REPORTING FROM SABCS 2017

SAN ANTONIO (FRONTLINE MEDICAL NEWS) – Adjuvant endocrine therapies improve outcomes of premenopausal breast cancer in the long term, with absolute benefit varying somewhat by therapy and by patient and disease characteristics, according to planned updates of a pair of pivotal phase 3 trials.

The trials – TEXT (Tamoxifen and Exemestane Trial) and SOFT (Suppression of Ovarian Function Trial) – are coordinated by the International Breast Cancer Study Group and together randomized more than 5,000 premenopausal women with early hormone receptor–positive breast cancer to 5 years of various types of adjuvant endocrine therapy. Their initial results, reported several years ago, form part of treatment guidelines that are used worldwide.

In an updated joint analysis of TEXT and SOFT, benefit of the aromatase inhibitor (AI) exemestane plus ovarian function suppression (OFS) over the selective estrogen-receptor modulator tamoxifen plus OFS was sustained, with a 4.0% absolute gain in 8-year disease-free survival, investigators reported at the San Antonio Breast Cancer Symposium. And in an updated analysis of SOFT, the benefit of tamoxifen plus OFS over tamoxifen alone became significant, with a 4.2% absolute gain in 8-year disease-free survival.

Relative benefits for various outcomes were generally similar across subgroups, but absolute benefits were greater for women having certain features increasing risk for poor outcomes.

Clinical implications

These updates, along with other emerging data, can be used to optimize endocrine therapy for younger women with breast cancer, according to invited discussant Ann H. Partridge, MD, of Dana Farber Cancer Institute in Boston.

“For higher-risk disease, we should be considering OFS. At this point in time, I don’t think HER2 status alone should drive this decision,” she commented. “If you are getting OFS, what do we do, AI versus tamoxifen? Well, we do see a large improvement in disease-free survival [with AIs], so many women will want to use AIs. Yet tamoxifen is still reasonable, especially in light of the survival data.”

Data on switch strategies and extended-duration therapy are generally lacking at present for the premenopausal population, Dr. Partridge noted. “That’s something that we still need to extrapolate from data that’s predominantly in postmenopausal women.”

Another compelling question is whether OFS can be used instead of chemo for some patients. “We are increasingly recognizing that women with higher-risk anatomy and lower-risk biology having endocrine-responsive tumors may get more bang for the buck from the optimizing of hormonal therapy, and chemo may not add much,” she said.

Both short- and long-term toxicities of the various endocrine therapies and, for aromatase inhibitors, the potential for breakthrough (return of estradiol levels to premenopausal levels) also need to be considered, Dr. Partridge stressed. “And ultimately, patient preference and tolerance are key. After all, the best treatment is the one the patient will take.”

“We need to follow these women on TEXT and SOFT very long term. It would be a crime not to follow these women further out,” she maintained. “We need to conduct real-world comparative effectiveness research to understand the risks and benefits of OFS more fully in our survivors. Then, as we start to suppress more ovaries in more women with breast cancer, we need to be aware clinically of these risks, and we need to share this awareness with their primary care providers because we need to optimize in particular their cardiovascular risk factors, and screen and treat for potential comorbidities that they may be at higher risk for.”

Joint TEXT and SOFT update

Initial results of the joint TEXT and SOFT analysis, reported after a median follow-up of 5.7 years, showed that exemestane plus OFS was superior to tamoxifen plus OFS for the primary outcome, providing a significant 3.8% absolute gain in 5-year disease-free survival (N Engl J Med. 2014;371:107-18).

The updated joint analysis, now with a median follow-up of 9 years and based on data from 4,690 women, showed that the 8-year rate of disease-free survival was 86.8% with exemestane plus OFS versus 82.8% with tamoxifen plus OFS (hazard ratio, 0.77; P = .0006), for a similar absolute benefit of 4.0%, reported Prudence Francis, MD, of the University of Melbourne, head of Medical Oncology in the Breast Service at the Peter MacCallum Cancer Centre, Melbourne.

In stratified analysis, absolute benefit tended to be greater among women in TEXT who received chemotherapy (6.0%); intermediate among women in TEXT who did not receive chemotherapy (3.7%) and women in SOFT who received prior chemotherapy (3.7%); and less among women in SOFT who did not receive chemotherapy (1.9%).

Exemestane plus OFS was also superior to tamoxifen plus OFS in terms of breast cancer–free interval, with an absolute 4.1% benefit (P = .0002), and distant recurrence–free interval, with an absolute 2.1% benefit (P = .02). Overall survival did not differ significantly between arms.

