Doctors say repurposing a common breast cancer drug could help men struggling with infertility. In an open-label clinical trial, men who used the drug, letrozole, increased their sperm count to a level that could facilitate less invasive fertility treatments.
The drug blocks estrogen production and is normally used as an adjunct therapy for hormone-receptor-positive postmenopausal breast cancer. Researchers in Shanghai suspected it might also benefit men with low sperm count, and last week they presented results from a preliminary clinical trial.
Fourteen percent of men who took letrozole for three months improved their sperm count scores under the World Health Organization’s classification system, leading to an upgrade in their fertility rating. That’s nearly three times the improvement seen in the control group.
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The average volunteer was aged around 30 and Han Chinese (96%). Participants had an average BMI of 25. Around one third admitted to being regular smokers and one in three said they liked a drink. Most of the men lived in a cooler region.
A Drug for Breast Cancer, Repurposed for Fertility
The team recruited 296 men aged 22 to 50 who had two consecutive sperm samples showing no sperm or very low concentration. Doctors excluded men with Y chromosome deletions, blocked tubes, gonadotrophin deficiency, or genetic mutations in sperm-making genes — conditions where boosting testosterone and lowering estrogen wouldn’t help.
Volunteers were randomized into two groups. One group took a daily 2.5 mg dose of letrozole, plus vitamin C and vitamin E pills, for 12 weeks. The other group got only the vitamins. Semen samples were collected at the start and end of the trial. Reproductive scientists independently scored each man using both the WHO Sperm Concentration Categories and the Dutch Society of Obstetrics and Gynaecology Total Motile Sperm Count Categories.
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The study was open label for participants and investigators, but the sperm assessors did not know which samples came from which group. That means the assessment of whether any change occurred was unbiased, even if a placebo effect could still factor in.
Who Benefited and Who Didn’t
After three months, 21 of the 147 men prescribed letrozole showed an upgrade in sperm production based on the WHO categories, compared with 8 of 147 men in the control group. The odds of success for a man using letrozole were calculated at 2.65 to 1. Under the Dutch categorization, 6 men in the letrozole group improved versus one man in the control group. Men with severe oligozoospermia benefitted the most, being 43 times more likely to show an upgrade, followed by those with cryptozoospermia.
But the researchers didn’t see any overall significant differences in semen parameters between the two groups. The reason? While 21 men showed a big change, 119 did not improve at all. When you average the change over all 147 participants, the upgrade disappears. That’s a common problem in clinical trials — more investigation is needed to figure out who the drug is likely to help before you can measure the magnitude.
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Men who used letrozole reported less sexual function and lower libido. Eighteen percent more men in the drug group said they felt less frisky compared to the control group. Some even developed proteinuria, a sign of kidney damage. No serious adverse events occurred and nobody died.
Clinical Relevance and Next Steps
The increase in sperm count, while modest for the group as a whole, is clinically useful for men who wish to undergo fertility treatment. Men who used letrozole were less likely to need surgical intervention to retrieve sperm — 11 out of 105 in the drug group versus 6 out of 113 in the control group. The researchers noted that the drug influenced men’s sex hormones, increasing testosterone levels and decreasing estradiol levels.
Overall, this appeared to be a successful treatment for a subgroup of men, but the mixed results mean the approach isn’t for everyone. The project continues, and scientists are already working to identify which patients are most likely to respond.
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