20 Hormonal Therapies

20.1 The roles of hormones in human cancers

20.1.2 Discovery of the role of hormones in cancer

George Beatson and Charles Huggins were the first to demonstrate that the control of tissues by sex hormones could be used in the therapy of hormone dependent cancers.

Figure 20.1: George Beatson and Charles Huggins were the first to demonstrate that the control of tissues by sex hormones could be used in the therapy of hormone dependent cancers.

In the late 19th century a new cancer hospital was opened in Glasgow, UK in which the surgeon George Beatson studied in detail the development and treatment of breast cancer. In 1896, he proceeded to publish a paper with the title ‘On Treatment of Inoperable Cases of Carcinoma of the Mamma: Suggestions for a New Method of Treatment, with Illustrative Cases,’ describing the treatment of three women with advanced breast cancer which had undergone a procedure to surgically remove their ovaries (‘oophorectomy’), leading to a regression of their tumours. This for the first time established a clear link between the hormonal production of the ovaries and their role in the progression in some forms of breast cancer.

Similarly, Charles Huggins and colleagues systematically studied the effects of hormones on prostate function. They discovered that surgical removal of the testicles (‘castration’) or hormone treatment led to a shrinking of the prostate gland (‘atrophy’) that could be reversed by administration of androgenic hormones. In 1941 the paper ‘Studies on prostate cancer. I. The effect of castration, of estrogen and of androgen injection on serum phosphatases in metastatic carcinoma of the prostate’ described the application of these insights to the control of prostate cancer by hormonal means for which he was awarded the Nobel Prize for Medicine in 1966.23

20.1.3 The mechanism of action of steroid hormones

As discussed above, cell cycle entry at R depends on the presence of growth factors including for example the ligands for receptor tyrosine kinases. In contrast to the receptor tyrosine kinase type of growth factors (EGFR/HER2 etc.) steroid hormone receptors are not found on the cell surfaces but within the cytoplasm, meaning the steroid hormones have to enter cells first before they can activate their receptors (Figure 20.2).

After entering the cell, steroid hormones bind to a specific receptor protein in the cytoplasm. The hormone binding to the receptor leads to the dissociation of inhibitory proteins (e.g., heat shock protein (HSP)) from the receptor complex which then allows the hormone–receptor complex to enter the cell nucleus (‘translocation’). After binding to recognition sites on specific genes their transcription is activated (or inhibited) resulting in downstream changes in the levels of the relevant proteins.

The mechanism of action of steroid hormones.

Figure 20.2: The mechanism of action of steroid hormones.

20.2 Oestrogen Hormones

The structure of oestrogenes (also "estrogenes") and of their precursor cholesterol

Figure 20.3: The structure of oestrogenes (also “estrogenes”) and of their precursor cholesterol

Oestrogens and androgens belong to the family of steroid hormones, together with progestagens (e.g., progesterone), the mineralocorticoids (aldosterone) and glucocorticoids (cortisol), all of which are known as steroid hormones and share a common core structure derived from the precursor molecule cholesterol (Figure 20.3).
The oestrogen hormones are all derived from cholesterol via first progesterones and then androgens. They are distinguished by modifications that add increasing numbers of hydroxyl functions to the molecule: oestrone (E1), oestradiol (E2), and oestratriole (E3). They fulfil slightly different roles and their relative regulatory importance changes, e.g., in post-menopausal women (E1), normal menstrual cycle (E2), or pregnancy (E3).

20.2.1 Oestrogens and oestrogen receptor function

The oestrogens function in an analogous fashion to other steroid hormones: Oestradiol (E2) binds to cytoplasmic oestrogen Receptor (ER) which triggers the release of co-repressors (inhibits gene expression). This allows a conformational change and dimerisation of the receptor molecules to occur and the binding of co-activators. Similarly to other pathways, the displacement of co-repressors and recruitment of co-factors allows for much more subtle control of downstream gene expression programs. This complex now translocates from the cytoplasm into the nucleus where it binds to oestrogen response element (ERE) in target genes. Transcriptional activation leads to the expression of the relevant target genes producing proteins with roles in cell proliferation. In addition, oestrogen also has non-genomic effects supporting increased cell survival.

