SURPASS-1 Trial: Tirzepatide Monotherapy Reduces HbA1c by 2.07% in Drug-Naive Type 2 Diabetes

SURPASS-1: The First Phase 3 Trial of Tirzepatide in Diabetes

The SURPASS-1 trial holds a special place in the tirzepatide story as the first Phase 3 study to evaluate this novel dual GIP/GLP-1 receptor agonist in patients with type 2 diabetes. Published in The Lancet in June 2021 by Rosenstock et al., this trial demonstrated that tirzepatide as monotherapy produced glycemic control and weight loss that exceeded anything previously achieved by a single injectable agent in drug-naive patients [1].

Study Design

SURPASS-1 was a 40-week, double-blind, randomized, placebo-controlled trial conducted across 52 sites in India, Japan, Mexico, and the United States. The study enrolled 478 adults with type 2 diabetes who were either drug-naive or had been treated with diet and exercise alone.

Key Inclusion Criteria:

• Age 18-80 years
• Type 2 diabetes diagnosed ≥6 months prior
• HbA1c 7.0-9.5%
• BMI ≥23 kg/m²
• No prior use of injectable glucose-lowering medications

Participants were randomized 1:1:1:1 to tirzepatide 5 mg, 10 mg, 15 mg, or placebo, administered once weekly by subcutaneous injection. Dose escalation followed the standard 2.5 mg starting dose with 2.5 mg increases every 4 weeks.

Glycemic Control Results

The HbA1c reductions were remarkable for a monotherapy trial:

Mean HbA1c Change from Baseline (8.06% average):

• Tirzepatide 5 mg: -1.87%
• Tirzepatide 10 mg: -1.89%
• Tirzepatide 15 mg: -2.07%
• Placebo: +0.04%

HbA1c Target Achievement:

• HbA1c <7.0%: 87% (5 mg), 92% (10 mg), 88% (15 mg) vs. 20% (placebo)
• HbA1c <6.5%: 82% (5 mg), 81% (10 mg), 86% (15 mg) vs. 10% (placebo)
• HbA1c <5.7% (normal range): 34% (5 mg), 31% (10 mg), 52% (15 mg) vs. 1% (placebo)

The fact that over half of patients on tirzepatide 15 mg achieved a normal HbA1c below 5.7% was extraordinary — this is a level of glycemic normalization rarely seen with any diabetes medication as monotherapy [1].

Weight Loss Results

Weight loss was substantial across all doses:

Mean Body Weight Change:

• Tirzepatide 5 mg: -7.0 kg (-7.0%)
• Tirzepatide 10 mg: -7.8 kg (-7.8%)
• Tirzepatide 15 mg: -9.5 kg (-9.5%)
• Placebo: -0.7 kg (-0.7%)

These weight loss results in a diabetes population were particularly notable because patients with type 2 diabetes typically experience less weight loss with GLP-1 receptor agonists compared to non-diabetic individuals, likely due to the weight-promoting effects of improved glycemic control [1].

Fasting Glucose and Insulin Sensitivity

Beyond HbA1c, tirzepatide demonstrated comprehensive glucose metabolism improvements:

• Fasting plasma glucose: Reduced by 43-59 mg/dL across tirzepatide doses vs. increased by 12 mg/dL with placebo
• Fasting insulin: Decreased significantly, suggesting improved insulin sensitivity rather than simply increased insulin secretion
• HOMA-IR: Improved, indicating reduced insulin resistance
• Fasting C-peptide: Maintained, suggesting preserved beta-cell function

These findings suggest that tirzepatide improves glycemic control through multiple mechanisms: enhancing insulin secretion, improving insulin sensitivity, and reducing body weight [1].

Safety Profile

The safety data were reassuring for a first Phase 3 trial:

Gastrointestinal Events:

• Nausea: 12-18% (tirzepatide) vs. 6% (placebo)
• Diarrhea: 12-14% vs. 8%
• Vomiting: 2-6% vs. 2%
• Decreased appetite: 6-11% vs. 0%

GI events were generally mild to moderate and occurred primarily during dose escalation.

Hypoglycemia:

• Clinically significant hypoglycemia (<54 mg/dL): 0% across all groups
• This is a critical safety advantage — tirzepatide achieved dramatic glucose lowering without causing dangerous hypoglycemia, reflecting its glucose-dependent mechanism of action

Discontinuation due to adverse events: 3-7% (tirzepatide) vs. 3% (placebo) [1].

Comparison to Other Monotherapy Trials

SURPASS-1 results compared favorably to other diabetes monotherapy trials:

The magnitude of HbA1c reduction with tirzepatide monotherapy exceeded what most combination therapies achieve [1].

Clinical Significance

SURPASS-1 established several important precedents:

1. Proof of concept: Validated the dual GIP/GLP-1 agonist mechanism in a rigorous Phase 3 setting
2. Monotherapy potential: Demonstrated that tirzepatide could be used as first-line therapy, not just as an add-on
3. Glycemic normalization: Showed that pharmacological normalization of HbA1c is achievable in a substantial proportion of patients
4. Foundation for SURPASS program: Provided the safety and efficacy foundation for the subsequent SURPASS-2 through SURPASS-5 trials

References

1. Rosenstock J, Wysham C, Frías JP, et al. "Efficacy and safety of a novel dual GIP and GLP-1 receptor agonist tirzepatide in patients with type 2 diabetes (SURPASS-1): a double-blind, randomised, phase 3 trial." The Lancet. 2021;398(10295):143-155. PubMed: 34186022

2. Coskun T, Sloop KW, Loghin C, et al. "LY3298176, a novel dual GIP and GLP-1 receptor agonist for the treatment of type 2 diabetes mellitus." Molecular Metabolism. 2018;18:3-14. PubMed: 30473097

3. Nauck MA, D'Alessio DA. "Tirzepatide, a dual GIP/GLP-1 receptor co-agonist for the treatment of type 2 diabetes with unmatched effectiveness regarding glycaemic control and body weight reduction." Cardiovascular Diabetology. 2022;21:169. PubMed: 36050763