/2 clinical trials, Z-endoxifen is expected to exhibit the greatest clinical benefit in CDK4/6i-naïve ER+ breast cancer.

In preclinical studies that evaluated the antitumor activity of Z-endoxifen with letrozole and tamoxifen in the aromatase-expressing MCF7 (MCF7AC1) breast cancer xenograft model, Z-endoxifen was shown to be superior to tamoxifen and letrozole both in the treatment of naïve xenografts, as well as in xenografts that developed resistance to letrozole (66). The MCF7AC1 model is a clinically relevant tumor model that has faithfully predicted the clinical efficacy of AI therapy over tamoxifen therapy as well as the clinical efficacy of fulvestrant and anastrozole combinatorial therapies over AI monotherapy in vivo (67, 68). Furthermore, in the letrozole-resistant MCF7AC1 xenograft tumor model, Z-endoxifen was superior not only to tamoxifen, but additionally to exemestane monotherapy or exemestane plus everolimus combinatorial therapies (66), all of which have a track record of proven clinical efficacy in the AI-resistant setting. Importantly, in the AI-resistant setting and unlike tamoxifen, Z-endoxifen was not cross-resistant, making Z-endoxifen a promising second-line therapy in the AI-resistant setting. Furthermore, in the MCF7AC1 model resistant to letrozole, a comparison of the transcriptome of tamoxifen and Z-endoxifen-treated tumors revealed that Z-endoxifen more potently inhibits ERα target genes than tamoxifen, further supporting Z-endoxifen’s potent antiestrogenic activity compared with tamoxifen. Ingenuity pathway analysis revealed that Z-endoxifen may uniquely target signaling kinases such as ATM and PI3K/AKT, with tumor tissues confirming reduced expression of phospho and total AKT in Z-endoxifen treated AI-resistant tumors at clinically relevant concentrations (5 µM) but not in tamoxifen-treated AI-resistant tumors (66). Collectively, these data certainly establish that Z-endoxifen is a superior endocrine agent compared with tamoxifen, particularly in suppressing the growth of endocrine-refractory ER+ breast cancer.

Endoxifen’s Antitumor Activity in Other Solid Tumors

The second phase 1 clinical trial study that evaluated the efficacy of Z-endoxifen at doses as high as 360 mg/d also investigated the antitumor activity of Z-endoxifen on other solid cancers including advanced gynecological and desmoid cancers (50). Of the 4 patients diagnosed with desmoid tumors, 1 who had previously progressed on tamoxifen and γ-secretase inhibitor therapy experienced a partial response with Z-endoxifen therapy. Remarkably, this patient remained on the study for as long as 62 cycles (ie, for nearly 5 years of Z-endoxifen treatment) and had improved pain levels. A second patient, who had progressed on multiple therapies as well as another patient who previously progressed on tamoxifen, showed prolonged disease stabilization (50). Given that desmoid tumors lack expression of ERα and progesterone receptors (69), this finding suggests that Z-endoxifen may be a substrate for non-ER cancer targets and may have beneficial effects in the treatment of ER- tumors. Although desmoid tumors lack ERα expression, it is noteworthy that these tumors express ER-beta (ERβ) (69), whose expression is reported to correlate with desmoid tumor growth (70). Preclinical studies have shown that endoxifen can stabilize ERβ protein expression and that ERβ presence enhances the sensitivity of breast cancer cells to antiestrogenic effects of endoxifen (71). Therefore, Z-endoxifen’s antitumor activity in desmoid tumors may involve an ERβ-dependent mechanism. Z-endoxifen therapy also led to partial response or stable disease for ≥ 6 cycles in patients diagnosed with gynecological malignancies (50). These findings call for future investigation of Z-endoxifen antitumor activity in other solid tumors. A comprehensive summary of the study design, clinical outcomes, including adverse effects in the 2 Z-endoxifen phase 1 clinical trials, is provided in Table 1, whereas a summary of Z-endoxifen pharmacokinetics is provided in Table 2.

Table 1.