Oestrogens support a range of different functions (Figure 20.4), including

  • Differentiation of secondary sexual characteristics
  • Maintenance of reproductive tract
  • Regulation of menstrual cycle
  • Promotion of growth and differentiation of the breast during maturation
  • Reduction of bone resorption and increase bone formation – bone maintenance
  • Regulation of liver production of cholesterol (+HDL -LDL)
  • Decrease plaque formation in coronary arteries
  • Cognitive function
Main organ with oestrogen function.

Figure 20.4: Main organ with oestrogen function.

Oestrogen has important physiological beneficial e.g. control of breast and uterus for sexual reproduction, limitation of cholesterol production to reduce build-up of plaque in the coronary arteries, and homoeostasis of bone balancing build-up and breakdown. However, inappropriate proliferation of cells in the breast and uterus linked to the proliferative stimulation by oestrogens can increase a woman’s chance of developing breast or uterine cancer.

While oestrogen levels drop dramatically after the menopause the beneficial effects remain important e.g. due to a cardio-protective role and counteracting osteoporosis and loss of cognitive function.

20.2.2 Oestrogen synthesis

In premenopausal women, the endogenous steroidal hormones (17β-estradiol (E2), Oestrone (E1), Oestriol (E3)) are predominantly synthesised in the developing follicles in the ovaries under the regulation of luteinising hormone (LH) and follicle stimulating hormones (FSH). Pulsatile release of gonadotropin releasing hormone (GnRH) from the hypothalamus triggers release of follicle-stimulating hormone (FSH) and luteinizing hormone (LH) in the pituitary gland at stimulate the production of oestrogen in the ovaries (different ratios dependent on stage of menstrual cycle). Stimulation of the stratum granulosum (FSH) induces a number of follicles leads to increased levels of aromatase enzyme responsible of the conversion of androgenic pre-cursors to E2. A positive feedback loop leads to development of a dominant follicle and a E2 surge which induces a negative feedback to reduce FSH and LH production followed by a LH surge that triggers the ovulation and then transition to the luteal phase which is dominated by the production of progesterone.

The synthesised of estradiol in the ovaries starts in theca interna cells with LH dependent synthesis of androstenedione from cholesterol. This then crosses the basal membrane into granulosa cells where conversion to estrone or estradiol occurs, either immediately or through testosterone. This conversion is catalysed by the enzyme aromatase (FSH induced).

In post-menopausal women this route of synthesis has seized to play any role and the production is sustained mainly from androgens in peripheral tissues (liver, adrenal glands, breast, and adipose tissue).

The mechanism of estrogen synthesis in the ovary

Figure 20.5: The mechanism of estrogen synthesis in the ovary

20.2.3 Oestrogen effects on breast cells and cancer risk

Once the ovaries start to produce oestrogens these stimulate the growth of the breasts. Specifically, the system of lobes and ducts develops that together will form the basis of the milk glands. After menarche higher levels and cyclical changes promote the growth milk ducts under the influence of oestrogen in the 1st half of the cycle, followed by the stimulation of secretory milk glands under the influence of progesteron in the 2nd half. In the absence of a pregnancy these changes revert back in preparation of the start of the next cycle. This proliferative stimulation of breast cells continues until menopause.

Increased levels of oestrogens thus increase the number of cell divisions. These have also been linked to an increased chance of DNA replication errors with associated risk or abnormal cells bearing mutations. This is therefore also linked to an increased risk of cancer. Oestrogens are thus potential driver of cancer proliferation and map on to the hallmark of sustained proliferative signalling. On fact, oestrogen treatment can induce mammary tumour formation in experimental animals.

These observations are also reflected in epidemiological observations that show the cumulative oestrogen exposure over a life span (e.g. early menarche and late menopause) is a breast cancer risk factor. Hormonal replacement therapy

In order to avoid some of the unpleasant symptoms caused by the lower oestrogen levels associated with the menopause the use of hormonal replacement therapy (HRT) has become a useful therapeutic option. However, reports appeared that suggested that long term use of HRT is linked to an increased risk of breast cancer and ovarian cancer. Despite a number of studies that have been carried out the data have however not been very clear. A recent meta analysis of a large number of studies/patients suggests that all types of HRT (except vaginal oestrogens) did indeed increase the breast cancer risks; risk also increased with longer use was higher for oestrogen-progestagen combinations than oestrogen alone (Collaborative Group on Hormonal Factors in Breast Cancer 2019).( Progestagen is a synthetic progesterone analogue used in HRT products.)

