Endocrine Abstracts

Nebido (testosterone undecanoate, 1000 mg) is a new, long acting, intramuscular (IM) preparation of testosterone (T) that can be administered every 10–14 weeks. We conducted a study to evaluate the factors that influence serum T levels after commencing Nebido and may therefore help clinicians estimate dosing frequency for this therapy.

Following the recommended loading regimen, 51 hypogonadal men (35, 68.6% secondary hypogonadism) had two injections of Nebido (1000 mg IM) 6 weeks apart. Serum T (assay: stable-isotope dilution liquid chromatography-tandem mass spectrometry) and SHBG were measured 12 weeks later. Forty-three (84%) had been previously treated with another testosterone preparation.

After commencing Nebido baseline T (mean 11.5 nmol/l, range 0.3–54.8) significantly increased (17.2 nmol/l (5.4–32.8), P=0.001). Serum T was within the healthy population reference range (8.0–25.0 nmol/l) in 75% of men. Patients with low testosterone levels were younger (33.4 vs 49.9 years, P=0.03) with an increased body mass (103.7 vs 85.2 kg, P=0.03). Subjects with primary hypogonadism had a higher T (20.9 nmol/l (9.8–32.8) vs 15.5 (5.4–32.6), P=0.02) and SHBG 39.2 nmol/l (11–82) vs 25.7 (9.0–60.0), P=0.003) although the calculated bioavailable testosterone was not significantly different (4.9 nmol/l (2.9–7.3) vs 4.2 (2.0–7.9), P=0.12).

On Nebido the serum T positively correlated with age (R=0.37, P=0.007) and negatively correlated with weight (R=−0.39, P=0.004), BMI (R=−0.40, P=0.003) and BSA (R=−0.36, P=0.009). Using multiple regression analysis age (t=2.04, P=0.05) and baseline T (t=−9.26, P<0.0001) were independent variables of the increase in T.

Conclusion: Age and baseline T are independent variables of the increase in T with Nebido and T levels correlated with body size. These results may then be used to estimate dosing frequency for this therapy.