Novel Non-Steroidal Aromatase Inhibitors: Are There New Perspectives in the Treatment of Breast Cancer?

Clinical Efficacy of the Aromatase Inhibitor Anastrozole in Relation to Prolactin Secretion in Heavily Pretreated Metastatic Breast Cancer

Aims and background

It is known that the aromatase inhibitors may act by decreasing estrogen levels. Moreover, it is known that estrogens may stimulate the release of prolactin (PRL), which is a growth factor for breast cancer. This phase II study was performed to evaluate the effects of the novel aromatase inhibitor anastrozole on PRL secretion in metastatic breast cancer and the possible influence of PRL pretreatment levels on the efficacy of therapy.

Methods

The study involved 14 pretreated metastatic breast cancer patients with a poor clinical status. Anastrozole was given orally once a day at 1 mg/day for at least 2 months. To evaluate PRL secretion, venous blood samples were collected before treatment and at 1-monthly intervals during treatment.

Results

The clinical response consisted of partial response (PR) in 2, stable disease (SD) in 5 and progressive disease (PD) in the remaining 7 patients. Abnormally high pretreatment levels of PRL were seen in 5/14 (36%) patients. Progressing patients showed significantly higher pretreatment levels of PRL than those who achieved PR or SD. None of the patients with high PRL pretreatment levels showed a decline in PRL levels on treatment with anastrozole.

Conclusions

This preliminary study suggests that anastrozole has no inhibitory effect on PRL secretion in metastatic breast cancer and that the evidence of abnormally elevated concentrations of PRL prior to therapy is generally associated with a lack of efficacy.

An overview of letrozole in postmenopausal women with hormone-responsive breast cancer

Third-generation aromatase inhibitors (AIs) have proven to be superior to tamoxifen in terms of time to disease progression in patients with hormone receptor (HR) positive (HR+) status and, nowadays, are used in the adjuvant and neoadjuvant settings, and first-line therapy for advanced breast cancer. Letrozole is a third generation AI, as are anastrozole and exemestane. In the past, clinical studies had demonstrated that letrozole was effective as a second-line treatment of metastatic breast cancer. In this paper, pharmacokinetic and pharmacodynamic properties of letrozole are reviewed along with its activity in preclinical and clinical settings. Additionally, the results of important clinical trials such as Breast International Group (BIG) 1-98, which tested the optimal initial adjuvant endocrine treatment and the sequential therapy with letrozole and tamoxifen, MA-17 that evaluates the benefits of extended adjuvant therapy, and other important studies in advanced and neoadjuvant disease, are reviewed. Safety comparisons of treatments are also addressed. Interestingly, about 50% of human epidermal growth factor receptor 2-positive (HER2+) breast cancers are HR+. However, HER2 positivity is a marker of antiestrogen treatment resistance. Because of this, a dual treatment is a logical approach when both receptors are overexpressed. The combination of lapatinib and letrozole leads to a significant improvement in the overall disease outcome. Also, the combination of everolimus plus letrozole has been tested in this setting. In fact, the coadministration of both agents seems to increase the efficacy of letrozole in newly-diagnosed HR+ patients. Once resistance to sequential trastuzumab and AI as monotherapy has been found, trastuzumab and letrozole combined in HR+ and HER2+ patients with advanced breast cancer can overcome resistance to both drugs administered as single agents, according to recently reported results.

Update on the current use of hormonals as therapy in advanced breast cancer

Hormonal agents have a confirmed role in the management of postmenopausal women with receptor-positive advanced breast cancer. Until recently, tamoxifen has been the accepted agent for treating these patients. However, accumulating evidence suggests that the new antiaromatase agents will replace the antiestrogens as the preferable option in hormone-naive patients. Comparative trials indicate that the aromatase inhibitors, anastrozole and letrozole, and the aromatase inactivator, exemestane, have at least equivalent efficacy to tamoxifen with similar or superior tolerability. These agents are also more effective than the progestin, megestrol acetate, when studied in patients progressing on tamoxifen. The improved aromatase selectivity and high potency of these antiaromatase agents when compared with earlier agents have resulted in improved efficacy and tolerability. Additionally, no cross-resistance has been reported between the antiaromatase agents and tamoxifen or, in some instances, among the antiaromatase agents themselves. The role of antiaromatase agents will certainly expand in the near future to include not only treatment of metastatic breast cancer, but use in the adjuvant and neoadjuvant settings as well, and, ultimately, breast cancer prevention. The results of ongoing investigations are awaited with interest.