If one assumes that the increased statistical risks are indeed caused by HRT then a woman aged 50 having 5 years of HRT would increase breast cancer incidence over the next 20 years depends on the preparation used between (1 in 50 to 1 in 200) and duration of treatment (longer treatment increases risk).

The risk needs to be considered in relation to the medical need or benefit but also other risk factors, e.g. the additional risk is similar to someone who drinks ( ≥2 units/day) but is much less than the additional risk due to obesity.24

20.3 Oestrogens and breast cancer

Breast cancer incidence rates and number of cases in men and women(UK)

Figure 20.6: Breast cancer incidence rates and number of cases in men and women(UK)

Breast cancer is the most common cancer in women accounting for around 30% of all cancer cases. The incidence of breast cancer rises with age; while the risk is much increased in older women the incidence rises from age 20-24 (Figure 20.6).

Breast cancer is not a uniform disease and one the most important clinical distinctions relates to the subtype. The sub-type is determined looking at histological markers of breast cancer cells e.g., after a needle biopsy, to distinguish cancer with high levels of certain biomarkers so that women can be allocated to the most appropriate treatment.

Around 70% of breast cancers have high levels of the marker for the oestrogen receptor (ER) (to be precise ERα) and are therefore classified as ER+; around 65% of these ER+ cancers also have higher levels of the progesteron receptor (PR+). The second largest group of breast cancers, around 20%, do not show high levels of ER but express high levels of the tyrosine kinase receptor HER2 on their cell surface instead (HER2+). Finally, about 10% of breast cancers do not have high levels of any of these receptors and therefore known as triple negative breast cancer (TNBC).As the cancer develops in some cases the status changes, e.g. ER+ cancer cells may loose the reliance on ER at a later stage which can lead to resistance to some hormone based therapies.

More recently additional classifications have been introduced based on molecular subtypes 20.7. These classifications are also of prognostic importance; here the cancers are divided based on the origin of the cancer cells, i.e., luminal, baso-luminal, or basal. The more luminal the cancer cells are in their characteristics the more likely they will be ER+. For this classification subtypes are divided (as before) into triple negative and HER2+ (representing more basal derived cells) but also Luminal A and Luminal B. Luminal A represents cells with ER+ and PR+ (‘double-positive’) and have a lower risk of metastasis and better a prognosis because when oestrogen and progesterone are present in the cell, the PR binds to the ER and alters gene expression of hundreds of genes with reduced rate of proliferation.

Breast cancer - histological and molcular subtypes (@MacMasterWong:2012)

Figure 20.7: Breast cancer - histological and molcular subtypes (Wong (2012))

20.4 Hormonal treatment of breast cancer

Determination of the breast cancer subtype is important as it identifies ER+ cancers where oestrogen plays a major role in driving cancer progression. As shown by Beatson over hundred years ago, removal of the ovaries — and thus the main source of oestrogen in pre-menopausal women could reduce breast cancer progression. The surgical approach can now be replaced with pharmacological strategies which include

  • Blocking the ER receptor
    • Selective estrogen receptor antagonists /modulators (SERMS)
  • Inhibition of the enzyme responsible for the biosynthesis of oestrogen
    • Aromatase inhibitors

20.4.1 Selective oestrogen receptor modulators (SERMS)

The ER of different target tissues vary in their precise structure. Furthermore, the type and levels of co-activators and co-repressors vary between tissues. SERMS are compounds that have both, agonist and antagonist effects on ER depending on the particular tissue. Typically, the anti-oestrogenic activity is selective to certain target tissues: e.g., inhibition of the ER in the breast, activation in the uterus or bone. Tamoxifen

Chemical structure of oestradiol, tamoxifen and fulvestrant

Figure 20.8: Chemical structure of oestradiol, tamoxifen and fulvestrant

Tamoxifen is the most commonly used hormone antagonist or modulator drug used in cancer treatment (Figure 20.8). Importantly, it is effective in pre-menopausal, peri-menopausal and post-menopausal women with ER+ breast cancer. In fact, Tamoxifen is a pro-drug and its mode of action involves a bioactivation step in the liver (CYP450). The resulting 4-hydroxytamoxifen, N-desmethyl-4-hydroxytamoxifen now have a 30-100x increased affinity to the ER. Depending on the ER confirmation it will lead to a tissue dependent partial activation of ER and recruitment of co-repressors and the overall co-activator/co-repressor balance. As a consequence such cells remain in G0/G1 and do not enter into the cell cycle (“R”) but remain in stasis.

