# Borrow from past control arms to shrink the control group in phase 3

Source: https://onco.cc/ideas/idea-tr1-bayesian-borrowing-smaller-controls/  
OnCo record `idea-tr1-bayesian-borrowing-smaller-controls` (Idea). Data CC BY-NC 4.0, attribute "Data from OnCo (onco.cc)"; commercial use needs a licence.

## TL;DR

When the standard treatment has been given to thousands of similar patients in earlier trials, a new trial could randomise fewer people to it and lean on that history, as long as the old data still matches.

## Summary

Hybrid control designs use Bayesian dynamic borrowing (power priors, meta-analytic-predictive priors, commensurate priors) from curated historical control arms and real-world cohorts to reduce concurrent control allocation to 1:2 or 1:3 while retaining randomisation. Borrowing is down-weighted automatically when the concurrent controls diverge from history. The FDA Complex Innovative Design programme has reviewed such designs; oncology uptake is limited.

## Fields

- Kind: Idea
- Last checked: 2026-09-08
- Hypothesis: Hybrid-control phase 3 trials in well-characterised indications will reach the same decision as a conventional 1:1 trial with 25-40 percent fewer control patients and shorter accrual, and the borrowing weight will correctly fall when historical drift exists.
- Rationale: Control-arm outcomes in first-line NSCLC, metastatic breast and CRC are stable across recent trials with the same regimen; requiring 400 fresh control patients every time is a cost paid by patients as well as sponsors.
- Proposed test: Re-analyse three completed phase 3 trials as if run with dynamic borrowing from pre-existing control data; confirm identical conclusions. Then run one prospective hybrid trial under the CID programme.
- Maturity: early-clinical
- Actor: industry

## Sources

- FDA Complex Innovative Trial Design meeting programme: https://www.fda.gov/drugs/development-resources/complex-innovative-trial-design-meeting-program

## Connected records

- cancers: [Colorectal cancer](https://onco.cc/cancers/colorectal/), [Non-small-cell lung cancer](https://onco.cc/cancers/nsclc/)
- terms: [Hazard ratio (HR)](https://onco.cc/terms/hazard-ratio/), [Standard of care](https://onco.cc/terms/standard-of-care/)
- bottlenecks: [Trial design, endpoints and cost](https://onco.cc/bottlenecks/b-trial-design/), [Trials enrol too few, too slowly](https://onco.cc/bottlenecks/b-trial-enrolment/)

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