Expert Shares How CDK 4/6 Inhibitors Broke the Mold in HR+/HER2- Breast Cancer

Richard S. Finn, MD

A trio of CDK 4/6 inhibitors—palbociclib (Ibrance), ribociclib (Kisqali), and abemaciclib (Verzenio)—have continued to revolutionize the field of breast cancer, explained Richard S. Finn, MD.

Palbociclib was approved by the FDA in February 2016 for use in combination with fulvestrant (Faslodex) in pretreated patients with hormone receptor (HR)-positive, HER2-negative metastatic breast cancer, based on results from the phase III PALOMA-3 trial. Palbociclib was initially granted approval by the FDA in February 2015 for use in combination with letrozole as a frontline treatment for postmenopausal women with estrogen receptor (ER)-positive, HER2-negative metastatic breast cancer.

The FDA approved ribociclib in March 2017 for use in combination with an aromatase inhibitor (AI) for the frontline treatment of postmenopausal women with HR-positive, HER2-negative advanced breast cancer. This decision was based on findings from the phase III MONALEESA-2 trial.

Finally, in September 2017, the FDA approved abemaciclib for use in combination with fulvestrant in women with HR-positive/HER2-negative advanced breast cancer with disease progression following endocrine therapy; the basis for the approval came from the MONARCH 2 trial. Abemaciclib was also approved as a single agent for patients with HR-positive/HER2-negative disease with metastatic disease who have previously received endocrine therapy and chemotherapy. The single-agent findings of which were considered from the MONARCH 1 trial.

All 3 CDK 4/6 inhibitors have demonstrated comparable levels of efficacy, but there are subtle mechanistic and toxicity differences, Finn added.

OncLive: Can you share the history of CDK 4/6 inhibitor development in breast cancer?

In an interview with OncLive, Finn, an associate professor of medicine at the Geffen School of Medicine at the University of California, Los Angeles, covers the evolution of CDK 4/6 inhibition in the field of breast cancer and appropriate patient selection for this type of therapy.Finn: The field has changed very quickly in a short period of time after decades of, really, no real novel approaches in first-line ER-positive breast cancer—let alone hormone-resistant breast cancer. We now have 3 CDK 4/6 inhibitors approved. Several years ago [we were] talking about CDK 4/6 inhibitors in a time where we only had very early data with palbociclib, which came out of preclinical work done by Dr Dennis Slamon and myself in collaboration with Pfizer. That led to the PALOMA-1 trial and, eventually, the accelerated approval of palbociclib and letrozole.

Now, we have a phase III study with ribociclib and letrozole in a similar population and phase III data with palbociclib with letrozole. Recently, at the 2017 ESMO Congress, we saw the data with abemaciclib and nonsteroidal aromatase inhibitors. All of these results, reassuringly, are very similar as far as efficacy. While there is some overlap with the toxicities, each molecule has a few unique aspects.

With these 3 CDK 4/6 inhibitors available, how is a patient selected to receive one regimen over the others?

Now, with fulvestrant in patients who are felt to be resistant to nonsteroidal AIs, we have data with palbociclib and fulvestrant from PALOMA-3—as well as data with abemaciclib and fulvestrant from their phase III study—the MONARCH studies. There are now 6 randomized studies—many of them reading out only in the past year or year and a half—with CDK 4/6 inhibitors and endocrine therapy. The landscape has changed very dramatically. As far as efficacy [goes], these combinations all seem very similar. The frontline regimens have similar efficacy, the second-line regimens have similar efficacy and, only recently, we now have abemaciclib approved as a single agent in patients who have never had a CDK 4/6 inhibitor in heavily pretreated patients from the single-agent MONARCH 1 study.

Therefore, we have not been able to identify a biomarker or a patient population that does not benefit from the addition of a CDK 4/6 inhibitor to endocrine therapy—whether that’s a nonsteroidal AI or fulvestrant. Somewhere, during the course of treatment, I would argue that a patient needs to get a CDK4/6 inhibitor. My bias is that, perhaps, the largest magnitude of benefit—this 10-month improvement we see in the frontline setting [with a CDK 4/6 inhibitor]—is maybe the place to get the greatest benefit.

Then, as far as distinguishing them on toxicities, there are probably some unique places where you would go to one versus the other. Ribociclib requires some monitoring of QT interval of patients who are on drugs known to create QT, or have some arrhythmia that is related to prolonged QT. It might not be your first choice.

What is the approach when a patient progression on a CDK4/6 inhibitor?

Could any of the CDK 4/6 inhibitors have potential in combination with immunotherapy?

There is deep vein thrombosis [DVT]-association that came out with abemaciclib. There was an increased risk of DVT in their phase III study with a nonsteroidal AI. All of [the CDK 4/6 inhibitors] cause neutropenia to some degree—less so with abemaciclib, but abemaciclib has more gastrointestinal toxicities. Therefore, there are subtle things. As far as efficacy, they are all fairly similar, as far as we can tell. There are some subtle toxicity differences that a clinician may take into account. Access might be an issue for some people. We would all like to see that, if we have 3 drugs competing in the same space, then the price should come down. But that is yet to be seen. There is a big interest in understanding acquired resistance to these drugs. What do you do after you progress on a CDK 4&6 inhibitor? All of the data we have with single-agent fulvestrant, exemestane, and exemestane with everolimus [Afinitor] were all generated in the pre-CDK 4/6 era. Therefore, we need some sense of how these patients do and that will probably come [more] from the phase III studies. We had some data from that on PALOMA-1—the phase II study. There is interest now in maybe targeting other CDK 4/6 inhibitors to overcome resistance. It’s an area that has been looked at for a while. It has been limited a little bit by medicinal chemistry, but there is a lot still in play. There is a strong rationale there. There has been a lot of data about the effect of CDK 4/6 on immune cells—not only just on the PD-L1 pathway within tumor cells and PD-1 expression or non-expression—but also CDK 4/6 activating the immune system for cancer. There was a scientific publication in the last few months that highlighted [this] as a potential avenue to go down.