The binding of oestradiaol and tamoxifen to the ER receptor triggers different gene expression programs

Figure 20.9: The binding of oestradiaol and tamoxifen to the ER receptor triggers different gene expression programs

Tamoxifen is thus mostly indicated for early and advanced breast cancer in pre-menopausal women. Typically, tamoxifen is taken as an adjuvant treatment (in support of the main therapy) and continues for many years after the initial therapy. In ER+ cancers treatment with Tamoxifen over 5 years reduces the risk of recurrence significantly from ~ 37% to 24.8% (Figure 20.9). However, in breast cancers with low expression of ER there is no beneficial effect. Further studies have shown that extension of this standard treatment to 10 years can bring additional benefit with a reduction of recurrence from ~ 25 to 21 % (2013 aTTom & ATLAS study).

Tamoxifen treatment is in general well-tolerated. However, there are short and long term side effects consistent with its mode of action as a SERM, i.e., tissue specific partial agonist and antagonist.

  • Short term side effects
    • Hot flushes
    • Irregular menses
    • Vaginal bleeding or discharge
  • Long-term side effects
    • Increased risk of endometrial cancer
    • Thromboembolism
    • Deep vein thrombosis
    • Pulmonary embolism (combination with cytotoxic therapy)
A comparison of the effect of tamoxifen treatement in ER+ and ER poor breast cancer [@EBCTCG:2011cq]

Figure 20.10: A comparison of the effect of tamoxifen treatement in ER+ and ER poor breast cancer (Early Breast Cancer Trialists’ Collaborative Group (EBCTCG) et al. 2011)

20.4.2 Aromatase inhibitors

An alternative approach to reducing the proliferative effect of oestrogen on ER+ breast cancer proliferation is focused on suppressing the endogenous production of oestrogen by inhibiting the enzyme(s) responsible.

A comparison of the effect of tamoxifen treatement in ER+ and ER poor breast cancer [@EBCTCG:2011cq]

Figure 20.11: A comparison of the effect of tamoxifen treatement in ER+ and ER poor breast cancer (Early Breast Cancer Trialists’ Collaborative Group (EBCTCG) et al. 2011)

This group of aromatase inhibitor (AIS) drugs includes Anastrozole, Letrozole, and Exemestrane. They are the first choice in the treatment of ER+ breast cancer in post-menopausal women (NICE guidelines 2009). They lower the oestrogen level in post-menopausal women with ER+ status but are not commonly used in premenopausal women (see below).

In pre-menopausal women the main source of oestrogen are the ovaries. Gonadotropin-releasing hormone (Gn-RH) from the hypothalamus induces the pulsatile secretion of follicle stimulating hormone (FSH) and luteinising hormone (LH) from the anterior pituitary in a ratio that changes during the course of the menstrual cycle. At the beginning of the cycle FSH induces proliferation of the stratum granulosum in ~ 20 follicles increasing secretion of the enzyme aromatase in their granulosa cells (8-10 fold increase of enzyme). A small amount of LH also stimulates Theca cell enzymes that produce the androgens needed for oestrogen synthesis (substrate). These androgens are absorbed by the granulosa cells and the aromatase catalyses the conversion oestrogens.

Attempts to interrupt ovarian oestrogen biosynthesis with first-generation AIS have failed because of the reflex increases in FSH and LH secretion, which counteract the inhibitory action of the drug (i.e. increasing secretion of FSH will increase the level of aromatase in the ovary). However, in post-menopausal women the pituitary - ovary axis is no longer active (and thus no feedback) and the main source of oestrogen production is now limited to peripheral tissues (adipose tissues). Recent works suggest however that AIS could be useful in pre-menopausal if at the same time a [LHRH analogue][LH-RH agonist drugs] is given which blocks the feedback release of FSH/LH (Pistelli et al. (2018)).