Summary of the study design, adverse events, and clinical outcomes from the 2 Z-endoxifen phase 1 clinical trials

Parameters	Goetz et al study, 2017 (49)	Takebe et al study, 2021 (50)
Patient enrollment details	Patients with metastatic or locally recurrent hormone receptor positive breast cancer.	Patients with hormone receptor positive solid breast tumors, desmoid tumors, or gynecologic tumors.
Prior treatment information	Progressed while receiving tamoxifen (if premenopausal) or an AI (if postmenopausal) in either the metastatic or adjuvant setting.	Progressed while receiving at least 1 prior chemotherapy regimen with or without at least 1 hormonal regimen in the metastatic setting. For HER2+ metastatic breast cancer, progressed on at least 1 prior HER2-directed regimen.
Limitations on prior therapy	None. Unlimited prior endocrine therapy regimens were allowed.	None.
Z-endoxifen dosages evaluated	20, 40, 60, 80, 100, 120 and 160 mg/d.	20, 40, 60, 100, 140, 200, 280 and 360 mg/d.
MTD	MTD not determined.	MTD not determined.
AE	Mostly grade 2 AE. Grade 4 hypertriglyceridemia noted at 60 mg/dose. No major AE reported at the highest dose (160 mg/d).	Most frequent AE were grades 2 and 3 lymphopenia and anemia. Grade 4 colonic perforation reported at the highest 360-mg dose.
Clinical benefit	Among the 25 patients with measurable disease, the overall response rate was 12.0% (95% CI, 2.6-31.2). Clinical benefit (stable disease ≥ 6 months) was observed in 19% of patients (3 of 16) who experienced progression during tamoxifen and 32% (7 of 22) who had no prior tamoxifen treatment or did not experience progression with adjuvant tamoxifen.	Breast cancer: 1/9 patients (11%) showed partial response; 3/9 patients (33%) experienced stable disease for ≥ 6 months. Desmoid tumors: 1/4 patients (25%) showed partial response; 2/4 patients (50%) experienced disease stabilization. Gynecologic tumors: 1/20 patients (5%) showed partial response; 3/20 patients (15%) experience disease stabilization of ≥ 6 cycles.

Abbreviations: AE, adverse event; AI, aromatase inhibitor; HER2, human epidermal growth factor 2 receptor; MTD, maximum tolerated dose.

Table 2.

Summary of the pharmacokinetics based on the dose level and treatment day from the 2 Z-endoxifen phase 1 clinical trials

