Human breast cancer cells.

Breast cancer cells.

Researchers have presented ‘striking’ data showing the power of a targeted drug to slow breast cancer growth.

This data is nothing short of phenomenal and will be practice changing.

– Dr Sara Hurvitz, study author

The breakthrough drug works by delivering high concentrations of chemotherapy directly to cancer cells that have a particular protein (HER2) on their surface. It’s been trialled for women with advanced HER2-positive breast cancer.

Patients taking the drug – trastuzumab deruxecan – were 72% less likely to see their cancer grow significantly or to die than those taking an existing breast cancer treatment.

“This exciting work is likely to change clinical practice and offer real benefits for patients with HER2-positive breast cancer,” commented Professor Charles Swanton, Cancer Research UK’s chief clinician.

‘Phenomenal’ data

The latest results from the DESTINY-Breast03 trial were presented at the European Society for Medical Oncology (ESMO) conference. It’s the first phase 3 trial to compare trastuzumab deruxecan to a standard therapy in people with advanced breast cancer.

Almost 80% of patients taking trastuzumab deruxecan saw their tumour shrink by more than a third, compared with 34% of those taking trastuzumab emtansine, a standard breast cancer treatment.

After 12 months, 76% of people taking the drug – trastuzumab deruxecan – had shown no disease progression. Only 34% of patients treated with trastuzumab emtansine saw the same results.

Swanton commented that HER2 positive breast cancers often become treatment resistant, so the fact that three quarters of patients on the new treatment saw no cancer progression after a year represents a significant success.

“This is a great example of how scientific progress made over the last three decades has resulted in a direct improvement for people affected by cancer.”

It’s too soon to know if trastuzumab deruxecan can improve survival compared with standard treatments, but at one year 94% of those taking the drug were alive, compared with 86% of those taking trastuzumab emtansine.

Both drugs combine a drug that targets the HER2 protein (trastuzumab) and a chemotherapy drug, delivering toxic doses of chemotherapy directly to cancer cells.

According to AstraZeneca, trastuzumab deruxecan delivers more than twice as much chemotherapy drug per trastuzumab than trastuzumb emtansine.

Changing the outlook for women with breast cancer

DESTINY-Breast03 compared the two drugs in over 500 patients with HER2 positive breast cancer that had spread to other parts of the body (metastatic breast cancer). Everyone taking part had already been treated with trastuzumab (Herceptin) and chemotherapy, but had seen their cancer progress despite treatment.

Dr Sara Hurvitz, senior author based at University of California Los Angeles (UCLA) said that since trastuzumab emtansine became a standard second-line treatment option almost a decade ago, no other therapy has gone up against it in a head-to-head trial. “Seeing a new therapy demonstrate such a substantial improvement in progression free survival is really exciting for our patients.”

Other experts agree. “The improvement in disease free survival seen in this study is very impressive,” added Professor David Cameron, a breast cancer expert at the University of Edinburgh. “And all subgroups of patients appear to benefit similarly which is important.”

Cameron added that the toxicity is also much more manageable now thanks to experience gained in recent years. Treatment-related lung disease was observed in around 10% of patients taking trastuzumab deruxecan, but most cases were less severe.

“I suspect this drug will be an important development for patients with HER2 positive breast cancer, adding to the growing number of drugs that are changing the outlook for these patients.”

AstraZeneca, the company that makes trastuzumab deruxtecan, is also studying if the drug could be used earlier for women with HER2 positive breast cancer and if it could be effective against other cancers, including HER2 positive lung tumours.

DESTINY-Breast03 was one of many trials that announced results at this year’s ESMO conference. Other highlights include data showing that combining existing drugs can improve prostate cancer survival and that a new drug could slow tumour regrowth in inoperable bowel cancer.

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