Aromatase inhibitors: from bench to bedside and back

Evolution of Medical Approaches and Prominent Therapies in Breast Cancer

An examination of the origins of medical approaches to breast cancer marks this disease as one of the most difficult to manage. As the early identification, diagnosis and treatment of breast cancer evolve, we will move to a time when each patient and their cancer can be assessed to determine unique patient-specific (personalized) approaches to therapy. Humans have attempted to manage breast cancer for millennia. Even today, the disease claims thousands of lives each year. In light of the increasingly sophisticated understanding of cancer diagnosis and treatment, together with our ultimate failure to offer a cure in the most difficult cases, it is instructive to reflect on the beginnings of our understanding.

Advances in the approach to novel drug clinical development for breast cancer

INTRODUCTION

In the post-genomic era clinical development of new agents to treat breast cancer (BC) can be a real challenge. Different from chemotherapy agents, with a broad but not specific spectrum of activity, novel drugs are being developed as 'targeted' agents, potentially benefiting a subgroup of patients. In BC, different clinically identifiable subtypes are now separately addressed in specific clinical trials.

AREAS COVERED

In this review, the authors discuss the clinical development of targeted drugs that have become part of the current treatment of BC. They also highlight the challenges that in other cases determined the failure of promising compounds. Furthermore, the article reports on how combinations of targeted agents have emerged as valid strategies to overcome acquired resistance. It also provides discussion of how 'old' therapies can be retargeted to certain patient populations or 'reinvented' as safer and more effective with the creation of drug conjugates. They also discuss how novel clinical trial designs are emerging to accelerate the successful matching of targeted drugs to the right patient population.

EXPERT OPINION

It is important not to forget that the development of BC therapeutics is a 'moving target', as its biology evolves in time under the pressure of ongoing treatments. There are currently a finite number of resources available for the development of new therapeutics, which means that resources need to be carefully allocated. There is also a need to prioritize clinical trials that can reduce the number of patients who are candidates for expensive treatments.

Aromatase inhibitor-associated bone loss and its management with bisphosphonates in patients with breast cancer

Postmenopausal women have an increased risk of osteopenia and osteoporosis due to loss of the bone-protective effects of estrogen. Disease-related processes may also contribute to the risk of bone loss in postmenopausal women with breast cancer. One of the most common and severe safety issues associated with cancer therapy for patients with breast cancer is bone loss and the associated increase in risk of fractures. This paper reviews the recent literature pertaining to aromatase inhibitor (AI)-associated bone loss, and discusses suggested management and preventative approaches that may help patients remain on therapy to derive maximum clinical benefit. A case study is presented to illustrate the discussion. We observed that AIs are in widespread use for women with hormone receptor-positive breast cancer and are now recommended as adjuvant therapy, either as primary therapy or sequential to tamoxifen, for postmenopausal women. AIs target the estrogen biosynthetic pathway and deprive tumor cells of the growth-promoting effects of estrogen, and AI therapies provide benefits to patients in terms of improved disease-free survival. However, there is a concern regarding the increased risk of bone loss with prolonged AI therapy, which can be managed in many cases with the use of bisphosphonates and other interventions (eg, calcium, vitamin D supplementation, exercise).

Management of patients with metastatic breast cancer

Hormone treatment is one of the key strategies in the management of metastatic breast cancer. Hormone treatment is one of the key strategies in the management of metastatic breast cancer. Aromatase inhibitors (AI) have been extensively studied in this setting. This section summarizes the key data regarding the use of AI in advanced breast cancer. In postmenopausal women, AI are the first line of treatment for untreated patients, or those who had prior AI treatment and progress after 12 months of adjuvant therapy. A longer disease-free interval and absence of visceral disease is associated with a better response. If tumors recur in less than 12 months, it is recommended that tamoxifen (TAM) or the estrogen-receptor antagonist fulvestrant (FUL) treatment be initiated. In the second-line setting, the best option after progression is the administration of either FUL or TAM. In the third-line setting, reintroduction of AI is considered an acceptable option. In premenopausal women who have not received prior treatment or who have progressed after 12 months following adjuvant treatment, it is recommended to initiate therapy with a combination of TAM and a luteinizing hormone-releasing hormone (LHRH) analog. If there is treatment failure with the use of this combination, megestrol acetate or an LHRH agonist plus an AI may be reasonable alternatives. Intensive research is ongoing to understand the mechanisms of resistance to hormone therapy. In human epidermal growth factor receptor 2 positive-patients, combinations with HER2 antagonists are associated with significant clinical activity.

