Diabetes Threatens Long-Term Survival in Metastatic Breast Cancer

For metastatic breast cancer patients, diabetes appeared to hurt outcomes only among long-term survivors, researchers reported.

In a retrospective study of people with metastatic breast cancer, diabetes wasn't associated with any increase in mortality over a 5-year follow-up (54% vs 56% without diabetes), according to Y.M. Melody Cheung, MBBS, of Brigham and Women's Hospital in Boston, and colleagues.

Time to next treatment for cancer was likewise similar, with 56% of both groups trying another treatment 1 year later, Cheung reported in Atlanta at ENDO 2022, the annual meeting of the Endocrine Society.

"These data provide some reassurance that in the first 5 years, glycemic control may not be a major contributor to overall mortality or cancer progression in most individuals with metastatic breast cancer," Cheung explained during a press conference.

Subgroup analyses showed that overall survival at year 5 also wasn't different between groups with good glycemic control (an HbA1c of 7% or less) compared with those with poor glycemic control (HbA1c above 7%). A total of 69% of both of these groups survived to year 5.

Likewise, 58% and 53% or the good glycemic and poor glycemic controls groups tried another treatment by year 1, respectively.

However, when measured according to a random blood glucose (RBG) test, there was a trend towards poorer survival seen with poorer glucose control at year 5:

• 55% with good glycemic control (median RBG ≤180 mg/dL)
• 48% with poor glycemic control (median RBG 180-200 mg/dL)
• 23% with very poor glycemic control (median RBG 200+ mg/dL)

The same trend emerged when it came to time to next treatment, with 57%, 47%, and 27% of the good, poor, and very poor glycemic control groups having tried another treatment by year 1.

Interestingly, certain differences began to emerge among long-term survivors.

In a landmark analysis of patients with metastatic breast cancer who survived over 8 years, diabetes was significantly associated with poorer survival. At year 10, 67% of those with diabetes survived versus 87% of patients free of diabetes (P=0.047). Likewise, 83% of those with good glycemic control (RBG ≤180 mg/dL) achieved 10-year survival versus 63% of those with poor glycemic control (RBG >180 mg/dL).

"As the clinical impact of diabetes may only be felt over this longer period of time, active management of glycemic control may be most relevant for patients who have better prognoses," Cheung suggested.

"Individualized diabetes goals and strategies -- taking into account patient prognosis -- should therefore be considered in patients with metastatic breast cancer," she added.

The analysis included 244 patients with diabetes and 244 in the control group, who appeared in institution databases from Brigham and Women's Hospital and Dana-Farber Cancer Institute. Case-control pairs were matched 1:1 based on age, sex, and ethnicity.

The average age of participants was 57, 99.6% were female, and 84% were white. The majority had HER2-/HR+ metastatic breast cancer (62.5%), following by triple-negative breast cancer (16.4%), HER2+/HR+ (11.7%), HER2+/HR- (7.6%), and unknown receptors status (1.9%).

As for diabetes type, half of patients with diabetes had type 2 (49%), followed by an unknown type (33%), drug-associated (15%), and type 1 diabetes (1%). Among patients with diabetes, median HbA1c was 6.6% and median RBG was 123 mg/dL.

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