PROTECT: Teplizumab in children and adolescents with recent-onset type 1 diabetes

What PROTECT tested and why it matters

In type 1 diabetes, the immune system attacks the insulin-making beta cells in the pancreas. At diagnosis, most people still have some working beta cells — a "honeymoon" window that fades over months to years. PROTECT asked a focused question: can a short course of the immune therapy teplizumab, given right after diagnosis, slow that loss and protect the body's own insulin production?¹

Teplizumab is an anti-CD3 monoclonal antibody that dampens the T cells driving the attack.¹ It is FDA-approved (as Tzield) to delay the onset of clinical (stage 3) diabetes in people aged 1 and older who have earlier-stage (stage 2) disease, and the FDA granted accelerated approval in 2026 for the newly diagnosed 8-17 age group PROTECT studied.² PROTECT asked the further question: does it help people who have already been diagnosed?¹

Who it was for

PROTECT enrolled 328 children and adolescents aged 8-17 who had been diagnosed with type 1 diabetes within the previous 6 weeks, had positive diabetes autoantibodies, and still had measurable insulin production.¹³ It ran across roughly 47 sites in the US, Canada, the UK, and several European countries.³

How it was designed

This was a Phase 3, randomized, double-blind, placebo-controlled trial.¹ Participants were assigned 2:1 to teplizumab (217) or placebo (111), each given as two 12-day intravenous courses six months apart.¹³ The main outcome was the change in stimulated C-peptide — a blood marker of how much insulin the pancreas can still make — measured at week 78 (about 18 months).¹³

Key results

PROTECT met its primary goal. At week 78, teplizumab-treated patients had significantly higher stimulated C-peptide than the placebo group (least-squares mean difference 0.13 pmol/mL; 95% CI 0.09-0.17; P<0.001).¹ Nearly all treated patients (94.9%) kept a clinically meaningful C-peptide level of at least 0.2 pmol/mL, versus 79.2% on placebo.¹

Importantly, the day-to-day diabetes measures that matter most to patients — insulin dose, HbA1c, time in target glucose range, and hypoglycemia — did not differ significantly between the groups.¹ The current label also carries a boxed warning about serious viral reactivation and has additional infection, immune-status and laboratory precautions.² Common side effects were headache, gastrointestinal symptoms, rash, lymphopenia (low white-cell counts), and mild cytokine release syndrome, mostly around the infusions.¹

September 2026 per-protocol analysis

A prespecified per-protocol analysis published on 10 September 2026 included 275 participants (180 teplizumab, 95 placebo). At week 78 it reported a C-peptide difference of 0.14 nmol/L, an insulin-dose difference of −0.17 U/kg/day and 6.17 percentage points more time in range (95% CI 0.13–12.2; nominal p<0.05). This selected analysis excluded participants with low adherence or other protocol deviations, including some whose dosing stopped for adverse events. It supports further interpretation but does not replace the original intention-to-treat result, whose key secondary endpoints were not significant.⁴

What it means and what's next

PROTECT shows teplizumab can preserve beta-cell function after diagnosis — biological proof that the disease can be slowed even after symptoms begin.¹ The original trial did not establish significant differences in its key clinical secondary outcomes at 18 months. The selected September per-protocol analysis adds context, but the long-term clinical meaning of preserving C-peptide remains a separate question.¹⁴