Guidelines for Osteoprotection in Breast Cancer Patients on an Aromatase Inhibitor

Postmenopausal women are at an increased risk of osteopenia and osteoporosis due to the physiologic loss of the bone protective effects of estrogen. Additionally, disease-related risk factors also contribute to the increased fracture risk. To further complicate matters, one of the most common and severe safety issues associated with cancer therapies for breast cancer patients is bone loss and the associated increased risk of fractures. These facts underscore the need to carefully monitor bone mineral density in patients with endocrine-responsive breast cancer, and to consider adjuvant therapy that may help manage and/or prevent bone loss and fracture. Aromatase inhibitors (AIs) are now in widespread clinical use for women with hormone receptor-positive breast cancer and have replaced tamoxifen as the gold standard of care. AIs target the estrogen biosynthetic pathway and deprive tumor cells of the growth-promoting effects of estrogen. These treatments provide significant benefit to patients in terms of improved disease-free and overall survival. Adversely, there is a concern of an increased risk of bone loss with prolonged therapy consequently leading to an increased fracture risk. This manuscript will review the recent literature pertaining to AI-associated bone loss and discuss suggested management and preventative approaches that may help patients remain on therapy to derive the most clinical benefits.

Challenges in clinical patient management

The use of endocrine therapy in breast cancer represents one of the earliest molecular targeting strategies used in cancer treatment. Tamoxifen, a selective estrogen-receptor (ER) modulator, has been the standard of care for women with receptor-positive breast cancer for the last 30 years. Tamoxifen suppresses the estrogen-dependent growth of breast cancer cells by specifically targeting the ER. Because of estrogenic effects, tamoxifen does not increase the risk of osteoporosis, but it can lead to endometrial cancer and thromboembolism. The third-generation aromatase inhibitors (AIs) exert their tumor antiproliferative action by targeting an enzyme critical for estrogen biosynthesis. The AIs thus have a different mechanism of action than tamoxifen, and a different safety profile. The majority of adverse events (AEs) related to the AIs are mild to moderate. Most of these AEs are common to menopause and are predictable and manageable. This review looks at AI-associated side effects and current clinical management strategies, with a particular emphasis on managing bone health. Compliance with long-term therapy, strategies to improve adherence, and considerations in elderly patients with hormone-responsive breast cancer are also discussed.

Clinical usefulness of high-dose toremifene in patients relapsed on treatment with an aromatase inhibitor

BACKGROUND

Aromatase inhibitors (AIs) have been employed as adjuvant therapy or as treatment for recurrent cases. However, when AI treatment fails, it is unclear which endocrine therapy is the most appropriate to introduce at this point and how effective it will be. In this study, we investigated the efficacy and safety of toremifene (TOR, Fareston(®)), a selective estrogen receptor modulator (SERM).

METHODS

Patients with recurrent or advanced breast cancer who had measurable or evaluable lesions, and were diagnosed as having progressive disease during AI treatment and subsequently given TOR at 120 mg/day (TOR120) as endocrine therapy were selected and analyzed retrospectively in relation to their medical history.

RESULTS

Of a total of 83 cases examined, 80 were evaluable. The objective response rate (ORR) was 15.0% (12/80), the clinical benefit (CB) rate was 45.0% (36/80), and median time to failure (TTF) was 7.8 months. TOR120 was also effective in the progressive disease cases relapsed on AI treatment. When TOR120 was used, as a first-, second- or third-line treatment, the CB rate was 57% (32/56); this fell to 17% (4/24) when TOR120 was used as a fourth-line or later treatment. There was no response in the five estrogen receptor (ER)-negative cases, compared with an ORR of 15% (10/67) in ER-positive cases. In cases with a human epidermal growth factor receptor 2 (HER2) score of 0, 1+, and 2+, the ORR was 11% (7/61), while there was no response in the five cases with scores of 3+. TOR120 was effective in cases previously treated with tamoxifen (TAM), with an ORR and CB rate of 12 and 29%, respectively. The last AI used was anastrozole in 30 cases and examestane in 46; the response rates to TOR120 were similar in both groups. With regard to adverse effects, hot flushes and/or night sweating was observed in 10 and 12 cases, respectively, but all of them were categorized as grade 1, and the treatment was rated excellent in acceptability.

CONCLUSIONS

TOR120 was rated excellent in acceptability, and high efficacy was observed when it was used up to third-line treatment for AI-failure cases, although this study may show some selection bias because of the retrospective study. In addition, it was also considered effective for TAM-failure cases.