Dose level (mg)	No. of patients	Day	Tmax (h)	Cmax (ng/mL)	C24h (ng/mL)	AUC0-24h (hours × µg/mL)	Accumulation (AUC)	Half-life (h)
Goetz et al study, 2017 (49)
20	3	1	4.3 ± 3.2	64.8 ± 13.2	38.1 ± 3.7	1.09 ± 0.21	3.47 ± 1.29	49.0 ± 21.7
	2	28	3.5 ± 3.5	215 ± 83	167 ± 49	4.19 ± 1.46
40	8	1	3.6 ± 2.3	169 ± 49	86.6 ± 22.6	2.49 ± 0.58	3.96 ± 0.89	57.2 ± 14.8
	8	28	1.7 ± 1.0	499 ± 49	414 ± 111a	9.66 ± 2.32
60	8	1	2.6 ± 0.9	348 ± 222	132 ± 93	4.33 ± 2.69	3.94 ± 1.86	56.5 ± 31.4
	5	28	2.8 ± 0.8	643 ± 332	421 ± 216	11.6 ± 5.5
80	8	1	6.8 ± 7.3	238 ± 49	152 ± 39	4.15 ± 0.91	4.14 ± 1.28	60.2 ± 21.4
	7	28	2.6 ± 1.7	913 ± 142	577 ± 122b	15.8 ± 2.3
100	8	1	4.0 ± 2.8	344 ± 104	194 ± 43	5.62 ± 1.18	4.62 ± 1.10	68.1 ± 18.4
	7	28	4.2 ± 3.1	1284 ± 364	952 ± 246	25.7 ± 6.8
120	3	1	3.0 ± 1.0	378 ± 155	243 ± 89	6.03 ± 2.73	3.64 ± 1.18	51.7 ± 19.7
	3	28	2.7 ± 1.1	1261 ± 453	813 ± 235	20.7 ± 6.3
160	3	1	3.7 ± 2.1	635 ± 39	333 ± 62	9.81 ± 6.3	3.81 ± 0.76	54.6 ± 12.8
	3	28	5.1 ± 4.0	1874 ± 633	1362 ± 379	37.9 ± 12.3
Takebe et al study, 2021 (50)
20	3	1	3.7 ± 0.6	68.6 ± 7.9	30.2 ± 4.3	1.08 ± 0.07	3.9 ± 0.9	55.9 ± 15.0
	3	28	0.7 ± 0.6	286.1 ± 86.8	131.7 ± 79.2	4.25 ± 1.19
40	5	1	4.0 ± 0.0	112.2 ± 30.2	52.5 ± 15.4	1.65 ± 0.36	3.0 ± 0.3	40.4 ± 4.4
	3	28	3.3 ± 2.5	257.9 ± 55.0	147.3 ± 42.6	4.76 ± 0.85
60	3	1	12.0 ± 10.4	121.6 ± 39.7	82.7 ± 9.3	2.14 ± 0.63	3.7 ± 1.5	52.4 ± 25.2
	3	28	10.8 ± 12.0	417.3 ± 188.6	345.4 ± 175.9	8.08 ± 4.05
100	3	1	4.3 ± 2.9	296.0 ± 30.0	155 ± 21	5.00 ± 0.46	2.7 ± N/A	36.8 ± N/A
	3	28	3.0 ± 1.4	1010.0 ± 411.5	428.0 ± N/A	13.29 ± N/A
140	3	1	4.3 ± 1.5	610.0 ± 306.0	311.0 ± 171.0	9.76 ± 4.24	2.4 ± 0.2	30.6 ± 3.5
	3	28	2.0 ± 1.0	1113.0 ± 325.0	563.0 ± 49.0	17.40 ± 1.15
200	3	1	4.0 ± 3.5	517.0 ± 232.0	336.0 ± 105.0	9.22 ± 3.11	3.1 ± 0.9	43.4 ± 15.8
	3	28	2.0 ± 0.0	1641.0 ± 830.0	1069.0 ± 652.0	30.75 ± 19.43
280	7	1	3.0 ± 0.8	930.0 ± 224.0	468.0 ± 128.0	14.96 ± 4.64	N/Ac	N/Ac
360	7	1	3.9 ± 1.6	1309.0 ± 349.0	676.0 ± 297.0	20.55 ± 7.33	N/Ac	N/Ac

Data presented as mean ± SD.

Abbreviations: AUC, area under the curve; AUC_0-24h, area under the curve over 24 h; C_24h, serum concentration after 24 h; C_max, peak serum concentration; N/A, not available; T_max, time to maximum serum concentration.

^a n = 7 as reported in Goetz et al study.

^b n = 5 as reported in Goetz et al study.

^c Pharmacokinetics on day 28 not reported for these doses in the Takebe et al study.

Additional in vitro studies have documented a role for endoxifen in effectively inhibiting growth of melanoma cancer cells and in vivo studies demonstrated superior antitumor activity over tamoxifen in reducing metastatic melanoma nodules (72). Endoxifen has also been shown to be superior over tamoxifen both as a single agent as well as in combination with all-trans-retinoic acid in inhibiting proliferation and migration of melanoma cancer cells, without inducing drug toxicity (73). These data suggest that endoxifen might be a promising treatment for melanoma. Like desmoid tumors, ERβ is expressed in melanoma, and thus is a potential target for therapeutic intervention (74). Further studies are needed to determine whether endoxifen’s therapeutic effects in melanoma are mediated via ERβ.

