Pooled SURPASS-5 and SURPASS-6 analysis supports adding tirzepatide to basal insulin

Researchers pooled data from 1,072 people with type 2 diabetes who took part in two clinical trials, SURPASS-5 and SURPASS-6, to see how adding tirzepatide to basal insulin performed across different types of patients ^[1]. The new analysis, a post hoc review published in a peer-reviewed diabetes journal, found that tirzepatide reduced HbA1c, fasting blood sugar, and the daily dose of insulin glargine needed, no matter a patient's age, how long they had lived with diabetes, their starting HbA1c, or how much insulin they were already using ^[1].

In SURPASS-5, tirzepatide was compared against a placebo added to insulin glargine, while in SURPASS-6 it was compared against insulin lispro added to the same basal insulin ^[1]. Both trials titrated insulin doses toward a target during the study ^[1]. The researchers split participants into subgroups: those with baseline HbA1c at or below 8.5% versus above it, those younger than 65 versus 65 and older, those with diabetes for less than 10 years versus 10 years or more, and those using less than 50 international units of insulin glargine per day versus 50 or more ^[1].

Across every one of these subgroups, tirzepatide's effect on HbA1c, fasting glucose, and insulin dose reduction was statistically significant, with p-values below 0.001 ^[1]. Importantly, the researchers found no significant differences in how well the drug worked between subgroups — a statistical result called "no significant heterogeneity," meaning the benefit held up consistently whether someone was older or younger, newly diagnosed or long-established, or on a low or high insulin dose ^[1]. Rates of clinically significant hypoglycemia, meaning dangerously low blood sugar, were described as low overall, though they were highest among younger participants who started with HbA1c above 8.5% and had lived with type 2 diabetes for 10 years or more ^[1].

Why it matters for patients

Many people with type 2 diabetes who use basal insulin still don't reach their blood sugar targets, and doctors often look for an add-on therapy rather than switching insulin regimens entirely ^[1]. This analysis suggests that adding tirzepatide, sold under the brand name Mounjaro for diabetes, works in a fairly predictable way across a wide range of patient profiles rather than only helping certain groups ^[1]. That kind of consistency can help patients and doctors have more confidence discussing whether this combination fits a particular treatment plan.

The finding that clinically significant hypoglycemia was more common in younger patients with higher starting HbA1c and longer-standing diabetes is a practical detail worth knowing, since insulin combinations always carry some risk of blood sugar dropping too low ^[1]. The study authors describe the overall hypoglycemia rate as low, but they do not report exact percentages in the portions of the abstract available here, so readers should not assume the risk is zero ^[1].

It is also worth noting this is a post hoc, exploratory analysis — meaning researchers looked back at existing trial data to ask a new question, rather than running a new randomized trial designed specifically to test these subgroups ^[1]. Several of the paper's authors are employees or shareholders of Eli Lilly and Company, which markets tirzepatide, and another author has received trial fees or consulting income from multiple drug makers including Lilly, a detail disclosed in the paper's conflict of interest statement ^[1].

What happens next

The original SURPASS-5 and SURPASS-6 trials are registered with the identifiers NCT04039503 and NCT04537923 ^[1]. The pooled subgroup analysis has now been published, but the sources reviewed here do not describe any new trials planned specifically to confirm these subgroup findings, so what regulatory or clinical practice changes might follow, if any, is not yet known.