These drugs do not increase the risk of:

  • Endometrial cancer
  • Stroke
  • Blood clots

but can cause:

  • Heart problems
  • Reduction in bone mass
  • Osteoporosis – bone fractures (spine, hip and wrist)
    • Requires constant monitoring - Dual energy X-ray absorptiometry (DEXA)25

20.4.3 Resistance to ER targeted therapies

Resistance to hormonal therapies, specifically the ER, are not understood well. Clinical evidence suggests that after good control for many years resistance can develop and cancer cells that were previously reliant on oestrogens and ER stimulation are able to progress despite low oestrogen levels or inability to access these receptors.

In principle resistance can be thought of as ‘intrinsic,’ i.e., the tumour does not respond well to hormonal therapies from the start (e.g., low ER expression or usually high levels of aromatase expression in he breast tissue). More relevant, in general is ‘acquired’ resistance. The risk of tumours acquiring resistance against any therapy is of course based on the underlying principles of [somatic evolution][Mutations and Somatic Evolution] which underpins the constant change and adaptation cancer cells are undergoing. These could involve the loss of ER expression/function which could be compensated for by the activation of other growth factor signalling pathways. Alternatively, increased levels of aromatase could lead to increased levels of oestrogens , etc.

20.4.4 New targeted therapies

The CDK inhibitors Ribociclib and Palbociclib are part of a new class of drugs that has become available for the treatment of breast cancer. This type of targeted therapy is based on inhibition of the cyclin D - CDK4/6 complex and thus inhibits progression through the cell cycle.

20.5 Androgen hormones

Androgens are a group of sex hormones that include testosterone and dihydrotestosterone (DHT) and which control the development and maintenance of male sexual characteristics but also have other function in the body (e.g. anabolic effects). Testosterone is predominatley produced in the testicles with a small quantitiy also being made in the adrenal glands.

The androgen hormones support physiological development and maintenance of the prostate by activation of the androgen receptor (AR) . Analogous to the ER , this cytoplasmic hormone receptor will translocate to the nucleus once the androgen hormones have entered the cells, and by binding displaced any co-repressors. Binding to androgen receptor response elements on DNA will then allow expression of downstream genes which are involved in growth of prostate cells.26

20.6 Androgens and Prostate cancer

Prostate cancer is one of the most common forms of cancer in men (~25%). It typically affects men over 65 years old and has been linked to a number of risk factors including, a family history of prostate cancer, ethnicity (more common in black men), and the presence of abnormal prostate cells or prostatic intraepithelial neoplasia (PIN).

The most common symptoms linked to prostate cancer are similar for patients suffering from benign prostate hypertrophy or hyperplasia (BPH), a non-cancerous enlargement of the prostate commonly found in older men, e.g. irregular urine flow and urinary problems. However, in prostate cancer the invasive nature can exacerbate those symptoms and also lead to erectile dysfunction, blood in the urine, and frequent pain in hips, lower back or thighs.

The diagnosis of prostate cancer and distinction from BHP is based on a combination of screening techniques, including a digital rectal exam and the prostate specific antigen (PSA) test. PSA can be detected in blood or semen and can be a useful biomarker to help monitoring potential prostate cancers. By itself it is not diagnostic as there is no defined normal range and PSA levels can very widely between individuals. Furthermore, PSA levels can be raised for unrelated reasons (false positive) or low, despite of the presence of prostate cancer (false negative). Its utility lies in allowing to monitor changes in an individual and the potential effects of treatment.

Positive diagnosis of prostate cancer relies on additional exams including transrectal ultrasound (TRUS) or MRI to image the internal structure of the prostate and a histological exam e.g. from biopsies obtained during a TRUS procedure; here up to 12 biopsies are taken in order to reduce the risk of missing smaller tumours in the prostate gland.

The benefits of screening are unclear, e.g. a large study looking at a single PSA test as a screening tool found no benefit, similarly meta analysis (Cochrane) suggests that benefits are unclear and need to be consdiered against the associated risks e.g. complications from biopsies and subsequent treatment, as well as the risk of overdiagnosis and overtreatment (Ilic et al. 2018). (It has been suggested that as much as 1000 screening tests would be required to potentially prevent 1 death.)