Endoxifen Effects on the Bone, Lipids, and Uterus

Long-term endocrine therapy is often associated with concerning side effects (75). Prolonged AI therapy is known to cause debilitating effects on bone health by inducing accelerated bone loss and increased rates of vertebral and hip fractures (76). In contrast, tamoxifen and the newer SERMs, including raloxifene, lasofoxifene, and arzoxifene (77), generally elicit beneficial effects on the bone by reducing bone loss and the risk of bone fractures and may exhibit some mild lipid-lowering effects (78-83). Given these data, it was of interest to evaluate the effects of endoxifen on these phenotypes. In ovariectomized mice, Z-endoxifen oral administration was shown to increase cancellous and cortical bone mass (84), eliciting similar beneficial effects on bone as noted with the other SERMs. Also, oral administration of Z-endoxifen not only reduced bone turnover in both ovary-intact and ovariectomized rats but also protected ovariectomized rats against bone loss (85), further corroborating endoxifen’s agonistic effects on the bone. Regarding the effect of endoxifen on lipids, there are few if any preclinical data pertaining to the effects of endoxifen on lipids. However, early data from the first phase 1 clinical study suggests that endoxifen may reduce total cholesterol levels (49).

An additional concerning side effect of prolonged tamoxifen therapy is the increased risk of uterine cancer (86, 87). This is a serious clinical problem and often an important consideration in the development of novel SERMs (88). Evaluation of Z-endoxifen’s uterotropic effects in both ovary-intact and ovariectomized rats have shown that endoxifen, unlike tamoxifen, elicits minimal proliferative activity in the uterus and notably its partial agnostic effects are less potent compared with estradiol (85, 89). Collectively, these data suggest that endoxifen’s beneficial effects may extend beyond its potent anticancer activity. Furthermore, these findings also suggest that in patients receiving AI or tamoxifen therapy, replacing with or switching over to endoxifen therapy for long-term treatment might be a safer option as it would alleviate some of the health concerns associated with the former treatments.

Endoxifen’s Antimanic Activity in Bipolar Disorder

Bipolar I disorder is a chronic mental illness characterized by the occurrence of acute mania or mixed episodes of shifts in mood, energy, and activity levels and the ability to carry out day-to-day tasks, thereby disabling normal life. The role of protein kinase C (PKC) signaling in the pathophysiology of bipolar disorder is well documented (90). Although clinical trials have previously demonstrated antimanic activity for tamoxifen, a known PKC inhibitor, in patients with bipolar I disorder (91, 92), reports have indicated that endoxifen is a more potent inhibitor of PKC compared with tamoxifen (93). Considering these data, endoxifen’s efficacy in the treatment of bipolar I disorder has been evaluated. A double-blind, active-controlled trial has demonstrated that endoxifen is well tolerated and exhibits promising antimanic activity in bipolar disorder patients, with its efficacy similar to that of divalproex, the commonly prescribed drug for the treatment of this disease (94). Furthermore, endoxifen treatment resulted in numerically fewer side effects compared with divalproex. The phase 3 multicenter, double-blind, active-controlled clinical trial that ensued further validated endoxifen safety and efficacy for the treatment of bipolar I disorder (Clinical Trials Registry-India/2017/07/009163) (95). This trial additionally showed that remission was achieved earlier with endoxifen, compared with divalproex, as early as day 4. Collectively, these findings highlight a novel antimanic activity for endoxifen in bipolar disorder and calls for further exploration of endoxifen actions in this disease.

Concluding Remarks

The current body of literature has clearly demonstrated that endoxifen is a potent antiestrogen with a unique mechanism of action that differs from tamoxifen. Phase 1/2 clinical trials evaluating Z-endoxifen have demonstrated encouraging antitumor activity in endocrine-refractory breast cancer as well as in nonbreast cancers. Early findings from the phase 2 clinical study directly comparing the efficacy of endoxifen vs tamoxifen monotherapy in ER+ metastatic breast cancer patients suggest that endoxifen may be an efficacious drug compared with tamoxifen.

