As I noted before, there are a large number of options in endocrine therapy, but one of the most promising is abemaciclib, a selective CDK inhibitor, that just completed FDA Fast Track and was approved early today (28 Sept. 2017) and commercially available for prescription under the name VERZENIO, and has shown, when the data is intensively mined as I have done, some of the best survival outcomes to data of any agent (even compared to non-endocrine targeted and chemotherapeutic approaches). Here’s my take on this extraordinary agent, and why:
Preliminary data from a phase I study of abemaciclib [SABCS 2014] in patients with five different tumor types including HR-positive metastatic breast cancer, with I note a median of seven prior systemic therapies (outliers out to 11 lines) found an objective response rate of 19% and a disease control rate (including stable disease (SD) under RECIST definition of >24 weeks of ) of 81% for HR-positive MBC patients.
This Phase I study was expanded to evaluate abemaciclib plus fulvestrant in HR-positive MBC [ASCO 2014.] with patients having a median of four prior systemic therapies (and outliers out to the eight line), finding an objective response rate of 62% confirmed PRs.
A subsequent Phase I study [Tolaney et al, J Clin Oncol 2015;33(Suppl 1).] of abemaciclib in combination with different endocrine therapies for HR-positive MBC (median lines of treatment = 3, with outliers out to 8) demonstrated a disease control rate (CR + PR + SD) was 67% for those on abemaciclib + AI therapy, and 75% for those on abemaciclib + tamoxifen.
The phase II MONARCH-1 trial evaluated abemaciclib MONOTHERAPY in HR-positive, HER2-negative MBC patients with a median of 3 lines of prior therapy for advanced disease, including a median of 2 lines of chemotherapy (as opposed to endocrine or biological therapy) for advanced disease, yielding an objective response rate (ORR) of 17.4% and a clinical benefit rate (CBR) of 42.4%. What's important to remember about MONARCH-1 is that many women in the study had already received multiple lines of endocrine treatment (median 3 - 5 previous lines in the metastatic setting, with outliers beyond that), developed resistance and were refractory to all of these treatments, AND progressed, many of whom also received chemotherapy, and these results were for MONOTHERAPY in a heavily pretreated population, which as I determined through examination of historical controls with any other agent achieves at best only 10 to 20% clinical benefit rates - half of that achieved by single-agent abemaciclib, and with responses of short duration.
Hence, the collective results of these trial, including MONARCH 1 (MONARCH-2 (with letrozole) and MONARCH-3 (with fulvestrant) are exceptionally exciting breakthrough in breast cancer therapeutics.
In addition, for me, the overall survival (OS) of these heavily pretreated MBC patients was 17 months, and that compares to historical control I review from all clinical trials to date, as significantly better than other survival results, which are usually are at best 13 months. Thus, consider that for the current champ, eribulin chemotherapy in this setting, the EMBRACE trial achieved a median overall survival (OS) was 13 months, we clearly have here a major advance and improvement in overall survival in the highly challenging later-line settings (past at least 3 lines of therapy).
The major limiting constraint with all CDK4/6 inhibitors to date, like palbociclib (Ibrance), is their most common adverse effect, that of myelotoxicity via neutropenia (low WBC neutrophils), with palbociclib (Ibrance inducing some degree of neutropenia in over half of women on it, with associated dose interruptions and sometimes dose reductions, a toxicity that so fat from the available data seems to also afflict ribociclib.
But abemaciclib (Verzenio) evades a good deal of this because although strictly a CDK4/6 inhibitor, it is more specific in inhibiting CDK4, that is it is CDK4-selective, and myelotoxicity including neutropenia is primarily associated with CDK6, making the incidence of neutropenia on abemaciclib significantly lower and more manageable.
CLINICAL LESSONS ON ABEMACICLIB (VERZENIO)
And unlike palbociclib (Ibrance):
- abemaciclib has strong single-agent activity in HR–positive MBC,
- has durable response in later line settings,
- a good tolerability profile with reduced neutropenia, and
- to date the highest OS in these challenging heavily pretreated settings of any oncotherapeutic agent, whether endocrine therapy, chemotherapy, or biological therapy.
Director, Medical Research, No Surrender Breast Cancer Foundation (NSBCF)Oncology Reviewer, Current Oncology [journal]Society for Integrative Oncology (SIO) European Association for Cancer Research (EACR)