Overview of the pharmacology of the aromatase inactivator exemestane

One third of all breast cancers and two thirds of postmenopausal breast cancers are estrogen dependent. Antiestrogen strategies, such as inhibition of estrogen-receptor binding and estrogen deprivation, are effective for the management of hormone-dependent breast cancer. Although currently available agents are effective, the development of more potent and selective agents continues. Both steroidal and nonsteroidal inhibitors of aromatase have been developed for clinical uses. A novel class of steroidal irreversible antiaromatase agents demonstrates a high degree of specificity for the aromatase enzyme and exhibits a unique pharmacokinetic profile. The ability of these agents to inactivate aromatase may explain their high degree of potency and lengthy duration of action. Exemestane, an orally active aromatase inactivator, has demonstrated excellent selectivity and tolerability and broad-based efficacy in the treatment of postmenopausal breast cancer. Current findings suggest that exemestane will be a valuable alternative for women with breast cancer, not only for those progressing on other hormonal therapies but in earlier stages of the disease and prevention.

First report of the investigation into the importance of pK(a) in the inhibition of estrone sulfatase by sulfamate containing compounds

In an effort to rationalise the inhibitory activity of a range of aminosulfamate compounds and to further investigate the recently reported definitive pharmacophore estrone sulfatase (ES), we undertook extensive synthesis, biochemical evaluation and physicochemical property determination of a range of similar compounds. Here, we report the initial results of our study into a series of simple (aminosulfonate based) substituted phenol derivatives. Using these compounds, we investigated the role of pK(a) in the inhibition of ES. The results of the study suggest that there is a strong correlation between the inhibitory activity and the stability of the resulting O(-) anion (i.e., the pK(a) of the starting phenol).

Structure-activity relationship study of steroidal and nonsteroidal inhibitors of the enzyme estrone sulfatase

We report the initial results of a series of molecular modelling studies to investigate the structural properties of non-steroidal inhibitors required for inhibitory activity against the enzyme estrone sulfatase (ES) [the enzyme responsible for the conversion of nonactive (sulfated) estrone to the active (nonsulfated) estrone]. From the results of the present study, we conclude that the C(17) polar group may not be necessary for inhibitory activity and that the only requirement appears to be the mimicking of the steroid C(3) sulfonate group. To test our hypotheses, we have designed novel straight chain inhibitors based upon alkyl alcohols, which upon evaluation, have been shown to possess inhibitory activity (e.g., an inhibitor based upon trichloroethanol has been shown to possess 46% inhibition at 0.76mM).

The aromatase inhibitor letrozole in advanced breast cancer: effects on serum insulin-like growth factor (IGF)-I and IGF-binding protein-3 levels

Serum insulin-like growth factor (IGF)-I and IGF-binding protein-3 levels were measured in two groups of postmenopausal women with advanced breast cancer, who received the aromatase inhibitor letrozole 0.5 or 2.5 mg p.o. once daily. Blood samples were obtained from 15 patients in each dose group at baseline, and one and three months after starting therapy. Circulating IGF-I and IGFBP-3 concentrations were determined by means of radioimmunoassay. In both dosage groups a statistically significant increase in the IGF-I levels was observed during three months of letrozole treatment (P=0.003). In addition, the multiple testing procedure yielded in the whole patient population a significant result in the comparison between mean IGF-I values after three months of therapy and those observed at baseline (P=0.004), the estimated average increase being of 24%. No significant result was obtained in the analysis for the dose effect (P=0.077) and for the time x dose interaction (P=0.208). Circulating IGFBP-3 levels did not appear to be affected by letrozole treatment in either of the dose groups. This is the first report concerning the short-term effects of letrozole on components of the IGF system in breast cancer patients; further investigations are warranted in order to confirm these preliminary data.