Among the 86% of patients with HER2-negative disease, exemestane plus OFS netted an absolute disease-free survival gain of 5.4% and an absolute distant recurrence–free interval gain of 3.4%. There was a consistent relative treatment benefit across subgroups, but larger absolute benefit, on the order of 5%-9%, in women given chemotherapy and in those younger than 35 years.

“Results for the HER2-positive subgroup require further investigation,” Dr. Francis said. “The trials enrolled both before and after the routine use of adjuvant trastuzumab, and a significant proportion of the patients with HER2-positive breast cancer did not receive adjuvant HER2-targeted therapy.”

In the entire joint-analysis population, exemestane plus OFS was associated with higher rates of musculoskeletal events of grade 3 or 4 (11% vs. 6%) and osteoporosis of grade 2-4 (15% vs. 7%), while tamoxifen plus OFS was associated with a higher rate of thrombosis/embolism of grade 2-4 (2.3% vs. 1.2%) and more cases of endometrial cancer (9 vs. 4 cases). At 4 years, early discontinuation of oral endocrine therapy was greater for exemestane than for tamoxifen (25% vs. 19%).

“After longer follow-up, with a median of 9 years, the combined analysis results confirm a statistically significant improvement in disease outcomes with exemestane plus ovarian suppression. As is critical given the long natural history of estrogen receptor–positive breast cancer, follow-up in these trials is currently continuing,” Dr. Francis summarized.

“To optimally translate the observed absolute trial improvements into clinical practice, oncologists need to discuss and weigh the potential benefits and toxicities in each individual patient who is premenopausal with hormone receptor–positive breast cancer,” she recommended.

Session attendee Hope S. Rugo, MD, of the University of California, San Francisco, Helen Diller Family Comprehensive Cancer Center, noted that exemestane had superior benefit despite the 25% rate of early discontinuation. “I wonder if one of the interpretations of that, given the toxicity of this therapy for very young women, is that we need some but maybe not so much. Maybe they don’t need 5 years altogether,” she said.

“The fact that they stopped their assigned endocrine therapy doesn’t mean that they didn’t continue any therapy. They may have switched over to tamoxifen or they may have decided they wanted to have a baby or there may have been many other things,” Dr. Francis replied, noting that analyses sorting out the reasons for early discontinuation are planned.

Session attendee Mark E. Sherman, MD, of the Mayo Clinic, Jacksonville, Fla., asked, “Do you have any ability to test for tamoxifen metabolites? It’s possible that a third to a half of patients got reduced benefit from that drug.”

Banked samples are available and a substudy is planned, according to Dr. Francis. “We haven’t got data on that yet, but yes, we are analyzing that.”

SOFT update

Initial results of the SOFT trial, reported after a median follow-up of 5.6 years, showed that adding OFS to tamoxifen did not significantly improve disease-free survival over tamoxifen alone in the entire trial population (N Engl J Med. 2015;372:436-46). However, there was benefit for women who received chemotherapy and remained premenopausal.

The updated SOFT analysis, now with a median follow-up of 8 years, focused mainly on the 1,018 women given tamoxifen alone and the 1,015 women given tamoxifen plus OFS. (Another 1,014 women were given exemestane plus OFS.)

“SOFT is now positive for its primary endpoint,” reported first author Gini Fleming, MD, director of the Medical Oncology Breast Program and medical oncology director of Gynecologic Oncology at University of Chicago Medicine. The 8-year disease-free survival rate was 83.2% with tamoxifen plus OFS, compared with 78.9% with tamoxifen alone (HR, 0.76; P = .009), corresponding to a 4.2% gain in this outcome. The relative benefit was identical whether patients had received chemotherapy or not, but absolute benefit was greater for those who had (5.3%), as well as for patients younger than 35 years (8.7%).

In addition, exemestane plus OFS was superior to tamoxifen alone (85.9% vs. 78.9%; HR, 0.65), with an absolute benefit of 7.0%. Again, absolute benefit was more pronounced among women who had received prior chemotherapy (9.0%) or were younger than 35 years (13.1%).

The relative disease-free survival benefit of tamoxifen plus OFS over tamoxifen alone was similar across most subgroups stratified by disease characteristics, but patients with HER2-positive disease derived greater relative benefit from the combination as compared with their HER2-negative counterparts (P = .04 for interaction). “When we look at the combination of exemestane plus OFS versus tamoxifen, this heterogeneity is no longer seen,” Dr. Fleming noted.

