Effect of Denosumab on Sperm Concentration in Men With Severe Oligospermia: A Randomized Controlled Trial

Denosumab 60 mg produced no improvement in sperm concentration over placebo in 39 men with severe oligospermia

Journal: Journal of Clinical Endocrinology & Metabolism | Published: 2026-02-20 | Type: Randomized Controlled Trial | PMID: 40858297 Authors: Yahyavi SK, Jorsal MJ, Wulff SM, Paredes CE, Bekker MC, Melsen LM, Nøhr B, Holt R, Blomberg Jensen M (Copenhagen University Hospital — Rigshospitalet) Funding/COI: Candys Foundation, The Innovation Foundation, Rigshospitalet, Novo Nordisk Foundation. No COI listed.

Summary

Denosumab — a RANKL inhibitor approved for osteoporosis — was proposed as a fertility treatment after a prior RCT reported semen quality improvements in a subset of infertile men with elevated AMH and small testes. This follow-up trial prospectively selected patients using those same biomarkers and found nothing: denosumab produced no advantage over placebo on any sperm parameter or reproductive hormone. Both groups improved from baseline, which is what regression to the mean looks like.

Claims

• No statistically significant difference in sperm concentration between denosumab and placebo at day 80
• No difference in total sperm count between groups
• No difference in serum testosterone, LH, FSH, inhibin B, or AMH
• Both groups showed increases in sperm concentration and total sperm count from baseline — but not in inhibin B
• n=39 completed (26 denosumab, 13 placebo); 3 dropouts from the original 42

Study Quality

Double-blind, placebo-controlled RCT with prospective biomarker-based patient selection — a methodologically appropriate design given the prior trial's hypothesis. The 2:1 randomization ratio is a defensible choice to gather more safety data on the active drug but reduces statistical power in the placebo arm. At n=39 completers this trial is underpowered; the prior RCT that generated the hypothesis was also small, so the field is stacking small trials on top of small trials. Eighty days is a reasonable window given the ~74-day human spermatogenesis cycle, though a longer follow-up would capture a full second cycle. Single-center limits generalizability.

Red Flags

• Very small sample: 26 treated, 13 placebo. Underpowered to detect anything but a large effect size
• Both groups improved from baseline with no between-group difference — a classic placebo response pattern, likely regression to the mean in a population selected for severe oligospermia
• The entire premise rests on a single prior small RCT; replication in a larger, independent cohort was never done before this confirmatory study
• Single-center (Copenhagen); all participants presumably on vitamin D and calcium supplementation per the protocol, which the authors themselves note confounds interpretation
• No conflict of interest listed, but Novo Nordisk Foundation funding is notable given Novo Nordisk's involvement in the denosumab market

Strengths

• Properly blinded and placebo-controlled — the gold standard for this type of question
• Prospective biomarker selection (AMH, testicular volume) attempted to enrich for likely responders, which was the right scientific approach
• Null result clearly reported without spin; authors conclude denosumab is "not superior to the vitamin D- and calcium-supplemented men in the placebo group"
• Outcome measures were pre-specified and objective (semen analysis, serum hormones)

Verdict

A clean null result that closes a door. The RANKL-inhibitor hypothesis for male infertility was always speculative, built on a single small trial, and this confirmatory RCT — while itself underpowered — found nothing across every measured endpoint. The fact that both groups improved equally from baseline is the real story: severe oligospermia fluctuates, and without adequate controls you'd have called this drug a success. The paper is worth reading precisely because it's a well-executed negative trial in a field that badly needs them.