At the 2022 San Antonio Breast Cancer Symposium, Ariella Hanker, PhD, UT Southwestern Medical Center, Dallas, Texas, presented on the biological mechanisms which cause CDK4/6 inhibitor resistance as well as novel approaches to overcome those resistances for patients with breast cancer.

Dr Hanker discussed the need for better laboratory models to reflect the resistance mechanisms seen in patients, to better understand CDK4/6 inhibitor resistance at the molecular level, and develop strategies for preventing and overcoming resistance and prolonging patient survival.

Transcript:

My name is Ariella Hanker. I'm an Assistant Professor in the Simmons Comprehensive Cancer Center at UT Southwestern Medical Center in Dallas, Texas. I am at the San Antonio Breast Cancer Symposium, and I gave a talk in the CDK4/6 Forum on CDK4/6 inhibitor resistance, biological mechanisms and novel approaches.

CDK4/6 inhibitors—ribociclib, palbociclib, and abemaciclib—in combination with endocrine therapies have revolutionized the treatment of ER-positive metastatic breast cancer. However, unfortunately this combination is not curative in most cases, so there is a need to understand CDK4/6 inhibitor resistance at the molecular level, to identify novel therapeutic strategies to prevent resistance or to overcome resistance, and to prolong survival even more.

So, what do CDK4 and 6 inhibitors do? Well, CDK4 and CDK6 are cyclin-dependent kinases. In complex with cyclin D1, this complex phosphorylates retinoblastoma tumor suppressor protein (RB) allowing release of the E2F transcription factor and cell cycle progression from the G1 to S6. And this allows cells to divide, and it can allow tumors to grow. So CDK4/6 inhibitors block that RB phosphorylation, blocking cell cycle progression and preventing tumor growth.

In the past few years, both laboratory studies and clinical biomarker studies have identified several different heterogeneous mechanisms of resistance to CDK4/6 Inhibitors. These… function alterations in RB1, which is the direct target of CDK4 and CDK6, also amplification of cyclin E and over expression of CDK6. So those are all cell cycle components. In addition, activation of growth factor signaling pathways such as FGFR1, HER2, KRAS, and loss of P10 can also promote resistance to CDK4/6 inhibitors. Each of these genomic mechanisms of resistance may engender therapeutic vulnerabilities. Loss of the RB tumor suppressor either by loss of function mutations or by deletion, is one of the best-known mechanisms of resistant to CDK4/6 inhibitors. If cells lose RB, blocking CDK 4/6 can no longer block E2F and can no longer block the cell cycle.

Studies have shown that loss of RB may confer enhanced dependency on components of the G2/M checkpoint and spindle assembly checkpoints such as aurora kinase and TTK. Another well-described mechanism of resistance is amplification of cyclin E1 or cyclin E2, which can sustain RB phosphorylation even when CDK4 and CDK6 are blocked. Since cyclin E partners with CDK2, tumors with cyclin E amplification may be particularly sensitive to CDK2 inhibitors, which are currently in clinical development. Genomic activation of growth factor signaling pathways is also well described to promote resistance to CDK4/6 inhibitors in some cases. For example, activating mutations in HER2 can promote resistance and these can be targeted by HER2 inhibitors. The SUMMIT trial showed that in HER2-mutant ER-positive metastatic breast cancers that had progressed on CDK4/6 inhibitors, the combination of HER2 inhibitors, neratinib and trastuzumab, together with fulvestrant was effective in that population.

To further understand CDK4/6 inhibitor resistance, I think we really need better patient derived laboratory models that fully reflect the heterogeneity of resistance mechanisms seen in patients. To address this in the lab, what we did is we collected ER-positive metastatic biopsies from patients following progression on CDK4/6 Inhibitors plus endocrine therapy. We digested these tumors, and we resuspended the digested cells in a Matrigel domes to establish organoid cultures. At this point, we've successfully established 14 CDK4/6-Inhibitor-resistant organoids that we're able to culture long term with about a 50% success rate. Genomic profiling of the match patient biopsy and the organoids showed that in most cases, we were able to see that the driver alterations found in the biopsy were retained in the organoid. Also, our organoids harbored several of the same genetic alterations that have been shown to promote resistance to either CDK4/6 inhibitors and/or endocrine therapy.

In collaboration with Tempus, we performed high throughput 3D drug screening using a fluorescence-based high content imaging approach. We screened 7 of our resistant organoids using 60 clinically relevant inhibitors, and we showed that the organoids again were still resistant to all 3 CDK4/6 inhibitors. As proof of concept, we found that the organoid that had a cyclin E1 amplification remains sensitive to a CDK2, 4 and 6 inhibitor. Again, because it had a cyclin E1 amplification, it may be more sensitive to CDK2 inhibition

There are more genomic and non-genomic mechanisms of resistance that have yet to be discovered. Also, we know that the tumor-immune microenvironment clearly plays a role in response to CDK4/6 inhibitors, but this needs to be better understood and better studied. Better laboratory models will be needed to study this.

Finally, one major challenge is that there can be multiple distinct drivers of resistance all present within a single patient, and these can be especially difficult to target. I think we really need to prevent that from happening in the first place. One approach to do that may be by tracking circulating tumor DNA over time, and when the ctDNA fraction starts to rise, or when a resistance mechanism is identified, therapy can be switched

Clearly, we have our work cut out for us, but I do think it's a very exciting time to be in the field and there is enormous opportunity for translational studies to push biomarker-defined therapeutic strategies into the clinic for patients following progression on CDK4/6 inhibitors.

Source:

Hanker A. “CDK4/6 inhibitor resistance: Biological mechanisms and novel approaches.” Presented at San Antonio Breast Cancer Symposium; December 6-10, 2022; San Antonio, Texas.