In premenopausal women with early-stage, ER+ breast cancer, an updated analysis of the International Breast Cancer Study Group 24-02 study demonstrated that therapies that more deeply suppress estrogen such as ovarian function suppression (OFS) drugs (eg, goserelin [Zoladex]) in addition to tamoxifen or AI therapy are superior to tamoxifen monotherapy (96, 97). However, deep estrogen deprivation can cause significant morbidity, often resulting in detrimental effects on women’s overall health. Follow-up studies of patients enrolled on the Suppression of Ovarian Function Trial and Tamoxifen and Exemestane Trial extending 8 years after surgery have reported that side effects associated with accelerated aging were doubled in premenopausal women treated with AI plus OFS compared with tamoxifen plus OFS (98). The superior estrogen-suppressive effects of endoxifen compared with tamoxifen (66) calls for future clinical investigation comparing the efficacy of endoxifen without OFS with that of tamoxifen or AI in combination with OFS therapy in premenopausal breast cancer patients. The results from such a study would determine whether endoxifen could be a replacement for the current standard of care that includes OFS, potentially eliminating the debilitating side effects associated with OFS therapy and considerably improving the quality of life in these women.

Additionally, the utility of combining Z-endoxifen with CDK4/6i is another interesting area of clinical investigation. Currently, CDK4/6i in combination with AI or fulvestrant is the standard of care for the treatment of metastatic hormone receptor positive breast cancer in patients, owing to their significant improvement in PFS (99-103). Although the PFS benefit in MONALEESA7 appeared similar comparing the tamoxifen/ribociclib and AI/ribociclib arms, given issues with prolonged QT, the standard approach is to use either fulvestrant or an AI in combination with CDK 4/6 inhibitors (104). Given that Z-endoxifen antitumor activity is superior to that of letrozole (66), further studies are needed to determine whether endoxifen and CDK 4/6i combinations provide better antitumor activity compared with either tamoxifen, fulvestrant, or AI combinations.

Finally, comprehensive transcriptome studies have indicated that endoxifen is a more potent inhibitor of estrogen-regulated genes compared with tamoxifen in ER+ breast cancer. However, endoxifen’s effect on the proteome remains poorly understood and calls for further investigation. Although emerging studies indicate that endoxifen may impact oncogenic signaling kinases (66), more in-depth phospho and total proteome-based mass spectroscopy analyses and kinase screening assays are necessary to comprehensively identify the signaling kinases and cellular proteins that are targeted by endoxifen. These findings are critical not only to our understanding of the molecular basis of endoxifen’s potent antitumor activity in ER+ breast cancer, but also will provide insights into its potential mode of action in nonbreast malignancies. Meanwhile, based on the encouraging findings from phase 1/2 clinical trials, the ongoing efforts to develop endoxifen for the treatment of ER+ breast cancer should continue (105).

Glossary

Abbreviations

4HT

4-hydroxy tamoxifen

AI

aromatase inhibitor

CDK4/6i

CDK4/6 inhibitor

CYP2D6

cytochrome P450 2D6

ERα

estrogen receptor alpha

ERβ

estrogen receptor-beta

ER+

estrogen receptor positive

HER2

human epidermal growth factor receptor 2

HR

hazard ratio

MCF7AC1

aromatase-expressing MCF7

NDT

N-desmethyltamoxifen

OFS

ovarian function suppression;

PFS

progression-free survival

PKC

protein kinase C

SERD

selective estrogen receptor degrader

SERM

selective estrogen receptor modulator

Additional Information

Disclosures: M.P.G. is the Erivan K. Haub Family Professor of Cancer Research Honoring Richard F. Emslander, MD; and reports personal fees continuing medical education activities from Research to Practice and Clinical Education Alliance, consulting fees to Mayo Clinic from Eagle Pharmaceuticals, Lilly, Biovica, Novartis, Pfizer, Sermonix, AstraZeneca, Blueprint Medicines, and Biotheranostics; and grant funding to Mayo Clinic from Pfizer, Lilly, and Sermonix. The other authors have nothing to disclose.

Data Availability

Data sharing is not applicable to this article because no data sets were generated or analyzed for this review.