Staging of the cancer in the prostate is based on the Gleason scores, a histolgoical assessment combining the size of the relative area (or volume) and the grade, a measure of histopathological signs of malignancy of the cancer cells (see also here). Ultimately a combination of imaging techniques, lymph node biopsy, and surgery provides a complete anatomical picture of the location and spread of the tumour and maps on to the familar TNM system (tumour, node, metastasis).

Stage I – localised to prostate, cannot be felt

Stage II – more advanced, but does not extend beyond prostate

Stage III – extends beyond prostate, into seminal vesicles, but not lymph nodes

Stage IV – has invaded bladder, seminal vesicles, lymph nodes, metastasis

20.6.1 Treatment options

During early stages of prostate cancer the cancer cells are dependent on higher levels of androgen hormones and are therefore sensitive to their withdrawal and such cancers are called androgen sensitive or castration sensitive. Therefore, surgery (orchiectomy) or pharmacological interventions are important options for treatment.

However, similar to ER dependent tumours prostate cancers dependent on AR tend to become ‘castration resistant’ and are able to grow despite only very low levels of androgens in the body. In some cases, these may still respond to some next generation anti-androgen drugs. Although, in some cases the prostate cancer cells also develop the capability for testosterone synthesis.

The treatment options for prostate cancer take account of the fact that these types of tumours tend not to be very aggressive and tend to grow relatively slowly. For many older men a prostate cancer may never become a medical problem in their life time. Therefore one treatment option is called ‘watchful waiting.’

Summary of options

  • Watchful waiting
  • Surgery (stages I and II)
  • Radiation therapy (all stages)
  • Hormone therapy (after surgery and/or radiation therapy)

20.6.2 Androgen deprivation therapy

Drugs used to suppress AR activation fall into two groups with different mode of action:

  • Luteinising hormone-releasing hormone (LH-RH) agonists (prevent testes from making testosterone)

  • Androgen receptor antagonists (block hormone action at the receptor) LH-RH / GnRH agonist drugs

Leuprolide (also known as Leuprorelin) is a synthetic gonadotropin releasing hormone (GnRH) analogue. The drugs are given as aqueous depot injection (s.c. or i.m.) and are readily absorbed.

GnRH is normally released in pulsatile form from neutrons in the hypothalamus to bind and activate the respective receptors in the pituitary (Figure 20.12). In response to this, the pituitary will release gonadotropins LH and FSH. In men FSH acts on the Sertoli cells to stimulate sperm production while LH will act on the Leydig cells to stimulate production of testosterone.

Binding of LH-RH agonist drugs will thus initially stimulate release of FSH and LH. This initial high release will trigger an initial ‘flare effect’. However, these drugs have a slow release from the receptor and thus disrupt the pulsatile signalling from hypothalamu· Tumour flare (potentially counteracted with of 5α-reductase inhibitors e.g. finasteride)

· Hot flushes

· Bone thinning – monitor bone density

· Impotence, lowered sex drive

Cardiovascular problemss to pituitary. This leads to subsequent desensitisation by down-regulation of the GnRH receptor and the consequent reduction in FSH and LH will over a few weeks lead to hypogonadism. This ‘shrinking’ of the testicles ultimately leads to a reduction in testosterone production.

The mechanism of action of LH-RH agonist drugs

Figure 20.12: The mechanism of action of LH-RH agonist drugs

Side effects:

  • Tumour flare (potentially counteracted with of 5α-reductase inhibitors e.g. finasteride)

  • Hot flushes

  • Bone thinning – monitor bone density

  • Impotence, lowered sex drive

  • Cardiovascular problems Androgen receptor antagonists

These drugs exist as steroidal or non-steroidal compounds; due to the structural similarity with other steroid hormones steroidal receptor antagonists may have interactions with a number of other steroid type receptors (side effects) . An example of non-steroidal an AR antagonists is bicalutamide which is approved for the treatment of metastatic prostate cancer (in combination with a GnRH analogue or surgical castration) and for locally advanced prostate cancer.

  1. https://www.nobelprize.org/nobel_prizes/medicine/laureates/1966/ [Nobel Prize for Physiology or Medicine 1966]↩︎

  2. HRT Risks↩︎

  3. NHS DEXA scan↩︎

  4. Both, AR and ER, also have non-genomic functions that do not require gene expression but are based on direct interactions with signaling pathways↩︎