This randomized controlled trial aims to assess whether treatment with denosumab can improve semen quality in infertile men selected by serum anti-mullerian hormone (AMH) as a positive predictive biomarker, and with severely impaired semen quality (concentrations between 0.01 million/mL to 2 million/mL).

Infertility is a common problem globally and impaired semen quality is responsible for up to 40% of all cases. Despite the high prevalence there are currently only very limited treatment options to improve semen quality for infertile men. Instead, almost all infertile couples are treated with inseminations or assisted reproductive techniques (ARTs) independently of the etiology of infertility. ARTs are very successful but expensive and associated with a significant treatment burden of the female partner due to the invasive methodology and the need for hormonal treatment often for several months.

RANKL is a ligand for the receptor activator of nuclear factor kappa beta (RANK), and their pathway plays a prominent role in the regulation of bone metabolism. The binding of RANKL to RANK on osteoclast precursors induces osteoclast maturation and activation, thereby stimulating bone resorption, and regulates cell cycle i.e., proliferation, differentiation, and apoptosis. Osteoprotegerin (OPG) is a secreted decoy receptor that controls RANKL-RANK interaction by binding RANKL and inhibits activation of RANK and preventing osteoclast differentiation and activation.

Denosumab, a drug used in millions of patients worldwide under trade names Prolia® and Xgeva®, inhibits the RANKL pathway and is used to treat osteoporosis and bone metastases. The drug's mechanism of action inhibits RANKL and thus inhibits bone resorption through reduced osteoclast activation. This reduces the loss of bone mineral density (BMD), which reduces the risk of bone loss and thereby the risk of fracture and osteoporosis. Denosumab has been shown in several clinical studies to be a safe and effective drug in both women and men and has been in clinical use in both sexes for many years. As Denosumab has a teratogenic effect, pharmacokinetic studies in both monkeys and healthy men were performed before approval of the drug as a treatment for osteoporosis in men. These studies showed that Denosumab concentration in semen does not pose a risk to the fetus during sexual intercourse with the pregnant woman and therefore is safe to use for the suggested infertility indication as there is no risk of fetal transmission.

In light of this, our research group has investigated the effect of denosumab in human testicular germ cell lines as well as in human testicular tissue ex vivo "hanging drop" cultures. Treatment with denosumab did in both cases increase the proliferation of the germinal cells, which is an indicator that denosumab treatment potential beneficial effect on sperm production by reducing apoptosis in the germ cells. This led to a pilot intervention study of 12 infertile men who were overall healthy and without comorbidities. The men were treated with a single-dose of 60 mg of denosumab subcutaneous (s.c.). The study showed that the response to RANKL inhibition was either bad or highly beneficial. This was an interesting finding and indicates that only a fraction of infertile men should be offered denosumab treatment and this fraction of beneficial responders should ideally be identified based on an easily accessible biomarker before initiation of treatment. On this knowledge, a randomized controlled trial, "First In Treating Male Infertility" (FITMI), is being conducted to explore whether treatment with denosumab can improve semen quality in infertile men who are selected by serum AMH, but already have a sperm concentration that is at least 2 million/mL. This is an important study, but unfortunately, it leaves out a solution for those with sperm concentration under 2 million/mL, who are the most vulnerable in this regard. Therefore, there is a need for a randomized controlled trial that addresses the specific concerns of individuals with sperm concentrations below 2 million/mL to provide a valuable option in an otherwise hopeless situation.

NAPO is a single-center, sponsor-investigator-initiated, placebo-controlled, double-blinded randomized trial. Following successful completion of screening procedures, subjects will be randomized in a 2:1 fashion to receive either denosumab 60 mg s.c. or a placebo. The study will be carried out at the Division of Translational Endocrinology, Copenhagen University Hospital, Herlev, Copenhagen, Denmark.

With the power to avoid a type II error set to 80% (1-β) at a two-sided 5% significance level, 42 men allocated 2:1 in each of the investigation arms are needed to detect a difference in sperm concentration of 100% between intervention and placebo group in the primary outcome. The primary analysis will be a covariance analysis in which day 80 measurements are analyzed, initially as crude values but also adjusted on baseline.

Inclusion Criteria:

• Infertile men ≥ 18 years and < 60 years of age
• Sperm concentration ≤ 2 million pr. mL
• Serum AMH levels ≥28 pmol/L.
• The participants must have appropriate Danish or English language skills and give written informed consent.

Exclusion Criteria:

• Chronic diseases, defined as diagnosis where signs, symptoms, and treatment imply an expected long duration and lack of a cure, such as diabetes mellitus, metabolism disorders, osteoporosis, colitis, etc.
• Sperm concentration <0.01 million pr. mL
• Men with current or previous malignancies, or at potential risk of testicular cancer after baseline examination and ultrasound will be excluded.
• Men with hypocalcemia at baseline, defined as albumin corrected calcium < 2,17 mmol/L or total calcium < 2.14 mmol/L
• Serum vitamin D (25OHD) levels < 25 nmol/L
• estimated Glomerular Filtration Rate (eGFR) < 60 mL/min/1,73 m2
• Insufficient dental status
• Vasectomy
• Hypersensitivity to latex, Denosumab, or to any of the excipients (acetic acid, sodium hydroxide, Sorbitol (E420), Polysorbate 20) will be excluded.
• Serum FSH <3 IU/L
• Testis size > 17 mL
• BMI ≥ 35 kg/m2