High-Dose Vitamin D3 Falls Short in Phase 3 Colorectal Cancer Trial

A supplement that once looked promising for extending survival in metastatic colorectal cancer (mCRC) failed to clear the bar in its confirmatory trial. For pharmacists fielding questions from patients who read about vitamin D and cancer outcomes, the phase 3 SOLARIS trial (NCT04094688) gives a clear, evidence-based answer: high-dose supplementation added to standard chemotherapy doesn't improve progression-free survival (PFS) compared with standard dosing.^1,2

The trial enrolled 455 patients with previously untreated mCRC across 151 US academic and community centers, randomly assigning them 1:1 to receive mFOLFOX6 or FOLFIRI plus bevacizumab, along with either high-dose vitamin D3 (an 8000 IU/d loading dose followed by 4000 IU/d) or standard-dose vitamin D3 (400 IU/d). The study was designed to confirm findings from the earlier phase 2 SUNSHINE trial (NCT03713619), in which high-dose supplementation showed a signal of PFS benefit.^1,3

However, that signal was not sustained. Median PFS was 11.8 months in the high-dose group versus 10.3 months in the standard-dose group—a difference that did not reach statistical significance (HR, 0.92; 95% CI, 0.73-1.16; 1-sided log-rank P = .25). Objective response rate and overall survival were similarly unmoved between arms, and a multivariable-adjusted analysis controlling for age, sex, race, BMI, and other factors told the same story.

What Pharmacists Should Take Away

The most practical takeaway is what this trial rules out: there's no efficacy justification for pushing mCRC patients toward high-dose vitamin D3 as an adjunct to chemotherapy. If a patient or caregiver raises the idea based on earlier headlines about SUNSHINE, pharmacists can now point to a much larger, better-powered trial that did not replicate the benefit.

Safety was not the differentiator either. Rates of grade 3 or higher adverse events, including neutropenia and hypertension, were comparable between arms, and vitamin D–specific toxicities—hypercalcemia, hyperphosphatemia, and renal calculi—were rare and not meaningfully different. High-dose supplementation was well tolerated, and adherence was strong in both groups (96% in each).

One finding worth flagging for the FAQ conversation, with appropriate caveats: a prespecified subgroup analysis found a significant interaction between primary tumor location and treatment effect (P = .02), with patients with left-sided tumors deriving more benefit from high-dose vitamin D3 (median PFS, 13.8 vs 10.2 months) than those with right-sided disease, who trended in the opposite direction. This tracks with prior epidemiologic and translational data suggesting sidedness matters for vitamin D biology in colorectal cancer. But the authors are explicit that this remains exploratory and shouldn't be used to guide practice yet.

The trial also offers a useful pharmacologic note independent of its primary result: the high-dose regimen reliably corrected vitamin D insufficiency into the sufficient range within one restaging cycle and sustained it through treatment, while standard dosing left patients insufficient throughout. That is relevant context if vitamin D repletion is being pursued for reasons unrelated to oncologic outcome—bone health, for instance—even though it doesn't move the needle on PFS.

For counseling purposes, SOLARIS closes the loop that SUNSHINE opened. High-dose vitamin D3 is safe alongside first-line chemotherapy for mCRC, but it is not an evidence-based lever for improving progression-free or overall survival in unselected patients.

REFERENCES

1. Ng K, Ou F, Zemla T, et al. Addition of High-Dose Vitamin D₃ to Standard Treatment in Patients With Metastatic Colorectal Cancer: The SOLARIS Randomized Clinical Trial (Alliance A021703). JAMA. Published online August 03, 2026. doi:10.1001/jama.2026.9350

2. Vitamin D3 With Chemotherapy and Bevacizumab in Treating Patients With Advanced or Metastatic Colorectal Cancer (SOLARIS). ClinicalTrials.gov identifier: NCT04094688. Updated March 3, 2026. Accessed August 7, 2026. https://clinicaltrials.gov/study/NCT04094688

3. This Was a Study of Efficacy and Safety of Two Secukinumab Dose Regimens in Subjects With Moderate to Severe Hidradenitis Suppurativa (HS). (SUNSHINE). ClinicalTrials.gov identifier: NCT03713619. Updated October 9, 2024. Accessed August 7, 2026. https://clinicaltrials.gov/study/NCT03713619