Who is this trial for?
This trial was for adults with hormone-receptor-positive (HR+), HER2-positive metastatic breast cancer whose cancer had not grown during or after first-line chemotherapy combined with HER2-targeted therapy.
People in the trial had:
- HR+/HER2+ metastatic breast cancer — meaning the cancer cells have receptors for hormones (estrogen or progesterone) AND an excess of the HER2 protein.
- No disease progression after 4–8 cycles of first-line treatment: chemotherapy (a taxane) plus HER2-targeted therapy (trastuzumab and pertuzumab).
- Ability to go on to maintenance (long-term) therapy.
What kind of trial is this?
This trial tested whether adding palbociclib to standard maintenance treatment could extend the time before cancer progressed in people with HR+/HER2+ metastatic breast cancer.
Background: why researchers asked this question
In HR+/HER2+ metastatic breast cancer, the standard first-line treatment is chemotherapy plus dual HER2-targeted therapy (trastuzumab and pertuzumab). For patients who respond, treatment typically continues with a “maintenance” approach: HER2-targeted therapy plus endocrine therapy (an anti-hormone pill, such as an aromatase inhibitor or tamoxifen), dropped once the intensive chemotherapy phase ends.
CDK4/6 inhibitors — pills like palbociclib — transformed treatment for HER2-negative HR+ metastatic breast cancer. They block a key driver of cancer cell division, and when added to endocrine therapy, they doubled the time before cancer progressed in that setting.
Researchers hypothesised these drugs might also work in HER2-positive disease. Lab studies suggested CDK4/6 inhibitors could help overcome resistance to HER2-targeted therapy. PATINA tested this by adding palbociclib to standard maintenance therapy after first-line chemotherapy.
The trial: what was tested and how
PATINA enrolled 518 people with HR+/HER2+ metastatic breast cancer who had received 4–8 cycles of first-line chemotherapy plus HER2-targeted therapy without the cancer progressing. They were randomly assigned 1:1 to maintenance therapy:
- Palbociclib + anti-HER2 therapy + endocrine therapy: palbociclib (orally, 21 days on / 7 days off per cycle) added to trastuzumab (with or without pertuzumab) plus an aromatase inhibitor or tamoxifen.
- Standard maintenance: anti-HER2 therapy (trastuzumab ± pertuzumab) plus endocrine therapy alone, without palbociclib.
The primary measure was progression-free survival (PFS) — how long before the cancer grew or the person died. The trial was open-label (participants and doctors knew which treatment was being given).
A note before the results
Alice was diagnosed with breast cancer about eight months ago — a rapidly growing mass in her left breast, with a bone scan revealing a few spots in her spine and pelvis. Her biopsy showed a cancer that was both hormone-receptor-positive and HER2-positive. She took chemotherapy with trastuzumab and pertuzumab like a champ, but after about six months, as fatigue and numbness and tingling in her fingers and toes worsened, it was time to stop the chemotherapy. The antibodies alone — trastuzumab and pertuzumab — were easy to tolerate, and we discussed adding an estrogen blocker, a simple pill, to help keep the cancer controlled while her hair grew back and she felt like herself again. This trial asked whether adding one more easy treatment, palbociclib, could help her stay in remission longer — and feel normal for longer — before tougher chemotherapy was needed again.
Results: what they found
Adding palbociclib to maintenance therapy meaningfully extended the time before cancer progressed — by more than 15 months on average — though this came with substantially more side effects.
Palbociclib + anti-HER2 + endocrine vs anti-HER2 + endocrine alone
On average, the cancer stayed controlled for more than 15 months longer in the palbociclib group (hazard ratio 0.75 — a 25% lower risk of the cancer growing or the person dying at any given time).
Serious side effects were far more common with palbociclib, mainly due to neutropenia (low white blood cell counts). This is an expected effect of CDK4/6 inhibitors and is usually managed with dose adjustments and monitoring rather than being medically dangerous.
The most severe (grade 4) side effects were also more common with palbociclib, again driven mainly by neutropenia.
Overall survival data were not yet mature at the time of this analysis. Whether palbociclib also extends life in this setting remains to be seen.
Is neutropenia from palbociclib dangerous?
Neutropenia from CDK4/6 inhibitors like palbociclib is different from neutropenia caused by traditional chemotherapy. It is usually less likely to cause serious infections, because palbociclib works at a different stage of cell development and immune function is less severely impaired. It is managed with blood count monitoring and temporary dose holds or reductions. Your oncologist and team will monitor this closely throughout treatment.
What do “grades” mean when describing side effects?
Doctors grade side effects on a scale from 1 to 5 based on severity. Grade 1 is mild and barely noticeable; grade 2 is moderate and may need some treatment; grade 3 is severe and usually requires medical care or a change in treatment; grade 4 is life-threatening and requires urgent care; grade 5 refers to a death related to the side effect. Most side effects in this trial were grade 1–2 and didn’t require any change in treatment. Grade 3–4 side effects are taken seriously and closely monitored, but as explained above, many — like the neutropenia seen here — are manageable with dose adjustments rather than being medically dangerous.
The bottom line
For HR+/HER2+ metastatic breast cancer that responded to first-line treatment, adding palbociclib to maintenance therapy extended the time before the cancer progressed by more than 15 months (44.3 vs 29.1 months, HR 0.75). This is a meaningful benefit in a metastatic setting. However, serious side effects — mainly manageable neutropenia — were substantially more common. Overall survival data are awaited.
What this could mean for you
- This applies after first-line treatment has worked. Patients are eligible after responding to (not progressing on) first-line chemotherapy + HER2 therapy.
- Side effects are real but typically manageable. Neutropenia is common; blood counts are checked regularly and doses are adjusted as needed.
- Survival data are still pending. Whether living longer (not just controlling cancer for longer) is also improved remains to be confirmed.
- CDK4/6 inhibitors require monitoring. Regular clinic visits and blood tests are part of the commitment to this therapy.
An oncologist’s perspective
This trial gives women like Alice a great option, since the addition of palbociclib to endocrine therapy is easy to tolerate, and is really the only time that we focus attention on the estrogen-sensitive portion of their cancers. The results of this trial are likely applicable to newer antibody-drug conjugates that are replacing traditional chemotherapy in first-line treatment, since even those drugs are tough, and when women need a break from them, the standard maintenance antibodies pertuzumab and trastuzumab will be offered. We eagerly await final results of this trial to hopefully confirm that the meaningful benefits described above also translate to a longer survival with triple positive breast cancer.
Questions & comments
Have a question about this trial? Ask below. Questions are read and answered by the site. We can’t give personal medical advice, but we’re glad to explain the research more clearly.