In the entire trial population, there was no significant benefit of tamoxifen plus OFS over tamoxifen alone for distant recurrence-free interval and a small, significant absolute 1.9% gain in overall survival.

“The cohort who had elected to receive no prior chemotherapy did exceedingly well regardless of therapy,” she said, with little difference in overall survival across the three arms. “There were only 24 deaths total in this cohort, and 12 of those deaths were in the setting of no distant recurrence.”

On the other hand, among the women who received chemotherapy, there were significant absolute overall survival benefits of 4.3% with tamoxifen plus OFS and 2.1% with exemestane plus OFS, over tamoxifen alone. “This late emergence of an overall survival benefit is consistent with the time course of events in estrogen receptor–positive breast cancer,” Dr. Fleming commented.

The proportion of patients who stopped their oral endocrine therapy early was 22.5% with tamoxifen alone and 18.5% with tamoxifen plus OFS. (It was 27.8% with exemestane plus OFS.) “Almost a quarter of the patients on either tamoxifen arm were using extended oral endocrine therapy at 6 years or later prior to any disease progression. Only about 12% of patients in the exemestane group were doing so,” she noted.

There were more cases of endometrial cancer with tamoxifen alone than with tamoxifen plus OFS (7 vs. 4 cases). Thrombosis/embolism of grade 2-4 occurred in 2.2% of each group (and 0.9% of the exemestane plus OFS group). Musculoskeletal symptoms of grade 3 or 4 occurred in 6.7% of patients with tamoxifen alone and 5.9% with tamoxifen plus OFS, but 12.0% with exemestane plus OFS. Respective rates of osteoporosis grade 2-4 were 3.9%, 6.1%, and 11.9%.

“The addition of OFS to tamoxifen significantly improves disease-free survival at 8 years’ median follow-up, and disease-free survival benefits are further improved by the use of exemestane plus OFS,” Dr. Fleming summarized. “Follow-up, which is critically important given the long natural history of ER-positive disease, continues.”

Session attendee Matthew P. Goetz, MD, of the Mayo Clinic, Rochester, Minn., commented, “For the primary endpoint, I was looking at the tail for tamoxifen. It seemed that there was a relatively rapid drop-off between year 5 and this 8-year follow-up. Have you looked carefully to see whether there is a difference between those who stayed on their therapy versus those who went off it per protocol? That is, extended versus not extended? The question is whether there is a carry-over effect, if you will, that is different in those with OFS versus those not.”

“The percent who went on to extended therapy between the tamoxifen and the tamoxifen plus OFS was fairly similar,” Dr. Fleming replied. “But the answer is no, we have not yet done any sort of per protocol analysis.”

Session attendee Steven Vogl, MD, of Montefiore Medical Center, New York, commented, “I worry about your control group. I’m worried, first, how many of your tamoxifen patients lost their menses and became postmenopausal in those 5 years? And of those, why didn’t they switch to an aromatase inhibitor? Only 25% of the patients continued after the 5 years according to your slide, and all of those patients should either have stayed on tamoxifen or switched to an aromatase inhibitor, now probably for 2 years at least.”

“We have not yet looked at data for who became amenorrheic during treatment, although we have it. However, it’s certainly possible to become amenorrheic on tamoxifen and not be postmenopausal, and we didn’t regularly collect estradiol levels on any but the very small subset of women in the SOFT-S trial. So I don’t know that we have exactly the data that you’re looking for,” Dr. Fleming said. “Many of these women are obviously at very, very low risk and have done well with 5 years of tamoxifen alone, and I don’t know, even given current guidelines, that extended tamoxifen would add a lot to that.”

Finally, session attendee Richard Gray, professor of medical statistics at the University of Oxford (England), wondered, “What is the certainty that follow-up will happen? Because obviously, prolonged follow-up is expensive and there are controversies about that. But this would be the one study you would really want to have 15- and 20-year data on.”

“We are working very, very hard on that,” Dr. Fleming replied. “NCI granted additional funds to institutions for prolonging follow-up, and IBCSG has been ceaselessly working to look for funding to continue it. So I think it’s relatively certain that it will happen.”

Dr. Francis disclosed that she has received fees for non-CME services from AstraZeneca and has given an overseas lecture for Pfizer. Dr. Fleming disclosed that she had no relevant financial relationships with commercial interests. The trials received financial support from Pfizer and Ipsen.

SOURCES: Francis et al. SABCS Abstract GS4-02 ; Fleming et al. SABCS Abstract GS4-03

op@frontlinemedcom.com

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